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Definition: The body's ability to maintain stable internal conditions despite changes in the external environment.
- Responses to Changes: Can be instantaneous or develop over time.

Positive Feedback Mechanism:
- Promotes (amplifies) a physiological trend.
Negative Feedback Mechanism:
- Reverses a trend to return to a set-point.
- Primary mechanism for maintaining homeostasis.
- Negative feedback loops consist of the following components:
  - Stimulus: A change detected in the body.
  - Receptor: Part of the system that senses the change.

Atoms: Smallest unit of matter, building blocks of everything.
Subatomic Particles:
- Protons: Positively charged; determines identity of the element.
- Neutrons: No charge; contributes to atomic mass.
- Electrons: Negatively charged; occupy energy shells around nucleus.
- Atomic Mass: Sum of protons and neutrons.

Isotopes: Atoms of the same element with different numbers of neutrons, affecting atomic weight.
Valence Electrons: Electrons in the outermost shell; involved in chemical bonding.
Bond Types:
- Covalent Bonds: Atoms share electrons. Can be:
  - Single: One pair of electrons shared.
  - Double: Two pairs of electrons shared.
  - Triple: Three pairs of electrons shared.
  - Polar Covalent: Unequal sharing, creating partial charges.
  - Nonpolar Covalent: Equal sharing.

Ions: Charged atoms; can be cations (positive, lose electrons) or anions (negative, gain electrons).
Dissociation in Water: Salts dissolve to produce ions; important for creating gradients across membranes, influencing nerve and muscle function.

Definition: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.
Importance: Contributes to macromolecular stability and properties of water.

Polarity: Water is polar, forming hydrogen bonds; interacts with other polar substances.
Chemical Reactivity: Water participates in hydration and hydrolysis reactions.
Thermal Stability: High specific heat capacity; resists rapid temperature changes.
Hydrophilic vs Hydrophobic:
- Hydrophilic: Water-loving substances.
- Hydrophobic: Water-repelling substances.

Kinetic Energy: Energy in motion; involved in diffusion processes.
- Diffusion: Passive movement of molecules from high to low concentration.
Exergonic Reactions: Release energy, typically during catabolism.
Endergonic Reactions: Required energy input, often seen in anabolic reactions.

Osmosis: Movement of water across membranes from an area of lower solute concentration to higher.
Transmembrane Proteins: Integral proteins allowing substance passage through membranes.
Peripheral Proteins: Loosely attached to surface of the membrane; often enzymes.
Processes:
- Endocytosis: Moving substances into cells via vesicles.
- Exocytosis: Release of substances from cells via secretory vesicles.

Cytoplasm: Contains organelles, cytoskeleton, and cytosol.
Cell Organelles:
- Membrane-bound: Nucleus, mitochondria, lysosomes, endoplasmic reticulum (RER & SER).
- Non-membrane-bound: Ribosomes, centrosomes.
- Nucleus: Control center of the cell; houses genetic material.

Major Muscles:
- Quadriceps Femoris: Extends the knee joint.
- Hamstrings: Flexes the knee joint.
- Gluteus Maximus: Extends and rotates the hip.
- Gastrocnemius: Plantarflexes the foot at the ankle.

Key Movements:
- Flexion/Extension: Decreasing/increasing the angle between two body parts.
- Abduction/Adduction: Moving away from/toward the body midline.
- Rotational Movements: Internal and external rotation of limbs.