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Chapter 3: The Structure and Function of Cells

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding their structure and function is essential for studying human biology and physiology.

• Cell Theory: All living things are composed of cells; the cell is the smallest unit exhibiting all characteristics of life; cells arise only from preexisting cells.

• Cellular Diversity: Cells vary in size, shape, and function, reflecting their specialized roles in the body.

CHNOPS: Essential Elements of Life

The acronym CHNOPS represents the six most common elements in living organisms:

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

• Oxygen (O)

• Phosphorus (P)

• Sulfur (S)

These elements are the building blocks of biomolecules such as proteins, nucleic acids, carbohydrates, and lipids.

Classification of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on their internal organization:

• Prokaryotic Cells: More primitive; lack membrane-bound organelles and a nucleus. Found in Bacteria and Archaea. Generally smaller in size.

• Eukaryotic Cells: More complex; possess a nucleus and membrane-bound organelles. Found in animals, plants, fungi, and protists.

Key Differences:

Feature	Prokaryotic	Eukaryotic
Nucleus	Absent	Present
Organelles	Absent	Present
Size	Smaller	Larger
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Cell Structure and Function

Major Components of Eukaryotic Cells

• Plasma Membrane: Encloses the cell, regulates entry and exit of substances.

• Nucleus: Contains genetic material (DNA); controls cellular activities.

• Cytoplasm: Gel-like fluid (cytosol) containing organelles and other cell components.

• Organelles: Specialized structures performing distinct functions (e.g., mitochondria, endoplasmic reticulum).

Specialized Cell Structures

Cell structure reflects function. For example:

• Muscle Cells: Numerous mitochondria for energy production.

• Nerve Cells: Long and thin to transmit impulses over distances.

Cell Size and Efficiency

Cells remain small to maximize efficiency in nutrient acquisition and waste disposal. The surface-to-volume ratio is crucial:

• Smaller cells have a higher surface-to-volume ratio, promoting efficient exchange.

• Structures like microvilli increase surface area.

Visualizing Cells

Microscopy

Microscopes are essential for studying cells:

• Light Microscope: Magnifies up to 1000x; can view living cells.

• Transmission Electron Microscope (TEM): Magnifies up to 100,000x; reveals internal structures.

• Scanning Electron Microscope (SEM): Magnifies up to 100,000x; provides 3D images of cell surfaces.

Internal Structures and Organelles

Nucleus

The nucleus is the control center of the cell:

• Double-layered nuclear membrane with pores for RNA and protein passage.

• Contains chromosomes (DNA) and the nucleolus (site of ribosome synthesis).

Ribosomes

Sites of protein synthesis, composed of RNA and protein:

• Free Ribosomes: Float in cytoplasm; synthesize proteins for immediate use.

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

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; manufactures and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and hormones, packages proteins and lipids for delivery.

Golgi Apparatus

Refines, packages, and ships products from the ER to other locations or for export.

Vesicles

• Secretory Vesicles: Export products from the cell.

• Endocytic Vesicles: Import substances into the cell.

• Peroxisomes: Detoxify wastes.

• Lysosomes: Contain digestive enzymes.

Mitochondria

"Power plants" of the cell; site of cellular respiration and ATP production.

• Double membrane structure; inner membrane highly folded.

• Utilizes oxygen and produces carbon dioxide.

• Generates ATP.

Energy Storage: Fat and Glycogen

• Triglycerides: Long-term energy storage in fat cells.

• Glycogen: Short-term energy storage in muscle and liver cells.

Support and Movement Structures

• Cytoskeleton: Internal scaffolding made of microtubules and microfilaments; maintains cell shape and anchors organelles.

• Cilia: Short, numerous; move substances across cell surfaces.

• Flagella: Long, single; enable sperm movement.

• Centrioles: Play a role in cell division.

Plasma Membrane and Transport

Plasma Membrane Structure

The plasma membrane is a selectively permeable lipid bilayer:

• Phospholipids: Form a bilayer with polar heads facing outward and nonpolar tails inward.

• Cholesterol: Adds mechanical strength.

• Proteins: Facilitate transport and communication.

Fluid Mosaic Model: Membrane components drift relative to each other.

Transport Across the Plasma Membrane

• Passive Transport: No energy required; moves substances down concentration gradient.

• Active Transport: Requires energy (ATP); moves substances against concentration gradient.

• Bulk Transport: Uses vesicles for endocytosis (import) and exocytosis (export).

Forms of Passive Transport

Type	Description	Examples
Simple Diffusion	Directly through lipid bilayer	O2, CO2, urea
Diffusion through Protein Channels	Via channel proteins	H2O, ions
Facilitated Transport	Via transporter proteins	Glucose, amino acids

Osmosis

• Osmosis: Diffusion of water across a selectively permeable membrane from low to high solute concentration.

• Osmotic Pressure: Pressure required to oppose osmosis.

Active Transport

• Moves substances from low to high concentration using energy and transport proteins (pumps).

• Sodium-Potassium Pump: Expels 3 Na+ for every 2 K+ imported; maintains cell volume and ion balance.

• Equation: $\text{ATP} \rightarrow \text{ADP} + P_i + \text{energy}$

Bulk Transport

• Endocytosis: Imports substances by engulfing them in vesicles.

• Exocytosis: Exports substances by fusing vesicles with the plasma membrane.

Cell Volume Regulation

• Isotonic: Equal solute concentration inside and outside; cell volume remains stable.

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

• Hypotonic: Lower solute outside; water enters cell, causing swelling.

Cellular Metabolism and Energy Production

Metabolism

Metabolism is the sum of all chemical reactions in an organism, organized into pathways:

• Anabolism: Builds larger molecules from smaller ones; requires energy.

• Catabolism: Breaks down larger molecules; releases energy.

ATP: Cellular Energy Currency

• ATP is produced by catabolism and used for cellular work.

• Equation: $\text{ATP} \leftrightarrow \text{ADP} + P_i + \text{energy}$

Cellular Respiration

Cellular respiration is the process of generating ATP from glucose, involving four stages:

1. Glycolysis: Occurs in cytoplasm; splits glucose into two pyruvate molecules; net yield of 2 ATP.

2. Preparatory Step: Pyruvate converted to acetyl CoA in mitochondria; produces NADH and CO2.

3. Citric Acid Cycle: Cyclic series of reactions in mitochondria; produces 2 ATP, NADH, FADH2, and CO2.

4. Electron Transport Chain (ETC) & Oxidative Phosphorylation: Located in inner mitochondrial membrane; uses NADH and FADH2 to generate ATP; O2 is final electron acceptor.

Overall ATP Yield: Up to 36 ATP per glucose molecule.

Alternative Energy Sources

• Glycogen: Rapidly catabolized to glucose.

• Fats: Broken down to glycerol and fatty acids; fatty acids enter citric acid cycle as acetyl CoA.

• Proteins: Broken down to amino acids; carbon backbones enter citric acid cycle; primary role is structural, not energy.

Anaerobic Metabolism

• Occurs when oxygen is unavailable.

• Glycolysis continues; pyruvate is converted to lactic acid.

• Limited ATP production (2 ATP per glucose).

• Lactic acid is metabolized aerobically when oxygen becomes available.

Summary Table: Energy Sources in Cells

Energy Source	Storage Form	ATP Yield	Notes
Glucose	Glycogen	36 ATP/glucose	Primary, rapid source
Fats	Triglycerides	Twice as much as carbs	Long-term storage
Proteins	Structural/enzymes	Similar to carbs	Used during starvation

Key Terms

• Cell Theory

• Prokaryote

• Eukaryote

• Organelle

• Plasma Membrane

• Osmosis

• Diffusion

• Active Transport

• ATP

• Glycolysis

• Citric Acid Cycle

• Electron Transport Chain

Additional info: Some details and examples have been expanded for clarity and completeness based on standard academic context.