Cell Structure and Function: Study Notes for Anatomy & Physiology

Cells: The Living Units

3.1 Cells are the Smallest Unit of Life

Cells are the fundamental building blocks of all living organisms. Understanding their structure and function is essential for studying anatomy and physiology.

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

• Cellular Organization:

  • All organisms are made from one or more cells.

  • Cells arise only from other cells.

• 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.

• Extracellular Materials: Substances found outside the cell, including interstitial fluid, blood plasma, and cerebrospinal fluid. These fluids transport nutrients and waste.

• Cellular Secretions: Include substances like saliva and mucus.

• Extracellular Matrix: A network of proteins and polysaccharides that help bind cells together.

PART 1: PLASMA MEMBRANE

3.2 The Plasma Membrane

The plasma membrane is a double layer of phospholipids with embedded proteins, providing both structure and selective permeability to the cell.

• Chemical Composition:

  • Phospholipids form a bilayer; hydrophilic heads face outward, hydrophobic tails face inward.

  • Cholesterol and glycolipids are interspersed, affecting fluidity and function.

• Membrane Proteins:

  • Integral proteins span the membrane and function as channels or carriers.

  • Peripheral proteins are attached to the membrane surface and have structural or enzymatic roles.

• Cell Junctions:

  • Tight Junctions: Prevent leakage between cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Gap Junctions: Allow communication between cells via ion passage.

3.3 Passive Membrane Transport

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

• 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 across a selectively permeable membrane.

• Factors Affecting Diffusion:

  • Concentration gradient

  • Temperature

  • Molecule size

• Osmosis and Tonicity:

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

  • Hypertonic: Higher solute concentration outside; water leaves the cell.

  • Hypotonic: Lower solute concentration outside; water enters the cell.

Equation for Diffusion Rate:

$J = -D \frac{dC}{dx}$

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

3.4 Active Membrane Transport

Active transport requires energy, usually in the form of 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 the movement of another substance down its gradient.

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

• Types of Endocytosis:

  • Phagocytosis: Cell engulfs large particles.

  • Pinocytosis: Cell engulfs fluid.

  • Receptor-mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Equation for Sodium-Potassium Pump:

$3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

3.5 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, maintained by ion gradients (mainly K+).

• Role of Na+/K+ Pump: Maintains gradients and membrane potential.

Equation for Membrane Potential (Nernst Equation):

$E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

3.6 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 structures and signal changes.

• Membrane Receptors: Proteins that bind specific molecules (ligands) and initiate cellular responses.

• G Protein-Coupled Receptors: Activate intracellular signaling cascades.

PART 2: THE CYTOPLASM

3.7 Cytoplasmic Organelles

Organelles are specialized structures within the cytoplasm that perform distinct tasks essential for cell survival.

• Cytosol: Fluid material between the cell membrane and nucleus.

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

• Ribosomes: Sites of protein synthesis; can be free or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Synthesizes lipids, detoxifies chemicals, stores calcium.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Peroxisomes: Contain enzymes for detoxification and lipid metabolism.

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

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support and facilitate movement.

3.8 Cilia and Microvilli

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

• Cilia: Whip-like, motile extensions that move substances across the cell surface.

• Flagella: Longer extensions that propel the cell (e.g., sperm).

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

Summary Table: Major Cell Organelles and Their Functions

Organelle	Main Function
Mitochondria	ATP production via cellular respiration
Ribosomes	Protein synthesis
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis, detoxification, calcium storage
Golgi Apparatus	Sorting, packaging, and modification of proteins and lipids
Lysosomes	Digestion of cellular waste and debris
Peroxisomes	Detoxification and lipid metabolism
Cytoskeleton	Structural support, movement, and intracellular transport

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.