Week 2 - Cells and Tissue

Introduction to the Cellular Level of Organisation

• Cell Biology: The scientific study of cells.

• The Cell: The cell serves as the basic structural and functional unit of the body. Cellular structure and function form the foundation for higher levels of organisation including tissues, organs, and organ systems.

• Three Main Parts of Human Cells:

  • Plasma Membrane: Forms the outer boundary of the cell and regulates the movement of substances into and out of the cell.

  • Cytoplasm: Contains the cytosol and various organelles required for cellular functions.

  • Nucleus: Contains the cell's DNA and acts as the control center for many cellular activities.

Comparison of Prokaryotic and Eukaryotic Cells

• Prokaryotic Cells:

  • Nucleus: No true nucleus present.

  • DNA: Circular DNA orientation.

  • Organelles: No membrane-bound organelles.

  • Size: Smaller cells.

  • Examples: Bacteria.

• Eukaryotic Cells:

  • Nucleus: True nucleus present.

  • DNA: Linear chromosomes.

  • Organelles: Membrane-bound organelles are present.

  • Size: Larger and more complex cells.

  • Examples: Human, animal, and plant cells.

Structure and Functions of Key Eukaryotic Organelles

• General Purpose: Organelles are specialised internal structures that perform specific functions to support cellular activity and complex processes such as energy production and protein synthesis.

• Nucleus: Contains DNA and controls cellular activities.

• Mitochondria: Produces ATP and generates cellular energy.

• Ribosomes: Acts as the primary site of protein synthesis.

• Golgi Apparatus: Modifies, packages, and transports proteins.

• Rough Endoplasmic Reticulum (RER): Synthesises and transports proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesises lipids and detoxifies chemicals.

• Lysosomes: Contain digestive enzymes used to break down wastes and damaged cell parts.

Structure of the Plasma Membrane

• Nature: A flexible boundary surrounding the cell.

• The Lipid Bilayer: The basic structural framework consisting of three main lipid types:

  • Phospholipids: Comprise approximately $75\,$ of the membrane lipids.

  • Cholesterol: Comprises approximately $20\,$ and assists in maintaining membrane stability.

  • Glycolipids: Comprise approximately $5\,$ and contribute to cell recognition.

• Amphipathic Nature: Membrane lipids contain both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Membrane Proteins:

  • Integral Proteins: Embedded within the lipid bilayer, often spanning the entire membrane.

  • Peripheral Proteins: Loosely attached to either the inner or outer surface of the membrane.

• Glycocalyx: Formed by the carbohydrate chains of glycoproteins (proteins with carbohydrate groups) and glycolipids. It enables cell recognition and supports cell adhesion.

Functions of Membrane Proteins

• Ion Channel (Integral): Forms a pore through which a specific ion can flow across the membrane. Most membranes have specific channels for common ions.

• Carrier / Transporters (Integral): Transports specific substances by undergoing a change in shape (e.g., transporting amino acids needed for protein synthesis).

• Receptor (Integral): Recognises a specific ligand and alters the cell's function. For instance, antidiuretic hormone binds to kidney receptors to change water permeability.

• Enzyme (Integral and Peripheral): Catalyses reactions inside or outside the cell (e.g., lactase in the small intestine).

• Linker (Integral and Peripheral): Anchors filaments inside and outside the plasma membrane to provide structural stability and shape; can participate in cell movement or linking two cells.

• Cell Identity Marker (Glycoprotein): Distinguishes personal cells from those of others (e.g., Major Histocompatibility (MHC) proteins).

Transport Across the Plasma Membrane

• Passive Transport: Does not require cellular energy (ATP). Substances move down their concentration gradient.

  • Simple Diffusion: Movement of molecules directly through the lipid bilayer (e.g., $O_2$ and $CO_2$).

  • Facilitated Diffusion: Movement across the membrane with the assistance of carrier or channel proteins (e.g., glucose transport).

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Requires energy, usually in the form of ATP.

  • Primary Active Transport: Uses energy directly from ATP to move substances (e.g., $Na^+$, $K^+$, $Ca^{2+}$, $H^+$, $Cl^-$ ions).

  • Secondary Active Transport: Uses energy from ion gradients created by primary active transport (includes antiport and symport systems).

  • Transport in Vesicles: Substances move in or out via vesicles budding from the membrane.

    • Endocytosis: Movement into the cell.

    • Exocytosis: Movement out of the cell.

    • Phagocytosis: "Cell eating."

    • Transcytosis: Movement through the cell.

Clinical Connections: Tonicity and Chemicals

• Tonicity in Red Blood Cells:

  • Isotonic: Balance of water.

  • Hypertonic: Solutions cause cells to lose water and shrink (crenation).

  • Hypotonic: Solutions cause cells to gain water and swell, potentially leading to cell rupture.

• Blue-ringed Octopus Envenomation: The venom contains tetrodotoxin, which blocks sodium ($Na^+$) ion channels in nerve cell membranes. This prevents nerve impulse transmission, leading to paralysis and respiratory failure.

• Free Radicals: Unstable molecules with unpaired electrons that damage lipids, proteins, and nucleic acids. Produced during metabolism (mitochondria) or external sources (pollution, radiation).

  • Antioxidants: Neutralise free radicals. Examples: Vitamin E, Vitamin C, beta carotene, zinc, selenium.

• Biopsy: Removal of a small sample of living tissue for microscopic examination to diagnose diseases like cancer or identify infections.

Cell Division: Mitosis versus Meiosis

• Somatic Cell Division (Mitosis):

  • Occurs in: Somatic cells (all cells except germ cells).

  • Function: Replaces dead/injured cells and adds new cells during growth.

  • Result: Two genetically identical "daughter" cells.

  • Components: Division of the nucleus (mitosis) and division of the cytoplasm (cytokinesis).

• Reproductive Cell Division (Meiosis):

  • Occurs in: Germ cells (gametes and their precursors).

  • Function: Producing gametes (eggs and sperm).

  • Process: Parent cell is diploid ($2n = 46$ chromosomes or $23$ pairs). Two cycles of division occur with one cycle of chromosome duplication.

  • Result: Four genetically distinct daughter cells that are haploid ($n = 23$ chromosomes).

  • Genetic Exchange: Occurs between chromosome pairs during the process.

Cell Death Mechanisms: Necrosis and Apoptosis

• Situations for Cell Death: Aging/replacement, destruction of abnormal cells, damage, or injury/disease.

• Necrosis (Uncontrolled Cell Death):

  • Nature: Traumatic death.

  • Process: The cell lyses (bursts), and contents are released.

  • Result: Causes an inflammatory response in surrounding tissue.

  • Causes: Extreme conditions (heat) or interaction with toxins/invading organisms.

• Apoptosis (Controlled Cell Death):

  • Nature: Programmed/controlled shut down.

  • Process: Cell is killed from the inside out; cell shrinks, membrane "blebs" off, and then collapses. No contents are released uncontrollably.

  • Uses: Maintaining balance with new cell generation, removing unneeded fetal cells, and eliminating abnormal/cancer cells.

Tissues: The Four Main Types

• Definition of Tissue: A collection of similar cells that together carry out a specific function.

• 1. Epithelial Tissue:

  • Structure: Tightly packed cells in continuous sheets (single or multiple layers).

  • Properties: Has polarity with an apical surface (exposed) and a basal surface (attached to basement membrane). It is avascular (no blood supply) but innervated (has nerves).

  • Classification: By shape (squamous, cuboidal, columnar) and arrangement of layers.

  • Types:

    • Covering and Lining: Skin, blood vessels, respiratory/digestive/urinary/reproductive interiors.

    • Glandular: Involved in secretion. Includes Endocrine (ductless, secretes hormones into blood) and Exocrine (has ducts, secretes sweat/mucus/sebum/enzymes to surfaces).

• 2. Connective Tissue:

  • Structure: Consists of widely spaced cells and an extracellular matrix (ground substance + protein fibres like collagen). Includes substances like hyaluronic acid and chondroitin sulphate.

  • Properties: Highly vascular (with exceptions) and innervated. Can bear weight and withstand trauma.

  • Types:

    • Loose: Areolar (packing material), Adipose (fat storage/BAT for heat), Reticular.

    • Dense: Regular (tendons/ligaments), Irregular (dermis/fasciae), Elastic (lung tissue/aorta).

    • Cartilage: Hyaline (glassy/support), Fibrocartilage (strongest/intervertebral discs), Elastic (ear).

    • Bone: Supports soft tissue and works with muscle for movement.

    • Liquid: Blood (plasma matrix) and Lymph.

• 3. Muscle Tissue:

  • Structure: Elongated cells (myocytes) containing myofilaments (actin and myosin).

  • Types:

    • Skeletal: Long, striated, multinucleated, voluntary control.

    • Cardiac: Branched, striated, single nucleus, intercalated discs (gap junctions/desmosomes), involuntary.

    • Smooth: Spindle-shaped, non-striated, involuntary, found in hollow organs (intestines, bladder).

• 4. Nervous Tissue:

  • Function: Responds to stimulus and transmits electrical impulses.

  • Composition: Neurons (conductive cells) and Neuroglia (support cells).

Tissue Repair Capacity

• Epithelial Tissue: Continuous capacity for renewal.

• Connective Tissue: Bone has high renewal; cartilage has much lower capacity.

• Muscle Tissue: Poor capacity for renewal as cells generally do not divide.

• Nervous Tissue: Poorest capacity; neurons normally do not undergo mitosis to replace damage.

Questions & Discussion

• Question: If all humans have the same $23$ pairs of chromosomes and nearly the same genes, why do individuals show so much variation in appearance, physiology, and disease susceptibility?

• Lecture Survival Check: "Which type of cell death is happening to your brain right now?"

  • Apoptosis: Calm, controlled shutdown.

  • Necrosis: Everything exploding from too much information.