Cell Structure, Membrane Transport, and Cell Communication: Study Notes for Anatomy & Physiology

Chapter 3: Cells and Their Functions

3.1 Cells are the Smallest Unit of Life

Cells are the fundamental structural and functional units of all living organisms. They exhibit a variety of shapes and sizes, each adapted to specific functions.

• Definition of a Cell: The basic unit of life, capable of performing all vital physiological functions.

• Cell Shapes: Common shapes include squamous (flat), cuboidal (cube-shaped), and columnar (tall and cylindrical).

• Extracellular Materials: Substances found outside the cells, such as extracellular fluid (ECF), extracellular matrix, and interstitial fluid.

• Generalized Cell Structure: Most human cells have three main parts:

  1. Plasma membrane – the outer boundary

  2. Cytoplasm – the intracellular fluid containing organelles

  3. Nucleus – the control center containing genetic material

3.2 The Plasma Membrane: Structure and Function

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed primarily of a phospholipid bilayer with embedded proteins.

• Chemical Composition: The membrane is a phospholipid bilayer with hydrophilic (polar) heads facing outward and hydrophobic (nonpolar) tails facing inward.

• Cholesterol: Stabilizes membrane fluidity and integrity.

• Glycocalyx: A carbohydrate-rich area on the cell surface important for cell recognition and protection.

• Membrane Proteins: Responsible for transport, signal transduction, cell recognition, and intercellular joining.

• Functions of the Plasma Membrane:

  • Physical barrier

  • Selective permeability

  • Communication

  • Cell recognition

3.3 Intercellular Junctions

Cells are connected by specialized junctions that allow communication, adhesion, and barrier formation.

Cell Junction	Description
Tight junction	Impermeable junction; prevents molecules from passing between cells
Desmosome	Anchoring junction; helps keep cells from separating during stress
Gap junction	Communicating junction; allows ions and small molecules to pass from cell to cell

3.4 Passive Membrane Transport: Diffusion and Osmosis

Passive transport moves substances across the membrane without energy input, driven by concentration gradients.

• Passive Transport: Movement of substances down their concentration gradient (from high to low concentration) without energy expenditure.

• Concentration Gradient: The difference in concentration of a substance across a space or membrane.

• Simple Diffusion: Movement of small or lipid-soluble molecules directly through the phospholipid bilayer.

• Facilitated Diffusion: Movement of larger or charged molecules via protein channels or carriers.

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

Comparison of Simple and Facilitated Diffusion:

• Both move substances down their concentration gradient.

• Facilitated diffusion requires membrane proteins; simple diffusion does not.

Osmosis Key Points:

• Water moves from areas of low solute concentration to high solute concentration.

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypertonic: Higher solute concentration outside; cell shrinks (crenates).

• Hypotonic: Lower solute concentration outside; cell swells (may lyse).

Type of Transport	Substance Transported
Simple diffusion	Oxygen molecules
Carrier-mediated facilitated diffusion	Amino acids, glucose molecules
Channel-mediated facilitated diffusion	Sodium ions (Na+), potassium ions (K+)
Simple diffusion	Steroid hormones

3.5 Active Membrane Transport

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

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

• Secondary Active Transport: Uses energy from the gradient created by primary active transport.

• Sodium-Potassium Pump: For each ATP, 3 Na+ are pumped out and 2 K+ are pumped in.

• Vesicular Transport: Bulk movement of substances via vesicles (endocytosis, exocytosis, phagocytosis, pinocytosis, receptor-mediated endocytosis).

	Primary active transport only	Vesicular transport only	Both primary active transport and vesicular transport
Requires ATP	X	X	X
Involves transport proteins	X		X
Moves substances across the membrane	X	X	X
Moves large substances or bulk transport of substances		X
Involves vesicles		X
Moves substances against their electrochemical gradient	X		X

Types of Vesicular Transport:

• Endocytosis: Uptake of materials into the cell.

• Exocytosis: Release of materials from the cell.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

• Receptor-mediated endocytosis: Selective uptake via receptor binding.

3.6 Membrane Potential

The membrane potential is the voltage difference across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential: Typically around -70 mV in neurons; inside is negative relative to outside.

• Major Ions: Na+ is the main extracellular cation; K+ is the main intracellular cation.

• Leakage Channels: More K+ channels than Na+ channels, contributing to the negative resting potential.

• Sodium-Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in per ATP.

3.7 Cell Adhesion Molecules and Membrane Receptors

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

• CAMs: Proteins such as cadherins and integrins that mediate cell-cell and cell-matrix adhesion.

• Functions: Aid in embryonic development, wound healing, and immune responses.

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

G Protein Signaling (Additional info: Expanded for clarity)

G protein-coupled receptors (GPCRs) are a major class of membrane receptors involved in signal transduction.

1. Ligand binds to receptor, causing a conformational change.

2. Receptor activates G protein by exchanging GDP for GTP.

3. Activated G protein activates (or inactivates) an effector protein (e.g., enzyme or ion channel).

4. Effector protein generates second messengers (e.g., cAMP, Ca2+).

5. Second messengers activate other enzymes or ion channels, leading to a cellular response.

6. G protein hydrolyzes GTP to GDP, returning to inactive state.

Part 2: The Cytoplasm

Composition of the Cytosol

The cytosol is the viscous, semitransparent fluid portion of the cytoplasm in which organelles and other inclusions are suspended.

• Pigments: Melanin granules in skin and hair cells are examples of nonmembranous organelles.

• Cytosol: Contains water, dissolved ions, proteins, and nutrients.