AP Lecture Ch 3

Introduction

• All organisms are composed of cells.

  • Range from single-cell organisms to complex multicellular organisms like humans.

• Cells are essential for all structural and functional properties of living organisms.

  • Govern the workings of the human body, mechanisms of disease, and rationale of therapy.

Development of the Cell Theory

• Modern Cell Theory includes:

  • All organisms are composed of cells and their products.

  • Cells are the simplest structural and functional units of life.

  • An organism's structure and function depend on the activities of its cells.

  • Cells arise only from preexisting cells, not from nonliving matter.

  • Fundamental similarities exist in the chemical composition and metabolic mechanisms of cells across species.

Cell Shapes and Sizes

• Human body comprises about 200 types of cells, including:

  • Squamous: Thin and flat with a bulging nucleus.

  • Polygonal: Irregularly angular shapes.

  • Stellate: Star-like shape.

  • Cuboidal: Squarish and equal in height and width.

  • Columnar: Taller than wide.

  • Spheroid to Ovoid: Round to oval.

  • Discoid: Disc-shaped.

  • Fusiform: Thick in the middle, tapered at the ends.

  • Fibrous: Thread-like shape.

• Some shapes appear differently in tissue sections compared to their 3D shapes.

Human Cell Size

• Most human cells range from 10–15 micrometers (µm) in diameter.

  • Egg cells are considerably larger at around 100 µm.

  • Nerve cells can be up to 1 meter long, making them the longest in the human body.

Limitations on Cell Size

• As cells grow, volume increases more than surface area, leading to:

  • Surface area proportional to the square of the diameter; volume proportional to the cube.

  • Nutrient absorption and waste removal rely on surface area - large cells risk rupture under pressure (like an overfilled balloon).

Basic Components of a Cell

Plasma Membrane

• Surrounds the cell, defining its boundaries.

• Composed of proteins and lipids, with varying composition and function in different cell regions.

Cytoplasm

• Contains organelles, including:

  • Cytoskeleton: Provides structure and support.

  • Cytosol: Intracellular fluid (ICF).

  • Extracellular Fluid (ECF): The fluid outside the cell.

The Plasma Membrane

• Composed largely of lipids with various embedded proteins.

  • Phospholipid Bilayer: 75% of membrane lipids; amphiphilic properties.

  • Hydrophilic phosphate heads face water; hydrophobic tails avoid water, maintaining membrane fluidity.

Membrane Proteins

• Constitute about 2% of the molecules in the plasma membrane but make up 50% of its weight.

  • Types include:

    • Transmembrane proteins: Span the membrane, containing both hydrophilic and hydrophobic regions.

    • Peripheral proteins: Adhere to one side of the membrane, often tethered to the cytoskeleton.

• Functions vary: receptors, second-messenger systems, enzymes, ion channels, carriers, markers for cell identity, and adhesion molecules.

Cell-Identity Markers

• Glycoproteins that contribute to the glycocalyx: the carbohydrate coating on cell surfaces.

• Serve to identify self from foreign cells, essential for immune response.

Second Messengers

• First messenger (like epinephrine) binds to surface receptors, activating intracellular signals.

• G-proteins relay the signal, activating enzymes like adenylate cyclase, resulting in cAMP production, which serves as a second messenger.

The Glycocalyx

• Unique fuzzy coat on cell surfaces made of carbohydrate portions of glycoproteins and glycolipids.

• Functions include:

  • Protection and immunity.

  • Facilitation of cell adhesion and fertilization.

  • Defense against cancer and contribution to transplant compatibility.

Microvilli

• Extensions of the membrane (1–2 mm) increase surface area, especially in absorption.

  • Act as sensory receptors and are supported by actin filaments, which aid in movement and absorption.

Cilia

• Hair-like structures, typically 7–10 mm long.

  • Primary cilium serves sensory functions; motile cilia move substances in a coordinated fashion (e.g., respiratory tract).

Flagella

• The only functional flagellum in humans is the tail of a sperm.

  • Longer than cilia and provides a whiplike movement.

Membrane Transport

• Plasma membrane acts as a selective barrier.

  • Passive Transport: Requires no ATP, includes diffusion, osmosis, and filtration.

  • Active Transport: Consumes ATP; includes pumping and vesicular transport.

Vesicular Transport

• Utilize vesicles to transport material into and out of cells:

  • Endocytosis: Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

  • Exocytosis: Discharges materials from the cell.

The Cytoskeleton

• Composed of filaments and cylinders that maintain structural integrity and support cellular organization.

  • Types:

    • Microfilaments: Smallest, composed of actin.

    • Intermediate fibers: Provide structural support.

    • Microtubules: Largest, composed of tubulin and facilitate movement.

Organelles

• Internal structures that perform specific metabolic functions, including:

  • Membranous organelles: Nucleus, mitochondria, etc.

  • Non-membranous organelles: Ribosomes, centrioles, etc.

The Nucleus

• Largest organelle; regulates gene expression and protein synthesis, surrounded by a double membrane (nuclear envelope) with pores for material passage.

Endoplasmic Reticulum

• Network of membranes involved in protein (rough ER) and lipid synthesis (smooth ER).

  • Rough ER: Studded with ribosomes for protein synthesis.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification tasks.

Ribosomes

• Granules of protein and RNA; sites of protein synthesis based on genetic code.

Golgi Complex

• Modifies, sorts, and packages proteins and lipids into vesicles.

  • Receives proteins from rough ER and finalizes their processing.

Lysosomes

• Membrane-bound organelles containing enzymes for digestion of cellular waste and foreign material.

Peroxisomes

• Similar to lysosomes but contain enzymes for oxidation of molecules; detoxify harmful substances.

Mitochondria

• Organelles responsible for ATP production, encapsulated by a double membrane with internal folds (cristae) for increased surface area.