Photosynthesis, Questions and Answers

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Photosynthesis, Questions andAnswers

Photosynthesis, Questions andAnswers

This paper exploits the processof photosynthesis by providing responses to posted question whileusing the work of Allen(2010) as the reference material.

6. (a) Atwhich stages of theelectron transport chain are protons translocated and/or releasedinto the lumen?

This occurs at photosystem II andI.

(b)What is the point of increasing the proton concentration in thelumen?

This increases an electrochemicalproton gradient in the lumen hence easy transport of protons from oneside of the membrane to the next.

(c)What enzyme uses this electrochemical potential gradient?

Adenosine Triphosphate synthase(ATP synthase)

(d)What does it make?

Adenosine Triphosphate (ATP)

7. (a) Whatis the role of Plastoquinone in the electron transport chain?

It carries the electrons fromphotosystem II to the cytochrome bfcomplex, which is a membrane bound protein reminiscent of the complexIII of the electron transport chain in the mitochondria (Allen,2010).

(b)What type of molecule is it?

It is a quinone molecule.

(c)How many electrons does it carry?

It carries one or two electrons

(d)Which complexes does it connect?

Photosystem II (PSII) andCytochrome b6fcomplex

(e)How is Plastoquinone reduced?

By accepting two electrons fromphotosystem II and two hydrogen cations H+from the stroma matrix of the chloroplast this leads to the formationof plastoquinol.

(f) Whatpart of the Plastid does Plastoquinone move through?

Through the lumen of thylakoiddiscs

8. (a) Whatis the role of Plastocyanin in the electron transport chain?

Its major function is to carryelectrons from the cytochrome bfto photosystem I

(b)What type of molecule is it?

It is a soluble electron carrierfound within the thylakoid lumen of chloroplasts.

(c)How many electrons does it carry?

Two electrons

(d)Which complexes does it connect?

Cytochrome b6fcomplex and photosystem I

(e)How is Plastocyanin reduced?

Plastocyanin is reduced byplastoquinol-1 within the chloroplast, specifically via thecytochrome f whichcontains two sites aiding the interaction between redox chemicalsi.e. a hydrophilic site where Plastocyanin reacts by electrontransfer and a hydrophilic site in which plastoquinol reacts throughthe transfer of hydrogen ion (Allen,2010)..

(f) Whation does it contain?

Copper ion

(g)What part of the Plastid does Plastocyanin move through?

It moves through the inner spaceof the thylakoids

9. (a) Whathappens to the electrons from reduced PQBH2when they aregiven to the Cytochrome b6fcomplex?

There will be an increase inelectrochemical potential gradient across the entire membrane.

(b)How does this split pathway increase the number of protonstranslocated to the lumen?

This occurs due to residing ofpositive end gradient on the side of the membrane to which theprotons are transferred.

10. (a) Whatenzyme uses this electrochemical potential gradient?

Photosystem II (PSII)

(b)What does it make?

Plastoquinol (QH2)

11. (a) Thewater splitting enzyme is part of which photosystem?

Photosystem II

(b)Which side of the thylakoid membrane is it situated?

It is embedded within the lipidsurroundings of the thylakoid membrane of the plants.

(c) Why?

The location of water splittingenzyme in the thylakoid membrane is because at the core of thisstructure is the reaction centre (RC) where light energy istransformed to electrochemical potential energy which thenfacilitates enzyme activation and consequently water splitting(Allen,2010).

(d)What metal ions are associated with this enzyme?

Manganese and Calcium ions

(e)How many water molecules are bound initially?

Two molecules

(f)What happens each time photosystem II absorbs a photon?

This energizes electrons that aretransferred through various coenzymes and cofactors to reducePlastoquinone to plastoquinol.

(g)What is the end result of the water splitting enzyme?

The hydrogen ions (protons)produced by the oxidation of water creates proton gradient which willbe used by ATP synthase to generate ATP

12. (a) Anoverabundance of reduced PQBH2can causewhat to happen to the light harvesting complex normally associatedwith photosystem II?

There will be colour changes in aplant to a pale green appearance showing reduction in chlorophyllratio.

13. (a) Howmany electrons are transferred at each step in the electron transportchain?

Four Electrons

(b)How many protons are translocated from the stroma to the lumen ateach step in the chain?

Two protons.

14. (a) Whatis photorespiration and why does it occur?

It is a metabolic process bywhich plants consumes oxygen while releasing carbon (iv) oxide. Ittakes place when the Carbon (iv) Oxide level reduces inside a leafand hence Rubisco utilizes Oxygen as a substitute of CO2. It majorlytakes place during hot, dry, bright days then stomata closes upleading to an increase in oxygen concentration then the CO2(Allen,2010).

(b)Which organelles take part in photorespiration?

Chloroplasts, peroxisomes andmitochondria

(c)How much reduced carbon is “saved” compared to that which is“lost”?

The amount of stored carbon as aresult of photosynthesis is in equilibrium with the amount lostduring respiration plants either store or release 6 carbon moleculesfor every respiratory or photosynthetic process (Allen,2010)..

(d) Whatare the amino acids involved in the mitochondrial reaction where CO2is lost?

TriCarboxylic Acid, Glycerol Acidand Fatty acids are the three primary amino acids involved in themitochondrial reactions involving the loss of CO2.

15. (a) Whatis C4 metabolism?

It a process where carbonfixation begins within the mesophyll cells converting CO2 intobicarbonate, this will be added o the three-carbon acidphosphoenolpyruvate through an enzyme called phosphoenolpyruvatecarboxylase leading to production of four-carbon acid calledoxaloacetate, which is further reduced to malate which is anotherform of four-carbon acid in form of C4 pathways (Allen,2010).

(b)What special type of anatomy is associated with C4 plants?

The Krans anatomy

(c)What is interesting or surprising about the evolution of C4metabolizing plants?

Their leaves have the Kransanatomy, they lack photosystem II, and they consist of more than twochloroplasts e.g. granal mesophyll cells and agranal in under sheathcells with mesophylls only performing photosynthesis even when theleaves stomata are closed (Allen,2010).

(d) Whatmolecule is added to phosphoenolpyruvate to form oxaloacetate duringC4 metabolism?

Phosphoenolpyruvate carboxylase

(e)What is different between this reaction and the reaction(s) thatRuBisCO can do?

C4 cycle reactions occur in thedark stage of photosynthesis using ATP from light stage however,RuBisCo reactions are light depended and takes place inphotosynthesis I.

(f)What happens to the C4 molecule?

The fate of C4 in the C4 isdecarboxylation into pyruvate

(g) Wheredoes the Calvin cycle take place in C4 plants?

Within the chloroplast of a thin–walled mesophyll cell and a 4-crbon acid

(h)What is a unique feature of Chloroplasts of the C4 cells that aredoing the Calvin Cycle?

They have Bundle-sheath cellsthat are always tightly packed around veins in the leaf thus allowingfor the occurrence of Calvin Cycle in these cells.

16. (a) Howdoes light affect the pH of the stroma, which regulates the RuBisCOenzyme?

Light acting as regulatory signalby activating certain carbon assimilation enzymes, this affects theambient conditions in the chloroplast stroma such as pH. This takesplace when light driven electron transport protons from the stromaacross the thylakoid membrane into the thylakoid lumen increasingstroma pH from about 7 in the dark to 8 in the light (Allen,2010).

(b)What is Thioredoxin?

It is simply a class of smallerredox proteins that are usually present in all organism playing arole in important biological processes e.g. redox signaling in human.

(c)How many Ferredoxins does it take to reduce one Thioredoxin?

Two Ferredoxins are requiredthis is because each Ferredoxin is a single electron donor while thereduction process of Thioredoxin requires 2 electrons.

(d)What is the difference between the reduced and oxidized states ofThioredoxin?

The difference is structuralunlike reduced Thioredoxin, the oxidized Thioredoxin residues form aflat hydrophobic ends lying closer to the S-S Bridge where theassociations with other protein molecules occur (Allen,2010).

(e)Do you think there would be more reduced or oxidized Thioredoxinduring the day?

During the day there will be morereduction of Thioredoxin because the light stage of photosynthesisgenerates more Ferredoxins which in turn catalyze the reduction ofthe former.

17. (a) Whatis the first hexose sugar produced from the triose phosphates createdduring the Calvin cycle?

Hexose Phosphates

(b)Using a phosphatase, this is converted to what?

Fructose 1, 6-bisphosphate

(c)What is the hexose-phosphate pool of sugars?

Glucose1-phosphate, Glucose6-phosphate and Fructose 6-phosphate

(d)What are the hexose substrates used to make sucrose?

Glucose and Fructose

18.What is typically the major non-water component in phloem sap?


19. (a) Whatstructure is at the end of sieve tube element cells?

Sieve tube elements

(b)What does this structure do for the sieve tube?

They transport sugars throughoutthe various plant parts


Allen,J. F. (2010).&nbspPhotosynthesis.Energy from the Sun: 14th International Congress onPhotosynthesis&nbsp(Vol.2). Berlin, Heidelberg, Germany: Springer Science &amp BusinessMedia.

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