Study Guide: Cells – The Living Units (Anatomy & Physiology)

Cells: The Living Units

Cell Theory

The cell theory is a foundational concept in biology and anatomy, stating that all living things are composed of cells. Cells are the smallest units of life, and all organisms are made of one or more cells. Cells only arise from other cells.

• Definition: A cell is the basic structural and functional unit of all living organisms.

• Unifying Principle: All living things are made of one or more cells, and all cells arise from pre-existing cells.

• Example: Humans, plants, and bacteria are all composed of cells.

Structure and Function of the Cell Membrane

The cell membrane (also called the plasma membrane) is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Phospholipids: Form the basic structure of the membrane; hydrophobic tails prevent water-soluble substances from crossing, forming a boundary.

• Cholesterol: Stabilizes the membrane, further decreasing water solubility and providing a ringed structure.

• Transport Proteins: Determine what functions the membrane can perform, such as transport, communication, and joining cells to each other and to the extracellular matrix.

• Receptors: Bind to chemical messenger molecules and send signals to the cell.

• Carbohydrates: Act as identity molecules, allowing cells to recognize each other.

Junctions Between Cells

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural integrity.

• Tight Junctions: Impermeable junctions that form a continuous seal around the cell, preventing molecules from passing between cells.

• Desmosomes: Anchoring junctions between cells that act like molecular Velcro, preventing cells from tearing apart.

• Gap Junctions: Communicating junctions that allow ions and small molecules to pass between cells, important in synchronizing electrical activity in heart and embryonic cells.

Membrane Transport Mechanisms

Cells exchange materials with their environment through various transport mechanisms.

• Simple Diffusion: Passive movement of molecules (gases, steroid hormones, fatty acids, O2, CO2) directly through the plasma membrane. Molecules must be lipid-soluble or small enough to pass through.

• Facilitated Diffusion: Passive transport using a larger or charged molecule (e.g., glucose, Na+), which moves through a protein channel or carrier in the membrane.

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient. Example: Sodium-potassium pump exchanges Na+ for K+ across the membrane.

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

Osmotic Environments: Isotonic, Hypotonic, Hypertonic

Cells can be exposed to different osmotic environments, affecting water movement.

• Isotonic: The concentration of solutes is the same inside and outside the cell; no net water movement.

• Hypotonic: The solution has a lower solute concentration than the cell; water enters the cell, which may swell and burst.

• Hypertonic: The solution has a higher solute concentration than the cell; water leaves the cell, causing it to shrink.

Vesicular Transport: Endocytosis and Exocytosis

Cells use vesicles to transport large molecules or particles across the membrane.

• Endocytosis: Cell import; the cell engulfs material into a vesicle.

• Pinocytosis: "Cell drinking"; the cell membrane folds to take in liquids.

• Phagocytosis: "Cell eating"; the cell engulfs large particles or debris.

• Exocytosis: Cell export; vesicles fuse with the membrane to release contents outside the cell.

Major Cell Organelles and Their Functions

Organelles are specialized structures within cells that perform distinct functions.

Organelle	Function
Nucleus	"Brain" of the cell; holds genetic blueprint DNA.
Endoplasmic Reticulum	Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies.
Ribosomes	"Factory"; reads instructions from mRNA and uses amino acids to build proteins.
Golgi Complex	Distributes proteins and lipids via secretory vesicles.
Lysosomes	Break down waste and renew cells.
Cytoskeleton	Internal muscular and skeletal system; gives shape to cell.
Mitochondria	"Powerhouse"; energy production and cell respiration.
Centrioles	Coordinate cell division; establish polarity.
Chromatin	DNA and protein that form chromosomes; genetic material.

Phases of the Cell Cycle

The cell cycle consists of a series of phases that prepare the cell for division and replication.

• G1 (Gap 1): Preparation for DNA synthesis.

• S (Synthesis): DNA replication.

• G2 (Gap 2): Growth and preparation for mitosis.

Interphase includes G1, S, and G2 phases; it is the period from cell formation to cell division.

Phases of Mitosis

Mitosis is the process by which a cell divides to produce two identical daughter cells. The main phases are:

• Prophase: Chromatin condenses into chromosomes; spindle fibers form; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Key Equations and Concepts

• Osmosis: Water moves from an area of low solute concentration to high solute concentration across a semipermeable membrane.

• Sodium-Potassium Pump: $3\text{Na}^+ \text{out} / 2\text{K}^+ \text{in}$ per ATP hydrolyzed.

• Diffusion Rate: $\text{Rate} = k \times [\text{Concentration Gradient}]$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table images have been converted to text format for accessibility.