Cellular Respiration and Photosynthesis: Core Pathways of Energy Transformation in Cells

Cellular Respiration

Overview

Cellular respiration is the process by which cells convert glucose (food) into ATP, the usable form of energy. This process involves a series of metabolic pathways that break down glucose in the presence of oxygen, releasing carbon dioxide, water, and energy.

• Overall equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP} + \text{Heat}$

• Purpose: Convert chemical energy in glucose to ATP for cellular work.

Glycolysis (Occurs in Cytoplasm)

Glycolysis is the first step in cellular respiration, breaking down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each).

• Key steps:

  • Start with 1 glucose molecule.

  • Through a series of steps, glucose is split in half.

  • End with 2 pyruvate molecules.

  • Energy yield: 2 ATP (net) and 2 NADH.

• Analogy: Like breaking a $100 bill into smaller bills.

Pyruvate Oxidation (Occurs in Mitochondrial Matrix)

Each pyruvate molecule is transported into the mitochondria and converted into Acetyl CoA, releasing one molecule of CO2 and generating NADH.

• Each 3-carbon pyruvate loses one carbon as CO2.

• The remaining 2-carbon fragment attaches to Coenzyme A, forming Acetyl CoA.

• Energy yield: 2 NADH (one per pyruvate).

• Analogy: Like unwrapping a piece of candy before eating it.

Citric Acid Cycle (Krebs Cycle) (Occurs in Mitochondrial Matrix)

The Acetyl CoA enters the citric acid cycle, where it is completely broken down, releasing CO2 and transferring high-energy electrons to NADH and FADH2.

• Each turn of the cycle removes carbons as CO2.

• Energy carriers (NADH and FADH2) are loaded with high-energy electrons.

• Energy yield per glucose: 6 NADH, 2 FADH2, 2 ATP.

• Since 1 glucose makes 2 Acetyl CoA, the cycle turns twice per glucose.

• Analogy: Like squeezing every last drop of juice from an orange.

Oxidative Phosphorylation (Occurs at Inner Mitochondrial Membrane)

This stage includes the electron transport chain and chemiosmosis, producing the majority of ATP during cellular respiration.

Part A: Electron Transport Chain

• NADH and FADH2 donate electrons to protein complexes (Complexes I-IV).

• Electrons move through the complexes, releasing energy to pump H+ ions into the intermembrane space.

• Oxygen acts as the final electron acceptor, forming water:

$\text{O}_2 + 4e^- + 4\text{H}^+ \rightarrow 2\text{H}_2\text{O}$

• Analogy: Like a bucket brigade passing water, with each complex pumping H+ into a 'fire'.

Part B: Chemiosmosis

• H+ ions flow back into the matrix through ATP synthase, driving ATP production.

• This process generates about 26-28 ATP per glucose.

• Analogy: Like water flowing through a dam’s turbine to make electricity.

Fermentation (Backup Plan When No Oxygen Is Present)

When oxygen is unavailable, cells use fermentation to regenerate NAD+ so glycolysis can continue.

• Lactic Acid Fermentation (in muscles):

  • Pyruvate + NADH → Lactic acid + NAD+

  • Occurs during intense exercise.

• Alcohol Fermentation (in yeast):

  • Pyruvate + NADH → Ethanol + CO2 + NAD+

  • Used in bread and alcohol production.

• Analogy: Like using a backup generator when the power goes out.

Photosynthesis

Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. It is essentially cellular respiration in reverse.

• Overall equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Light Reactions (Occur in Thylakoid Membranes)

Light reactions capture solar energy and convert it into chemical energy in the form of ATP and NADPH.

• Photosystem II:

  • Light excites chlorophyll, energizing electrons.

  • Water is split to replace lost electrons: $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4e^-$

  • O2 is released as a byproduct.

• Electron Transport Chain:

  • Electrons move through proteins, pumping H+ into the thylakoid space.

  • Creates a proton gradient for ATP production.

• Photosystem I:

  • Electrons are re-energized by light and transferred to NADP+, forming NADPH.

• ATP Synthase: H+ flows out, making ATP.

• Products: ATP, NADPH, O2

• Note: Visible light is a small part of the electromagnetic spectrum.

Calvin Cycle (Occurs in Stroma/Matrix of Chloroplast)

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO2 into glucose.

• Step 1: Carbon Fixation

  • Rubisco enzyme attaches CO2 to a 5-carbon molecule (RuBP), forming a 6-carbon molecule that splits into two 3-carbon molecules.

• Step 2: Reduction

  • ATP and NADPH are used to convert 3-carbon molecules into G3P (a sugar).

  • Some G3P is used to make glucose.

• Step 3: Regeneration

  • Most G3P is used to regenerate RuBP, requiring ATP.

• Net result: 3 CO2 + ATP + NADPH → 1 G3P (which becomes glucose)

• Analogy: Like a factory that takes in raw materials (CO2) and uses energy (ATP/NADPH) to assemble a product (sugar).

Special Plant Adaptations

Plants have evolved different strategies to optimize photosynthesis under various environmental conditions.

C3 Plants (e.g., rice, wheat)

• Use regular photosynthesis.

• In hot, dry conditions, stomata close to save water, causing O2 to build up and CO2 to run low.

• Rubisco may use O2 instead of CO2 (photorespiration), which is wasteful.

C4 Plants (e.g., corn, sugarcane)

• Spatial separation: Use two different cell types.

• CO2 is fixed by PEP carboxylase in mesophyll cells (does not use O2).

• 4-carbon product moves to bundle sheath cells, where CO2 is released for the Calvin cycle.

• Prevents photorespiration.

CAM Plants (e.g., cacti, pineapples)

• Temporal separation: Carbon fixation and Calvin cycle occur at different times.

• Stomata open at night (less water loss), CO2 is fixed and stored as a 4-carbon acid.

• During the day, stomata close, and CO2 is released for the Calvin cycle.

Key Molecules to Know

• NAD+/NADH: Electron carrier (NAD+ = empty, NADH = full of electrons)

• FAD/FADH2: Electron carrier (similar to NAD+/NADH, but different location in the pathway)

• NADP+/NADPH: Electron carrier used in photosynthesis

• ATP: Main energy currency of the cell

Quick Location Guide

Process	Location
Glycolysis	Cytoplasm
Pyruvate Oxidation	Mitochondrial matrix
Citric Acid Cycle	Mitochondrial matrix
Electron Transport Chain	Inner mitochondrial membrane
Light Reactions	Thylakoid membrane (chloroplast)
Calvin Cycle	Stroma (chloroplast)

The Big Picture

Cellular Respiration: Glucose + O2 → CO2 + H2O + ATP (energy OUT)

Photosynthesis: CO2 + H2O + Light → Glucose + O2 (energy IN)

These are opposite processes: Plants use photosynthesis to make food, and all organisms (including plants) use cellular respiration to break down food for energy.