Substances can enter and leave cells through the cell membrane. The 3 main ways substances are transported in and out of cells are:

1. Diffusion
2. Osmosis
3. Active transport

For IGCSE Biology, it is important to understand the processes of these 3 modes of transport.

Diffusion

• Diffusion is the movement of particles from a region of high concentration to a region of low concentration.
• We say that particles are moving down a concentration gradient through diffusion.
• No energy is required for diffusion, so we call it a passive process.
• Only small molecules can diffuse across the cell membrane, such as oxygen, carbon dioxide, glucose, amino acid and ions. Large molecules, such as protein and starch, cannot diffuse across the cell membrane.

For example, if we look at the picture above ☝️

• The concentration of a particular substance is high outside of the cell (extracellular space), and the concentration is low inside the cell (intracellular space).
• As time goes by, the substance will gradually diffuse from the extracellular space (where the concentration is high) to the intracellular space (where the concentration is low).
• After sufficient time, the concentration will become equal outside and inside of the cell.

Osmosis

• Osmosis is the movement of water molecules from a low concentration to a high concentration through a semi-permeable membrane. Semi-permeable membranes (also called partially permeable membranes) only allow certain particles to pass through it while blocking other particles to pass through.
• At dilute solutions, the concentration of water is high (high water potential). At concentrated solutions, the concentration of water is low (low water potential).
• Through osmosis, water molecules move from high water potential to low water potential.

• When a concentrated sugar solution and dilute sugar solution are separated by a partially permeable membrane, only water molecules are free to move between the two solutions.
• Water molecules move from dilute sugar solution (high water potential) to concentrated sugar solution (low water potential).
• Water level will increase on the concentrated sugar solution side, and water level will decrease on the dilute sugar solution side.
• Over time, the concentration of sugar will become equal on both sides.

Osmosis in animal and plant cells

When animal and plant cells are placed in a dilute or concentrated solution, the results of osmosis are different. This topic is frequently asked in IGCSE Biology. 

Animal cell

• When an animal cell is placed in a solution that has a lower concentration than inside the cell (hypotonic solution), water will move into the cell. Eventually, the animal cell will burst and die (lysed).
• When an animal cell is placed in a solution that has the same concentration as inside the cell (isotonic solution), the amount of water leaving and entering the cell is same. The animal cell can maintain its normal shape in an isotonic solution.
• When an animal cell is placed in a solution that has a higher concentration than inside the cell (hypertonic solution), water will move out of the cell. Too much water leaving the cell will cause the animal cell to become shriveled.

Plant cell

• When a plant cell is placed in a hypotonic solution, water will move into the cell. Unlike animal cells, plant cells have a strong cell wall that supports the cell. Instead of bursting, the cytoplasm will push against the cell wall, causing the plant cell to become turgid. Turgidity is essential for maintaining the rigidity of plant cells.
• When a plant cell is placed in a isotonic solution, the plasma membrane is not as tightly in touch with the cell wall as a turgid cell. The plant cell in this case is called flaccid.
• When a plant cell is placed in a hypertonic solution, water leaves the cell, causing the cytoplasm to shrink away from the cell wall (plasmolyzed).

☝️ Results of osmosis in animal and plant cells in one diagram ☝️

Active Transport

• Active transport is the movement of particles from a region of low concentration to a region of high concentration. Active transport requires energy (ATP) generated from respiration.
• Transport proteins, which are embedded in cell membranes, can use energy to transport the particles against the concentration gradient.

Summary of Diffusion, Osmosis and Active Transport

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