Photosynthesis

Question 1

What is the size and shape of chloroplasts

Answer 1

• Lens shaped
• About 5-10 µm in length and 4-7µm in width

Question 2

Describe the features of the chloroplast envelope

Answer 2

• Made up of a double membrane
• The outer membrane is selectively permeable to some solutes
• The inner membrane is highly permeable. Substances pass through with the aid of transporters

Question 3

Describe the features of the Stroma

Answer 3

• A gel-like matrix enclosed by the chloroplast envelope
• Contains circular DNA, 70S ribosomes, starch granules, oil droplets and enzymes involved in the calvin cycle

Question 4

Describe the features of the thylakoids

Answer 4

• A third membrane system within the stroma consisting of flattened sacs or pouches
• photosynthetic pigments and electron carriers are embedded within the membrane
• The space enclosed within the thylakoid is known as the thylakoid lumen or thylakoid space
• This compartmentalisation allows chemiosis to take place and for ATP to be produced by photophosphorylation

Question 5

Describe the features of the granum

Answer 5

• A stack of thylakoids
• THis increases the surface area and the amount of pigments available for the light-dependent reaction of photosytnthesis
• Connecting the grana are flattened tubular thylakoids known as intergranal lamellae. These lamellae connect the thylakoid compartments into a single, continuous compartment within the stroma.

Question 6

What is chlorophyll?

Answer 6

• The main pigment utilised in photosynthesis
• Chlorophyll absorbs mainly red and blue-violet light. It reflects green light which gives most plants their characteristic green colour.
• Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in the thylakoid membrane

Question 7

What does each molecule of chlorophyll consist of?

Answer 7

1. A hydrophyllic porphyrin ring that functions in light absorption
   
   • The porphyrin ring has a flat, light-absorbing hydrophyllic head which contains a magnesium atom at its center.
   • Magnesium defiency in plants reduces chlorophyll production and causes yellowing (ie. chlorosis)
2. A hydrophobic hydrocarbon tail that projects into the thylakoid membrane to keep the chlorophyll embedded in the thylakoid membrane

Different chlorophyll have different side chains on their hydrophyllic head and this modifies their absorption spectra, increasing the range of wavelengths of light absorbed

Question 8

What is chlorophyll a?

Answer 8

It is a major pigment in photoautotrophs and it absorbs blue and red light.

• Only chlorophyll a can participate directly in the light-dependent reaction, which converts light energy to chemical energy.
• The other pigments in the thylkaoid membrane can absorb light and transfer energy to chlorophyll a, which initiates the light-dependent reaction

Question 9

What are carotenoids

Answer 9

• They are accessory pigments, as they pass the light energy they absorb onto chlorophyll a of the reaction centre
• They are yellow, orange, red or brown pigments that absorb strongly in the blue-violet range
• The two main types of carotenoids are:
  
  • carotene
  • xanthophylls
• Both absorb light in the 460 to 550 nm of the visible light spectrum

Question 10

What are the 2 main fucntions of carotenoids and other accessory pigments

Answer 10

1. Broadening spectrum of light for photosynthesis
   
   • Accessory pigments absorb the intermediate wavelngths of light which chlorophyll cannot thus broadening the spectrum of colours that can drive photosynthesis
   • However, carotenoids are not very effective as a photosynthetic pigment and transfer only about 10% of their absorbed energy
2. Photoprotection
   
   • Absorbs excessive light and prevents auto-oxidation of chlorophyll and hence, preventing photobleaching. This function is known as photoprotection
   • Excessive light intensity can damage the chlorophyll pigments, so instead of transmitting energy to the chlorophyll, some carotenoids abosrb and dissipate excess light energy from chlorophyll, protecting them from destrucion by light.

Question 11

Give a brief overview of the 3 main stages of photosynthesis

Answer 11

1. Light harvesting stage
   
   • Light energy is captured by the plant using a mixture of pigments including chlorophyll
2. Light dependent reaction
   
   • light energy is harnessed to excite and displace an electoron from chlorophyll
   • light energy is converted to chemical energy through a flow of electrons that is coupled to ATP synthesis
   • NADPH is produced
   • photolysis of water - Light is involved in the splitting of water into hydrogen ions and oxygen
3. Light independent reaction
   
   • chemical energy of ATP and NADPH (from the light-dependent stage) is used in the reduction of caarbon dioxide and hence, the manufacure of sugar

Question 12

How are the first 2 stages different from the last?

Answer 12

The first 2 stages require light and occur in the thylakoid membranes of chloroplasts. However, the third stage does not require light and takes place in the stroma of chloroplasts

Question 13

When a molecule of chlorophyll or some other photosynthetic pigment absorbs light, it changes from its ground state to excited state.

What are the 3 ways it tends to return to its original ground state?

Answer 13

1. By transferring the energy - but not the electron- directly to a neighbouring chlorophyll molecule by a process called resonance energy transfer (happens in light harvesting)
2. By boosting an electron to a higher energy level then transferring it to a nearby molecule capable of accepting electrons, the elctron acceptor. The molecule returns to its original state by taking up a low-energy electron from another molecule, an electron donor (ie. electron transfer). This occurs in light-dependent reaction
   
   • In chloroplasts, water serves as a weak electron donor. When water is oxidised this way, oxygen is released along with 2 protons.
3. Finally, energy is lost when excess energy is converted to heat or a combination of heat and light of a longer wavelength. This occurs when light energy is absorbed by an isolated chlorophyll molecule in solution

Question 14

What are photosystems?

Answer 14

In the thylakoid membrane, photosynthetic pigments that trap light energy are arranged into photosystems

These multiprotein complexes convert the captured light energy into useful forms.

Question 15

What are the 3 closely-linked components of photosystems?`

Answer 15

1. Light-harvesting complexes (LHCs)
   
   • light is collected by the 200 to 300 pigment molecules that are bound to them
   • They are important in capturing light. They absorb light energy and transfer the light energy to the reaction center
2. Reaction center
   
   • contains a pair of special chlorophyll a molecules which act as irreversible trap for energy. An excited electron is immediately passed to an adjacent chain of electron acceptors in the same complex
3. A primary electron acceptor
   
   • It is found in the reaction centre and is involved in electron transfer

Question 16

How are 2 functionally and spatially distinct photosystems different and similar?

Answer 16

Photosystem II (PSII) and photosystem I (PSI) differ in the wavelengths that they absorb

• In PSII, the special chlorophyll a is known as P680 as it absorbs light maximally at wavelengths of 680nm
• In PSI, the special chlorophyll a is known as P700 as it absorbs light maximally at wavelength of 700nm

The 2 photosystems P680 and P700 are thus

• identical in their special chlorophyll a molecule
• but differ in their light-absorbing properties becuase of association with different accessory pigments and proteins in the thylakoid membrane, hence affecting the electron distribution

Question 17

What are Electron transport chains (ETC) ?

Answer 17

They are found at the thylakoid membranes, in between the photosystems

Electron carriers play an important role in many redox reactions by transferring electrons from one carrier to another. They can be coenzymes or protein molecules.

• Some electron carriers are arranged at the membrane to form ETC.

Question 18

What happens at the ETC?

Answer 18

At the a ETC, electrons are passed down the carriers by a series of redox reactions. Each carrier molecule receives an electron (reduction), and in turn donates it (oxidation) to the next carrier down the chain.

Thus, an ETC allows the transfer of electrons to be done in several energy-releasing steps instead of one.

• An electron progressively loses energy as it is transferred from one carrier to another
• Some of the energy released is used to make ATP

Question 19

What is the coenzyme electron carriers in chloroplast

Answer 19

nicotinamide adenine dinucleotide phosphate (NADP)

it can pick up a pair of electrons and a proton to be reduced to NADPH.

Question 20

Describe light dependent reaction

Answer 20

Also known as he light reaction

• The reaction occurs in the thylakoid membrane of chloroplasts
• The role of this stage is to synthesise NADPH and ATP using captured light energy from the light-harvesting stage. Chemical energy is trapped in ATP and NADPH

The NADPH and ATP produced are used in the light-independent reactiion to fix carbon dioxide and finally trap energy in glucose

Finally ATP is synthesised through chemiosmosis

Question 21

What are the 2 ways a light dependent reaction can proceed

Answer 21

1. Non-cyclic photophosphorylation
   
   • The 2 photosystems and ETC work cooperatively to build a chemiosmotic gradient for ATP synthesis and reduce NADP to NADPH
2. Cyclic photophosphorylation
   
   • photosystem I can act alone to build the chemiosmotic gradient for ATP synthesis

Question 22

Describe steps 1-3 of non-cyclic photophosphorylation

Answer 22

1. A photon of light strikes a pigment molecule in LHC and energy is relayed via resonance energy transfer until it reaches one of the 2 special chlorophyll a molecules in the PSII reaction centre. It excites one of the electrons in P680 to a higher energy state.
2. This photoexcited electron from P680 is captured by the primary electron acceptor in the reaction centre. Now each P680 is missing an electron.
3. An enzyme splits a water molecule into two electrons, two hydrogen ions and an oxygen atom. This process involves light and is known as photolysis of water. The electrons released are used to replenish the deficit of electrons from the reaction centre of PSII. The oxygen atom immediately combines with another oxygen atom, releasing O2 as a by-product.

Equation: H₂O→2e⁺ + 2H⁺ + 1/2O₂

Question 23

Describe step 4 of non-cyclic photophosphorylation

Answer 23

From the primary electron acceptor, the energised electron passes from PSII to PSI via a first electron transport chain (ETC) consisting of the following electron carrier molecules:

• From plastoquinone (Pq)
• Down a cytochrome (b-f) complex
• Then to plastocyanin (Pc)

Through a series of oxidation-reduction reactions.

These electron carrier molecules are arranged in increasing electron affinity so that transport of electrons down the ETC is unidirectional

Question 24

Describe step 5 of non-cyclic photophosphorylation

Answer 24

As electrons flow from molecule to molecule, it drops to lower energy levels. Free energy released from this exergonic reaction is used to pump protons against concentration gradient from the stroma into the thylakoid space. A proton gradient will be generated across the thylakoid membrane, which is used to drive ATP synthesis.

This synthesis of ATP is called photophosphorylation because it uses light energy to phosphorylate (addition of phosphate) ADP

Question 25

Describe step 6 of non-cyclic photophosphorylation

Answer 25

## Footnote Meanwhile, light energy from another photon of light strikes a pigment molecule in a light- harvesting complex of PSI, exciting an electron of one of the two special chlorophyll a in the PSI reaction centre. The excited electron is then captured by the PSI’s primary electron acceptor, creating an electron deficit in the P700. This electron deficit is replenished by the electron from PSII that reaches the last electron acceptor of the first ETC,

Question 26

Describe step 7 and 8 of non-cyclic photophosphorylation

Answer 26

* The excited electron is passed from PSI's primary electron acceptor down a second ETC through ferredoxin (Fd) * The enzyme NADP reductase transfers electrons from Fd to NADP. Two electrons are required for its reduction to NADPH.

Question 27

describe the steps of cyclic photophosphorylation

Answer 27

1. Light is absorbed by the LHC and passed on to chlorophyll a (P700) in the reaction centre of PSI 2. 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 3. The energised electrons from PSI are passed to ferredoxin (Fd), cycled back to cytochrome (b-f) complex on the first electron transport chain and from there, back to PSI 4. As these electrons are passed along the first electron transport chain, enough energy is released to synthesise ATP from ADP and P_¡.Consequently, ATP is produced * The ATP is needed in the light-independent stage of photosynthesis

Question 28

What is chemiosmosis?

Answer 28

* The process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to frive 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.

Question 29

What is the Z-scheme? (continuation of chemiosmosis) (holy this one is long but idk how to shorten)

Answer 29

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. * The proton gradient drives the synthesis of ATP by ATP synthetase complex * Hydrogen ions diffuse down this gradient from the thylakoid space across the thylakoid membrane into the stroma through the ATP synthetase 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

Question 30

What is the Calvin cycle?

Answer 30

* The light-independent reaction's other name * 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-independent reaction

Question 31

What are the 3 phases of the Calvin cycle of a C3 plant?

Answer 31

1. Carbon dioxide uptake and fixation 2. Reduction of phosphoglyrceric acid (PGA) and 3. Regeneration of carbon dioxide acceptor (RuBP), which ultimately leads to product synthesis and sugar formation

Question 32

Describe carbon dioxide fixation

Answer 32

* Carbon dioxide diffuses through the stomatat and into the cytoplasm of the mesophyll cells and into the chloroplasts * Carbon dioxide is fixed when it combines with a five-carbon carbon dioxide acceptor , ribulose biphosphate (RuBP), to form an unstable six-carbon intermediate. This reaction (aka carboxylation of RuBP) is catalysed by an enzyme, ribulose bisphosphate carboxylase oxygenase (rubisco) * The unstable six-carbon intermediate breaks down spontaneously into 2 molecules of a three-carbon compound called phosphoglyceric acid (PGA) / 3-phophoglycerate/ glycerate-3-phosphate (GP) Equation: RuBP + CO₂ + H₂O → (rubisco) 2PGA

Question 33

describe reduction of PGA

Answer 33

* Each molecule of PGA is phosphorylated by ATP forming 1,3-biphosphoglycerate * A pair of electrons donated from NADPH further reduces 1,3-biphosphoglycerate to form glyceraldehyde-3-phosphate (GALP or G3P) or triose phosphate (TP). The hydrogen for the reduction comes from the NADPH while the energy for this step comes from ATP

Question 34

Describe regeneration of carbon dioxide acceptor (RuBP)

Answer 34

* For every three molecules of carbon diosxide that enters the Calvin cycle, three molecules of RuBP are carboxylated and a total of six molecules of TP are formed * Only one molecule can be counted as a net gain of carbohydrate. The other five molecules of TP must be used to regenerate the 3 molecules of RuBP used in the fixation step 1 * To accomplish this, three molecules of ATP are invested * RuBP is regenerated and the Calvin cycle continues

Question 35

Describe product synthesis and sugar formation

Answer 35

* The TP spun off from the light-independent reaction becomes the starting materal for metabolic pathways that synthesise other organic compounds, including glucose and other carbohydrates * 2 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 come from carbon dioxide while the hydrogen atoms come from NADPH.