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Cambridge IGCSE Biology · 0610

Chapter 3: Movement into and out of cells

Diffusion

Definition
The net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.
Energy source
The energy for diffusion comes from the kinetic energy of random movement of molecules and ions.
Movement across membranes
Some substances move into and out of cells by diffusion through the cell membrane.
Importance
Crucial for the movement of gases and solutes in living organisms. For example, oxygen and carbon dioxide diffuse in and out of cells for metabolic reactions like respiration and photosynthesis.

Factors affecting the rate of diffusion:

  1. Surface area: As surface area increases, the rate increases because there is more space available for substances to diffuse through.
  2. Temperature: Increasing temperature increases the rate as molecules gain kinetic energy and move faster.
  3. Concentration gradient: A steeper gradient (greater difference in concentration) increases the rate.
  4. Distance: A greater diffusion distance slows the rate as molecules must travel further.
Diffusion of solute particles across a partially permeable membrane from a region of high concentration to low concentration until equilibrium is reached
Diagram 1: Diffusion across a cell membrane. Solute particles are shown at a higher concentration on one side of a partially permeable membrane. Arrows indicate net movement down the concentration gradient until particles are evenly distributed on both sides at equilibrium.

Exam Traps

  • Do not confuse diffusion of solutes with osmosis, which is the net movement of water only.

Osmosis

Role of water
Water acts as a solvent in organisms, which is vital for digestion, excretion, and transport. It also provides a medium for metabolic reactions and acts as a temperature buffer.
Definition
The net movement of water molecules through a partially permeable membrane.
Water potential
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
Investigation
Osmosis can be studied using dialysis tubing (an artificial partially permeable membrane) or by immersing plant tissues (e.g., potato) in solutions of different concentrations and measuring changes in mass or length.

Effects of osmosis on plant tissues

Plants rely on osmosis for structural support:

Turgid
Cells are described as turgid when they are swollen due to high water content.
Turgor pressure
The pressure of water inside the cells pressing outwards on the cell wall, which supports the plant structure.
Flaccid
Occurs when water moves out of the cell; the cell shrinks, but the membrane remains attached to the wall.
Plasmolysis
Occurs when too much water leaves the cell; the cell membrane peels away from the cell wall.
Three plant cells showing turgid, flaccid, and plasmolysed states in solutions of different water potential
Diagram 2: Effects of osmosis on plant cells. (A) Turgid cell in pure water — water enters by osmosis, the vacuole expands, and the cell membrane is pressed firmly against the cell wall. (B) Flaccid cell in a slightly concentrated solution — water leaves the cell and the vacuole shrinks, but the membrane remains in contact with the wall. (C) Plasmolysed cell in a highly concentrated solution — extensive water loss causes the cell membrane to pull away from the cell wall.

Active transport

Definition
The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration.
Protein carriers
Active transport is facilitated by protein carriers embedded in the cell membrane. These proteins bind to specific molecules, change shape using energy, and force the molecules across the membrane.
Importance and examples
  • Ion uptake by root hairs: Plants use active transport to take up ions like nitrates and magnesium from the soil even when the concentration in the soil is lower than inside the root hair cell.
  • Uptake of glucose: Glucose is absorbed against the concentration gradient in the small intestine and kidney tubules.
Carrier protein in the cell membrane transporting a molecule against a concentration gradient using energy from ATP
Diagram 3: Active transport via a carrier protein. A molecule binds to the carrier on the side of lower concentration. Energy from respiration (ATP) causes the protein to change shape, transporting the molecule across the membrane and releasing it on the side of higher concentration.

Exam Traps

  • Water uptake by root hairs is osmosis, not active transport — only ion uptake against the gradient is active transport.

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