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2.6.2Unit 2 — Multicellular Organisms

Red Blood Cells

5 min read 5 parts Lesson 28 of 39

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Section 1 of 5Structure of the Red Blood Cell

2.6.2 Red Blood Cells
Haemoglobin
  • Each cell is packed with this red protein, which combines with oxygen to form oxyhaemoglobin.
No nucleus
  • Mature red blood cells have no nucleus, which leaves more room inside the cell for haemoglobin.
Biconcave shape
  • The cell is a flattened disc that is thinner in the middle than at the edges.
Small size
  • The cells are small and flexible enough to pass along very narrow capillaries.

Section 2 of 5Adaptations to Function

Adaptations to Function
More oxygen carried
  • Packing in more haemoglobin means that each cell can transport a larger amount of oxygen.
Large surface area
  • The biconcave shape gives a high surface area to volume ratio, so oxygen can diffuse in and out quickly.
Efficient uptake
  • In the lungs oxygen diffuses into the cells and combines with haemoglobin to form oxyhaemoglobin.
Release in tissues
  • Where oxygen concentration is low, oxyhaemoglobin releases oxygen, which then diffuses into the respiring cells.

Section 3 of 5Oxygen Transport Sequence

1
Air breathed into the lungs has a high concentration of oxygen, which diffuses into the blood across the thin walls of the alveoli.
2
Oxygen enters the red blood cells and combines with haemoglobin to make oxyhaemoglobin.
3
The heart pumps the blood to the body, so the red blood cells travel to the capillaries of the tissues.
4
Oxygen separates from haemoglobin and diffuses out of the red cells and into the respiring cells.

Section 4 of 5Why Adaptations Matter

Why Adaptations Matter
Faster exchange
  • A larger surface area to volume ratio means a shorter distance for oxygen to travel to reach all the haemoglobin.
Capacity
  • The more haemoglobin in each cell, the more oxygen is delivered to respiring tissues every time blood circulates.
Flexible shape
  • Flattened, flexible cells can squeeze through the narrowest capillaries, so oxygen reaches every tissue.
Link to respiration
  • Oxygen delivered by red cells is used in respiration, which releases energy for the cell to carry out its work.

Section 5 of 5Exam Checklist & Tips

2.6.2 – Key Points for the Exam

  • Haemoglobin A protein in red blood cells that binds oxygen.
  • Oxyhaemoglobin The compound formed when haemoglobin combines with oxygen.
  • Biconcave Having a disc shape that is thinner in the centre than at the edge.
  • Surface area to volume ratio
    • The surface area of a cell compared with its volume;
      • a large ratio speeds diffusion.

Common Mistakes

  • Saying that red blood cells contain a nucleus; mature ones do not.
  • Stating that haemoglobin is a cell; it is a protein inside the cell.
  • Using the term biconcave without explaining that it increases surface area for diffusion.
  • Confusing haemoglobin, which is the protein, with oxyhaemoglobin, which is haemoglobin combined with oxygen.
Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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