Chapter 3: An Introduction to Cell Biology – Cell Types, Organelles, Plasma Membrane, and Genetic Code

Cell Classifications and Cell Theory

Somatic Cells vs Sex Cells

Cells are classified into two main types: somatic cells and sex cells. Somatic cells constitute all body cells except those involved in reproduction, and they possess an identical genome (DNA). Sex cells, also known as germ cells (sperm and ovum), contain half the genetic material of somatic cells, enabling genetic diversity through sexual reproduction.

• Somatic cells: Diploid, make up tissues and organs.

• Sex cells: Haploid, involved in fertilization.

Cell Theory

• All cells arise from pre-existing cells (principle of evolution).

• Cells are the building blocks of all living things.

• All living things are composed of at least one cell.

• Cells are the smallest units that perform all vital physiological functions.

• Cells maintain homeostasis.

Cellular Organelles

Overview of Main Organelles

Organelles are specialized structures within cells that perform distinct functions necessary for cellular survival and activity.

• Nucleus: Control center, stores genetic information, regulates metabolism and protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Protein synthesis (contains ribosomes).

  • Smooth ER: Synthesis of lipids, storage of Ca2+, detoxification.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.

• Ribosomes: Sites of protein synthesis (bound to ER or free in cytoplasm).

• Mitochondria: ATP production via cellular respiration.

• Lysosomes: Breakdown of cellular debris and foreign material.

• Vesicles: Transport materials within and outside the cell.

Plasma Membrane Structure and Function

Plasma Membrane (PM)

The plasma membrane is the outer boundary of the cell, functioning as a selective barrier between the internal cytoplasm and the external environment.

• Cytoplasm: The internal environment, consisting of cytosol (intracellular fluid, high in K+) and organelles.

• Extracellular Fluid (ECF): The external fluid, high in Na+, where nutrients and wastes are exchanged.

Functions of the Plasma Membrane

• Physical Isolation: Separates internal and external environments.

• Regulation of Exchange: Controls movement of ions, nutrients, and wastes via transport proteins.

• Sensitivity/Response: Embedded protein receptors detect chemical and mechanical signals.

• Structural Support: Anchors cells and tissues through membrane proteins.

Membrane Proteins

Types and Functions

• Integral Proteins: Span the membrane, may be transmembrane.

• Peripheral Proteins: Located on inner or outer surfaces of the membrane.

• Anchoring Proteins: Stabilize the membrane.

• Receptor Proteins: Bind and respond to ligands (hormones, ions).

• Recognition Proteins: Label cells as normal or abnormal.

• Enzyme Proteins: Catalyze reactions near or within the membrane.

• Carrier & Channel Proteins: Facilitate transport of substances; carriers bind to molecules, channels allow passage without binding.

Cytoskeleton and Surface Structures

Cytoskeletal Proteins

The cytoskeleton maintains cell shape and includes microfilaments, intermediate filaments, and microtubules.

• Cilia: Short, numerous, move substances across cell surfaces (e.g., trachea).

• Flagella: Long, singular, whip-like motion for cell movement (e.g., sperm).

• Microvilli: Increase surface area for absorption.

Membranous Organelles

Functions and Interconnections

• Endoplasmic Reticulum: Synthesis, storage, transport, and detoxification.

• Golgi Apparatus: Protein modification and shipping.

• Ribosomes: Protein synthesis.

• Mitochondria: ATP production via aerobic metabolism. Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 34-36\text{ATP}$

• Lysosomes: Breakdown of debris, autolysis, autophagy, apoptosis.

• Vesicles: Internal and external transport.

Lysosome Function

• Primary Lysosome: Contains inactive enzymes.

• Secondary Lysosome: Contains active enzymes after fusion with debris.

• Autolysis: Destruction of cell after death.

• Autophagy: Programmed cell death for recycling components.

• Apoptosis: Cell death for developmental and maturational purposes.

Mitochondria Function

• ATP Production: Cellular respiration occurs in three phases: glycolysis, citric acid cycle, electron transport chain.

• Aerobic Metabolism: Utilizes O2 to break down glucose, yielding 34-36 ATP per glucose molecule.

Nucleus and Genetic Code

Nucleus Structure and Function

• Nuclear Pores: Allow exchange and communication.

• Nucleolus: Assembles rRNA and ribosomal subunits.

• Nucleosomes: DNA wrapped around histones for condensation.

DNA, Chromatin, and Chromosomes

• DNA: Long thread of genetic material with genes.

• Chromatin: Loosely coiled DNA + protein, active in non-dividing cells.

• Chromosomes: Highly coiled during division, protect DNA.

• Genes: Segments of DNA coding for proteins.

Genetic Code and Protein Synthesis

• Genetic Code: Chemical language of DNA; 3 bases = 1 amino acid.

• Central Dogma:

  • Transcription: DNA is copied into mRNA.

  • Translation: Ribosomes use mRNA to synthesize proteins by linking amino acids.

Control of Protein Synthesis

• Direct Control: Synthesis of structural and secretory proteins.

• Indirect Control: Regulation of metabolism via enzymes.

• Homeostasis: Protein synthesis maintains cellular balance.

Plasma Membrane Permeability and Transport

Permeability

• Impermeable: No substances pass.

• Freely Permeable: All substances pass.

• Selectively Permeable: Only certain substances pass, regulated by membrane proteins.

Molecules Passing the Plasma Membrane

• Hydrophobic (nonpolar) molecules: Pass easily.

• Hydrophilic (polar) molecules: Require transport proteins.

• Lipid solubility: Lipids can pass through the membrane.

Modes of Transport

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

• Facilitated Diffusion: Diffusion via transport proteins.

• Osmosis: Movement of water across the membrane from high to low concentration.

• Active Transport: Uses energy and transport proteins to move substances against concentration gradient. Example: Sodium-potassium pump ($3\text{Na}^+$ out, $2\text{K}^+$ in per ATP).

• Vesicular Transport: Bulk movement via vesicles (endocytosis and exocytosis).

Tonicity and Cellular Response

• Isotonic Solution: No net movement of water; cell remains unchanged.

• Hypertonic Solution: Water moves out; cell shrinks.

• Hypotonic Solution: Water moves in; cell swells.

Summary Table: Membranous Organelles and Functions

Organelle	Main Function
Nucleus	Genetic information storage, control center
Endoplasmic Reticulum	Macromolecule synthesis, storage, transport, detoxification
Golgi Apparatus	Protein modification and shipping
Ribosomes	Protein synthesis
Mitochondria	ATP production via cellular respiration
Lysosomes	Breakdown of debris, autolysis, autophagy, apoptosis
Vesicles	Transport within and outside the cell

Additional info: Figures referenced in the notes (e.g., 3-1, 3-7, 3-8, 3-9, 3-10, 3-11, 3-12) provide visual models and diagrams for organelle structure and function, protein synthesis, and membrane transport mechanisms.