Photosynthesis Overview

Study Notes

Photosynthesis Introduction

Photosynthesis is the process of using light energy to create carbohydrates, which are also known as sugars.

Photosynthesis Equation

The chemical equation for photosynthesis is:
- 6H₂O + 6CO₂ + light energy → C₆H₁₂O₆ + 6O₂
Reactants:
- Water (H₂O) enters the plant through the roots, and carbon dioxide (CO₂) enters the plant through tiny openings called stomata on the leaves.
Products:
- Glucose (C₆H₁₂O₆) is a simple sugar that is produced, and oxygen gas (O₂) is released through the stomata.

Chloroplasts

Chloroplasts are organelles that carry out photosynthesis.
They have two membranes: an outer membrane and an inner membrane.
The space between the membranes is called the intermembrane space.
The inner membrane encloses a fluid called the stroma, which contains a network of interconnected sacs called thylakoids.
A stack of thylakoids is called a granum (plural: grana).
The fluid inside a thylakoid is called the lumen.

Chlorophyll

Chlorophyll is the pigment that absorbs light energy and is found in the thylakoids.
It absorbs blue and red light and reflects green light, which is why most plants appear green.

Photosynthesis Stages

Photosynthesis consists of two main stages:

Light-Dependent Reactions

Occur within the thylakoids.
Steps:
- Light strikes photosystem II, exciting its electrons and causing them to flow to a mobile electron carrier called plastoquinone.
- Photosystem II replaces its lost electrons by oxidizing water, producing oxygen gas, hydrogen ions (H⁺), and electrons.
- Plastoquinone carries the electrons to the cytochrome b6f complex, pumping protons from the stroma into the lumen, creating a concentration gradient.
- Electrons continue to a copper-containing protein called plastocyanin, which transfers them to photosystem I.
- Photosystem I absorbs light energy, boosting the electrons, which then move to an iron-sulfur protein called ferredoxin.
- Ferredoxin transfers the electrons to NADP reductase, which reduces NADP⁺ to NADPH.
- The proton gradient drives hydrogen ions through ATP synthase, producing ATP.
Products:
- Oxygen (O₂)
- ATP
- NADPH

Light-Independent Reactions (Calvin Cycle)

Occur in the stroma.
Steps:
- Carbon fixation: Carbon dioxide reacts with ribulose bisphosphate (RuBP), a five-carbon sugar, catalyzed by the enzyme rubisco, producing two molecules of 3-phosphoglycerate (PGA).
- Reduction: ATP and NADPH are used to convert PGA into glyceraldehyde-3-phosphate (G3P).
- Regeneration of RuBP: Most G3P is used to regenerate RuBP, while some is used to produce sugars like glucose.

Summary of Calvin Cycle Reactions

Three molecules of CO₂ produce one molecule of G3P.
This requires nine ATP and six NADPH molecules.
Six molecules of CO₂ are needed to produce one molecule of glucose, requiring 18 ATP and 12 NADPH molecules.

Description

Explore the essential process of photosynthesis, where light energy is converted into carbohydrates. This quiz covers the photosynthesis equation, the role of reactants like water and carbon dioxide, and the function of chloroplasts. Test your knowledge of this fundamental biological process!