Chapter 2: Foundations – The Cell (Cellular Structure and Function)

Foundations: The Cell

Introduction to Cells

Cells are the fundamental units of life, forming the structural and functional basis of all living organisms. The human body contains trillions of cells, each performing vital functions necessary for survival.

• Cells as Building Blocks: All plants and animals are composed of cells.

• Cell Division: New cells are produced by the division of preexisting cells.

• Vital Functions: Cells carry out all essential functions of the body.

Types of Cells

There are two primary categories of cells in the human body:

• Sex Cells (Germ Cells): Sperm in males and oocytes in females; involved in reproduction.

• Somatic Cells: All other cells in the body; responsible for forming tissues and organs.

Cellular Anatomy

Basic Components of the Cell

Each cell consists of several key structures:

• Cytoplasm: The material within the cell, including cytosol and organelles.

• Cytosol: The intracellular fluid containing nutrients, proteins, and waste products.

• Organelles: Specialized structures performing specific functions.

• Plasma Membrane: The cell's boundary, composed of various molecules.

The Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates interactions.

• Functions:

  • Physical barrier

  • Regulates exchange of materials

  • Sensitivity to extracellular changes

  • Cell-to-cell communication, adhesion, and structural support

• Components:

  • Phospholipids (form a bilayer)

  • Proteins (peripheral and integral)

  • Glycolipids (outer layer only)

  • Sterols (e.g., cholesterol)

Phospholipid Bilayer

• Hydrophilic heads face outward and inward surfaces.

• Hydrophobic tails face each other, forming the membrane's core.

• Outer layer contains glycolipids and glycoproteins, forming the glycocalyx.

Proteins in the Membrane

• Peripheral Proteins: Attached to either surface of the membrane.

• Integral Proteins: Embedded within the membrane; some form channels (including gated channels).

Glycolipids and Sterols

• Glycolipids: Only in the outer layer; serve as receptors.

• Sterols: Stabilize membrane structure; cholesterol is a key example.

Membrane Permeability: Passive and Active Processes

The plasma membrane's permeability determines how substances move in and out of the cell.

• Impermeable: No substances cross.

• Freely Permeable: All substances cross easily.

• Selectively Permeable: Only certain substances are allowed through.

Passive Processes

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

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

• Facilitated Diffusion: Solutes move passively via carrier proteins; affected by concentration gradient, solute size/charge, temperature, and number of carrier proteins. Examples: glucose, amino acids.

Active Processes

• Active Transport: Solutes are moved against their concentration gradient using carrier proteins and ATP. Examples: sodium, potassium, calcium, magnesium ions.

• Endocytosis: Uptake of materials via vesicles.

  • Pinocytosis: Uptake of small molecules and fluid.

  • Phagocytosis: Uptake of solid particles (e.g., bacteria, cell debris).

  • Receptor-mediated Endocytosis: Specific molecules (ligands) bind to membrane receptors.

• Exocytosis: Release of intracellular material to the outside; requires ATP and calcium ions.

Extensions of the Plasma Membrane: Microvilli

Microvilli are fingerlike projections that increase the cell's surface area for absorption.

• Absorb material from extracellular fluid (ECF).

• Movement helps circulate ECF and absorb nutrients.

The Cytoplasm

Cytosol

The cytosol is the fluid portion of the cytoplasm, containing dissolved substances and inclusions.

• High concentration of potassium ions, low sodium ions (compared to ECF).

• Net negative charge inside the cell.

• High protein concentration, small amount of carbohydrates, large reserve of amino acids and lipids.

• Contains inclusions: masses of insoluble substances.

Organelles

Organelles are specialized structures within the cell, divided into nonmembranous and membranous types.

• Nonmembranous Organelles: Cytoskeleton, centrioles, cilia, flagella, ribosomes.

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

Nonmembranous Organelles

The Cytoskeleton

The cytoskeleton provides structural support and facilitates movement within the cell.

• Microfilaments: Made of actin; anchor cytoskeleton, stabilize membrane proteins, produce movement (actin interacts with myosin).

• Intermediate Filaments: Provide strength, stabilize organelles, transport material; neurofilaments support nerve axons.

• Thick Filaments: Composed of myosin; abundant in muscle cells, involved in contraction.

• Microtubules: Made of tubulin; main component, change cell shape, move organelles, form centrioles, cilia, flagella.

Centrioles, Cilia, and Flagella

• Centrioles: Located in the centrosome; produce microtubules for chromosome movement during cell division.

• Cilia: Surface structures; beat rhythmically to move fluid or secretions.

• Flagella: Surface structures; propel sperm cells.

Ribosomes

• Free Ribosomes: Float in cytoplasm; synthesize proteins for the cell.

• Attached Ribosomes: Bound to endoplasmic reticulum; synthesize proteins for export or membrane.

Membranous Organelles

Double-layered Membranous Organelles

• Mitochondria: Produce ATP (energy currency of the cell).

• Nucleus: Contains chromosomes; control center for cell activities.

Single-layered Membranous Organelles

• Endoplasmic Reticulum (ER): Network of hollow tubes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids, steroids, carbohydrates, stores calcium, detoxifies toxins.

• Golgi Apparatus: Modifies and packages proteins, renews cell membrane.

• Lysosomes: Contain digestive enzymes; remove bacteria, debris, recycle materials, involved in autolysis.

• Peroxisomes: Contain catalase; abundant in liver cells, convert hydrogen peroxide to water and oxidants.

Nucleus Structure

• Nuclear Envelope: Consists of perinuclear space and nuclear pores.

• Nucleoplasm: Contains nuclear matrix filaments, nucleoli, chromatin, chromosomes.

• Chromosomes: Made of DNA wrapped around histones; DNA-histone complex is a nucleosome.

Membrane Flow

Membrane flow refers to the continuous movement and recycling of membrane components within the cell.

• Transport vesicles connect ER and Golgi apparatus.

• Secretory vesicles connect Golgi apparatus and plasma membrane.

• Vesicles remove and recycle segments of the plasma membrane.

Intercellular Attachment

Cells attach to each other via various mechanisms to form tissues.

• Cell Adhesion Molecules (CAMs): Mediate cell attachment.

• Cellular Cement: Proteoglycans like hyaluronan.

• Cell Junctions:

  • Gap junctions

  • Tight junctions (e.g., desmosomes)

  • Spot desmosome

  • Hemidesmosome

The Cell Life Cycle

Cell Reproduction

Cell reproduction involves a series of events leading to the formation of new cells. Mitosis is one part of this process.

• Interphase: Cell prepares for division; consists of G0, G1, S, and G2 phases.

• Mitosis: Division of the nucleus; includes prophase, metaphase, anaphase, telophase.

• Cytokinesis: Division of the cytoplasm; overlaps with anaphase and telophase.

Interphase

• G0 Phase: Cell is not preparing for division; performs normal functions.

• G1 Phase: Duplication of organelles, protein synthesis.

• S Phase: DNA and histone synthesis, chromosome replication.

• G2 Phase: Protein synthesis.

DNA Replication

• Weak bonds between nitrogenous bases break; DNA strands unwind.

• DNA polymerase binds to bases; ligases link complementary nucleotides.

• Results in duplicated DNA molecules.

Mitosis

• Mitotic Rate: Frequency of cell division.

• Stem Cells: Somatic cells capable of division.

• Phases:

  • Prophase: First phase; chromosomes condense.

  • Metaphase: Chromatids align at the cell's equator.

  • Anaphase: Chromatids separate to opposite poles.

  • Telophase: New nuclear membranes form.

Cytokinesis

• Division of cytoplasm; overlaps with anaphase and telophase.

Summary Table: Membranous vs. Nonmembranous Organelles

Type	Examples	Main Functions
Nonmembranous	Cytoskeleton, centrioles, cilia, flagella, ribosomes	Structural support, movement, protein synthesis
Membranous	Mitochondria, nucleus, ER, Golgi apparatus, lysosomes, peroxisomes	Energy production, genetic control, synthesis, packaging, digestion

Key Equations

• Diffusion Rate: Fick's Law (for academic context) $J = -D \frac{dC}{dx}$ Where: J = flux, D = diffusion coefficient, dC/dx = concentration gradient

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

Additional info: Academic context and equations were added to clarify passive transport mechanisms and organelle functions.