8.3 Photosynthesis

Question 1

Where does light dependant reactions take place?

Answer 1

Light dependent reactions take place in the intermembrane space of the thylakoids.

Question 2

Where does light independant reactions take place?

Answer 2

Light independent reactions take place in the stroma

Question 3

What is photosynthesis?

Answer 3

Photosynthesis is the process by which cells synthesise organic molecules (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, some bacteria) In plants, photosynthesis occurs within a specialised organelle called the chloroplast

Question 4

Photosynthesis is a two step process:

Answer 4

1. The light dependent reactions convert light energy from the Sun into chemical energy (ATP) 2. The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)

Question 5

What are light dependent reactions?

Answer 5

Light is absorbed by chlorophyll, which releases energised electrons that are used to produce ATP (chemical energy) The electrons are donated to carrier molecules (NADP+), which is used (along with ATP) in the light independent reactions The electrons lost from the chlorophyll are replaced by water, which is split (photolysis) to produce oxygen and hydrogen The light dependent reactions occur in the intermembrane space of membranous discs called thylakoids

Question 6

What are light independent reactions?

Answer 6

ATP and hydrogen / electrons (carried by NADPH) are transferred to the site of the light independent reactions The hydrogen / electrons are combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates) The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together The light independent reactions occur within the fluid-filled interior of the chloroplast called the stroma

Question 7

What does absorption of light by photosystems generate?

Answer 7

Excited electrons

Question 8

Where does transfer of excited electrons occur?

Answer 8

Transfer of excited electrons occurs between carriers in thylakoid membranes

Question 9

Steps of light dependant reactions:

Answer 9

The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH) These reactions occur within specialised membrane discs within the chloroplast called thylakoids and involve three steps: 1. Excitation of photosystems by light energy 2. Production of ATP via an electron transport chain 3. Reduction of NADP+ and the photolysis of water

Question 10

First step of light dependant reactions:

Answer 10

Step 1: Excitation of Photosystems by Light Energy * Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane * Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm) * When a photosystem absorbs light energy, delocalised electrons within the pigments become energised or ‘excited’ * These excited electrons are transferred to carrier molecules within the thylakoid membrane

Question 11

Second step of light dependant reactions:

Answer 11

Step 2: Production of ATP via an Electron Transport Chain * Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane * As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the thylakoid * This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force * The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis) * ATP synthase uses the passage of H+ ions to catalyse the synthesis of ATP (from ADP + Pi) * This process is called photophosphorylation – as light provided the initial energy source for ATP production * The newly de-energised electrons from Photosystem II are taken up by Photosystem I

Question 12

What do ATP synthase in thylakoids generate?

Answer 12

ATP synthase in thylakoids generates ATP using the proton gradient

Question 13

What does excited electrons from Photosystem II contribute to?

Answer 13

Excited electrons from Photosystem II are used to contribute to generate a proton gradient

Question 14

Third step of light dependant reactions:

Answer 14

Step 3: Reduction of NADP+ and the Photolysis of Water * Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+ * This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions * The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II * The electrons lost from Photosystem II are replaced by electrons released from water via photolysis * Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product)

Question 15

Overview of light dependant reactions

Answer 15

* The light dependent reactions occur within the intermembrane space of the thylakoids * Chlorophyll in Photosystems I and II absorb light, which triggers the release of high energy electrons (photo activation) * Excited electrons from Photosystem II are transferred between carrier molecules in an electron transport chain * The electron transport chain translocates H+ ions from the stroma to within the thylakoid, creating a proton gradient * The protons are returned to the stroma via ATP synthase, which uses their passage (via chemiosmosis) to synthesise ATP * Excited electrons from Photosystem I are used to reduce NADP+ (forming NADPH) * The electrons lost from Photosystem I are replaced by the de-energised electrons from Photosystem II * The electrons lost from Photosystem II are replaced following the photolysis of water * The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions

Question 16

Z Scheme

Answer 16

The energy changes (oxidation / reduction) that occur during photosynthesis may be represented as a Z scheme: * First vertical bar: Photosystem II electrons are energised by light (electrons replaced by photolysis of water molecules) * Diagonal bar: Electrons lose energy as they pass through an electron transport chain (synthesising ATP) * Second vertical bar: Photosystem I electrons are energised by light (electrons used to reduce NADP+)

Question 17

What is produced in the light dependent reactions?

Answer 17

Reduced NADP and ATP are produced in the light dependent reactions

Question 18

Photophosphorylation

Answer 18

The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source Photophosphorylation may be either a cyclic process or a non-cyclic process

Question 19

Cyclic Photophosphorylation

Answer 19

* Cyclic photophosphorylation involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+ * When light is absorbed by Photosystem I, the excited electron may enter into an electron transport chain to produce ATP * Following this, the de-energised electron returns to the photosystem, restoring its electron supply (hence: cyclic) * As the electron returns to the photosystem, NADP+ is not reduced and water is not needed to replenish the electron supply

Question 20

Non-Cyclic Photophosphorylation

Answer 20

* Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+ * When light is absorbed by Photosystem II, the excited electrons enter into an electron transport chain to produce ATP * Concurrently, photoactivation of Photosystem I results in the release of electrons which reduce NADP+ (forms NADPH) * The photolysis of water releases electrons which replace those lost by Photosystem II (PS I electrons replaced by PS II)

Question 21

Cyclic vs Non-Cyclic Photophosphorylation

Answer 21

* Cyclic photophosphorylation can be used to produce a steady supply of ATP in the presence of sunlight * However, ATP is a highly reactive molecule and hence cannot be readily stored within the cell * Non-cyclic photophosphorylation produces NADPH in addition to ATP (this requires the presence of water) * Both NADPH and ATP are required to produce organic molecules via the light independent reactions * Hence, only non-cyclic photophosphorylation allows for the synthesis of organic molecules and long term energy storage

Question 22

Steps of light dependant reactions:

Answer 22

The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma The light independent reactions are collectively known as the Calvin cycle and involve three main steps: 1. Carboxylation of ribulose bisphosphate 2. Reduction of glycerate-3-phosphate 3. Regeneration of ribulose bisphosphate

Question 23

First step of light independant reactions:

Answer 23

Step 1: Carbon Fixation * The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP) * An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP * The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP) * A single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP

Question 24

Second step of light independant reactions:

Answer 24

* Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP * Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy * Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule

Question 25

Third step of light independant reactions:

Answer 25

Step 3: Regeneration of RuBP * Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule * Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch * The remaining five TP molecules are recombined to regenerate stocks of RuBP (5 × 3C = 3 × 5C) * The regeneration of RuBP requires energy derived from the hydrolysis of ATP

Question 26

Calvin cycle

Answer 26

The light independent reactions are also collectively known as the Calvin cycle – named after American chemist Melvin Calvin Calvin mapped the complete conversion of carbon within a plant during the process of photosynthesis Calvin’s elucidation of photosynthetic carbon compounds is commonly classed the ‘lollipop experiment’ This is due to the fact that the apparatus he utilised was thought to resemble an upside-down lollipop

Question 27

Key events of the Calvin cycle

Answer 27

The Calvin cycle outlines the events that result in the formation of organic molecules from inorganic sources (CO2) * Ribulose bisphosphate (RuBP) is carboxylated by carbon dioxide (CO2) to form a hexose biphosphate compound * The hexose biphosphate compound immediately breaks down into molecules of glycerate-3-phosphate (GP) * The GP is converted by ATP and NADPH into molecules of triose phosphate (TP) * TP can be used to form organic molecules or can be recombined by ATP to reform stocks of RuBP

Question 28

Chloroplast functions

Answer 28

* Thylakoids – flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation * Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane * Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption * Stroma – central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur * Lamellae – connects and separates thylakoid stacks (grana), maximising photosynthetic efficiency

Question 29

Chloroplast diagram

Answer 29