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GCSE Biology Unit 1 Summary

Dec 3, 2025

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Overview

  • Summary of key GCSE Biology Unit 1 topics: cells, microscopy, photosynthesis, diffusion/osmosis, enzymes, respiration, homeostasis, nervous/endocrine systems, plant hormones, ecosystems, nutrient cycles, and human impacts.
  • Covers practical methods, definitions, formulas, and examples relevant for higher and foundation tiers.

Cells and Microscopy

  • All living organisms are made of cells.
  • Light microscope: views whole cells, sometimes nucleus; limited subcellular detail.
  • Electron microscope: higher resolution and resolving power; shows organelles clearly.
  • Magnification formula: magnification = image size / object size

Cell Types and Organelles

  • Two main groups: eukaryotic (nucleus; plants, animals) and prokaryotic (no nucleus; bacteria).
  • Cell membrane: selectively permeable; controls entry/exit of substances.
  • Cell wall: plant cell walls made of cellulose; bacterial walls not cellulose.
  • Cytoplasm: site of most chemical reactions.
  • Mitochondria: site of respiration, releases energy.
  • Ribosomes: site of protein synthesis (assemble amino acids).
  • Chloroplasts (plants): site of photosynthesis; contain chlorophyll.
  • Permanent vacuole (plants): stores sap.

Cell Specialization and Stem Cells

  • Cells differentiate to fulfill functions (e.g., nerve, muscle, root hair, xylem, phloem).
  • Stem cells: unspecialized; found in embryos, plant meristems, and bone marrow.
  • Bone marrow stem cells mainly produce blood cells.
  • Uses: treating diabetes, paralysis, cloning debates (ethical considerations).
  • Plant cloning: conserve species, produce crops with desired traits.

Photosynthesis

  • Occurs in chloroplasts (chlorophyll).
  • Reaction requires light energy; endothermic process.
  • Uses glucose for respiration, starch/fat storage, cellulose for walls, and amino acids for proteins.
  • Limiting factors: light intensity, CO2 concentration, temperature.
  • Graph interpretation: curve plateaus when a different factor becomes limiting.

Practical: Measuring photosynthesis

  • Use submerged pondweed and inverted measuring cylinder to collect O2.
  • Independent variable: light intensity (changes with distance from lamp).
  • Light intensity ∝ 1/(distance^2); doubling distance quarters intensity.

Leaf Structure (function per layer)

  • Waxy cuticle: reduces water evaporation.
  • Upper epidermis: transparent, allows light through.
  • Palisade mesophyll: many chloroplasts; main photosynthesis site.
  • Spongy mesophyll: air gaps for gas exchange.
  • Vascular bundle: xylem and phloem.
  • Lower epidermis with stomata: gas exchange; guard cells control stomatal opening.

Diffusion, Osmosis, and Surface Area

  • Diffusion: passive movement down concentration gradient (no energy).
  • Occurs across partially permeable membranes if molecule size permits.
  • Osmosis: diffusion of water across a partially permeable membrane.
  • Rate increases with greater concentration difference, higher temperature, and larger surface area.
  • High surface area to volume ratio examples: villi (intestine), alveoli (lung), root hair cells.

Practical: Osmosis in potato

  • Cut equal cylinders, weigh, place in sugar solutions of varying concentration.
  • After incubation, reweigh; calculate percentage change in mass.
  • Plot % change vs concentration; x-intercept ≈ internal potato solution concentration.

Active Transport

  • Carrier proteins use energy to move substances against concentration gradients.
  • Example: mineral ion uptake into root hair cells.

Food Tests

  • Iodine: orange → black for starch.
  • Benedict’s: blue → green/orange/brick red for reducing sugars (semi-quantitative).
  • Biuret reagent: blue → purple for proteins.
  • Ethanol (cold): goes cloudy for lipids (fats).

Enzymes

  • Biological catalysts; specific to substrates (lock-and-key).
  • Types: carbohydrases (e.g., amylase), proteases, lipases.
  • Enzyme activity rises with temperature until denaturation changes active site shape.
  • Optimum temperature and pH give maximum activity; extremes cause denaturation.

Practical: Enzyme activity (amylase)

  • Mix amylase and starch at varied temperatures/pH buffers.
  • Sample at intervals; use iodine on spotting tile to detect remaining starch.
  • Record time until iodine stays orange (no starch). Plot data to find optimum range.

Respiration and Metabolism

  • Breathing supplies O2 for cellular respiration; air path: trachea → bronchi → bronchioles → alveoli.
  • Alveoli: large surface area for diffusion; O2 binds to hemoglobin in RBCs.
  • Aerobic respiration: glucose + oxygen → CO2 + water (+ energy).
  • Anaerobic respiration (animals): glucose → lactic acid (+ less energy).
    • Causes muscle ache and oxygen debt; lactic acid later converted to glucose in liver.
  • Anaerobic (plants/yeast): glucose → ethanol + CO2 (fermentation); used in baking and alcohol production.
  • Metabolism: sum of chemical reactions in organisms (e.g., respiration, synthesis/breakdown of macromolecules).

Homeostasis

  • Maintenance of stable internal conditions (temperature, blood glucose, water).
  • Negative feedback restores variables toward normal.

Nervous System and Reflexes

  • CNS: brain and spinal cord. PNS: nerves throughout body.
  • Receptor → sensory neuron → synapse (neurotransmitter) → relay neuron → motor neuron → effector (muscle/gland).
  • Reflex arc: bypasses brain via spinal cord for rapid responses.
  • Reaction time practical: ruler drop test; calculate mean; can test effects of stimulants/depressants.

Eye and Vision

  • Accommodation: lens shape changes to focus light on retina.
    • Distant objects: ciliary muscles relax, suspensory ligaments tighten, lens thinner.
    • Near objects: ciliary muscles contract, ligaments slacken, lens thicker.
  • Pupil size (iris) adjusts to light intensity.
  • Cornea: transparent outer layer, slight refraction.
  • Retina: contains rods (light intensity) and cones (three types: red, green, blue wavelengths).
  • Defects:
    • Myopia (short-sighted): difficulty focusing distant objects.
    • Hyperopia (long-sighted): difficulty focusing near objects.
  • Corrections: glasses, contact lenses, or laser eye surgery (reshapes cornea).

Endocrine System and Hormones

  • Glands secrete hormones into blood; slower signaling than nervous system.
  • Pituitary: master gland controlling other glands.
  • Examples: pancreas (insulin, glucagon), thyroid, adrenal (adrenaline), ovaries/testes.

Blood Glucose Regulation

  • High blood glucose → pancreas secretes insulin → cells take up glucose; liver converts excess to glycogen.
  • Low blood glucose → pancreas secretes glucagon → liver breaks glycogen to glucose.
  • Diabetes:
    • Type 1: pancreas produces little/no insulin; treated with insulin injections.
    • Type 2: cells less responsive; linked to obesity; diet and lifestyle management.

Kidney Function and Water Balance

  • Kidneys filter blood, reabsorb useful substances and water; remove urea (from amino acid breakdown).
  • ADH (from pituitary) increases kidney water reabsorption; low ADH → more urine produced.
  • Dialysis replaces kidney function if kidneys fail.

Plant Hormones and Tropisms

  • Gibberellins: promote seed germination, flowering, fruit size.
  • Ethene: promotes fruit ripening.
  • Auxins (auxins/oxins mentioned): control shoot and root growth; destroyed by light.
    • Phototropism: auxin gathers on shaded side of shoot → cell elongation → bends toward light.
    • Geotropism: auxin distribution causes roots to grow downward.
  • Uses: weed killers, rooting powders, tissue culture growth.

Practical: Geotropism test with seeds on damp cotton wool in a petri dish; rotate and observe root bending.

Ecology: Food Chains and Populations

  • Food chain shows flow of biomass/energy across trophic levels.
    • Producers → primary consumers (herbivores) → secondary consumers (predators) → tertiary/apex predators.
  • Organisms compete for food, water, space, light, shelter, and mates.
  • Interdependence: species rely on others; form communities.
  • Abiotic factors: non-living (light, temperature, moisture, soil pH, CO2/O2).
  • Biotic factors: living influences (food availability, predators, pathogens).

Sampling Methods

  • Quadrats: estimate population size by sampling part of area, calculating mean, scaling up.
  • Transects: study distribution change over distance.

Pyramid Of Biomass

  • Represents relative biomass at each trophic level; always a pyramid shape.
  • Calculate percentage transferred: (amount at next level / amount at previous level) × 100.

Nutrient Cycles

Carbon Cycle

  • Decomposition releases CO2 used by plants.
  • Respiration by organisms and microbes returns CO2 to atmosphere.

Nitrogen Cycle

  • N2 in atmosphere → nitrates by nitrogen-fixing bacteria (soil/root nodules).
  • Decomposition: proteins → ammonia by bacteria/fungi.
  • Nitrifying bacteria convert ammonia → nitrates.
  • Plants absorb nitrates for growth.
  • Denitrifying bacteria convert nitrates → N2 gas.
  • Human input: artificial fertilizers add nitrates.

Eutrophication

  • Excess fertilizer runoff causes algal blooms.
  • Surface algae block light; underwater plants die; bacteria decompose dead plants, using oxygen.
  • Low oxygen levels kill aquatic animals.

Biodiversity and Human Impact

  • Biodiversity: variety of organisms in ecosystem; higher biodiversity increases stability.
  • Human activities lowering biodiversity: urban development, deforestation, peat bog destruction, pollution.
  • Waste management and reducing pollution are key to minimizing environmental impact.

Key Terms and Definitions

  • Magnification: image size / object size
  • Eukaryote: cell with nucleus
  • Prokaryote: cell without nucleus
  • Diffusion: passive movement down concentration gradient
  • Osmosis: diffusion of water across partially permeable membrane
  • Active Transport: energy-driven movement against gradient
  • Photosynthesis: light-driven synthesis of glucose in chloroplasts
  • Respiration: release of energy from glucose (aerobic or anaerobic)
  • Metabolism: sum of all chemical reactions
  • Homeostasis: maintaining stable internal conditions
  • Negative Feedback: response that restores variable to normal

Action Items / Practical Skills To Revise

  • Practice magnification and actual size calculations.
  • Draw and label leaf cross-section and pyramids of biomass from data.
  • Plan and interpret photosynthesis, osmosis, and enzyme practicals.
  • Memorize food tests and expected colour changes.
  • Revise steps of nervous reflex arc and hormone feedback loops.
  • Understand experimental controls, independent/dependent variables, and repeats/means.
  • Familiarize with nutrient cycles and causes/effects of eutrophication.

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