B4 - Transport across cell membranes

Question 1

The movement of substance across cell membranes is affected by membrane structure. Describe how.

Answer 1

1. Phospholipid allows movement/ diffusion of non-polar/ lipid soluble substances
2. Phospholipid prevents movement/ diffusion of polar/ charged/ lipid insoluble substances OR proteins allow polar/charged substances to cross the membrane membrane/ bilayer
3. Carrier protein allow active transport
4. Channel/ carrier proteins allow facilitated diffusion/ co-transport
5. Shape/ charge of channel/ carrier determines which substances move
6. Number of channel/ carriers determine how much movement occurs
7. Membrane surface area determines how much diffusion/ movement
8. Cholesterol affects fluidity/ rigidity/ permeable

Question 2

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.

Answer 2

1. (Simple/facilitated) diffusion from high to low concentration/ down concentration
2. Small/ non-polar/ lipid-soluble molecules pass via phospholipids/ bilayer OR large/ polar/ water-soluble molecules go through proteins
3. Water moves by osmosis from high water potential to low water potential
4. Active transport is movement from low to high concentration against the concentration gradient and requires ATP)
5. Active transport/facilitated diffusion involve proteins/ carriers
6. Active transport requires energy (ATP)
7. Glucose/Na+ ions for co-trasport
8. Endocytosis/ Exocytosis

Question 3

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Answer 3

1. (Simple) diffusion of small/non-polar molecules down a concentration gradient
2. Facilitated diffusion down a concentration gradient via a protein (carrier/channel)
3. Osmosis of water down a water potential gradient
4. Active transport against a concentration gradient via protein carrier using ATP
5. Co-transport of 2 different substances using a carrier protein)

Question 4

Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane.

Answer 4

Osmosis:
1. From a high water potential to low water potential (down a water potential gradient)
2. Through aquaporin/ water channels

Facilitated diffusion:
3. Channel/ carrier protein
4. Down a concentration gradient

Active transport:
5. Carrier protein/ protein pumps
6. Against concentration gradient
7. using ATP/ energy

Phagocytosis:
8. Engulfing by cell surface membrane to form vesicle

Exocytosis:
9. Fusion of vesicle with cell surface membrane

Question 5

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein. Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not.

Answer 5

1. Chloride ions are water soluble/ charged/ polar
2. Cannot cross lipid bilayer
3. Chloride ions transported by facilitated diffusion (channel/carrier protein)
4. Oxygen is not charged/ polar
5. Oxygen is lipid soluble, so can diffuse across lipid bilayer