Lesson 7 Photosynthesis

Question 1

Light-Dependent Reactions

Answer 1

Take place in the intermembrane space of the thylakoids and across the
thylakoid membrane.

Light is absorbed by photosynthetic pigments which are found in the thylakoid
membrane.

The pigments are associated with proteins that are involved in electron
transport, proton pumping and chemiosmosis.

The photosynthetic pigments are combined into two complex groups called
photosystem I and photosystem II, which absorb light energy and use it to
boost electrons into an excited state.

Question 2

Step 1of the Light –Dependent Pathway

Answer 2

Photoactivation of photosystem II

Pigment molecules in the photosystem absorb light and boost electrons in a
molecule of chlorophyll to a higher energy level.

The electrons are accepted by an electron carrier protein at the start of the
electron transport chain.

This electron carrier protein is called plastoquinone. Plastoquinone accepts
two excited electrons from photosystem II and moves away to another
position in the membrane.

Plastoquinone carries the electrons to the start of the ETC.

Question 3

Step 2 of the Light –Dependent Pathway

Answer 3

2. Photolysis

These electrons must be replaced and they are taken from water.

Water is split into electrons, protons (H+) and an oxygen atom.

The oxygen is released as an excretory product.

2H2O 🡪 O2 + 4H+ + 4 e-

Question 4

Step 3 of the Light –Dependent Pathway

Answer 4

3. Photophosphorylation

Excited electrons travel along the ETC into photosystem I.

The energy the electrons lose as they travel is used to pump protons into the
thylakoid interior.

The interior is small so the concentration of protons builds quickly. The
protons flow through a large protein which contains ATP synthase.

This occurs between photosystem II and I

When the electrons reach the end of the ETC they are passed to
plastocyanin.

Question 5

Cyclic and Non-Cyclic Photophosphorylation

Answer 5

When ATP is produced through energy from excited electrons flowing through
photosystem II to NADP+, the process is called non cyclic photophosphorylation.

Cyclic photophosphorylation occurs when the supply of NADP+ runs out and the
electrons from photosystem I have no acceptor.

This causes them to rejoin the ETC and generate more ATP.

Cyclic photophosphorylation does not generate oxygen or NADPH + H+

Question 6

Step 4 of the Light –Dependent Pathway

Answer 6

Absorption of light energy causes photoactivation of photosystem I, boosting
more electrons to a higher energy level.

Electrons arriving from photosystem II replace these electrons.

The electrons are passed to ferredoxin, which is a protein in the fluid outside
the thylakoid.

Two molecules of reduced ferredoxin are used to reduce NADP+ to NADPH +
H+.

This reduced NADP+ is required for the light independent reactions.

Question 7

Light Independent Reactions

Answer 7

The light independent reactions take place in the stroma of the chloroplast and
make up the cyclic pathway known as the Calvin Cycle.

ATP and NADPH + H+ from the light reactions supply the energy and the
electrons.

Carbohydrate is the final product of the Calvin cycle.

Question 8

Step 1 of the Calvin Cycle

Answer 8

The cycle begins with the acceptor molecule, ribulose bisphosphate (RuBP),
combines with CO2 from the atmosphere to form glycerate-3-phosphate (GP).

This is process is called carbon fixation and is catalysed by the enzyme, RuBP
carboxylase, also known as Rubisco.

Each CO2 combines with a RuBP to make two molecules of GP according to
the reaction below:

3 RuBP + 3 CO2 🡪 6 GP

Question 9

Step 2 of the Calvin Cycle

Answer 9

The ATP and NADPH + H+ from the light dependent reactions reduces the
glycerate-3-phosphate into triose phosphate (TP).

No phosphate is added to the compound so the only input from ATP is
energy.

6 molecules of TP are formed and 6 ATP and 6 NADPH + H+ are used during
this step.

Question 10

Step 3 of the Calvin Cycle

Answer 10

Six molecules of triose phosphate are produced in step 2 but only five are
needed to reform ribulose bisphosphate.

One molecule of TP leaves the cycle and is used to create glucose or other
organic molecules.

Question 11

Step 4 of the Calvin Cycle

Answer 11

The triose phosphate that leaves the cycle takes a phosphate with it which has
to be replaced with a phosphate from ATP.

This replacement is done while the five remaining triose phosphates are
converted back into three ribulose bisphosphate molecules.Six turns of the Calvin cycle are used to produce one molecule of glucose.

Other triose phosphate molecules are used to produce other products of
photosynthesis such as amino acids, fatty acids, vitamins and cellulose.