photosynthesis

Question 1

photosynthesis

Answer 1

Green plants use the energy of sunlight to produce sugars from the inorganic raw materials carbon dioxide and water, by a process called photosynthesis.

Question 2

photosynthesis waste product

Answer 2

The waste product is oxygen

Question 3

Photosynthesis occurs in plant cells containing chloroplasts – typically,

Answer 3

these are found mainly in the leaves of green plants

Question 4

in chioroplast , light energy is trapped by the green pigment

Answer 4

chlorophyll, and becomes the chemical energy in molecules such as glucose and ATP

Question 5

light energy is transferred to

Answer 5

organic compounds in photosynthesis,

Question 6

Sugar formed in photosynthesis may temporarily be stored as

Answer 6

starch, but sooner or later most is used in metabolism.

Question 7

chloroplasts are the organelles where the

Answer 7

reactions of photosynthesis occur.

Question 8

chloroplasts are members of a group of organelles

Answer 8

called plastids.

Question 9

In the light-dependent reactions,

Answer 9

light energy is used directly to split water (a process known as ‘photolysis’, for obvious reasons). Hydrogen is then removed and retained by the photosynthetic-specific hydrogen acceptor, known as NADP . At the same time, ATP is generated from ADP and phosphate, also using energy from light. This is known as photophosphorylation. Oxygen is given off as a waste product of the light-dependent reactions. This stage occurs in the grana of the chloroplasts.k

Question 10

In the light-independent reactions,

Answer 10

This stage occurs in the stroma of the chloroplast. Of course, the light-independent reactions require a continuous supply of the products of the light-dependent reactions (ATP and reduced hydrogen acceptor NADPH H ), but do not directly involve light energy (hence the name). Names can be misleading, however, because sugar production is an integral part of photosynthesis, and photosynthesis is a process that is powered by transfer of light energy.

Question 11

ld Chlorophyll molecules are in

Answer 11

they are grouped together in structures called photosystems, held in the thylakoid membranes of the grana

Question 12

ld steps

Answer 12

1 enzymes catalysing the splitting of water into hydrogen ions, electrons and oxygen atoms 2 enzymes catalysing the formation of ATP from ADP and phosphate (Pi) 3 enzymes catalysing the conversion of oxidised H-carrier (NADP ) to reduced carrier (NADPH H ) 4 electron-carrier molecules (these are large proteins).

Question 13

excited electrons

Answer 13

When light energy reaches a reaction centre, ‘ground-state’ electrons in the key chlorophyll molecule are raised to an ‘excited’ state by the light energy received. As a result, high-energy electrons are released from this chlorophyll molecule, and these electrons bring about the biochemical changes of the light-dependent reactions (Figure 5.6). The spaces vacated by the high-energy (excited) electrons are continuously refilled by non-excited or ‘ground-state’ electrons.

Question 14

Firstly, the excited electrons from photosystem II are picked up by,

Answer 14

and passed along, a chain of electron-carriers. As these excited electrons pass, some of the energy causes the pumping of hydrogen ions (protons) from the chloroplast’s matrix into the thylakoid spaces. Here they accumulate – incidentally, causing the pH to drop. The result is a proton gradient that is created across the thylakoid membrane, and which sustains the synthesis of ATP. This is an example of chemiosmosis

Question 15

As a result of these energy transfers, the excitation level of the electrons

Answer 15

falls back to ‘ground state’ and they come to fill the vacancies in the reaction centre of photosystem I. Thus, electrons have been transferred from photosystem II to photosystem I.

Question 16

Meanwhile the ‘holes’ in the reaction centre of photosystem II are filled by electrons (in their ground state) from water molecules. In fact,

Answer 16

the positively charged ‘vacancies’ in photosystem II are powerful enough to cause the splitting of water (photolysis), in the presence of a specific enzyme. The reaction this enzyme catalyses then triggers the release of hydrogen ions and oxygen atoms, as well as ground-state electrons.

Question 17

The oxygen atoms combine to form molecular oxygen, the waste product of photosynthesis. The hydrogen ions are used in the reduction of NADP (see below). In the grana of the chloroplasts, the synthesis of ATP

Answer 17

is coupled to electron transport via the movement of protons by chemiosmosis. Here, the hydrogen ions trapped within the thylakoid space flow out via ATPase enzymes, down their electrochemical gradient. At the same time, ATP is synthesised from ADP and Pi. This is called photophosphorylation.

Question 18

We have seen that the excited electrons that

Answer 18

eventually provide the energy for ATP synthesis originate from water. They fill the vacancies in the reaction centre of photosystem II, and are subsequently moved on to the reaction centre in photosystem I. Finally, they are used to reduce NADP . The photophosphorylation reaction in which they are involved is described as non-cyclic photophosphorylation, because the pathway of electrons is linear.

Question 19

Secondly, the excited electrons from photosystem I

Answer 19

are picked up by a different electron acceptor. Two at a time, they are passed to NADP , which – with the addition of hydrogen ions from photolysis – is reduced to form NADPH H . By this sequence of reactions, repeated again and again at very great speed throughout every second of daylight, the products of the light-dependent reactions (ATP and NADPH H ) are formed.

Question 20

ATP and reduced NADP do not

Answer 20

normally accumulate, however, as they are immediately used in the fixation of carbon dioxide in the surrounding stroma (in the light-independent reactions). Then the ADP and NADP diffuse back into the grana for re-use in the light-dependent reactions.