Cell Structure and Function: Study Notes for Anatomy & Physiology

Cells: The Living Units

Cells are the Smallest Unit of Life

Cells are the fundamental building blocks of all living organisms. They perform essential functions necessary for life and are organized into tissues, organs, and systems.

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

• Extracellular Materials: Substances found outside the cell, such as body fluids (interstitial fluid, blood plasma, cerebrospinal fluid), cellular secretions, and extracellular matrix.

• Major Regions of a Generalized Cell:

  1. Plasma membrane: Selectively permeable outer boundary.

  2. Cytoplasm: Contains organelles and cytosol.

  3. Nucleus: Organelle that controls cellular activity.

Example: Human cells such as neurons, muscle cells, and epithelial cells all share these basic structural features.

PART 1: PLASMA MEMBRANE

The Plasma Membrane Structure and Function

The plasma membrane is a double layer of phospholipids with embedded proteins, forming the boundary of the cell and regulating the movement of substances in and out.

• Chemical Composition: Primarily phospholipids, cholesterol, proteins, and carbohydrates.

• Phospholipid Bilayer:

  • Hydrophilic (water-loving) heads face outward; hydrophobic (water-fearing) tails face inward.

  • Cholesterol stabilizes the membrane.

• Membrane Proteins:

  • Integral proteins: Span the membrane, involved in transport and signaling.

  • Peripheral proteins: Attached to the surface, involved in support and signaling.

• Cell Junctions:

  • Tight junctions: Prevent leakage between cells.

  • Desmosomes: Anchor cells together.

  • Gap junctions: Allow communication between cells.

Example: Intestinal epithelial cells use tight junctions to prevent leakage of digestive enzymes.

Passive Membrane Transport

Passive transport is the movement of substances across the membrane without energy input from the cell, driven by concentration gradients.

• Types of Passive Transport:

  1. Simple diffusion: Movement of molecules from high to low concentration.

  2. Facilitated diffusion: Movement via protein channels or carriers.

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

• Factors Affecting Diffusion: Steepness of gradient, temperature, molecule size.

• Osmosis:

  • Water moves toward higher solute concentration.

  • Can cause cells to swell (hypotonic) or shrink (hypertonic).

Equation:

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

Active Membrane Transport

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

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

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

• Vesicular Transport: Movement of large particles via vesicles (endocytosis, exocytosis).

• Types of Endocytosis:

  • Phagocytosis: Cell engulfs large particles.

  • Pinocytosis: Cell engulfs fluid.

  • Receptor-mediated endocytosis: Specific uptake via receptors.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Selective Diffusion and Membrane Potential

The membrane potential is the voltage difference across the cell membrane, established by selective diffusion of ions.

• Resting Membrane Potential: Typically -50 to -100 mV, due to K+ and Na+ gradients.

• Key Equation:

$E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{\text{outside}}}{[K^+]_{\text{inside}}} \right)$

Example: Neurons use changes in membrane potential to transmit electrical signals.

Cell Adhesion Molecules and Membrane Receptors

Cell adhesion molecules (CAMs) and membrane receptors allow cells to interact with their environment and communicate.

• CAMs: Glycoproteins that attach cells to extracellular matrix and other cells.

• Membrane Receptors: Proteins that bind signaling molecules and initiate cellular responses.

• G Protein-Coupled Receptors: Initiate signal transduction via activation of G proteins.

Example: Immune cells use CAMs to migrate to sites of infection.

PART 2: THE CYTOPLASM

Cytoplasm and Cytosol

The cytoplasm is the material between the plasma membrane and the nucleus, containing cytosol, organelles, and inclusions.

• Cytosol: Fluid portion containing dissolved substances.

• Cytoskeletal Elements: Microfilaments, intermediate filaments, microtubules.

Example: Cytoskeletal elements maintain cell shape and enable movement.

Cytoplasmic Organelles

Organelles perform specialized tasks within the cell.

• Mitochondria: Powerhouse of the cell, site of ATP production.

• 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 packages proteins and lipids.

• Lysosomes: Contain digestive enzymes.

• Peroxisomes: Detoxify harmful substances.

Example: Liver cells have abundant smooth ER for detoxification.

Cilia and Microvilli

Cilia and microvilli are cellular extensions that increase surface area or aid in movement.

• Cilia: Whip-like extensions that move substances across cell surfaces.

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

Example: Ciliated cells in the respiratory tract move mucus; microvilli in intestinal cells increase nutrient absorption.

Summary Table: Major Cell Structures and Functions

Structure	Main Function	Example/Application
Plasma Membrane	Selective barrier, communication	Regulates entry/exit of substances
Nucleus	Genetic control center	Contains DNA
Mitochondria	ATP production	Energy for cellular processes
Ribosomes	Protein synthesis	Translation of mRNA
Endoplasmic Reticulum	Protein/lipid synthesis	Rough ER: proteins; Smooth ER: lipids
Golgi Apparatus	Modification, sorting, packaging	Secretion of proteins
Lysosomes	Digestion of macromolecules	Breakdown of waste
Peroxisomes	Detoxification	Breakdown of hydrogen peroxide
Cilia	Movement	Respiratory tract
Microvilli	Increase surface area	Intestinal absorption