Cell Structure and Function: Essentials of Anatomy & Physiology (Chapter 3 Study Guide)

Cell Structure and Function

Introduction

This chapter explores the fundamental unit of life—the cell. It covers the structure and function of cells, the plasma membrane, transport mechanisms, organelles, the cell cycle, and cellular differentiation, providing a foundation for understanding human anatomy and physiology.

Cell Theory and the Study of Cells

Cell Theory

• All living things are composed of cells.

• The cell is the basic structural and functional unit of life.

• All cells arise from pre-existing cells.

These principles form the basis for understanding biological structure and function.

Cytology

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

• Cells are studied using microscopes:

  • Light microscopy (LM): Uses visible light to observe cells and tissues.

  • Electron microscopy (EM): Uses electron beams for much higher resolution, revealing ultrastructure.

Diversity of Cells in the Human Body

• Human cells vary widely in shape, size, and function (e.g., blood cells, neurons, fat cells, bone cells, epithelial cells).

• This diversity allows for specialization and efficient functioning of tissues and organs.

Overview of Cell Anatomy

• Despite their diversity, all cells share common features:

  • Plasma membrane (cell membrane): Separates the cell's internal environment (cytoplasm) from the external environment (extracellular fluid).

  • Cytoplasm: The material within the cell membrane, excluding the nucleus.

The Plasma Membrane

Functions of the Plasma Membrane

• Physical isolation from the extracellular environment

• Regulation of exchange with the environment (controls entry/exit of substances)

• Sensitivity to environmental changes (contains receptors)

• Structural support (anchors cells and tissues)

Structure of the Plasma Membrane

• Extremely thin (6–10 nm)

• Composed of lipids, proteins, and carbohydrates

Membrane Lipids

• Phospholipids: Major component; form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Adds stiffness, reduces fluidity and permeability.

• Lipid-soluble substances cross easily; water-soluble substances require channels or carriers.

Phospholipid Bilayer

• Hydrophilic heads face water (extracellular and intracellular fluids).

• Hydrophobic tails face each other, forming a selective barrier.

Membrane Proteins

• Transmembrane proteins: Span the membrane; most common.

• Peripheral proteins: Loosely bound to the surface or partially embedded.

• Functions can change over time.

Functions of Membrane Proteins

• Receptors: Bind specific molecules (ligands).

• Channels: Allow passage of ions and small molecules.

• Carriers: Transport substances across the membrane.

• Enzymes: Catalyze reactions.

• Anchors: Attach to other cells or structures.

• Identifiers: Mark cell identity (important for immune recognition).

Membrane Carbohydrates

• Combine with proteins (glycoproteins) or lipids (glycolipids).

• Functions: lubrication, adhesion, receptors, immune system recognition.

Membrane Permeability and Transport

Permeability

• Permeability: Ease with which substances cross a membrane.

• Impermeable: Nothing passes.

• Freely permeable: Anything passes.

• Selectively permeable: Some substances pass; others do not (describes plasma membranes).

Movement Across the Membrane

• Passive processes: No energy required (e.g., diffusion, osmosis, facilitated diffusion).

• Active processes: Require energy (usually ATP; e.g., active transport, vesicular transport).

• Carrier-mediated transport: Can be passive or active, involves specific membrane proteins.

Diffusion

• Movement of molecules from high to low concentration (down the concentration gradient).

• Concentration gradient: Difference in concentration between two areas.

Diffusion Across the Plasma Membrane

• Lipid-soluble substances cross the lipid bilayer directly.

• Small water-soluble substances and ions pass through channel proteins.

• Large water-soluble molecules require carrier proteins.

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves from low solute concentration to high solute concentration.

• Osmotic pressure: Force of water movement into a solution due to solute concentration.

• Can be measured as the hydrostatic pressure needed to oppose water flow.

Tonicity

• Describes the effect of solute concentration on cell shape:

  • Isotonic: No net water movement; cell retains normal shape.

  • Hypotonic: Cell gains water, swells, and may burst (hemolysis in RBCs).

  • Hypertonic: Cell loses water, shrivels (crenation in RBCs).

Carrier-Mediated Transport

• Uses membrane proteins to move specific ions or molecules.

• Can be passive (facilitated diffusion) or active (requires ATP).

• Carrier proteins are specific and can be reused.

• Types:

  • Cotransport: Two substances moved in the same direction.

  • Countertransport: Two substances moved in opposite directions.

Facilitated Diffusion

• Carrier proteins passively move larger compounds down their concentration gradient.

• Compound binds to carrier, which changes shape and moves it across the membrane.

Active Transport

• Requires ATP to move substances against their concentration gradient.

• Example: Sodium-potassium exchange pump (Na+/K+ pump).

Vesicular Transport

• Moves materials in membrane-enclosed sacs (vesicles).

• Types:

  • Endocytosis: Brings materials into the cell.

  • Exocytosis: Expels materials from the cell.

Types of Endocytosis

• Receptor-mediated endocytosis: Specific ligands bind to receptors, triggering vesicle formation.

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid.

• Phagocytosis: "Cell eating"; cell engulfs large particles or cells.

Summary Table: Membrane Transport Processes

Process	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose transport
Osmosis	No	Water down gradient	Water movement
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Endocytosis	Yes (ATP)	Into cell	Phagocytosis
Exocytosis	Yes (ATP)	Out of cell	Neurotransmitter release

Cytoplasm and Organelles

Cytoplasm

• Material between the plasma membrane and the nucleus.

• Contains cytosol (intracellular fluid) and organelles.

Cytosol

• Fluid portion of cytoplasm; contains dissolved nutrients, ions, proteins, and waste products.

Organelles

• Specialized structures performing specific cellular functions.

• Membranous organelles: Surrounded by membranes (e.g., nucleus, ER, Golgi apparatus, mitochondria, lysosomes, peroxisomes).

• Non-membranous organelles: Not surrounded by membranes (e.g., cytoskeleton, centrioles, ribosomes).

Cytoskeleton

• Internal protein framework; provides strength and flexibility.

• Components: microfilaments, intermediate filaments, microtubules.

Microvilli

• Small, finger-like projections of the plasma membrane.

• Increase surface area for absorption.

Centrioles

• Cylindrical structures involved in cell division; produce spindle fibers.

Cilia and Flagella

• Cilia: Move substances across cell surfaces.

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

Ribosomes

• Sites of protein synthesis.

• Types: free (in cytosol) and fixed (attached to rough ER).

Proteasomes

• Contain proteolytic enzymes; degrade and recycle damaged proteins.

Endoplasmic Reticulum (ER)

• Network of membranes continuous with the nuclear envelope.

• Smooth ER (SER): Synthesizes lipids and carbohydrates.

• Rough ER (RER): Studded with ribosomes; synthesizes proteins.

Golgi Apparatus

• Stack of flattened membranes (cisternae).

• Modifies, packages, and ships proteins and lipids.

Lysosomes

• Vesicles containing digestive enzymes; break down waste and cellular debris.

Peroxisomes

• Vesicles with enzymes that break down fatty acids and neutralize toxins (produce hydrogen peroxide).

Mitochondria

• Double-membraned organelles; site of aerobic metabolism (cellular respiration).

• Produce 95% of cellular ATP.

The Nucleus

• Largest organelle; control center of the cell.

• Surrounded by a double membrane (nuclear envelope) with nuclear pores.

• Contains nucleoplasm, nucleoli (synthesize rRNA), and chromosomes (DNA).

• DNA stores genetic instructions for protein synthesis.

Genetic Information and Protein Synthesis

• Gene: Functional unit of heredity; sequence of DNA that codes for a protein.

• Protein synthesis occurs in two stages:

  • Transcription: DNA is used to make messenger RNA (mRNA).

  • Translation: mRNA is used by ribosomes to assemble amino acids into proteins.

Genetic Code Table (Example)

DNA Triplet	mRNA Codon	tRNA Anticodon	Amino Acid
AAA	UUU	AAA	Phenylalanine
TAC	AUG	UAC	Methionine (start codon)

Cell Life Cycle

Stages of the Cell Cycle

• Interphase: Cell grows, performs normal functions, and prepares for division.

• Mitosis: Division of the nucleus (prophase, metaphase, anaphase, telophase).

• Cytokinesis: Division of the cytoplasm, producing two daughter cells.

DNA Replication

• DNA strands separate and each serves as a template for a new strand.

• Results in two identical DNA molecules.

Cell Division

• Mitosis: Produces two genetically identical somatic cells.

• Meiosis: Produces gametes (sperm and ova) with half the chromosome number.

• Apoptosis: Programmed cell death.

Cancer and Tumors

• Cancer: Uncontrolled cell division due to gene mutations; can metastasize (spread) to other tissues.

• Tumor (neoplasm): Mass of abnormal cells.

• Benign tumor: Does not spread; usually not life-threatening.

• Malignant tumor: Invades surrounding tissues; can metastasize.

Cellular Differentiation

• Process by which cells become specialized in structure and function.

• Occurs through selective gene activation or repression.

• All somatic cells have the same DNA but express different genes.

• Groups of specialized cells form tissues.