Foundations of Anatomy & Physiology: Cells, Homeostasis, and Membrane Transport

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The human body is organized into hierarchical levels, each with increasing complexity and specialization.

Atoms and Molecules: Atoms combine to form molecules, which are the chemical building blocks of the body.
Cells: The smallest structural and functional unit of an organism capable of independent functioning.
Tissues: Groups of similar cells that perform a specific function.
Organs: Structures composed of two or more tissue types working together to perform specific functions.
Organ Systems: Groups of organs that work together to accomplish a common purpose.
Organism: The complete living being.

Homeostasis is the maintenance of a stable internal environment. The body uses feedback mechanisms to regulate physiological variables.

Negative Feedback: A process that returns a variable to its set point, counteracting deviations (e.g., blood pressure regulation, blood glucose concentration, blood pH regulation, respiratory rate).
Positive Feedback: A process that amplifies a change, moving the variable further from its set point (e.g., platelet aggregation in blood clotting, childbirth).

Key Components of a Homeostatic Feedback Loop:

Understanding anatomical terminology is essential for describing locations and relationships in the body.

Planes: Sagittal (right and left), Frontal (anterior and posterior), Transverse (top and bottom).
Directional Terms: Superior/inferior, anterior/posterior, medial/lateral, proximal/distal, superficial/deep.
Examples: The fingers are distal to the elbow. The nose is superior to the mouth.

Cells contain specialized structures called organelles, each with distinct functions necessary for cell survival and activity.

Smooth ER: Site of lipid synthesis and detoxification.
Rough ER: Studded with ribosomes; synthesizes proteins for export or membrane insertion.
Lysosomes: Contain digestive enzymes to degrade cellular waste.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
Nucleus: Contains genetic material; site of transcription.
Nucleolus: Produces ribosomal RNA.
Plasma Membrane: Regulates entry and exit of substances.
Mitochondria: Site of cellular respiration and ATP production.
Ribosomes: Synthesize proteins.
Centrioles: Involved in cell division.
Peroxisome: Neutralizes free radicals.
Cytoskeletal Elements: Provide structural support.

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.
Proteins: Serve as channels, receptors, enzymes, and anchors.
Cholesterol: Stabilizes membrane fluidity.
Carbohydrates: Attach to proteins/lipids, forming glycoproteins/glycolipids for cell recognition.

Cell junctions connect adjacent cells and regulate communication and adhesion.

Cell Junction	Description	Example
Desmosome	Anchoring junctions that bind cells together	Skin epithelium
Tight junction	Seal adjacent cells to prevent leakage	Intestinal lining
Gap junction	Allow direct communication between cells	Cardiac muscle

Substances move across the plasma membrane by passive or active processes.

Transport Type	Energy Source	Insert high or low into the blanks	Types	Description
Simple Diffusion	Passive	From high concentration to low concentration	1. Simple diffusion	Movement of small molecules through the phospholipid bilayer from an area of higher concentration to an area of their lower concentration, that is along their concentration gradient.
Osmosis	Passive	From high water concentration to low water concentration	2. Osmosis	Movement of water from high to low pressure/concentration.
Primary Active Transport	Active	From low concentration to high concentration	3. Primary active transport	Movement of molecules against their concentration gradient using ATP.
Pinocytosis	Active		4. Pinocytosis	Cell "drinking"; uptake of extracellular fluid.
Phagocytosis	Active		5. Phagocytosis	Binding of a molecule to an external cell receptor causes endocytosis of this molecule into the cell and the membrane will pinch off around the object and become a vesicle.
Exocytosis	Active		6. Exocytosis	Vesicles fuse with the plasma membrane to release contents outside the cell.

Osmosis is the movement of water across a semi-permeable membrane. Tonicity describes the effect of a solution on cell volume.

Hypotonic Solution: Lower solute concentration than the cell; water enters the cell, causing it to swell.
Hypertonic Solution: Higher solute concentration than the cell; water leaves the cell, causing it to shrink.
Isotonic Solution: Equal solute concentration; no net movement of water.

Example: A red blood cell placed in distilled water (hypotonic) will swell; in a concentrated salt solution (hypertonic), it will shrink.

Simple Diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer.
Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

Primary Active Transport: Direct use of ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).
Secondary Active Transport: Uses the energy from the movement of another substance down its gradient (often established by primary active transport) to move a different substance against its gradient.

Filtration: Movement of water and solutes across a membrane due to hydrostatic pressure, not concentration gradients (e.g., filtration in kidney glomeruli).

Osmosis: Where is the flux, is the permeability, and is the concentration difference across the membrane.
Fick's Law of Diffusion: Where is the rate of diffusion, is the diffusion coefficient, and is the concentration gradient.

Translation is the process by which ribosomes synthesize proteins using mRNA as a template, occurring in the cytoplasm.
Cellular respiration in mitochondria converts glucose to ATP, the cell's energy currency.