Cell Structure and Function

Membrane Composition

The cell membrane is a dynamic structure that separates the interior of the cell from its external environment. It is primarily composed of:

• Phospholipids: Form a bilayer, providing a semi-permeable barrier.

• Proteins: Embedded or attached to the membrane, serving as channels, receptors, or enzymes.

• Cholesterol: Stabilizes membrane fluidity and integrity.

Membrane Junctions

Cells are connected by specialized junctions:

• Tight Junctions: Seal adjacent cells, preventing passage of substances between them.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells via channels.

Membrane Transport

Substances move across the cell membrane by various mechanisms:

• Passive Transport: Does not require energy; moves substances down their concentration gradient.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement of larger or polar molecules via protein channels (e.g., glucose).

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

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Uses protein pumps (e.g., Na+/K+ pump).

• Vesicular Transport: Moves large particles or volumes via vesicles.

  • Endocytosis: Uptake of materials into the cell.

    • Phagocytosis: "Cell eating" of solids.

    • Pinocytosis: "Cell drinking" of liquids.

    • Receptor-mediated Endocytosis: Specific uptake using receptors.

  • Exocytosis: Release of substances from the cell.

Tonicity refers to the effect of a solution on cell volume:

• Isotonic: No net movement of water; cell size remains constant.

• Hypertonic: Water leaves the cell; cell shrinks.

• Hypotonic: Water enters the cell; cell swells.

Organelle Functions

• Mitochondria: Site of ATP production.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

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

• Lysosomes: Digest cellular waste.

• Ribosomes: Protein synthesis.

• Nucleus: Contains genetic material (DNA).

• Nucleolus: Produces ribosomal RNA.

• Cytoskeleton: Provides structural support and movement.

• Centrioles: Organize microtubules during cell division.

Parts of the Cytoskeleton

• Microfilaments: Actin filaments; involved in cell movement.

• Intermediate Filaments: Provide structural support; anchor organelles.

• Microtubules: Tubulin; assist with cell movement and mitosis; form centrioles, cilia, and flagella.

Cell Extensions

• Cilia: Move substances across cell surfaces.

• Flagella: Propel cells (e.g., sperm).

• Microvilli: Increase surface area for absorption.

Cell Cycle and DNA

Cell Cycle

• Interphase: Cell grows and DNA replicates.

• Mitosis: Division of the nucleus; four phases:

  1. Prophase: Chromosomes condense; spindle forms.

  2. Metaphase: Chromosomes align at the cell equator.

  3. Anaphase: Sister chromatids separate.

  4. Telophase: Nuclear envelope reforms; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm; results in two daughter cells.

DNA Replication

• Each new DNA molecule consists of one old and one new strand (semi-conservative replication).

• Base Pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

• Example: If one strand is ATG-CAG, the complementary strand is TAC-GTC.

Gene Definition

• A gene is a segment of DNA that codes for a specific protein.

Protein Synthesis

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

• Translation: mRNA is decoded by ribosomes to assemble proteins in the cytoplasm.

• Requirements for translation: mRNA, tRNA, ribosomes.

• Codon: Three-base sequence on mRNA; specifies an amino acid.

• Anticodon: Three-base sequence on tRNA; pairs with codon.

• Example: DNA sequence ATG-CAG transcribes to mRNA UAC-GUC (A pairs with U in RNA).

Tissues

Epithelial Tissue

• Structure: Apical surface (exposed) and basement membrane (attached).

• Named by layers and cell shape:

  • Simple: One layer.

  • Stratified: Multiple layers.

  • Squamous: Flat cells.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall cells.

• Locations:

  • Stratified squamous: Skin.

  • Simple squamous: Serous membranes.

  • Pseudostratified columnar: Respiratory tract.

  • Transitional: Urinary tract.

• Endocrine glands: Secrete hormones into blood.

• Exocrine glands: Secrete products onto surfaces or into ducts.

Connective Tissue

• Cells are surrounded by an extracellular matrix.

• Three types of fibers:

  • Collagen: Strong, provides support.

  • Elastic: Stretchy, provides flexibility.

  • Reticular: Fine, forms networks.

• Types and locations:

  • Adipose: Hypodermis.

  • Dense regular: Tendons & ligaments.

  • Dense irregular: Dermis (reticular layer).

Nervous Tissue

• Composed of neurons and supporting cells; specialized for communication.

Muscle Tissue

• Skeletal: Voluntary, striated, attached to bones.

• Cardiac: Involuntary, striated, found in heart.

• Smooth: Involuntary, non-striated, found in walls of organs.

Tissue Repair

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

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

• Steps:

  1. Inflammation

  2. Organization (restores blood supply)

  3. Regeneration and fibrosis

Body Membranes and Skin

Body Membrane Types

• Cutaneous: Skin; dry membrane.

• Mucous: Lines cavities open to exterior; moist.

• Serous: Lines closed cavities; moist; two layers:

  • Parietal: Lines cavity wall.

  • Visceral: Covers organs.

• Synovial: Lines joint cavities; produces lubricating fluid.

Skin Functions

• Protection, temperature regulation, sensation, vitamin D synthesis.

Cells of the Epidermis

• Keratinocytes: Produce keratin; provide strength.

• Melanocytes: Produce melanin; protect against UV.

• Dendritic cells: Immune defense.

• Tactile cells: Sensory receptors.

Layers of the Skin

• Epidermis: Stratified squamous epithelium.

• Dermis: Two layers:

  • Papillary: Loose connective tissue; forms fingerprints.

  • Reticular: Dense irregular connective tissue; provides strength.

Layers (Strata) of the Epidermis

1. Stratum basale: Mitosis occurs; deepest layer.

2. Stratum spinosum: Cells begin to flatten.

3. Stratum granulosum: Cells accumulate keratin.

4. Stratum lucidum: Only in thick skin (palms, soles).

5. Stratum corneum: Dead, keratinized cells; outermost layer.

Hair Structure

• Shaft: Visible part.

• Root: Embedded in skin.

• Three layers: Medulla (center), Cortex (middle), Cuticle (outer).

• Arrector pili muscles: Cause hair to stand up.

Nail Structure

• Bed: Skin beneath nail.

• Lunule: Crescent-shaped area.

• Cuticle: Protective fold.

• Matrix: Growth area.

• Nails are composed of keratin.

Sweat and Sebaceous Glands

• Eccrine glands: Produce watery sweat; regulate temperature.

• Apocrine glands: Produce thicker sweat; found in armpits and groin.

• Sebaceous glands: Secrete sebum (oil); lubricates skin and hair.

Homeostatic Imbalances of Skin

• Includes infections, allergies, and cancer.

Skin Cancer

• Primary cause: UV radiation.

• Types:

  • Basal cell carcinoma: Most common, least dangerous.

  • Squamous cell carcinoma: Can metastasize.

  • Melanoma: Most dangerous; arises from melanocytes.

• ABCDE rule: Asymmetry, Border, Color, Diameter, Evolving.

• Metastasized: Cancer has spread to other parts of the body.

Burns

• Immediate threats: Fluid loss, infection.

• Types:

  • First-degree: Only epidermis; redness.

  • Second-degree: Epidermis and part of dermis; blisters.

  • Third-degree: Entire thickness; requires skin grafts.

• Rule of Nines: Used to estimate extent of burns.

Additional info: Academic context was added to expand brief points, clarify examples, and provide definitions for key terms and processes.