Chapter 7 Study Guide Answers PDF

Preview

Summary

This is the study guide answers for chapter 7 in Berndtson's class...

Description

Answers to Chapter 7 Study Questions 1. The plasma membrane of a cell exhibits selective permeability. What does this mean and why is selective permeability a critical property of a cell? The plasma membrane allows some substances to cross more easily than other substances, which enables the cell to have a unique internal environment. 2. Explain why a phospholipid is an amphipathic molecule. Amphipathic molecules have both hydrophilic and hydrophobic regions. The head (choline, phosphate, and glycerol) of a phospholipid is hydrophilic while the tail (hydrocarbons) is hydrophobic. Why do phospholipids form a bi-layer in an aqueous environment? This arrangement places the hydrophilic heads in contact with water, while the hydrophobic tails are in contact with each other and away from water. 3. A fluid mosaic model is use to describe the plasma membrane. What part of the membrane contributes to its fluidity, and what part of the membrane contributes to the mosaic component? The phospholipids assembled in a bi-layer arrangement provide the fluidity, while the proteins that are embedded or transvers the membrane are the mosaics components. Remember, that both the phospholipids and many of the integral proteins move laterally within the plasma membrane. 4. What type of bond holds the phospholipid membranes together? Hydrophobic interactions The individual phospholipid molecules are “fluid” meaning that they can move within the membrane. In what direction do they usually move? The adjacent phospholipid molecules will switch position laterally about 107 times/sec enabling them to travel about 2 μm/sec. On very rare occasions (~1 time/month) phospholipid molecules will flip vertically within the membrane. 5. Membrane proteins can also move within membranes, and some membrane proteins are frequently grouped together is a specialized patch. Below is a list of the membrane proteins that are involved in transmitting the adrenaline “signal” into a liver cell. In terms of hormone signal transduction, adrenaline is considered the “primary message”. It is a small amine hormone that functions in the “fight or flight” response, thus it is also considered a “stress” hormone. In general, stress hormones such as adrenaline (also called epinephrine) act by mobilizing energy (glucose) from storage in the liver to ATP production in the muscles, which leads to an increase in heart rate, blood pressure, and breathing rate. Membrane Protein Adrenaline Receptor (Adrenergic Receptor)

Integral or Peripheral protein Integral

G Protein (composed of α-, β-, and γ-subunits)

Peripheral

Adenylyl Cyclase

Integral

Which of these membrane proteins moves within the membrane? The G-Protein What is the function of the G-protein? Once activated, the α-subunit moves laterally within the membrane to activate the membrane protein adenylyl cyclase. What is the function of adenylyl cyclase? This enzyme functions to generate cAMP from ATP. What is the function of cAMP within the cell? cAMP is the “secondary messenger” molecule that functions to initiate a cascade of enzymes that ultimately stimulate glycogenolysis within a liver cell. Within muscle cells, glucose is oxidized via cellular respiration to generate ATP that is used in the “fight or flight” response. 6. How does the plasma membrane of animal cells stays fluid at low temperatures? Animal cells have cholesterol molecules embedded within the hydrophobic part of the membrane. These molecules prevent the hydrocarbon tails from packing close together and solidifying at low temperatures. Animals that live in

extremely cold environments (e.g. artic chard) also have both saturated and unsaturated fatty acid in their phospholipids. The unsaturated fatty acids prevent the phospholipids from packing close together and solidifying at low temperature. Why is fluidity important for membranes? Membranes must be fluid to function (but not too fluid). Enzymatic proteins in the membrane are inactive when the membrane solidifies. Some of these enzymes also need to move laterally to function. 7. There are many (~200) types of cells in the body and each type has a unique function. Similarly each cell type has a unique cell membrane. How do cell membranes differ between the various cell types? Each membrane has a unique set of proteins that reflects the unique functions of each membrane. What are the two major types of membrane proteins? Integral proteins (e.g. Integrins) and peripheral proteins (e.g. associated proteins) 8. Cite an example of a membrane protein that has the following function: Hint: Review slides 14 and 15 and other lecture slides. • Transport: Glut2 transporter, aquaporin channel proteins • Enzyme activity: Metabolic enzymes such as ATP synthase • Signal Transduction: Hormone receptors such as the β adrenergic receptor or the insulin receptor • Cell-Cell Recognition: The ABO glycoproteins on RBC • Intercellular Joining: Proteins found in gap junctions such as the gap junction alpha-1 protein • Attachment to the cytoskeleton and ECM: Integral proteins (cytoskeleton) or fibronectin (ECM) 9. Many cells have transmembrane glycoproteins in which the carbohydrate component is located on the extracellular side of plasma membrane (see figure below).

Where within the endomembrane system of the cell are these monosachharides added to these proteins. Within both the ER and Golgi. When a transmembrane glycoprotein moves through the endomembrane system, is its carbohydrate component facing inward (e.g. placing it inside the lumen of the organelles) or face outward (e.g. placing it in contact with the cytoplasm that surrounds the organelle)? Facing inside the lumen of the ER and Golgi, and facing inside the transport vesicles. 10. In terms of polarity, what type of molecule moves easily through the plasma membrane and what type of molecule does not? Small nonpolar molecules such as molecular oxygen, carbon dioxide and molecules consisting of mostly hydrocarbons move through the membrane easily while polar molecules do not. Examples of polar molecules are monosaccharides, ions, AAs, and water. Large biological molecules such as proteins are way too large to cross the membrane. These large molecules are secreted from cells via exocytosis and frequently enter the cell via endocytosis. 11. What is passive transport? Passive transport is the movement of molecules down a concentration gradient (from high to low concentration). Molecules move by random thermal movements. The three

different types are simple diffusion, osmosis, and facilitated diffusion. This process does not require energy and the molecule that are transported are moving towards equilibrium. 12. What is osmosis? Osmosis is the movement of water molecules across a selectively permeable membrane. What are the two types of water? Free water and bound water (H-bonded to hydrophilic parts of solute molecules). Which type of water is transported by osmosis? Free water. In what direction does water move by osmosis? Water moves from an area of higher to lower free water concentrations or from an area of lower to higher solute concentration. What integral protein helps speed up osmosis? Aquaporin water channel. 13. What is active transport? Active transport is the movement of molecules across membranes against their concentration gradient. Active transport requires energy and uses carrier proteins. Why does life require active transport? Active transport allows cells to maintain internal concentrations of small molecules that differ from concentrations in the environment. This compartmentalization also occurs within the organelles of cells. These different environments are critical for cell function. For instance oxidative phosphorylation (a process the generates ATP) depends on different chemical environments on either side of the inner wall of the mitochondria. 14. Know the steps involved in one cycle (crank) of the Sodium-Potassium pump. Why is the membrane potential of a cell -70 millivolts? There is an unequal distribution of ions across the membrane, more negative ions inside than outside the membrane. This difference favors the transport of cations (+) into the cell and anions (-) out of the cell. What two forces are important for maintaining the electrochemical gradient of a cell? Chemical force, an ion’s concentration gradient, and electrical force, the effect of membrane potential (+ or -) on the ions movement. 15. What is the major electrogenic pump in animal cells? Sodium-potassium pump. More + ions (3 Na+) leave the cell then enter the cell (2 K+). What is the major electrogenic pump in plant, fungi and bacterial cells? Proton pump 16. Plants use a cotransport mechanism to move sucrose into nonphotosynthetic plant cells such as ones found in the roots of a plant. What transport protein directly function in the co-transport of sucrose? Sucrose-H+ Cotransporter What transport protein indirectly functions in the co-transport of sucrose? Proton Pump Which transport protein generates the [H+] gradient within the plant cell? Proton Pump What molecule is co-transported into the cell against its concentration gradient? Sucrose Where does the energy come from to drive the movement of this molecule into the cell? When H+ flow into the cell (e.g. down their concentration gradient), the energy that is stored across the membrane is released and this energy is used to cotransport sucrose into the cell (against its concentration gradient). 17. Complete the following tables to help you understand the different mechanisms that transport small molecules across membranes.! Name of Process

Involves a Transport Protein

Simple Diffusion: nonpolar molecules

No

Examples: O2, CO2, Steroids, fatty acids

Concentration Gradient (moves down or against gradient) Down

Energy Required

No

Facilitated Diffusion: polar molecules or ions

Channel and carrier proteins

Down

No

Specific Example: Glucose Osmosis: H2O Slow: Fast:

Glut2 Transporter

Down

No

No Aquaporin

Down Down

No No

Transport Protein

Concentration Gradient (moves down or against gradient)

Energy Required

Examples: Na+, Cl-

Sodium-Potassium Pump

Against

Yes (ATP)

H+

Proton Pump

Against

Yes (ATP)

Active Transport (small molecules): Name of Process

Active Transport: Many polar molecules or ions

Cotransport (small molecules): Name of Process Transport Protein Sucrose-H+ cotransporter Cotransport: Sucrose

H+

Concentration Gradient

Energy Required

Against

Yes, but energy comes from the flow of H+ into the cell (down its gradient)

Down

Down (provides the energy to move sucrose into cell against its gradient) Remember, a separate proton pump is required to generate the [H+] gradient within the plant cell. Thus, ATP indirectly provides the energy necessary for cotransport

18. Fill in the missing components of the following table to review the different mechanisms that transport large macromolecules across membranes. Remember all of these processes require a large amount of energy! Use of process Release secretory protein Incorporates proteins and lipids into the plasma membrane

Type of macromolecule transported Protein Hormones Integral protein (glycoproteins) Membrane Lipids (glycolipids)

Endocytosis: Phagocytosis

Uptake via pseudopodium of large particles

Bacteria, Food

Endocytosis: Pinocytosis

Uptake of droplets of extracellular fluid that include dissolved solutes

Non specific solutes in extracellular fluid

Endocytosis: Receptor Mediated Endocytosis

Uptake of ligands that are specific for a receptor in the coated pits as well as nonspecific molecules. Key point is that

Specific ligand (high concentration) Non specific solutes (low concentrations)

Exocytosis

the receptor allows for concentrated ligand uptake

Example: LDL

19. What is the cause of!familial hypercholesterolemia? LDL receptor proteins are defective or missing in an individual. These receptors are critical for the movement of LDL into the cell via receptor-mediated endocytosis. When cells do not take up LDLs, they remain in the blood and can contribute to the build up of lipid droplets in the blood. What is a Low density Lipoprotein (LDL)? An LDL is a chylomicron that transports cholesterol within the blood stream. It contains a core of cholesteryl ester molecules (very hydrophobic) that is surrounded by phospholipids, unesterfied cholesterol, and proteins. Why are LDLs considered the “bad cholesterol”? They transport and deposit cholesterol into the walls of arteries which will attract macrophages and drive atherosclerosis (clogged blood vessels). Why are High Density Lipoproteins (HDLs) considered “good” cholesterol? These chylomicrons can remove fat from macrophages in the walls of arteries....