Module 3.1. Introduction to Cells

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and significance of cells in living organisms.

• All organisms are made up of one or more cells: Cells are the basic building blocks of life.

• Cells are the fundamental unit of structure and function: All physiological processes occur at the cellular level.

• Cells arise from preexisting cells: New cells are produced by the division of existing cells.

Overview of Cell Structure

A typical cell consists of three main parts, each with distinct functions:

• Plasma Membrane: Acts as a "fence" that separates the cell from its environment.

  • Intracellular fluid: Fluid inside the cell.

  • Extracellular fluid: Fluid outside the cell.

• Cytoplasm: Everything inside the cell except the nucleus.

  • Cytosol: Fluid portion, mostly water, also called intracellular fluid.

  • Organelles: Specialized structures/machines with specific jobs inside the cell.

  • Cytoskeleton: Provides structural framework, keeping organelles in place.

• Nucleus: The "instructional manual" of the cell, containing DNA. DNA provides the instructions for cell function and replication. Red blood cells lack a nucleus to maximize oxygen-carrying capacity.

Module 3.2. Plasma Membrane

Functions

The plasma membrane is a selectively permeable barrier that controls the movement of substances in and out of the cell.

• Physical isolation: Separates the cell from its environment.

• Regulation of exchange: Controls entry and exit of ions, nutrients, and waste.

• Sensitivity: Responds to changes in the environment.

• Communication: Allows cells to communicate and transport messages.

• Identification: Contains markers that identify the cell as part of the organism.

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which provides fluidity and selective permeability.

• Phospholipids: Each molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) lipid tails.

• Fluid Mosaic Model: The membrane is a dynamic structure with proteins, carbohydrates, and lipids scattered throughout, allowing lateral movement of components.

Membrane Proteins

Proteins embedded in the membrane perform various functions:

• Integral proteins: Span the membrane; cannot be removed without damaging the membrane. Involved in transport and signaling.

• Peripheral proteins: Attached to the membrane surface; can be removed without damaging the membrane.

• Channel proteins: Form open tunnels for ions and molecules to pass through; often non-specific.

• Carrier proteins: Bind to specific substances and transport them across the membrane; may require energy.

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Structural support proteins: Anchor the membrane to the cytoskeleton or extracellular matrix.

• Linker proteins: Connect adjacent cell membranes, aiding in tissue formation.

Other Membrane Components

• Cholesterol: Stabilizes membrane structure, especially as temperature changes.

• Glycolipids and Glycoproteins: Carbohydrate-containing molecules that extend from the cell surface, important for cell recognition and communication.

Module 3.3. Membrane Transport

Vocabulary

• Selectively permeable: Allows some substances to pass while blocking others.

• Passive processes: Do not require energy; substances move down their concentration gradient.

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

Passive Processes

• Diffusion: Movement of solute from high to low concentration until equilibrium is reached.

  • Factors affecting rate: molecule size, temperature, distance, electrical forces.

  • Simple diffusion: Direct movement through the membrane (e.g., gases like CO2).

  • Facilitated diffusion: Uses carrier or channel proteins for substances that cannot diffuse directly.

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

  • Water moves from an area of low solute concentration to high solute concentration.

  • Osmotic pressure: The force required to prevent water movement by osmosis.

  • Hydrostatic pressure: The pressure exerted by a fluid due to its weight.

  • Isotonic solution: Equal solute concentration inside and outside the cell; no net water movement.

  • Hypertonic solution: Higher solute concentration outside the cell; water moves out, causing cell shrinkage (crenation).

  • Hypotonic solution: Lower solute concentration outside the cell; water moves in, causing cell swelling and possible lysis (bursting).

Active Processes

• Require energy (ATP) to move substances against their concentration gradient (low to high concentration).

• Sodium-Potassium Exchange Pump: Maintains concentration gradients by transporting 3 sodium ions out and 2 potassium ions in for every ATP molecule hydrolyzed.

  • Essential for maintaining cell potential and volume.

  • Equation:

• Vesicular Transport: Movement of large particles or fluids via vesicles.

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

  • Pinocytosis: "Cell drinking"; ingestion of fluid droplets.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

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

Membrane Potential

• Electrical (membrane) potential: Separation of charges across the plasma membrane.

• Resting membrane potential: The voltage across the membrane when the cell is at rest, typically measured in millivolts (mV).

• Resting potential is essential for nerve impulse transmission and muscle contraction.

Module 3.7. Protein Synthesis

Overview

Protein synthesis is the process by which cells manufacture proteins using the genetic information encoded in DNA.

Key Steps in Protein Synthesis

1. Gene: A segment of DNA containing the code for a specific protein (sequence of DNA triplets).

2. Nucleotides: The building blocks of DNA and RNA. DNA contains adenine (A), guanine (G), cytosine (C), and thymine (T). RNA contains uracil (U) instead of thymine.

   • DNA base pairs: C-G, A-T

   • RNA base pairs: C-G, A-U

3. Genetic code: Each set of three nucleotides (triplet or codon) specifies a particular amino acid.

Transcription

• Process by which a gene's DNA sequence is copied to make messenger RNA (mRNA).

• Occurs in the nucleus.

• mRNA carries the genetic code from DNA to the ribosome for protein synthesis.

Translation

• Process by which the mRNA sequence is used to assemble amino acids into a polypeptide chain (protein).

• Occurs at the ribosome in the cytoplasm.

• Transfer RNA (tRNA) brings the correct amino acids to the ribosome according to the mRNA codons.

Summary Table: DNA vs. RNA

Practice: Tonicity Diagrams

For each diagram of a cell in solution, determine the direction of water movement and classify the solution as hypotonic, hypertonic, or isotonic:

• Hypotonic: Water moves into the cell (cell may swell).

• Hypertonic: Water moves out of the cell (cell may shrink).

• Isotonic: No net movement of water (cell remains the same size).

Additional info: Practice questions reinforce understanding of osmosis and tonicity, which are critical for understanding cell volume regulation in physiology.