Module 3.1 Introduction to Cells

Basic Processes of Cells

Cells are the fundamental units of life, performing essential processes to maintain homeostasis and support organismal function.

• Cell metabolism: The sum of all chemical reactions occurring within a cell, including catabolic and anabolic pathways.

• Transport of substances: Movement of molecules into and out of cells, crucial for nutrient uptake and waste removal.

• Communication: Cells communicate via chemical signals, such as hormones and neurotransmitters, to coordinate activities.

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

Module 3.1 Overview of Cell Structure

Main Components of Cells

Cells have distinct structural components that perform specialized functions.

• Plasma membrane: A selectively permeable barrier that surrounds the cell, controlling entry and exit of substances.

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

  • Cytosol: The fluid portion of cytoplasm.

  • Inclusions: Non-living substances stored in the cytoplasm (e.g., glycogen, lipid droplets).

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

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

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

Module 3.2 Structure of the Plasma Membrane

The Phospholipid Bilayer

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

• Molecules must have hydrophilic (water-attracting) and hydrophobic (water-repelling) regions to interact with both intracellular and extracellular fluids.

• The bilayer separates the extracellular fluid (ECF) from the cytosol.

Membrane Proteins

Proteins embedded in the plasma membrane serve various functions, including transport, signaling, and structural support.

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

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

Module 3.3 Transport Across the Plasma Membrane

Selective Permeability and Transport Mechanisms

The plasma membrane allows selective movement of substances via passive and active transport mechanisms.

• Passive transport mechanisms: Do not require energy; substances move down their concentration gradients.

  • Simple diffusion: Movement of nonpolar solutes (e.g., oxygen, carbon dioxide, lipids) directly through the bilayer.

  • Facilitated diffusion: Movement of polar or charged solutes (e.g., ions, glucose) via membrane proteins (channels or carriers).

  • Osmosis: Diffusion of water across a membrane from lower to higher solute concentration.

    • Osmotic pressure: The force driving water movement due to solute concentration differences.

    • Water moves via channel proteins called aquaporins or directly through the bilayer.

• Tonicity: Describes the effect of solution concentration on cell volume.

  • Isotonic: No net water movement; cell volume remains constant.

  • Hypertonic: Water leaves the cell; cell shrinks.

  • Hypotonic: Water enters the cell; cell swells.

• Active transport mechanisms: Require energy (usually ATP) to move substances against their concentration gradients.

  • Use carrier proteins called pumps.

  • Examples include sodium-potassium pump and calcium pump.

Module 3.4 Cytoplasmic Organelles

Major Organelles and Their Functions

Organelles are specialized structures within the cytoplasm that perform essential cellular functions.

• Mitochondria: Site of oxidative catabolism; produce ATP via cellular respiration.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Ribosomes: Synthesize proteins by translating mRNA.

• The Endomembrane System:

  • Rough endoplasmic reticulum (RER): Studded with ribosomes; synthesizes and processes proteins.

  • Smooth endoplasmic reticulum (SER): Synthesizes lipids and detoxifies chemicals.

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

  • Lysosomes: Contain digestive enzymes for breaking down cellular waste and foreign material.

Module 3.5 The Cytoskeleton

Types and Functions of Cytoskeletal Filaments

The cytoskeleton provides structural support, facilitates movement, and organizes cellular components.

• Actin filaments (microfilaments): Support cell shape and enable movement.

• Intermediate filaments: Provide mechanical strength.

• Microtubules and centrioles: Involved in cell division and intracellular transport.

Cellular Extensions

• Microvilli: Increase surface area for absorption.

• Cilia: Move substances across cell surfaces.

• Flagella: Enable cell motility (e.g., sperm cells).

Module 3.6 The Nucleus

Structure and Function of the Nucleus

The nucleus contains the cell's genetic material and directs cellular activities.

• DNA: Encodes genetic information for protein synthesis.

• Nuclear envelope: Double membrane that surrounds the nucleus.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Chromosomes: Structures containing DNA; humans have 23 pairs.

• Nucleolus: Region within the nucleus responsible for ribosomal RNA synthesis and ribosome assembly.

Module 3.7 Protein Synthesis

Overview of Protein Synthesis

Protein synthesis is the process by which cells manufacture proteins using genetic instructions from DNA.

• Gene expression: Production of a protein from a specific gene.

• Transcription: DNA is copied into messenger RNA (mRNA).

  • Occurs in the nucleus.

  • Involves stages: initiation, elongation, termination.

• Translation: mRNA is decoded at the ribosome to assemble amino acids into a polypeptide.

  • Transfer RNA (tRNA) brings amino acids to the ribosome.

  • Each set of three nucleotides (codon) codes for a specific amino acid.

• Summary equation:

Module 3.8 The Cell Cycle

Phases of the Cell Cycle

The cell cycle consists of two main phases: interphase (cell growth and DNA replication) and M phase (cell division).

• Interphase:

  • G1 phase: Cell growth.

  • S phase: DNA replication.

  • G2 phase: Preparation for division.

• M phase:

  • Mitosis: Division of the nucleus; includes four stages:

    1. Prophase

    2. Metaphase

    3. Anaphase

    4. Telophase

  • Cytokinesis: Division of the cytoplasm.

Cell Cycle Control and Cancer

• Cell cycle checkpoints ensure proper division and prevent errors.

• Cells that fail checkpoints may undergo programmed cell death (apoptosis).

• Loss of cell cycle control can result in uncontrolled cell division (tumor).

• Metastasis: The ability of cancer cells to spread throughout the body.

Summary Table: Types of Cytoskeletal Filaments

Summary Table: Membrane Transport Mechanisms

Key Equations

• Osmotic Pressure: where is osmotic pressure, is the van 't Hoff factor, is molarity, is the gas constant, and is temperature.

• Summary of Protein Synthesis:

Additional info: Some explanations and examples have been expanded for clarity and completeness.