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

Cell Diversity and Structure

Diversity of Cells in Size and Shape

Cells in the human body exhibit remarkable diversity in both size and shape, reflecting their specialized functions.

• Size: Most human cells range from 10 to 100 micrometers in diameter, but some (e.g., neurons) can be much longer due to their processes.

• Shape: Cell shapes include spherical (e.g., ovum), flat (e.g., epithelial cells), elongated (e.g., muscle fibers), and star-shaped (e.g., neurons).

• Function: The shape of a cell is closely related to its function; for example, red blood cells are biconcave to maximize gas exchange.

Cellular Compartments and Membranes

Intracellular and Extracellular Fluid

The body’s fluids are divided into intracellular fluid (ICF) and extracellular fluid (ECF), separated by the plasma membrane.

• Intracellular Fluid (ICF): The fluid within cells, containing nutrients, ions, and organelles.

• Extracellular Fluid (ECF): The fluid outside cells, including interstitial fluid and plasma.

• Plasma Membrane: A selectively permeable barrier that separates ICF from ECF.

Cell Organelles: Structure and Function

Nucleus

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

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleoli: Sites of ribosome synthesis.

• Nucleoplasm: Semi-fluid matrix inside the nucleus.

• Nuclear Pores: Allow passage of molecules between nucleus and cytoplasm.

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes and processes proteins.

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

Ribosomes

Ribosomes are molecular machines that synthesize proteins from amino acids.

• Free Ribosomes: Float in cytoplasm; produce proteins for intracellular use.

• Bound Ribosomes: Attached to rough ER; produce proteins for export or membranes.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Cisternae: Flattened membrane sacs.

• Function: Receives products from ER, processes them, and sends them to destinations.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Function: Break down waste materials, cellular debris, and foreign substances.

• Role in Disease: Malfunction can lead to lysosomal storage disorders.

Mitochondria

Mitochondria are the cell’s powerhouses, generating ATP through cellular respiration.

• Structure: Double membrane, inner folds called cristae.

• Function: Produce energy (ATP) via oxidative phosphorylation.

Cytoskeleton

The cytoskeleton provides structural support, facilitates movement, and organizes cell contents.

• Microfilaments: Actin filaments for cell movement and shape.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Tubulin structures for transport and cell division.

Centrosomes and Centrioles

Centrosomes organize microtubules and contain a pair of centrioles, crucial for cell division.

• Centrosome: Microtubule organizing center.

• Centrioles: Cylindrical structures involved in spindle formation during mitosis.

Cilia and Flagella

Cilia and flagella are hair-like structures that aid in cell movement or moving substances across cell surfaces.

• Cilia: Short, numerous; move substances over cell surfaces (e.g., respiratory tract).

• Flagella: Long, single; propel cells (e.g., sperm cell).

Plasma Membrane Structure and Components

Molecular Structure of the Plasma Membrane

The plasma membrane is a fluid mosaic composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Carbohydrates: Glycoproteins and glycolipids involved in cell recognition.

Hydrophilic and Hydrophobic Portions

Phospholipids have distinct hydrophilic and hydrophobic regions, critical for membrane function.

• Hydrophilic (Water-loving): Phosphate heads face the aqueous environments inside and outside the cell.

• Hydrophobic (Water-fearing): Fatty acid tails face each other within the membrane, creating a barrier to water-soluble substances.

Components of Cell Membranes

Component	Function
Phospholipids	Form bilayer, barrier to water-soluble molecules
Cholesterol	Stabilizes membrane fluidity
Proteins	Transport, signaling, structural support
Carbohydrates	Cell recognition, adhesion

Membrane Permeability and Transport

Factors Affecting Membrane Permeability

Several factors influence how easily substances cross the plasma membrane.

• Molecule Size: Smaller molecules cross more easily.

• Lipid Solubility: Lipid-soluble substances pass through the bilayer.

• Charge: Charged particles require channels or carriers.

• Presence of Transport Proteins: Facilitate movement of specific molecules.

Solvent and Solute

Solutions consist of solvents (usually water) and solutes (dissolved substances).

• Solvent: The medium in which solutes are dissolved (e.g., water).

• Solute: The particles dissolved in the solvent (e.g., ions, glucose).

Passive vs. Active Transport

Transport across membranes can be passive (no energy required) or active (energy required).

• Passive Transport: Movement down a concentration gradient; includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Movement against a concentration gradient; requires ATP.

Types of Membrane Transport Processes

• Simple Diffusion: Movement of molecules from high to low concentration without assistance.

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

• Facilitated Diffusion: Movement of molecules via transport proteins.

• Active Transport: Uses energy (ATP) to move substances against their gradient.

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

• Pinocytosis: "Cell drinking"; uptake of fluid and small molecules.

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

• Exocytosis: Release of substances from the cell via vesicles.

Sodium-Potassium (Na+/K+) Pump

Mechanism and Significance

The Na+/K+ pump is an active transport mechanism crucial for maintaining cellular homeostasis.

• Mechanism: Uses ATP to move 3 Na+ ions out and 2 K+ ions into the cell per cycle.

• Significance: Maintains membrane potential, regulates cell volume, and supports nerve impulse transmission.

Equation:

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

Tonicity and Osmosis

Tonicity

Tonicity describes the effect of a solution on cell volume, based on solute concentration relative to the cell.

• Isotonic: Equal solute concentration; no net water movement; cell remains the same size.

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

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

Determining Water Movement by Osmosis

Water moves from areas of lower solute concentration to higher solute concentration across the membrane.

• If ECF is hypotonic: Water enters cell; cell swells.

• If ECF is hypertonic: Water leaves cell; cell shrinks.

• If ECF is isotonic: No net water movement; cell stays the same.

Equation for Osmosis:

$\text{Osmosis}: \quad \text{Water moves from} \; \text{low solute} \; \text{to high solute concentration}$

Additional info: Academic context and definitions have been expanded for clarity and completeness.