The Cell: Structure, Membrane, and Transport Processes

3.1 Basic Processes of Cells

Cell Metabolism

Cell metabolism encompasses all chemical reactions that occur within a cell to maintain life. These reactions are essential for energy production, synthesis of new molecules, and breakdown of waste products.

• Catabolic Reactions: Break down molecules to release energy.

• Anabolic Reactions: Build complex molecules from simpler ones, requiring energy input.

• Oxidation-Reduction Reactions: Involve the transfer of electrons between molecules, crucial for energy production.

Substance Transport

Cells must move substances across their membranes to maintain homeostasis. This includes the import of nutrients and export of waste products.

• Transport Mechanisms: Include passive (no energy required) and active (energy required) processes.

Communication

Cells communicate with each other and their environment through chemical signals and direct contact.

• Cell Signaling: Involves receptors and signaling molecules.

Cell Reproduction

Many cells undergo cell division to reproduce, repair tissues, and support growth.

3.1 Overview of Cell Structure

Basic Components of Most Animal Cells

• Plasma Membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol and organelles.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Functions of the Plasma Membrane

• Physical Barrier: Separates the cell from its surroundings.

• Communication: Contains proteins that allow the cell to communicate with its environment.

• Transport: Regulates movement of substances in and out of the cell.

• Defines Fluid Compartments: Separates intracellular fluid (ICF) from extracellular fluid (ECF).

Components of the Cytoplasm

• Cytosol (Intracellular Fluid): The fluid portion of the cytoplasm, containing water, ions, and dissolved molecules.

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

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

Nucleus

• Surrounded by a double membrane (nuclear envelope).

• Contains DNA, which directs cellular activities by coding for proteins.

3.2 The Phospholipid Bilayer

Structure and Function

The plasma membrane is primarily composed of a phospholipid bilayer, which forms an effective barrier between the cell's internal and external environments.

• Phospholipids: Have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Arrange themselves so that the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward toward water.

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which states that the membrane is a flexible, dynamic structure with proteins and other molecules embedded within the phospholipid bilayer.

• Proteins: Integral and peripheral proteins serve various functions, including transport, signaling, and structural support.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Proteins

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

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

• Types of Membrane Proteins:

  • Channels: Allow specific ions or molecules to pass through.

  • Carriers: Bind and transport substances across the membrane.

  • Receptors: Bind signaling molecules and initiate cellular responses.

  • Enzymes: Catalyze chemical reactions.

  • Structural Support: Maintain cell shape and structure.

3.3 Transport Across the Plasma Membrane

Types of Transport

• 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

• Diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.

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

• Facilitated Diffusion: Movement of substances across the membrane via specific transport proteins.

Osmosis and Tonicity

• Osmosis: Water moves from an area of lower solute concentration to an area of higher solute concentration.

• Tonicity: Describes the ability of a solution to cause a cell to gain or lose water.

  • Isotonic Solution: No net movement of water; cell size remains the same.

  • Hypertonic Solution: Cell loses water and shrinks.

  • Hypotonic Solution: Cell gains water and swells.

Hydrostatic and Osmotic Pressure

• Hydrostatic Pressure: The force exerted by a fluid against a wall.

• Osmotic Pressure: The pressure required to prevent the movement of water by osmosis.

Active Transport Processes

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

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

Sodium-Potassium Pump (Na+/K+ ATPase)

• Maintains the electrochemical gradient across the plasma membrane.

• For every 3 sodium ions pumped out, 2 potassium ions are pumped in.

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

Vesicular Transport

• Endocytosis: Uptake of substances into the cell via vesicles.

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

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

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

Consequences of Ion Transport: Introduction to Electrophysiology

• Resting Membrane Potential: The electrical potential difference across the plasma membrane, typically -70 mV in neurons.

• Maintained by the sodium-potassium pump and differential permeability of the membrane to ions.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down water gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Glucose/Na+ cotransport
Endocytosis/Exocytosis	Yes (ATP)	Bulk transport	Neurotransmitter release

Key Terms and Definitions

• Phospholipid Bilayer: Double layer of phospholipids forming the basic structure of the plasma membrane.

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

• Isotonic, Hypertonic, Hypotonic: Terms describing the relative solute concentration of solutions.

• Electrochemical Gradient: Combined effect of concentration and electrical gradients on ion movement.

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