Energy and Metabolism

Key Definitions

• Energy: The capacity to do work or cause change. In biological systems, energy is required for processes such as growth, movement, and cellular transport.

• Endergonic Reaction: A chemical reaction that absorbs free energy from its surroundings; non-spontaneous.

• Exergonic Reaction: A chemical reaction that releases free energy; spontaneous.

• Activation Energy: The minimum amount of energy required to start a chemical reaction.

• Catalysts: Substances that speed up chemical reactions without being consumed; in biology, these are usually enzymes.

• Catabolism: Metabolic pathways that break down molecules into smaller units and release energy.

• Anabolism: Metabolic pathways that construct molecules from smaller units, requiring energy input.

• Metabolism: The sum of all chemical reactions in an organism, including both catabolic and anabolic pathways.

Thermodynamics in Biology

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; energy conversions are inefficient, with some energy lost as heat.

Enzymes and Regulation

• How Enzymes Work: Enzymes lower the activation energy of reactions, increasing reaction rates. They bind substrates at their active sites, forming enzyme-substrate complexes.

• Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration, and presence of inhibitors or activators.

• Inhibitors: Molecules that decrease enzyme activity. Competitive inhibitors bind to the active site; noncompetitive inhibitors bind elsewhere, changing enzyme shape.

• Activators: Molecules that increase enzyme activity, often by stabilizing the active form of the enzyme.

• Feedback Inhibition: A regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

Glucose Catabolism (Cellular Respiration)

Overall Chemical Reaction

The overall reaction for aerobic cellular respiration is:

• Oxidized: Glucose (C6H12O6) is oxidized to CO2.

• Reduced: Oxygen (O2) is reduced to H2O.

Four Steps of Glucose Catabolism

1. Glycolysis

   • Location: Cytoplasm

   • Substrates: Glucose

   • Products: 2 Pyruvate, 2 ATP (net), 2 NADH

   • Key Reaction: Glucose is split into two 3-carbon molecules (pyruvate).

2. Pyruvate Oxidation (Formation of Acetyl CoA)

   • Location: Mitochondrial matrix

   • Substrates: 2 Pyruvate

   • Products: 2 Acetyl CoA, 2 CO2, 2 NADH

   • Importance of Acetyl CoA: Acetyl CoA is the entry molecule for the citric acid cycle.

3. Krebs Cycle (Citric Acid Cycle)

   • Location: Mitochondrial matrix

   • Substrates: 2 Acetyl CoA

   • Products: 4 CO2, 2 ATP, 6 NADH, 2 FADH2

   • Key Reaction: Acetyl CoA is oxidized, releasing CO2 and transferring electrons to carriers.

4. Electron Transport Chain (ETC) & Oxidative Phosphorylation

   • Location: Inner mitochondrial membrane

   • Substrates: NADH, FADH2, O2

   • Products: H2O, ~28 ATP

   • Key Reaction: Electrons are transferred through protein complexes, creating a proton gradient used to generate ATP.

Electron Carriers

• NAD+ (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) are electron carriers. They accept electrons during catabolic reactions and deliver them to the ETC.

Electron Transport Chain Principles

1. Electrons move through a series of protein complexes, releasing energy used to pump protons (H+) across the membrane.

2. The resulting proton gradient drives ATP synthesis via ATP synthase (the channel protein that generates ATP).

Regulation Points

• Glycolysis (Phosphofructokinase): Allosterically regulated by ATP and citrate.

• Pyruvate Dehydrogenase: Regulated by feedback inhibition from its products.

Fermentation

• Occurs when oxygen is absent; regenerates NAD+ by reducing pyruvate to lactate or ethanol.

Role of Oxygen

• Oxygen is the final electron acceptor in the ETC, allowing the chain to continue and ATP to be produced.

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP to form ATP (occurs in glycolysis and Krebs cycle).

• Oxidative Phosphorylation: ATP synthesis powered by the proton gradient generated by the ETC.

Photosynthesis

Overall Chemical Reaction

• Oxidized: H2O is oxidized to O2.

• Reduced: CO2 is reduced to glucose.

Two Steps of Photosynthesis

1. Light-Dependent Reactions

   • Location: Thylakoid membranes of chloroplasts

   • Substrates: H2O, light, NADP+, ADP

   • Products: O2, ATP, NADPH

   • Light Dependency: Yes

   • Key Reaction: Splitting of water, generation of ATP and NADPH

2. Calvin Cycle (Light-Independent Reactions)

   • Location: Stroma of chloroplasts

   • Substrates: CO2, ATP, NADPH

   • Products: Glucose (or G3P), ADP, NADP+

   • Light Dependency: No (but depends on products of light reactions)

   • Key Reaction: Carbon fixation and reduction to form sugars

Pigments in Photosynthesis

• Chlorophyll a: Main pigment; absorbs violet-blue and red light, reflects green.

• Chlorophyll b: Accessory pigment; absorbs blue and orange, reflects yellow-green.

• Carotenoids: Accessory pigments; absorb blue-green, reflect yellow-orange.

Definitions

• Pigment: A molecule that absorbs certain wavelengths of light and reflects others.

• Photosystem: A complex of proteins and pigments that captures light energy and initiates electron transport.

• Electron Carriers: NADP+ is reduced to NADPH during light reactions, carrying electrons to the Calvin cycle.

Cell Transport

Tonicity Problems

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration outside; water moves in, cell swells.

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

Types of Transport

• Passive Transport: Movement of substances across membranes without energy input (e.g., diffusion, facilitated diffusion, osmosis).

• Active Transport: Movement against a concentration gradient, requiring energy (e.g., proton pump).

Transport Mechanisms

• Diffusion: Movement of molecules from high to low concentration.

• Ion Diffusion: Movement of ions across membranes, often through channels.

• Facilitated Diffusion: Passive transport via membrane proteins.

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

• Proton Pump: Active transport protein that moves protons (H+) across membranes, creating electrochemical gradients.

• Endocytosis: Uptake of materials into the cell by vesicle formation.

• Exocytosis: Release of materials from the cell by vesicle fusion with the membrane.

Cell Signaling

Overview

• Cells communicate via chemical signals that bind to receptors, triggering responses.

• Signaling pathways regulate metabolism, growth, and cellular responses to the environment.

Additional info: Where specific details were not provided, standard textbook explanations were included for completeness and clarity.