 |
Biology Guttman | |||||
|---|---|---|---|---|---|---|
|
Student
Online Learning Center
| ||||||
 | ||||||
|
|
Biology Guttman | |||||
|---|---|---|---|---|---|---|
|
Student
Online Learning Center
| ||||||
|
| ||||||
| Extended Lecture Outline |
Chapter 11: The Dynamic Cell |
SECTION A. CELLS REGULATE THEMSELVES
11.1 Cell growth means both synthesizing new biomolecules and dividing to form more cells.
a. A cell is growing when it makes more of its own structure. Adding mass by adding water would not constitute growth.
b. Growth means synthesizing the characteristic biomolecules of cell structure–proteins, nucleic acids, polysaccharides, and lipids.
c. The cellular stores of energy, ATP, and NADPH are used to synthesize new biomolecules.
d. Growth of bacteria or yeast can be easily observed when such organisms are placed into a nutrient medium.
e. A nutrient medium is defined as a mixture of the materials that cells need for growth.
f. Bacterial binary fission occurs when a cell divides down the middle to become two cells (Figure 11.1).
g. Growth means two things: cells get larger and they proliferate–increase in number.
h. Cell culture provides the opportunity to observe the growth of plant and animal cells in nutrient media (Figure 11.2).
11.2 Cells grow by assimilation materials from their environment through biosynthetic pathways.
a. Organisms are said to assimilate materials from their surroundings into their structure.
b. Metabolic pathways in cells build atoms of raw materials into monomers such as amino acids, nucleotides, sugars, and fatty acids.
c. Biosynthesis begins with metabolites of the central metabolic pathways of glycolysis and the Krebs cycle.
d. Keto-acids are converted into amino acids through reactions called transaminations.
e. Enzymes transfer amino and keto groups from one acid to another by using the coenzyme pyridoxal, which we obtain as vitamin B6 (Figure 11.3).
f. As Figure 11.4 shows, the three amino acids–alanine, aspartate, and glutamate–are the gateways to pathways used to synthesize most of the other amino acids, as well as the purine and pyrimidine nucleotides of nucleic acids.
g. Animals differ from most other organisms because they lack the enzymes for many biosynthetic reactions, so they must obtain eight of the twenty amino acids from their food.
11.3 Organisms maintain themselves in steady-state conditions through homeostatic mechanisms.
a. An organism's structure and composition can remain virtually the same even though it continuously takes in nutrients and produces wastes.
1. A steady-state condition is achieved when materials constantly flow one way through the system: incoming nutrients are converted into more of the organism's structure or into wastes, with no reverse flow.
2. In a steady-state, when metabolic flow is balanced, the rates of all reactions are so tuned to one another that the concentrations of metabolites remain quite constant, even though no one molecule remains in any pool for long (Figure 11.5).
b. Organisms have evolved mechanisms for survival in the face of harsh and constantly changing environmental conditions.
1. Homeostasis is the constant internal state achieved by an organism regardless of changes in the surroundings.
2. All cells in an organism must keep their internal conditions constant within a narrow range.
3. Cells slow down or speed up processes to maintain a balanced, steady state flow.
c. Homeostasis is achieved by regulating certain variables, such as temperature or the concentration of an ion.
1. Regulation always depends on closed circuits that employ negative feedback.
2. Feedback is a process in which information from the output of a device is sent back to control the device, and in negative feedback the information is used to keep the regulated variable close to some desired point.
3. A controlled variable is one that can be managed based on information received.
4. Sensors monitor the variable from moment to moment
5. The sensor in a system communicates with a comparator, which compares the actual situation with the desired one (set point).
6. The effector in a system receives an activating signal if work is needed to return the system to the desired set point.
d. Negative feedback is fundamental to physiology (Figure 11.6).
1. Homeostatic systems in our bodies monitor conditions from moment to moment, both internally and externally, and send out signals to correct any problems.
2. Homeostatic systems keep human body temperature at about 37û C, our blood at about pH 7.4, maintain concentrations of CO2, glucose, calcium, and other materials in the blood and govern many other processes.
3. Most homeostatic mechanisms operate on fundamentally chemical information.
e. A regulatory circuit requires at least one transducer, a device that converts one form of energy or information into another.
1. An example of a biological transducer would be the receptor cells of the eye transducing light signals into nerve impulses.
2. Organisms include some transducers, such as muscles, that only convert energy, while many others convert information.
3. All biological transducers are chemical in nature.
f. A control system with negative feedback leads to homeostasis and stability.
1. Positive feedback creates amplification and leads to instability: a small change leads to increasingly greater effects.
2. A biological case of positive feedback occurs during childbirth when the uterine muscles respond to the growing fetus, the pituitary gland produces the hormone oxytocin. Oxytocin signals the uterine muscles to contract further, which in turn brings about production of more oxytocin in a cycle that culminates with birth.
3. Long-term regulation of a system requires negative feedback.
11.4 Allosteric proteins are general informational transducers.
a. Allosteric protein can bind two different molecules, or ligands, with quite distinct shapes, at sites with correspondingly distinct shapes, so that the interaction at one site affects the other site (Figure 11.7).
1. One ligand is commonly a small molecule or ion; the other may be a different small molecule or a macromolecule such as another protein or a nucleic acid.
2. Ligands bind to proteins through weak bonds, so the interaction between the two molecules is readily reversible.
3. Ligands interact stereospecifically with proteins and tend to stabilize the protein in one conformation.
b. Allosteric proteins are informational transducers.
11.5 Allosteric enzymes at critical points regulate the activity of metabolic pathways.
a. To maintain a steady state, a cell must coordinate its hundreds of metabolic reactions so they stay in step with one another.
1. In cells, allosteric enzymes are located at key points in pathways. These enzymes' activities are regulated by specific metabolites.
2. Most often, enzymes are inhibited by their product, and the process is called end-product inhibition or feedback inhibition (Figure 11.8).
b. Regulatory circuits show how remarkably well a system can be designed by evolution, because feedback controls in cells are placed for maximum efficiency (Figure 11.9).
1. In a branched biosynthetic pathway, each end-product inhibits the first enzyme specific to its synthesis.
2. Sometimes different end-products inhibit the same early reaction: a cell may have two distinct enzymes to catalyze the same reaction, each one inhibited by a different end-product.
3. Regulatory ligands may also activate allosteric enzymes, rather than inhibiting them.
4. The main pathways that ultimately produce ATP are regulated at several points by ATP, ADP, and AMP (Figure 11.10).
11.6 Cells can recognize and respond to external ligands.
a. To maintain their integrity, organisms require information about their external environment as well as about internal conditions.
b. A stimulus is any environmental factor that an organism recognizes and responds to, and the most basic stimuli are chemical.
c. Flagellated bacteria will swim in response to chemical stimuli as shown in Figure 11.11. This movement is called chemotaxis.
1. Positive chemotaxis occurs when an organism moves toward a stimulus.
2. Negative chemotaxis occurs when an organism retreats from a stimulus.
d. Organisms detect each kind of external ligand by means of a distinctive receptor protein.
1. A receptor protein is an allosteric protein whose binding site on the external surface of the plasma membrane is stereospecific for some ligand.
2. When the ligand binds, the receptor changes its shape, setting other events in motion (Figure 11.12).
3. Biologists say that the receptor recognizes a ligand that binds to it, or allows an organism to recognize the ligand.
4. Bacteria have a chemoreceptor for each type of ligand they can recognize (Figure 11.13).
11.7 Signal ligands carry information at different levels of activity.
a. Organisms use chemical signals, and chemoreceptors, to carry information between parts of a cell, between the cells of a multicellular organism, and between individuals of the same species or different species.
b. Signal ligands are generally unusual molecules that are not catabolized for energy and materials.
1. Hormones, pheromones, and alarmones are all signal ligands.
2. The first signal ligands to be discovered were hormones. Hormones carry signals between cells of a multicellular organism to elicit a distinctive response by other types of cells (Figure 11.14).
c. Pheromones carry signals between organisms of a species. Males and females of a species use sex pheromones to find and recognize one another and to stimulate mating behavior (Figure 11.15).
d. Intracellular signal ligands called alarmones trigger specific responses inside cells when certain threatening conditions arise.
e. In addition to signal ligands, many comparable compounds have roles in ecological communities.
1. Antibiotics are prime examples of allomones that have detrimental effects on other species.
2. Other compounds know as kairomones benefit the individual that detects them, not the one that produces them. Some organisms hunt by detecting pheromones of their prey.
11.8 Eucaryotic cells respond to external signals through a common transduction pathway.
a. G-proteins, or guanine-nucleotide binding proteins, are located in the cytoplasmic face of the cell membrane and bind the nucleotides guanosine diphosphate (GDP) and guanosine triphosphate (GTP) (Figure 11.16).
1. G-proteins are used as signals, not for the energy that they carry.
2. G-proteins are general transducers that convert an external stimulus into an internal signal.
b. First and second messengers are another means of communication found in cells.
1. First messengers move from cell to cell, stimulating target cells to produce intracellular messengers.
2. Second messengers are produced as a result of a cells contact with a first messenger, and they initiate the cell's specific action.
c. In the 1960s, Earl Sutherland discovered cyclic AMP (cAMP, adenosine 3', 5'-monophosphate), the second messenger in which the phosphate group is attached in a ring structure to two points on the ribose.
d. Calcium ions (Ca) are second messengers that frequently act by binding to intracellular proteins, such as calmodulin, which initiate various processes (Figure 11.17).
e. Protein phosphorylation is a transducing device that illustrates that some proteins in a cell are activated or inactivated by the addition and removal of small, simple chemical groups.
1. The activity of a protein is changed by phosphorylation–adding a phosphoryl group –and the enzymes that phosphorylate other proteins are called protein kinases.
2. Eucaryotic cells carry an assortment of specialized protein kinases to regulate their many activities; each kinase phosphorylates its target protein on either a serine or a tyrosine residue.
SECTION B. CELLS MOVE AND CHANGE THEIR FORM
a. A eucaryotic cell's shape and most of its movements are controlled by its cytoskeleton (Figure 11.18).
1. The cytoskeleton is a fibrous framework that stretches throughout a cell.
2. The cytoskeleton is made of three main elements: actin filaments (also called microfilaments), microtubules, and intermediate filaments.
11.9 Actin filaments effect many cell movements.
a. Much of the cytoskeleton consists of 6-nm-wide microfilaments of the protein actin, which may constitute 5—20 percent of the cellular protein.
1. Actin filaments are responsible for muscle contraction and movements during embryonic development in animals.
2. Fluorescent antibodies against actin bind to these filaments and reveal them as a spectacular, glowing network (Figure 11.19).
3. Actin filaments also form the cell cortex, a layer just under the cell surface, which holds the cell in shape and accounts for much of its movements.
b. Some animal cells have minute, fingerlike microvilli projecting from one face, creating a huge surface for absorption of food molecules or for other processes (Figure 11.20).
c. Animal cells in cell cultures contact the substratum at many adhesion plaques, where actin filaments attach to the plasma membrane. Adhesion plaques are complexes of at least six proteins besides actin (Figure 11.21).
d. Actin filaments are dynamic structures that can quickly change their form, and this is one way they effect cellular movements.
1. Actin is composed of 45-kDa subunits called G (globular) actin, which polymerize into long, double-stranded filaments of F (fibrous) actin (Figure 11.22).
2. Treadmilling is a process where actin filaments change their form by the addition and removal of subunits (Figure 11.22 and Figure 11.23).
e. Actin filaments also produce movement by interacting with filaments of myosin, a long, fibrous protein whose globular end is an ATPase (Figure 11.24).
1. Myosin and actin are organized so that myosin molecules pull on actin molecules by activating this ATPase (Figure 11.25).
2. Actin-myosin complexes are responsible for the movement called cyclosis or cytoplasmic streaming (Figure 11.26).
3. In animal cells, much of the cytoskeleton breaks down just before cell division, so the cell relaxes and becomes round; then some filaments assemble in a contractile ring that closes to pinch the cell in two (Figure 11.27).
4. In amoeboid movement, the pseudopods have stiff cortical layers, and a stream of cytoplasm seems to flow through the middle of a pseudopod, changing from a sol at the rear of the cell into a gel as it reaches the tip of the pseudopod (Figure 11.28).
11.10 Microtubules shape cells and are used for movement.
a. Microtubules were discovered in 1963 when Keith R. Porter and Myron Ledbetter introduced the fixative glutaraldehyde into electron microscopy.
1. They discovered that the cytoplasm contains many small tubules, 25 nm in diameter with a central 14-nm hole.
2. Microtubules are built from a- and b-tubulin (50 kDa each), which form an a—b dimer.
3. A general principle of cytoplasmic organization is that the minus end of a microtubule is protected by a microtubule-organizing center (MTOC).
b. Microtubular structure can be quite remarkable (Figure 11.29). Microtubules are the elements of all the following structures and processes:
1. they supplement the actin filament cytoskeleton to maintain the shapes of many cells,
2. they form the motile skeletal structure of cilia and flagella,
3. they form the core of organelles called centrioles and basal bodies,
4. they form structures that some protozoans use for movement,
5. they are the spindle fibers of the mitotic apparatus, which separates chromosomes during cell division,
6. they organize the cell wall between daughter cells in plant cell division,
7. they help to develop and maintain the long, thin extensions of nerve cells called axons that carry nerve impulses over long distances,
8. they help to move many materials in cells, such as releasing secretion granules during exocytosis and moving pigment granules in pigment cells (chromatocytes).
11.11 Microtubule structures are organized by special centers.
a. Many microtubule complexes shift around as cells change their activities.
b. MTOCs orient microtubules and initiate their polymerization from tubulin dimers.
c. The centrosome, a diffuse region near the nucleus, serves as an MTOC.
d. The centrosome contains a centriole, a structure that seems to be the same as the basal body at the base of every cilium and flagellum (Figure 11.30).
e. The centrosome, with or without a centriole, organizes a loose cytoskeleton of microtubules in non-dividing cells (Figure 11.31).
11.12 Objects move on microtubules by means of specialized motors.
a. Cells use at least two kinds of molecular motors.
b. Molecular motors are specialized microtubule-associated ATPases that hydrolyze ATP and use the energy to move along the microtubule.
c. The microtubule motors form two families: dyneins and kinesins (Figure 11.32).
d. Dyneins move from the plus end to the minus end of a microtubule and kinesins do the reverse.
e. Dyneins and kinesins can be used to anchor organelles to microtubules and to move them at speeds up to 50 µm per minute.
11.13 Cilia and flagella are movable bundles of microtubules.
a. Many eucaryotic cells bear extensions of the cell surface that are used for rapid movement and are called kinetids or undulipodia.
b. Kinetids are called cilia when they are short and flagella when they are long.
c. Human sperm cells typically have one flagellum, many algae have two and some protozoans have many.
d. The entire external surfaces of some animals, such as flatworms, are covered with cilia.
e. Cilia and flagella have nine doublets of microtubules plus two single tubules in the center making a characteristic 9 + 2 pattern (Figure 11.33).
f. The doublets found in cilia and flagella are linked by the protein nexin and by other interconnecting filaments to make a framework about 200 nm in diameter called an axoneme (Figure 11.34).
11.14 Bacterial flagella are made of flagellin, a different globular protein.
a. The procaryotic flagellum is very different from eucaryotic flagella.
1. The bacterial flagellum is a long, naked helix of the protein flagellin that protrudes from the bacterial wall and membrane, where its hooked end is anchored into a system of rings. (Figure 11.35).
2. The bacterial flagellum rotates inside the rings by one structure pulling on the other.
11.15 Materials can be moved across membranes by bulk transport.
a. Cells can move larger amounts of material in and out by means of vesicles (Figure 11.36).
1. In exocytosis, a vesicle inside the cell fuses with the plasma membrane and disgorges its contents.
2. Through exocytosis, cells export materials that they have synthesized in the ER-Golgi system.
b. Cells import bulk material in at least two distinct ways: endocytosis and phagocytosis.
1. In phagocytosis, cells engulf relatively larger bits of material through movements of the cell membrane directed by actin filaments (Figure 11.37).
2. Many protists, such as amoebas, feed by phagocytosis.
c. In endocytosis, a vesicle from the plasma membrane invaginates and pinches off.
1. Endocytosis begins with coated pits, depressions in the cell surface formed by networks of the protein clathrin.
2. Coated pits regularly invaginate, round up, and pinch off to make a coated vesicle, which is covered by a regular cage of clatharin (Figure 11.38).
d. In the process of receptor-mediated endocytosis, receptors on animal cell surfaces bind proteins in the blood that carry nutrients such as iron or lipids, and coated vesicles bring these receptor-protein complexes into the cytoplasm (Figure 11.39).
1 Coated vesicles then carry their contents to endosomes, which are larger vesicles that lie beneath the cell surface (Figure 11.40).
2. Receptor proteins and clathrin networks are recycled to the cell surface for reuse and then other vesicles deliver hydrolytic enzymes from the Golgi apparatus to the endosomes, converting them into lysosomes.
3. Since coated vesicles always enclose a certain amount of fluid when they close, endocytosis has also been called pinocytosis, or "cell drinking."
e. Lysosomes are also used in autophagy or "eating self."
1. Autophagy is a way to recycle selected portions of a cell without destroying the whole cell.
2. Lysosomes contain digestive enzymes that break down ingested foreign materials and allow cells to recycle cellular components.
MHHE Home | About MHHE | Help Desk | Legal Policies and Info | Order Info | What's New | Get Involved
