Photosynthesis and Cellular Respiration: A Comprehensive Overview

This page provides a detailed comparison of photosynthesis and cellular respiration, highlighting their key steps and components. It emphasizes the interconnected nature of these processes in energy production and utilization within living organisms.

Steps of Photosynthesis

Photosynthesis is divided into two main phases: the light reaction and the Calvin cycle.

Light Reaction

The light reaction occurs in the thylakoid membrane and involves the following steps:

1. Photons of light excite electrons, causing them to move along the electron transport chain.
2. Water molecules split, donating hydrogen ions $H+$ while oxygen (O₂) is released as a byproduct.
3. ATP and NADPH are produced and move to the stroma for use in the Calvin cycle.

Highlight: The light reaction is crucial for converting light energy into chemical energy in the form of ATP and NADPH.

Calvin Cycle (Light-Independent Reaction)

The Calvin cycle takes place in the stroma and involves these key steps:

1. Carbon dioxide (CO₂) enters the stroma, initiating carbon fixation.
2. ATP and NADPH from the light reaction are used to help convert CO₂ into glucose.
3. Multiple cycles are required to create glucose components, which are then assembled into a single glucose molecule.
4. ADP, phosphate, and NADP+ are recycled back to the thylakoid for reuse in the light reaction.

Vocabulary: Carbon fixation is the process of incorporating carbon dioxide into organic compounds.

Cellular Respiration

Cellular respiration is the process by which cells break down glucose to release energy. It consists of three main stages:

1. Glycolysis: Occurs in the cytoplasm

   • Glucose is converted to pyruvate, producing ATP and NADH

2. Krebs Cycle: Takes place in the inner mitochondrial space

   • Pyruvate is further broken down, yielding NADH, FADH₂, ATP, and CO₂

3. Electron Transport Chain: Located in the mitochondrial membrane

   • NADH and FADH₂ are oxidized, and oxygen is reduced to water, producing ATP

Definition: The electron transport chain is a series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions, coupled with the transfer of protons across a membrane to create a proton gradient that drives ATP synthesis.

The overall equation for cellular respiration is:

C6H12O6 + O₂ → CO₂ + H₂O + ATP

Fermentation

In the absence of oxygen, cells can undergo fermentation after glycolysis:

• Alcoholic fermentation produces ethanol
• Lactic acid fermentation produces lactic acid in muscles

Example: During intense exercise, when oxygen supply is limited, muscle cells may switch to lactic acid fermentation to continue producing ATP.

The page concludes by noting that anabolic reactions build up molecules, while catabolic reactions break them down.

Highlight: Understanding the relationship between photosynthesis and cellular respiration is crucial for comprehending energy flow in ecosystems and cellular metabolism.