Chapter 3: Cells – Structure, Function, and Division

Cells: The Fundamental Units of Life

Definition and Overview

Cells are the basic structural and functional units of all living organisms. Every organism, from the simplest bacteria to complex humans, is composed of cells. Understanding cell structure and function is essential for studying anatomy and physiology.

• Cell Membrane: A protective barrier that regulates what enters and exits the cell.

• Cytoskeleton: A network of protein filaments that maintains cell shape, secures organelles, and enables movement.

• Organelles: Specialized structures within the cell that perform distinct functions.

Types of Cells

Cells vary in shape, size, and function. Most human cells are not perfectly round and are specialized for their roles, such as muscle cells, nerve cells, and blood cells.

• Muscle Cells: Elongated for contraction.

• Nerve Cells: Long extensions for transmitting signals.

• Blood Cells: Disc-shaped for transport.

Cellular Organelles and Their Functions

Major Organelles

Each organelle within a eukaryotic cell has a unique function essential for cell survival and activity.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Synthesizes lipids; detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.

• Mitochondria: "Powerhouse" of the cell; produces ATP (energy).

• Lysosomes: Break down waste and cellular debris.

• Vesicles: Transport materials within the cell.

Organelles Table

Organelle	Main Function
Nucleus	Genetic information storage and regulation
Ribosomes	Protein synthesis
Rough ER	Protein synthesis and processing
Smooth ER	Lipid synthesis and detoxification
Golgi Apparatus	Protein and lipid modification and shipping
Mitochondria	ATP production (cellular energy)
Lysosomes	Breakdown of waste and cellular debris
Vesicles	Transport of substances

Cytoskeleton and Extracellular Matrix

Cytoskeleton

The cytoskeleton is a dynamic network that provides structural support and facilitates movement within the cell.

• Microtubules: Hollow tubes for cell shape and transport.

• Microfilaments: Thin filaments for movement and support.

• Intermediate Filaments: Provide mechanical strength.

• Centrioles: Involved in cell division.

Extracellular Matrix (ECM)

The ECM is material secreted by cells into their environment, providing structural and biochemical support.

• Functions: Cell adhesion, communication, and tissue formation.

• Examples:

  • Blood cells create a fluid ECM (plasma).

  • Skin cells create a flexible ECM.

  • Bone cells create a hard ECM.

Plasma Membrane Structure and Function

Composition

The plasma membrane surrounds all cells, regulating the movement of substances and facilitating communication.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Cholesterol: Maintains membrane fluidity.

• Proteins: Serve as anchors, transporters, and receptors.

• Glycolipids and Glycoproteins: Involved in cell recognition and immunity.

Cell Junctions and Receptors

• Cell Junctions: Connect cells to each other and the ECM.

• Receptors: Proteins that receive signals from the environment or other cells.

Movement Across the Cellular Membrane

Passive Transport

Passive transport does not require energy; substances move down their concentration gradient.

• Simple Diffusion: Movement of nonpolar molecules (e.g., gases, alcohol) directly through the membrane.

• Facilitated Diffusion: Movement of polar molecules (e.g., glucose) via protein channels.

• Osmosis: Movement of water through aquaporins from low to high solute concentration.

Osmotic Pressure Equation

Osmotic pressure can be described by:

$ext{Osmotic Pressure} = iMRT$

where i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature.

Tonicity

Tonicity describes the relative concentration of solutes in a solution compared to the cell.

Type	Description
Isotonic	Equal solute concentration inside and outside the cell
Hypotonic	Lower solute concentration outside the cell
Hypertonic	Higher solute concentration outside the cell

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Pumps/Carrier Proteins: e.g., sodium-potassium pump.

• Endocytosis: Uptake of large molecules or quantities via vesicles.

• Exocytosis: Release of large molecules or quantities via vesicles.

Cell Cycle and Division

Phases of the Cell Cycle

The cell cycle is the series of events leading to cell division and replication.

• Interphase: Cell grows, performs normal functions, and duplicates DNA (about 90% of the cycle).

• Mitotic Phase: Cell divides via mitosis and cytokinesis, producing two identical daughter cells.

Mitosis and Cytokinesis

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell center.

• Anaphase: Chromatids separate to opposite poles.

• Telophase: Nuclear membranes reform.

• Cytokinesis: Cytoplasm divides, forming two cells.

Cancer: Uncontrolled Cell Division

Genetic Regulation of the Cell Cycle

• Proto-oncogenes: "Gas pedal" genes that promote cell cycle progression. When mutated, become oncogenes, accelerating division.

• Tumor Suppressor Genes: "Brake pedal" genes that inhibit cell division. Mutation leads to uncontrolled growth.

Cell Death and Tumor Formation

• Apoptosis: Programmed cell death for damaged cells.

• Tumors:

  • Benign: Cells remain localized.

  • Malignant: Cells divide rapidly and may spread (metastasize).

Treatment Options

• Surgery

• Radiation therapy

• Chemotherapy

Example: Hydration and Tonicity in Sports Medicine

During strenuous exercise, water and electrolytes are lost. Drinking plain water (hypotonic) can quickly rehydrate cells, but excessive intake may cause cellular swelling. Sports drinks (isotonic) contain sodium and sugar, replenishing electrolytes but may not hydrate as rapidly as water. Overconsumption can be harmful.

Additional info: The sodium-potassium pump is a classic example of active transport, maintaining cellular electrochemical gradients essential for nerve and muscle function.