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Chapter 13: Variation and Natural Selection

Biology Stage 9 Comprehensive Review — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 8 1 quiz

📚 Alhussan Biology - Stage 9 Hub / Topical Revision
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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 8Plants and photosynthesis

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
  • Leaves they are adapted with a large surface area, stomata and chlorophyll.
Minerals
  • Minerals nitrate is needed for proteins and magnesium for chlorophyll.
  • A plant grown with every mineral except magnesium has yellow leaves.
Transport
  • Transport water moves up the xylem in the transpiration stream and sugars move in the phloem.

Section 2 of 8Excretion and the kidney

Excretion
  • Excretion it removes metabolic waste: carbon dioxide via the lungs, and urea, excess water and salts via the kidneys.
Urea
  • Urea it is made in the liver by deamination.
Nephron
  • 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
  • ADH it controls water reabsorption.
Kidney failure
  • Kidney failure it can be treated by dialysis or by a transplant.

Section 3 of 8Reproduction, genetics and development

1Gametes
  • Gametes sperm and egg cells have 23 chromosomes.
2Development
  • 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.
3Sex
  • Sex it is determined by sex chromosomes: XX female and XY male.
4Genes and alleles
  • Genes and alleles genes are sections of DNA on chromosomes, and alleles are versions of genes.
5Healthy pregnancy
  • Healthy pregnancy a healthy diet, no smoking or alcohol, and antenatal care.

Section 4 of 8Carbon cycle and climate

1Carbon dioxide balance
  • Carbon dioxide balance photosynthesis removes it, while respiration, decomposition and combustion return it.
2Human impact
  • 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.
3Evidence
  • Evidence ice cores and temperature records.
4Models
  • Models climate models make predictions with uncertainty.

Section 5 of 8Variation and evolution

Variation
  • Variation it is caused by genes, environment or both.
Natural selection
  • 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
  • Extinction habitat loss and other pressures can lead to extinction.
Conservation
  • Conservation reserves, captive breeding and seed banks.

Section 6 of 8Links between topics

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

Section 7 of 8Key terms

TermMeaning
PhotosynthesisMaking glucose from carbon dioxide and water using light.
DeaminationRemoval of the amino group from amino acids in the liver.
ZygoteThe cell formed when sperm and egg nuclei fuse.
AlleleA version of a gene.
Greenhouse effectWarming caused by gases that absorb infrared radiation.
Natural selectionSurvival and reproduction of the best adapted individuals.

Section 8 of 8Exam technique

1
Mixing up filtration and reabsorption.
2
Describing evolution as an individual changing.
3
Saying weather is the same as climate.
4
Forgetting units in answers.

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