Biology 1.1 Movement across Membranes

Why can substances pass through cell membranes?

Cell membranes are partially permeable, enabling some molecules to pass through.

By what three methods can different substances pass through cell membranes?

Diffusion

Osmosis

Active transport

What is diffusion?

The net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient.

Why is diffusion described as a passive process?

It does not require energy.

What substances can pass through cell membranes by diffusion?

Small molecules, e.g., carbon dioxide, oxygen.

What two factors affect the rate of diffusion?

Temperature

Concentration gradient

How does temperature affect the rate of diffusion?

The higher the temperature, the more energy possessed by molecules, and the faster the rate of diffusion.

How does concentration gradient affect the rate of diffusion?

The steeper the concentration gradient, the faster the rate of diffusion.

What is osmosis?

The net movement of water molecules from an area of high water (low solute) concentration to an area of low water (high solute) concentration across a partially permeable membrane.

Describe what happens to an animal cell if it is placed into a more dilute solution.

Higher concentration of water in the surrounding solution.

Water enters the cell by osmosis.

Pressure inside the cell increases, causing the cell to burst.

Describe what happens to a plant cell if it is placed into a more dilute solution.

Higher concentration of water in the surrounding solution.

Water enters the cell by osmosis.

The cell wall resists the increase in pressure, causing the cell to become turgid.

Describe what happens to an animal cell if it is placed into a more concentrated solution.

Lower concentration of water in the surrounding solution.

Water leaves the cell by osmosis.

Pressure inside the cell decreases, causing the cell to shrink.

Describe what happens to a plant cell if it is placed into a more concentrated solution.

Lower concentration of water in the surrounding solution.

Water leaves the cell by osmosis.

Pressure inside the cell decreases, causing the cytoplasm to shrink.

The cytoplasm pulls away from the cell wall (plasmolysis).

Describe what happens to a cell if it is placed into a solution of equal water concentration.

No net movement of water molecules into or out of the cell.

What is active transport? (Higher)

The movement of molecules across a cell membrane from an area of low concentration to an area of high concentration, against the concentration gradient, using energy./