Cell Structure and Microscopy

Light Microscopy

Light microscopy is a technique that uses visible light and lenses to magnify small objects, allowing observation of cells and tissues.

• Types of Light Microscopes: Bright-field, phase-contrast, fluorescence, and differential interference contrast.

• Phase-contrast microscopy: Enhances contrast in transparent specimens without staining.

• SEM vs. TEM: Scanning Electron Microscopy (SEM) provides 3D images of surfaces; Transmission Electron Microscopy (TEM) reveals internal structures at high resolution.

Example: TEM is used to view organelles inside a cell, while SEM is used to examine the cell's surface.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and organization.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotic cells: Have a nucleus and organelles (e.g., plants, animals, fungi).

Example: Escherichia coli is a prokaryote; human cells are eukaryotic.

Endomembrane System and Organelles

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Main components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

Example: Secretory proteins are synthesized in the rough ER, processed in the Golgi, and exported via vesicles.

Other Organelles

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

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

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Autophagy: Process by which cells recycle their own components using lysosomes.

Example: Mitochondria convert glucose into ATP, providing energy for cellular processes.

Cytoskeleton and Cell Junctions

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, shape, and movement for cells.

• Main types: Microtubules, microfilaments (actin filaments), intermediate filaments.

• Functions: Cell shape, intracellular transport, cell division, and motility.

Example: Microtubules form the mitotic spindle during cell division.

Cell Junctions

Cell junctions are specialized structures that connect cells to each other and to the extracellular matrix.

• Types: Tight junctions, desmosomes, gap junctions (animals); plasmodesmata (plants).

• Functions: Communication, adhesion, and barrier formation.

Example: Gap junctions allow ions and small molecules to pass between animal cells.

Plasma Membrane Structure and Function

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts a flexible bilayer of lipids with embedded proteins.

• Phospholipid bilayer: Amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Proteins: Integral (span the membrane) and peripheral (attached to surface).

• Cholesterol: Modulates membrane fluidity and stability.

Example: Membrane proteins act as receptors, channels, or enzymes.

Membrane Carbohydrates

Carbohydrates attached to proteins and lipids on the cell membrane surface play roles in cell recognition and signaling.

• Glycoproteins and glycolipids: Involved in cell-cell recognition and immune response.

Membrane Transport Mechanisms

Passive Transport

Passive transport is the movement of substances across the membrane without energy input.

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: Uses channel or carrier proteins for transport of specific molecules.

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

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Carrier proteins: Use ATP to transport ions and molecules.

• Sodium-potassium pump: Maintains electrochemical gradients in animal cells.

Equation: pump exchanges 3 out for 2 in per ATP hydrolyzed.

Bulk Transport

Bulk transport involves movement of large particles or volumes via vesicles.

• Endocytosis: Uptake of materials into the cell (phagocytosis, pinocytosis, receptor-mediated).

• Exocytosis: Release of materials from the cell.

Example: White blood cells engulf bacteria via phagocytosis.

Water Balance and Osmosis

Osmotic Terms

Cells regulate water balance to maintain homeostasis.

• 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.

Example: Plant cells become turgid in hypotonic solutions.

Energy, Thermodynamics, and Enzymes

Forms of Energy

Cells use and transform energy to perform work.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy.

• Thermal energy: Energy from heat.

• Chemical energy: Energy stored in chemical bonds.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics.

• First Law: Energy cannot be created or destroyed.

• Second Law: Entropy (disorder) increases in closed systems.

• Free energy equation:

• Spontaneous reactions: Negative ; non-spontaneous: positive .

Enzymes and Catalysis

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy ().

• Active site: Region where substrate binds and reaction occurs.

• Enzyme specificity: Enzymes bind specific substrates due to their unique active site shape.

• Enzyme regulation: Allosteric regulation, competitive and noncompetitive inhibition.

• Factors affecting activity: Temperature, pH, cofactors, and coenzymes.

• Localization: Enzymes are found in specific cellular compartments (e.g., mitochondria, membranes).

Example: ATP synthase is localized in the mitochondrial membrane and synthesizes ATP.

HTML Table: Comparison of Membrane Transport Mechanisms

HTML Table: Types of Cell Junctions

Summary

This study guide covers essential topics in general biology, including cell structure, membrane function, transport mechanisms, energy, and enzymes. Understanding these concepts is fundamental for further study in cell biology, physiology, and biochemistry.