Basic Principles

All matter is made of particles such as atoms, molecules or ions. These particles are always moving, bumping into each other and bouncing about. This causes the gas to spread outwards through a process called diffusion.

Concentration Gradient

Concentration gradient is the difference in concentration between two regions. Particles diffuse down the concentration gradient and become evenly spaced out after some time.

The steeper the concentration gradient, the faster the particles will move and the faster the rate of diffusion.

Across Membranes

A permeable membrane allows the solvent (water) and solutes (dissolved substances) to pass through. The cell membrane is a partially permeable membrane that only allows some substances to pass through. Diffusion is therefore an important way by which oxygen and carbon dioxide move into and out of cells.

Factors affecting the rate of diffusion

The table below shows factors affecting the rate of diffusion and what favours the process.

Role of diffusion in living systems

Common examples of diffusion: nutrient uptake in animals and gaseous exchange in both plants and animals. 

In animals 
- Diffusion of nutrients from the gut (high concentration of nutrients) to blood stream (lower concentration of nutrients)
- Gaseous exchange at the lungs, CO2 from the blood to air, O2 from air to blood
In plants
- Oxygen generated during photosynthesis diffuse out of the leaf and CO2 diffuse into the leaf through stomata

Water Potential

Water potential is a measure of the tendency of water to move from one region to another. A dilute solution contains more water molecules per unit volume than a concentrated solution, so it has a higher water potential.

Water Potential Gradient

When a partially permeable membrane separates two solutions of different water potential, a water potential gradient is established. Water moves from a solution with higher water potential (less concentrated) to one with lower water potential (more concentrated).

Dilute vs Concentrated

Solutions can be classified as dilute or concentrated. Dilute solutions have relatively small amount of solute particles in a given amount of water whereas concentrated solutions have a relatively large amount of solute particles.

Partially permeable membrane

The membrane allows only certain small molecules (like water) to go through, thus disallowing the larger ones to pass through. As osmosis occurs, particles small enough can still pass through by diffusion.

Effect on Animal Cells

In a dilute solution, the cytoplasm of the animal cell has lower water potential than the outside solution, and water enters the cell by osmosis. The cytoplasm expands, and the cell bursts and dies eventually when too much water enters. This lack of cell wall results in overexpansion of cell membrane.

In a concentrated solution, water leaves the animal cell by osmosis. The cell shrinks and becomes dehydrated. The membrane forms spikes and the cell is crenated.

Effect on Plant Cells

In a dilute solution, the cell sap in the vacuole of the plant cell has lower water potential than that of the solution. Water enters the cell by osmosis and the vacuole increases in size.
The cell wall prevents overexpansion of the cell membrane by exerting an opposing force. The cell becomes turgid.

Effect on Plant Cells (cont.)

In a concentrated solution, the cell sap in the vacuole of the plant cell has higher water potential. Water from the vacuole and cytoplasm leave the cell and the cell decreases in size, becoming flaccid.

The vacuole and cytoplasm shrink away from the cell wall and the cell becomes plasmolysed.

Definition

Active transport is the process in which energy is used to move a substance from a region of low concentration to a region of high concentration. Cells need to perform active transport to acquire essential substances (eg. nutrients, ions) and excrete toxic substance from the cell. 

Examples of active transport include the uptake of ions of root hair cells and the uptake of glucose of cells in the intestine.

Process

Active transport only takes place in living cells as living cells undergo respiration to release energy. Energy is needed in active transport to move substances against a concentration. The video below highlights the differences between the different movement of substances into or out of cells.

Effects of SA : Volume ratio

The smaller the size of an object, the greater the surface area to volume ratio. Hence, the faster the movement of substances in and out.

Summing Up

To sum up the entire chapter, this table summarises the key differences between the three different types of movement mentioned.