2.3/2.4 Osmosis & Diffusion

Question 1

What happens when a cell is in a solution that has less water molecules than its cytoplasm?

Answer 1

• In animal cells, the entire cell shrivels
• In plant cells, the vacuole and cytoplasm shrink but the cell wall maintains overall shape of the cell

Question 2

What is osmosis?

Answer 2

• The net movement of water from a region of lower concentration to a region of higher concentration through a partially permeable membrane
• Water molecules move down the conc. gradient

Question 3

What is diffusion?

Answer 3

• It is PASSIVE transport
• Net movement of substance from a region of high concentration to a region of low concentration
• It occurs until both sides of the membrane are in equillibrium

Question 4

Explain simple diffusion?

Answer 4

• Molecules are small - so they can fit between phospholipids
• Molecules are non polar - so they can interact with non polar phospholipid tails

Question 5

What factors affect the rate of diffusion?

Answer 5

1. Concentration gradient - The larger the difference in conc. on both sides of the surface, the faster molecules move
2. Temperature - molecules/ions have more KE so move faster
3. Surface area - by increasing SA with eg. folding, the smaller the diffusion distance, and more spaces to travel across
4. Ion size - smaller, non-polar molecules diffuse more quickly

Question 6

Why does facilitated diffusion occur?

Answer 6

• Certain molecules and ions cannot diffuse directly though the phospholipid bilayer of cell membranes
• Large and polar molecules cannot diffuse across
• A transport protein is required to facilitate diffusion

eg. glucose and amino acids

Question 7

Explain how channel proteins enable facilitated diffusion

Answer 7

There are pores in the cell membrane that charged particles diffuse through
They are embedded in the plasma membrane

Question 8

Explain how carrier proteins enable facilitated diffusion

Answer 8

• Unlike channel proteins, these switch between 2 shapes
• A large molecule attaches to a carrier proteins binding site
• The binding site opens to one side of the membrane first
• Then open to the other side of the membrane when the carrier protein switches shape

Question 9

What is active transport?

Answer 9

It is the movement of molecules/ions across a membrane AGAINST the conc. gradient
This requires energy in the form of ATP
Only uses a carrier protein is needed for this process and ATP is needed for the CP to change shape

Question 10

How is ATP used to provide energy?

Answer 10

ATP is produced from respiration and is hydrolysed to release energy

eg inc. reabsorption of useful molecules/ions into blood after filtration in kidneys, taking sugars from cells into the phloem tissue

Question 11

Explain endocytosis

Answer 11

• A cell can surround a substance with a section of membrane
• The membrane engulfs the substance and pinches off to produce a temp. vacuole to form a vesicle
• This is an active process and requires energy

Question 12

Explain exocytosis

Answer 12

• Cells need to secrete substances
• Vesicles pinch off from sacs of the Golgi apparatus
• These vesicles are moved toward the cell surface and fuses with the cell membrane
• The cell releases its contents
• This is an active process requiring energy