Chapter 4: Cell Structure

Overview of Cell Types

Cells are the basic units of life, and they can be classified as prokaryotic or eukaryotic. Understanding their differences and similarities is fundamental in biology.

• Prokaryotic cells: Simple cells without a nucleus or membrane-bound organelles. Example: Bacteria.

• Eukaryotic cells: Complex cells with a nucleus and various organelles. Example: Animal and plant cells.

Key Differences Between Prokaryotic and Eukaryotic Cells

Common Features of All Cells

• Plasma membrane: Encloses the cell, controls movement of substances.

• Cytoplasm/cytosol: Fluid where cellular processes occur.

• Ribosomes: Sites of protein synthesis.

• Genetic material (DNA): Contains instructions for cell function.

Cell Organelles and Their Functions

Organelles are specialized structures within eukaryotic cells that perform distinct functions.

• Nucleus: Stores genetic material; controls cell activities.

• Ribosomes: Synthesize proteins.

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digest cellular waste and foreign material.

• Mitochondria: Produce ATP via cellular respiration.

• Chloroplasts: (in plants) Conduct photosynthesis.

• Cell wall: (in plants, fungi, bacteria) Provides structure and protection.

• Flagella/Cilia: Enable cell movement.

• Plasmodesmata: Channels between plant cells for communication.

Surface Area-to-Volume Ratio

The surface area-to-volume ratio affects the efficiency of diffusion and transport in cells. Smaller or elongated cells have higher ratios, facilitating better exchange of materials.

• Formula:

• Formula:

Chapter 5: Membrane Transport

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Fluid mosaic model: Describes the dynamic arrangement of lipids and proteins.

• Phospholipids: Have hydrophilic heads and hydrophobic tails, forming a bilayer.

• Marker proteins (glycoproteins): Involved in cell recognition.

• Transport proteins: Facilitate movement of substances across the membrane.

Types of Membrane Transport

Cells transport materials across membranes via several mechanisms, classified as passive or active transport.

• Passive transport: Movement without energy input; includes diffusion, osmosis, and facilitated diffusion.

• Active transport: Requires energy (ATP) to move substances against their concentration gradient.

Passive Transport Mechanisms

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Facilitated diffusion: Movement via transport proteins (e.g., aquaporins for water).

Active Transport Mechanisms

• Pumps: (e.g., sodium-potassium pump) Use ATP to move ions.

• Endocytosis: Cell engulfs substances into vesicles.

• Exocytosis: Vesicles fuse with membrane to release contents.

Membrane Potential and Cell Communication

The membrane potential is the voltage difference across the cell membrane, essential for nerve impulse transmission and muscle contraction.

• Ion channels: Allow selective passage of ions, contributing to potential.

• Cell junctions: Structures that connect cells and facilitate communication (e.g., gap junctions, plasmodesmata).

Osmotic Terms

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypotonic: Lower solute concentration outside; water enters cell.

• Hypertonic: Higher solute concentration outside; water leaves cell.

• Turgid: Swollen cell (plant cells in hypotonic solution).

• Flaccid: Limp cell (plant cells in isotonic solution).

Cell Adaptations and Specialized Functions

Cellular Adaptations for Function

Cells adapt their structure to suit their function. For example, muscle cells have abundant mitochondria for energy, while liver cells have many lysosomes for detoxification.

• Microvilli: Increase surface area for absorption (e.g., gut epithelial cells).

• Golgi apparatus: Abundant in cells producing and secreting proteins.

• Mitochondria: Numerous in cells with high energy demands.

• Ribosomes on ER: High in cells synthesizing large amounts of protein.

Example: Gut Epithelial Cell Adaptations

• Active transport: Microvilli increase surface area for transport proteins.

• Enzyme synthesis: Ribosomes on rough ER produce digestive enzymes.

• Rapid diffusion: Thin cell membrane and microvilli facilitate quick absorption.

Cell Life Cycle and Specialization

Life Cycle of a Fly (Example)

Organisms undergo distinct life stages, with cellular changes and specialization at each stage.

• Larva: Growth and storage of nutrients.

• Pupa: Breakdown of old tissues, formation of new tissues (lysosomes active).

• Adult: Specialized functions and reproduction.

Surface Area-to-Volume Ratio and Diffusion Efficiency

Effect of Cell Shape on Diffusion

Cell shape influences the efficiency of diffusion. Higher surface area-to-volume ratios support faster diffusion.

• Cells with higher surface area-to-volume ratios (e.g., small cubes, flattened shapes) diffuse substances more efficiently.

• As cell size increases, the SAV ratio decreases, reducing diffusion efficiency.

Membrane Structure and Transport of Materials

Features Facilitating Transport

• Selective permeability: Only certain molecules can cross the membrane.

• Fluidity: Double bonds in phospholipids increase membrane fluidity, aiding transport.

• Transport proteins: Enable movement of ions and large molecules.

Antibiotics and Cell Structure

• Vancomycin: Acts on bacterial cell walls; does not affect human cells (which lack cell walls).

• Tetracycline: Inhibits protein synthesis by acting on bacterial ribosomes.

Summary Table: Key Terms and Concepts

Additional info: Some explanations and examples have been expanded for clarity and completeness, including formulas and context for cell adaptations, membrane transport, and antibiotic action.