Photosynthesis

Photosynthesis: An Overview

• Definition: A series of chemical reactions occurring in producers (plants, algae).

  • Converts light energy into chemical energy, stored in biomass.

  • Light energy splits water molecules (H_2O), releasing hydrogen (H) and oxygen (O).

  • Oxygen is a waste product, while hydrogen combines with carbon dioxide (CO_2) to produce glucose.

  • Chemical energy is contained within glucose molecules, serving as fuel for respiration.

ATP as a Supply of Energy

• Need for Energy: All organisms need a constant supply to maintain cellular functions such as:

  • Building new molecules from digestion products (anabolic reactions).

  • Active transport across cell membranes.

  • Muscle contraction and nerve impulse conduction.

• ATP Structure:

  • Adenosine Triphosphate (ATP), the universal energy currency.

  • Type of nucleic acid, similar to nucleotides in DNA/RNA.

  • Consists of:

  • Nitrogenous Base: Adenine

  • Sugar: Ribose

  • Phosphate Groups: Three, hence "triphosphate".

• Energy Transfer:

  • Removal of one phosphate forms Adenosine Diphosphate (ADP), removal of two forms Adenosine Monophosphate (AMP).

  • ATP is produced during respiration:

  • ADP + Pi ightarrow ATP (where Pi is inorganic phosphate).

  • Breakdown of glucose releases energy to phosphorylate ADP:

  • ATP
    ightarrow ADP + P_i.

  • Hydrolysis of ATP is catalyzed by ATPase enzyme.

  • By-product ADP and inorganic phosphate can regenerate ATP.

Chloroplasts: Structure & Function

• Role of Chloroplasts: Organelles in plant cells where photosynthesis occurs.

• Structure:

  • Surrounded by a double membrane (chloroplast envelope) made of phospholipid bilayers.

  • Contains:

  • Stroma: Gel-like fluid with enzymes, sugars, ribosomes, chloroplast DNA, starch grains, or lipid droplets (if photosynthetic).

  • Thylakoids: Flattened sacs forming stacks (grana) connected by lamellae, housing photosystems with pigments such as chlorophyll a, chlorophyll b, and carotene.

• Functional Implications:

  • Chloroplast envelope keeps necessary components close for photosynthesis.

  • Transport proteins in the inner membrane control molecule flow between stroma and cytoplasm.

  • Large surface area of grana enhances light absorption, facilitating ATP production.

  • Electron carriers and ATP synthase proteins are integral to the thylakoid membrane.

Light-dependent Reactions

• Overview: First stage of photosynthesis, occurs across thylakoid membranes and uses light energy.

• Photolysis:

  • Light energy splits water (H_2O), yielding:

  • 2 hydrogen ions (2H^+).

  • 2 electrons (2e^-).

  • 1 oxygen atom (O) released as waste.

• Energy Conversion:

  • Light energy transforms into chemical energy as ATP and reduced NADP (NADPH).

  • NADP accepts hydrogen, becoming reduced NADP, which assists in transferring hydrogen molecules.

  • Reduction and Oxidation Definitions:

  • Reduction: Gain of electrons/hydrogens or loss of oxygen.

  • Oxidation: Loss of electrons/hydrogens or gain of oxygen.

• Products:

  • ATP, NADPH, and oxygen as by-products.

  • ADP and NADP can be recycled back to the light-dependent reactions for further usage.

Production of ATP and NADPH

• Photophosphorylation: The process of synthesizing ATP during light-dependent reactions; involves:

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

  • Cyclic Photophosphorylation: Produces ATP only.

• ** Electron Transport Chain**: A series of proteins facilitating electron transfer, releasing energy along the way.

• Chemiosmosis:

  • Energy from electrons leads to the creation of a proton gradient.

  • Protons (H^+) are pumped from low to high concentration across the thylakoid membrane.

  • Movement back into the stroma through ATP synthase catalyzes ATP production.

• Final Steps:

  • Electrons from photosystem II pass to photosystem I, combining with hydrogen ions and NADP to form NADPH:

  • H^+ + 2e^- + NADP
    ightarrow NADPH.

Light-independent Reactions (Calvin Cycle)

• Overview: Often referred to as the Calvin Cycle; does not require light directly but depends on ATP and NADPH from the light-dependent reactions.

• Three Main Steps:

  1. Carbon Dioxide and RuBP Combination:

  • CO2 combines with ribulose bisphosphate (RuBP, a 5C compound) via enzyme rubisco, forming two molecules of glycerate 3-phosphate (GP, a 3C compound).

  1. Reduction of GP:

  • GP is reduced to glyceraldehyde 3-phosphate (GALP, another 3C compound) using energy from ATP and hydrogen from NADPH.

  1. RuBP Regeneration:

  • Some GALP is used to regenerate RuBP, requiring ATP.

  • Producing one glucose molecule requires six turns of the Calvin Cycle (due to 6-carbon requirement of glucose).

Products of Photosynthesis

• Types of Organic Molecules Produced:

  • Intermediate Products:

  • GP is converted into amino acids for proteins and fatty acids for lipids.

  • GALP can form hexose sugars, which can further convert to transport sugars (e.g., sucrose) and polysaccharides (e.g., starch, cellulose).

• Biomass Functionality:

  • Glucose is used in respiration, while other nutrients contribute to plant biomass growth, which is then passed on to consumers.

Practical: The Hill Reaction

• Purpose: Investigate the rate of light-dependent reactions using chlorophyll.

• Indicators Used: DCPIP and methylene blue undergo color change to indicate electron acceptance.

  • DCPIP changes from blue (oxidized) to colorless (reduced).

• Experimental Setup:

  • Materials: Spinach leaves, isolation solution, glassware, colorimeter, etc.

  • Procedure: Mashing leaves for chloroplast extraction, measuring absorbance over time to track reaction rate.

• Expected Results: Absorbance decreases over time reflecting DCPIP reduction and chloroplast activity. Changing variables adjustments help analyze reactions under different conditions.