VCE Biology - Unit 1: AOS1 How do cells function?

1. Classification and Fundamental Cell Theory
Criteria for Life (MRS GREEN)

To be classified as a living organism, a biological entity must typically demonstrate the following eight processes:

  • M (Movement): Self-initiated change in position or internal transport.

  • R (Respiration): Biochemical processes (aerobic or anaerobic) to convert chemical energy from nutrients into ATP.

  • S (Sensitivity): The ability to detect and respond to stimuli in the external or internal environment.

  • G (Growth): A permanent increase in size or cell number.

  • R (Reproduction): The production of offspring, either sexually or asexually.

  • E (Equilibrium): Homeostasis; maintaining a stable internal environment despite external changes.

  • E (Excretion): The removal of metabolic waste products (e.g., CO_{2}, urea).

  • N (Nutrition): Obtaining compounds necessary for energy and structural growth.

The Three Tenets of Cell Theory

  1. All living organisms are composed of one or more cells.

  2. Cells are the basic structural and functional units of life.

  3. All cells arise from pre-existing cells through cellular division.

2. Cellular Diversity: Prokaryotes vs. Eukaryotes
Prokaryotic Cells (Domains Archaea and Bacteria)

  • Structure: Lack a membrane-bound nucleus and specialized organelles. Genetic material is located in an irregular region called the nucleoid.

  • Genetic Material: Consists of a single, circular chromosome and small, independent circles of DNA called plasmids.

  • Size: Very small (typically 0.1-5.0 \mu m), allowing for rapid diffusion.

  • Cell Wall: Most bacteria possess a rigid cell wall composed of peptidoglycan.

  • Replication: Divide via binary fission, a rapid asexual process involving DNA replication and cytoplasm partitioning without spindle fibers.

Eukaryotic Cells (Domain Eukarya: Protists, Fungi, Plants, Animals)

  • Structure: Contain a distinct nucleus and various membrane-bound organelles that compartmentalize specific biochemical reactions.

  • Genetic Material: Multiple linear chromosomes stored within the double-membraned nuclear envelope.

  • Size: Significantly larger (10-100 \mu m).

  • Replication: Complex division involving mitosis (for somatic growth) and meiosis (for gamete production).

3. Detailed Organelle Functions

  • The Nucleus:

    • Encased in a double-layered nuclear envelope with nuclear pores that regulate mRNA and protein traffic.

    • The Nucleolus is a dense region inside responsible for synthesizing ribosomal RNA (rRNA).

  • Ribosomes:

    • Composed of two subunits (large and small) made of rRNA and proteins. They translate mRNA into polypeptide chains.

  • The Endomembrane System:

    • Rough ER (RER): Studded with ribosomes; site of folding and modifying proteins intended for secretion or membrane insertion.

    • Smooth ER (SER): Lacks ribosomes; synthesizes lipids (phospholipids, steroids) and detoxifies metabolic byproducts.

    • Golgi Apparatus: A series of flattened sacs (cisternae) that modify, sort, and package proteins into secretory vesicles. It has a 'cis' face (receiving) and a 'trans' face (shipping).

  • Mitochondria:

    • The 'powerhouse' of the cell. Inner membrane folds (cristae) increase surface area for the Electron Transport Chain. Contains its own circular DNA and 70S ribosomes, supporting the Endosymbiotic Theory.

  • Chloroplasts (Plants and Algae):

    • Contain thylakoid stacks (grana) where light-dependent reactions occur and the stroma for the Calvin Cycle. They capture light energy to synthesize glucose.

  • Lysosomes:

    • Spherical sacs containing hydrolytic (digestive) enzymes that perform autophagy (breaking down old organelles) and apoptosis (programmed cell death).

4. The Plasma Membrane: The Fluid Mosaic Model

The plasma membrane is a semi-permeable boundary consisting of several components:

  1. Phospholipid Bilayer: Formed by amphipathic molecules with hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails. This creates a barrier to most water-soluble substances.

  2. Proteins:

    • Integral/Transmembrane Proteins: Span the bilayer; act as transporters (channels/carriers).

    • Peripheral Proteins: Attached to the exterior/interior; used for signaling and structural support.

  3. Cholesterol: Embedded between phospholipids to regulate membrane fluidity (stabilizing it at high temperatures and preventing freezing at low temperatures).

  4. Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), acting as markers for cell-to-cell recognition and communication.

5. Mathematics of Cell Size: SA:V Ratio

As a cell grows in size, its volume increases much faster than its surface area. This limits the efficiency of nutrient intake and waste disposal.

  • Surface Area (SA): For a cube/prism: SA = 2(lw + lh + hw)

  • Volume (V): V = l \times w \times h

  • Ratio: \text{SA:V} = \frac{SA}{V}

Biological Significance:

  • A high SA:V ratio (found in small cells) ensures that the distance internal molecules must travel to reach the membrane is short, and the relative area for exchange is large.

  • Cells that must be large often develop specialized shapes, such as microvilli (folds) or flattened shapes, to artificially increase their surface area without significantly increasing volume.

6. Membrane Transport Mechanisms
Passive Transport (No ATP required)

  1. Simple Diffusion: Movement of small, non-polar molecules (e.g., O{2}, CO{2}) directly through the bilayer down a concentration gradient.

  2. Facilitated Diffusion: Movement of large or polar molecules (e.g., Glucose, Ions) via specific protein channels or carrier proteins.

  3. Osmosis: The net diffusion of free water molecules across a semi-permeable membrane from a region of low solute concentration to high solute concentration.

    • Tonicity:

      • Isotonic: Equal solute concentration; no net water movement.

      • Hypertonic: Higher solute concentration outside; water leaves the cell (cell shrinks/plasmolysis).

      • Hypotonic: Lower solute concentration outside; water enters the cell (cell swells/turgid).

Active Transport (Requires ATP)

  1. Protein-Mediated: Using ATP-powered pumps (like the Sodium-Potassium pump) to move solutes against their concentration gradient.

  2. Bulk Transport:

    • Exocytosis: Vesicles fuse with the plasma membrane to expel large quantities of materials (e.g., hormones).

    • Endocytosis: The membrane invaginates to take in materials. Includes Phagocytosis (cell eating/solids) and Pinocytosis (cell drinking/liquids).