Cell Membranes, Thermodynamics, and Cellular Respiration: Study Guide

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Cell Membranes

Structure and Fluidity of Cell Membranes

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that separates the interior of the cell from its external environment. Its structure and fluidity are influenced by the types of fatty acids and cholesterol present.

Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
Saturated Fatty Acids: Have no double bonds; pack tightly, decreasing membrane fluidity, especially at lower temperatures.
Unsaturated Fatty Acids: Contain one or more double bonds; create kinks, preventing tight packing and increasing fluidity.
Cholesterol: Acts as a fluidity buffer; at high temperatures, it stabilizes the membrane and reduces fluidity; at low temperatures, it prevents tight packing, increasing fluidity.

Example: In cold environments, organisms often increase unsaturated fatty acids in their membranes to maintain fluidity.

Transport Across Cell Membranes

Transport processes allow substances to move across the membrane, either with or without energy input.

Passive Transport: Does not require energy; substances move down their concentration gradient.
Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.
Facilitated Diffusion: Movement of polar or charged molecules via protein channels or carriers (e.g., glucose, ions).
Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient (e.g., Na+/K+ pump).
Ligand-Gated Channels: Open in response to binding of a specific molecule (ligand).
Voltage-Gated Channels: Open or close in response to changes in membrane potential.

Example: The sodium-potassium pump uses ATP to move Na+ out and K+ into the cell against their gradients.

Permeability of the Membrane to Different Molecules

Gases (O2, CO2): Pass easily by simple diffusion.
Hydrophobic Molecules: (e.g., steroids) diffuse freely.
Small Polar Molecules: (e.g., H2O) pass slowly or via channels (aquaporins).
Large Polar Molecules: (e.g., glucose) require facilitated diffusion.
Charged Molecules (ions): Require protein channels or pumps.

Thermodynamics and Energy in Biology

First and Second Laws of Thermodynamics

First Law: Energy cannot be created or destroyed, only transformed.
Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Gibbs Free Energy (G) and Spontaneity

Gibbs free energy determines whether a reaction can occur spontaneously.

Free Energy (G): The energy in a system available to do work.
Change in Free Energy (ΔG):
Spontaneous Reactions: Occur without energy input; (exergonic).
Non-Spontaneous Reactions: Require energy input; (endergonic).
Entropy (S): A measure of disorder; higher entropy means lower free energy.

Example: Hydrolysis of ATP is exergonic and spontaneous.

ATP and Energy Coupling

ATP Hydrolysis:
Energy released from ATP hydrolysis is used to drive endergonic reactions (energy coupling).
ATP breakdown powers cellular processes such as muscle contraction and active transport.

Enzymes and Activation Energy

Enzymes: Biological catalysts that lower activation energy, increasing reaction rates.
Activation Energy (Ea): The energy barrier that must be overcome for a reaction to proceed.
Enzymes do not change ΔG; they only speed up the rate at which equilibrium is reached.
Inhibitors:
- Competitive: Bind to the active site, blocking substrate binding.
- Noncompetitive: Bind elsewhere, changing enzyme shape and function.

Cellular Respiration, Anaerobic Respiration, and Fermentation

Metabolism: Anabolism and Catabolism

Metabolism: The sum of all chemical reactions in a cell.
Anabolism: Building complex molecules from simpler ones (requires energy).
Catabolism: Breaking down complex molecules into simpler ones (releases energy).

Redox Reactions in Metabolism

Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Oxidizing Agent: Accepts electrons (is reduced).
Reducing Agent: Donates electrons (is oxidized).
Role of Oxygen: Final electron acceptor in aerobic respiration.

Example: NAD+ is reduced to NADH during glycolysis and the citric acid cycle.

Electron Carriers

NAD+/NADH: NAD+ (oxidized), NADH (reduced); carries electrons to the electron transport chain.
FAD/FADH2: FAD (oxidized), FADH2 (reduced); also carries electrons.

Controlled Energy Release and ATP Synthesis

Energy is released in small steps to maximize ATP production and minimize heat loss.
ATP is produced by substrate-level phosphorylation (direct transfer of phosphate) and oxidative phosphorylation (via electron transport chain and chemiosmosis).

Major Metabolic Pathways

Pathway	Main Stages	Location (Eukaryote)	ATP Yield	Final Electron Acceptor
Aerobic Respiration	Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Electron Transport Chain	Cytosol (glycolysis), Mitochondria (others)	~30-32 ATP/glucose	O2
Anaerobic Respiration	Glycolysis, ETC (with alternative acceptor)	Cytosol, Plasma Membrane (prokaryotes)	Varies (<30 ATP)	Other than O2 (e.g., NO3-)
Fermentation	Glycolysis, Fermentation	Cytosol	2 ATP/glucose	Organic molecule (e.g., pyruvate, acetaldehyde)

Stages of Aerobic Cellular Respiration

Glycolysis: Occurs in cytosol; glucose → 2 pyruvate, 2 ATP (net), 2 NADH.
Pyruvate Oxidation: Mitochondrial matrix; pyruvate → acetyl-CoA, NADH, CO2.
Citric Acid Cycle: Mitochondrial matrix; acetyl-CoA → CO2, NADH, FADH2, ATP (or GTP).
Oxidative Phosphorylation: Inner mitochondrial membrane; electron transport chain and chemiosmosis produce most ATP.

Substrate-Level vs. Oxidative Phosphorylation: Substrate-level occurs in glycolysis and citric acid cycle; oxidative occurs in the electron transport chain.

Key Enzymes in Metabolic Pathways

Kinase: Transfers phosphate groups (e.g., hexokinase in glycolysis).
Isomerase: Rearranges molecules (e.g., phosphoglucose isomerase).
Dehydrogenase: Removes hydrogen atoms (e.g., glyceraldehyde-3-phosphate dehydrogenase).

Fermentation Types and Functions

Alcoholic Fermentation: Pyruvate → ethanol + CO2 (e.g., yeast).
Lactic Acid Fermentation: Pyruvate → lactate (e.g., muscle cells under low O2).
Both regenerate NAD+ for glycolysis.

Summary Table: Metabolic Pathways

Pathway	Oxygen Required?	ATP Yield	Final Electron Acceptor
Aerobic Respiration	Yes	High (~30-32)	O2
Anaerobic Respiration	No	Moderate	Inorganic molecule (not O2)
Fermentation	No	Low (2)	Organic molecule

Electron Transport Chain and ATP Production

Electron Transport Chain (ETC): Series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to O2.
Proton Motive Force: ETC pumps protons (H+) into the intermembrane space, creating an electrochemical gradient.
ATP Synthase: Protons flow back into the matrix through ATP synthase, driving ATP production (chemiosmosis).

Equation for Cellular Respiration:

Key Questions and Answers

Main difference between aerobic respiration, anaerobic respiration, and fermentation: The final electron acceptor (O2 for aerobic, other inorganic for anaerobic, organic for fermentation) and ATP yield.
Role of kinases, dehydrogenases, and isomerases in glycolysis: Kinases transfer phosphates, dehydrogenases remove hydrogens/electrons, isomerases rearrange molecules.
Why a reduced compound is necessary for cellular respiration: Reduced compounds (e.g., glucose) have high-energy electrons that can be transferred to electron carriers for ATP production.
Function of NADH and FADH2: Carry electrons to the ETC for ATP synthesis.
Location and net result of glycolysis: Cytosol; 2 ATP (net), 2 NADH, 2 pyruvate per glucose.
Role of ETC in oxidative phosphorylation: Transfers electrons, pumps protons, creates gradient for ATP synthesis.
Proton motive force: Electrochemical gradient of protons used by ATP synthase to make ATP.
Flow of electrons in ETC: Electrons move from NADH/FADH2 through complexes I-IV to O2, generating ATP.
Final electron acceptor in aerobic respiration: O2; essential to keep ETC functioning and prevent backup of electrons.
Final electron acceptor in fermentation: Organic molecule (e.g., pyruvate); differs from O2 in aerobic and inorganic in anaerobic respiration.

Additional info: In prokaryotes, the electron transport chain is located in the plasma membrane, and some can use alternative electron acceptors (e.g., nitrate) for anaerobic respiration.