Study Notes on Photosynthesis

Introduction to Photosynthesis

Definition: Photosynthesis is a process that converts light energy into chemical energy, producing oxygen and glucose.
Importance: Essential for survival, as it provides oxygen needed for respiration and food for organisms.
Found in: Plants, bacteria, algae, and protists; it is ubiquitous in nature.

Chloroplasts are organelles found in eukaryotic cells where photosynthesis occurs.
- Contain structures called thylakoids, which are organized in stacks known as granum.
- The space surrounding the thylakoids is filled with a liquid called stroma; the Calvin cycle occurs here.

Several pigments contribute to photosynthesis, most notably:
- Chlorophyll A: Primary pigment in photosynthesis.
- Chlorophyll B: Accessory pigment that helps in capturing light.
- Other pigments: Carotene and Xanthophylls, which assist in light absorption and exhibit colors (orange and yellow respectively).
Chromatography: A method to separate pigments from leaves, revealing different components of photosynthesis.
Absorption Spectrum: Chlorophyll absorbs blue and red light significantly but not green (appears green due to reflection of green light).
Explanation for color: Being green prevents overheating as black pigments would absorb excessive light.

Photosynthesis is a chemical reaction defined by the equation:
6 CO2 + 6 H2O + light
ightarrow C6H{12}O6 + 6 O2
Reactants: Water (H2O), Carbon Dioxide (CO2), Light.
Products: Glucose (C6H12O6) and Oxygen (O2).
Purpose: Plants use glucose for energy and structural components (e.g., cellulose) and release oxygen as a byproduct.

Photosynthesis consists of two main steps:
- Light Reactions: Occurs in the thylakoid membrane.
- Calvin Cycle: Takes place in the stroma.

Inputs: Light and water enter the thylakoid membrane.
Outputs: Oxygen, NADPH, and ATP are produced.
Process:
- Light energy is absorbed, energizing electrons.
- Electrons pass through an electron transport chain, creating a proton gradient.
- ATP is formed via ATP synthase as protons flow back into the stroma.
- Water is split (photolysis), releasing oxygen as a waste product and generating protons.

Inputs: ATP, NADPH, and Carbon Dioxide (CO2) are needed.
Outputs: Glucose and other carbohydrates are produced.
Process:
- Carbon dioxide (a one-carbon molecule) is fixed to RUBP (a five-carbon molecule) by the enzyme rubisco.
- The resulting six-carbon compound is split into two three-carbon molecules called G3P.
- ATP and NADPH provide the energy needed to convert G3P into glucose.
- Some G3P is used to regenerate RUBP, facilitating the cyclic process of the Calvin cycle.

Definition: Occurs when there is insufficient carbon dioxide, causing oxygen to bind instead.
Consequences: Results in a non-functional compound that must be broken down by the plant, leading to energy loss and reduced glucose production.
C3 Plants: Majority of plants that undergo this process.

Initially not a problem as oxygen levels were low; evolved later when atmospheric oxygen increased.
Environmental conditions affecting carbon dioxide availability:
- Stomata close to prevent water loss during hot weather, leading to photorespiration when they are closed.

Example plants: Jade plant, pineapple.
Mechanism: Open stomata at night to take in CO2, converting it into malic acid stored for use during the day.

Example plant: Corn.
Mechanism: Capture CO2 in a four-carbon molecule, transporting it to bundle sheath cells to avoid photorespiration under high temperatures.
C4 photosynthesis helps reduce the risk of photorespiration by effectively managing CO2 intake.

Photosynthesis is a critical biological process sustaining life on earth, and understanding its mechanisms, from chloroplast structure to light-dependent reactions and adaptations to environmental constraints, is vital for grasping ecological and biological principles.