Curriculum
Course: SQA National 5 Biology
Login

Curriculum

SQA National 5 Biology

1. Unit 1 — Cell Biology

0/28

2. Unit 2 — Multicellular Organisms

0/39

3. Unit 3 — Life on Earth

0/31
Text lesson

2.7.1 Exchange Surfaces

2.7.1 Exchange Surfaces: Explain why cells need exchange of materials with the blood, and describe how oxygen, nutrients and wastes move between exchange surfaces, blood and respiring cells.

2.7.1 Exchange Surfaces
The Need for Exchange
Respiration
  • Cells need a constant supply of oxygen and nutrients for respiration, which releases energy in the form of ATP.
Oxygen uptake
  • Oxygen is absorbed in the lungs, where it diffuses from the air sacs called alveoli into the blood.
Nutrient uptake
  • Glucose and amino acids are absorbed from digested food in the small intestine and pass into the blood through the villi.
Delivery
  • The heart pumps the oxygenated, nutrient-rich blood in arteries, arterioles and capillaries to the cells that need them.
1

Oxygen is carried around the body by the red blood cells, whereas dissolved nutrients are carried in the blood plasma.
2

At the capillaries within tissues, substances move between the blood and the cells, mostly by diffusion.
3

Respiring cells take in oxygen, glucose, amino acids, water and minerals from the blood.
4

Cells release carbon dioxide, urea and some water into the blood, which carries these wastes away.
5

Carbon dioxide is mostly carried dissolved in plasma to the lungs, where it diffuses into the alveoli and is breathed out.
Close to cells
  • Capillary networks surround the tissues, so substances only have a short way to move between blood and cells.
Very thin walls
  • The capillary wall is one cell thick, so the diffusion distance is short.
Narrow lumen
  • The lumen is narrower than a red blood cell, which slows blood flow and gives more time for exchange.
Large total area
  • There are so many capillaries that their combined surface area is very large, giving rapid exchange of materials.
Moves into cells Moves out of cells
Oxygen, glucose, amino acids Carbon dioxide
Water and minerals Urea and some water
Maintaining Diffusion
Ventilation
  • Breathing keeps the carbon dioxide concentration in the alveoli low so that diffusion out of the blood continues.
Blood flow
  • Moving blood carries absorbed substances away, so the concentration gradient at each exchange surface stays steep.
Waste substances
  • Other waste substances carried in the blood include urea, excess ions and excess water.
Overall idea
  • Exchange surfaces, the heart and blood vessels work together as one system to supply cells and remove wastes.

2.7.1 – Key Points for the Exam

  • Exchange surface: A surface across which substances are absorbed or released.
  • Diffusion: The net movement of particles from a region of higher concentration to lower concentration.
  • Alveolus: A tiny air sac in the lungs where gas exchange occurs.
  • Villus: A finger-like projection of the small intestine that absorbs nutrients.
  • ATP: The molecule that provides the energy released by respiration for cell activities.

Common Mistakes

  • Saying that oxygen is carried in the plasma, when it is carried by red blood cells.
  • Writing that exchange takes place in arteries and veins; it takes place in the capillaries.
  • Forgetting that waste substances such as urea must be removed as well as carbon dioxide.
  • Stating that cells make oxygen for respiration, instead of taking it up from the blood.
YOUR STUDY PATH

2.7.1 Exchange Surfaces

SQA National 5 Biology / 2. Unit 2 — Multicellular Organisms

Accepted payment methods