Chapter 3: The Cells

Cell Theory

The cell is the fundamental structural and functional unit of living organisms. All life processes depend on cellular activity.

• Organismal functions depend on individual and collective cell functions.

• Biochemical activities of cells are dictated by their specific subcellular structures.

• Continuity of life depends on cells.

Generalized Cell Structure

All human cells share basic structural features, though they may differ in function.

• Plasma membrane: Outer boundary of the cell, separating internal and external environments.

• Cytoplasm: Contains organelles and cytosol.

• Nucleus: Control center housing genetic material.

Plasma Membrane

Structure and Function

The plasma membrane is a dynamic bilayer of lipids and proteins, providing selective permeability and compartmentalization.

• Phospholipid bilayer: Impermeable to most water-soluble molecules.

• Proteins: Integral and peripheral, involved in transport, signaling, and structural support.

• Fluid mosaic model: Describes the flexible, ever-changing nature of the membrane.

• Extracellular fluid (ECF) and intracellular fluid (ICF) are separated by the plasma membrane.

Membrane Lipids

• Phospholipids: Form the basic structure; hydrophilic heads and hydrophobic tails.

• Glycolipids: Contribute to cell recognition.

• Cholesterol: Stabilizes membrane, affects fluidity.

Membrane Proteins

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

• Peripheral proteins: Loosely attached, function in support, enzymes, and cell signaling.

Functions of Membrane Proteins

• Molecular transport

• Signal transduction

• Attachment to cytoskeleton and extracellular matrix

• Enzymatic activity

• Intercellular joining

• Cell-cell recognition

Membrane Junctions

• Tight junctions: Prevent leakage between cells.

• Desmosomes: Anchor cells together, provide mechanical strength.

• Gap junctions: Allow communication via ion and molecule passage.

Membrane Transport

Types of Membrane Transport

The plasma membrane is selectively permeable, allowing certain molecules to pass while restricting others.

• Passive processes: No energy required; movement down concentration gradients.

• Active processes: Require energy (ATP); movement against concentration gradients.

Passive Processes

• Simple diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Transport via carrier or channel proteins.

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

Tonicity

Tonicity describes the ability of a solution to change the shape of cells by altering their internal water volume.

• Isotonic: Same solute concentration as the cell; no net water movement.

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

• Hypotonic: Lower solute concentration; water enters the cell, causing swelling and possible lysis.

Active Processes

• Primary active transport: Direct use of ATP to move substances (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from ion gradients created by primary active transport.

Vesicular Transport

• Transport of large particles, macromolecules, and fluids via membrane-bound vesicles.

• Requires cellular energy (ATP).

• Exocytosis: Discharge of substances from the cell.

• Endocytosis: Uptake of substances into the cell.

• Transcytosis: Movement into, across, and out of the cell.

Types of Endocytosis

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

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

• Receptor-mediated endocytosis: Specific uptake via receptor binding.

Summary Table: Active Processes

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the cellular region between the plasma membrane and nucleus, containing cytosol, organelles, and inclusions.

• Cytosol: Fluid portion containing dissolved solutes.

• Organelles: Specialized structures performing cellular functions.

• Inclusions: Stored nutrients, pigments, or other materials.

Cytoplasmic Organelles

• Membranous: Mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, nucleus.

• Nonmembranous: Ribosomes, cytoskeleton, centrioles.

Mitochondria

Mitochondria are double-membrane organelles responsible for producing most of the cell's ATP via cellular respiration.

• Contain their own DNA and RNA.

• Known as the "powerhouse" of the cell.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for export and membrane.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

The Golgi apparatus modifies, concentrates, and packages proteins and lipids for secretion or delivery to other organelles.

• Acts as the cell's "packaging and shipping plant."

Lysosomes

Lysosomes are membrane-bound sacs containing digestive enzymes for breaking down waste, toxins, and cellular debris.

• Involved in autophagy and apoptosis.

Peroxisomes

Peroxisomes contain enzymes for detoxifying harmful substances and neutralizing free radicals.

Ribosomes

Ribosomes are non-membranous organelles composed of rRNA and proteins, responsible for protein synthesis.

• Free ribosomes synthesize cytosolic proteins.

• Membrane-bound ribosomes synthesize proteins for export or membranes.

Cytoskeleton

The cytoskeleton is a network of protein filaments providing structural support, organization, and movement.

• Microfilaments: Actin filaments for cell movement and shape.

• Intermediate filaments: Provide tensile strength.

• Microtubules: Hollow tubes for organelle movement and cell division.

Centrosomes and Centrioles

• Organize microtubules and mitotic spindle during cell division.

Cellular Extensions

• Cilia and flagella: Motile extensions for movement.

• Microvilli: Increase surface area for absorption.

Nucleus and Genetic Material

Nucleus

The nucleus is the control center of the cell, containing genetic material (DNA) and directing cellular activities.

• Double-membrane envelope with nuclear pores.

• Contains nucleoli (site of rRNA synthesis) and chromatin (DNA and proteins).

Nucleolus

• Dark-staining body within the nucleus.

• Site of rRNA synthesis and ribosome assembly.

Chromatin

• Composed of DNA (30%), histone proteins (60%), and RNA (10%).

• Condenses to form chromosomes during cell division.

Cell Cycle and Division

Interphase

Interphase is the period of cell growth and DNA replication between cell divisions.

• G1 phase: Growth and normal metabolic roles.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis.

DNA Replication

DNA replication is the process by which a cell copies its DNA before division.

• Helicase unwinds the double helix.

• DNA polymerase synthesizes new strands using each original strand as a template.

• Process is semiconservative: each new DNA molecule contains one old and one new strand.

Cell Division

Cell division includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Mitosis: Produces two genetically identical daughter cells.

• Cytokinesis: Division of cytoplasm, begins during late anaphase.

Phases of Mitosis

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Cleavage furrow forms, cytoplasm divides.

Key Equation: Cell Cycle Duration

The duration of the cell cycle varies by cell type and physiological conditions.

• Example equation for cell cycle time (T):

Where , , , and are the durations of G1, S, G2, and M phases, respectively.

Additional info:

• Some content (e.g., figures, specific table numbers) referenced in the slides is not included here but can be found in standard Anatomy & Physiology textbooks.

• Cell cycle regulation is crucial for tissue growth, repair, and homeostasis.