General Biology Study Guide: Energy, Metabolism, Cellular Respiration, and Photosynthesis (Chapters 6-8)

Module 3 Lesson 1: Energy and Metabolism

Definition and Types of Energy

Energy is the capacity to do work or cause change. In biological systems, energy exists primarily as kinetic (energy of motion) and potential (stored energy).

• Kinetic Energy: Energy associated with movement, such as the motion of molecules.

• Potential Energy: Stored energy, such as chemical energy in bonds.

• Measurement of Energy: Energy is measured in units such as joules or calories.

• Source of Energy: Most energy for living organisms originates from the sun.

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, which is fundamental to energy transformations in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Example: Cellular respiration involves oxidation of glucose and reduction of oxygen.

Thermodynamics in Biology

Thermodynamics governs energy changes in biological systems.

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

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

• Gibbs Free Energy ($\Delta G$): Indicates whether a reaction is spontaneous. Negative $\Delta G$ means the reaction releases energy and is spontaneous.

Activation Energy

Activation energy is the minimum energy required to start a chemical reaction.

• Role of Enzymes: Enzymes lower activation energy, increasing reaction rates.

ATP: Structure and Function

Adenosine triphosphate (ATP) is the primary energy currency of the cell.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Function: Provides energy for cellular processes by hydrolysis of its terminal phosphate bond.

Catalysts and Enzymes

Catalysts speed up chemical reactions without being consumed. Enzymes are biological catalysts.

• Active Site: Region on the enzyme where substrate binds.

• Enzyme Specificity: Shape of the active site determines enzyme specificity.

• Inhibition: Competitive inhibitors bind to the active site; noncompetitive inhibitors bind elsewhere, altering enzyme function.

Metabolism

Metabolism encompasses all chemical reactions in a cell, divided into two forms:

• Anabolism: Building up molecules (requires energy).

• Catabolism: Breaking down molecules (releases energy).

Module 3 Lesson 2: Cellular Respiration and Fermentation

Autotrophs vs. Heterotrophs

Organisms are classified based on how they obtain energy.

• Autotrophs: Produce their own food (e.g., plants via photosynthesis).

• Heterotrophs: Obtain energy by consuming other organisms.

Potential Energy in Organic Molecules

Organic molecules store energy in C-H bonds, which is released during cellular respiration.

• Cellular Respiration: Process by which cells extract energy from organic molecules.

Electron Carriers

Electron carriers transport electrons during cellular respiration.

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons to become NADH.

• FAD (Flavin adenine dinucleotide): Accepts electrons to become FADH2.

Stages of Cellular Respiration

Cellular respiration consists of glycolysis, Krebs cycle, and electron transport chain.

• Aerobic Respiration: Uses oxygen as the final electron acceptor.

• Anaerobic Respiration/Fermentation: Uses other molecules (e.g., organic molecules) as electron acceptors.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm.

• Process: Glucose is broken into two molecules of pyruvate.

• Net ATP Produced: 2 ATP per glucose molecule.

Products of Pyruvate Oxidation

• CO2

• NADH

• Acetyl-CoA

Krebs Cycle Products

Product	Per Glucose
CO2	4
NADH	6
FADH2	2
ATP	2

Electron Transport Chain (ETC)

The ETC is a series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons to generate ATP.

• Final Electron Acceptor: Oxygen in aerobic respiration.

• ATP Yield: Theoretical yield is about 36-38 ATP per glucose; actual yield is lower due to losses.

Fermentation

Fermentation allows cells to produce ATP without oxygen.

• Products: Lactic acid (in animals), ethanol and CO2 (in yeast).

Module 3 Lessons 3-4: Photosynthesis

Photosynthesis Equation

Photosynthesis converts light energy into chemical energy in plants.

• General Equation:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• Comparison: Photosynthesis is essentially the reverse of cellular respiration.

Light-Dependent and Light-Independent Reactions

Photosynthesis consists of two main stages:

• Light-Dependent Reactions: Occur in the thylakoid membranes; produce ATP and NADPH.

• Light-Independent Reactions (Calvin Cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

Pigments in Photosynthesis

Pigments absorb light energy for photosynthesis.

• Chlorophyll a: Main pigment; absorbs blue and red light.

• Chlorophyll b, Carotenoids: Accessory pigments; absorb other wavelengths.

• Location: Pigments are found in photosystems within the thylakoid membrane.

Photosystems and Electron Flow

Photosystems are complexes of pigments and proteins that capture light energy.

• Photosystem II (PSII): Absorbs light, splits water, releases O2.

• Photosystem I (PSI): Absorbs light, produces NADPH.

• Electron Transport: Electrons flow from water to NADP+, producing ATP and NADPH.

Calvin Cycle

The Calvin Cycle uses ATP and NADPH to fix CO2 into glucose.

• Phases: Carbon fixation, reduction, regeneration.

• Turns Required: Three turns of the cycle produce one molecule of G3P (glyceraldehyde-3-phosphate).

• Products: Glucose, ADP, NADP+.

Comparing Photosynthesis and Cellular Respiration

Process	Location	Reactants	Products
Photosynthesis	Chloroplast	CO2, H2O, light	Glucose, O2
Cellular Respiration	Mitochondria	Glucose, O2	CO2, H2O, ATP

• Key Difference: Photosynthesis stores energy; cellular respiration releases energy.

Additional info:

• CAM and C4 photosynthesis are adaptations for plants in hot, dry environments, allowing them to minimize water loss.

• ATP synthase is the enzyme that synthesizes ATP using the proton gradient generated by the electron transport chain.