Homeostasis

Definition and Importance

Homeostasis refers to the maintenance of a stable internal environment within the body, despite external changes. This is essential for the proper functioning of cells and organs, and for overall health.

• Definition: Maintaining a relatively constant internal environment (e.g., temperature, pH, glucose levels).

• Survival Needs: Nutrients, oxygen, water, normal body temperature, and appropriate atmospheric pressure are required for survival.

• Homeostatic Imbalance: Too much or too little of any survival need can be harmful and may lead to disease or dysfunction.

Homeostatic Control Mechanisms

• Receptor: Detects changes (stimuli) in the environment.

• Control Center: Processes information and determines the appropriate response.

• Effector: Carries out the response to restore balance.

Feedback Mechanisms

• Negative Feedback: The response reduces or eliminates the original stimulus (e.g., body temperature regulation).

• Positive Feedback: The response enhances or amplifies the original stimulus (e.g., labor contractions, blood clotting).

Basic Chemistry Concepts

Elements and Compounds

• Elements: Substances that cannot be broken down into simpler substances by chemical means. Major elements in the human body: O (65%), C (18%), H (10%), N (3%).

• Atoms: Smallest units of elements with unique properties.

• Molecules: Two or more atoms bonded together (e.g., O2).

• Compounds: Two or more different atoms bonded together (e.g., H2O, C6H12O6).

Mixtures

• Solutions: Homogeneous mixtures; solute dissolved in solvent (e.g., glucose in blood).

• Colloids: Heterogeneous, cloudy, particles do not settle (e.g., cytosol).

• Suspensions: Heterogeneous, particles settle out (e.g., blood cells in plasma).

Chemical Bonds and Reactions

• Ionic Bonds: Transfer of electrons from one atom to another, forming ions (cation = +, anion = -).

• Covalent Bonds: Sharing of electrons between atoms (single, double, triple bonds).

• Polar vs. Nonpolar: Unequal (polar) vs. equal (nonpolar) electron sharing.

• Reactions:

  • Synthesis (anabolic): A + B → AB

  • Decomposition (catabolic): AB → A + B

  • Exchange (displacement): AB + C → AC + B

• Energy Flow: Endergonic = absorbs energy; exergonic = releases energy.

• Rate Influencers: Temperature, concentration, particle size, catalysts (enzymes).

Basic Human Biochemistry

Inorganic Compounds

• Water: High heat capacity, heat of vaporization, solvent, cushioning, reactivity.

• Salts: Electrolytes (Na+, K+, Ca2+) vital for nerve impulses and muscle function.

• Acids & Bases: Acids release H+; bases accept H+ (release OH-).

• pH Scale: = acidic, $7 = basic.

Organic Compounds

• Carbohydrates: C, H, O; main energy source.

  • Monosaccharides: Glucose

  • Disaccharides: Sucrose

  • Polysaccharides: Glycogen, starch

• Lipids: C, H, O; energy storage, insulation, cell membranes.

  • Triglycerides: Fats, oils; saturated vs. unsaturated

  • Phospholipids: Major component of plasma membranes

  • Steroids: Cholesterol, vitamin D, hormones

• Proteins: Polymers of amino acids; structural (fibrous) or functional (globular)

  • Structure: Primary → Secondary → Tertiary → Quaternary

  • Enzymes: Biological catalysts, lower activation energy, substrate-specific

• Nucleic Acids: DNA (genetic blueprint) & RNA (protein synthesis)

• ATP: Energy currency of cell (adenosine triphosphate)

Cytology: Plasma Membrane & Transport

Cell Theory

• Cells are the structural and functional unit of life.

• All living organisms are composed of cells.

• All cells come from pre-existing cells.

Generalized Cell Structure

• Plasma Membrane: Selectively permeable barrier; fluid mosaic model.

• Cytoplasm: Intracellular fluid and organelles.

• Nucleus: DNA control center.

Plasma Membrane Components

• Phospholipid Bilayer: Polar heads (hydrophilic) outside, nonpolar tails (hydrophobic) inside.

• Proteins: Integral (channels, carriers, receptors) and peripheral (enzymes, support, cell recognition).

• Glycocalyx: Identity markers.

Cell Junctions

• Tight Junctions: Impermeable, prevent leakage.

• Desmosomes: Strong, allow flexibility (skin, heart).

• Gap Junctions: Allow communication (ions, small molecules).

Membrane Transport

• Passive Transport (no ATP):

  • Simple diffusion

  • Facilitated diffusion (via carriers/channels; e.g., glucose, ions)

  • Osmosis (water movement; low solute → high solute)

  • Filtration (pressure-driven across capillary walls)

• Tonicity:

  • Isotonic: Cell unchanged

  • Hypertonic: Cell shrinks (crenation)

  • Hypotonic: Cell swells (lysis)

• Active Transport (uses ATP):

  • Primary: Na+/K+ pump, Ca2+ pump

  • Secondary: Uses gradient from primary to move molecules (symport/antiport)

  • Vesicular: Endocytosis (phagocytosis, pinocytosis), exocytosis (secretion)

Table: Comparison of Major Organic Compounds