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2.3.1Unit 2 — Multicellular Organisms

The Gametes — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 39

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Part 1 of 5Haploid and Diploid

2.3.1 The Gametes
Diploid
  • A diploid cell has two complete sets of chromosomes, written 2n;
    • human body cells have 23 pairs, making 46 chromosomes.
Haploid
  • A haploid cell has one complete set of chromosomes, written n;
    • human gametes have 23 chromosomes with no pairs.
Gametes
  • Gametes are the sex cells used in sexual reproduction, and they are haploid.
Why halve
  • If gametes were diploid, the chromosome number would double in every generation after fertilisation.

Part 2 of 5Where Gametes Are Made

Where Gametes Are Made
Plant male gametes
  • Pollen grains produced in the anthers contain the male gametes.
Plant female gametes
  • Egg cells are found in the ovules, which are inside the ovary.
Human male gametes
  • Sperm cells are made in the testes.
Human female gametes
  • Egg cells (ova) are made in the ovaries.

Part 3 of 5Sperm Cell Adaptations

Tail
  • The tail allows the sperm to swim towards the egg.
Mitochondria
  • Many mitochondria in the middle section supply energy from aerobic respiration for swimming.
Acrosome
  • The head contains enzymes that digest the outer layers of the egg to allow entry.
Nucleus
  • The haploid nucleus carries the father's genetic information.

Part 4 of 5Egg Cell Adaptations

1
The egg is much larger than the sperm because it contains a store of food in the cytoplasm for the early embryo.
2
It contains a haploid nucleus carrying the mother's genetic information.
3
The egg has an outer jelly layer and membrane that change after a sperm enters, which helps stop other sperm entering.
4
The egg is not able to move itself, so it relies on the oviduct to be moved along.

Part 5 of 5Exam Checklist & Tips

2.3.1 – Key Points for the Exam

  • Gamete A sex cell, such as a sperm or egg.
  • Haploid Having one set of chromosomes (n).
  • Diploid Having two sets of chromosomes (2n).
  • Acrosome The part of a sperm head containing digestive enzymes.
  • Ovule The part of a flower that contains the egg cell.

Common Mistakes

  • Saying that gametes are diploid; they have half the normal number of chromosomes.
  • Confusing pollen grains with the ovule; pollen is male and the ovule is female.
  • Writing that the egg has a tail; only the sperm cell has a tail.
  • Saying the sperm gets energy from the acrosome; mitochondria provide the energy.

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