B5-1 Photosynthesis Flashcards

Question

Cyclic Photophosphorylation

Answer 1

Cyclic photophosphorylation involves Photosystem I only. No NADPH is produced. Light is absorbed by the LHC and passed on to chlorophyll a (P700) in the reaction centre of PSI. This causes the P700 molecule to emit an energised electron which is raised to a higher energy level and picked up by the primary electron acceptor in the reaction centre. The energised electrons from PSl are passed to ferredoxin (Fd), cycled back to cytochrome (b-f) complex on the first ETC and from there, back to PSI. As these electrons are passed along the first ETC, enough energy is released to synthesise ATP from ADP and Pi. Consequently, ATP is produced. The ATP is needed in the light-independent stage of photosynthesis.

Answer 2

Chemiosmosis is the process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP. The thylakoid membrane is impermeable to H+, thus as the light reaction proceeds, accumulation of hydrogen Ions occurs in the thylakoid space. In the 'Z-scheme' of electron flow in non-cyclic photophosphorylation, electrons flow down energy levels along the electron transport chain from PSII to PSI, and release free energy. Using this free energy, the cytochrome complex pumps hydrogen ions against the concentration gradient from the stroma, across the thylakoid membrane, into the thylakoid space. Photolysis of water produces H+ which also contributes to the proton concentration in the thylakoid space. This results in an electrochemical and concentration gradient, known as a proton gradient, where there are more hydrogen ions inside the thylakoid space than there are in the stroma.

Answer 3

Energy-coupling The proton gradient drives the synthesis of ATP by ATP synthase complex (enzyme embedded in thylakoid membrane). Hydrogen ions diffuse down this gradient from the thylakoid space across the thylakoid membrane into the stroma through the ATP synthase complex. This drives the formation of ATP catalysed by the enzyme ATP synthase, where one ATP is synthesised for every 2 H+ that return to the stroma through the ATP synthase complex. ATP synthesised is used for active transport of protons into thylakoid (endogonic). Diffusion of H+ is energy-releasing (exergonic).

Answer 4

The reaction occurs in the stroma of chloroplasts. Its purpose is to reduce carbon dioxide using ATP (energy source) and NADPH (reducing power), produced in the light-dependent reaction. The reactions of the Calvin cycle for a C3 plant can be classified into three phases: (1) carbon dioxide uptake and fixation, (2) reduction of phosphoglyceric acid (PCA) and (3) regeneration of carbon dioxide acceptor (RuBP), which ultimately leads to product synthesis and sugar formation. Carbon Dioxide Fixation Carbon dioxide diffuses through the stomata and into the cytoplasm of the mesophyll cells and into the chloroplasts. Carbon dioxide is fixed when it combines with a five-carbon CO2 acceptor, ribulose bisphosphate (RuBP), to form an unstable six-carbon intermediate. This reaction (carboxylation of RuBP) is catalysed by an enzyme, ribulose bisphosphate carboxylase oxygenase (rubisco). The unstable six-carbon intermediate breaks down spontaneously Into two molecules of a three-carbon compound called phosphoglyceric acid (PGA) / 3-phosphoglycerate / glycerate-3-phosphate (GP). RuBP (5C) + CO2 (1C) + H20 ---rubisco--> 2PGA (2 x 3C) Reduction of PGA Each molecule of PGA is phosphorylated by ATP (i.e. receives an additional phosphate group from ATP) forming 1,3-bisphosphoglycerate. A pair of electrons donated from NADPH further reduces 1,3-bisphosphoglycerate to form glyceraldehyde-3-phosphate (GALP or G3P) or triose phosphate (TP). The hydrogen for the reduction comes from the NADPH (which is oxidised) while the energy for this step comes from ATP. Regeneration of Carbon Dioxide Acceptor (RuBP) For every three molecules of carbon dioxide that enters the Calvin cycle, three molecules of RuBP are carboxylated and a total of six molecules of TP are formed. Only one molecule of TP can be counted as a net gain of carbohydrate. The other five molecules of TP must be used to regenerate the three molecules of RuBP used in the fixation step 1 (see above). To accomplish this, three more molecules of ATP are invested. RuBP is regenerated and the Calvin cycle continues. Production Synthesis and Sugar Formation The TP spun off from the light-independent reaction becomes the starting material for metabolic pathways that synthesise other organic compounds, including glucose and other carbohydrates. Two molecules of TP are utilised to synthesise one molecule of hexose sugar. Hence, the formation of one molecule of hexose sugar requires six turns of the Calvin cycle. Note: The carbon and oxygen atoms of hexose sugars (C6H1206) come from carbon dioxide as well as RuBP while the hydrogen atoms come from NADPH.

Answer 5

Although TP is the end product of the light-independent reaction, it does not accumulate in large quantities. Both PGA and TP are also intermediates in glycolysis. Both glycolysis and the Krebs cycle occupy a central role in metabolism. TP can be used by the cell outside of the chloroplasts in the synthesis of all other forms of carbon-containing substances including other carbohydrates, lipids, proteins. nucleic acids and chlorophyll.

Answer 6

A large proportion of TP is converted to hexose sugars, particularly glucose and fructose which are respiratory substrates for the ATP production. For storage purposes, many glucose molecules are in turn covalently linked together to form starch molecules that are stored as starch granules. Glucose molecules are also polymerised to form cellulose which is needed for the structural growth of plants (i.e. in the formation of new cell walls)

Answer 7

PGA can enter the glycolytic pathway and is converted to an acetyl group, which is added to coenzyme A to form acetyl CoA. Acetyl CoA is converted to fatty acids in both the cytoplasm and chloroplasts. Glycerol is made from TP. Fatty acids and glycerol combine to form triglycerides (for storage) and phospholipids (for cellular membranes).

Answer 8

PGA and TP are important precursors in the synthesis of amino acids (and hence proteins). In the process, nitrogen is incorporated into the product molecules. Plants in turn use these molecules to make other nitrogen-containing compounds, including nucleotides for DNA and RNA synthesis.

Answer 9

The point beyond which an increase in light intensity will cause no further increase in the rate of photosynthesis.

Answer 10

Measured in lux (lx), a measure of luminosity as perceived by the human eye. As seen in 27, at low light intensities, the rate of photosynthesis increases linearly with light intensity. Photosynthesis is no longer limited by light - it is said to be light-saturated. Other factors become limiting. Except for in shaded plants, light is not normally a major limiting factor. Very high light intensities may damage the chlorophyll (i.e. photobleaching) and decrease the rate of photosynthesis. However, plants that have been naturally exposed to such conditions are usually protected by thick cuticles and hairy leaves.

Answer 11

Point at which the rate of photosynthesis is equal to the rate of respiration. At the compensation point, all the carbon dioxide produced during respiration is used for photosynthesis and all the oxygen produced during photosynthesis is used for respiration. Thus, there is no NET gaseous exchange between the plant and its environment at compensation point. The compensation point is reached at quite low light intensities, usually at sunrise and at sunset. Below the compensation point, the rate of photosynthesis is less than the rate of respiration so there is a net release of carbon dioxide and absorption of oxygen from the atmosphere. Above the compensation point, the rate of photosynthesis is greater than the rate of respiration and there is net absorption of carbon dioxide and release of oxygen into the atmosphere.

Answer 12

Shade plants have a lower rate of respiration than that of sun plants. Shade leaves are thin with fewer palisade mesophyll layers. Have fewer cells so require less energy for maintenance. Thus, shade plants reach compensation point at a lower light intensity and much sooner than sun plants. Sun plants have a higher level of respiration and a much higher rate of photosynthesis than shade plants. Sun leaves are thick with more palisade mesophyll layers. Have more cells so require more energy invested into maintenance. However, they are also able to absorb higher light intensities. By producing more palisade layers, the sun plants can trap more light energy for the production of carbohydrates.

Answer 13

Carbon dioxide is needed in the light-independent reaction where it is used to make sugar. As the concentration of carbon dioxide increases, the rate of photosynthesis also increases, until carbon dioxide becomes saturated and is no longer limiting. Under normal conditions, carbon dioxide is the major limiting factor in photosynthesis; its concentration in the atmosphere varies between 0.03% and 0.04%, but increased photosynthetic rate can be achieved by increasing this percentage.

Answer 14

The light-independent reactions are enzyme-controlied and thus temperature-sensitive. The rate of reaction doubles for every 10°C rise up to about 35°C. As temperature rises towards the optimum, the rate of reaction increases because the molecules Involved move more rapidly and have a greater chance of colliding. The rate of photosynthesis decreases at higher temperatures as enzymes start to denature.

Answer 15

Chlorophyll concentration is not normally a limiting factor, but reduction in chlorophyll levels can be induced by several factors, including: diseases, mineral deficiency, normal ageing process, lack of light because light is needed for the final stage of chlorophyll synthesis. If the leaf turns yellow, it is termed to be chlorotic and this yellowing process is called chlorosis

Answer 16

An obvious way of killing a plant is to inhibit photosynthesis, and various herbicides have been used to do this.

Answer 17

Periods of temporary wilting can lead to severe losses in crop yield. Plants usually close their stomata in response to wilting, and this would prevent access of carbon dioxide for photosynthesis. Even slight water deficiency, with no visible effects, might significantly reduce crop yield.

Answer 18

The concentration of oxygen in the Earth's atmosphere is 21%, as compared to 0.03-0.04% carbon dioxide. At any one time, there is much more oxygen than carbon dioxide In many plants, the initial fixation of carbon in the Calvin cycle (light-independent reaction) is catalysed by the enzyme rubisco. However, oxygen competes with carbon dioxide for the active site in rubisco, Less carbon dioxide will be fixed, and previously fixed carbon in RuBP will be lost as carbon dioxide i.e. there is a net loss of carbon. Less PGA will be synthesized and photosynthetic output is decreased This process is known as photorespiration (photo: occurs in the light, respiration: consumes oxygen while producing carbon dioxide)

Answer 19

Low levels of certain gases of industrial origin, notably ozone and sulfur dioxide, are very damaging to the leaves of some plants. It is estimated, that cereal crop losses as high as 15% may occur in badly polluted areas. Soot can block stomata and reduce the transparency of the leaf epidermis.