AB

AP Biology Unit Review: Cell Communication, Cell Cycle, and Cell Structure

Cell Communication

Signal Transduction Pathways

  • A signal transduction pathway is a series of molecular events triggered when a signaling molecule binds to a receptor, leading to a specific cellular response.

  • Example: Yeast mating—Yeast cells of opposite types (a and α) release signaling molecules that bind to receptors, initiating a cascade that allows them to fuse into a diploid cell.

Types of Cell Signaling

Type

Example

Paracrine (local)

Growth factors stimulate nearby cells to grow and divide.

Synaptic (local)

Neurotransmitters (e.g., acetylcholine) transmit signals across synapses.

Endocrine (long-distance)

Hormones travel via the bloodstream to distant target cells.

Three Stages of Signal Transduction

  1. Reception – A signaling molecule (ligand) binds to a receptor.

  2. Transduction – The signal is relayed and amplified (e.g., phosphorylation cascades).

  3. Response – The cell performs a specific action (e.g., gene expression, enzyme activation).

Types of Receptors

  • G-Protein Coupled Receptors (GPCRs) – Activate G-proteins, which relay signals via second messengers.

  • Receptor Tyrosine Kinases (RTKs) – Phosphorylate themselves and initiate a cascade.

  • Ion Channel Receptors – Open in response to ligand binding, allowing ions to flow (e.g., in nerve transmission).

Intracellular Receptors

  • Example: Testosterone – A steroid hormone that diffuses through the membrane, binds to an intracellular receptor, and regulates gene expression.

Regulation of Signal Transduction

  • Protein Kinases phosphorylate proteins (activate).

  • Protein Phosphatases remove phosphates (deactivate).

  • Amplification – A single ligand can activate many molecules, increasing efficiency.

Cell Cycle

Phases of the Cell Cycle

  1. Interphase (90% of the cycle)

    • G1 (Gap 1) – Cell grows and prepares for DNA replication.

    • S (Synthesis) – DNA replication occurs.

    • G2 (Gap 2) – Cell prepares for mitosis, fixing replication errors.

    • G0 (Resting phase) – Cells that stop dividing enter this phase (e.g., nerve cells).

  2. Mitotic Phase (M Phase)

    • Mitosis: Division of the nucleus.

      • Prophase – Chromosomes condense, spindle forms.

      • Metaphase – Chromosomes align at the center.

      • Anaphase – Sister chromatids separate.

      • Telophase – Nuclear envelopes re-form.

    • Cytokinesis – Cytoplasm divides, forming two identical daughter cells.

Regulation of the Cell Cycle

  • Checkpoints (G1, G2, M)

    • Ensure the environment and DNA integrity are suitable for division.

    • Malfunctioning checkpoints can lead to cancer (uncontrolled cell division).

Cancer and the Cell Cycle

  • Cancer cells lack normal regulation, leading to rapid and uncontrolled division.

  • HeLa Cells: The immortal cells from Henrietta Lacks, used in medical research without consent, highlighting ethical concerns in science.

Cell Structure

Cell Theory

  1. All living things are made of cells.

  2. Cells come only from pre-existing cells (biogenesis).

  3. Cells are the basic units of life.

Prokaryotic vs. Eukaryotic Cells

Feature

Prokaryotic Cells

Eukaryotic Cells

Nucleus

No nucleus

Has a nucleus

Organelles

No membrane-bound organelles

Membrane-bound organelles

Examples

Bacteria, Archaea

Plants, Animals, Fungi, Protists

Key Organelles & Their Functions

  • Cell Membrane – A phospholipid bilayer regulating transport (fluid mosaic model).

  • Nucleus – Contains DNA; directs cell functions.

  • Mitochondria – Site of cellular respiration (powerhouse of the cell).

  • Endoplasmic Reticulum (ER)

    • Rough ER: Has ribosomes; protein synthesis.

    • Smooth ER: Makes lipids; detoxifies.

  • Golgi Apparatus – Modifies, packages, and ships proteins.

  • Lysosomes – Break down waste; contain digestive enzymes.

  • Cytoskeleton – Maintains shape, helps in movement (microtubules & microfilaments).

  • Chloroplasts (Plant cells) – Site of photosynthesis.

  • Cell Wall (Plants & Bacteria) – Provides structure and support.

Cell Membrane & Transport

Membrane Structure

  • Composed of phospholipids (hydrophilic heads, hydrophobic tails).

  • Proteins (integral & peripheral) help in transport.

  • Cholesterol maintains fluidity.

Types of Transport

Type

Description

Example

Passive Transport

No energy required; moves down the gradient

Diffusion, Osmosis

Facilitated Diffusion

Uses transport proteins; still passive

Glucose transport

Active Transport

Requires ATP; moves against the gradient

Sodium-Potassium Pump

Endocytosis

Cell engulfs large molecules

Phagocytosis, Pinocytosis

Exocytosis

Cell releases substances

Neurotransmitter release

Water Potential & Osmosis

  • Ψ (Psi) = Ψs + Ψp

    • Ψs = -iCRT

    • i = Ionization constant

    • C = Concentration

    • R = Pressure constant

    • T = Temperature (Kelvin)

  • Osmosis: Water moves from high to low water potential.

  • Tonicity:

    • Hypotonic – Water enters the cell (can burst).

    • Hypertonic – Water leaves the cell (shrivels).

    • Isotonic – No net water movement.

Cell Communication & Quorum Sensing

  • Quorum Sensing: Bacteria release autoinducers to coordinate group behaviors (e.g., forming biofilms).

  • Neurotransmitters are released via exocytosis in neurons.

  • Hormones (Endocrine signaling) travel through blood to distant cells.

  • Transmembrane Proteins relay signals across the membrane.

Examples of Cell Communication

  • Adrenaline (Epinephrine)

    • Hormone & neurotransmitter; regulates fight-or-flight response.

  • Insulin

    • Regulates glucose uptake via receptor-mediated endocytosis.

Final Notes

  • Thermodynamics & Cells: Cells require energy to maintain order (entropy increases without energy input).

  • Bioethics: Henrietta Lacks’ story highlights the importance of consent in medical research.