Chapter 5: Cell Membranes

Structure and Functions of Cell Membranes

The cell membrane is a dynamic structure that separates the interior of the cell from its external environment, regulating the movement of substances in and out of the cell.

• Phospholipid Bilayer: Composed of amphipathic phospholipids with hydrophilic heads and hydrophobic tails.

• Proteins: Integral, transmembrane, and peripheral proteins serve roles in transport, signaling, and structural support.

• Cholesterol: Interspersed within the bilayer, modulates membrane fluidity and stability.

• Glycoproteins and Glycolipids: Involved in cell recognition and signaling.

Example: The plasma membrane of animal cells contains cholesterol, which helps maintain fluidity across temperature changes.

Phospholipid Structure and Membrane Formation

Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions. This property drives the spontaneous formation of bilayers in aqueous environments.

• Amphipathic: Molecules with both polar (water-loving) and nonpolar (water-fearing) parts.

• Self-Assembly: Phospholipids arrange themselves so that hydrophobic tails are shielded from water, forming a bilayer.

Example: Cell membranes form naturally when phospholipids are placed in water.

Membrane Transport Mechanisms

Transport across membranes can be passive or active, depending on energy requirements and the nature of the molecules involved.

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes diffusion, osmosis, and facilitated diffusion.

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

• Facilitated Diffusion: Passive movement of molecules via transport proteins; does not require ATP.

• Active Transport: Movement against a concentration gradient, requiring ATP and transport proteins.

Example: Sodium-potassium pump uses ATP to move Na+ and K+ ions across the membrane.

Osmotic Solutions and Cell Response

Cells respond differently to hypertonic, hypotonic, and isotonic environments.

• Hypertonic: Higher solute concentration outside; water leaves the cell, causing shrinkage (plasmolysis in plant cells).

• Hypotonic: Lower solute concentration outside; water enters the cell, causing swelling (turgid in plant cells).

• Isotonic: Equal solute concentration; no net water movement.

Example: Red blood cells placed in a hypotonic solution swell and may burst.

Bulk Transport Processes

Cells use vesicles for the transport of large molecules and particles.

• Endocytosis: Uptake of materials via vesicle formation; requires ATP.

• Exocytosis: Release of materials from the cell via vesicle fusion; requires ATP.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids.

• Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor binding.

Example: White blood cells engulf bacteria via phagocytosis.

Chapter 6: Energy and Enzymes

Forms of Energy and Thermodynamics

Cells require energy to perform work, which is governed by the laws of thermodynamics.

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy due to position or structure.

• Chemical Energy: Energy stored in chemical bonds.

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

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

Example: Glucose contains chemical energy that is released during cellular respiration.

Cellular Reactions and ATP

Cells use ATP to couple exergonic and endergonic reactions, enabling cellular work.

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Requires energy input; non-spontaneous.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

Equation:

Example: Muscle contraction uses ATP hydrolysis for movement.

Enzymes and Regulation

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

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate.

• Activation Energy: Minimum energy required to start a reaction.

• Competitive Inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive Inhibitors: Bind elsewhere, altering enzyme shape and function.

• Feedback Inhibition: End product inhibits an earlier step in the pathway.

• Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration, inhibitors.

Example: Cyanide inhibits cytochrome c oxidase, halting cellular respiration.

Chapter 7: Cellular Respiration

Overview and Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP.

• Stages: Glycolysis, Krebs Cycle (Citric Acid Cycle), Electron Transport Chain (ETC).

• Balanced Equation:

• Glycolysis: Occurs in cytoplasm; breaks glucose into pyruvate.

• Krebs Cycle: Occurs in mitochondrial matrix; oxidizes acetyl-CoA to CO2.

• ETC: Occurs in inner mitochondrial membrane; produces most ATP.

Example: Aerobic respiration yields up to 36-38 ATP per glucose molecule.

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells can produce ATP via fermentation.

• Alcohol Fermentation: Produces ethanol and CO2.

• Lactic Acid Fermentation: Produces lactic acid.

• Anaerobes: Organisms that do not require oxygen.

• Facultative Anaerobes: Can survive with or without oxygen.

Example: Yeast cells perform alcohol fermentation in the absence of oxygen.

Role of Electron Carriers

Electron carriers such as NAD+ and FAD are essential for transferring electrons during respiration.

• NADH: Reduced form; carries electrons to ETC.

• FADH2: Another electron carrier.

Example: NADH donates electrons to the ETC, driving ATP synthesis.

Chapter 8: Photosynthesis

Photosynthetic Organisms and Structures

Photosynthesis is the process by which autotrophs convert light energy into chemical energy.

• Autotrophs: Organisms that produce their own food (e.g., plants, algae, cyanobacteria).

• Chloroplast: Organelle where photosynthesis occurs; contains thylakoids, stroma, and chlorophyll.

Example: Green plants use chloroplasts to capture sunlight and produce glucose.

Light Reactions and Calvin Cycle

Photosynthesis consists of light-dependent reactions and the Calvin cycle.

• Light Reactions: Occur in thylakoid membranes; produce ATP and NADPH, release O2.

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

• Balanced Equation:

• Chlorophyll: Main pigment; absorbs light energy.

• Photosystem I vs II: Both capture light, but differ in electron acceptors and products.

Example: Oxygen produced during photosynthesis comes from the splitting of water in the light reactions.

Adaptations and Environmental Impact

Plants have evolved various mechanisms to optimize photosynthesis and respond to environmental changes.

• C3, C4, CAM Plants: Different pathways for carbon fixation to adapt to climate.

• Greenhouse Effect: Accumulation of greenhouse gases affects global climate.

• Deforestation and Fossil Fuels: Human activities impact photosynthetic organisms and climate.

Example: CAM plants open stomata at night to reduce water loss.

Additional Info

• Cell Membrane Synthesis: Most membrane components are synthesized in the endoplasmic reticulum.

• Freely Permeable Molecules: Small nonpolar molecules (e.g., O2, CO2) can cross the membrane without assistance.

• Practice Questions: The notes include practice questions to reinforce concepts such as membrane transport, enzyme function, cellular respiration, and photosynthesis.