B2 - Movement Across Membranes

Define diffusion

• spreading out of particles from an area of high concentration to an area of low concentration

What type of process is diffusion, and why?

Passive process - doesn’t require energy

Name 5 factors affecting diffusion rate

1. Concentration gradient

2. Temperature

3. Distance

4. Size of particles

5. Surface area

How does concentration gradient affect the rate of diffusion?

Greater difference = faster rate of diffusion

How does temperature affect rate of diffusion?

Higher temperature = more kinetic energy = faster rate of diffusion

How does distance affect rate of diffusion?

Further the particles need to travel = longer it takes to diffuse, so slower rate of diffusion

How does size of particles affect rate of diffusion?

Smaller particles diffuse faster = faster diffusion rate

How does surface area affect diffusion rate?

Larger surface area = faster rate of diffusion

Define osmosis

Net movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration

What happens when a cell is placed in pure water?

There is a higher water potential outside the cell compared to inside, so water enters the cell via osmosis, across a partially permeable membrane

What happens when a plant cell is placed in water?

The flow of water into the cell exerts pressure on the cytoplasm, which presses against the cell wall

• known as TURGOR

What happens when an animal cell is placed in water?

Animal cells don’t contain cell walls, so the membrane bursts from the pressure

What happens if you place cells in a more concentrated solution?

The outside solution has a lower water potential than the cytoplasm, so the cell loses water

What happens if you place a plant cell in a more concentrated solution?

It becomes FLACCID as pressure reduces, and the cell membrane and cytoplasm shrink away from cell wall

What happens when you place an animal cell in a more concentrated solution?

It becomes CRENATED as cytoplasm decreases

Define active transport

Movement of molecules against a concentration gradient, using energy from respiration

What type of process is active transport?

Active - requires energy

If a cell uses active transport, what feature does it tend to have?

Lots of mitochondria to provide energy for active transport

How is active transport achieved?

Carrier proteins found in the cell membrane