Section 5.5 – Life Is Supported by the Sun; Light Energy Captured During Photosynthesis Converts Carbon Dioxide to Carbohydrates

Briefly describe the two main sets of reactions involved in photosynthesis.

The two main sets of reactions are: (1) Light reactions - convert light energy into chemical energy in the form of ATP and NADPH; (2) Carbon-fixation reactions (Calvin cycle) - use ATP, NADPH, and CO₂ to produce carbohydrates. Light reactions occur first, followed by the Calvin cycle.;

Write the chemical formula for photosynthesis. You may use words instead of symbols. Then, explain which component of photosynthesis uses each reactant and which component produces each product.

The chemical formula for photosynthesis is: 6 CO₂ + 12 H₂O + light → C₆H₁₂O₆ + 6 O₂ + 6 H₂O. CO₂ is used in the Calvin cycle to make sugar. H₂O is split in light reactions, producing O₂. Light powers the light reactions. Glucose is made in the Calvin cycle. O₂ is made from water during the light reaction.;

Draw a diagram that shows the role of photons, transported electrons, NADPH, ATP, H₂O, O₂, CO₂, and sugars in photosynthesis.

Students should draw a diagram illustrating how light (photons) excites electrons in chlorophyll within photosystems, leading to ATP and NADPH production in light reactions. These energy carriers are then used in the Calvin cycle to fix CO₂ into sugars, while H₂O provides electrons (releasing O₂).;

Summarize the light reactions of photosynthesis, including the location where they occur, the components in order, and the overall goal of the process.

Light reactions occur in the thylakoid membrane. The sequence is: (1) Photosystem II splits water to release O₂, (2) Electron transport chain moves electrons, (3) Photosystem I makes NADPH, (4) ATP synthase makes ATP. The goal is to convert sunlight into ATP and NADPH for the Calvin cycle.;

Draw a cell with a chloroplast, labeling the following parts of the chloroplast: stroma, thylakoid, thylakoid membrane.

Students should label the chloroplast showing the stroma (fluid-filled area), thylakoids (flattened sacs), and thylakoid membranes (where the light reactions occur).;

List the components of photophosphorylation and describe the events occurring in each.

Components: Photosystem II → Electron Transport Chain → Photosystem I → ATP Synthase. Events: (1) Light excites electrons in chlorophyll, (2) Water is split to replace electrons, releasing O₂, (3) Electron transport chain pumps protons to create a gradient, (4) ATP synthase uses the gradient to produce ATP.;

Compare and contrast oxidative phosphorylation (from aerobic respiration) and photophosphorylation.

Both processes use electron transport chains and chemiosmosis to generate ATP via ATP synthase. Oxidative phosphorylation uses energy from the oxidation of nutrients with oxygen as the final electron acceptor, while photophosphorylation uses light energy to excite electrons from water, producing oxygen as a by-product.;

Describe the role of photosystem I and photosystem II in relation to the electron transport chain during photosynthesis.

Photosystem II (P680) absorbs light, splits water, releases O₂, and sends electrons to the electron transport chain. Photosystem I (P700) re-excites electrons and transfers them to reduce NADP⁺ into NADPH. Together, they function in the Z-scheme to move electrons from water to NADPH.;

Summarize the carbon-fixation reactions of photosynthesis, including the location where they occur, the components in order, and the overall goal of the process.

The Calvin cycle (carbon-fixation reactions) occurs in the stroma of the chloroplast. Steps: (1) Carbon fixation: CO₂ + RuBP → 3-phosphoglycerate (3PG); (2) Reduction: 3PG → G3P using ATP and NADPH; (3) Regeneration: G3P → RuBP using ATP. The goal is to convert CO₂ into carbohydrates (G3P/glucose).;

What is another term that is often used in place of the term "carbon-fixation reactions"?

Calvin cycle or light-independent (dark) reactions.;

Describe the role of phosphorylation, carboxylation, and redox reactions in the carbon fixation reactions of photosynthesis.

Phosphorylation: ATP donates phosphate groups to intermediates. Carboxylation: CO₂ attaches to RuBP forming 3PG (catalyzed by rubisco). Redox: NADPH reduces 3PG to G3P by donating electrons and energy.;

Name the enzyme that catalyzes the rate limiting step in the carbon fixation reactions of photosynthesis and describe that reaction. Explain what it means to say that this step is "rate-limiting."

The enzyme is rubisco (ribulose bisphosphate carboxylase/oxygenase). Reaction: RuBP + CO₂ → two molecules of 3-phosphoglycerate (3PG). It is rate-limiting because rubisco works slowly (2-3 reactions per second), controlling the overall speed of the Calvin cycle.;

Explain the importance of photosynthesis to the earth's biosphere.

The Calvin cycle produces carbohydrates (G3P/glucose), storing energy in C-H bonds. These carbohydrates feed both autotrophs and heterotrophs, providing energy for almost all life on Earth. Photosynthesis also produces oxygen, essential for respiration.;

Describe the difference between an autotroph and a heterotroph.

Autotrophs ("self-feeders") perform photosynthesis to make their own food from CO₂ and light energy. Heterotrophs ("other-feeders") cannot photosynthesize; they obtain energy by consuming other organisms.;

Summarize the relationship between photosynthesis and cellular respiration in relation to release and capture of energy.

Photosynthesis captures light energy and stores it in glucose. Cellular respiration releases stored energy from glucose to produce ATP. The products of one process are the reactants of the other, making them complementary.;

Explain to a nonscientist why plants carry out both cellular respiration and photosynthesis but animals only carry out cellular respiration.

Plants perform photosynthesis to make glucose and cellular respiration to break that glucose down for usable energy (ATP). Animals lack chloroplasts to capture light energy, so they obtain glucose by eating other organisms and perform only respiration.;

photosynthesis

Process where light energy converts CO₂ and H₂O into carbohydrates and O₂.;

light reactions

First phase of photosynthesis that converts light energy into ATP and NADPH in the thylakoids.;

carbon-fixation reactions

Second phase (Calvin cycle) that uses ATP, NADPH, and CO₂ to form carbohydrates.;

wavelength

Distance between peaks of light waves; shorter wavelength means more energy.;

photon

Packet of light energy absorbed by pigments.;

pigment

Molecule that absorbs specific wavelengths of light.;

chlorophyll a

Main pigment in photosynthesis that absorbs blue and red light.;

chlorophyll b

Accessory pigment that absorbs additional wavelengths to assist chlorophyll a.;

light-harvesting complex

Group of pigment molecules that capture light and transfer energy to the reaction center.;

photosystem I

Contains P700 chlorophyll; re-excites electrons to form NADPH.;

photosystem II

Contains P680 chlorophyll; splits water and sends electrons through the chain.;

reaction center

Site in the photosystem where excited electrons are transferred to an electron acceptor.;

NADP⁺/NADPH

Electron carrier pair; NADP⁺ accepts electrons to become NADPH, which is used in carbon fixation.;

photophosphorylation

ATP synthesis powered by light-driven electron transport.;

cyclical electron transport

Electron flow using only Photosystem I to make extra ATP.;

Calvin cycle

Series of carbon-fixation reactions that produce G3P from CO₂.;

carbon fixation

Incorporation of CO₂ into organic molecules (RuBP → 3PG).;

rubisco

Enzyme that catalyzes CO₂ fixation to RuBP in the Calvin cycle.;

RuBP

Five-carbon molecule that reacts with CO₂ during carbon fixation.;

3PG

First stable three-carbon product of CO₂ fixation.;

G3P

Three-carbon sugar product of the Calvin cycle used to make glucose.;

autotroph

Organism that makes its own food through photosynthesis.;

heterotroph

Organism that must consume other organisms for energy.;