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Cells and Tissues: Structure, Function, and Repair

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Cells and Tissues

Plasma/Cell Membrane

The plasma membrane is a dynamic structure that separates the cell from its external environment and regulates the movement of substances in and out of the cell.

  • Fluid Mosaic Model: The plasma membrane is described by the fluid mosaic model, which depicts it as a flexible bilayer of lipids with embedded proteins.

  • Lipid Bilayer: Composed of phospholipids with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

  • Membrane Proteins:

    • Integral proteins: Span the membrane and are involved in transport and signaling.

    • Peripheral proteins: Attached to the surface and play roles in cell signaling and structural support.

Specializations of Plasma Membrane

  • Microvilli: Increase surface area for absorption.

  • Membrane Junctions:

    • Tight junctions: Prevent leakage of extracellular fluid.

    • Desmosomes: Anchor cells together.

    • Gap junctions: Allow communication between cells.

Plasma Membrane Functions

The plasma membrane controls the movement of substances and facilitates communication between cells.

  • Membrane Transport: Selective permeability allows only certain molecules to pass.

  • Passive Processes:

    • Diffusion: Movement of molecules from high to low concentration.

    • Facilitated Diffusion: Transport of substances via membrane proteins.

    • Osmosis: Diffusion of water across a membrane.

      • Osmotic Pressure: Pressure exerted by water movement.

      • Tonicity: Effect of solution on cell volume.

        • Isotonic: No net water movement.

        • Hypotonic: Water enters cell; cell may swell.

        • Hypertonic: Water leaves cell; cell shrinks.

    • Filtration: Movement of water and solutes due to hydrostatic pressure.

  • Active Processes:

    • Active Transport: Movement against concentration gradient using energy (ATP).

    • Vesicular Transport:

      • Exocytosis: Release of substances from cell.

      • Endocytosis: Uptake of substances into cell.

        • Phagocytosis: "Cell eating" of large particles.

        • Pinocytosis: "Cell drinking" of fluids.

        • Receptor-mediated endocytosis: Specific uptake via receptors.

  • Other Membrane Functions:

    • Membrane Potential: Electrical charge difference across membrane.

    • Communication: Contact signaling, electrical, and chemical signaling between cells.

Cytoplasm

Definition and Components

The cytoplasm is the cellular material between the plasma membrane and the nucleus, containing organelles and cytosol.

  • Cytosol: Fluid portion containing dissolved substances.

  • Organelles: Specialized structures performing specific functions.

Organelles of the Cytoplasm

  • Mitochondria: Site of ATP production (cellular respiration).

  • Ribosomes: Sites of protein synthesis.

  • Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes; synthesizes proteins.

    • Smooth ER: Synthesizes lipids and detoxifies chemicals.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Contain digestive enzymes for breakdown of waste.

  • Peroxisomes: Detoxify harmful substances.

  • Cytoskeleton: Provides structural support and facilitates movement.

  • Centrioles: Involved in cell division.

  • Cilia: Specialized for movement in certain cells.

The Nucleus

Definition and Structure

The nucleus is the control center of the cell, containing genetic material (DNA).

  • Location: Usually central within the cell.

  • Composition:

    • Nucleoplasm: Fluid inside the nucleus.

    • Nuclear Envelope: Double membrane with pores.

    • Nucleoli: Sites of ribosome synthesis.

    • Chromatin: DNA and associated proteins.

Cell Growth and Reproduction: Mitosis

The Life Cycle of the Cell

Cells undergo a cycle of growth, DNA replication, and division to produce new cells.

  • Interphase: Period of cell growth and DNA replication.

    • G1 phase: Cell growth.

    • S phase: DNA synthesis.

    • G2 phase: Preparation for division.

  • DNA Replication:

    • Complementary Base Pairing: Adenine pairs with Thymine, Cytosine with Guanine.

    • Double Helix: DNA structure described by Watson and Crick.

    • Semi-Conservative Process: Each new DNA molecule contains one old and one new strand.

    Equation:

  • Cell Division Events:

    • Mitosis (M phase):

      • Prophase: Chromatin condenses into chromosomes.

      • Metaphase: Chromosomes align at the cell equator.

      • Anaphase: Sister chromatids separate.

      • Telophase: Nuclear envelopes reform.

    • Cytokinesis: Division of cytoplasm to form two daughter cells.

Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions.

  • Three Forms of RNA:

    • mRNA (messenger RNA): Carries genetic code from DNA to ribosome.

    • tRNA (transfer RNA): Brings amino acids to ribosome.

    • rRNA (ribosomal RNA): Forms part of ribosome structure.

  • Steps of Protein Synthesis:

    • Transcription: DNA is transcribed into mRNA in the nucleus.

    • Translation: mRNA is translated into a polypeptide chain at the ribosome.

Tissues

Overview of Tissues

Tissues are groups of cells with similar structure and function. There are four primary types:

  • Epithelial Tissue

  • Connective Tissue

  • Muscle Tissue

  • Nervous Tissue

Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands.

  • Locations:

    • Body surfaces (skin)

    • Internal cavities (lining of digestive tract)

  • Functions:

    • Protection

    • Absorption

    • Filtration

    • Excretion

    • Secretion

    • Sensory reception

  • Characteristics:

    • Cellularity: Composed almost entirely of cells.

    • Specialized contacts: Cells joined by junctions.

    • Polarity: Apical and basal surfaces.

    • Basement membrane: Supports epithelium.

    • Innervated but avascular: Has nerves but no blood vessels.

    • Regeneration: Rapidly replaces lost cells.

  • Classification:

    • First part: Number of cell layers

      • Simple (one layer)

      • Stratified (two or more layers)

      • Pseudostratified (false layers)

    • Second part: Shape of cells

      • Squamous (flat)

      • Cuboidal (cube-shaped)

      • Columnar (tall)

  • Types of Epithelium:

    • Simple squamous

    • Simple cuboidal

    • Simple columnar

    • Pseudostratified columnar

    • Stratified squamous

    • Transitional

  • Glandular Epithelium:

    • Endocrine: Secretes hormones into blood.

    • Exocrine: Secretes products into ducts.

      • Unicellular (e.g., goblet cells)

      • Multicellular (e.g., sweat glands)

Connective Tissue Proper

Major Functions of Connective Tissue

Connective tissue supports, protects, and binds other tissues together.

  • Binding and support

  • Protection

  • Insulation

  • Transportation of substances (blood)

Characteristics of Connective Tissue

  • Common origin: Derived from mesenchyme.

  • Extracellular matrix: Composed of ground substance and fibers.

Structural Elements of Connective Tissue

  • Ground Substance (Matrix): Fills space between cells and fibers.

  • Fibers:

    • Collagen: Provides strength.

    • Elastic: Provides elasticity.

    • Reticular: Supports soft tissue.

  • Cells: Fibroblasts, adipocytes, immune cells, etc.

Types of Connective Tissue

  • Loose Connective Tissue:

    • Areolar: Supports and binds other tissues.

    • Adipose: Stores fat.

    • Reticular: Supports lymphoid organs.

  • Dense Connective Tissue:

    • Dense regular: Tendons and ligaments.

    • Dense irregular: Dermis of skin.

  • Cartilage:

    • Matrix: Firm but flexible.

    • Cells: Chondrocytes.

    • Types:

      • Hyaline: Most common, supports and reinforces.

      • Elastic: Maintains shape, flexible.

      • Fibrocartilage: Strong, resists compression.

  • Bone: Rigid support and protection.

  • Blood: Transports nutrients, gases, and wastes.

Muscle Tissue

Structural Overview

Muscle tissue is specialized for contraction and movement.

  • Skeletal Muscle: Voluntary movement, striated.

  • Cardiac Muscle: Heart muscle, involuntary, striated.

  • Smooth Muscle: Walls of hollow organs, involuntary, non-striated.

Nervous Tissue

Nervous tissue is specialized for communication via electrical and chemical signals.

  • Neurons: Transmit impulses.

  • Neuroglia: Support and protect neurons.

Tissue Repair

Regeneration and Fibrosis

Tissue repair restores structure and function after injury. It occurs by regeneration (replacement with same tissue) or fibrosis (replacement with scar tissue).

  • Regeneration: Replacement of destroyed tissue by the same kind of cells.

  • Fibrosis: Replacement by dense connective tissue (scar formation).

Factors That Affect Repair

  • Type of tissue

  • Severity of injury

  • Nutrition

  • Blood supply

  • Health

Additional info: Where the original notes were incomplete, standard academic context and definitions have been added for clarity and completeness.

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