Chapter 7 - Membrane Structure and Function

Question 1

What is selective permeability?

Answer 1

The plasma membrane allows some substances to cross it more easily than others. This is the ability of the cell to discriminate in its chemical exchanges with its environment, which is fundamental to life.

Question 2

What are aquaporins?

Answer 2

Helical regions of a membrane protein. One molecule of this protein enables billions of water molecules to pass through the membrane every second.

Question 3

What is an amphipathic molecule?

Answer 3

A molecule that has both a hydrophilic region and a hydrophobic region. Most membrane proteins are amphipathic and can reside in the phospholipid bilayer with their hydrophilic regions protruding.

Question 4

What are the most abundant lipids of membranes?

Answer 4

Lipids and proteins are the staple ingredients of membranes and the most abundant lipids in most membranes are phospholipids.

Question 5

What is the stable boundary between two aqueous compartments?

Answer 5

A phospholipid bilayer.

Question 6

What is the fluid mosaic model?

Answer 6

States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.

Question 7

Are proteins randomly distributed in the membrane?

Answer 7

No. Groups of proteins are often associated in long lasting, specialized patches, where they carry out common functions.

*The lipids appear to form defined regions as well.

Question 8

Discuss the fluidity of membranes.

Answer 8

• Phospholipids in the plasma membrane can move within the bilayer
• Most of the lipids, and some proteins, drift laterally
• Rarely, a lipid may flip-flop transversely across the membrane
• As temperatures cool, membranes switch from a fluid state to a solid state
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
• Membranes must be fluid to work properly. Most membranes are about as fluid as salad oil

Question 9

How does the steroid cholesterol effect membrane fluidity at different temperatures?

Answer 9

• At warm temperatures, cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing

Question 10

What are membrane proteins and their functions?

Answer 10

A membrane is a collage of different proteins grouped together and embedded in the matrix of the lipid bilayer. Different membrane proteins determine most of the membranes specific functions.

Question 11

What are integral proteins?

Answer 11

Penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane.

The hydrophobic regions of an integral protein often consist of one or more stretches of non polar amino acids, often coiled into alpha helices.

Question 12

What are peripheral proteins?

Answer 12

Protein “appendages” that are loosely bound to the surface of the membrane.

Question 13

What are the six major functions of membrane proteins?

Answer 13

1. Transport - a protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.
2. Enzymatic activity
3. Signal transduction - a membrane receptor protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger may cause the protein to change shape, allowing it to relay the message to the inside of the cell.
4. Cell-cell recognition - some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. A type of cell-cell binding that is short lived.
5. Intercellular joining - membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. This type of cell-cell binding is more long lasting.
6. Attachment to the cytoskeleton and extracellular matrix (ECM) - microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes.

Question 14

What is cell-cell recognition?

Answer 14

A cell’s ability to distinguish one type of neighboring cell from another. Crucial to the functioning of an organism.

Question 15

What are glycolipids?

Answer 15

Carbohydrates that are covalently bonded to lipids.

Question 16

What are glycoproteins?

Answer 16

Carbohydrates that are covalently bonded to proteins.

Question 17

Discuss the permeability of the lipid bilayer.

Answer 17

• Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
• Hydrophilic molecules including ions and polar molecules do not cross the membrane easily

Question 18

What are transport proteins?

Answer 18

Proteins that allow for the passage of hydrophilic substances across the membrane. Channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane.

*A transport protein is specific for the substance it moves.

Question 19

What channel proteins allow for the passage of water molecules through the membrane certain cells?

Answer 19

Aquaporins - each aquaporin allows entry of up to 3 billion water molecules per second, passing single file through its central channel, which fits ten at a time.

Question 20

What are carrier proteins?

Answer 20

Transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane.

Question 21

What is passive transport?

Answer 21

A diffusion of a substance across a membrane with no energy investment.

Question 22

What is diffusion?

Answer 22

The movement of particles of any substance so that they spread out into the available space.

• A simple rule of diffusion; in the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated.
• Diffusion is a spontaneous process*

Question 23

When is dynamic equilibrium reached?

Answer 23

When as many molecules cross the membrane in one direction as in the other.

Question 24

What is the concentration gradient?

Answer 24

Substances diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decreases.

Question 25

What is osmosis?

Answer 25

The diffusion of free water across a selectively permeable membrane, whether artificial or cellular. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides.

Question 26

What is tonicity?

Answer 26

The ability of a surrounding solution to cause a cell to gain or lose water.

Question 27

What is an isotonic solution?

Answer 27

The solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.

Question 28

What is a hypertonic solution?

Answer 28

The solute concentration is greater than that inside the cell and the cell loses water.

*hyper means “more”

Question 29

What is a hypotonic solution?

Answer 29

The solute concentration is less than that inside the cell and the cell gains water.

Question 30

What is osmoregulation?

Answer 30

The control of solute concentrations and water balance. A necessary adaptation for life.

Question 31

What is turgor pressure?

Answer 31

As water enters a plant cell, the plant cell swells. However, the inelastic cell wall will expand only so much before it exerts a back pressure on the cell, called turgor pressure. Turgor pressure opposes further water uptake.

The cell is now turgid (very firm), which is the healthy state for most plant cells.

Question 32

When does a place cell become flaccid?

Answer 32

If a plant’s cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp).

Question 33

When do plant cells lose water?

Answer 33

In a hypertonic environment.

Question 34

What is the effect called plasmolysis?

Answer 34

When the plant cell loses water and begins to shrivel, it’s plasma membrane pulls away from the cell wall at multiple places, which causes the plant to wilt and can lead to plant death.

Question 35

What is facilitated diffusion?

Answer 35

Transport proteins speed the passive movement of molecules across the plasma membrane.

Question 36

What are ion channels?

Answer 36

Channel proteins that transport ions and facilitate the diffusion of ions.

Question 37

What are gated channels?

Answer 37

Channels that open or close in response to a stimulus. For some gated channels, the stimulus is electrical.

Question 38

What is active transport?

Answer 38

To pump a solute across a membrane against its gradient requires work. Active transport uses energy to move solutes against their gradients.

• The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins.
• ATP powers active transport.
• One way ATP can power active transport is by transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane.

Question 39

What is the sodium-potassium pump?

Answer 39

A transport system that exchanges NA+ for K+ across the plasma membrane of animal cells.

This transport system pumps ions against steep concentration gradients; sodium ion concentration is high outside the cell and low inside, while potassium ion concentration is low outside the cell and high inside.The pump oscillates between two shapes in a cycle.

Question 40

What is membrane potential?

Answer 40

The voltage difference across a membrane. Voltage is created by differences in the distribution of positive and negative ions across a membrane.

*The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides.

Question 41

What is the electrochemical gradient?

Answer 41

Two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the ion’s movement).

Question 42

What is an electrogenic pump?

Answer 42

A transport protein that generates voltage across a membrane.

Electrogenic pumps help store energy that can be used for cellular work.

Question 43

What is the major electrogenic pump of animal cells?

Answer 43

The sodium-potassium pump

Question 44

What is the main electrogenic pump of plants, fungi, and bacteria?

Answer 44

The proton pump; which actively transports protons (hydrogen ions, H+) out of the cell

Question 45

What is cotransport?

Answer 45

Occurs when active transport of a solute indirectly drives transport of other substances.

A transport protein (a cotransporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient.

Question 46

Discuss bulk transport across the plasma membrane.

Answer 46

• Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
• Bulk transport requires energy

Question 47

What is exocytosis?

Answer 47

When the cell secretes certain molecules by the fusion of vesicles with the plasma membrane in order to export their products.

Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

Question 48

What is endocytosis?

Answer 48

The cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane.

The reverse of exocytosis.

Question 49

What are the three types of endocytosis?

Answer 49

1. Phagocytosis - cellular eating; a cell engulfs a particle in a vacuole and the vacuole fuses with a lysosome to digest the particle
2. Pinocytosis - cellular drinking; molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
3. Receptor-mediated endocytosis - binding of ligands to receptors triggers vesicle formation; enables the cell to acquire bulk quantities of specific substances. Embedded in the plasma membrane are proteins with receptor sites exposed to the extracellular fluid. Specific solutes bind to the sites. The receptor proteins then cluster in coated pits, and each coated pit forms a vesicle containing the bound molecules. After the ingested material is liberated from the vesicle, the emptied receptors are recycle to the plasma membrane by the same vesicle. (i.e. human cells taking in cholesterol for membrane synthesis and the synthesis of other steroids)

Question 50

What is a ligand?

Answer 50

Any molecule that binds specifically to a receptor site of another molecule.