GCSE BIOLOGY OCR GATEWAY - Google Docs

B1.1 Cell Structures: Plant & Animal Cells

Differences between Prokaryotic and Eukaryotic Cells

• Prokaryotic Cells

  • No nucleus

  • No mitochondria

  • Genetic material floats in cytoplasm

  • Simple and smaller than eukaryotic cells

  • Size: 1-10 micrometers

  • Example: Bacterial cells

• Eukaryotic Cells

  • Genetic material contained in the nucleus

  • Complex and relatively larger

  • Size: 10-100 micrometers

  • Examples: Plant and Animal cells

Structures of Eukaryotic Cells

• Nucleus

  • Controls activities of the cell.

  • Arranged into chromosomes containing instructions for cell/organism formation.

• Mitochondria

  • Site where respiration occurs, combining glucose and oxygen.

• Cell Membrane

  • Selective barrier controlling substance movement into and out of the cell.

  • Contains receptor molecules.

• Cytoplasm

  • Jelly-like substance for chemical reactions necessary for cell life.

Bacterial Cells

Examples of Prokaryotes

• E. coli - causes food poisoning

• Streptococcus - associated with sore throats

• Streptomyces - known for killing disease-causing bacteria

Features of a Prokaryotic Cell

• Cell Wall

  • Provides structure and protection.

• Cytoplasm

  • Jelly-like medium where chemical reactions occur.

• Cell Membrane

  • Regulates substance movement.

• Genetic Material

  • Floats in cytoplasm as a long strand of DNA (bacterial chromosome).

  • Typically circular.

Subcellular Structures of Bacteria

• Flagella

  • Tail-like structure for movement.

• Pili

  • Hairlike structures for attachment.

• Slime Capsule

  • Protects from dehydration, allows attachment.

• Plasmid

  • Circular DNA used to store additional genes.

Light Microscopy

Components of a Light Microscope

• Allows observation of small structures in detail through light.

• Objective lenses magnify the object on a slide placed on the stage.

Steps to Observe Cells

1. Move stage to lowest position.

2. Select lowest magnification objective lens.

3. Place slide with cells on the stage.

4. Raise stage carefully, avoiding lens contact.

5. Lower stage slowly with coarse focus until the object comes into view (initially blurred).

6. Use fine focus to achieve clarity.

7. Switch to a higher magnification lens for greater detail if needed.

Total Magnification Formula

• Total Magnification = Eyepiece Lens Magnification x Objective Lens Magnification

Why Stain Cells?

• Staining highlights specific subcellular structures.

Common Stains

• Methylene Blue - stains animal cell nuclei (appears blue).

• Iodine - stains plant cell nuclei.

• Crystal Violet - stains bacterial cell walls.

Electron Microscopy

Transmission Electron Microscope (TEM)

• Uses a beam of electrons to pass through a thin slice of sample, producing an image.

Scanning Electron Microscope (SEM)

• Provides 3D images by sending electrons across a specimen's surface.

Advantages of Electron Microscopes

• Higher magnification and resolution than light microscopes.

• Enhanced understanding of subcellular structures, such as chloroplasts.

B1.2 What Happens in Cells?

DNA

• Functions in the body:

  • Each DNA molecule (chromosome, 46 per cell) contains genes that code for specific proteins.

• Structure of DNA:

  • Composed of two strands (double helix).

  • Nucleotides form the structure, joined together, each containing:

    • Deoxyribose sugar

    • Phosphate group

    • Base

Transcription & Translation

• Messenger RNA (mRNA)

  • DNA can't leave the nucleus; a copy is made via transcription.

  • Single-strand of DNA copies itself, travels to ribosomes in the cytoplasm.

• Translation Process

  • mRNA binds to a ribosome; base triplets are read to form proteins:

    1. Ribosome reads mRNA triplets.

    2. Amino acids join to form a chain (protein).

Enzymes

• Enzymes are proteins acting as biological catalysts to speed up reactions without being consumed.

• Made up of amino acids; specificity lies in unique active sites (Lock and Key hypothesis).

B1.3 Respiration

Metabolic Rate

• Represents the speed of chemical reactions for energy transfer from food.

Carbohydrates, Proteins, & Lipids

• Carbohydrates:

  • Polymers made from smaller sugar units; broken down by carbohydrase enzymes (e.g., amylase breaks down starch).

• Proteins:

  • Formed from amino acids; sequence determines the synthesized protein.

• Lipids:

  • Composed of 3 fatty acids and glycerol, broken down by lipase into fatty acids and glycerol.

Aerobic Respiration

• Equation:Glucose + Oxygen -> Carbon Dioxide + WaterC6H12O6 + 6O2 -> 6CO2 + 6H2O (Exothermic)

• Takes place in mitochondria, transferring energy to ATP for cell functions like muscle contraction.

Anaerobic Respiration

• Laboratory Process: Conditions where oxygen is limited lead to anaerobic pathways, producing different byproducts e.g.:

  • Lactic acid in muscles.

  • In yeast: Ethanol and CO2 (Fermentation).Equation: Glucose -> Ethanol + Carbon DioxideC6H12O6 -> 2C2H5OH + 2CO2.

B1.4 Photosynthesis

Equation

• Photosynthesis Reaction:Carbon Dioxide + Water -> Glucose + Oxygen6CO2 + 6H2O -> C6H12O6 + 6O2

  • Occurs in chloroplasts using light to convert reactants into glucose.

Main Stages of Photosynthesis

1. Light-dependent reactions - water splits into oxygen and hydrogen.

2. Light-independent reactions - carbon dioxide combines to create glucose.

Testing for Starch

• Yellow/Brown: No starch

• Blue/Black: Yes starch

Factors Affecting Photosynthesis

Light Intensity

• Higher light intensity increases the rate until saturation is reached.

Carbon Dioxide

• Increased concentration speeds up the rate as CO2 is a key reactant.

Temperature

• Enzyme-controlled reaction; increased temperature enhances reaction rates until denaturation occurs at too high levels.

B2.1 Supplying the Cell

Diffusion

• Movement from high to low concentration, known as passive transport.

• Continues until equilibrium is reached.

Increasing Rate of Diffusion

• Decrease distance for travel.

• Increase concentration gradient.

• Increase surface area available for diffusion.

Osmosis

• Water diffusion across selectively permeable membranes.

• Water potential definitions: high potential means high water concentration.

In Plant Cells

• Water uptake creates turgor pressure making cells firm.

In Animal Cells

• Excess water can cause cells to swell and burst (lysis) or lose water leading to crenation.

Active Transport

• Movement against concentration gradients, requiring energy (ATP).

• Carrier proteins assist in moving molecules into cells.

• Example: Root hair cells taking up minerals against concentration gradients.

Mitosis

• Purpose: Cell division into two identical daughter cells.

Stages of Mitosis

1. DNA unzips to form two strands.

2. Nucleotides pair with exposed bases to form two DNA molecules.

3. Cytokinesis occurs, separating the cells.

Cell Differentiation

• Process where cells become specialized for specific functions.

Examples:

• Sperm Cell: Has flagellum and mitochondria for energy.

• Fat Cell: Large fat storage capacity.

Stem Cells

• Location: Undifferentiated cells found in embryos and some adult tissues.

  • Embryonic Stem Cells: Can differentiate into all cell types.

  • Adult Stem Cells: Limited differentiation capacity.

  • Plant Stem Cells: Found in meristems, capable of division.