Cellular Energy: Respiration and Photosynthesis Study Guide

Cellular Energy

Overview

Cellular energy processes are fundamental to life, involving the transformation of energy through metabolic pathways such as cellular respiration and photosynthesis. This section covers the substrates, products, and mechanisms of these pathways, as well as the molecular details of ATP production and energy transfer.

Cellular Respiration

Substrates and Products of Major Stages

• Glycolysis: Substrate: Glucose; Products: 2 Pyruvate, 2 ATP, 2 NADH

• Pyruvate Oxidation: Substrate: Pyruvate; Products: Acetyl CoA, NADH, CO2

• Krebs Cycle (Citric Acid Cycle): Substrate: Acetyl CoA; Products: 2 CO2, 3 NADH, 1 FADH2, 1 ATP (per turn)

• Electron Transport Chain (ETC): Substrates: NADH, FADH2, O2; Products: H2O, ATP

ATP Production

• ATP (Adenosine Triphosphate): The main energy currency of the cell, produced by substrate-level phosphorylation (glycolysis, Krebs cycle) and oxidative phosphorylation (ETC).

• How many ATP from one glucose? Up to 38 ATP (theoretical maximum), but typically 30-32 ATP due to losses.

Key Molecules

• Acetyl CoA: Central molecule entering the Krebs cycle, formed from pyruvate.

• NAD+ and FAD: Electron carriers reduced during glycolysis and the Krebs cycle to NADH and FADH2.

Electron Transport Chain (ETC) and Chemiosmosis

• Electron Flow: Electrons from NADH and FADH2 pass through protein complexes, releasing energy.

• Proton Gradient: Energy from electrons pumps protons (H+) across the inner mitochondrial membrane, creating an electrochemical gradient.

• Chemiosmosis: Protons flow back through ATP synthase, driving ATP production.

• Oxygen's Role: Final electron acceptor, forming water ($2H^+ + 2e^- + 1/2O_2 \rightarrow H_2O$).

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox Reaction: Coupled process where one molecule is oxidized and another is reduced.

Fermentation

• Fermentation in Yeast: Anaerobic process converting pyruvate to ethanol and CO2, regenerating NAD+.

Metabolic Pathways

• Order of Reactions: Glycolysis → Pyruvate Oxidation → Krebs Cycle → ETC

• Common Pathway: Glycolysis is common to both aerobic respiration and fermentation.

High-Energy Foods

• Carbohydrates and Fats: Contain many C-H bonds, which release energy when oxidized.

Photosynthesis

Light Reactions

• Location: Thylakoid membranes of chloroplasts.

• Products: ATP, NADPH, O2

• Electron Carriers: NADP+ is reduced to NADPH.

Calvin Cycle

• Location: Stroma of chloroplasts.

• Function: Uses ATP and NADPH to fix CO2 into sugars.

Light Absorption and Energy

• Wavelength and Absorbance: Different pigments absorb specific wavelengths; absorption spectrum affects photosynthetic rate.

• Energy per Photon: Inversely related to wavelength ($E = \frac{hc}{\lambda}$).

Summary Table: Key Features of Respiration and Photosynthesis

Process	Main Location	Key Inputs	Key Outputs	Electron Carrier
Cellular Respiration	Mitochondria	Glucose, O2	CO2, H2O, ATP	NADH, FADH2
Photosynthesis (Light Rxns)	Thylakoid Membrane	Light, H2O	O2, ATP, NADPH	NADPH
Photosynthesis (Calvin Cycle)	Stroma	CO2, ATP, NADPH	Glucose (or Sucrose)	—

Additional info:

• The Calvin cycle and light reactions are both essential for photosynthesis; the Calvin cycle is often tested.

• In some experiments, sucrose is used as the sugar product rather than glucose.