Chapter 3-The Cellular Level of Organization

I. GENERALIZED ANIMAL CELL

A cell is the basic, living, structural and functional unit of the body.

Cells are measured in micrometers and vary in size.

Cells vary in shape - often determined by their function.

A generalized cell is a composite that represents various cells of the body.

The principal parts of a cell are the:

nucleus - where DNA is, surrounded by a double membrane called the nuclear envelope

(not found in bacterial cells - prokaryotic, only in eukaryotic cells)

Cytoplasm - between cell and nuclear membranes.

contains:

cytosol - contains proteins, enzymes, nutrients, ions, and other small molecules

organelles - highly organized structures with characteristic shapes that are specialized for specific cellular activities.

plasma (cell) membrane - separates inside of cell from external environment.

II. PLASMA (CELL) MEMBRANE

A. Membrane Chemistry and Anatomy

The plasma (cell) membrane surrounds the cell and separates it from other cells and the external environment. It regulates everything that enters and leaves the cell. It is composed primarily of phospholipids and proteins.

Phospholipid bilayer - like ping-pong balls on water. (with rubber ducks) form a bilayer because phospholipids are amphipathic. Most of the lipids in the membrane are phospholipids.

Also find glycolipids and cholesterol.

Selectively permeable- allows fat soluble to pass - steroid hormones, Oxygen and CO₂.

Signal transduction - receive and respond to incoming messages (hormones, nerves)

Glycolipids (not in book)(5% of the membrane) are also amphipathic and are found on the external surface of the cell. They are the target of certain bacterial toxins, they are important for adhesion among cells, they may mediate cell-to-cell recognition and communication, and contribute to regulation of cell growth and development.

Cholesterol is a steroid which makes up about 20% of the membrane lipid. These molecules strengthen the membrane, but decrease its flexibility.

The proteins are of two types :integral and peripheral. It is the proteins and glycoproteins that to a large extent, determine what the cell can do.

Integral proteins extend from one side of the membrane to the other. Most or all of these proteins are glycoproteins. The sugar portion faces the extracellular fluid. These proteins can serve as channels for the passage of small water soluble molecules through the membrane. Others become more active and act as transporters or carriers to move certain ions and molecules in or out of the cell.

Peripheral proteins are loosely attached to the inner or outer surfaces of the cell membrane. They can serve as receptors for hormones, neurotransmitters, nutrients or antibodies, or are enzymes. Membrane glycoproteins and glycolipids are often cell identity markers. "UPC code" These glycoproteins mark red blood cells as having become too old and in need of being removed from circulation, and cells with viruses mark themselves so protective cells will kill them and the viruses within them before the viruses can reproduce and infect other body cells.

Intercellular junctions

Some cells have no contact with other cells

Other cells are tightly packed.

tight junctions - membranes of adjacent cells come together and fuse. e.g. between cells of blood vessels in the brain.

desmosomes - "spot welds" in skin and in cardiac muscle

gap junctions - tubular channels that connect cytoplasm of adjacent cells

Cellular Adhesion Molecules-or CAMs are molecules that help cells form temporary attachments to other cells - e.g. white blood cells.

B. Membrane Physiology

The plasma membrane functions in:

cellular communication

the establishment of an electrochemical gradient (not in book)

inside has more negative charges - resulting in a membrane potential of

-20 to -200 mV especially important in muscle and nerve cells

selective permeability a membrane is permeable if it allows substances to pass through it. (Sieve and water) Plasma membranes are not completely permeable to any substance, but they do allow some substances to pass more readily than others. Water passes easily.

Substances can pass through the membrane depending on:

lipid solubility - nonpolar, hydrophobic molecules can pass through the membrane.

Steroid hormones

important in drug formulation

size - only a few, small uncharged polar molecules can pass through the phospholipid bilayer.

electrical charges - these prevent passage through the phospholipid bilayer, but some smaller charged molecules pass through protein channels or are transported. Charge of cell makes it easier for cations to enter the cell.

the presence of channels and transporters - some molecules pass through the water filled pore in a protein channel, others are shuttled across the membrane by transporters. Most channels and transporters are very selective, helping only a specific solute to cross the membrane.

III. MOVEMENT OF MATERIALS ACROSS PLASMA MEMBRANES

There are two ways that nutrients and wastes, etc. pass across membranes:

Passive processes ( no energy required) and active processes (energy from ATP required).

A. Passive Processes

Passive processes depend on the concentration of substances and their kinetic energy. (also distance withing the body) . We measure the kinetic energy of the molecules as temperature. The higher the temperature, the more energy molecules have and the more they move. (Can see this under a microscope as Brownian movement).

The second factor is the concentration gradient. Concentration refers to how many molecules of a certain type there are in an area. The term gradient simply means that there are more (molecules, temperature, pressure) in one area than another.

The random action of molecules moves them from an area of high concentration of similar molecules to an area of low concentration. This process is called diffusion. Ex. dissolving sugar in coffee, or opening a bottle of perfume in one corner of the room. If a liquid contains more than one substance, each substance will diffuse in the direction set up by its own concentration gradient. Because no outside energy is involved, this is a passive process. Simple diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration until an equilibrium is reached.

In living cells, diffusional equilibrium does not normally occur. Instead, we talk of physiologically steady states, where concentrations of diffusing substances are unequal, but stable. Diffusion is important the movement of oxygen and carbon dioxide into and out of cells.

Small solutes that are not lipid soluble can pass through protein channels in the cell membrane. Because of the use of protein "helpers" this is a special type of diffusion called facilitated diffusion .Larger molecules like glucose and amino acids may be carried across the membrane by protein carrier molecules. The number of carrier molecules will also help determine the rate of diffusion.

Osmosis is the movement of water through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. When a selectively permeable membrane does not allow a solute to pass through it, water moves down a concentration gradient. Osmotic pressure is defined as the pressure required to stop the movement of pure water into a solution containing solutes when the two are separated by a membrane permeable only to water. A solution with a higher osmotic pressure will take water from a solution with lower osmotic pressure. Osmotic pressure is an important force in moving water between the fluid compartments in the body.

By itself, water cannot become more or less concentrated. To change the concentration of water, we must dissolve something in it. In that case the water becomes less concentrated. When we talk about how concentrated one solution is in relation to another solution we use the term tonicity. If two solutions have the same amount of a substance dissolved in them, they are said to be isotonic. If one solution has more of the substance than the other it is hypertonic, and if it has less than the other solution it is said to be hypotonic. How does this affect cells?

In an isotonic solution, red blood cells maintain their normal shape; in a hypotonic solution, they undergo hemolysis; in a hypertonic solution, they undergo crenation.

Filtration is a type of bulk flow where the movement of water and dissolved substances across a membrane due to gravity or hydrostatic pressure. Example: kidneys

B. ActiveTransport

Active processes depend on the use of ATP by the cell. The two principal types are active transport and vesicular transport.

Active transport is the movement of a substance across a cell membrane from lower to higher concentration using energy derived from ATP either directly (primary active transport) or indirectly (secondary active transport). Many of these systems are called "pumps" because they move substances uphill, just as a water pump moves water up hill against the force of gravity. The most prevalent primary active transport pump is the sodium pump or also the sodium/potassium pump. Each cell has hundreds of these sodium pumps, which must work continuously, since Na+ and K+ flow back across the membrane through leakage channels.

Endocytosis

Exocytosis

3 forms of endocytosis: pinocytosis, phagocytosis, and receptor-mediated endocytosis

Pinocytosis " cell drinking" is the ingestion of fluid. In this process, the membrane folds inward, and the fluid becomes surrounded by a pinocytic vesicle. The membrane eventually breaks down, and the contents of the vesicle becomes part of the cytoplasm. Most cells carry on pinocytosis.

Phagocytosis is the ingestion of solid particles. "cell eating" pseudopods, phagocytic vesicle or phagosome - combines with a lysosome. It is an important process used by some white blood cells phagocytes to destroy bacteria that enter the body.

Receptor-mediated endocytosis is the selective uptake of large molecules and particles (ligands) by cells. Similar to pinocytosis, but only occurs when specific substances bind to receptors on the cell surface. Sometimes viruses , such as the HIV virus sneak into cells by attaching to receptors and being taken in by R-M endocytosis.

1. ligand binds to integral protein

2. endocytic vesicle forms

3. joins with others to form endosome

4. receptors separate from ligands

5. receptor portion pinches off and returns to membrane

6. remaining endosome is joined by a lysosome or releases contents into cell.

Exocytosis - secretory vesicles form inside the cell, and release contents outside the cell. Important for release of neurotransmitters, and secretory cells which produce protein hormones or mucus.

Transcytosis- combines exo- and endocytosis to move substances across a cell, especially cells that have tight junctions. HIV across epithelial cells that would normally prevent the passage of such organisms.

IV. CYTOPLASM

The semifluid, gel-like, intracellular fluid is termed cytosol. The cytosol is mostly water, but has a lot of protein, which gives it the consistency of jello. Cytosol also contains carbohydrates, lipids, and inorganic substances. Many important metabolic reactions take place in the cytosol. Cytoplasm is the substance inside the cell between the plasma membrane and nucleus. It includes the cytosol plus organelles, except the nucleus.

V. ORGANELLES

Organelles are specialized structures suspended in the cytosol that have characteristic appearances and functions. "little organs". They play specific roles in cellular growth, maintenance, repair, and control. The numbers and types of organelles vary in different cells, depending on the function of the cells.

A. Ribosomes

Ribosomes are tiny granules consisting of ribosomal RNA and ribosomal proteins. Some ribosomes (free) float in the cytosol and have no attachments to other organelles; these make proteins for use in the cell. Other ribosomes attach to endoplasmic reticulum. These ribosomes make proteins insertion into the plasma membrane, or for export outside the cell. Functionally, ribosomes are the sites of protein synthesis.

B. Endoplasmic Reticulum

The ER is a network of membrane-enclosed channels continuous with the nuclear membrane. Rough ER has ribosomes attached to it. Smooth ER does

not contain ribosomes. The ribosomes of the rough ER synthesize proteins. Rough ER may also add sugars to certain proteins, forming glycoproteins. Together with the Golgi complex, it synthesizes and packages molecules that will be secreted from the cell.

Smooth ER is the site of fatty acid, phospholipid and steroid synthesis. In certain cells it also detoxifies a variety of chemicals, such as alcohol, pesticides and carcinogens. In muscle cells, calcium ions released from the sarcoplasmic reticulum, which resembles smooth ER, causes muscle contraction.

C. Golgi Apparatus(Complex, body)

The Golgi complex consists of several stacked, flattened membranous sacs (cisterns) referred to as cis, medial, and trans. The principal function of the Golgi complex is to

process, sort, and deliver proteins and lipids to the plasma membrane, and to form secretory vesicles and lysosomes and new cell membrane.

D. Mitochondria

Mitochondria consist of a smooth outer membrane and a folded inner membrane surrounding the interior matrix. The inner folds are called cristae. This is where complex reactions harness energy for use by the cell. Enzymes in the walls if the mitochondria use oxygen to break down glucose and other nutrients to release energy. This process is called aerobic or cellular respiration.(breathing, needs O2) The mitochondria are called 'powerhouses' of the cell because they produce most of a cell's ATP. They have their own DNA and many unique RNA's, but they depend on the cell for certain vital enzymes. They are believed to once have been free living organisms.

E. Lysosomes

Lysosomes are membrane-enclosed vesicles that are formed by the Golgi complex and contain digestive enzymes. Tay-Sachs disease is an inherited trait (Eastern European Jews) where one lysosomal enzyme is not formed. This enzyme breaks down a membrane gylcolipid found in nerve cells. Thus, the glycolipid accumulates in the nerve cells, resulting in blindness, dementia, loss of coordination and death, usually before the age of 5.

Lysosomal enzymes, like stomach enzymes, work best a low pH. The lysosomal membranes have transport pumps which concentrate H+ ions inside the lysosome. They are found in large numbers in white blood cells, which carry on phagocytosis. Lysosomes function in intracellular digestion(phagocytosis, pinocytosis and receptor- mediated endocytosis), digestion of worn-out organelles (autophagy), digestion of cellular contents (autolysis) after death and in some pathological conditions , and extracellular digestion (debris at infection site).

F. Peroxisomes

Peroxisomes are similar to lysosomes but smaller. They contain enzymes (for example, catalase) that use molecular oxygen to oxidize various organic substances such as phenol, formaldehyde and alcohol. These reactions produce hydrogen peroxide. They are especially important in kidney and liver cells where they detoxify many harmful substances. In the liver, they also synthesize bile acids, which aid in the digestion of fat.

G. Centrosome and Centrioles

The dense area of cytoplasm containing the centrioles is called a centrosome. Centrioles are paired cylinders arranged at right angles to one another made up of nine clusters of three tubules. Centrioles play a role in the formation and regeneration of flagella and cilia. Centrosomes serve as centers for organizing microtubules in the mitotic spindle during cell division.

H. Flagella and Cilia

These cellular projections have the same basic structure and are used in movement. If projections are few (typically occurring singly or in pairs) and long, they are called flagella. If they are numerous and short, they are called cilia (eyelashes) . The flagellum on a sperm cell moves the entire cell. The cilia on cells of the respiratory tract move foreign matter trapped in mucus along the cell surfaces toward the throat for elimination.

I. Vesicles

Vesicles are membrane sacs found within the cell. They may be formed during endocytosis or may be formed by the Golgi apparatus or ER. The transport of substances around and in and out of the cell by means of vesicles is called vesicle trafficking.

J. Microfilaments and microtubles - The Cytoskeleton

The cytoskeleton gives a cell shape and allows it to move, much like the body’s skeleton does. Microfilaments, and microtubules form the cytoskeleton.

Microfilaments are formed from the protein actin, and give support to the cell. They assist in cell movements and movements within cells (secretion, phagocytosis, pinocytosis)

Microtubules are larger than microfilaments and are made of the protein tubulin. They support and shape the cell, help move pseudopods, and act as conveyor belts to move substances and organelles through the cytosol.

K. CELL INCLUSIONS

Cell inclusions are chemical substances produced by cells. They may have recognizable shapes, but are not surrounded by a membrane. These substances are mostly organic molecules, and may appear and disappear over the life of the cell. Examples of cell inclusions are melanin, glycogen, and triglycerides.

IV. Nucleus

Usually the largest organelle, the nucleus is a round or oval structure inside the cell. It is bound by a double membrane that is penetrated by pores which allow the passage of material between the nucleus and the cytoplasm. This is called the nuclear envelope or nuclear membrane. The nucleus contains a special type of cytoplasm called the nucleoplasm. Contained in the nucleoplasm are the Chromatin granules which are the threadlike structures made of DNA and associated proteins. During cell division these molecules become tightly coiled into individual rodlike structures called chromosomes. The chromosomes contain genes, which are the instructions for making proteins. (Nucleus is like a library) Chromosomes consist of subunits called nucleosomes that are composed of DNA (genetic material) and histone proteins.

There may be one or more nucleoli nu KLEE o lus(nucleolus): these structures are associated with regions of the DNA which code for special RNA and proteins. The nucleolus the RNA and proteins together to form ribosomes. These ribosomes are produced in the nucleus and pass into he cytoplasm through the pores in the nuclear membrane.

The nucleus is essential for the survival of the cell. It controls the cell by determining which of the enzymes and other proteins are made and when. Cells, such as red blood cells, which lose their nuclei, die shortly thereafter., the nucleus, controls cellular activities and contains the genetic information. Most body cells have a single nucleus; some (red blood cells) have none, whereas others (skeletal muscle cells) have several.

THE CELL CYCLE:

Cell division is the process by which cells reproduce themselves. It consists of nuclear division (mitosis or meiosis) and cytoplasmic division (cytokinesis). Cell division that results in an increase in body cells is called somatic cell division and involves a nuclear division called mitosis plus cytokinesis. Cell division that results in the production of sperm and oocytes is called reproductive cell division and consists of a nuclear division called meiosis plus cytokinesis.

Somatic Cell Division

Because we reproduce sexually, our DNA comes in pairs: we receive one member of a pair from our mother, and the other member of the pair from our father. The members of a pair of chromosomes are called homologous chromosomes. Each chromosome of a pair codes for the same type of information, but each chromosome is somewhat different from its homologous chromosome. (Like a set of 23 books on how to build a house. You get one set from your mother and one from your father. They have the same basic information, but your mother may have different ideas than your father about storage space, plumbing, and floor tiles.) Gene 1 may code for eye color, and the chromosome we get from mom says "make the eyes blue", whereas the chromosome we got from dad says"make the eyes brown". Humans have 46 chromosomes, or 23 pairs. When we divide our cells, it is important that the new cells have a copy of each of the 46 chromosomes, so that the cell can perform all the functions required of it.

The cell cycle is composed of all the processes a cell goes through from one division to the next. Your book breaks this cycle into two parts: interphase and cell division.

Interphase : this is often thought off as the "resting stage". However, the cell is doing its job, and may be quite busy; it is just not in the process of dividing. It is during this stage that the replication of the DNA, centrosome and centrioles, and other organelles takes place. How long a cell spends in interphase varies greatly with the type of cell. Nerve cells spend nearly their entire life in interphase (G₀ phase), and cancer cells spend very little time in interphase.

Interphase can be further divided into three parts:

The first part is called G₁, the gap or growth phase, and goes from the completion of the division of the cell up until the beginning of DNA synthesis or duplication.

The next part is called S-phase, for DNA synthesis. This is when the DNA makes a copy of itself. Once a cell enters the S-phase it is committed to dividing.

The part just before mitosis is called G₂, the gap between DNA replication and the beginning of mitosis.

Mitosis in this case refers specifically to the division of the chromosomes and the nucleus.

Your book refers to division of the nuclear contents as karyokinesis.

Mitosis is divided into four stages, even though it is one continuous process:

Prophase: this is the earliest stage of mitosis. During interphase, the DNA is spread out in the nucleus and is all but invisible, and is referred to as chromatin. When mitosis begins, the DNA folds up or condenses, and becomes visible under the microscope. Take note that the duplicated chromosomes are linked together by a small spherical body called a centromere. Attached to the centromere is a protein complex called the kinetochore. At this point we refer to the chromosomes as sister chromatids. Any nucleoli disappear, the nuclear envelope breaks down, and the mitotic spindle forms. The mitotic spindle starts at the organelles called centrioles. Centrioles are pairs of cylinders at right angles to each other. These cylinders are made up of fused microtubules. The fibers which make up the mitotic spindle are also microtubules.

(Late prophase or) Prometaphase is the transition between prophase and the next stage called metaphase. This is when nuclear membrane disappears, and the chromatids attach to the fibers of the mitotic spindle by means of a specialized structure called a kinetochore.

Metaphase is the easiest stage of mitosis to recognize, because all the chromosomes line up in the middle of the cell due to the action of the microtubules.

During Anaphase the centromere of each chromatid pair splits, and the kinetochore microtubules shorten and pull one of each duplicated chromosome to opposite ends of the cell.

Telophase is the last phase of mitosis. Here the mitotic spindle and kinetochores disappear, and new nuclear membranes form. The nucleoli can be seen again, and the chromosomes unwind.

Cytokinesis usually accompanies mitosis, beginning in anaphase and finishing after telophase. In anaphase, dividing animal cells form a contractile ring of actin microfilaments perpendicular to the axis of the mitotic spindle. This ring pinches the cell into two, using some of the same proteins used in muscle tissue. The indentation in the cells is called the cleavage furrow. Finally the two cells separate into two daughter cells with identical DNA.

We all start out as a single cell, formed from the union of a sperm and an egg. That cell then divides, and those cells divide. If all the cells did was simply to divide, we would be a big ball of similar cells. Along the way, the cells begin to specialize, and become adapted for certain functions. This is what we call differentiation. The way this comes about is due to which genes on the DNA are expressed and which are turned off. A bone cell doesn’t produce neurotransmitters, and brain cells do not produce actin and myosin. Each cell becomes specialized in its function, and then interacts with other cells to form tissues and organs.

How many times a cell divides depends on the type of cell.

Skin cells and cells in areas that can be abraded divide continuously.

Liver cells divide a certain # of times, then stop. However, if part of the liver is damaged or removed, some of the cells may again divide to replace that tissue.

Nerve cells do not reproduce after they mature and differentiate, so damage to nervous tissue is usually permanent.

Most organs do have some stem cells, that retain their ability to divide and differentiate.

Most cells of the body experience cell senescence that is, they will divide for a certain number of times and then stop. The number of cell divisions of a connective tissue cell from a human fetus in culture ranges from 35-63, with the average being 50 times. The same type of cell take from an adult and put into culture will divide only 14- 29 times.

The reason for this appears to be telomeres. Telomeres are made up of repeating units of six nucleotides at the ends of the chromosomes. Some of these sequences are lost each time the cell divides. When the ends wear down to a certain point, the cells stop dividing. Telomerase ?

Other factors that influence cell division:

1. Levels of proteins call kinases and cyclins which rise and fall and trigger other proteins that cause cell division.

{ Maturation promoting factor (MPF) induces cell division, both mitosis and meiosis. MPF consists of cdc2 proteins and cyclin. Cyclin builds up during interphase and activates cdc2 proteins and MPF, which induce mitosis (or meiosis). Cyclin is low at the end of end of mitosis and the cdc2 proteins are not activated.}

2. Cell size- or the ratio between cell volume and surface area.

Volume increases faster than surface area, and surface area is need for diffusion etc.

3. External factors such as hormones and growth factors. E.g.estrogen and uterine lining.
Growth factors are similar, but act more locally - epidermal growth factor stimulates growth of new skin beneath a scab. Salivary glands also produce this growth factor.

4. Contact inhibition (density dependent inhibition)

LOSS OF CONTROL OF CELL DIVISION:

When cells lose their ability to control their division, we end up with a mass of cells called a neoplasm, or tumor.

Benign tumors remain in a single location. Doesn’t mean they can’t be fatal if occur in the wrong place.

Malignant tumors are invasive, resembling a crab with outstretched claws (that’s where the name cancer comes from). The cells may reach the circulatory system, break off and spread to other organs - metastasize.

At least two types of genes cause cancer.

Oncogenes activate other genes that increase cell division rate.

Tumor suppressor genes - keep mitosis in check. When these genes are lost or shut off, the cell divides out of control, leading to cancer.

Cell death, or apoptosis (ap-oe- TO-sis) is also regulated, and appears to be under genetic control. We know that cells grown outside the body divide a certain number of times then stop. In apoptosis, the DNA breaks up, the mitochondria cease functioning, and the cytoplasm shrinks, but the cell membrane remains unbroken. The cell is then consumed by phagocytes. (vs. necrosis where cells burst and inflammation results) Apoptosis removes unneeded cells during embryological development, regulates the number of cells in a tissue, and eliminates potential cancer cells.

Sometimes the genes which control cell division are lost or shut off by viruses.( gene p53) The result is uncontrolled growth or cancer. Some genes called BCL, produce proteins that block apoptosis. In some tumors, these genes are left turned on, inhibiting cell death and decreasing the effectiveness of anticancer drugs.