Unit 2 - Cell membranes, Transport

Question 1

Describe what is meant by cell membranes.

Answer 1

• Membranes are described as a fluid mosaic model
  & contain components like
  =Phospholipids
  =Glycoproteins & Glycolipids
  =Proteins
  =Cholestrol
• All cells and organelle membranes have the same structure

Question 2

Describe the main component of cell membranes

Answer 2

Phospholipids aligned as a bilayer allows for movement
=Hydrophillic heads attracted to water
=Hydrophobic tails repelled by water

Question 3

Describe the role of cholesterol in the cell membrane

Answer 3

• Restrict lateral movement of other molecules in the membrane
• Therefore membrane less fluid at high temperatures

Question 4

Describe the 2 types of protein in the cell membrane & briefly outline function.

Answer 4

EXTRINSIC PROTEIN
- Provide mechanical support
- OR Attached to proteins or lipids to form glycolipids & glycoproteins
- Involved in cell signaling, as receptors and help cells attach to one another to
form tissues
INTRINSIC
- Either Channel or carrier protein
- Involved in the transport of molecules across the membrane

Question 5

Describe the role of channel and carrier proteins

- mainly in terms of diffusion.

Answer 5

• Channel proteins form
• aqueous hydrophilic channels that allow
• specific (small, polar & charged particles) water-soluble molecules to diffuse through the membrane
  (MAINLY WATER)
• Carrier proteins transport specific molecules across the membrane
• by changing shape when the molecules bind to the protein
• releasing the molecule at the other side of the membrane

Question 6

Define diffusion

Answer 6

• Diffusion is a passive process
• That involves the overall net movement of small molecules from a region of higher concentration to a lower concentration
• Down the concentration gradient, until equilibrium

Question 7

Describe what type of molecules can simply diffuse across the bilayer?

Answer 7

• Lipid soluble, Non-polar & Very small

e. g CO2 & H20

Question 8

Define Facilitated Diffusion

Answer 8

• Facilitated diffusion is a passive process
• That involves the overall net movement of molecules/ions from a region of higher concentration to a lower concentration
• Down the concentration gradient across a partially permeable membrane
• Via channel or carrier proteins

Question 9

Describe what type of molecules can be transported with facilitated diffusion?
(CARRIER PROTEINS)

Answer 9

• Charged ions, Polar & Large molecules

e. g glucose & urea

Question 10

Describe the 4 factors increasing the rate of diffusion

Answer 10

+Higher Temperature - increased kinetic energy = molecules move faster = faster diffusion
+Larger Surface area - faster rate of absorption
+Higher difference in Concentration Gradient
+Short Diffusion distance

Question 11

Define Osmosis
water = solvent
substances dissolved in water = solute

Answer 11

• Osmosis is a passive process
• That involves the overall net movement of water molecules
• From an area of high water potential to an area of low water potential down water potential gradient
• through a partially/selectively permeable membrane until equilibrium

Question 12

Define Water Potential

PURE WATER = 0Ѱ

Answer 12

• the more dilute a solution, the higher (less negative) the water potential (Ѱ)

Question 13

Describe 3 factors that affect the rate of osmosis

Answer 13

• The larger the surface area, the faster the rate of osmosis
• The thinner the exchange surface, the faster the rate of osmosis
• The higher the water potential gradient, the faster the rate of osmosis

Question 14

Define Active Transport

Answer 14

• Active transport is an active process requiring ATP from respiration
• Which involves the movement of molecules/ions from an area of low concentration to a high concentration against the concentration gradient
• Across a cell membrane, using carrier proteins

Question 15

Describe the process of active transport across a cell membrane.

Answer 15

• Complementary molecule to be transported binds to the specific site of carrier protein
• ATP on the inside of the membrane directly bonds to a carrier protein and hydrolysed to form ADP and Pi(phosphate)
• Causing carrier protein to change shape and move molecule across membrane & release
• Pi ion is released causing carrier protein to go back to its original shape

Question 16

Define Co-transport

Answer 16

• Co-transport is the transport of one substance coupled with the transport of another substance across a semi permeable membrane
• Using a carrier protein
• Both molecules move in same direction against concentration gradient

Question 17

Explain how Co- transport is involved in the absorption of glucose in the small intestine

ROUGH DIAGRAM:

LUMEN OF ILEUM
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
EPITHELIAL CELL
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
BLOOD

Answer 17

• Na+ ions are actively transported out of epithelial cells into bloodstream by carrier protein using hydrolysis of ATP
• So lower conc of Na+ ions in epithelial cell than lumen Generateing a concentration gradient for Na+ ions from ileum into epithelial cell
• Na+ ions move in by facilitated diffusion in co transporter bringing glucose with it against its concentration gradient

Question 18

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

Answer 18

• A large number of protein channels in membrane
  for facilitated diffusion
• Large number of protein carriers in membrane for active transport
• Folded membrane so larger surface area for absorption