AQA Biology GCSE - Topic 1: Cell Biology - Notes

Cell Structure (1.1)

  • Eukaryotes and Prokaryotes (1.1.1)

    • All living things are made of cells: prokaryotic or eukaryotic.

    • Animal and plant cells are eukaryotic, containing:

      • Cell membrane

      • Cytoplasm

      • Nucleus with DNA

    • Bacterial cells are prokaryotic, smaller, and have:

      • Cell wall

      • Cell membrane

      • Cytoplasm

      • Single circular DNA strand and plasmids

    • Organelles are structures in a cell with different functions.

    • Orders of magnitude:

      • 10 times bigger: 10110^1

      • 1000 times bigger: 10310^3

      • 10 times smaller: 10−110^{-1}

    • Prefixes:

      • Centi: 0.01

      • Milli: 0.001

      • Micro: 0.000,001

      • Nano: 0.000, 000, 001

Animals and Plants (1.1.2)

  • Animal and Plant Cells:

    • Nucleus: Contains DNA, enclosed in a nuclear membrane.

    • Cytoplasm: Liquid, contains enzymes and organelles.

    • Cell membrane: Controls entry and exit.

    • Mitochondria: Aerobic respiration.

    • Ribosomes: Protein synthesis.

  • Plant Cells Only:

    • Chloroplasts: Photosynthesis, contains chlorophyll.

    • Permanent vacuole: Contains cell sap, improves rigidity.

    • Cell wall: Made of cellulose, provides strength.

  • Bacterial Cells:

    • Cytoplasm: (See above)

    • Cell membrane: (See above)

    • Cell wall: Peptidoglycan.

    • Single circular DNA: Floats in cytoplasm.

    • Plasmids: Small DNA rings.

  • Calculating Size:

    • Use shapes like circles or rectangles to estimate size/area.

Cell Specialisation (1.1.3)

  • Differentiation: Cells gain structures to suit their role.

    • Animal cells differentiate once.

    • Plant cells can differentiate throughout life (stem cells).

  • Examples of Specialised Cells in Animals:

    • Sperm cells:

      • Streamlined head and long tail.

      • Many mitochondria.

      • Acrosome with digestive enzymes.

    • Nerve cells:

      • Long axon.

      • Dendrites for connections.

      • Mitochondria for neurotransmitters.

    • Muscle cells:

      • Myosin and actin for contraction.

      • Many mitochondria.

      • Store glycogen.

  • Examples of Specialised Cells in Plants:

    • Root hair cells:

      • Large surface area.

      • Large permanent vacuole.

      • Mitochondria for active transport.

    • Xylem cells:

      • Lignin for strength; cells die, forming a continuous tube.

    • Phloem cells:

      • Sieve plates for substance movement.

      • Companion cells provide energy.

Cell Differentiation (1.1.4)

  • Stem cells differentiate by switching genes on/off to produce specific proteins.

  • Animal cells differentiate early and lose the ability.

  • Plant cells retain the ability to differentiate throughout life.

Microscopy (1.1.5)

  • Microscopes enlarge images of small structures.

  • Light Microscope:

    • Uses objective and eyepiece lenses.

    • Max magnification: x2000.

    • Resolving power: 200nm.

    • Views tissues, cells, and large subcellular structures.

  • Electron Microscope:

    • Uses electrons to form images.

    • Scanning EM (SEM): 3D images.

    • Transmission EM (TEM): 2D images.

    • Magnification: up to x2,000,000.

    • Resolving power: 10nm (SEM), 0.2nm (TEM).

  • Calculations:

    • Object size = image size / magnification

Culturing Microorganisms (1.1.6 - Biology Only)

  • Microorganisms are cultured in labs using nutrients.

    • Culture medium contains carbohydrates, minerals, proteins, and vitamins.

  • Methods:

    • Nutrient broth solution: Bacteria suspension in sterile broth, shaken regularly.

    • Agar gel plate: Bacteria form colonies on agar surface.

  • Agar Plate Preparation:

    • Sterilize agar jelly and Petri dish.

    • Spread microorganism using inoculating loops.

    • Tape lid and incubate upside down.

  • Reasons for Steps:

    • Sterilization: Prevents contamination.

    • Sealing (partially): Prevents airborne contamination but allows oxygen.

    • Upside down storage: Prevents condensation disrupting growth.

    • Incubation at 25 degrees: Prevents growth of harmful bacteria.

  • Binary Fission:

    • Bacteria multiply by binary fission (every 20 minutes).

    • Formula: bacteria at beginning x 2number of divisions2^{\text{number of divisions}} = bacteria at end

    • Number of divisions = time / mean division time.

  • Testing Antibiotics:

    • Paper discs with antibiotics on agar plate with bacteria.

    • Inhibition zone size indicates antibiotic effectiveness.

    • Control disc with sterile water.

  • Calculations:

    • Cross-sectional area of colonies/inhibition zones: πr2πr^2

Cell Division (1.2)

  • Chromosomes (1.2.1)

    • Nucleus contains genetic information in chromosomes.

    • Genes are sections of DNA coding for proteins.

    • 23 pairs of chromosomes (46 total) in each cell, except gametes (23).

  • Mitosis and the Cell Cycle (1.2.2)

    • Cell cycle stages:

      • Interphase: Growth, organelle duplication, DNA replication.

      • Mitosis: Chromosomes align and are pulled apart.

      • Cytokinesis: Two identical daughter cells form.

    • Importance: Growth, development, repair, asexual reproduction.

Stem Cells (1.2.3)

  • Stem cells: Undifferentiated cells that divide and differentiate.

  • Types:

    • Embryonic stem cells:

      • From zygotes.

      • Can differentiate into any cell type.

    • Adult stem cells:

      • In bone marrow, form many cell types.

    • Meristems in plants:

      • In root and shoot tips.

      • Can differentiate into any plant cell type.

  • Therapeutic Cloning:

    • Embryo with patient's genes, used for stem cells.

    • Advantage: No rejection.

  • Benefits vs Problems:

    • Benefits: Replace damaged parts, research into differentiation.

    • Problems: Ethical concerns, embryo destruction, contamination.

Transport in Cells (1.3)

  • Diffusion (1.3.1)

    • Movement of particles from high to low concentration.

    • Passive (no energy required).

    • Small molecules move across cell membranes e.g., oxygen, glucose.

    • Examples: Gas exchange in lungs, urea removal.

  • Factors Affecting Rate:

    • Concentration gradient: Higher difference = faster diffusion.

    • Temperature: Higher = faster diffusion.

    • Surface area: More area = faster diffusion.

  • Surface Area to Volume Ratio:

    • High ratio: Sufficient diffusion.

    • Low ratio: Requires adaptations.

  • Adaptations for Diffusion:

    • Large surface area: Alveoli, villi, gill lamellae, flattened leaves.

    • Thin membrane: Alveoli and capillary walls.

    • Efficient blood supply/ventilation: Steep concentration gradient.

Osmosis (1.3.2)

  • Movement of water from high to low water potential through a partially permeable membrane.

  • Passive (no energy required).

  • Dilute solution: high water potential.

  • Concentrated solution: low water potential.

  • Effects:

    • Animal cells: Swell and burst in dilute solution; shrivel in concentrated solution.

    • Plant cells: Become turgid in dilute solution; plasmolysis in concentrated solution.

  • Experiment:

    • Potato tubers in sugar solutions: Mass changes indicate water movement.

Active Transport (1.3.3)

  • Movement of particles from low to high concentration (against gradient).

  • Requires energy from respiration.

  • Examples:

    • Root hairs: Mineral ions from soil.

    • Gut: Glucose and amino acids into bloodstream.