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Diffusion is net movement of particles from higher to lower concentration due to their random motion. It does not require energy supplied by respiration. Higher temperature, larger surface area and a steeper gradient can increase rate. Thin exchange surfaces reduce diffusion distance. As an organism grows, its surface area to volume ratio decreases, so diffusion across the outer surface alone may not supply every cell quickly enough.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Compare three transport mechanisms
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which transport mechanism can move against a concentration gradient?
Active transport. It requires an energy supply.
After watching 2: What moves in osmosis?
Water. A partially permeable membrane is required.
What you'll be able to do
Diffusion is net movement of particles from higher to lower concentration due to their random motion. It does not require energy supplied by respiration. Higher temperature, larger surface area and a steeper gradient can increase rate. Thin exchange surfaces reduce diffusion distance. As an organism grows, its surface area to volume ratio decreases, so diffusion across the outer surface alone may not supply every cell quickly enough.
Osmosis is net water movement across a partially permeable membrane from a more dilute solution toward a more concentrated solution. Plant cells become turgid as water enters and the wall resists expansion. In a concentrated external solution they lose water and may become plasmolysed. For an investigation, standardise tissue size, blot before weighing, control temperature and compare percentage mass change so starting masses can differ fairly.
Active transport moves substances against a concentration gradient using energy transferred by respiration and membrane carrier proteins. Root cells can absorb mineral ions from dilute soil solution. Glucose absorption can also require active processes. The transported material is not itself necessarily the energy source. A cell poisoned so respiration stops may still show diffusion while energy-dependent transport declines. Distinguish oxygen use in respiration from the transport mechanism itself.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why does a thin-walled alveolus improve gas exchange?
It supports rapid diffusion between air and blood.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.