Cell Structure & Microscopy

Prokaryotic vs Eukaryotic Cells

Cells are the fundamental units of life, classified as prokaryotic or eukaryotic based on structural differences.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles. Found in Bacteria and Archaea.

• Eukaryotic cells: Possess a nucleus and various membrane-bound organelles. Found in Eukarya (plants, animals, fungi, protists).

• Organelles common to all cells: Plasma membrane, cytoplasm, ribosomes.

• Specialized organelles: Mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus (present only in eukaryotes).

Domains of Life: Bacteria, Archaea, Eukarya.

• Bacteria: Prokaryotic, diverse metabolic pathways.

• Archaea: Prokaryotic, often extremophiles.

• Eukarya: Eukaryotic, includes multicellular organisms.

Microscopy: Light microscopes can visualize cells, nuclei, and some organelles, but not most internal structures or viruses.

• Limits of light microscopy: Cannot resolve structures smaller than ~200 nm (e.g., ribosomes, viruses).

Common Exam Traps: Viruses are not cells; ribosomes are not membrane-bound organelles.

Example: Ribosomes are visible only with electron microscopy, not light microscopy.

Endomembrane System & Protein Trafficking

Pathways and Functions

The endomembrane system coordinates protein synthesis, modification, and transport within eukaryotic cells.

• Rough Endoplasmic Reticulum (ER): Site of protein synthesis for secreted and membrane proteins.

• Golgi Apparatus: Modifies, sorts, and ships proteins received from the ER.

• Protein Pathway: Proteins synthesized in rough ER → transported to Golgi → packaged into vesicles → delivered to plasma membrane or other destinations.

• Integral membrane proteins: Amphipathic (contain both hydrophobic and hydrophilic regions), allowing them to embed in membranes.

Common Exam Traps: Reversing the order of protein trafficking; assuming all proteins remain in the cytosol.

Example: A hormone is synthesized in the rough ER, processed in the Golgi, and secreted via vesicle fusion with the plasma membrane.

Membrane Structure & Transport

Fluid Mosaic Model and Transport Mechanisms

Cell membranes are dynamic structures composed of lipids, proteins, and carbohydrates, facilitating selective transport and communication.

• Fluid Mosaic Model: Describes the membrane as a flexible bilayer of phospholipids with embedded proteins.

• Carbohydrates: Attached to proteins/lipids, important for cell-cell recognition.

• Osmosis: Movement of water across a membrane toward higher solute concentration.

• Tonicity: Hypotonic (lower solute), Hypertonic (higher solute), Isotonic (equal solute).

• Endocytosis: Uptake of materials via vesicles; includes phagocytosis (solid particles), pinocytosis (fluid), receptor-mediated endocytosis (specific molecules).

Common Exam Traps: Confusing diffusion (movement of solutes) with osmosis (movement of water); water moves toward higher solute concentration.

Example: In a hypotonic solution, animal cells swell as water enters; plant cells become turgid.

Metabolism, Energy, & Enzymes

Pathways and Enzyme Function

Metabolism encompasses all chemical reactions in cells, organized into pathways that build or break down molecules.

• Anabolic pathways: Build complex molecules from simpler ones (require energy).

• Catabolic pathways: Break down complex molecules into simpler ones (release energy).

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy; do not change (free energy change).

• Saturation: When all enzyme active sites are occupied, increasing substrate does not increase rate.

• Optimal temperature: Each enzyme works best at a specific temperature.

• Feedback inhibition: End product of a pathway inhibits an earlier enzyme, regulating pathway activity.

Common Exam Traps: Enzymes do not alter ; anabolic and catabolic pathways are opposites.

Example: Feedback inhibition: ATP inhibits phosphofructokinase in glycolysis.

Equation:

Where: = change in free energy, = change in enthalpy, = temperature, = change in entropy.

Cellular Respiration

Stages and Products

Cellular respiration is the process by which cells extract energy from glucose, producing ATP.

• Stages: Glycolysis → Pyruvate oxidation → Citric acid cycle → Electron transport chain & chemiosmosis.

• Glycolysis: Occurs in cytosol; net products: 2 ATP, 2 NADH, 2 pyruvate.

• CO2 release: Occurs during pyruvate oxidation and citric acid cycle, not glycolysis.

• ATP synthesis: Driven by chemiosmosis, using a proton gradient across the mitochondrial membrane.

Common Exam Traps: CO2 is not released during glycolysis; electron transport chain and ATP synthase are distinct.

Example: Oxidative phosphorylation produces most cellular ATP via ATP synthase.

Equation:

Photosynthesis

Light Reactions and Calvin Cycle

Photosynthesis converts light energy into chemical energy in plants, algae, and some bacteria.

• Light reactions: Occur in thylakoid membranes; produce ATP and NADPH; oxygen is generated by splitting water.

• Calvin cycle: Occurs in stroma; uses ATP and NADPH to fix carbon dioxide into sugars.

• Photosynthesis vs cellular respiration: Photosynthesis occurs in light; respiration occurs continuously.

Common Exam Traps: Calvin cycle does not produce oxygen; thylakoid and stroma are distinct locations.

Example: The Calvin cycle uses ATP and NADPH from light reactions to synthesize glucose.

Equation:

Cell Cycle & Mitosis

Stages and Mechanisms

The cell cycle describes the sequence of events in cell growth and division, including DNA replication and mitosis.

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

• DNA replication: Occurs during S phase, not mitosis.

• Mitosis stages: Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis: Cleavage furrow (animals) vs cell plate (plants).

• Binary fission: Division in prokaryotes; mitosis in eukaryotes.

Common Exam Traps: DNA replicates in S phase, not during mitosis; metaphase and anaphase are distinct stages.

Example: The mitotic spindle separates sister chromatids during anaphase.