Mastering HW 5

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. Drag the labels onto the table to indicate when each statement is true. Labels can be used once, more than once, or not at all.

1. ALWAYS 2. ONLY BEFORE EQUILIBRIUM 3. ONLY BEFORE EQUILIBRIUM 4. NEVER 5. ONLY AT EQUILIBRIUM

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 below. 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. 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+.

Phospholipids form the main fabric of the plasma membrane. One feature of phospholipids is that when they are placed in an aqueous solution, they will self-assemble into a double layer (bilayer) that resembles the bilayer of the plasma membrane. This self-assembly occurs because phospholipids are hydrophilic at one end (the phospholipid head) and hydrophobic at the other end (the phospholipid tails). Drag the labels to their appropriate locations in the figure.

A) hydrophilic B) hydrophilic C) grey on top, yellow on bottom D) grey on bottom, yellow on top E) hydrophilic F) hydrophobic G)hydrophilic

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. Complete the diagram below using the following steps.

A. 3 Na+ ^ 2 K+ v B.[Na+]high [K+]low C. excess + charge D. [Na+]low [K+]high E. excess - charge

A red blood cell placed in a hypertonic solution will shrink in a process called crenation. A red blood cell placed in a hypotonic solution will swell and potentially burst in a process called hemolysis. To prevent crenation or hemolysis, a cell must be placed in an isotonic solution such as 0.9% (m/v) NaCl or 5.0% (m/v) glucose. This does not mean that a cell has a 5.0% (m/v) glucose concentration; it just means that 5.0% (m/v) glucose will exert the same osmotic pressure as the solution inside the cell, which contains several different solutes. A red blood cell is placed into each of the following solutions. Indicate whether crenation, hemolysis, or neither will occur. Solution A: 3.21% (m/v) NaCl Solution B: 1.65% (m/v) glucose Solution C: distilled H2O Solution D: 6.97% (m/v) glucose Solution E: 5.0% (m/v) glucose and 0.9%(m/v) NaCl Drag each solution to the appropriate bin.

Crenation: A, D, E Hemolysis: B, C Neither: none

Drag the terms on the left to the appropriate blanks on the right to complete the sentence. Terms may be used more than once.

During osmosis, water diffuses across a selectively permeable membrane from the region of higher FREE WATER concentration and lower SOLUTE concentration to the side with lower FREE WATER concentration and higher SOLUTE concentration.

Sort the phrases into the appropriate bins depending on whether they describe exocytosis, endocytosis, or both.

Exocytosis: - secretes large molecules out of the cell -increases the surface area of the plasma membrane -requires fusion of vesicles with the plasma membrane Endocytosis: -decreases the surface area of the plasma membrane -forms vesicles from inward folding of the plasma membrane Both: - requires cellular energy - transported substances never physically cross the plasma membrane

A red blood cell has been placed into three different solutions. One solution is isotonic to the cell, one solution is hypotonic to the cell, and one solution is hypertonic to the cell. Determine which type of solution is in each beaker based on the cell's reaction. Drag each item to the appropriate bin.

Hypertonic: crenation (knotches) Isotonic: even/ normal Hypotonic: hemolysis (burst)

A critical feature of the plasma membrane is that it is selectively permeable. This allows the plasma membrane to regulate transport across cellular boundaries--a function essential to any cell's existence. How does phospholipid structure prevent certain molecules from crossing the plasma membrane freely? Drag the labels to fill in the table.

NONPOLAR: Hydrophobic, Can cross easily, No transport protein required POLAR: Hydrophilic, Have difficulty in crossing the hydrophobic part, Transport protein required to cross efficiently IONS: Hydrophilic, Have difficulty in crossing the hydrophobic part, Transport protein required to cross efficiently

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. 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.

Only Channels: -Allow water molecules and small ions to flow quickly across the membrane. -Provide a continuous path 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:~ -Are integral membrane proteins. -Transport solutes down a concentration gradient or electrochemical gradient. -Provide a hydrophilic path across the membrane.

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.

Oxygen Water Carbon dioxide Lipids

1. A phospholipid has a "head" made up of a glycerol molecule attached to a single ______________, which is attached to another small molecule

Phosphate group

A semipermeable membrane is placed between the following solutions. Which solution will decrease in volume?

Solution A: 1.4% (m/v) starc

A semipermeable membrane is placed between the following solutions. Which solution will increase in volume?

Solution D: 12.4% (m/v) NaCl

If the pores in the selectively permeable membrane became larger, but still not large enough to let the sugar pass through, what would happen during osmosis in the U-shaped tube compared to what is shown in the figure?

The final water levels would be the same as shown in the figure, but the solutions would reach near equal concentrations faster.

If more sugar were added to the solution on the left side of the tube, what would happen to the water level on the right side of the tube?

The water level would go down.

Which of the following statements about the conditions shown in the U-shaped tube are true?

There is less free water in the right arm of the tube than in the left arm of the tube. Water is tightly clustered around the hydrophilic solute molecules on both sides of the membrane.

In many animal cells, the uptake of glucose into the cell occurs by a cotransport mechanism, in which glucose is co-transported with Na+ ions. Complete the diagram below using the following steps.

a) [glucose] low b) glucose down, Na+ down c) [glucose] high 1. down 2. against

2. Phospholipids vary in the small molecules attached to the phosphate group. The phospholipid shown in the figure has a ____________ attached to phosphate.

choline group

4. A phospholipid also has two "tails" made up of two _________________ molecules, which consist of a carboxyl group with a long hydrocarbon chain attached

fatty acid

3. Because the phosphate group and its attachments are either charged or polar, the phospholipid head is _______________, which means it has an affinity for water.

hydrophilic

5. Because the C-H bonds in the fatty acid tails are relatively nonpolar, the phospholipid tails are ____________________, which means they are excluded from water.

hydrophobic