Topic 2.3 Movement across membranes

Question 1

How does membrane structure affect movement of substances?

Answer 1

• Phospholipid Bilayer allows diffusion of non polar substances
• PB prevents diffusion of polar substances
• Carrier proteins allow active transport
• Channel proteins allow FD/ co-transport
• Cholesterol affects fluidity

Question 2

Describe 5 ways substances can move across the membrane

Answer 2

• Simple diffusion of small/ non polar molecules
• Facilitated diffusion down a concentration gradient via protein carrier/channel
• Osmosis of water down a water potential gradient
• Active transport against a concentration gradient using ATP
• Co transport of 2 different substances using a carrier protein

Question 3

Describe how substances cross the cell membrane by osmosis

Answer 3

Down a WP gradient through water channels

Question 4

Describe how substances cross the cell membrane by facilitated diffusion

Answer 4

Down the concentration gradient through channel/ carrier proteins

Question 5

Describe how substances cross the cell membrane by active transport

Answer 5

Through carrier/channel proteins against concentration gradient using ATP

Question 6

Describe how substances cross the cell membrane by simple diffusion

Answer 6

Down the concentration gradient directly through the membrane

Question 7

Explain co- transport of glucose with sodium ions (5 marks)

Answer 7

• Na+ ions leave epithelial cell and enter blood
• by active transport via carrier protein using ATP
• so Na+ concentration in cell is lower than in lumen of ileum
•Na+ enter by FD
•Glucose is absorbed with Na+ against their concentration gradient