Photosynthesis

Definition

• Photosynthesis is the process by which plants, some bacteria, and protistans convert sunlight into glucose from carbon dioxide and water.

• Oxygen is released as a byproduct.

Summary Equation

• Word equation: carbon dioxide + water → glucose + oxygen.

Role of Chlorophyll

• Chlorophyll: A green pigment critical for capturing sunlight energy.

• Different types of chlorophyll include chlorophyll a (found in all photosynthetic organisms) and accessory pigments (e.g., chlorophyll b, xanthophylls, carotenoids).

• Chlorophyll a absorbs from violet-blue (400-500 nm) and reddish orange-red (600-700 nm) wavelengths, with limited absorption in the green range.

• Structure of chlorophyll:

  • Lipid-soluble hydrocarbon tail (C20H39 -)

  • Flat hydrophilic head containing magnesium

  • Connection via an ester bond.

Leaf Structure

Function of Leaves

• Leaves act as solar collectors filled with photosynthetic cells.

• Water and carbon dioxide enter through stomata, while sugar and oxygen exit.

Water Transport

• Water enters the plant at the roots and travels to the leaves via xylem vessels.

Stomata

• Stomata: Specialized structures allowing gas exchange (CO2 in, O2 out).

• Guard cells regulate the opening and closing of stomata to prevent water loss.

• Example: Cottonwood trees can lose 100 gallons of water per hour in hot conditions.

Chloroplast Structure

• Chloroplasts: Eukaryotic organelles containing thylakoids (where photosynthesis occurs) arranged in stacks called grana, surrounded by stroma.

• Thylakoids contain photosynthetic chemicals.

Stages of Photosynthesis

Light-Dependent Reactions

• Occur in the grana of chloroplasts; require light energy.

• Process highlights:

  • Chlorophyll a absorbs light, causing an electron to become excited and transferred to a primary electron acceptor.

  • Water is split (photolysis) to provide electrons, releasing O2 and H+ ions.

  • ATP is produced through photophosphorylation, while NADP+ is reduced to NADPH.

Light-Independent Reactions (Calvin Cycle)

• Occur in the stroma; use ATP and NADPH to synthesize carbohydrates from carbon dioxide.

• Processes involved:

  • Carbon fixation: CO2 is combined with ribulose 1,5-biphosphate (RuBP).

  • Glycerate 3-phosphate (GP) is created and subsequently reduced to glyceraldehyde 3-phosphate (GALP).

• Each pair of GALP will:

  • Convert to glucose or other macromolecules.

  • Recycle to reform RuBP.

Photosystems and Electron Transport

Photosystem II and I

• Photosystem II (PSII) absorbs light at 680 nm and initiates the process.

• Photosystem I (PSI) absorbs light at 700 nm, acting later in the chain.

• Two processes generate a Z shape visually depicting energy changes during electron transfer, allowing ATP formation from ADP and inorganic phosphate.

Non-Cyclic vs. Cyclic Phosphorylation

• Non-cyclic phosphorylation: Produces both ATP and NADPH.

• Cyclic phosphorylation: PSI uses energy to generate more ATP without producing NADPH or oxygen, supporting the light-independent stage.

Chemiosmosis

• Electrochemical gradients established by H+ ions pumping across the thylakoid membranes result in ATP synthesis in the chloroplast through chemiosmosis.

Factors Affecting Photosynthesis

• Light Intensity: Higher intensity increases the rate until limited by another factor.

• Carbon Dioxide Concentration: Increased CO2 enhances carbohydrate formation during light-independent reactions.

• Temperature: Enzyme-catalyzed reactions have optimal temperatures; activity declines at higher temperatures.