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, including 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 involving the plasma membrane.

• Communication: Cells communicate with their environment and other cells via chemical and electrical signals.

• 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 basic structural components:

• Plasma membrane: Surrounds the cell, isolating its internal structures and processes from the external environment. It provides structural support, communication, and cell identification.

• Cytoplasm: Contains the cytosol (intracellular fluid), organelles (specialized structures with specific functions), and the cytoskeleton (network of protein filaments for support and transport).

• Nucleus: A roughly spherical organelle that contains most of the cell's DNA and is the primary site for RNA synthesis. It is enclosed by a phospholipid bilayer called the nuclear envelope.

Cell Size and Diversity

Cells vary widely in size and structure to enable 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).

• The phospholipid bilayer has two key properties:

  1. Molecules must have parts that can interact with water in both fluid compartments without falling apart.

  2. Molecules must have parts that repel water, keeping the ECF and cytosol separated.

Fluid Mosaic Model

This model describes the plasma membrane as a dynamic, fluid structure with multiple components (proteins, lipids, carbohydrates) that can move within the bilayer.

• Membrane proteins: Integral proteins span the membrane; peripheral proteins are found on one side only.

• Other components: Cholesterol stabilizes the membrane; glycolipids and glycoproteins are involved in cell recognition.

Module 3.3: Transport Across the Plasma Membrane

Passive Transport Processes

Passive transport does not require energy and includes:

• Diffusion: Movement of solute molecules from high to low concentration 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 lower to higher solute concentration, often through aquaporins.

• Tonicity: Compares the solute concentration of two solutions, affecting cell volume (isotonic, hypertonic, hypotonic).

Active Transport Processes

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

• Primary active transport: Uses pumps (e.g., Na+/K+ ATPase) to maintain ion gradients.

• Secondary active transport: Uses ATP indirectly to transport substances by harnessing the energy from another substance's gradient.

Transport via Vesicles

• Endocytosis: Cell engulfs ECF, large molecules, or whole cells (phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Large molecules exit the cell via vesicles fusing with the plasma membrane.

Introduction to Electrophysiology

• Separation of charges across the plasma membrane creates an electrical potential.

• Membrane potential: The electrical potential across the membrane at rest, measured in millivolts (mV).

• The inside of the cell is usually more negative than the outside.

Module 3.4: Cytoplasmic Organelles

Overview of Organelles

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

• Mitochondria: The "power plant" of the cell, responsible for ATP production via oxidative catabolism. Contains its own DNA and a double membrane.

• Peroxisomes: Use oxygen to carry out reactions that produce hydrogen peroxide (H2O2), important for detoxification.

• Ribosomes: Granular organelles composed of protein and ribosomal RNA (rRNA); sites of protein synthesis. Found free in the cytosol or bound to membranes.

• Endomembrane system: Includes the plasma membrane, nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, and vesicles. Functions in synthesis, modification, and transport of proteins and lipids.

• Endoplasmic reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes and transports proteins.

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

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for export or delivery to other organelles.

Table: Comparison of Major Organelles

Key Equations and Concepts

• Diffusion rate (Fick's Law): Where is the diffusion flux, is the diffusion coefficient, and is the concentration gradient.

• Osmotic pressure: Where is osmotic pressure, is the van 't Hoff factor, is molarity, is the gas constant, and is temperature in Kelvin.

• Na+/K+ ATPase pump: Maintains the electrochemical gradients of sodium and potassium across the plasma membrane, essential for cell function.

Summary

• Cells are the basic units of life, with specialized structures and processes for maintaining homeostasis.

• The plasma membrane controls the movement of substances and communication with the environment.

• Organelles perform distinct functions necessary for cell survival and specialization.