3.2.3 Transport Across Cell Membranes

Question 1

What molecules are in the cell membrane

Answer 1

• proteins
• glycoproteins
• glycolipids
• cholesterol

Question 2

Describe the phospholipid bilayer

Answer 2

• hydrophilic heads and outside of bilayer - form intermolecular forces with water
• hydrophobic fatty acid tails inside bilayer - form intermolecular forces with each other
• form selective permeable barrier - only non-polar molecules can diffuse through fatty acid core
• phospholipids move - rotate and laterally through membrane

Question 3

Role of proteins in cell membranes

Answer 3

• channel and carrier (transmembrane) for movement of substances across membrane.
• other transmembrane and peripheral proteins - many functions, including receptors, signalling molecules, increasing membrane stability.

Question 4

Role of glycoproteins in cell membranes

Answer 4

• protein with short carbohydrate chain
• cell recognition and signalling; cell attachment

Question 5

Role of glycolipids in cell membranes

Answer 5

• phospholipid with short carbohydrate chain
• cell recognition and attachment

Question 6

Role of cholesterol in cell membranes

Answer 6

• reduces membrane fluidity by restricting the movement of other molecules

Question 7

What are the different ways that molecules move across membranes

Answer 7

• simple diffusion
• facilitated diffusion
• osmosis
• active transport
• co-transport

Question 8

Describe simple diffusion

Answer 8

• passive movement from higher to lower concentration
• across phospholipid bilayer
• non-polar molecules only

Question 9

Facilitated diffusion

Answer 9

• passive movement from higher to lower concentration
• requires channel or carrier proteins - specific to substrate
• polar molecules and ions

Question 10

Osmosis

Answer 10

Diffusion of water molecules from higher to lower water potential
. Limited across phospholipid bilayer - rapid movement requires channel proteins
. Water potential = ability of water molecules to move freely (no IMF attracting to solute)
. Pure water = 0kPa. All solutions have -tive water potentials. More solutes (more conc) = lower water potential.
. In plants: water potential = solute potential + hydrostatic pressure

Question 11

Active transport

Answer 11

• movement from lower conc to higher conc
• requires specific carrier proteins in membrane
• requires hydrolysis of ATP - phosphorylates carrier protein, causing shape change.

Question 12

Co-transport

Answer 12

• movement of 2 substances simultaneously through a carrier protein
• one substances moves against conc gradient (active transport)
• one substance moves down conc gradient (facilitated diffusion)

Question 13

Adaptions to increase the rate of transport across membranes

Answer 13

• increased number of channel &b carrier proteins
• increased S.A of membrane
• mechanisms to increase conc gradients
• increased number of mitochondria to supply more ATP for active transport