Plant systems and photosynthesis Flashcards

Question

light reactions - step 2

Answer 1

2. In the thylakoid space, water is split into: 1. 2 e–, which replace chlorophyll's electron twice, 2. 2 H+, which contribute to the high [H+] in the thylakoid space, and 3. an O atom, which joins another O, forming O2 gas.

Answer 2

As electrons travel down the electron + transport chain, 4 H are pumped into the thylakoid space, increasing [H+], decreasing the pH.

Answer 3

In chemiosmosis, the diffusion of H+ from the thylakoid space back to the stroma down the [H+] gradient powers ATP synthase, producing ATP.

Answer 4

5. Low-energy e– from water end up in NADPH (high-energy e–). 1. NADPH formation removes H+ from the stroma, decreasing its [H+], increasing the pH.

Answer 5

uses the chemical energy of ATP and NADPH to reduce CO2 to sugar; light-independent, as it doesn’t require light directly

Answer 6

1. The Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (which is abbreviated RuBP). 2. The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or rubisco (This is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.) 3. The product of the reaction is a six-carbon intermediate that is short-lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed).

Answer 7

1. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate. 2. Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming glyceraldehyde 3-phosphate (G3P). 3. Specifically, the electrons from NADPH reduce a carboxyl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar - the same three-carbon sugar formed in glycolysis by the splitting of glucose. 4. For every *three* molecules of CO2 that enter the cycle, there are *six* molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to complete the cycle. 5. The cycle began with 15 carbons’ worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. 6. Now there are 18 carbons’ worth of carbohydrate in the form of six molecules of G3P. 7. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuBP.

Answer 8

1. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. 2. To accomplish this, the cycle spends three more molecules of ATP. 3. The RuBP is now prepared to receive CO2 again, and the cycle continues. One molecule of G3P exits the cycle per three CO2 molecules fixed and is converted to glucose and other organic molecules.

Answer 9

- Light is a form of electromagnetic energy. - The colors we see as visible light include those wavelengths that drive photosynthesis. - A pigment absorbs light of specific wavelengths; chlorophyll *a* is the main photosynthetic pigment in plants. - Other accessory pigments absorb different wavelengths of light and pass the energy on to chlorophyll *a*. - A pigment goes from a ground state to an excited state when a photon of light boosts one of the pigment’s electrons to a higher-energy orbital. This excited state is unstable. - Electrons from isolated pigments tend to fall back to the ground state, giving off heat and/or light. - A photo system is composed of a reaction-center complex surrounded by light-harvesting complexes that funnel the energy of photons to the reaction-center complex. - When a special pair of reaction-center chlorophyll *a* molecules absorbs energy, one of its electrons is boosted to a higher energy level and transferred to the primary electron acceptor. - Photosystem II contains P680 chlorophyll *a* molecules in the reaction-center complex; photosystem I contains P700 molecules. - Linear electron flow during the light reactions uses both photosystems and produces NADPH, ATP, and oxygen. - Cyclic electron flow employs only one photosystem, producing ATP but no NADPH or O2. - During chemiosmosis in both mitochondria and chloroplasts, electron transport chains generate an H+ gradient across a membrane. - ATP synthase uses this proton-motive force to make ATP.

Answer 10

composed of a reaction-centre complex surrounded by several light-harvesting complex

Answer 11

consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins; the number and variety of pigment molecules enable a photosystem to harvest light over a larger surface area and a larger portion of the spectrum than could any single pigment molecule alone

Answer 12

a molecule capable of accepting electrons and becoming reduced

Answer 13

light drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the thylakoid membranes of chloroplasts - the key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane

Answer 14

part of the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth

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Plant systems and photosynthesis Flashcards

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