Cell Structure and Function: An Overview for Anatomy & Physiology

Module 3.1: Introduction to Cells

Basic Processes of Cells

All cells perform several essential processes to maintain life. Understanding these processes is fundamental to cell biology and physiology.

• Cell Metabolism: The sum of all chemical reactions that a cell carries out to maintain life. This includes anabolic (building up) and catabolic (breaking down) reactions, as well as oxidation-reduction reactions.

• Transport of Substances: The movement of substances into, out of, or within a cell, often across the plasma membrane. This is vital for nutrient uptake, waste removal, and maintaining homeostasis.

• Communication: Cells communicate with their environment and other cells via chemical and electrical signals, which is essential for coordination and function.

• Cell Reproduction: The process by which cells divide to produce new cells, necessary for growth, development, and tissue repair.

Overview of Cell Structure

Most animal cells share three main structural components:

1. Plasma Membrane: Encloses the cell, isolating its internal environment from the external environment. It provides structural support, communication, and cell identification.

2. Cytoplasm: The region inside the plasma membrane, containing:

   • Cytosol: The intracellular fluid, rich in dissolved solutes, inclusions, and proteins. It is the site of many metabolic reactions.

   • Organelles: Specialized structures (e.g., mitochondria, endoplasmic reticulum) that perform specific cellular functions.

   • Cytoskeleton: A network of protein filaments that supports cell shape, anchors organelles, and enables movement.

3. Nucleus: A roughly spherical organelle containing most of the cell's DNA. It is surrounded by a nuclear envelope (a double membrane) and is the site of RNA synthesis.

Cell Size and Diversity

Cells vary widely in size and structure, allowing them to perform specialized functions.

Module 3.2: Structure of the Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which separates the intracellular fluid (ICF) from the extracellular fluid (ECF).

• Amphipathic Nature: Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, allowing the membrane to form a stable barrier between aqueous compartments.

• Fluid Mosaic Model: The membrane is a dynamic, fluid structure with proteins and other molecules (cholesterol, glycolipids, glycoproteins) embedded within the bilayer. Components can move laterally within the membrane.

Membrane Proteins

• Integral Proteins: Span the membrane and are involved in transport and communication.

• Peripheral Proteins: Attached to one side of the membrane, often involved in signaling or maintaining cell shape.

Other Membrane Components

• Cholesterol: Stabilizes membrane fluidity, especially during temperature changes.

• Glycolipids and Glycoproteins: Involved in cell recognition and signaling.

Module 3.3: Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane allows certain molecules to cross while restricting others, which is critical for cell survival.

Passive Transport Processes

• Diffusion: Movement of solute molecules from high to low concentration (down their concentration gradient) until equilibrium is reached.

  • Simple Diffusion: Nonpolar solutes (e.g., O2, CO2, lipids) pass directly through the membrane.

  • Facilitated Diffusion: Polar solutes (e.g., ions, glucose) cross via membrane proteins (channels or carriers).

• Osmosis: Movement of water across a membrane from low to high solute concentration, often through aquaporins.

• Tonicity: Describes the relative solute concentration of two solutions separated by a membrane (isotonic, hypertonic, hypotonic).

Active Transport Processes

• Primary Active Transport: Uses ATP to move solutes against their concentration gradient via pumps (e.g., Na+/K+ ATPase).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Vesicular Transport

• Endocytosis: Uptake of large molecules or particles via vesicles (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Release of substances from the cell via vesicles fusing with the plasma membrane.

Electrophysiology

• Membrane Potential: The separation of charges across the plasma membrane creates an electrical potential, measured in millivolts (mV).

• Resting Membrane Potential: The value of the membrane potential when a cell is at rest; typically, the inside of the cell is more negative than the outside.

Module 3.4: Cytoplasmic Organelles

Overview of Organelles

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival and homeostasis.

Organelle	Main Function
Mitochondria	ATP production via oxidative catabolism; contains its own DNA and ribosomes
Peroxisomes	Carry out oxidation reactions, producing hydrogen peroxide (H2O2)
Ribosomes	Protein synthesis; can be free in cytosol or bound to rough ER
Endoplasmic Reticulum (ER)	Rough ER: protein synthesis and transport; Smooth ER: lipid synthesis, detoxification, calcium storage
Golgi Apparatus	Modifies, sorts, and packages proteins and lipids for export or delivery to other organelles

Endomembrane System

• Includes the plasma membrane, nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles.

• Functions in synthesis, modification, and transport of proteins and lipids.

Rough vs. Smooth Endoplasmic Reticulum

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

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium ion storage.

Golgi Apparatus

• Receives proteins and lipids from the ER, modifies them, and packages them into vesicles for transport.

Mitochondria

• Double-membraned organelles; site of most ATP production via aerobic respiration.

• Contain their own DNA and ribosomes.

Peroxisomes

• Membrane-bound organelles that use oxygen to carry out oxidation reactions, producing hydrogen peroxide.

Ribosomes

• Composed of protein and ribosomal RNA (rRNA).

• Sites of protein synthesis; can be free in the cytosol or bound to the ER.

Key Terms and Definitions

• Homeostasis: The maintenance of a stable internal environment.

• Organelle: A specialized subunit within a cell with a specific function.

• Phospholipid Bilayer: The fundamental structure of the plasma membrane, composed of two layers of phospholipids.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Endocytosis/Exocytosis: Processes for moving large substances into or out of the cell via vesicles.

Important Equations

• Diffusion Rate (Fick's Law):

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

• Osmotic Pressure:

$\Pi = iMRT$

• Resting Membrane Potential (Nernst Equation):

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

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Osmosis	No	Water movement	Water via aquaporins
Primary Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Secondary Active Transport	Yes (indirectly)	Low to High (coupled)	Na+-glucose symport
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, Exocytosis

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.