Ch 7: Membrane Structure and Function

Question 1

What is the plasma membrane and why is it selectively permeable?

Answer 1

The plasma membrane is the outer barrier that encloses a cell. Plasma membranes are selectively permeable because they regulate the materials that can pass through a cell, such as the passage of oxygen, nutrients, and wastes.

Question 2

What are the “staple” ingredients of membranes? What other molecules might you also find involved?

Answer 2

Lipids and proteins are the staple ingredients of membranes, but carbohydrates are also involved. Phospholipids are especially abundant in cell membranes, due to their hydrophobic and hydrophilic components allowing them to exist as stable boundaries for cells.

Question 3

Describe the “fluid mosaic model” of membranes.

Answer 3

The fluid mosaic model shows different proteins embedded in a fluid layer of phospholipids, cholesterol, and other components of the membrane.

Question 4

Explain what influence each of the following will have on the fluidity of the molecule.

• • Decreasing temperature
• • Saturated v. unsaturated fatty acid tails
• • Cholesterol

Answer 4

• • Decreasing temperature causes the membrane to solidify. As temperature decreases, the phospholipids of the membrane become more closely packed. In contrast, increasing temperature causes the membrane to become more fluid.
• • Saturated fatty acid tails have fewer double hydrogen bonds, thus having fewer kinks, so the hydrocarbon tails are able to pack more tightly. Unsaturated fatty acid tails have more kinks, so they cannot pack as tightly as saturated hydrocarbon tails, thus being more fluid at lower temperatures.
• • The steroid cholesterol is located between phospholipid molecules in the plasma membranes of animal cells, and has different effects on membrane fluidity at different temperatures. At high temperatures, cholesterol limits the movement of phospholipids to reduce membrane fluidity. At low temperatures, it disrupts the close packing of phospholipids to prevent them from solidifying, acting like a “fluid buffer.”

Question 5

Why is this fluidity important?

Answer 5

Fluidity is important because it affects its permeability and enzymatic processes. If a membrane is solidified, proteins at its surface will find it more difficult to move to different locations within the membrane to function, and become inactive. Additionally, a membrane that is too fluid is also not ideal.

Question 6

Describe some of the variation in cell membrane lipid composition seen in various organisms, such as bacteria, fish & wheat.

Answer 6

Some fish that live in extremely cold environments have higher amounts of unsaturated hydrocarbon tails, which enable their membranes to remain fluid enough to survive in low temperatures. Some bacteria that live in extremely hot temperatures have membranes that contain lipids that prevent them from becoming excessively fluid. Like some plants, such as winter wheat, certain bacteria can also adjust the amount of unsaturated phospholipids depending on the temperature of the environment.

Question 7

List and describe the 6 major functions of membrane proteins.

Answer 7

Transport

• — Can provide hydrophilic channel across membrane to transport selective solutes
• — Can change shape to transport substances across the membrane

Enzymatic activity
—- Can be a protein with its active site (where the reactant binds) exposed to substances in the adjacent solution

Signal transduction

• — A receptor protein can bind to a chemical messenger with a specific shape, and then changes its shape according to the chemical messenger
• — Then relays the message to inside the cell, usually by binding to the cytoplasm

Cell-cell recognition
—- Identification tags specifically recognized by membrane protein of other cells

Intercellular joining
—- Membrane proteins of adjacent cells can hook/bind together

Attachment to the cytoskeleton and ECM
—- Cytoskeleton can be noncovalently bound to membrane proteins that can help maintain cell shape and stabilize location of certain proteins

Question 8

How are scientists using their knowledge of CD4 and CCR5 cell surface proteins to develop drug treatments for HIV?

Answer 8

HIV can infect cells with CCR5 on its surfaces, but cannot bind to and infect a cell lacking CCR5 on its surface. Because CD4 has so many functions in cells, it is risky to mess around with it, so scientists are aiming to develop drugs involving CCR5 instead.

Question 9

What is cell-cell recognition and how are membrane carbohydrates involved?

Answer 9

Cell-cell recognition is a cell’s ability to distinguish one type of cell from another. Cells can recognize others by binding to molecules, usually containing carbohydrates, on the extracellular surface of the plasma membrane. Some membrane carbohydrates are covalently bonded to lipids and form glycolipids. Some are covalently bonded to proteins, and are called glycoproteins.

Question 10

Carbohydrates are attached to plasma membrane proteins in the ER (see Fig.7.9). On which side of the vesicle membrane are the carbohydrates during transport to the cell surface?

Answer 10

The carbohydrates are on the extracellular side.

Question 11

Selective permeability means that some molecules will pass through the membrane while others won’t. Molecules that can pass through will also do so at different rates.
Describe what molecules will pass EASILY through the membrane. Which molecules will pass through the membrane with difficulty?

Answer 11

Hydrophobic molecules, or nonpolar molecules like hydrocarbons, CO2 and O2, can dissolve in the lipid bilayer and easily cross through the membrane. Polar molecules like water and glucose pass through the membrane with difficulty because their passage is impeded by the hydrophobic interior of the membrane.

Question 12

What are transport proteins? Compare channel proteins with carrier proteins.

Answer 12

Transport proteins are proteins on the surface of plasma membranes that transport specific ions and polar molecules through the membrane while avoiding contact with the lipid bilayer. Channel proteins are proteins that can provide a hydrophilic tunnel that certain molecules can travel through. Carrier proteins hold onto their passengers and shuttles them through the plasma membrane.

Question 13

What is an aquaporin? Why are they necessary for the transport of water across a membrane (think about water’s chemical bonding and chemical properties)?

Answer 13

Aquaporin is a transport channel protein that helps with the passage of water molecules. Most aquaporin proteins contain 4 identical polypeptide subunits, in which each form a channel water molecules use to pass through.

Question 14

Define diffusion. How is dynamic equilibrium related to diffusion?

Answer 14

Diffusion is the movement of particles so that they randomly spread out into the available space. Dynamic equilibrium is reached when particles are diffused to equal concentrations.

Question 15

Why is diffusion considered “passive” transport? What is the energy that fuels diffusion?

Answer 15

Diffusion is considered passive transport because it requires no energy for a substance to diffuse across a membrane. The concentration gradient, a region along which the density of a chemical substance increases or decreases, represents potential energy and is what drives diffusion. Passive transport also moves down the concentration gradient, another explanation for why it requires no energy.

Question 16

Study figure 7.11 (A U tube) Explain what the water does and why it does it. Why doesn’t the sugar pass through the membrane?

Answer 16

The sugar molecules are too large to pass through the membrane, but water molecules are able to. The water molecules can diffuse across the membrane in random directions, but overall, the water diffuses from the side with a higher concentration of sugar to the side with lower concentration, thus achieving equal concentration. The sides have a similar concentration of sugar, but the side with more solute molecules have fewer free water molecules to offset the initially higher concentration.

Question 17

Isotonic

Answer 17

An isotonic solution contains the same concentration of solutes as the inside of the cell. If a cell is placed in an isotonic solution, there will be no net movement of water across the membrane. Water diffuses across the membrane at the same rate in both directions.

Question 18

Hypertonic

Answer 18

A hypertonic solution has more non penetrating solutes than the inside of the cell. If a cell is placed in a hypertonic solution, the water will diffuse out of the cell, and the cell will lose water and shrivel.

Question 19

Hypotonic

Answer 19

A hypotonic solution has less non penetrating solutes than inside of the cell. If a cell is placed in a hypotonic solution, water will diffuse into the cell faster than it leaves, and the cell will swell and burst.

Question 20

Why is it necessary to distinguish between cells without and with cell walls? What problems does a cell without cell walls face?

Answer 20

It is necessary to distinguish between cells with and without cell walls because cells without cell walls can’t tolerate excessive uptake or loss of water. However, this does not become a problem when the cell lives in an isotonic environment. These organisms must have other adaptations for osmoregulation, which is the control of solute concentration and water balance.

Question 21

How does a single-celled organism without a cell wall, such as a paramecium, survive?

Answer 21

Paramecium lives in pond water, which is hypotonic to the cell. However, paramecium cells do not burst because its contractile vacuole pumps water out of the cell as it enters by osmosis.

Question 22

Plant cells have cell walls. Describe the conditions that would cause a plant cell to be turgid, flaccid or plasmolyzed.

Answer 22

Plant cells swell as water enters, and the cell wall will expand enough until it exerts turgor pressure back onto the cell, which prevents any more water intake. The cell is turgid (very firm) when surrounded by a hypotonic solution, which helps hold up the plant. The plant cell is flaccid when its surroundings are isotonic, there isn’t water and the plant wilts. In a hypertonic solution, the plant cell loses water and also wilts.

Question 23

What is facilitated diffusion?

Answer 23

Facilitated diffusion is when certain ions and molecules diffuse across the cell membrane with the assistance of transport proteins. This is also considered a passive transport because it moves down the concentration and requires no energy.

Question 24

How does active transport compare to passive transport? Why might active transport be necessary for a cell?

Answer 24

Active transport is when a cell moves a solute against its concentration gradient, which requires work, and thus energy. Carrier proteins carry solutes against their concentration gradients rather than channel proteins because they pick them up rather than letting them pass through. Active transport enables a cell to maintain internal concentrations of small solutes that differ from their environment. For example, an animal cell has a high concentration of potassium ions (K+) and lower sodium ions (Na+). The plasma membrane can help maintain these by pumping Na+ out and K+ in.

Question 25

The sodium-potassium pump is an example of active transport. Study figure 7.15 and explain how the ATP functions to operate the membrane proteins and what ions are moved against their gradients. When the pump is on, what happens to the balance of charges across the membrane?

Answer 25

ATP hydrolysis provides energy for active transport. When its terminal phosphate group is transferred directly to the transport protein, and can cause the protein to change its shape that relays a solute bound to the transport protein across the membrane. This works with the sodium-potassium pump, which exchanges Na+ for K+ across the membrane.

Question 26

What is voltage? How is it related to membrane potential?

Answer 26

Voltage is electrical potential energy, or a separation of opposite charges. The cytoplasmic side of a membrane is negative relative to the extracellular side because of an unequal distribution of anions and cations. The membrane potential is the voltage across the membrane, and ranges from -50 to -200 mV.The membrane potential favors the passive transport of cations into the cell and anions out because the inside is negative. So, technically two forces drive diffusion: the concentration gradient and electrical force→ electrochemical gradient.

Question 27

What is a proton pump? Explain why they are called electrogenic.

Answer 27

An electrogenic pump is a transport protein that generates voltage across a membrane. A proton pump is the main electrogenic pump that transports protons (hydrogen ions, H+) out of the cell. Electrogenic pumps help store energy by generating voltage across membranes.

Question 28

Cotransport is when an electrogenic pump, which is actively transporting a solute across a membrane, also indirectly transports other molecules across the membrane. Describe the cotransport of sucrose in plant cells.

Answer 28

A carrier protein, like a H+/sucrose cotransporter is able to use the diffusion of H+ down its electrochemical gradient into the cell to drive the uptake of sucrose. An H+ gradient is concentrated through an ATP-driven proton pump, and represents potential energy that can be used for active transport. So, ATP hydrolysis indirectly provides energy for cotransport.

Question 29

Exocytosis

Answer 29

Exocytosis is the process by which the cell secretes certain molecules when fusing vesicles with the plasma membrane. A transport vesicle from the Golgi apparatus moves along a microtubule of the cytoskeleton to the plasma membrane. When they come into contact, proteins rearrange the lipid molecules of the bilayers to fuse the membranes together. The contents spill out of the vesicle, and the vesicle becomes part of the membrane.

Question 30

Endocytosis

Answer 30

Endocytosis is when the cell takes in molecules by forming new vesicles from the plasma membrane. A small area of the plasma membrane forms a pocket, sinks and pinches in, and forms a vesicle containing the material outside the cell.

Question 31

Both endocytosis and exocytosis might be associated with what organelles? Which process would result in the expansion of the plasma membrane? Which one will cause the membrane to become smaller? Explain.

Answer 31

Both are associated with the Golgi apparatus. Exocytosis would expand the plasma membrane because it fuses the membrane with a transport vesicle. Endocytosis would cause the membrane to become smaller because it uses the plasma membrane to form vesicles.

Question 32

Explain the role of endocytosis in the inherited disease familial hypercholesterolemia.

Answer 32

Cholesterol travels in the blood in low-density lipoproteins (LDLs), which bind to LDL receptors on plasma membranes and enter by endocytosis. In hypercholesterolemia, LDLs cannot enter cells due to missing or defective LDL receptor proteins, which cause accumulation of cholesterol in the blood. This contributes to early atherosclerosis, which is the buildup of lipids within blood vessel walls, narrowing space in vessels and impeding blood flow and potentially causing heart damage and stroke.