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BIOLOGY · Stage 9 · Chapter 13: Variation and Natural Selection

Biology Stage 9 Comprehensive Review

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Biology Stage 9 Comprehensive Review — Alhussan International Academy — Biology (Stages 7–9) notes: Draw together the Stage 9 biology topics: plants and photosynthesis, excretion, reproduction and genetics, the carbon cycle, climate change, variation and natural selection.Photosy…

  • Kidney failure
  • Development
  • Genes and alleles
  • Healthy pregnancy
  • Carbon dioxide balance
  • Human impact

Section 1 of 6Plants and photosynthesis

  • Photosynthesis
    • plants make glucose in chloroplasts using light energy: carbon dioxide + water → glucose + oxygen.
    • Without carbon dioxide a plant cannot photosynthesise, so it cannot make new biomass from the air.
  • Leaves
    • they are adapted with a large surface area, stomata and chlorophyll.
  • Minerals
    • nitrate is needed for proteins and magnesium for chlorophyll.
    • A plant grown with every mineral except magnesium has yellow leaves.
  • Transport
    • water moves up the xylem in the transpiration stream and sugars move in the phloem.

Section 2 of 6Excretion and the kidney

  • Excretion
    • it removes metabolic waste: carbon dioxide via the lungs, and urea, excess water and salts via the kidneys.
  • Urea
    • it is made in the liver by deamination.
  • Nephron
    • high-pressure filtration at the glomerulus forces small molecules into the capsule.
    • Selective reabsorption returns all the glucose and as much water and salt as needed to the blood.
    • The rest is urine, which passes through the ureters to the bladder and out through the urethra.
  • ADH
    • it controls water reabsorption.
  • Kidney failure
    • it can be treated by dialysis or by a transplant.

Section 3 of 6Reproduction, genetics and development

  • Gametes
    • sperm and egg cells have 23 chromosomes.
  • Development
    • fertilisation in the oviduct forms a zygote with 46 chromosomes,
    • which divides, implants and develops into a fetus supported by the placenta and amniotic fluid.
  • Sex
    • it is determined by sex chromosomes: XX female and XY male.
  • Genes and alleles
    • genes are sections of DNA on chromosomes, and alleles are versions of genes.
  • Healthy pregnancy
    • a healthy diet, no smoking or alcohol, and antenatal care.

Section 4 of 6Carbon cycle and climate

  • Carbon dioxide balance
    • photosynthesis removes it, while respiration, decomposition and combustion return it.
  • Human impact
    • burning fossil fuels and deforestation increase carbon dioxide, enhancing the greenhouse effect and warming the climate.
    • Consequences are sea level rise, extreme weather and habitat change.
  • Evidence
    • ice cores and temperature records.
  • Models
    • climate models make predictions with uncertainty.

Section 5 of 6Variation and evolution

  • Variation
    • it is caused by genes, environment or both.
  • Natural selection
    • it acts on variation, so advantageous alleles become more common because their owners survive and reproduce.
    • Examples include the peppered moth and antibiotic-resistant bacteria.
    • A rat poison introduced in 1950 had little effect by 2000 because resistant rats were selected.
  • Extinction
    • habitat loss and other pressures can lead to extinction.
  • Conservation
    • reserves, captive breeding and seed banks.

Section 6 of 6Links between topics

  • Photosynthesis connects the carbon cycle, food chains and climate.

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