Module 3.1: Introduction to Cells

Basic Processes of Cells

Cells carry out several fundamental processes essential for life. These processes are common to all cell types and underpin cellular function and survival.

• Cell metabolism: All chemical reactions that occur within a cell to maintain life, including anabolic (building) and catabolic (breaking down) reactions.

• Transport of substances: Movement of molecules into and out of the cell, necessary for nutrient uptake and waste removal.

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

• Cell reproduction: Cells divide to produce new cells for growth, development, and repair.

Additional info: Catabolic reactions release energy, often captured in ATP; anabolic reactions consume energy to build cellular components.

Overview of Cell Structure

Most animal cells share several key structural components that enable their diverse functions.

• Plasma membrane: Surrounds each cell, isolating its internal environment and regulating exchange with the external environment.

• Cytoplasm: Contains cytosol (fluid), organelles, and the cytoskeleton.

• Nucleus: Contains most of the cell's DNA and is the primary site for genetic information storage and processing.

Additional info: The cytoskeleton provides structural support and facilitates intracellular transport.

Module 3.2: Structure of the Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a double layer of phospholipids, which forms a barrier between the cell and its environment.

• Phospholipids: Each molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

• Bilayer arrangement: Hydrophilic heads face outward toward water, while hydrophobic tails face inward, away from water.

Fluid Mosaic Model: The membrane is described as a fluid mosaic because proteins and other molecules float within or on the bilayer, allowing flexibility and dynamic function.

Functions of Membrane Proteins

Membrane proteins are integral to plasma membrane function, contributing to transport, communication, and structural integrity.

• Transport proteins: Move substances across the membrane.

• Receptor proteins: Bind to signaling molecules and initiate cellular responses.

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Structural support: Maintain cell shape and stabilize membrane structure.

• Linking proteins: Connect adjacent cells or anchor the membrane to the cytoskeleton.

Other Membrane Components

• Carbohydrates: Attach to proteins and lipids, forming glycoproteins and glycolipids involved in cell recognition.

• Cholesterol: Stabilizes membrane fluidity and structure.

Module 3.3: Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane is selectively permeable, allowing certain molecules to cross while restricting others. This property is essential for maintaining cellular homeostasis.

• Passive transport: Does not require energy; substances move down their concentration gradient.

• Active transport: Requires energy (usually ATP); substances move against their concentration gradient.

Passive Transport Processes

Passive transport includes several mechanisms by which substances move across the membrane without energy input.

• Diffusion: Movement of molecules from high to low concentration.

• Simple diffusion: Direct movement of small, nonpolar molecules through the lipid bilayer.

• Facilitated diffusion: Movement of charged or polar molecules via membrane proteins (channels or carriers).

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

Key Concepts in Diffusion and Osmosis

• Concentration gradient: Difference in concentration of a substance across a space.

• Equilibrium: State where concentrations are equal on both sides of the membrane.

• Osmotic pressure: Pressure exerted by water moving across a membrane due to differences in solute concentration.

• Tonicity: Describes the effect of a solution on cell volume (isotonic, hypotonic, hypertonic).

Table: Types of Passive Transport

Drugs and Membrane Receptors

Many drugs are designed to interact with membrane receptors, which are proteins that bind specific ligands and trigger cellular responses.

• Agonists: Activate receptors to produce a biological response.

• Antagonists: Block receptors and inhibit biological responses.

Example: Antihistamines block histamine receptors to reduce allergy symptoms.

Key Equations

• Fick's Law of Diffusion:

• Osmotic Pressure (van't Hoff equation):

Where is the rate of diffusion, is the diffusion coefficient, is the concentration gradient, is osmotic pressure, is the van't Hoff factor, is molarity, is the gas constant, and is temperature.

Summary Table: Cell Structure Components

Review Questions

• What are the main processes common to all cells?

• Describe the structure and function of the plasma membrane.

• Explain the difference between passive and active transport.

• How do membrane proteins contribute to cell function?

• What is the role of osmosis in maintaining cell homeostasis?