Unit 3: Cell Structure and Function – Comprehensive Study Notes

Unit 3: Cell Structure and Function

Topic 3.1: Cell Structure and Function

This topic explores the structure and function of subcellular components and organelles, and how they contribute to the overall function of cells. Key organelles include ribosomes, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, vacuoles, nuclear envelope, plasma membrane, and cytoplasm.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or bound to rough ER.

• Endoplasmic Reticulum (ER): Rough ER is studded with ribosomes and synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Powerhouse of the cell; site of cellular respiration and ATP production.

• Lysosomes: Contain digestive enzymes to break down macromolecules and recycle cell components.

• Vacuoles: Storage organelles; large central vacuole in plants maintains turgor pressure.

• Chloroplasts: Found in plant cells; site of photosynthesis.

• Plasma Membrane: Selectively permeable barrier; regulates movement of substances in and out of the cell.

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pores for transport.

Example: The rough ER and Golgi apparatus work together to synthesize and modify proteins for secretion.

Topic 3.2: Cell Size

Cell size is limited by the surface area-to-volume ratio, which affects the rate of exchange of materials with the environment. Smaller cells have a higher surface area-to-volume ratio, facilitating efficient exchange.

• Surface Area: Total area of the cell's outer membrane.

• Volume: Space inside the cell.

• Surface Area-to-Volume Ratio: Determines efficiency of material exchange.

Formula:

• Surface Area of a Sphere: $4\pi r^2$

• Volume of a Sphere: $\frac{4}{3}\pi r^3$

• Surface Area of a Cube: $6a^2$

• Volume of a Cube: $a^3$

Example: Red blood cells are small and biconcave, maximizing their surface area for gas exchange.

Topic 3.3: Plasma Membrane Structure

The plasma membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. Each component plays a role in maintaining the cell's internal environment.

• Phospholipids: Form the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition.

Example: Channel proteins facilitate the movement of ions across the membrane.

Topic 3.4: Plasma Membrane Functions

The plasma membrane maintains homeostasis by controlling the movement of substances. The Fluid Mosaic Model describes its dynamic nature.

• Fluid Mosaic Model: Membrane is flexible and composed of various molecules.

• Selective Permeability: Allows certain molecules to pass while blocking others.

• Transport Proteins: Facilitate movement of substances.

Example: Aquaporins are channel proteins that facilitate water transport.

Topic 3.5: Membrane Permeability

Membrane permeability is determined by the structure of the membrane and the properties of molecules. Small, nonpolar molecules pass easily; large or charged molecules require transport proteins.

• Passive Transport: Diffusion and facilitated diffusion; no energy required.

• Active Transport: Requires energy (ATP) to move substances against concentration gradients.

Example: Oxygen diffuses freely across the membrane, while glucose requires a transporter.

Topic 3.6: Cell Wall

The cell wall provides structural support and protection in plant, fungal, and some prokaryotic cells. It also helps maintain cell shape and prevents excessive water uptake.

• Composition: Mainly cellulose in plants; chitin in fungi; peptidoglycan in bacteria.

• Function: Maintains cell shape, prevents lysis in hypotonic environments.

Example: Plant cells do not burst in hypotonic solutions due to the rigid cell wall.

Topic 3.7: Membrane Transport

Cells use various mechanisms to transport molecules across membranes, maintaining homeostasis and responding to environmental changes.

• Passive Transport: Simple diffusion, facilitated diffusion, osmosis.

• Active Transport: Uses energy to move substances against gradients.

Example: Sodium-potassium pump maintains ion gradients in animal cells.

Topic 3.8: Bulk Membrane Transport

Bulk transport moves large molecules or particles across the membrane via endocytosis and exocytosis.

• Endocytosis: Cell engulfs material via vesicles (phagocytosis, pinocytosis, receptor-mediated).

• Exocytosis: Vesicles fuse with membrane to release contents outside the cell.

Example: White blood cells ingest bacteria via phagocytosis.

Topic 3.9: Facilitated Diffusion & Active Transport

Facilitated diffusion uses transport proteins to move molecules down their concentration gradient, while active transport moves molecules against the gradient using energy.

• Facilitated Diffusion: For large or charged molecules; uses channel or carrier proteins.

• Active Transport: Requires ATP; e.g., sodium-potassium pump.

Example: Glucose enters cells via facilitated diffusion through GLUT transporters.

Topic 3.10: Tonicity

Tonicity describes the effect of a solution on cell volume due to osmosis. Solutions can be isotonic, hypotonic, or hypertonic relative to the cell.

• Isotonic: No net water movement; cell volume remains constant.

• Hypotonic: Water enters cell; cell may swell or burst.

• Hypertonic: Water leaves cell; cell shrinks.

Intracellular Environment	Extracellular Environment	Direction of Water Flow	Result to the Cell
Hypotonic	Isotonic	Out	Shrink
Isotonic	Hypotonic	In	Swell
Hypertonic	Isotonic	Out	Shrink

Formula: Water Potential: $\Psi = \Psi_s + \Psi_p$

Example: Plant cells become turgid in hypotonic solutions due to water uptake.

Topic 3.11: Osmoregulation

Osmoregulation is the process by which organisms maintain water and solute balance. It is essential for survival in varying environments.

• Water Potential ($\Psi$): Determines direction of water movement.

• Solute Potential ($\Psi_s$): $\Psi_s = -iCRT$

• Pressure Potential ($\Psi_p$): Physical pressure on solution.

Example: Freshwater fish excrete dilute urine to maintain osmotic balance.

Topic 3.12: Mechanisms of Transport

Various mechanisms allow ions and other molecules to cross membranes, including simple diffusion, facilitated diffusion, and active transport.

• Simple Diffusion: Movement of small, nonpolar molecules down concentration gradient.

• Facilitated Diffusion: Uses transport proteins for larger or charged molecules.

• Active Transport: Moves substances against gradient using ATP.

Example: Calcium ions are pumped out of cells via active transport.

Topic 3.13: Membrane-Bound Organelles

Eukaryotic cells contain membrane-bound organelles that compartmentalize functions, increasing efficiency and specialization.

• Nucleus: Contains genetic material; site of transcription.

• Mitochondria: Site of ATP production.

• Endoplasmic Reticulum: Protein and lipid synthesis.

• Golgi Apparatus: Protein modification and sorting.

Example: Lysosomes digest cellular waste within their membrane-bound compartment.

Topic 3.14: Cell Compartmentalization

Compartmentalization allows eukaryotic cells to carry out specialized functions in distinct organelles, increasing metabolic efficiency.

• Prokaryotes: Lack membrane-bound organelles; metabolic processes occur in cytoplasm.

• Eukaryotes: Have organelles such as nucleus, mitochondria, ER, and Golgi.

Example: Enzymes for cellular respiration are localized in mitochondria.

Topic 3.15: Origins of Cell Compartmentalization

Comparing prokaryotic and eukaryotic cells highlights differences in compartmentalization and complexity.

Feature	Prokaryotic Cells	Eukaryotic Cells
Organelles	None	Membrane-bound
DNA Location	Nucleoid	Nucleus
Size	Smaller	Larger

Example: Bacteria are prokaryotes; plant and animal cells are eukaryotes.

Topic 3.16: Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes similar to bacteria.

• Implication: Mitochondria and chloroplasts replicate independently within cells.

Example: Chloroplasts in plants share similarities with cyanobacteria.

Additional info: These notes expand upon the provided outlines and questions, offering definitions, examples, and formulas for key concepts in cell structure and function. Tables have been recreated to compare cell types and tonicity effects. All equations are provided in LaTeX format for clarity.