Transport Across Cell Membranes

Question 1

Describe the fluid mosaic model of membranes

Answer 1

Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has a flexible shape

Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol and glycolipids in membranes

Answer 2

Cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable

Glycolipids: cell signalling and cell recognition

Question 3

Explain the functions of extrinsic and intrinsic proteins in membranes

Answer 3

Extrinsic:

Binding sites
Antigens
Bind cells together
Involved in cell signalling

Intrinsic:

Electron carries
Channel proteins
Carrier proteins

Question 4

Explain the functions of membranes within cells

Answer 4

Provide internal transport system

Selectively permeable to regulate passage of molecules into and out of organelle

Provide reaction surface

Isolate organelles from cytoplasm for specific metabolic reactions

Question 5

Explain the functions of the cell surface membrane

Answer 5

Isolates cytoplasm from extracellular environment

Selectively permeable to regulate transport of substances

Involved in cell signalling/ recognition

Question 6

Name 3 factors that affect membrane permeability

Answer 6

Temperature: high temperature denatures membrane proteins / phospholipid molecules have more kinetic energy and move further apart

pH: changes tertiary structure of membrane proteins

Use of solvent: may dissolve membrane

Question 7

Define osmosis

Answer 7

Water diffuses across a semi permeable membrane from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is reached

Question 8

What is water potential?

Answer 8

Pressure created by water molecules measured in kPa

More solute = water potential is more negative

Question 9

How does osmosis affect plant and animal cells?

Answer 9

Osmosis into cell:
Plant - protoplast swells = cell turgid
Animal - lysis

Osmosis out of cell:
Plant - protoplast shrinks = cell flaccid
Animal - crenation

Question 10

Define simple diffusion

Answer 10

Passive process requires no energy from ATP hydrolysis

Net movement of small, lipid soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration

Question 11

Define facilitated diffusion

Answer 11

Passive process

Specific channel or carrier protein with complementary binding sites transport large and polar molecules down the concentration gradient

Question 12

Explain how channel and carrier proteins work

Answer 12

Channel: hydrophilic channels bind to specific ions = one side of the protein closes and the other opens

Carrier: binds to complementary molecule = conformational change releases the molecule on the other side of the membrane

Question 13

Name 5 factors that affect the rate of diffusion

Answer 13

Temperature
Diffusion distance 
Surface area
Size of molecule
Difference in concentration

Question 14

State Fick’s law

Answer 14

surface area x difference in concentration / diffusion distance

Question 15

How are cells adapted to maximise the rate of transport across their membranes?

Answer 15

Many carrier / channel proteins

Folded membrane increases surface area

Question 16

Define active transport

Answer 16

Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape

Specific carrier protein transports molecules from areas of low concentration to areas of high concentration

Question 17

Compare and contrast active transport and facilitated diffusion

Answer 17

Both may involve carrier proteins

Active transport requires energy from ATP hydrolysis; facilitated diffusion is a passive process

Facilitated diffusion may also involve channel proteins

Question 18

Define co-transport

Answer 18

Movement of substances against the concentration gradient, coupled up with the movement of another substance down its concentration gradient

Question 19

Explain how co transport is involved in the absorption of glucose / amino acids in the small intestine

Answer 19

1. Na+ actively transported out of the epithelial cells and into the bloodstream
2. Na+ concentration lower in the epithelial cells than lumen of the gut
3. Transport of glucose/amino acids from lumen to epithelial cells is coupled to facilitate diffusion of Na+ down electrochemical gradient