AQA GCSE Biology - Notes

Cell Biology
Cells can be prokaryotic (bacteria, no nucleus) or eukaryotic (animal, plant, fungi, have a nucleus).
Animal cells: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
Plant cells: animal cell parts + cell wall, chloroplasts, permanent vacuole.
Microscopy: light microscopes (lower resolution), electron microscopes (higher resolution).
Cell division: mitosis creates identical cells for growth and repair; the cell cycle includes interphase (growth) and mitosis (division).
Stem cells: undifferentiated cells from embryos (pluripotent) or adult bone marrow (multipotent), used in medicine.

Transport in Cells
Diffusion: movement of particles from high to low concentration.
Osmosis: diffusion of water across a partially permeable membrane from dilute to concentrated solution.
Active transport: movement of substances against a concentration gradient, requires energy from respiration (e.g., root hair cells absorb minerals).

Organisation
Cells → tissues → organs → organ systems.
Enzymes are biological catalysts, made of proteins, that speed up reactions by lowering activation energy.
Digestive enzymes:

• Amylase breaks starch → sugars (mouth, small intestine).

• Protease breaks protein → amino acids (stomach, pancreas, small intestine).

• Lipase breaks fats → glycerol + fatty acids (pancreas, small intestine).
  The heart is a double circulatory system (right side pumps to lungs, left side pumps to body).
  Arteries carry blood away (high pressure), veins carry blood to heart (valves), capillaries exchange substances.
  Blood: red blood cells (carry oxygen), white blood cells (defence), platelets (clotting), plasma (transports substances).
  Non-communicable diseases (e.g., cancer, coronary heart disease) can be caused by lifestyle (e.g., smoking, diet).

Infection and Response
Pathogens: microorganisms that cause disease (bacteria, viruses, fungi, protists).
Spread by air, water, direct contact.
Examples:

• Bacterial: salmonella (food poisoning), gonorrhoea (STD).

• Viral: measles (rash, fever), HIV (immune system damage), TMV (plants, discolouration).

• Fungal: rose black spot (plants, black spots reduce photosynthesis).
  Defence systems: skin (barrier), nose (mucus), trachea and bronchi (cilia, mucus), stomach (acid).
  Immune system:

• Phagocytosis: white blood cells engulf pathogens.

• Antibody production: specific to antigens.

• Antitoxin production: neutralises toxins.
  Vaccination: injects dead/inactive pathogens to stimulate antibody production.
  Antibiotics treat bacterial infections, not viruses; overuse leads to resistance.

Bioenergetics
Photosynthesis: occurs in chloroplasts, converts light energy to chemical energy.
Equation: carbon dioxide + water → glucose + oxygen.
Limiting factors: light intensity, carbon dioxide concentration, temperature.
Glucose uses: respiration, starch storage, cellulose for cell walls, amino acids (with nitrates), lipids for seeds.
Respiration:

• Aerobic: glucose + oxygen → carbon dioxide + water (+energy).

• Anaerobic (animals): glucose → lactic acid (+small energy).

• Anaerobic (plants/yeast): glucose → ethanol + carbon dioxide.
  Metabolism: all the chemical reactions in the body, including conversion of glucose, breakdown of excess proteins, respiration.

Homeostasis and Response
Homeostasis: maintaining a stable internal environment.
Controlled conditions: blood glucose, body temperature, water levels.
Nervous system: stimulus → receptor → sensory neurone → CNS → motor neurone → effector.
Reflex arc: automatic, fast response (no conscious brain involvement).
Synapse: gap between neurones where neurotransmitters pass signals.
Endocrine system: glands that secrete hormones into the blood.

• Pituitary gland: master gland (controls other glands).

• Pancreas: insulin (lowers blood glucose) and glucagon (raises blood glucose).

• Adrenal gland: adrenaline (fight or flight).

• Ovaries: oestrogen (female sex hormone).

• Testes: testosterone (male sex hormone).
  Menstrual cycle:

• FSH: stimulates egg maturation.

• LH: triggers ovulation.

• Oestrogen: thickens uterus lining, inhibits FSH.

• Progesterone: maintains lining.
  Contraception: hormonal (pill, implant) or barrier (condom, diaphragm).
  IVF: fertilisation outside body; involves hormone treatment.

Inheritance, Variation and Evolution
DNA: genetic material in chromosomes, double helix structure.
Gene: small section of DNA coding for a protein.
Genome: entire genetic material of an organism.
Mitosis: produces identical diploid cells for growth.
Meiosis: produces non-identical haploid gametes (sex cells).
Sex determination: XX = female, XY = male.
Genetic diagrams: Punnett squares show inheritance of alleles.
Dominant allele: always expressed.
Recessive allele: only expressed if two copies present.
Genetic disorders:

• Cystic fibrosis: recessive disorder (mucus build-up).

• Polydactyly: dominant disorder (extra fingers/toes).
  Genetic screening: checks embryos or adults for genetic disorders.
  Variation: differences in characteristics due to genetics and environment.
  Evolution: gradual change in species over time by natural selection.
  Selective breeding: humans breed plants/animals for desired traits.
  Genetic engineering: changing an organism’s DNA (e.g., insulin-producing bacteria).
  GM crops: increased yield, disease resistance, longer shelf life.

Ecology
Ecosystem: community of living organisms and abiotic environment.
Biotic factors: living (predators, food availability).
Abiotic factors: non-living (temperature, light, moisture).
Adaptations: structural, behavioural, functional to survive environment.
Food chains: producer → primary consumer → secondary consumer → tertiary consumer.
Energy lost at each trophic level (only ~10% passed on).
Biodiversity: variety of species; important for stable ecosystems.
Human impacts: deforestation, pollution (land, air, water), climate change.
Conservation: protecting ecosystems and biodiversity (e.g., breeding programs, habitat protection).
Waste management: reducing landfill, recycling, treating sewage.