General Biology: Cellular Energetics, Photosynthesis, and Cell Signaling Study Guide

Section 8.1: Thermodynamics and Metabolism

First and Second Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed (law of conservation of energy).

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

Anabolic vs. Catabolic Pathways

• Anabolic pathways build complex molecules from simpler ones (e.g., protein synthesis); they require energy input.

• Catabolic pathways break down complex molecules into simpler ones (e.g., cellular respiration); they release energy.

Energy Release and Consumption

• Catabolic pathways release energy; anabolic pathways consume energy.

• Example: Cellular respiration is catabolic and releases energy; photosynthesis is anabolic and consumes energy.

Cellular Respiration Equation

• The general chemical equation for cellular respiration is: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$

• Direction: Catabolic (breakdown of glucose).

• Anabolic direction would be the reverse (e.g., photosynthesis).

Kinetic vs. Potential Energy

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Potential energy: Stored energy (e.g., chemical bonds in ATP).

• Cells convert potential energy in food molecules to kinetic energy for cellular work.

Section 8.3: ATP and Cellular Work

ATP Coupling and Reactions

• ATP powers cellular work by coupling exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

• Catabolic reactions are generally exergonic; anabolic reactions are endergonic.

ATP Structure and Hydrolysis

• ATP is composed of three main parts: adenine (a nitrogenous base), ribose (a sugar), and three phosphate groups.

• Hydrolysis of ATP (breaking the terminal phosphate bond) releases energy: $ATP + H_2O \rightarrow ADP + P_i + \text{Energy}$

• Phosphorylation: The transfer of a phosphate group to another molecule, making it more reactive (less stable, higher free energy).

Nucleoside vs. Nucleotide

• Nucleoside: Base + sugar.

• Nucleotide: Base + sugar + phosphate(s) (e.g., ATP).

ATP Cycle

• ATP is regenerated from ADP and inorganic phosphate via cellular respiration.

Section 8.4: Enzymes and Activation Energy

Enzyme Function and Activation Energy

• Enzymes are biological catalysts (usually proteins) that speed up reactions by lowering activation energy.

• Enzymes are specific to substrates and reactions.

• Activation energy is the energy required to start a reaction.

• Reactions can be exothermic (release energy) or endothermic (absorb energy).

• Enzymes do not change the overall free energy change ($\Delta G$) of a reaction.

Enzyme Structure and Denaturation

• Enzymes have primary, secondary, tertiary, and sometimes quaternary structure.

• Denaturation (loss of structure) affects secondary, tertiary, and quaternary levels, leading to loss of function.

Enzyme Inhibition and Regulation

• Competitive inhibition: Inhibitor binds to the active site.

• Noncompetitive inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Feedback inhibition: End product of a pathway inhibits an earlier step, regulating pathway activity.

Cofactors and Compartmentalization

• Cofactors are non-protein helpers (e.g., metal ions, vitamins) that assist enzyme function.

• Compartmentalization within cells allows for specialized environments and regulation of metabolic pathways.

Section 9.1–9.6: Cellular Respiration

Redox Reactions and Electron Carriers

• Oxidation: Loss of electrons; Reduction: Gain of electrons.

• Electron acceptors (oxidizing agents) and donors (reducing agents) are involved in redox reactions.

• NAD+ and FAD are key electron carriers; they become NADH and FADH2 when reduced.

Glycolysis

• Occurs in the cytoplasm; splits glucose into two pyruvate molecules.

• Has two phases: Energy Investment and Energy Payoff.

• Does not require oxygen (anaerobic); produces a net gain of 2 ATP and 2 NADH per glucose.

Pyruvate Oxidation and Citric Acid Cycle

• Pyruvate oxidation occurs in the mitochondrial matrix; pyruvate is converted to acetyl CoA.

• Citric Acid Cycle (Krebs Cycle) occurs in the matrix; acetyl CoA combines with oxaloacetate to form citrate.

• Produces CO2, ATP (or GTP), NADH, and FADH2.

Electron Transport Chain (ETC) and Chemiosmosis

• ETC is located in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 pass through complexes, creating a proton gradient.

• Protons flow back through ATP synthase, driving ATP production (chemiosmosis).

• Oxidative phosphorylation produces most of the ATP in cellular respiration.

Fermentation and Anaerobic Respiration

• Fermentation occurs when oxygen is absent; regenerates NAD+ to allow glycolysis to continue.

• Types: Lactic acid fermentation (in muscles), alcoholic fermentation (in yeast).

Pathway Integration and Regulation

• Glycolysis and the citric acid cycle connect to other metabolic pathways.

• Feedback inhibition regulates pathway activity to prevent overproduction.

Section 10.1–10.4: Photosynthesis

Overview and Organelles

• Photosynthesis occurs in the chloroplasts of plant cells.

• Converts light energy into chemical energy (glucose).

• Autotrophs (producers) perform photosynthesis; heterotrophs (consumers) rely on them for food.

Light Reactions and Calvin Cycle

• Light reactions occur in the thylakoid membranes; convert light energy to ATP and NADPH.

• Calvin Cycle occurs in the stroma; uses ATP and NADPH to fix CO2 into sugars (G3P).

• Three turns of the Calvin Cycle produce one G3P molecule; five G3P molecules are recycled to regenerate RuBP.

Photosynthetic Pigments and Light Absorption

• Chlorophyll is the main pigment; absorbs blue and red light, reflects green.

• Accessory pigments (carotenoids) absorb additional wavelengths.

• Light energy excites electrons in pigments, initiating electron transport.

Electron Flow and ATP/NADPH Production

• Photosystems II and I capture light energy and transfer electrons through the electron transport chain.

• Proton gradient is established across the thylakoid membrane; ATP synthase produces ATP.

• NADP+ is reduced to NADPH at the end of the chain.

Calvin Cycle Details

• Three stages: Carbon fixation, reduction, and regeneration of RuBP.

• CO2 combines with RuBP (catalyzed by Rubisco) to form 3-phosphoglycerate.

• ATP and NADPH are used to convert 3-phosphoglycerate to G3P.

Section 11.1–11.2: Cell Signaling

Types of Cell Signaling

• Paracrine signaling: Local signaling between nearby cells.

• Synaptic signaling: Specialized local signaling in neurons.

• Endocrine signaling: Long-distance signaling via hormones in the bloodstream.

Stages of Cell Signaling

• Reception: Signal molecule binds to receptor.

• Transduction: Signal is relayed and amplified inside the cell.

• Response: Cell carries out a specific activity.

Receptors and Signal Transduction

• Hydrophilic ligands bind to plasma membrane receptors; hydrophobic ligands bind to intracellular receptors.

• Types of receptors: G protein-coupled receptors, receptor tyrosine kinases, ion channel receptors.

• cAMP is a second messenger that activates protein kinases.

Table: Comparison of Anabolic and Catabolic Pathways

Pathway	Direction	Energy	Example
Anabolic	Builds molecules	Consumes energy	Photosynthesis, protein synthesis
Catabolic	Breaks down molecules	Releases energy	Cellular respiration, digestion

Table: Summary of Cellular Respiration Stages

Stage	Location	O2 Required?	Main Products
Glycolysis	Cytoplasm	No	2 ATP, 2 NADH, 2 Pyruvate
Pyruvate Oxidation	Mitochondrial matrix	Yes	2 Acetyl CoA, 2 NADH, 2 CO2
Citric Acid Cycle	Mitochondrial matrix	Yes	2 ATP, 6 NADH, 2 FADH2, 4 CO2
ETC & Chemiosmosis	Inner mitochondrial membrane	Yes	~26-28 ATP, H2O

Table: Types of Cell Signaling

Type	Distance	Example
Paracrine	Local	Growth factors
Synaptic	Local (neurons)	Neurotransmitters
Endocrine	Long-distance	Hormones

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard General Biology curriculum.