The Cellular Level of Organization: Structure and Function of Cells

Cell Theory and Cell Biology

Introduction to Cell Theory

• Cells are the fundamental building blocks of all living organisms.

• All organisms are composed of one or more cells.

• Cells arise only from the division of preexisting cells.

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

• Homeostasis at the tissue, organ, and organism level is maintained by the coordinated activity of cells.

Cell Biology (Cytology)

• Cytology is the study of cells, their structure, and function.

• All animal cells have three main parts:

  • Nucleus

  • Cytoplasm (cytosol)

  • Cell Membrane

• Cells are organized into tissues to perform specific functions.

Types of Cells

• Sex cells (germ cells): Sperm (male) and oocytes (female).

• Somatic cells: All other body cells except sex cells.

The Cell Structure

Plasma Membrane

• The plasma membrane (cell membrane) forms the outer boundary of the cell and separates the cytoplasm from the extracellular fluid.

• It is extremely thin (6–10 nm) and contains lipids, proteins, and carbohydrates.

Plasma Membrane: Lipids

• Composed mainly of a phospholipid bilayer:

  • Hydrophilic heads face outward toward watery environments.

  • Hydrophobic fatty-acid tails face inward, away from water.

  • This arrangement forms a barrier to ions and water-soluble compounds.

  • Cholesterol in the membrane makes it less fluid and less permeable.

Plasma Membrane: Proteins

• Integral proteins: Embedded within the membrane.

• Peripheral proteins: Bound to the inner or outer surface of the membrane.

• Functions of membrane proteins:

  • Anchoring proteins: Attach to inside or outside structures.

  • Recognition proteins: Label cells as normal or abnormal.

  • Enzymes: Catalyze reactions inside or outside the cell.

  • Receptor proteins: Bind and respond to ligands (e.g., hormones).

  • Carrier proteins: Transport specific solutes through the membrane.

  • Channels: Regulate water flow and solutes passing through the membrane.

Plasma Membrane: Carbohydrates

• Carbohydrates are found as proteoglycans, glycoproteins, and glycolipids on the cell surface, forming a sticky "sugar coat" (glycocalyx).

• Functions of the glycocalyx:

  • Lubrication and protection

  • Anchoring and locomotion

  • Specificity in binding (receptors)

  • Recognition (immune response)

Plasma Membrane: Functions

• Barrier: Separates the cell from its environment.

• Regulation: Controls exchange of ions, nutrients, and wastes.

• Sensitivity: Responds to environmental changes.

• Structural support: Stabilizes the cell and tissues.

Cytoplasm and Organelles

Cytoplasm

• The material inside the cell, excluding the nucleus.

• Contains:

  • Cytosol: Intracellular fluid with dissolved materials.

  • Organelles: Structures with specific functions.

Nonmembranous Organelles

• Not completely enclosed by membrane; direct contact with cytosol.

• Include: cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, and flagella.

Cytoskeleton

• Provides strength, flexibility, and structural support.

• Three main components:

  • Microfilaments: Thin filaments of actin; provide mechanical strength and interact with myosin for muscle contraction.

  • Intermediate filaments: Durable, stabilize organelles and cell position.

  • Microtubules: Hollow tubes of tubulin; move organelles, form spindle apparatus, and structural components of cilia and flagella.

Other Nonmembranous Organelles

• Microvilli: Increase surface area for absorption.

• Centrioles: Form spindle apparatus during cell division.

• Cilia: Move fluids across cell surface; important in respiratory and reproductive tracts.

• Flagella: Whip-like extension for cell movement (e.g., sperm).

• Ribosomes: Synthesize proteins; can be free in cytoplasm or fixed to endoplasmic reticulum.

• Proteasomes: Contain enzymes to degrade damaged proteins.

Membranous Organelles

• Enclosed by a phospholipid membrane; isolated from cytosol.

• Include: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, mitochondria, and nucleus.

Endoplasmic Reticulum (ER)

• Network of intracellular membranes continuous with the nuclear envelope.

• Smooth ER: Synthesizes lipids, phospholipids, and cholesterol; stores calcium; detoxifies drugs.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

Golgi Apparatus

• Stacks of flattened membranes (cisternae); modifies, packages, and sorts proteins and lipids for delivery.

Lysosomes

• Vesicles containing digestive enzymes; break down damaged organelles and pathogens.

• Primary lysosomes: Contain inactive enzymes.

• Secondary lysosomes: Formed when primary lysosomes fuse with damaged organelles or endocytic vesicles.

• Lysosomal storage disorders: Occur when lysosomes cannot clear toxic material (e.g., Tay-Sachs disease).

Peroxisomes

• Small vesicles containing enzymes that break down fatty acids and organic compounds.

Mitochondria

• Produce ATP through aerobic metabolism (cellular respiration).

• Contain an outer membrane and a highly folded inner membrane (cristae).

• Key steps:

  1. Glycolysis (in cytosol): Glucose → 2 pyruvate.

  2. Citric acid cycle (in mitochondrial matrix): Pyruvate → CO2 + high-energy electrons.

  3. Electron transport chain (on cristae): Electrons → ATP production.

• Mitochondrial disorders can cause muscle weakness, developmental delays, and respiratory problems.

Nucleus

• Largest organelle; stores DNA as genetic code.

• Controls cell metabolism, protein synthesis, and genetic information storage.

• Contains:

  • Nucleoplasm: Fluid inside the nucleus.

  • Nuclear matrix: Network of filaments for support.

  • Nucleolus: Synthesizes rRNA and assembles ribosomal subunits.

  • Nucleosomes: DNA coiled around histones; form chromatin (loose) or chromosomes (condensed).

Genetic Information and Protein Synthesis

DNA Organization and Function

• Genetic code: Sequence of bases (A, T, C, G) in DNA.

• Triplet code: Three bases = one amino acid.

• Gene: DNA sequence coding for a protein.

• DNA → RNA → Protein

Stages of Protein Synthesis

• Transcription: DNA → mRNA (in nucleus)

• Translation: mRNA → polypeptide (in ribosome)

Equilibrium and Protein Synthesis

• Protein synthesis is regulated by enzymes and structural proteins.

• Equilibrium changes occur when the extracellular environment changes.

Plasma Membrane and Movement

Membrane Permeability

• The plasma membrane is selectively permeable—it allows some substances to pass while restricting others.

• Permeability depends on size, charge, molecular shape, and lipid solubility.

Types of Transport

• Passive transport: Does not require energy (e.g., diffusion, osmosis, facilitated diffusion).

• Active transport: Requires energy (e.g., carrier-mediated transport, vesicular transport).

Passive Transport: Diffusion

• Molecules move from high to low concentration (down the concentration gradient).

• Example: Gas exchange in the lungs.

Passive Transport: Facilitated Diffusion

• Used for molecules that cannot cross the membrane spontaneously.

• Transported by proteins (channels or carriers).

• Example: Glucose transport via specific protein channels.

Passive Transport: Osmosis

• Diffusion of water across a membrane from high to low water concentration.

• Osmosis continues until solute concentration is equal on both sides.

• Water can cross via aquaporins (water channels).

• Osmotic pressure: Force with which water moves into a solution due to solute concentration.

• Hydrostatic pressure: Pressure that opposes osmotic pressure.

Osmolarity and Tonicity

• Osmolarity: Total solute concentration in a solution.

• Tonicity: Describes how a solution affects cell volume:

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

  • Hypotonic: Lower solute concentration than cell; water enters cell, causing swelling.

  • Hypertonic: Higher solute concentration than cell; water leaves cell, causing shrinkage.

Clinical Connections and Case Studies

Primary Ciliary Dyskinesia (PCD) and Situs Inversus

• PCD is a genetic disorder affecting cilia structure and function, leading to respiratory issues and sometimes situs inversus (reversed organ placement).

• Cilia are essential for moving mucus and particles in the respiratory tract and for moving eggs in the reproductive tract.

• Defective cilia can cause chronic respiratory infections, fertility issues, and abnormal organ positioning.

Cystic Fibrosis and Channel Proteins

• Cystic fibrosis is caused by defective chloride channel proteins in the cell membrane, leading to thick mucus and respiratory problems.

Lysosomal Storage Disorders

• Occur when lysosomes cannot degrade certain materials, leading to accumulation and cell dysfunction (e.g., Tay-Sachs disease).

Mitochondrial Disorders

• Result from dysfunctional mitochondria, causing energy deficits and symptoms such as muscle weakness and developmental delays.

Summary Table: Types of Cell Transport

Type of Transport	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen/CO2 exchange in lungs
Facilitated Diffusion	No	High to Low	Glucose transport
Osmosis	No	High to Low (water)	Water movement in kidneys
Active Transport	Yes	Low to High	Na+/K+ pump
Vesicular Transport	Yes	Varies	Endocytosis, exocytosis

Key Equations

• Osmotic Pressure: $\Pi = iMRT$ Where $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K)

Additional info:

• Clinical case studies (e.g., Mackenzie) are used to illustrate the importance of cell structure and function in health and disease.

• Some details about protein synthesis and genetic code are expanded for clarity.