Cells: The Basic Unit of Life

Cell Theory and Cellular Composition

The cell is the fundamental structural and functional unit of all living organisms. Cell theory outlines four main tenets:

• All living organisms are composed of cells.

• The activity of an organism depends on the collective activities of its cells.

• Principle of complementarity: The structure of a cell determines its function.

• Continuity of life: All cells arise from pre-existing cells.

Most cells are composed primarily of carbon, hydrogen, oxygen, and nitrogen, with water accounting for about 60% of cell mass.

Types of Cells

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotic cells: Possess a nucleus and membrane-bound organelles (e.g., animal, plant, fungi, protist cells).

Anatomy of a Generalized Animal Cell

Main Components

• Plasma membrane: Surrounds the cell, regulates entry/exit of substances.

• Nucleus: Control center, contains DNA.

• Cytoplasm: Area between plasma membrane and nucleus, contains organelles.

Plasma Membrane Structure

• Fluid mosaic model: Double layer of phospholipids arranged tail-to-tail, with embedded proteins, cholesterol, glycoproteins, and glycolipids.

• Phospholipid bilayer: Hydrophilic heads face water; hydrophobic tails face inward, making the membrane selectively permeable.

• Proteins: Function as enzymes, receptors, channels, and carriers.

• Glycoproteins/glycocalyx: Cell recognition, adhesion, immunity.

Cell Junctions

• Tight junctions: Impermeable, bind cells into leak-proof sheets.

• Desmosomes: Anchoring junctions, resist mechanical stress.

• Gap junctions: Allow communication via connexons (protein channels).

Nucleus

• Nuclear envelope: Double membrane with nuclear pores for selective exchange.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA wound around histones; condenses to chromosomes during cell division.

Cytoplasm and Organelles

• Cytosol: Fluid suspending organelles.

• Inclusions: Stored nutrients, cell products.

• Organelles: Metabolic machinery, often membrane-bound.

Major Organelles

• Mitochondria: Powerhouse, site of cellular respiration, produces ATP.

• Rough ER: Studded with ribosomes, synthesizes proteins.

• Smooth ER: Lipid metabolism, detoxification.

• Golgi apparatus: Modifies, packages proteins for secretion, membrane incorporation, or lysosome formation.

• Lysosomes: Digest worn-out organelles, bacteria, debris.

• Peroxisomes: Detoxify harmful substances, break down free radicals.

• Cytoskeleton: Internal framework; microfilaments (actin), intermediate filaments, microtubules (tubulin).

• Centrioles: Organize mitotic spindle during cell division.

Cell Extensions

• Cilia: Move materials across cell surface (e.g., respiratory tract).

• Flagella: Propel cell (e.g., sperm).

• Microvilli: Increase surface area for absorption (e.g., digestive tract).

Cell Diversity

Specialized Cell Types

• Fibroblasts: Connect body parts, secrete fibers.

• Erythrocytes: Red blood cells, carry oxygen.

• Epithelial cells: Cover/line organs, resist tearing.

• Muscle cells: Skeletal and smooth, contractile filaments.

• Fat cells: Store nutrients (lipids).

• Macrophages: White blood cells, digest pathogens.

• Neurons: Gather information, transmit messages.

• Oocytes and sperm: Reproductive cells.

Cell Physiology: Membrane Transport

Solution Terminology

• Solution: Homogeneous mixture.

• Solvent: Dissolving medium (water in the body).

• Solute: Dissolved substance.

Fluid Compartments

• Intracellular fluid: Nucleoplasm, cytosol.

• Extracellular fluid: Interstitial fluid, outside cell.

Membrane Transport Mechanisms

• Passive transport: No ATP required.

• Active transport: Requires ATP.

Passive Transport

• Diffusion: Movement from high to low concentration (down gradient).

• Simple diffusion: Unassisted, for small/lipid-soluble molecules.

• Osmosis: Diffusion of water via aquaporins.

• Facilitated diffusion: Assisted by protein channels/carriers (e.g., glucose, ions).

• Filtration: Movement by hydrostatic pressure (e.g., kidney filtration).

Tonicity

Active Transport

• Solute pumps: Move substances against gradient (e.g., sodium-potassium pump).

• Vesicular transport: Bulk movement via vesicles.

• Exocytosis: Secretion of substances out of cell.

• Endocytosis: Uptake of substances into cell (phagocytosis, pinocytosis, receptor-mediated).

Sodium-Potassium Pump Equation

For each cycle:

• 3 Na+ out

• 2 K+ in

• 1 ATP hydrolyzed

Equation:

Cell Division

Cell Cycle

• Interphase: Growth, metabolic activity, DNA replication.

• Cell division: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

DNA Replication

• DNA uncoils, each strand serves as template.

• Complementary base pairing: A-T, G-C.

Mitosis Stages

Cytokinesis: Cleavage furrow forms, cytoplasm divides, two daughter cells result.

Protein Synthesis

Central Dogma

• DNA → RNA → Protein

• Gene: DNA segment coding for a protein.

RNA Types

• mRNA: Messenger, carries code from nucleus to ribosome.

• rRNA: Ribosomal, forms ribosomes.

• tRNA: Transfer, brings amino acids to ribosome.

Transcription

• DNA base sequence transcribed to mRNA.

• Base pairing: A-U, C-G (in RNA).

• Example: DNA AAT-CGT-TCG → mRNA UUA-GCA-AGC

Translation

• mRNA attaches to ribosome.

• tRNA anticodon binds mRNA codon, brings correct amino acid.

• Ribosome moves along mRNA, amino acids joined by peptide bonds.

Body Tissues

Four Primary Tissue Types

• Epithelial: Covers, lines, forms glands.

• Connective: Supports, binds, protects, stores, transports.

• Muscle: Contracts for movement.

• Nervous: Receives, conducts impulses.

Epithelial Tissue

• Locations: Body coverings, linings, glands.

• Functions: Protection, absorption, filtration, secretion.

• Hallmarks: Apical surface, basement membrane, avascular, regenerates well.

Classification

Glandular Epithelium

• Endocrine glands: Ductless, secrete hormones into blood (e.g., thyroid).

• Exocrine glands: Secrete via ducts to surface (e.g., sweat, oil glands).

Connective Tissue

• Functions: Protect, support, cushion, insulate.

• Characteristics: Varying vascularity, abundant extracellular matrix (ground substance + fibers).

Types of Connective Tissue

Muscle Tissue

• Skeletal: Voluntary, striated, multinucleate, attached to bones.

• Cardiac: Involuntary, striated, one nucleus, branching cells, intercalated discs, heart only.

• Smooth: Involuntary, no striations, one nucleus, spindle-shaped, walls of hollow organs.

Nervous Tissue

• Function: Receive and conduct electrochemical impulses.

• Components: Neurons (message senders), neuroglia (support, protect, insulate).

• Locations: Brain, spinal cord, nerves.

Summary Table: Four Tissue Types

Tissue Repair and Developmental Aspects

Tissue Repair (Wound Healing)

• Regeneration: Replacement by same cell type.

• Fibrosis: Replacement by scar tissue (dense connective tissue).

• Depends on tissue type and injury severity.

• Sequence: Inflammation → clot formation → granulation tissue → regeneration/fibrosis → scab detaches.

• Epithelial, bone, fibrous connective tissues regenerate well; skeletal muscle regenerates poorly; cardiac muscle and nervous tissue replaced by scar tissue.

Developmental Aspects

• Growth via mitosis continues through puberty.

• Epithelium and connective tissue remain mitotic; muscle and nervous tissue become amitotic (lose ability to divide).

• Injury severely handicaps amitotic tissues.

• Aging leads to loss of cell division ability; causes include chemical, physical insults, and genetics.

• Neoplasms (tumors) result from uncontrolled mitosis; hyperplasia (increase in size) from excessive stimulation; atrophy (decrease in size) from lack of stimulation.

Key Equations and Concepts

Diffusion Equation

Rate of diffusion is proportional to concentration gradient and inversely proportional to distance:

Where is flux, is diffusion coefficient, is concentration gradient.

Osmosis

Water moves toward higher solute concentration:

Sodium-Potassium Pump

Central Dogma

Additional info:

• Epithelium is avascular but regenerates well due to proximity to connective tissue.

• Connective tissue matrix composition determines rigidity (bone vs. blood).

• Muscle tissue contractility is due to actin and myosin filaments.

• Nervous tissue conductivity is due to ion channels and neurotransmitter release.