The Cell

Overview

The cell is the basic structural and functional unit of the body, ranging in size from 2 to 120 μm. Cells carry out essential life processes and are the foundation of all tissues and organs.

• Components:

  • Cell membrane: Made of a fluid phospholipid bilayer, proteins, cholesterol, and associated carbohydrates.

  • Proteins: Integral (embedded in membrane) and peripheral (associated with membrane surface).

  • Carbohydrates: Serve as receptors and channel gates.

Movement of Particles Across the Membrane

Cells regulate the movement of substances to maintain homeostasis. Transport mechanisms include:

• Diffusion: Movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached.

  • Simple diffusion: Allows small, non-polar molecules to move freely (e.g., oxygen, carbon dioxide).

  • Facilitated diffusion: Uses protein channels or carriers for polar or charged molecules (e.g., glucose, ions).

• Osmosis: Net diffusion of water down its concentration gradient through a selectively permeable membrane.

• Tonicity: Relative concentration of non-penetrating solutes in solution.

  • Isotonic: Same solute concentration as cell interior.

  • Hypotonic: Lower solute concentration than cell interior (cell swells).

  • Hypertonic: Higher solute concentration than cell interior (cell shrinks).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

  • Example: Sodium-potassium pump ()

• Endocytosis and Exocytosis:

  • Endocytosis: Cell membrane forms a vesicle to bring substances into the cell.

  • Exocytosis: Vesicle fuses with the cell membrane to release substances outside the cell.

Membrane Potentials

Overview

The unequal charge distribution across the membrane creates a voltage, known as the membrane potential. This is essential for nerve impulse transmission and muscle contraction.

• Key factors:

  • Permeability properties of the membrane.

  • Action of the Na-K pump.

  • Presence of non-diffusible negatively charged compounds within the cell.

Membrane Junctions

Types

• Desmosomes: Cell membranes (20 nm separation) anchored to each other by cytoskeletal protein and fibers; provide mechanical support.

• Tight junctions: Cell membranes with virtually no extracellular space; prevent passage of substances between cells.

• Gap junctions: Cell membranes in close contact (2-4 nm apart) with protein channels allowing ions and molecules to pass directly from one cell to another.

Cytoplasm

Overview

The cytoplasm is the fluid found inside the cell external to the nucleus and its constituent molecules; consists of organelles and the cytosol.

Organelles

• Nucleus: Contains DNA (chromatin) and is the control center for cell activities.

  • Double membrane (nuclear envelope) with pores.

  • Contains nucleolus (site of rRNA synthesis).

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins.

  • Free ribosomes: Synthesize proteins for use within the cell.

  • Bound ribosomes (on rough ER): Synthesize proteins for export or membrane insertion.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and packages proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Complex: Stack of membrane-bound sacs; modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Double-membraned, rod-shaped organelles; site of ATP production via aerobic respiration.

  • Contains its own DNA and ribosomes.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

  • Microfilaments (actin): 7 nm diameter; cell movement and shape.

  • Intermediate filaments: 15 nm diameter; mechanical strength.

  • Microtubules: 25 nm diameter; cell division, transport, and structure.

• Centrioles: Nine triplets of microtubules; function in cell division by organizing spindle fibers.

• Flagella and Cilia: Microtubule-based structures for movement.

  • Flagella: Long, few in number; propel cells (e.g., sperm).

  • Cilia: Short, numerous; move substances over cell surface.

• Vesicles: Membrane-bound sacs for transport and storage.

  • Lysosomes: Contain digestive enzymes for breaking down waste.

  • Peroxisomes: Contain enzymes for neutralizing free radicals and detoxification.

• Microvilli: Finger-like extensions of the cell membrane; increase surface area for absorption and secretion.

Cell Division

Overview

Cell division is the process by which cells reproduce. It includes mitosis (somatic cell division) and meiosis (reduction division for gamete formation).

• Mitosis: Produces two daughter cells with the same number of chromosomes as the parent cell.

  • Stages: Prophase, Metaphase, Anaphase, Telophase.

  • Chromosomes: Composed of DNA and proteins; duplicated before division.

  • Chromatid: Each duplicated chromosome consists of two sister chromatids attached at the centromere.

• Meiosis: Reduces chromosome number by half; produces gametes for sexual reproduction.

Cell Cycle Phases

• G1 phase: Synthesis of proteins necessary for gene replication.

• S phase: DNA synthesis (replication).

• G2 phase: Synthesis of proteins necessary for mitosis.

• M phase: Mitosis (division of cell).

Aging

Overview

Aging is the progressive failure of the body's homeostatic adaptive responses, leading to loss of function and capacity to respond to stress.

• Cellular changes include:

  • Decrease in protein synthesis.

  • Decrease in water content.

  • Increase in cross-linking and other chemical damage.

  • Increase in age pigments (e.g., lipofuscin).

Theories of Aging

• Collagen cross-linking: Extracellular proteins increasingly bind to one another with age, reducing cell flexibility and nutrient exchange.

• Programmed Theories: Genetic control of aging; biological clocks (e.g., pineal gland, immune system) regulate lifespan.

• Somatic Mutation: Environmental damage to DNA or proteins impairs cell function.

• Free-radical theory: Highly reactive oxygen molecules generated by metabolism cause damage to vital enzymes and DNA.

Table: Types of Membrane Transport

Table: Major Cell Organelles and Functions

Key Equations

• ATP Hydrolysis (energy for active transport):

• Osmotic Pressure: where = osmotic pressure, = van't Hoff factor, = molarity, = gas constant, = temperature

Example Applications

• Example: The sodium-potassium pump maintains the resting membrane potential in neurons by actively transporting Na+ out and K+ into the cell against their concentration gradients.

• Example: Mitochondria produce ATP, which is used for muscle contraction and active transport processes.

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.