State that some substances move into and out of cells by diffusion through the cell membrane.

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Diffusion

3.1 Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. This process occurs in all living things and is essential for the transport of substances into and out of cells.

Key Concepts

  • Concentration Gradient: The difference in concentration of a substance between two areas. Diffusion occurs down a concentration gradient.
  • Net Movement: Diffusion is a net movement, meaning that while particles move in both directions, there is a greater movement from high to low concentration.
  • No Energy Required: Diffusion is a passive process and does not require any energy input from the cell.

Diffusion Across the Cell Membrane

The cell membrane is a selectively permeable barrier. This means that it allows some substances to pass through easily, while restricting the passage of others. Diffusion is a primary way in which substances cross the cell membrane.

Substance Movement Explanation
Oxygen (O2) From high concentration (air) to low concentration (inside the cell) Cells need oxygen for respiration. Oxygen diffuses into the cell where its concentration is lower.
Carbon Dioxide (CO2) From high concentration (inside the cell) to low concentration (air) Carbon dioxide is a waste product of respiration. It diffuses out of the cell where its concentration is higher.
Water (H2O) Can move in both directions depending on the water potential Water moves across the cell membrane due to differences in water potential. This is important for maintaining cell turgor pressure.
Nutrients (e.g., glucose, amino acids) From high concentration (extracellular fluid) to low concentration (inside the cell) Cells require nutrients for energy and growth. Nutrients diffuse into the cell where their concentration is lower.

Figure: Suggested diagram: A simple diagram showing oxygen molecules moving from an area of high concentration (outside the cell) to an area of low concentration (inside the cell) across a cell membrane. Label the high and low concentration areas and the cell membrane.

Example: In the lungs, oxygen diffuses from the alveoli (air sacs) into the blood, and carbon dioxide diffuses from the blood into the alveoli to be exhaled.

Factors affecting diffusion rate:

  • Temperature: Higher temperatures increase the rate of diffusion.
  • Surface area: A larger surface area allows for faster diffusion.
  • Concentration gradient: A steeper concentration gradient leads to faster diffusion.
  • Distance: A shorter distance allows for faster diffusion.