Chapter 2: Movement of substances

Diffusion

• Passive process

• Net movement of particles from a region of higher concentration to a region of lower concentration, which is down a concentration gradient

• Can happen with or without a partially permeable membrane

Concentration gradient

• Difference in concentration between two regions

Factors that affect rate of diffusion

• Concentration gradient

• Diffusion distance

• Surface area-to-volume ratio

How does diffusion distance affect the rate of diffusion?

• The shorter the diffusion distance, the less time is needed for substances to travel, and hence rate of diffusion is higher

How does surface area-to-volume ratio affect diffusion?

• The greater the surface area-to-volume ratio of a cell, the higher the rate at which substances move in and out of it, resulting in an increase in rate of diffusion

Osmosis

• Net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

What is water potential?

• A measure of the tendency of water molecules to move from one place to another

Factors affecting rate of osmosis

• Water potential gradient

• Distance over which water molecules need to move

• Surface area-to-volume ratio

What happens to a cell in a solution with higher water potential?

1. When a plant cell is placed in a solution of higher water potential, the cell sap has lower water potential than that of the solution outside the cell.

2. By osmosis, water molecules will move from a region of higher water potential outside the cell into a region of lower water potential inside the cell.

3. A plant cell will expand or swell, which is said to become turgid, as it is protected by an inelastic cell wall. Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.

4. An animal cell will swell and may even burst in a solution of high water potential as it lacks the protection of a cell wall.

What happens to a cell in a solution with the same water potential?

• The cell will not change its shape as there is no net movement of water molecules in or out of the cell.

What happens to a cell in a solution with lower water potential?

1. When a plant cell is immersed in a solution with lower water potential, the cell sap has a higher water potential than the solution outside the cell.

2. By osmosis, water molecules will move from a region of higher water potential inside the cell to a region of lower water potential outside the cell.

3. A plant cell will decrease in size and become flaccid. The cytoplasm and cell membrane shrinks away from the cell wall, also known as plasmolysis. A plasmolysed can be restored by placing it in water or a solution with higher water potential.

4. An animal cell will lose water and shrink. Spikes will appear in the cell, also known as crenation. The cell will eventually die due to dehydration. 

Importance of turgor in plants

• Turgor plays an important role in maintaining the shape of soft tissues in plants

• It keeps herbaceous (non-woody) plants firm and upright

• Prevents plant cells from becoming flaccid and plasmolysed

• Movements of certain plant parts are due to changes in turgor like regulating the opening and closing of stomata

Active transport

• The process in which energy is used to move the particles of a substance across a membrane against its concentration gradient

• Particles travel from a region where they are of lower concentration to a region where they are of higher concentration

Differences between diffusion and active transport

• Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient while active transport is the net movement of particles from a region of lower concentration to a region of higher concentration, against a concentration gradient

• Energy from respiration is not required for diffusion but required for active transport

• Partially permeable membrane is not needed for diffusion but is needed for active transport

where does active transport occur

- active transport occur only in living cells because living cells respire

- it is during respiration that energy is released, and part of this energy is used in active transport

human body examples

> uptake of glucose by the microvilli of epithelial cells in the small intestine, and from the kidney tubules into the blood capillaries

plants examples

> root hair cells can take in mineral salts from the soil solution by diffusion or active transport, depending on the concentration of mineral salts in the soil solution

differences between osmosis and diffusion

osmosis

- refers to only water (solvent molecules)

- takes place across a partially permeable membrane

vs

diffusion

- refers to any substance, gas, or liquid

- does not require a partially permeable membrane