OCR Biology A-Level Complete Revision Notes

Cell Structure and Microscopy

• Key Conversions: $1\,\text{mm} = 1000\,\mu\text{m}$, $1\,\mu\text{m} = 1000\,\text{nm}$.

• Microscopy Definitions:

  • Magnification: Number of times larger an image is compared to the actual specimen.

  • Resolution: Minimum distance between two points seen as separate entities.

  • Artefact: Structural detail caused by specimen processing (e.g., bubbles).

  • Differential Staining: Multiple stains used to highlight different cell structures.

• Calibration: Eyepiece graticule is a ruler-like scale; Stage micrometer is a slide with a known length used for calibration. Relationship: $\text{M1D1} = \text{M2D2}$.

• Organelles:

  • Ribosomes: Non-membrane bound structures for protein synthesis.

  • Cytoskeleton: Provides mechanical strength and movement; comprises microtubules (largest), intermediate filaments, and microfilaments (smallest).

  • Protein Secretion: Synthesised on ribosomes at rough ER \textrightarrow packaged into transport vesicles \textrightarrow move via cytoskeleton \textrightarrow Golgi apparatus (processing) \textrightarrow trans face \textrightarrow exocytosis.

• Cell Size: Prokaryotes ($0.1\,\mu\text{m} - 10\,\mu\text{m}$), Eukaryotes ($10\,\mu\text{m} - 100\,\mu\text{m}$).

• Endosymbiotic Theory: Mitochondria and chloroplasts were prokaryotes; evidenced by loops of DNA and prokaryote-sized ribosomes.

Biological Molecules

• Water: Polar molecule with uneven charge ($\delta^{+}$ hydrogen and $\delta^{-}$ oxygen). Forms up to $4$ hydrogen bonds. Supports life via high surface tension, polar solvent properties, and high density ($ice$ is less dense than $water$).

• Carbohydrates: Formula $\text{C}_x(\text{H}_2\text{O})_y$. Monomer: monosaccharides (alpha and beta glucose are isomers). Bond: Glycosidic bond.

  • Starch: Storage in plants ($1-4$ bonds, helical amylose, and branched amylopectin).

  • Glycogen: Storage in animals (highly branched $1-4$ and $1-6$ bonds).

  • Cellulose: Structural ($\beta$-glucose, alternate monomers flipped, linear microfibrils).

• Lipids: Macromolecules (not polymers) made of glycerol and fatty acids. Bond: Ester bond.

  • Triglycerides: One glycerol and $3$ fatty acids. Saturated (solid at room temperature) or unsaturated (liquid, contains $\text{C}=\text{C}$ bonds).

  • Phospholipids: Amphipathic (hydrophilic head, hydrophobic tail); form bilayers.

• Proteins: Polymers of amino acids joined by peptide bonds ($\text{CO-NH}$).

  • Structure Levels: Primary (sequence), Secondary ($\alpha$-helix/$\beta$-sheet, hydrogen bonds), Tertiary ($3\text{D}$ shape, disulfide/ionic/hydrogen bonds), Quaternary (multiple polypeptides).

  • Globular: Spherical, water-soluble (e.g., Haemoglobin, Insulin, Catalase).

  • Fibrous: Insoluble, structural (e.g., Collagen, Keratin, Elastin).

• Tests:

  • Starch: Iodine (blue-black).

  • Reducing Sugars: Benedict's reagent + heat (brick-red).

  • Lipids: Emulsion test (milky white).

  • Proteins: Biuret reagent (purple).

Nucleotides and Nucleic Acids

• Nucleotide Components: Phosphate group, pentose sugar (ribose/deoxyribose), nitrogenous base.

• Bases: Purines (Adenine, Guanine - double ring); Pyrimidines (Thymine, Cytosine, Uracil - single ring).

• DNA Structure: Antiparallel double helix ($3'$ to $5'$ and $5'$ to $3'$). Complementary pairing: $A-T$ ($2$ bonds), $C-G$ ($3$ bonds).

• Replication: Semiconservative. Helicase unzips; DNA polymerase forms phosphodiester bonds ($3'$ to $5'$ direction). Lagging strand forms Okazaki fragments joined by DNA ligase.

• Genetic Code: Triplet, non-overlapping, degenerate, and universal.

• ATP: Phosphorylated nucleotide ($adenine$, $ribose$, $3$ phosphates). High-energy phosphoanhydride bonds.

Enzymes

• Nature: Biological catalysts; globular proteins. Lower activation energy.

• Models: Lock and Key (active site perfectly matches substrate); Induced Fit (active site changes shape slightly).

• Factors: Temperature, $\text{pH}$, [Substrate], [Enzyme].

• $Q_{10}$ Equation: Rate doubles for every $10\,^{\circ}\text{C}$ rise.

• Inhibition:

  • Competitive: Binds to active site.

  • Non-competitive: Binds to allosteric site.

• Cofactors: Prosthetic groups (permanent) and coenzymes (temporary, e.g., $\text{Cl}^{-}$ for amylase).

Biological Membranes and Cell Cycle

• Fluid Mosaic Model: Fluid (moving phospholipids) and Mosaic (proteins/cholesterol components).

• Transport: Simple diffusion, facilitated diffusion (channel/carrier proteins), Osmosis ($\psi$ moves from high to low), and Active Transport (requires ATP).

• Cell Cycle: $G_1$ (growth), $S$ (DNA replication), $G_2$ (preparation), and $M$ (mitosis). Checkpoints ensure DNA integrity.

• Mitosis Stages: Prophase, Metaphase, Anaphase, Telophase. Result: $2$ genetically identical diploid cells.

• Meiosis: Two divisions ($I$ and $II$). Results in $4$ genetically different haploid gametes via independent assortment and crossing over.

• Stem Cells: Totipotent (any cell), Pluripotent (most cells), Multipotent (limited range).

Exchange Surfaces and Transport Systems

• Mammalian Exchange: Trachea \textrightarrow Bronchi \textrightarrow Bronchioles \textrightarrow Alveoli ($200 - 300\,\mu\text{m}$ diameter). Tidal volume is resting breath; Vital capacity is maximum intake.

• Fish: Countercurrent flow in gill lamellae maintains constant concentration gradient.

• Insects: Spiracles lead to tracheae and tracheoles. Tracheal fluid levels change during activity.

• Animal Transport: Open vs. Closed; Single vs. Double (human). Artery (thick walls), Vein (valves, large lumen), Capillary (one-cell thick).

• Cardiac Cycle: Atrial systole \textrightarrow Ventricular systole \textrightarrow Diastole. Myogenic contraction initiated by SAN (Sinoatrial node).

• Plant Transport: Xylem (dead cells, lignin, water/minerals); Phloem (living cells, sieve plates, translocation of assimilates).

• Xylem Pathway: Apoplast (cell walls), Symplast (cytoplasm via plasmodesmata). Casparian strip forces water into symplast.

• Transpiration: Driven by evaporation from stomata. Cohesion-Tension theory involves hydrogen bonding between water molecules.

Disease, Biodiversity, and Ecosystems

• Pathogens: Bacteria (toxins), Viruses (cell hijacking), Protoctista (cell digestion), Fungi (saprophytes/spores).

• Examples: Tuberculosis (Mycobacterium tuberculosis), Malaria (Plasmodium falciparum), HIV/AIDS (Virus).

• Immunity: Phagocytosis (Neutrophils/Macrophages). MHC (Major histocompatibility complex) presents antigens. T-cells (Thymus) and B-cells (Bone marrow).

• Biodiversity: Species richness (number of species) and evenness (relative abundance). Simpson’s Index measures biodiversity ($near\,1$ is high).

• Conservation: In-situ (habitat) and Ex-situ (zoos/seed banks).

• Classification: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Ecosystems: Efficiency at producer level is $1-3\%$. Energy transfer between consumers is approximately $10\%$. Net Production = Gross Production $-$ Respiratory Losses.

Questions & Discussion

• Question: What is the rule for genes with more than $2$ alleles?

• Answer: They are defined as polymorphic.

• Question: Why might an increase in core body temperature be harmful?

• Answer: It affects enzyme activity. If too high (e.g., $40\,^{\circ}\text{C}$), enzymes may become denatured as hydrogen bonds break, changing the active site shape and stopping metabolic reactions.

• Question: Describe the role of the adrenal gland cortex in the long-term response to stress.

• Answer: The cortex secretes steroid hormones such as cortisol and aldosterone. These stimulate the breakdown of fats and proteins into glucose, increase blood volume/pressure via sodium/water uptake in kidneys, and suppress the immune system.

• Question: Explain how stimulation of the adrenal gland's medulla causes an increase in glycogen to glucose conversion.

• Answer: The medulla secretes adrenaline which binds to receptors on cell membranes, activating adenylyl cyclase. This catalyses the production of cyclic AMP ($\text{cAMP}$) from $\text{ATP}$, activating a cascade of reactions to convert glycogen into glucose.

• Question: How do you calculate the Mitotic Index?

• Answer: $\frac{\text{number of cells with condensed chromosomes}}{\text{total number of cells}} \times 100$.