Biomolecules and Membranes II

Sort the phrases into the appropriate bins depending on whether they describe exocytosis, endocytosis, or both. 1. transported substances never physically cross the plasma membrane 2. forms vesicles from inwards folding of the plasma membrane 3. requires fusion of vesicles with the plasma membrane 4. decreases the surface area of the plasma membrane 5. requires cellular energy 6. increases the surface area of the plasma membrane 7. secretes large molecules out of the cell

-Exocytosis: 7. secretes large molecules out of the plasma membrane, 6. increases the surface area of the plasma membrane, 3. requires fusion of vesicles with the plasma membrane -Endocytosis: 4. decreases the surface area of the plasma membrane, 2. forms vesicles from inwards folding of the plasma membrane Both: 5. requires cellular energy, 1. transported substances never physically cross the plasma membrane -In exocytosis, substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell. In endocytosis, substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle. Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell. http://session.masteringbiology.com/problemAsset/1125909/10/1125909_001.jpg

Facilitated diffusion via channels and carrier proteins The majority of solutes that diffuse across the plasma membrane cannot move directly through the lipid bilayer. The passive movement of such solutes (down their concentration gradients without the input of cellular energy) requires the presence of specific transport proteins, either channels or carrier proteins. Diffusion through a transport protein in the plasma membrane is called facilitated diffusion. Diagram showing facilitated diffusion across the plasma membrane. A channel protein embedded in the membrane allows yellow balls to travel through a channel from the outside of the cell to the inside. A carrier protein embedded in the membrane undergoes a shape change allowing red balls to travel from the outside of the cell to the inside. http://session.masteringbiology.com/problemAsset/1119629/22/1119629_007.jpg Sort the phrases into the appropriate bins depending on whether they are true only for channels, true only for carrier proteins, or true for both channels and carriers. -Provide a continuous path across the membrane, -undergo a change in shape to transport solutes across the membrane, -Are integral membrane proteins, -Allow water molecules and small ions to flow quickly across the membrane, -Transport solutes down a concentration or electrochemical gradient, -Transport primarily small polar organic molecules, -Provide a hydrophilic path across the membrane Only channels: Only carriers: Both channels and carriers:

-Only channels: -Provide a continuous path across the membrane, -Allow water molecules and small ions to flow quickly across the membrane -Only carriers: -Undergo a change in shape to transport solutes across the membrane, -Transport primarily small polar organic molecules -Both channels and carriers: -Transport solutes down a concentration or electrochemical gradient, -Are integral membrane proteins, -Provide a hydrophilic path across the membrane Carrier proteins and channels are both transport proteins involved in facilitated diffusion, the passive transport of solutes across a membrane down their concentration or electrochemical gradient. As integral membrane proteins, both carriers and channels protect polar or charged solutes from coming into contact with the hydrophobic interior of the lipid bilayer. Furthermore, all transport proteins are specific for the solutes they transport, owing to the specificity of the interactions between the solute and the transport protein. Channels are protein-lined pores across the membrane. A channel may be open at all times (non-gated), or may be gated such that the channel opens and closes under specific conditions. Channels transport inorganic ions or water http://session.masteringbiology.com/problemAsset/1119629/22/1119629_008.jpg In contrast, carrier proteins do not have a pore. Binding of the transported solute to the carrier protein on one side of the membrane induces a conformational change in the protein that exposes the solute binding site to the opposite side of the membrane, where the solute is released. Carriers transport small polar solutes such as sugars and amino acids. http://session.masteringbiology.com/problemAsset/1119629/22/1119629_009.jpg

Part A - Diffusion All molecules have energy that causes thermal motion. One result of thermal motion is diffusion: the tendency of substances to spread out evenly in the available space. Although the motion of each individual molecule is random, there can be directional motion of an entire population of molecules. Consider a chamber containing two different types of dye molecules, purple and orange. The chamber is divided into two compartments (A and B) by a membrane that is permeable to both types of dye. Initially (left image), the concentration of the orange dye is greater on side A, and the concentration of the purple dye is greater on side B. With time, the dye molecules diffuse to a final, equilibrium state (right image) where they are evenly distributed throughout the chamber. http://session.masteringbiology.com/problemAsset/1119629/22/1119629_001.jpg -(never),(always),(only before equilibrium is reached),(only at equilibrium) 1. orange dye moves independently of purple dye.___ 2. concentration of gradients exist that drive diffusion of both dyes.___ 3. there is a net movement of orange dye from side A to side B. ___ 4. purple dye ONLY moves from side B to side A.____ 5. there is no net movement of purple dye.____

1. (always) 2. (only before equilibrium is reached) 3. (only before equilibrium is reached) 4. (never) 5. (only at equilibrium) Each dye molecule and the water molecules that surround it are in constant motion due to their thermal energy. Any individual molecule's motion is random because of the frequent collisions among all of the molecules. If a concentration gradient exists for a population of molecules, the motion of the individual molecules in that population will result in a net (directional) movement from higher to lower concentration. For example, in the initial condition, there is a concentration gradient for the orange dye. As a result, the orange dye molecules diffuse down the concentration gradient, with net movement from side A to side B. Once diffusion has eliminated the concentration gradient and equilibrium is reached, net movement stops, but the random motion of each molecule continues (as indicated by the red arrows in the image below).

Because ions carry a charge (positive or negative), their transport across a membrane is governed not only by concentration gradients across the membrane but also by differences in charge across the membrane (also referred to as membrane potential). Together, the concentration (chemical) gradient and the charge difference (electrical gradient) across the plasma membrane make up the electrochemical gradient. Consider the plasma membrane of an animal cell that contains a sodium-potassium pump as well as two non-gated (always open) ion channels: a Na+ channel and a K+ channel. The effect of the sodium-potassium pump on the concentrations of Na+ and K+ as well as the distribution of charge across the plasma membrane is indicated in the figure bhttp://session.masteringbiology.com/problemAsset/1119630/24/1119630_010.jpgelow. Which of the following statements correctly describe(s) the driving forces for diffusion of Na+ and K+ ions through their respective channels? Select all that apply. 1. The diffusion of Na+ ions into the cell is facilitated by the Na+ concentration gradient across the plasma membrane. 2. The diffusion of Na+ ions into the cell is impeded by the electrical gradient across the plasma membrane. 3. The diffusion of K+ ions out of the cell is impeded by the K+ concentration gradient across the plasma membrane. 4. The diffusion of K+ ions out of the cell is impeded by the electrical gradient across the plasma membrane. 5.The electrochemical gradient is larger for Na+ than for K+.

1. The diffusion of Na+ ions into the cell is facilitated by the Na+ concentration gradient across the plasma membrane. 4. The diffusion of K+ ions out of the cell is impeded by the electrical gradient across the plasma membrane. 5.The electrochemical gradient is larger for Na+ than for K+. -The concentration gradient of Na+ ions across the membrane (higher Na+ concentration outside) facilitates the diffusion of Na+ into the cell. At the same time, the electrical gradient across the membrane (excess positive charge outside) drives Na+ into the cell. The concentration gradient of K+ ions across the membrane (higher K+ concentration inside) facilitates the diffusion of K+ out of the cell. However, the electrical gradient across the membrane (excess positive charge outside) impedes the diffusion of K+ out of the cell. The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and the electrical gradient (charge difference) across the membrane. For Na+ ions, diffusion through the Na+ channel is driven by both the concentration gradient and the electrical gradient. But for K+ ions, the electrical gradient opposes the concentration gradient. Therefore, the electrochemical gradient for Na+ is greater than the electrochemical gradient for K+.

Which of these is a polysaccharide? 1. cellulose 2. sucrose 3. lactose 4. galactose 5. glucose

1. cellulose Cellulose is a carbohydrate composed of many monomers.

_____ is the most abundant organic compound on Earth. 1. Glucose 2. Starch 3. Cellulose 4. Glycogen 5. Lactose

3. cellulose Cellulose, a component of plant cell walls, is the most abundant organic compound found on earth.

Some solutes are able to pass directly through the lipid bilayer of a plasma membrane, whereas other solutes require a transport protein or other mechanism to cross between the inside and the outside of a cell. The fact that the plasma membrane is permeable to some solutes but not others is what is referred to as selective permeability. Which of the following molecules can cross the lipid bilayer of a membrane directly, without a transport protein or other mechanism? Select all that apply. 1. proteins 2. sucrose 3. lipids 4. carbon dioxide 5. water 6. ions 7. oxygen

3. lipids, 4. carbon dioxide, 5.water, 7. oxygen Some solutes pass readily through the lipid bilayer of a cell membrane, whereas others pass through much more slowly, or not at all. Small nonpolar (hydrophobic) molecules, such as dissolved gases (O2, CO2, N2) and small lipids, can pass directly through the membrane. They do so by interacting directly with the hydrophobic interior of the lipid bilayer. Very small polar molecules such as water and glycerol can pass directly through the membrane, but much more slowly than small nonpolar molecules. The mechanism that permits small polar molecules to cross the hydrophobic interior of the lipid bilayer is not completely understood, but it must involve the molecules squeezing between the hydrophobic tails of the lipids that make up the bilayer.Polar molecules such as glucose and sucrose have very limited permeability.Large molecules such as proteins cannot pass through the lipid bilayer.Ions and charged molecules of any size are essentially impermeable to the lipid bilayer because they are much more soluble in water than in the interior of the membrane. http://session.masteringbiology.com/problemAsset/1119629/22/1119629_006.jpg

glucose + glucose —> _____ by _____. 1. starch + water ... dehydration synthesis 2. lactose + water ... hydrolysis 3. maltose + water ... dehydration synthesis 4. cellulose + water ... hydrolysis 5. sucrose + water ... dehydration synthesis

3. maltose + water ... dehydration synthesis Maltose is the disaccharide formed when two glucose molecules are linked by dehydration synthesis.

Glycogen is _____. 1. a transport protein that carries oxygen 2. a source of saturated fat 3. the form in which plants store sugars 4. a polysaccharide found in animals 5. a polysaccharide found in plant cell walls

4. a polysaccharide found in animals Animals store energy in the form of glycogen.

Polymers that contain sugars ... 1. (a) may store hereditary information. 2. (b) may store energy. 3. (c) may protect cells. 4. Both (b) and (c). 5. (a), (b), and (c).

5. (a), (b), and (c). Good choice! Polymers that contain sugars do all the named functions and more. For example, they also lubricate the path of roots through soil and they glue plant cells together.

Which of these is a source of lactose? 1. sugar cane 2. sugar beets 3. starch 4. potatoes 5. milk

5. milk Lactose is the sugar found in milk.

In many animal cells, the uptake of glucose into the cell occurs by a cotransport mechanism, in which glucose is cotransported with Na+ ions. -OUTSIDE CELL A). [Na+]high (Glucose [high] or [low]) -(BETWEEN) B). Glucose-sodium cotransporter (^[glucose]v[Na+]),(v[glucose]^[Na+]),(v[glucose]v[Na+]) -INSIDE CELL C). [Na+]low (Glucose [high] or [low]) D. 1. Na+ moves ([down] or [against]) its electrochemical gradient 2. Glucose moves ([down] or [against]) its concentration gradient

A). Glucose low B). v glucose v Na+ C). Glucose high D). 1.down 2.against In cotransport, the energy required to move one solute against its concentration or electrochemical gradient is provided by an ion moving into the cell down its electrochemical gradient. The ion that moves into the cell down its gradient is usually the same ion that is pumped out of the cell by an active transport pump: for example, Na+ in animal cells using the sodium-potassium pump, or H+ in plants and prokaryotes using the proton pump. In the case of the glucose-sodium cotransporter in animals, Na+ moves back into the cell down its electrochemical gradient, providing the energy for glucose to move into the cell against its concentration gradient. The energy for glucose transport into the cell is supplied indirectly by the sodium-potassium pump's hydrolysis of ATP, and directly by the Na+ electrochemical gradient created by the pump. http://session.masteringbiology.com/problemAsset/1125908/12/1125908_007.jpg

All cells contain ion pumps that use the energy of ATP hydrolysis to pump ions across the plasma membrane. These pumps create an electrochemical gradient across the plasma membrane that is used to power other processes at the plasma membrane, including some transport processes. In animal cells, the main ion pump is the sodium-potassium pump. -OUTSIDE CELL B. ([Na+]low, [K+]high) or ([Na+]high,[K+]low) C. (excess + charge) or (excess - charge) -(BETWEEN) A. (3[Na+]v,2[K+]^)，（2[Na+]^,3[K+]v), (3[Na+]^,2[K+]v) -INSIDE CELL D. ([Na+]low,[K+]high) or ([Na+]high,[K+]low) E. (excess + charge) or (excess - charge)

B.[Na+]high [K+]low C. excess + charge A. 3 Na+ ^ 2 K+ v D. [Na+]low [K+]high E. excess - charge -Active transport by the sodium-potassium pump follows this cycle. 1. Three Na+ ions from the cytosol bind to the pump. 2. The binding of Na+ stimulates the phosphorylation of the pump protein by ATP. 3. Phosphorylation causes a conformational change in the pump that moves the three Na+ ions against their concentration gradient and releases them outside the cell. 4. The release of the Na+ ions permits two K+ ions from outside the cell to bind to the pump, and the phosphate group is released. 5. Release of the phosphate group causes another conformational change in the pump. 6. The conformational change in the pump moves the two K+ ions against their concentration gradient and releases them into the cytosol. http://session.masteringbiology.com/problemAsset/1119630/24/1119630_009.jpg The net result is that the concentration of Na+ is higher outside the cell and the concentration of K+ is higher inside the cell. In addition, one more positive charge has been transported out of the cell than into the cell, leaving the outside of the cell with an excess positive charge and the inside with an excess negative charge. Thus, the sodium-potassium pump creates both chemical gradients and charge differences across the plasma membrane. The function of the sodium-potassium pump in animal cells (and the proton pump in bacteria and plant cells) is essential to many cell functions. It prevents chemical and electrical gradients across the plasma membrane from reaching equilibrium (at which point the cell would be dead) and powers many types of active transport across the plasma membrane.

a simple sugar is composed of composed of equal parts carbon and water, which gave rise to the general name of any sugar as a ____

Carbohydrate

lactose, the sugar in milk is a ___ because it can be split into two monosaccharides

disaccharide

A____ cannot be hydrolyzed any further

monosaccharide

A carbohydrate that yields many monosaccharides when hydrolyzed is a____

polysaccharide