Information

3.10: Studying Cells - Biology


3.10: Studying Cells

3.10: Studying Cells - Biology

A cell is the smallest unit of a living thing and is the basic building block of all organisms.

Learning Objectives

State the general characteristics of a cell

Key Takeaways

Key Points

  • A living thing can be composed of either one cell or many cells.
  • There are two broad categories of cells: prokaryotic and eukaryotic cells.
  • Cells can be highly specialized with specific functions and characteristics.

Key Terms

  • prokaryotic: Small cells in the domains Bacteria and Archaea that do not contain a membrane-bound nucleus or other membrane-bound organelles.
  • eukaryotic: Having complex cells in which the genetic material is contained within membrane-bound nuclei.
  • cell: The basic unit of a living organism, consisting of a quantity of protoplasm surrounded by a cell membrane, which is able to synthesize proteins and replicate itself.

Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks, and the building blocks of your body are cells.

Cells as Building Blocks

A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Several cells of one kind that interconnect with each other and perform a shared function form tissues several tissues combine to form an organ (your stomach, heart, or brain) and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). There are many types of cells all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and plant cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic.

Types of Specialized Cells

Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover the organs and body cavities within. Bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.

Various Cell Types: (a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope), and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure.


3.10: Studying Cells - Biology

cell 'sel n 1 : the basic unit of structure & function in living things

In just a second we'll review some of the dudes who were among the first to study cells. In order for any of these guys to make any observations or discoveries, there had to have been certain technological tools available to them. The biggest of these tools was certainly the compound light microscope. An understanding of the microscope is "a must" in any biology course.
I have dedicated a page to the microscope in my "LAB REVIEW" Pages. To look at that now, click here .

For now, here is a quick exercise to review some important tools & techniques related to cell study. Match the tool or technique with its description.

Tools & Techniques
For Cell Study
compound microscope
electron microscope
microdissection apparatus
phase-contrast microscope
simple microscope
staining
stereomicroscope
ultracentrifuge

1. microscope composed of one lens
2. microscope that creates an image using two lenses
3. adding a chemical that makes certain cell structures easier to see, usually kills the cells
4. a high resolution microscope used to study living cells
5. microscope that provides images of the greatest magnification & resolution
6. microscope with two oculars, usually used during dissections to observe relatively large structures in more detail
7. small tools used to remove or transplant cell organelles
8. machine that can be used to separate cell organelles according to their densities

After you've done your best, check your answers < herE >.

OK, let's continue our review/study of the cell by reviewing the guys who first studied the cell. Number from 1-7 on a piece of paper & match each dude with his claim to fame.

Anton van Leeuwenhoek (1670)
Robert Hooke (1665)
Robert Brown (1831)
Matthias Schleiden (1838)
Theodor Schwann (1838)
Johannes Purkinje (1839)
Rudolf Virchow (1858)

  • the exact wording of the cell theory may vary depending on what textbook you use, but it's always composed of those 3 ideas
  • the "life functions" referred to in statement #2 are the same ones covered on my life functions page
  • if you peek back to the scientists review, you may notice that statements #1 & 2 are primarily a combo of Schleiden & Schwann's research, & statement #3 is Virchow's big idea
  • keep in mind that The Cell Theory is based on over 300 years of scientific investigations, beginning with Hooke in 1665 and continuing through today
  • some notable questions or "exceptions" to the Cell Theory do exist

Exceptions to the Cell Theory :

1. Viruses - are they alive ?
According to the Cell Theory we have to say "no" because a virus is not a cell. Viruses are made of two chemicals, protein & nucleic acid, but have no membranes, nucleus, or protoplasm. They appear to be alive when they reproduce after infecting a host cell.
2. Mitochondria & chloroplasts.
These cell organelles (small structures inside the cell) have their own genetic material & reproduce independently from the rest of the cell.
3. Where did the first cell come from ?
According to statement #3 of the cell theory, all cells come from other living cells. So how did the first cell ever appear ? It's the old "chicken & egg" dilemma. We will investigate this question (& its possible answer) in more detail during the Evolution Unit.

CELL STRUCTURE & ORGANELLES

In this course we concern ourselves mostly with the differences between prokaryotic cells & eukaryotic cells, and between animal cells & plant cells.

The prokaryotic-eukaryotic difference is easy prokaryotic cells do not have a nucleus & eukaryotic cells do. Remember that all prokaryotic organisms are classified in the Moneran Kingdom. The organisms in the other four Kingdoms have eukaryotic cells.
(If you want to review the 5 Kingdoms, check out my "5 Kingdoms Page".)

The animal-plant cell differences aren't too bad either. Basically, some organelles are found in plant cells and not animal cells & vice versa. More on that in a minute.

LET'S LABEL SOME ORGANELLES .

ANIMAL CELL

WORD BANK
cytoplasm
centrioles
chromosomes (DNA)
endoplasmic reticulum
golgi body
lysosome
mitochondria
nucleolus
nucleus
plasma membrane
ribosome
vacuole

Check here to click your work ---
I mean, click here to check your work.

Before we label a plant cell, let's take a
sTudY BReaK .

OK, time to label a typical plant cell (notice the green?). Usually, plant cell diagrams focus on structures that distinguish plant cells from animal cells. So some of the organelles that animal & plant cells have in common (like ribosomes, golgi bodies, endoplasmic reticulum) get left out all together. Label the diagram below & see what I mean .

PLANT CELL

WORD BANK
cell wall
chloroplast
cytoplasm
nucleolus
nucleus
plasma membrane
vacuole (large)

PLANT CELLS vs ANIMAL CELLS

If you can label diagrams of a plant or animal cell, then you pretty much know what the differences are between them.

This table summarizes the differences :

ORGANELLE ANIMAL CELL PLANT CELL
centrioles visible none (not visible)
cell wall none present
chloroplasts none present
vacuole small large

ORGANELLES & THEIR FUNCTIONS

Now that you know what each organelle looks like, it's time to get the functions of each organelle to stick to your brain somewhere. Choose an organelle from the word bank for each description in #1-15.

WORD BANK
cell membrane
cell wall
chloroplast
centrioles
centrosome
cytoplasm
endoplasmic reticulum
golgi apparatus
lysosome
mitochondria
nuclear membrane
nucleolus
nucleus
ribosomes
vacuole

  • Some organelles have two names : golgi apparatus = golgi bodies cell membrane = plasma membrane.
  • The organelles that you, an average biology student, would see in lab using a compound light microscope would be : the cell membrane, cell wall, nucleus, nucleolus, chloroplasts, large vacuoles. Other organelles require higher magnification and better resolution microscopes that typical high schools don't have on hand.
  • You should know a few more details about the plasma (cell) membrane. It is composed of lipids (fat molecules) & protein in what is described as the "Fluid Mosaic Model".

Now that you know everything there is to know about cells, may I suggest you brush up on your understanding of the microscope ?
Check out my Microscope Page.

Back to Biology Topics Outline

IF YOU HAVE COMMENTS (GOOD OR BAD) ABOUT THIS OR ANY OF MY BIOLOGY PAGES, CLASSES, OR ANYTHING ELSE IN GENERAL, DROP ME A NOTE :
[email protected]

StuDyiNG the CeLL - ANSWER PAGES

Answers to "Tool & Techniques for Cell Study" :
1. simple microscope
2. compound microscope
3. staining
4. phase-contrast microscope
5. electron microscope
6. stereomicroscope
7. microdissection apparatus
8. ultracentrifuge
<-- back

Answers to "Cell Scientists Matching" :
1) J. Purkinje
2) R. Hooke
3) R. Virchow
4) A. von Leeuwenhoek
5) M. Schleiden
6) M. Schwann (has an "a" in last name - animal cells)
7) R. Brown
<--back

Answers to "Animal Cell Diagram" :

1. lysosome
2. endoplasmic reticulum
3. chromosome (DNA)
4. golgi body (apparatus)
5. vacuole
6. mitochondria
7. ribosome
8. nucleolus
9. nucleus (nuclear membrane would also be OK)
10. centrioles
11. plasma membrane
12. cytoplasm

Answers to "Plant Cell Diagram":

1. nucleus
2. nucleolus
3. plasma membrane
4. cytoplasm
5. cell wall
6. vacuole
7. chloroplast

ANSWERS to "Organelle Functions" Matching :
1. cytoplasm
2. cell membrane
3. nucleus
4. cell wall
5. ribosomes
6. endoplasmic reticulum
7. golgi apparatus
8. mitochondria
9. vacuole
10. nuclear membrane
11. nucleolus
12. lysosome
13. centrioles
14. centrosome
15. chloroplast
<-- back


3.3 Eukaryotic Cells

At this point, it should be clear that eukaryotic cells have a more complex structure than do prokaryotic cells. Organelles allow for various functions to occur in the cell at the same time. Before discussing the functions of organelles within a eukaryotic cell, let us first examine two important components of the cell: the plasma membrane and the cytoplasm.

Visual Connection

What structures does a plant cell have that an animal cell does not have? What structures does an animal cell have that a plant cell does not have?

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 3.8) made up of a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule composed of two fatty acid chains, a glycerol backbone, and a phosphate group. The plasma membrane regulates the passage of some substances, such as organic molecules, ions, and water, preventing the passage of some to maintain internal conditions, while actively bringing in or removing others. Other compounds move passively across the membrane.

The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus). This folding increases the surface area of the plasma membrane. Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form matching the function of a structure.

People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.

The Cytoplasm

The cytoplasm comprises the contents of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals (Figure 3.7). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The Cytoskeleton

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that helps to maintain the shape of the cell, secures certain organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently. Collectively, this network of protein fibers is known as the cytoskeleton . There are three types of fibers within the cytoskeleton: microfilaments, also known as actin filaments, intermediate filaments, and microtubules (Figure 3.9).

Microfilaments are the thinnest of the cytoskeletal fibers and function in moving cellular components, for example, during cell division. They also maintain the structure of microvilli, the extensive folding of the plasma membrane found in cells dedicated to absorption. These components are also common in muscle cells and are responsible for muscle cell contraction. Intermediate filaments are of intermediate diameter and have structural functions, such as maintaining the shape of the cell and anchoring organelles. Keratin, the compound that strengthens hair and nails, forms one type of intermediate filament. Microtubules are the thickest of the cytoskeletal fibers. These are hollow tubes that can dissolve and reform quickly. Microtubules guide organelle movement and are the structures that pull chromosomes to their poles during cell division. They are also the structural components of flagella and cilia. In cilia and flagella, the microtubules are organized as a circle of nine double microtubules on the outside and two microtubules in the center.

The centrosome is a region near the nucleus of animal cells that functions as a microtubule-organizing center. It contains a pair of centrioles, two structures that lie perpendicular to each other. Each centriole is a cylinder of nine triplets of microtubules.

The centrosome replicates itself before a cell divides, and the centrioles play a role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division is not clear, since cells that have the centrioles removed can still divide, and plant cells, which lack centrioles, are capable of cell division.

Flagella and Cilia

Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell, (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, they are many in number and extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecium) or move substances along the outer surface of the cell (for example, the cilia of cells lining the fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that move particulate matter toward the throat that mucus has trapped).

The Endomembrane System

The endomembrane system (endo = within) is a group of membranes and organelles (Figure 3.13) in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell (Figure 3.7). The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 3.10).

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 3.10). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material, and proteins. This combination of DNA and proteins is called chromatin. In eukaryotes, chromosomes are linear structures. Every species has a specific number of chromosomes in the nucleus of its body cells. For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is eight.

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes resemble an unwound, jumbled bunch of threads.

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus, called the nucleolus (plural = nucleoli), aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (Figure 3.13) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: the rough endoplasmic reticulum and the smooth endoplasmic reticulum, respectively.

The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.

The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER where they undergo modifications such as folding or addition of sugars. The RER also makes phospholipids for cell membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane (Figure 3.13). Since the RER is engaged in modifying proteins that will be secreted from the cell, it is abundant in cells that secrete proteins, such as the liver.

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (see Figure 3.7). The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and steroid hormones detoxification of medications and poisons alcohol metabolism and storage of calcium ions.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place. The sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs (Figure 3.11).

The Golgi apparatus has a receiving face near the endoplasmic reticulum and a releasing face on the side away from the ER, toward the cell membrane. The transport vesicles that form from the ER travel to the receiving face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications. The most frequent modification is the addition of short chains of sugar molecules. The newly modified proteins and lipids are then tagged with small molecular groups to enable them to be routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into vesicles that bud from the opposite face of the Golgi. While some of these vesicles, transport vesicles, deposit their contents into other parts of the cell where they will be used, others, secretory vesicles, fuse with the plasma membrane and release their contents outside the cell.

The amount of Golgi in different cell types again illustrates that form follows function within cells. Cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundant number of Golgi.

In plant cells, the Golgi has an additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.

Lysosomes

In animal cells, the lysosomes are the cell’s “garbage disposal.” Digestive enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. In single-celled eukaryotes, lysosomes are important for digestion of the food they ingest and the recycling of organelles. These enzymes are active at a much lower pH (more acidic) than those located in the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, thus the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

Lysosomes also use their hydrolytic enzymes to destroy disease-causing organisms that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (Figure 3.12).

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Vacuoles are somewhat larger than vesicles, and the membrane of a vacuole does not fuse with the membranes of other cellular components. Vesicles can fuse with other membranes within the cell system. Additionally, enzymes within plant vacuoles can break down macromolecules.

Visual Connection

Why does the cis face of the Golgi not face the plasma membrane?

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, free ribosomes appear as either clusters or single tiny dots floating freely in the cytoplasm. Ribosomes may be attached to either the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum (Figure 3.7). Electron microscopy has shown that ribosomes consist of large and small subunits. Ribosomes are enzyme complexes that are responsible for protein synthesis.

Because protein synthesis is essential for all cells, ribosomes are found in practically every cell, although they are smaller in prokaryotic cells. They are particularly abundant in immature red blood cells for the synthesis of hemoglobin, which functions in the transport of oxygen throughout the body.

Mitochondria

Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. The formation of ATP from the breakdown of glucose is known as cellular respiration. Mitochondria are oval-shaped, double-membrane organelles (Figure 3.14) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae, which increase the surface area of the inner membrane. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria because muscle cells need a lot of energy to contract.

Peroxisomes

Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. Alcohol is detoxified by peroxisomes in liver cells. A byproduct of these oxidation reactions is hydrogen peroxide, H2O2, which is contained within the peroxisomes to prevent the chemical from causing damage to cellular components outside of the organelle. Hydrogen peroxide is safely broken down by peroxisomal enzymes into water and oxygen.

Animal Cells versus Plant Cells

Despite their fundamental similarities, there are some striking differences between animal and plant cells (see Table 3.1). Animal cells have centrioles, centrosomes (discussed under the cytoskeleton), and lysosomes, whereas plant cells do not. Plant cells have a cell wall, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not.

The Cell Wall

In Figure 3.7b, the diagram of a plant cell, you see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protist cells also have cell walls.

While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose, a polysaccharide made up of long, straight chains of glucose units. When nutritional information refers to dietary fiber, it is referring to the cellulose content of food.

Chloroplasts

Like mitochondria, chloroplasts also have their own DNA and ribosomes. Chloroplasts function in photosynthesis and can be found in eukaryotic cells such as plants and algae. In photosynthesis, carbon dioxide, water, and light energy are used to make glucose and oxygen. This is the major difference between plants and animals: Plants (autotrophs) are able to make their own food, like glucose, whereas animals (heterotrophs) must rely on other organisms for their organic compounds or food source.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked, fluid-filled membrane sacs called thylakoids (Figure 3.15). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane and surrounding the grana is called the stroma.

The chloroplasts contain a green pigment called chlorophyll, which captures the energy of sunlight for photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria also perform photosynthesis, but they do not have chloroplasts. Their photosynthetic pigments are located in the thylakoid membrane within the cell itself.

Evolution Connection

Endosymbiosis

We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species live in close association and typically exhibit specific adaptations to each other. Endosymbiosis (endo-= within) is a relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. Microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and are provided a stable habitat and abundant food by living within the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that mitochondria and chloroplasts have DNA and ribosomes, just as bacteria do. Scientists believe that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them. Through evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the photosynthetic bacteria becoming chloroplasts.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 3.7, you will see that plant cells each have a large, central vacuole that occupies most of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. In plant cells, the liquid inside the central vacuole provides turgor pressure, which is the outward pressure caused by the fluid inside the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That is because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm and into the soil. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of a plant results in the wilted appearance. Additionally, this fluid has a very bitter taste, which discourages consumption by insects and animals. The central vacuole also functions to store proteins in developing seed cells.

Extracellular Matrix of Animal Cells

Most animal cells release materials into the extracellular space. The primary components of these materials are glycoproteins and the protein collagen. Collectively, these materials are called the extracellular matrix (Figure 3.16). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other.

Blood clotting provides an example of the role of the extracellular matrix in cell communication. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel, stimulates adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors.

Intercellular Junctions

Cells can also communicate with each other by direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata (singular = plasmodesma) are junctions between plant cells, whereas animal cell contacts include tight and gap junctions, and desmosomes.

In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell walls surrounding each cell. Plasmodesmata are numerous channels that pass between the cell walls of adjacent plant cells, connecting their cytoplasm and enabling signal molecules and nutrients to be transported from cell to cell (Figure 3.17a).

A tight junction is a watertight seal between two adjacent animal cells (Figure 3.17b). Proteins hold the cells tightly against each other. This tight adhesion prevents materials from leaking between the cells. Tight junctions are typically found in the epithelial tissue that lines internal organs and cavities, and composes most of the skin. For example, the tight junctions of the epithelial cells lining the urinary bladder prevent urine from leaking into the extracellular space.

Also found only in animal cells are desmosomes , which act like spot welds between adjacent epithelial cells (Figure 3.17c). They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.

Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 3.17d). Structurally, however, gap junctions and plasmodesmata differ.

Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell Yes Yes Yes
Cytoplasm Provides structure to cell site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleoid Location of DNA Yes No No
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidizes and breaks down fatty acids and amino acids, and detoxifies poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells organizing center of microtubules in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes No
Cell wall Protection, structural support and maintenance of cell shape Yes, primarily peptidoglycan in bacteria but not Archaea No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm.
Cilia Cellular locomotion, movement of particles along extracellular surface of plasma membrane, and filtration No Some No

This table provides the components of prokaryotic and eukaryotic cells and their respective functions.


3.10 Summary

  • Biochemical reactions are chemical reactions that take place inside of living things. The sum of all of the biochemical reactions in an organism is called metabolism.
  • Metabolism includes catabolic reactions, which are energy-releasing (exothermic) reactions, as well as anabolic reactions, which are energy-absorbing (endothermic) reactions.
  • Most biochemical reactions need a biological catalyst called an enzyme to speed up the reaction. Enzymes reduce the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, which is called the enzyme’s substrate.
  • There are many inherited metabolic disorders in humans. Most of them are caused by a single defective or missing enzyme.

3. Protein-protein and protein-DNA interactions

An important task in deciphering protein function is the identification of other entities with which it interacts. Although C. elegans has not been exploited as an organism for biochemical analysis it is clearly amenable to such studies. Below are protocols describing immunoprecipitation (IP) and chromatin immunoprecipitation (ChIP) that should serve as general guidelines for in vivo interaction studies in C. elegans . These interactions can often be confirmed by standard in vitro techniques such as two-hybrid, GST pull-down studies, and electrophoresis mobility shift assays (EMSA).

Protocol 16: Immunoprecipitation from embryo lysates (Ray Chan and Barbara Meyer)

Seeding asynchronous liquid worm culture

Float adults worms off 9-cm NGM plates with 5 ml of M9. Approximately 6𔃆 plates saturated with asynchronous population of worms, but not starved, will be needed to seed each liter of liquid culture.

Add 10󈝻 ml of saturated HB101 and monitor the food supply at least once per day. Spot 1 drop of culture onto a 5-cm NGM plate and allow the liquid to evaporate for about 1𔃀 minutes until the worms can crawl. If there is sufficient food, the worms should not scatter or forage. Alternatively, starved worms appear somewhat translucent.

Harvesting asynchronous culture and seeding synchronous cultures

Filter culture through a 35-μm miracloth (Calbiochem) to collect gravid hermaphrodites.

Wash with approximately 0.5 L of dH2O.

Treat each liter of culture with 100 ml of freshly made alkaline-bleach solution. Mix on a stir-plate for 5󈝶 minutes (for mutant worms, it may be advisable to bleach for less than 5 minutes). Stop the bleaching process when the adult worms start to break open.

Alkaline-bleach solution (100 mL):
75 mL H2O
20 mL commercial bleach
5 mL 10 N NaOH

Weigh a 250-mL conical centrifuge tube. Record weight __________ g.

Centrifuge the bleached worms at 1𔃀,000 rpm in a tabletop centrifuge. Stop the centrifuge as soon as the speed reaches 2,000 rpm. (It takes several minutes for the rotor to come to a complete stop).

Resuspend the worm in M9 and centrifuge as in step 5. Repeat this wash step once more for a total of two M9 washes.

Weigh the centrifuge tube with the washed embryos.

Combined weight of centrifuge tube and embryos __________ g.

Weight of the embryos __________ g.

Seed 0.5 to 2 grams of embryos per 1-L completed S-basal medium. Generally use 0.5𔂿 g for N2 and 1𔃀 g for mutant worms. Allow the embryos to hatch overnight without food and feed the synchronized L1 larvae the next day.

Wash unused embryos in homogenization buffer, resuspend in 1 mL of homogenization buffer per gram of embryo and store in − 80°C freezer.

All steps are performed at 4°C.

Thaw 10󈝻 mL of embryos frozen in homogenization buffer.

Place the tube on ice and sonicate it with ten 30-second bursts at 15% power on a Heat System XL2020 sonicator with a standard tapered microtip (cat.#419, 3 mm diameter). Wait 1 minute in between bursts for cooling.

Pellet debris by centrifugation at 6,500 rpm (5,000 × g) in a SS-34 rotor for 20 minutes. Collect the supernatant in a clean 50-mL tube.

Sonicate as described in step 2 to shear the DNA. Pellet debris by centrifugation at 14,500 rpm (25,000 × g) in a SS-34 rotor for 20 minutes. Collect and quick-freeze the supernatant in 0.5𔂿 mL aliquots.

All steps performed at 4°C.

Thaw embryo lysates on ice. Incubate lysates with Protein A Sepharaose beads (100 μL per mL lysates Amersham) or IgGsorb (100 μL per mL lysates The Enzyme Center) for 10󈞊 minutes. Spin in a microcentrifuge for 2 min. at 500 × g to pellet the Sepharose beads or IgGsorb. Transfer the supernatant to a new microfuge tube and spin in a microcentrifuge at top speed for 10 min. Use the supernatant for IP.

Incubate approximately 5 μg of affinity purified antibodies with 3 mg total protein of embryo lysates for 2 hrs. Bring the final volume up to 1.0 to 1.4 mL using ChIP buffer with 140 mM KCl.

Pellet non-specific precipitates by spinning in a microcentrifuge at top speed (approximately 16,000 × g ) for 10 min.

Transfer the supernatant to a new microfuge tube with 25 μL of Protein A Sepharose and place on a rocker/nutator for 30 min.

Pellet the antibody-antigen complexes captured on the Protein A Sepharose beads by spinning in a microcentrifuge for 2 min. at 500 × g . Remove the supernatant.

Wash the beads by adding 1 mL of ChIP buffer and spinning in a microcentrifuge for 2 min. at 500 × g . Remove the wash buffer. Repeat this step for a total of four washes.

Elute by boiling the beads with 1 x SDS sample buffer and loading directly onto an SDS-PAGE gel. Alternatively, incubate the beads with 200 μL of 0.1 M glycine (pH 3.0) at room temp. Pellet the beads as described above, remove and save the supernatant. Precipitate the eluate with trichloroacetic acids.

For buffer recipes see the Chromatin Immunoprecipitation protocol below.

Protocol 17: Chromatin Immunoprecipitation From Embryo Lysates (Ray Chan and Barbara Meyer)

Harvest embryos by bleaching gravid hermaphrodites. [A typical yield for N2 is 2𔃃 grams from 25 to 30 9-cm NGM plates].

Wash the embryos extensively with 1x M9 to remove the alkaline bleach solution.

Prepare 100 mL of formaldehyde solution [1 x M9 solution with 2% (v/v) formaldehyde]

Wash the embryos once in the formaldehyde solution. Aspirate away the wash solution. Add fresh formaldehyde solution to 50 mL and gently shake (using a nutator) at room temp for 30 minutes.

Wash the cross-linked embryos once with 50 mL of 0.1 M Tris-HCl (pH 7.5), followed by two 50-mL washes of 1x M9.

Wash the embryos once with homogenization buffer. Add 1 mL of homogenization buffer per gram embryo (based on the starting amount), quick-freeze and store at − 80°C.

Thaw 10󈝻 mL of cross-linked embryos and add fresh protease inhibitors.

Place the tube on ice and sonicate it with ten 30-second bursts at 15% power on a Heat System XL2020 sonicator with a standard tapered microtip (cat.#419, 3 mm diameter). Wait 1 minute in between bursts for cooling.

Pellet debris by centrifugation at 6,500 rpm (5,000 × g) in a SS-34 rotor for 20 minutes. Collect the supernatant in a clean 50-mL tube.

Sonicate as described in step 2 to shear the DNA. Pellet debris by centrifugation at 14,500 rpm (25,000 × g) in a SS-34 rotor for 20 minutes. Collect and quick-freeze the supernatant in 0.5𔂿 mL aliquots.

Thaw embryo lysates on ice. Incubate lysates with Protein A Sepharaose beads (100 μL per mL lysates Amersham) or IgGsorb (100 μL per mL lysates The Enzyme Center) for 10󈞊 minutes. Spin in a microcentrifuge for 2 min. at 2,000 × g to pellet the Sepharose beads or IgGsorb. Transfer the supernatant to a new microfuge tube and spin in a microcentrifuge at top speed for 10 min. Use the supernatant for IP.

Incubate approximately 5 μg of affinity purified antibodies with 3 mg total protein of embryo lysates for 2 hrs.

Pellet non-specific precipitates by spinning in a microcentrifuge at top speed (approximately 16,000 × g ) for 10 min.

Transfer the supernatant to a new microfuge tube with 25 μL of Protein A Sepharose and place on a rocker/nutator for 30 min.

Pellet the antibody-antigen complexes captured on the Protein A Sepharose beads by spinning in a microcentrifuge for 2 min. at 500 × g . Remove the supernatant.

Wash the beads as follows:

2x 1-mL of ChIP buffer with 100 mM KCl

2x 1-mL of ChIP buffer with 1 M KCl

Add 200 μL elution buffer [10 mM Tris-HCl (pH8), 1% (w/v) SDS]. Spin to pellet the beads. Transfer eluate to a clean microfuge tube. Repeat elution step once more. Combine the eluates (400 μL total vol).

Add 16 μL of 5 M NaCl and heat overnight at 65°C to reverse the formaldehyde cross-links. [Use a PCR machine with a heated top or a Hybaid oven to avoid condensation at top of the tube].

Adding 8 μL of 0.5 M EDTA and 16 μL of 1 M Tris-HCl (pH 6.8). Mix. Digest proteins with 20 μg of proteinase K (Boehringer Mannheim) for 1 hour at 45°C.

Phenol-chloroform extractions. Add 10󈞀 μg of glycogen. Mix. EtOH precipitate. Resuspend in 100 μL TE.

Buffer Recipes

M9 buffer: 6 g Na2HPO4, 3 g KH2PO4, 5 g NaCl, 0.25 g of MgSO47H2O per liter. Autoclave.

1 M potassium citrate, pH 6.0: Per liter solution, add 268.8 g tripotassium citrate, 26.3 g citric acid monohydrate and adjust pH with KOH. Autoclave to sterilize.

Trace metals solution: Per liter solution, add 1.86 g Na2EDTA, 0.69 g FeSO4·7H2O, 0.2 g MnCl2·4H2O, 0.29 g ZnSO4·7H2O, 0.016g CuSO4. Autoclave to sterilize and store in the dark.

1 M potassium phosphate, pH 6.0: Per liter solution, dissolve 136 g KH2PO4 in 900 mL dH2O and adjust pH with KOH. Autoclave to sterilize.

10 x S basal medium: Per liter solution, add 59 g of NaCl, 500 mL of 1 M potassium phosphate (pH 6), 10 mL of cholesterol (5 mg/mL in EtOH). Autoclave to sterilize. [Note: the cholesterol will not go into solution].

Complete S medium: Per liter, add 100 mL of 10 x S basal medium, 10 mL 1 M potassium citrate (pH 6), 10 mL trace metals solution, 3 mL 1 M MgSO4, 3 mL 1 M CaCl2. Autoclave to sterilize.

Homogenization buffer: 50 mM HEPES-KOH, pH 7.6 1 mM EDTA 140 mM KCl 0.5% NP-40 10% glycerol. Add fresh protease inhibitors (aprotinin, pepstatin A, leupeptin, PMSF) and 5 mM DTT before use.

ChIP buffer: 50 mM HEPES-KOH, pH 7.6 1 mM EDTA 0.05% NP-40. Add KCl to the desired concentration. Add fresh protease inhibitors (aprotinin, pepstatin A, leupeptin, PMSF) and 1 mM DTT before use. [This buffer contains less NP-40 and no glycerol compared to the homogenization buffer].

Protocol 18: Chromatin immunoprecipitation (Johnathan Whetstine and Yang Shi)

This protocol is a modified version of a protocol from Upstate Biotechnologies (www.upstate.com).

Chromatin is isolated from 3×10 5 embryos/ChIP reaction, which equates to no less than 400 μg chromatin per immunoprecipitation (IP). There is no harm in scaling up for cleaner results, especially with poor antibodies.

Adult N2 worms grown on either HB101 or RNAi expressing bacteria are bleached and washed before being immersed and rotated in 1.5% Formaldehyde/M9 buffer for 30 minutes at 16 °C.

Embryos are washed extensively with M9 (at least 3 times) and gently centrifuged. Be careful not to rupture embryos.

Add warm chromatin SDS lysis solution (30 °C Upstate biotechnologies cat. # 20-163) to the embryos (minimum of 3×10 5 embryos per 200 μl solution) and dounce homogonize at least 30 times.

Place the slurry on ice for at least 20󈞊 minutes.

Combine the total amount of embryos or adults and sonicate 25 times per 2ml of extract used for an approximately 500bp smear (15 sec constant, 20% output on some sonicators). Do not over heat or allow to foam excessively. Keep on ice.

Centrifuge the samples, and keep the supernatant for DNA quantification before processing.

Aliquot the supernatant (200 μl) and dilute 10-fold into Dilution buffer (Upstate Biotechnologies cat. # 20-153).

Pre-clear each sample twice with 80 μl of ssDNA/protein A agarose beads from Upstate Biotechnologies (cat. # 16-157).

Incubate samples with the indicated antibody overnight at 4 °C with constant rotation.

The next morning add 60 μl of beads and incubate for 1 hour. Centrifuge and wash the beads once at room temperature with constant rotation with each of the following buffers (Note: You can make these buffers, see Upstate recipe, but I find that the beads are cleaner when these products are used): 1.0 ml High Salt Solution (Upstate Biotechnologies cat. # 20-155), 1.0 ml Low Salt buffer Solution (Upstate Biotechnologies cat. # 20-154), 1.0 ml LiCl Solution (Upstate Biotechnologies cat. # 20-156). After the last wash, the beads are washed three times with 1X TE, pH 8.0, which makes the beads slightly translucent.

Prepare and add fresh elution buffer (1%SDS and 0.1M NaHCO3 250 μl) to the beads at room temperature with constant agitation. Keep the supernatant and repeat this step once more.

Reverse the elution with 0.2 M NaCl for 4 hours at 65 °C.

Treat the samples with proteinase K (10 μl 0.5M EDTA, 20 μl 1M Tris-HCl, pH 6.5, and 2 μl 10mg/ml proteinase K) for 1 hour at 45 °C and then extract with phenol:chloroform:isoamyl alcohol.

Precipitate the extracted solution with 20 μg of yeast tRNA, and resuspend each sample in 50 μl warm 10 mM Tris-HCl, pH 8.0. The samples are now ready for PCR reactions.


Contents

With continual improvements made to microscopes over time, magnification technology advanced enough to discover cells. This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, known as cell biology. When observing a piece of cork under the scope and he was able to see pores. This was shocking at the time because it was believed no one else had seen these.To further support his theory, Matthias Schleiden and Theodor Schwann both studied cells of both animal and plants. What they discovered was there were significant differences between the two types of cells. This put forth the idea that cells were not only fundamental to plants, but animals as well. [3]

Robert Hooke's microscope was a recreation of Leeuwenhoek's microscope in the 17th century, except his was 300x magnification [30].The discovery of the cell was made possible through the invention of the microscope. In the first century BC, Romans were able to make glass. They discovered that objects appeared to be larger under the glass. In Italy during the 12th century, Salvino D’Armate made a piece of glass fit over one eye, allowing for a magnification effect to that eye. The expanded use of lenses in eyeglasses in the 13th century probably led to wider spread use of simple microscopes (magnifying glasses) with limited magnification. Compound microscopes, which combine an objective lens with an eyepiece to view a real image achieving much higher magnification, first appeared in Europe around 1620. In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia. Hooke also used a simpler microscope with a single lens for examining specimens with directly transmitted light, because this allowed for a clearer image. [5]

An extensive microscopic study was done by Anton van Leeuwenhoek, a draper who took the interest in microscopes after seeing one while on an apprenticeship in Amsterdam in 1648. At some point in his life before 1668, he was able to learn how to grind lenses. This eventually led to Leeuwenhoek making his own unique microscope. He made one with a single lens. He was able to use a single lens that was a small glass sphere but allowed for a magnification of 270x. This was a large progression since the magnification before was only a maximum of 50x. After Leeuwenhoek, there was not much progress in microscope technology until the 1850s, two hundred years later. Carl Zeiss, a German engineer who manufactured microscopes, began to make changes to the lenses used. But the optical quality did not improve until the 1880s when he hired Otto Schott and eventually Ernst Abbe. [6]

Optical microscopes can focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than the wavelengths of visible light. The development of the electron microscope in the 1920s made it possible to view objects that are smaller than optical wavelengths, once again opening up new possibilities in science. [6]

Drawing of the structure of cork by Robert Hooke that appeared in Micrographia. The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia. In this book, he gave 60 ‘observations’ in detail of various objects under a coarse, compound microscope. One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning ‘a small room’ like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb. However, Hooke did not know their real structure or function. What Hooke had thought were cells, were actually empty cell walls of plant tissues. With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing. Therefore, he did not think the "cellulae" were alive. His cell observations gave no indication of the nucleus and other organelles found in most living cells. In Micrographia, Hooke also observed mould, bluish in color, found on leather. After studying it under his microscope, he was unable to observe “seeds” that would have indicated how the mould was multiplying in quantity. This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause. Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovered that generation was achieved otherwise. [5]

Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did. He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x. Under these microscopes, Leeuwenhoek found motile objects. In a letter to The Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms. Over time, he wrote many more papers in which described many specific forms of microorganisms. Leeuwenhoek named these “animalcules,” which included protozoa and other unicellular organisms, like bacteria. Though he did not have much formal education, he was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676. He also found for the first time the sperm cells of animals and humans. Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter the egg cell. This put an end to the previous theory of spontaneous generation. After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries. [5]

The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying. Biologists believed that there was a fundamental unit to life, but were unsure what this was. It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants. [7] This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization”, [8] Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit.

In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells", meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen. [9] Before, it had been thought that cells shared walls and the fluid passed between them this way.

Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden. [10] While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it. In 1839, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells. He also suggested that cells were made by a crystallization process either within other cells or from the outside. [11] However, this was not an original idea of Schleiden. He claimed this theory as his own, though Barthelemy Dumortier had stated it years before him. This crystallization process is no longer accepted with modern cell theory. In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures. [12] This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants. From these conclusions about plants and animals, two of the three tenets of cell theory were postulated. [7]

1. All living organisms are composed of one or more cells

2. The cell is the most basic unit of life

Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker. [6] In 1855, Rudolf Virchow added the third tenet to cell theory. In Latin, this tenet states Omnis cellula e cellula. [7] This translated to:

3. All cells arise only from pre-existing cells

However, the idea that all cells come from pre-existing cells had in fact already been proposed by Robert Remak it has been suggested that Virchow plagiarized Remak and did not give him credit. [13] Remak published observations in 1852 on cell division, claiming Schleiden and Schawnn were incorrect about generation schemes. He instead said that binary fission, which was first introduced by Dumortier, was how reproduction of new animal cells were made. Once this tenet was added, the classical cell theory was complete.

The generally accepted parts of modern cell theory include:

  1. All known living things are made up of one or more cells [14]
  2. All living cells arise from pre-existing cells by division.
  3. The cell is the fundamental unit of structure and function in all living organisms. [15]
  4. The activity of an organism depends on the total activity of independent cells. [16]
  5. Energy flow (metabolism and biochemistry) occurs within cells. [17]
  6. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm. [18]
  7. All cells are basically the same in chemical composition in organisms of similar species. [17]

The modern version of the cell theory includes the ideas that:

  • Energy flow occurs within cells. [17]
  • Heredity information (DNA) is passed on from cell to cell. [17]
  • All cells have the same basic chemical composition. [17]

The cell was first discovered by Robert Hooke in 1665 using a microscope. The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s. In this theory the internal contents of cells were called protoplasm and described as a jelly-like substance, sometimes called living jelly. At about the same time, colloidal chemistry began its development, and the concepts of bound water emerged. A colloid being something between a solution and a suspension, where Brownian motion is sufficient to prevent sedimentation. The idea of a semipermeable membrane, a barrier that is permeable to solvent but impermeable to solute molecules was developed at about the same time. The term osmosis originated in 1827 and its importance to physiological phenomena realized, but it wasn’t until 1877, when the botanist Pfeffer proposed the membrane theory of cell physiology. In this view, the cell was seen to be enclosed by a thin surface, the plasma membrane, and cell water and solutes such as a potassium ion existed in a physical state like that of a dilute solution. In 1889 Hamburger used hemolysis of erythrocytes to determine the permeability of various solutes. By measuring the time required for the cells to swell past their elastic limit, the rate at which solutes entered the cells could be estimated by the accompanying change in cell volume. He also found that there was an apparent nonsolvent volume of about 50% in red blood cells and later showed that this includes water of hydration in addition to the protein and other nonsolvent components of the cells.

Evolution of the membrane and bulk phase theories

Two opposing concepts developed within the context of studies on osmosis, permeability, and electrical properties of cells. [19] The first held that these properties all belonged to the plasma membrane whereas the other predominant view was that the protoplasm was responsible for these properties. The membrane theory developed as a succession of ad-hoc additions and changes to the theory to overcome experimental hurdles. Overton (a distant cousin of Charles Darwin) first proposed the concept of a lipid (oil) plasma membrane in 1899. The major weakness of the lipid membrane was the lack of an explanation of the high permeability to water, so Nathansohn (1904) proposed the mosaic theory. In this view, the membrane is not a pure lipid layer, but a mosaic of areas with lipid and areas with semipermeable gel. Ruhland refined the mosaic theory to include pores to allow additional passage of small molecules. Since membranes are generally less permeable to anions, Leonor Michaelis concluded that ions are adsorbed to the walls of the pores, changing the permeability of the pores to ions by electrostatic repulsion. Michaelis demonstrated the membrane potential (1926) and proposed that it was related to the distribution of ions across the membrane. [20]

Harvey and Danielli (1939) proposed a lipid bilayer membrane covered on each side with a layer of protein to account for measurements of surface tension. In 1941 Boyle & Conway showed that the membrane of frog muscle was permeable to both K +
and Cl −
, but apparently not to Na +
, so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to K +
, H +
, and Cl −
as well as the impermeability to Na +
, Ca +
, and Mg 2+
. Over the same time period, it was shown (Procter & Wilson, 1916) that gels, which do not have a semipermeable membrane, would swell in dilute solutions.

Loeb (1920) also studied gelatin extensively, with and without a membrane, showing that more of the properties attributed to the plasma membrane could be duplicated in gels without a membrane. In particular, he found that an electrical potential difference between the gelatin and the outside medium could be developed, based on the H +
concentration. Some criticisms of the membrane theory developed in the 1930s, based on observations such as the ability of some cells to swell and increase their surface area by a factor of 1000. A lipid layer cannot stretch to that extent without becoming a patchwork (thereby losing its barrier properties). Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties.

In 1938, Fischer and Suer proposed that water in the protoplasm is not free but in a chemically combined form—the protoplasm represents a combination of protein, salt and water—and demonstrated the basic similarity between swelling in living tissues and the swelling of gelatin and fibrin gels. Dimitri Nasonov (1944) viewed proteins as the central components responsible for many properties of the cell, including electrical properties. By the 1940s, the bulk phase theories were not as well developed as the membrane theories. In 1941, Brooks & Brooks published a monograph, "The Permeability of Living Cells", which rejects the bulk phase theories.

Emergence of the steady-state membrane pump concept

With the development of radioactive tracers, it was shown that cells are not impermeable to Na +
. This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove Na +
as it permeates cells. This drove the concept that cells are in a state of dynamic equilibrium, constantly using energy to maintain ion gradients. In 1935, Karl Lohmann [de] discovered ATP and its role as a source of energy for cells, so the concept of a metabolically-driven sodium pump was proposed. The tremendous success of Hodgkin, Huxley, and Katz in the development of the membrane theory of cellular membrane potentials, with differential equations that modeled the phenomena correctly, provided even more support for the membrane pump hypothesis.

The modern view of the plasma membrane is of a fluid lipid bilayer that has protein components embedded within it. The structure of the membrane is now known in great detail, including 3D models of many of the hundreds of different proteins that are bound to the membrane. These major developments in cell physiology placed the membrane theory in a position of dominance and stimulated the imagination of most physiologists, who now apparently accept the theory as fact—there are, however, a few dissenters. [ citation needed ]

The reemergence of the bulk phase theories

In 1956, Afanasy S. Troshin published a book, The Problems of Cell Permeability, in Russian (1958 in German, 1961 in Chinese, 1966 in English) in which he found that permeability was of secondary importance in determination of the patterns of equilibrium between the cell and its environment. Troshin showed that cell water decreased in solutions of galactose or urea although these compounds did slowly permeate cells. Since the membrane theory requires an impermanent solute to sustain cell shrinkage, these experiments cast doubt on the theory. Others questioned whether the cell has enough energy to sustain the sodium/potassium pump. Such questions became even more urgent as dozens of new metabolic pumps were added as new chemical gradients were discovered.

In 1962, Gilbert Ling became the champion of the bulk phase theories and proposed his association-induction hypothesis of living cells.


Talk Overview

Life on Earth evolved once – this means that all biological systems on our planet are rooted in the same fundamental framework. This framework is extremely complex and we have yet to fully understand the processes inside each living cell. One way of understanding complex systems is to break them down into simpler parts. This is the principle of engineering the synthetic cell: to use our current knowledge of biology for building a living cell with the least amount of parts and complexity. Synthetic cells can be used to teach us about the basic principles of life and evolution, and they hold promise for a range of applications including biomaterials and drug development. Dr. Kate Adamala narrates an introduction to this exciting field.


Please be aware that due to COVID-19 safety guidelines all in-person exams have been suspended. As such, all final exams are currently being delivered through ProctorU, which has an approximate fee of $35 involved. There will be more information in your course shell, on how to apply, if your course has a final exam.

A series of 5 assignments and a final exam are used to formally measure student success in meeting the course learning objectives. To successfully complete this course, students must obtain at least 50% on the final mandatory examination and 50% overall. Students who do not submit an assignment will be assigned a mark of zero (0) for that assignment.

Assignment 1 8%
Assignment 2 12%
Assignment 3 10%
Assignment 4 10%
Assignment 5 10%
Final Exam 50%
Total 100%


Watch the video: Δυναμικό ενέργειας (January 2022).