Chapter 7 - Membrane Structure and Function

Question 1

Selective Permeability

Answer 1

A property of biological membranes that allows them to regulate the passage of substances across them.

Question 2

Amphipathic Molecule

Answer 2

• A molecule with a hydrophilic region and a hydrophobic region
• Example is phospholipids
  
  • Most abundant lipid in the plasma membrane

Question 3

Fluid Mosaic Model

Answer 3

• The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.
• In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

Question 4

Freeze-Fracture

Answer 4

• A specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

Question 5

How do phospholipids in the plasma membrane move within the bilayer?

Answer 5

• Most of the lipids, and some proteins, drift laterally
• Rarely does a molecule flip-flop transversely across the membrane

Question 6

Do membrane proteins move?

Answer 6

• Yes

Question 7

Membrane Fluidity

Answer 7

• As temperatures cool, membranes switch from a fluid state to a solid state
• The temperature at which a membrane solidifies depends on the types of lipids
  
  • Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
  • The steroid cholesterol has different effects on membrane fluidity at different temperatures
    
    • At warm temperatures (such as 37ºC) it restrains movement of phospholipids
    • At cool temperatures, it maintains flyidity by preventing tight packing
• Membranes must be fluid to work properly
  
  • They are usually about as fluid as salad oil
• Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
  
  • Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

Question 8

Membrane Proteins

Answer 8

• A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer.
• Proteins determine most of the membrane’s specific functions
• Types of membrane proteins
  
  • Peripheral proteins
  • Integral proteins

Question 9

Peripheral Protein

Answer 9

A protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer

Question 10

Integral Protein

Answer 10

• A transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both sides of the membrane (or lining the channel in the case of a channel protein)
• The hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

Question 11

The six major functions of membrane proteins:

Answer 11

Question 12

The Role of Membrane Carbohydrates in Cell-Cell Recognition

Answer 12

• Cells recognize eachother by binding to surface molecules, often containing carbohydrates, on the extracellular surface of th eplasma membrane
• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly proteins (forming glycoproteins)
• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Question 13

Glycolipid

Answer 13

A lipid with one or more covalently attached carbohydrates

Question 14

Glycoprotein

Answer 14

A protein with one or more covalently attached carbohydrates

Question 15

Why do membranes have distinct inside and outside faces?

Answer 15

• The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

Question 16

The Permeability of the Lipid Bilayer

Answer 16

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

Question 17

Transport Protein

Answer 17

• A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane
  
  • A transport protein is specific for the substance it moves
• Allow passage of hydrophilic substances across the membrane
• Types of transport proteins
  
  • channel proteins
  • Carrier proteins

Question 18

Channel Proteins

Answer 18

• A type of transport protein
• Have a hydrophilic channel that certain molecules or ions can use as a tunnel to get through the membrane
  
  • aquaporins
  • Ion channels (gated channels)

Question 19

Aquaporin

Answer 19

A channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane.

Question 20

Ion Channel (Gated Channel)

Answer 20

A transmembrane protein channel that allows a specific ion to diffuse across the membrane down to its concentration or electrochemical gradient

Question 21

Carrier Protein

Answer 21

• Proteins that undergo a subtle change in shape that translocates the solute-binding site across the membrane

Question 22

Passive Transport

Answer 22

• The diffusion of a substance across a biological membrane with no expenditure of energy.
• Some diseases are caused by malfunctions in specific transport systems
  
  • Kidney disease “cystinuria”

Question 23

Diffusion

Answer 23

• The spontaneous movement of a substance down to its concentration or electrochemical gradient, from a region where it is more concentrated to a region where it is less concentrated
• (Simplified) the tendency for molecules to spread out evenly into the available space
• Although each molecule moves randomly, diffusion of a population of molecules may be directional
• At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
• Substances diffuse down their concentration gradient

Question 24

Concentration Gradient

Answer 24

• A region along which the density of a chemical substance increases or decreases.
• No work must be done to move substances down the concentration gradient

Question 25

Osmosis

Answer 25

• The diffusion of free water across a selectively permeable membrane
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concenration is equal on both sides

Question 26

Tonicity

Answer 26

• The ability of a solution surrounding a cell to cause that cell to gain or lose water.
• Hypertonic or hypotonic environments create osmotic problems for organisms

Question 27

Isotonic Solution

Answer 27

• A solution that, when surrounding a cell, causes no net movement of water into or out of the cell.
• Solute concentration is the same as that inside the cell

Question 28

Hypertonic Solution

Answer 28

• A solution that, when surrounding a cell, will cause the cell to loose water
• Solute concentration is greater than that inside the cell
• Hypertonic environments create osmotic problems for organisms

Question 29

Hypotonic Solution

Answer 29

• A solution that, when surrounding a cell, will cause the cell to take up water
• Solute concentration is less than that inside the cell
• Hyportonic environments create osmotic problems for organisms

Question 30

Osmoregulation

Answer 30

• Regulation of solute concentrations and water balance by a cell or organism
• Necessary adaptation for life in hypertonic or hypotonic environments
  
  • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Question 31

Turgid

Answer 31

• Swollen or distended, as in plant cells
  
  • A walled cell becomes turgid if it has a lower water potential than its surroundings, resulting in entry of water
• A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

Question 32

Flaccid

Answer 32

• Lacking turgor (stiffness or firmness), as in a plant cell in surroundings where there is a tendency for water to leave the cell
  
  • A walled cell becomes flaccid if it has a higher water potential than its surroundings, resulting in the loss of water
• If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

Question 33

Plasmolysis

Answer 33

• A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall
• occurs when the cell looses water to a hypertonic environment

Question 34

Facilitated Diffusion

Answer 34

• The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure.
• Speeds the passive movement of molecules across the plasma membrane
• Passive because the solute moves down its concentration gradient.

Question 35

Active Transport

Answer 35

• The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy.
• Usually uses energy in the form of ATP
• Performed by specific proteins embedded in the membranes
• Allows cells to maintain concentration gradients that differ from their surroundings
• The sodium-potassium pump is one type of active transport system

Question 36

Sodium-Potassium Pump

Answer 36

• A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.
• The sodium-potassium pump is the major electrogenic pump of animal cells

Question 37

Membrane Potential

Answer 37

• The difference in electrical charge across a cell’s plasma membrane due to the differential distribution of positive and negative ions.
• Affects the activity of excitable cells and the transmembrane movement of all charged substances.

Question 38

Electrochemical Gradient

Answer 38

• The diffusion gradient of an ion
• affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion’s tendency to move relative to the membrane potential (an electrical force)

Question 39

Electrogenic Pump

Answer 39

• An active transport protein that generates voltage across a membrane while pumping ions
• Help store energy that can be used for cellular work
• The sodium-potassium pump is the major electrogenic pump of animal cells
• The proton pump is the main electrogenic pump of plants, fungi and bacteria

Question 40

Proton Pump

Answer 40

An active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process

Question 41

Cotransport

Answer 41

• The coupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient
• Occurs when active transport of a solute indirectly drives transport of other solutes
• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Question 42

Exocytosis

Answer 42

• The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.
• Bulk transport requires energy
• Many secretory cells use exocytosis to export their products

Question 43

Endocytosis

Answer 43

• Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane
• A reversal of exocytosis, involving different proteins
• Bulk transport requires energy
• Three types
  
  • Phagocytosis (cellular eating)
  • Pinocytosis (Cellular drinking)
  • Receptor-mediated endocytosis

Question 44

Phagocytosis

Answer 44

• A type of endocytosis in which large particulate substances or small organisms are taken up by a cell.
• It is carried out by some protists and certain immune cell or animals
  
  • In mammals, mainly macrophages, neutrophils, and dendritic cells
• Engulfs the particle in a vacuole which fuses with a lysosome which digests the particle
• “Cellular eating”

Question 45

Pinocytosis

Answer 45

• A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes
• Molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
• “Cellular drinking”

Question 46

Receptor-Mediated Endocytosis

Answer 46

• The movement of specific molecules into a cell by the inward budding of vesicles containing proteins with receptor sites specific to the molecules being taken in (ligands)
• Enables a cell to acquire bulk quantities of specific substances

Question 47

Ligand

Answer 47

A molecule that binds specifically to another molecule, usually a larger one.