AQA Biology GCSE Topic 1: Cell Biology

Cell Structure (1.1)

• Two main types of cells:
  • Eukaryotes:
  • Found in animals and plants.
  • Structures include:
    • Cell membrane
    • Cytoplasm
    • Nucleus containing DNA
  • Prokaryotes:
  • Found in bacterial cells, usually smaller.
  • Structures include:
    • Cell wall
    • Cell membrane
    • Cytoplasm
    • A single circular strand of DNA and plasmids (small rings of DNA).
• Organelles: Structures in cells with specific functions.

Orders of Magnitude

• Use orders of magnitude to compare cell sizes.
  • 10 times bigger: $10^1$ times bigger.
  • 1000 times bigger: $10^3$ times bigger.
  • 10 times smaller: $10^{-1}$ times smaller.
• Prefixes:
  • Centi: $0.01$
  • Milli: $0.001$
  • Micro: $0.000001$
  • Nano: $0.000000001$

Animals and Plants (1.1.2)

• Subcellular Structures: Functionality in animal and plant cells.
  • Nucleus:
  • Contains DNA for protein coding.
  • Enclosed in a nuclear membrane.
  • Cytoplasm:
  • Site of chemical reactions.
  • Contains enzymes that act as biological catalysts.
  • Cell Membrane:
  • Regulates entry and exit of substances.
  • Mitochondria:
  • Site for aerobic respiration; produces energy.
  • Ribosomes:
  • Site for protein synthesis, usually on rough endoplasmic reticulum.

Plant Cells Specific Structures:

• Chloroplasts: Photosynthesis occurs; contains chlorophyll for light absorption.
• Permanent Vacuole: Contains cell sap; aids cell rigidity.
• Cell Wall: Composed of cellulose; provides strength.

Bacterial Cells

• Structure and Function Review:
  • Made of peptidoglycan cell wall.
  • Circular DNA in the cytoplasm.
  • Plasmids present.

Cell Specialisation (1.1.3)

• Differentiation: Process where cells develop new structures for specialized functions.
  • Stem cells can differentiate throughout life.
  • Most animal cells differentiate only during early development.

Examples of Specialised Cells:

1. Sperm Cells:
   • Streamlined for swimming; many mitochondria for energy; acrosome for penetrating egg membranes.
2. Nerve Cells:
   • Long axons for signal transmission; numerous dendrites for connections; mitochondria for synthesizing neurotransmitters.
3. Muscle Cells:
   • Specialized for contraction; contain proteins myosin and actin; high energy needs met by mitochondria.

Plant Specialised Cells:

1. Root Hair Cells:
   • Large surface area for absorption of water and nutrients.
2. Xylem Cells:
   • Hollow tubes formed by dead cells to transport water.
3. Phloem Cells:
   • Transport photosynthesis products; form sieve plates for transport continuity.

Microscopy (1.1.5)

• Light Microscope:
  • Uses two lenses; maximum magnification of $x2000$; resolving power typically $200nm$.
• Electron Microscope:
  • Uses electrons; two types:
  • Scanning Electron Microscope (SEM): 3D images, $x2,000,000$ magnification, $10nm$ resolving power.
  • Transmission Electron Microscope (TEM): 2D images, $0.2nm$ resolving power.
• Calculations:
  • Magnification formula: eyepiece magnification x objective magnification.
  • Size of object: size of image / magnification.

Culturing Microorganisms (1.1.6)

• Nutrient media needed for growth.
• Growing methods:
  1. In nutrient broth solution.
  2. On agar gel plates (colonies formed on the surface).

Steps and Rationale

• Sterilization is critical to prevent contamination.
• Incubation at $25^ ext{o}C$ to limit harmful bacteria growth.
• Zone of Inhibition Measurement: Determines antibiotic effectiveness on bacterial growth.

Cell Division (1.2)

• Chromosomes: Genetic material in nucleus; 23 pairs (46 total) in body cells; gametes have 23.

Mitosis and the Cell Cycle:

1. Interphase: Cell grows, organelles increase, DNA replicated.
2. Mitosis: Chromosomes pulled apart.
3. Cytokinesis: Division forms two identical daughter cells.

Stem Cells (1.2.3)

• Types:
  1. Embryonic Stem Cells: Can differentiate into any cell type.
  2. Adult Stem Cells: Limited differentiation capabilities.
  3. Plant Meristems: Continuous differentiation possibilities.

Active Transport (1.3.3)

• Movement of substances against their concentration gradient; requires energy.
• Examples:
  • Root hair cells uptake minerals.
  • Glucose and amino acids from gut to blood.