Chapter 7: Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a dynamic combination of lipids, proteins, and carbohydrates. These components contribute to membrane fluidity and functionality.

• Components: Phospholipids, cholesterol, proteins (integral and peripheral), and carbohydrates.

• Fluidity: Phospholipids move laterally; cholesterol stabilizes membrane fluidity; unsaturated fatty acids increase fluidity.

• Cell Differences: Membrane composition varies between cell types and organisms, affecting permeability and function.

• Example: Animal cell membranes contain more cholesterol than plant cell membranes.

Tonicity

Tonicity refers to the relative concentration of solutes in two solutions separated by a membrane, affecting water movement.

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

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

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

• Predicting Water Movement: Water moves from hypotonic to hypertonic solutions.

• Equilibrium: At equilibrium, concentrations are equal on both sides.

• Example: Red blood cells in a hypotonic solution swell; in a hypertonic solution, they shrink.

Diffusion Across Membranes

Substances move across membranes by diffusion, either passively or with assistance.

• Simple Diffusion: Small, nonpolar molecules (e.g., O2, CO2) diffuse easily.

• Facilitated Diffusion: Polar or charged molecules require protein channels or carriers.

• Hydrophobic Interior: Prevents passage of ions and large polar molecules without help.

• Example: Glucose uses a carrier protein to cross the membrane.

Electrochemical Gradients

An electrochemical gradient is formed by differences in ion concentration and electrical charge across a membrane.

• Chemical Force: Difference in ion concentration.

• Electrical Force: Difference in charge (membrane potential).

• Pumps: Sodium/potassium pump and proton pump create gradients.

• Example: The Na+/K+ pump moves 3 Na+ out and 2 K+ in, maintaining cell potential.

Chapter 8: An Introduction to Metabolism

First and Second Laws of Thermodynamics

Living systems obey the laws of thermodynamics, governing energy transformations.

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

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

• Application: Cells convert chemical energy to work, releasing heat.

• Example: Cellular respiration releases heat and increases entropy.

Free Energy and Reaction Favorability

Free energy (G) determines whether a reaction is energetically favorable.

• Negative ΔG: Reaction is spontaneous (exergonic).

• Positive ΔG: Reaction is non-spontaneous (endergonic).

• Exergonic: Energy released; e.g., ATP hydrolysis.

• Endergonic: Energy required; e.g., synthesis of glucose.

• Equation:

• Graph Interpretation: Downward slope = exergonic; upward slope = endergonic.

Enzymes and Regulation

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

• Regulation: Enzyme activity is affected by temperature, pH, inhibitors, and activators.

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

• Non-competitive Inhibitors: Bind elsewhere, changing enzyme shape.

• Activators: Increase enzyme activity.

• Graph Interpretation: Rate of reaction changes with different factors.

• Example: Pepsin works best at low pH in the stomach.

Chapter 9: Cellular Respiration and Fermentation

Structure of Mitochondria

The mitochondrion is the site of cellular respiration, with distinct compartments for each stage.

• Outer Membrane

• Inner Membrane (folded into cristae)

• Intermembrane Space

• Matrix

• Stages Location:

  • Glycolysis: Cytosol

  • Pyruvate Oxidation & Citric Acid Cycle: Matrix

  • Electron Transport Chain: Inner membrane

Stages of Respiration: Inputs and Outputs

Cellular respiration consists of three main stages, each with specific inputs and outputs.

• Glycolysis: Input: Glucose; Output: 2 pyruvate, 2 ATP, 2 NADH

• Pyruvate Oxidation & Citric Acid Cycle: Input: Pyruvate; Output: CO2, NADH, FADH2, ATP

• Electron Transport Chain & Chemiosmosis: Input: NADH, FADH2, O2; Output: ATP, H2O

Electron Pathway: Glucose to Oxygen

Electrons from glucose are transferred to oxygen through a series of carriers.

• Path: Glucose → NADH/FADH2 → Electron Transport Chain → O2

• Example: NADH donates electrons to the chain, which ultimately reduce oxygen to water.

Chemiosmosis and ATP Synthesis

Chemiosmosis is the process by which a proton gradient drives ATP synthesis.

• Proton Gradient: Created by electron transport chain pumping H+ into intermembrane space.

• ATP Synthase: Uses gradient to synthesize ATP.

• Equation:

Fermentation and Anaerobic Respiration

Cells can generate ATP without oxygen via fermentation or anaerobic respiration.

• Fermentation: Glycolysis followed by conversion of pyruvate to lactic acid or alcohol.

• Anaerobic Respiration: Uses electron transport chain with a final electron acceptor other than O2.

• Similarities: Both occur without oxygen; both start with glycolysis.

• Differences: Fermentation does not use electron transport chain; anaerobic respiration does.

• Outputs:

  • Alcohol Fermentation: Ethanol, CO2, ATP

  • Lactic Acid Fermentation: Lactate, ATP

Chapter 10: Photosynthesis

Autotrophs vs. Heterotrophs

Autotrophs produce their own food via photosynthesis; heterotrophs consume other organisms for energy.

• Autotrophs: Plants, algae, some bacteria

• Heterotrophs: Animals, fungi, most bacteria

• Example: Green plants are autotrophs; humans are heterotrophs.

Structure of Chloroplasts

The chloroplast is the site of photosynthesis, with specialized structures for light reactions and the Calvin cycle.

• Outer and Inner Membranes

• Stroma: Fluid-filled space; site of Calvin cycle

• Thylakoids: Membranous sacs; site of light reactions

• Grana: Stacks of thylakoids

• Locations:

  • Light Reactions: Thylakoid membranes

  • Calvin Cycle: Stroma

Photosystems and Light Reactions

Photosystems II and I work together to capture light energy and drive electron flow, leading to ATP and NADPH production.

• PS II: Absorbs light, splits water, releases O2, transfers electrons to PS I

• PS I: Absorbs light, produces NADPH

• Chemiosmosis: Proton gradient across thylakoid membrane drives ATP synthesis (similar to mitochondria)

Outputs of Light Reactions and Calvin Cycle

Photosynthesis consists of two stages with distinct outputs.

• Light Reactions: ATP, NADPH, O2

• Calvin Cycle: Glucose (or G3P), ADP, NADP+

Electron Pathway: Water to CO2

Electrons flow from water to NADPH and ultimately to CO2 during photosynthesis.

• Path: H2O → PS II → PS I → NADPH → Calvin Cycle → CO2 (reduced to sugar)

• Example: Water is split in PS II, providing electrons for the light reactions.