Cells: The Living Units – Structure and Function in Anatomy & Physiology

Cells: The Living Units

3.1 Cells as the Smallest Unit of Life

Cells are the fundamental structural and functional units of all living organisms. Understanding their composition and organization is essential for studying anatomy and physiology.

• Definition: A cell is the basic structural and functional unit of life.

• Cell Theory:

  • All living things are made of one or more cells.

  • Cells arise only from other cells.

  • Cells carry out the basic functions of life.

• Human Cell Main Parts:

  1. Plasma Membrane: Selectively permeable barrier that regulates entry and exit of substances.

  2. Cytoplasm: Intracellular fluid containing organelles.

  3. Nucleus: Organelle that controls cellular activity.

• Extracellular Materials: Found outside the cell and include:

  • Extracellular fluids (e.g., blood plasma, cerebrospinal fluid).

  • Cellular secretions (e.g., digestive secretions, lubricating fluids).

  • Extracellular matrix (substances that act as cell glue or lubricants).

PART 1: PLASMA MEMBRANE

3.2 The Plasma Membrane

The plasma membrane is a double layer of phospholipids with embedded proteins, forming a dynamic boundary for the cell.

• Structure:

  • Composed of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.

  • Contains cholesterol and proteins (integral and peripheral).

• Functions:

  • Acts as a selective barrier, controlling the movement of substances.

  • Facilitates cell communication and adhesion.

• Membrane Proteins:

  • Integral proteins span the membrane and are involved in transport and signaling.

  • Peripheral proteins are attached to the membrane surface and function in support and signaling.

• Cell Junctions:

  • Glycocalyx: Carbohydrate-rich area on the cell surface for recognition.

  • Tight Junctions: Prevent passage of substances between cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Gap Junctions: Allow direct communication between adjacent cells.

3.3 Passive Membrane Transport

Passive transport is the movement of molecules down their concentration gradient without energy input from the cell.

• Types of Passive Transport:

  1. Simple Diffusion: Movement of small or lipid-soluble molecules directly through the membrane.

  2. Facilitated Diffusion: Movement of molecules via protein channels or carriers.

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

• Osmosis Equation: $J = -D \frac{dC}{dx}$ Where J is the flux, D is the diffusion coefficient, and dC/dx is the concentration gradient.

• Isotonic, Hypertonic, Hypotonic Solutions:

  • Isotonic: Same solute concentration as the cell; no net water movement.

  • Hypertonic: Higher solute concentration than the cell; water moves out, cell shrinks.

  • Hypotonic: Lower solute concentration than the cell; water moves in, cell swells.

3.4 Active Membrane Transport

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

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses energy from ion gradients created by primary active transport.

• Vesicular Transport:

  • Endocytosis: Cell takes in materials by engulfing them in vesicles.

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of fluid droplets.

  • Receptor-mediated Endocytosis: Specific uptake of molecules via receptor binding.

  • Exocytosis: Release of substances from the cell via vesicle fusion with the plasma membrane.

3.5 Membrane Potential

Selective diffusion of ions across the plasma membrane establishes the membrane potential, an electrical gradient essential for cell function.

• Resting Membrane Potential: The voltage across the cell membrane at rest, typically -50 to -100 mV.

• Key Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ Where $V_m$ is membrane potential, R is the gas constant, T is temperature, F is Faraday's constant, and [K+] are potassium concentrations.

• Role of Ion Pumps: Maintain gradients and membrane potential by active transport of ions.

3.6 Cell-Environment Interactions

Cells interact with their environment through adhesion molecules and membrane receptors, facilitating communication and response to external signals.

• Cell Adhesion Molecules (CAMs): Anchor cells to extracellular matrix and other cells.

• Plasma Membrane Receptors: Bind chemical signals (ligands) and initiate cellular responses.

• G Protein-Coupled Receptors: Activate second messenger systems, such as cyclic AMP.

PART 2: THE CYTOPLASM

3.7 Cytoplasmic Organelles

Organelles within the cytoplasm perform specialized tasks essential for cell survival and function.

• Mitochondria: Site of ATP production via cellular respiration.

• Ribosomes: Synthesize proteins; can be free or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for export.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

• Peroxisomes: Contain enzymes for detoxification and breakdown of fatty acids.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structural support and facilitating movement.

• Centrosomes and Centrioles: Organize microtubules and are important in cell division.

3.8 Cellular Extensions: Cilia and Microvilli

Cilia and microvilli are specialized extensions of the plasma membrane that increase surface area or aid in movement.

• Cilia: Motile projections that move substances across the cell surface.

• Microvilli: Finger-like extensions that increase surface area for absorption.

• Centrioles: Barrel-shaped organelles associated with cilia and flagella formation.

PART 3: NUCLEUS

3.9 The Nucleus

The nucleus is the control center of the cell, containing genetic material and regulating cellular activities.

• Nuclear Envelope: Double membrane with pores for transport of molecules.

• Nucleoli: Dark-staining bodies within the nucleus; site of ribosome assembly.

• Chromatin: DNA-protein complex; condenses into chromosomes during cell division.

• Histones: Proteins that package and organize DNA into nucleosomes.

• Chromosomes: Condensed chromatin protecting genetic material during cell division.

Summary Table: Major Cell Structures and Functions

Structure	Main Function	Key Components
Plasma Membrane	Selective barrier, communication	Phospholipids, proteins, cholesterol
Cytoplasm	Site of metabolic activity	Cytosol, organelles
Nucleus	Genetic control center	DNA, nucleoli, nuclear envelope
Mitochondria	ATP production	Double membrane, enzymes
Ribosomes	Protein synthesis	rRNA, proteins
Endoplasmic Reticulum	Protein/lipid synthesis, detoxification	Rough ER (ribosomes), Smooth ER
Golgi Apparatus	Modification, sorting, packaging	Stacked membranes
Lysosomes	Digestion, waste removal	Hydrolytic enzymes
Peroxisomes	Detoxification	Oxidases, catalases
Cytoskeleton	Structural support, movement	Microtubules, microfilaments, intermediate filaments
Cilia/Microvilli	Movement/absorption	Plasma membrane extensions

Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and table entries are inferred from standard cell biology knowledge.