Structure and Function of Cells: Human Biology Study Notes

Structure and Function of Cells

Cell Theory

The cell theory is a foundational concept in biology, stating that all living things are composed of cells, the cell is the smallest unit exhibiting all characteristics of life, and all cells arise from preexisting cells.

• All living things are made of cells.

• Cells are the basic unit of life.

• Cell division produces new cells from existing cells.

Classification of Cells

Cells are classified based on their internal organization into two main types: prokaryotic and eukaryotic cells.

• Prokaryotic cells: Lack membrane-bound compartments; include Bacteria and Archaea.

• Eukaryotic cells: Have membrane-bound organelles; include animals, plants, fungi, and protists.

Prokaryotic vs. Eukaryotic Cell Structure

• Prokaryotes: No nucleus, no organelles, smaller size, rigid cell wall.

• Eukaryotes: Nucleus, organelles, larger size, complex internal structure.

Eukaryotic Cell Structure

Eukaryotic cells contain a plasma membrane, nucleus, cytoplasm, and organelles, each with specialized functions.

• Plasma membrane: Encloses the cell.

• Nucleus: Contains genetic material.

• Cytoplasm: Gel-like fluid (cytosol) and organelles.

• Organelles: Specialized structures for cellular functions.

Cell Structure Reflects Cell Function

Cell structure is closely related to function. For example, muscle cells have many energy-producing organelles, while nerve cells are long and thin for impulse transmission.

• Muscle cells: Numerous mitochondria for energy.

• Nerve cells: Specialized for signal transmission.

Cell Size and Efficiency

Cells remain small to maximize efficiency in nutrient acquisition and waste disposal. The surface-to-volume ratio is critical for cellular function.

• Small cells: Higher surface-to-volume ratio.

• Microvilli: Increase surface area for absorption.

Visualizing Cells with Microscopes

Microscopes are essential tools for studying cells, allowing visualization of cell structure and function.

• Light microscope: Up to 1,000x magnification; views living samples.

• Transmission electron microscope: Up to 1,000,000x; reveals internal details.

• Scanning electron microscope: Up to 1,000,000x; provides 3D surface views.

Internal Structures of Cells

Cells contain various internal structures, both membrane-bound and non-membrane-bound, each with specific functions.

• Nucleus: Information center; controls cell activities.

• Ribosomes: Protein synthesis.

• Endoplasmic reticulum (ER): Manufacturing center; rough ER (protein synthesis), smooth ER (lipid synthesis).

• Golgi apparatus: Refines, packages, ships products.

• Vesicles: Storage and transport.

• Mitochondria: Energy production (ATP).

The Nucleus

The nucleus is the control center of the cell, containing genetic information and regulating cellular activities.

• Double-layered nuclear membrane protects DNA.

• Nucleolus: Ribosome component synthesis.

• Nuclear pores: Allow passage of RNA and proteins.

• Chromosomes: DNA storage.

Ribosomes

Ribosomes are responsible for synthesizing proteins, either for immediate use or for export.

• Free ribosomes: Float in cytoplasm; synthesize proteins for cell use.

• Bound ribosomes: Attached to ER; synthesize proteins for export or organelles.

Endoplasmic Reticulum (ER)

The ER is a highly folded network involved in protein and lipid synthesis.

• Rough ER: Has ribosomes; protein synthesis and modification.

• Smooth ER: No ribosomes; lipid synthesis and packaging.

Golgi Apparatus

The Golgi apparatus refines, packages, and ships cellular products.

• Packaging: Products are placed in vesicles for transport.

• Shipping: Vesicles deliver products within or outside the cell.

Vesicles

Vesicles are membrane-bound containers for storage and transport.

• Secretory vesicles: Export products.

• Endocytic vesicles: Import substances.

• Peroxisomes: Detoxify wastes.

• Lysosomes: Digestive enzymes.

Mitochondria

Mitochondria are the cell's power plants, generating ATP through cellular respiration.

• Double membrane: Inner membrane highly folded.

• ATP production: Quick energy source.

Fat and Glycogen Storage

Cells store energy as fat (triglycerides) and glycogen for long-term and short-term needs, respectively.

• Fat: Long-term energy; stored in fat cells.

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

Cell Support and Movement

The cytoskeleton provides structural support and enables movement through cilia, flagella, and centrioles.

• Cytoskeleton: Microtubules and microfilaments.

• Cilia: Short, numerous; move substances.

• Flagella: Long, single; enable cell movement.

• Centrioles: Important in cell division.

Plasma Membrane Structure and Function

The plasma membrane separates the cell from its environment and regulates the movement of substances.

• Lipid bilayer: Phospholipids, cholesterol, proteins.

• Fluid mosaic: Components drift relative to each other.

• Selectively permeable: Controls entry and exit of molecules.

Molecule Transport Across the Plasma Membrane

Molecules cross the plasma membrane via passive and active transport mechanisms.

• Passive transport: No energy required; includes diffusion, osmosis, facilitated diffusion.

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

• Bulk transport: Endocytosis and exocytosis for large molecules.

Diffusion and Osmosis

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Osmotic pressure: Pressure needed to oppose osmosis.

Forms of Passive Transport

• Direct diffusion: Through lipid bilayer (small, nonpolar molecules).

• Protein channels: Allow passage of ions and water.

• Facilitated transport: Specific molecules bind to transporter proteins.

Active Transport

• Requires ATP: Moves substances against gradient.

• Sodium-potassium pump: Expels sodium, brings in potassium.

Bulk Transport

• Endocytosis: Imports substances via vesicles.

• Exocytosis: Exports substances via vesicles.

Cell Volume Regulation

Cell volume is regulated by the sodium-potassium pump and extracellular fluid tonicity.

• Isotonic: Equal solute concentrations; normal cell volume.

• Hypertonic: Higher extracellular solute; cell shrinks.

• Hypotonic: Lower extracellular solute; cell swells.

Cellular Metabolism

Metabolism is the sum of all chemical reactions in a cell, organized into pathways.

• Anabolism: Builds larger molecules; requires energy.

• Catabolism: Breaks down molecules; releases energy.

• Enzymes: Catalyze reactions.

Glucose and Cellular Respiration

Glucose is a primary energy source, used to generate ATP through cellular respiration.

• One glucose: Yields ~36 ATP.

• Cellular respiration: Four stages: glycolysis, preparatory step, citric acid cycle, electron transport chain.

Stages of Cellular Respiration

• Glycolysis: Splits glucose into pyruvate; net 2 ATP.

• Preparatory step: Converts pyruvate to acetyl CoA.

• Citric acid cycle: Breaks down acetyl CoA; produces ATP, NADH.

• Electron transport chain: Transfers electrons; produces ATP via oxidative phosphorylation.

Alternative Energy Sources

Cells can use fats and proteins for energy when glucose is unavailable.

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

• Proteins: Broken down to amino acids; carbon backbones enter citric acid cycle.

Anaerobic Metabolism

In the absence of oxygen, cells use anaerobic pathways, producing limited ATP and lactic acid.

• Glycolysis continues: Pyruvate converted to lactic acid.

• Limited ATP: Only 2 ATP per glucose.

• Lactic acid: Causes muscle fatigue; metabolized when oxygen returns.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Absent	Present
Cell Size	Smaller	Larger
Cell Wall	Present	Variable
Domains	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Summary Table: Types of Membrane Transport

Transport Type	Energy Required	Direction	Examples
Diffusion	No	High to low concentration	Oxygen, CO2
Osmosis	No	Water movement	Water
Facilitated Diffusion	No	High to low concentration	Glucose, amino acids
Active Transport	Yes (ATP)	Low to high concentration	Sodium-potassium pump
Endocytosis/Exocytosis	Yes (ATP)	Bulk movement	Insulin, waste export

Summary Table: Cellular Respiration Stages

Stage	Location	Main Products
Glycolysis	Cytoplasm	Pyruvate, ATP, NADH
Preparatory Step	Mitochondria	Acetyl CoA, NADH
Citric Acid Cycle	Mitochondria	ATP, NADH, CO2
Electron Transport Chain	Mitochondria	ATP, H2O

Key Equations

• Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{ATP}$

• Diffusion: $J = -D \frac{dC}{dx}$ (where J = flux, D = diffusion coefficient, dC/dx = concentration gradient)

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