Photosynthesis

Question 1

What are the two types of chlorophyll?

Answer 1

Chlorophyll a (blue-green)
Chlorophyll b (yellow-green)

Question 2

What is the significance of the porphyrin ring that is attached to the chlorophyll molecule?

Answer 2

Porphyrin’s structure has alternating single and double bonds that contain delocalized electrons which absorb light energy and begin the photosynthetic process.

Question 3

Difference between chlorophyll a and b

Answer 3

Chlorophyll a and b have different groups at position -R:

• chlorophyll a has a methyl group ( - CH3)
• chlorophyll b has an aldehyde group ( - COH)

Question 4

Which type of chlorophyll is the primary light absorbing pigment for many photosynthetic organisms?

Answer 4

chlorophyll a

Question 5

Describe the chemical structure of chlorophyll.

Answer 5

Chlorophyll is made up of a porphyrin ring and a hydrocarbon tail. The hydrocarbon tail (aka phyto tail or phyto chain) anchors the molecule in a membrane due to an association between the hydrophobic end (hydrocarbon tail) and the phospholipid layer of the membrane.

Question 6

Where did chlorophyll get its name from?

Answer 6

The root word, chloros, means yellow-green

Question 7

What is the largest group of photosynthesizing prokaryotes?

Answer 7

cyanobacteria (blue-green algae)

Question 8

What is the effect of cyanobacterial blooms?

Answer 8

Cyanobacterial blooms can be identified in nutrient-rich water and can be toxic to mammals, humans, fish etc. The cyanobacterium, microcystis aeruginosa, in dense blooms can be a hazard to the environment but it is not poisonous. This organism produces a toxin called microcystin.

Question 9

What organism paved the way for heterotrophic life on Earth?

Answer 9

Unicellular prokaryotic organisms called cyanobacteria are the first group of cells to produce oxygen on a large scale and were estimated to be the first organisms on earth to effectively use carbon dioxide and water to carry out the photosynthetic process. Heterotrophs rely on this mass production of oxygen and the resulting food molecules for life.

Question 10

How were plant cells created?

Answer 10

Plant cells were formed by an endo-symbiotic relationship. It is reasoned that cyanobacteria (a prokaryotic cell) formed an endosymbiotic relationship with a eukaryotic organism where the cyanobacteria were protected from harsh conditions and the eukaryotic organism benefited from the food molecules that were produced from the photosynthetic bacterium inside its cell. Hence, this relationship evolved to create plant cells.

Question 11

Compare and differentiate between chloroplasts and cyanobacteria.

Answer 11

Chloroplasts and cyanobacteria both contain chlorophyll a and perform photosynthesis.

Cyanobacteria contain chlorophyll d which also carries out photosynthesis. It has a prokaryotic cell structure or make up that has no mitochondria or nuclei, unlike chloroplasts.

Question 12

What is the primary photosynthetic organ of most plants?

Answer 12

Leaf

Question 13

What is the overall chemical equation for photosynthesis?

Answer 13

6H20 + 6CO2 + light energy ——-> C6H12O6 + O2

Question 14

What is the generical form of the chemical equation for photosynthesis?

Answer 14

CO2 + H2O + light energy ———> [CH20] + O2

Question 15

Describe how the plant structures help to accomplish photosynthesis efficiently.

Answer 15

The leaves are thin to minimize the distance that the gases have to travel to reach the chloroplast, and broad to maximize the amount of sunlight that is absorbed. Inside the leaf are many layers such as the epidermis layers, the palisade mesophyll layer and guard cells.

The epidermis layers are right below the surface of the leaf and they are very transparent to allow all of light energy to pass through to the chloroplasts in the palisade mesophyll layer. The palisade mesophyll layer is where most of the photosynthesis occurs.

The shaded region of the leaf (underneath) is where the guard cells are located. The guard cells regulate the opening and closing of stomata due to environmental conditions such as daytime or nighttime weather and temperature. The stomata helps with regulating the entering and exiting of carbon dioxide and oxygen gases as well as water vapour during transpiration.

Question 16

What are vascular bundles?

Answer 16

Vascular bundles are veins in the leaf that transport water and minerals from the roots to the leaves and carbohydrates (starch) from the leaves to the roots.

Question 17

Although stomata cover only 1% to 2% of a leaf’s epidermal surface area, they may be responsible for more than 85% of the water lost by a plant. Explain why this is true?

Answer 17

Stomata are the pores or openings of the leaf; they are the one of the main channels through which substances such as water, enter and escape the surface of the leaf. The opening and closing of stomata is dependent on the movement of water caused by potassium ions. Therefore, most of the water lost by a plant is due to stomata.

Question 18

How does transpiration help with the photosynthetic process?

Answer 18

1) Transpiration creates a “transpiration pull” that helps with moving water and minerals from the roots to the leaves for photosynthesis.
2) The process of converting liquid to water vapour as it exits the stomata provides a cooling effect on the leaves which prevents them from rising to temperatures outside of their norm causing protein denaturation in the enzymes that catalyze the reactions of photosynthesis.

Question 19

What are the general conditions that promote transpiration?

Answer 19

• warm, dry and windy weather

- sunny weather

Question 20

Explain the process of opening and closing the stomata.

Answer 20

Opening: when K+ ions move into the guard cells, water follows by osmosis and the guard cells swell (turgid) causing the stomata to open.

Closing: when K+ ions move out of the guard cells, water follows by osmosis and the guard cells become limp (flaccid) causing the stomata to close.

Question 21

Why is the size of the stomata dependent on the availability of ATP?

Answer 21

The diffusion of K+ ions across the guard cell membranes causes an active transportation of H+ ions through correlated membrane proton pumps.

Question 22

Why do stomata generally open in the day and close at night?

Answer 22

Stomata open in the day because the blue light from the Sun activates the blue-light receptors in the guard cells membranes which stimulates the pumping of protons out of cells. The subsequent electrochemical gradient causes potassium ions to diffuse into the guard cells and water follows by osmosis. The guard cells become flaccid and the stomata open. The mesophyll cells begin to use the carbon dioxide that was stored in the air spaces overnight.

A reduction in the concentration of sucrose causes the stomata to close at night.

Question 23

Distinguish between the structure of a chloroplast and

the structure of a mitochondrion.

Answer 23

Chloroplasts are larger and more complex than a mitochondrion and is made up of pigments: chlorophyll, carotenoids and photosynthetic pigments. It is disc-shaped while mitochondria are bean-shaped.

The two chambers in the chloroplasts are: stroma and thylakoid
The two chambers in the mitochondrion are: matrix and cristae

Question 24

What are the three (3) stages of photosynthesis?

Answer 24

1. Capturing light energy
2. Using captured light energy to make ATP and reduce NADP to NADPH
3. Using the free energy of ATP and the reducing power of NADPH to make organic compounds like glucose from CO2

Question 25

Why are the first 2 stages of photosynthesis called ‘light reactions’?

Answer 25

They are reactions that get energy directly from light which require chlorophyll and occurs on the thylakoid membranes in the chloroplasts.

Question 26

What is carbon fixation?

Answer 26

A endergonic process that incorporates the carbon atom of CO2 into organic compounds for ex glucose using the free energy of ATP and the reducing power of NADPH.

Question 27

Where does the third stage of photosynthesis take place?

Answer 27

Carbon fixation takes place in the stroma by a cyclic sequence of enzyme-catalyzed reactions called the Calvin cycle or photosynthetic carbon reduction cycle.

Question 28

The reactions that take place in the Calvin Cycle used to be called ‘dark reactions’. Why is this not a suitable phrase for describing the Calvin cycle?

Answer 28

The Calvin cycle is activated by using light energy. Its reactions slow down or cease in the dark because the enzymes that catalyze these reactions were observed to be either inactive or operating at low activity in the dark.
The Calvin cycle also requires ATP and NADPH which are formed by light reactions.

Question 29

How are the wavelength and energy of a photon related?

Answer 29

The wavelength is inversely proportional to the energy of a photon. The shorter the wavelength the more energy a photon has and the longer the wavelength the less energy a photon possesses.

Question 30

Which light has more energy: red light or green light?

Answer 30

Red light has less energy than green light because it has a longer wavelength like that of radio waves. Therefore, green light has a higher energy value than red light.

Question 31

Explain the spectroscope.

Answer 31

The spectroscope separates the photons by their wavelengths or energies and forms the electromagnetic spectrum (spectrum of light).

Question 32

What are photosystems?

Answer 32

Photosystems are clusters of photosynthetic pigments in the thylakoid membranes that absorb photons of particular wavelengths and use their energies to synthesize ATP from ADP and Pi, and reduce NADP forming NADPH.

Question 33

Where does the supply of electrons (H atoms) come from to reduce NADP+ to NADPH?

Answer 33

It comes from water molecules that enter the stroma and move into the thylakoid membrane.

Question 34

Describe Van Helmont’s experiment and state the controls in his experiment

Answer 34

nnn

Question 35

What are bracts?What colour changes do bracts of poinsettia (Euphorbia pulcherrima) plants undergo?
At what time of year does this change normally occur?

Answer 35

mmm

Question 36

What is fluorescence?

Answer 36

When a electron that has been excited by a photon, releases this energy in the form of light as it returns to a ground state

Question 37

Why don’t most electrons fluoresce?

Answer 37

The excited electron is captured by a primary electron acceptor in the photosynthetic membrane and is used for redox reactions - the chlorophyll is oxidized and the primary electron acceptor is reduced.

Question 38

Name and describe the three (3) stages of light reactions.

Answer 38

1. Photoexcitation: the energy of a photon that strikes the photosynthetic membrane is absorbed by an electron of chlorophyll.
2. Electron transport: the excited electron is transferred through membrane-bound electron carriers, to pump protons to through the photosynthetic membrane creating an H+ reservoir that is used to reduce an electron acceptor.
3. Chemiosmosis: the H+ reservoir is used to fuel ATPase complexes and synthesize ATP from ADP and Pi (inorganic phosphate).

Question 39

How do photosystems act like primary light-harvesting units of chloroplasts?

Answer 39

Light is absorbed by tiny clusters of proteins called photosystems. Photosystems are made up of antennae complexes and a reaction center.

The antennae complex is composed of several chlorophyll molecules and accessory pigments which are set in a protein matrix located in the thylakoid membrane.

It absorbs a photon of energy and transfers it from pigment to pigment until it reaches the chlorophyll a molecule. The chlorophyll a molecule absorbs this energy and its electrons are excited to a higher energy level. An electron of chlorophyll a is transferred to a primary electron acceptor by redox reactions and the chlorophyll a molecule is oxidized.

Question 40

Why might some chlorophyll molecules fluoresce?

Answer 40

Chlorophyll molecules that have been isolated from the photosynthetic membrane might fluoresce because there are no primary electron acceptors present to receive their photoexcited electron.

Question 41

What is the difference between photosystem I and photosystem II?

Answer 41

Photosystem I contains the chlorophyll a molecule that is called p700 because it is best at absorbing or its absorption spectrum peaks at a wavelength of 700 nm (red light).
Photosystem II contains the chlorophyll a molecule called p680 because it absorbs photons with a wavelength of 680 nm (red light) best. Photosynthesis begins here, when a photon excites an electron of chlorophyll p680.

Question 42

Why are the absorption spectra of chlorophyll p700 and p680 peaked at different wavelengths?

Answer 42

Because of the different proteins that they are associated with in their reaction centers.

Question 43

What do plants used photosystems I and II?

Answer 43

Plants uses these two types of photosystems to produce ATP and NADPH in the non cyclic electron flow

Question 44

Explain the non cyclic electron flow and chemiosmosis.

Answer 44

1. Photosystem II is struck by a photon which excites an electron of chlorophyll p680 in its reaction center. The excited electron is captured to a primary electron acceptor called pheophytin which transfers the electron to an electron carrier, plastoquinone (PQ), through redox reactions. This process occurs twice, to transfer 2 electrons to the electron transport chain.
2. A ‘Z’ protein that is associated with photosystem II, splits water molecules into hydrogen ions, oxygen and electrons:
   Two (2) of these electrons are used to replace the ones that were lost by chlorophyll p680.
   The oxygen atom bonds to another oxygen atom to form molecular oxygen and leaves the chloroplast as oxygen gas.
   The resulting hydrogen ions increase the concentration of hydrogen ions or electrochemical gradient in the thylakoid space or lumen therefore increasing the production of ATP.
3. The two electrons from photosystem II pass through the Q cycle which transports or pumps protons from the stroma into the thylakoid space or lumen because protons cannot pass through the phospholipid bilayer of the thylakoid membrane. This creates an electrochemical gradient for chemiosmosis.

Four protons are transferred to the thylakoid lumen for each pair of electrons.

The electrons move through the plastocyanin (Pc) and other components of the electron transport chain to photosystem I, replacing the electrons that were lost by photosystem I when it was struck by photons.

4. The electrons from photosystem I passes through another electron transport chain that contains an iron-containing protein, ferredoxin (Fd). They are transferred to the enzyme NADP reductase for the reduction of a mobile electron carrier, NADP+ to NADPH. This process uses the 2 electrons and H+ ions from the stroma.
5. The protons in the thylakoid membrane create an electrochemical gradient that fuels the phosphorylation of from ADP to ATP, which is catalyzed by ATP synthase. Every ATP molecule is synthesized using four (4) H+ ions.

Question 45

Why is it called a non cyclic electron flow?

Answer 45

Some of electrons that are lost by the reaction centers of the photosystems are eventually used to reduce NADP+ to NADPH, which does not re-enter the cycle or return to the photosystems.

Question 46

What happens in the cyclic electron flow?

Answer 46

In the cyclic electron flow, excited electrons from the photosystem I are passed to Fd (ferredoxin), goes through the Q cycle and back to chlorophyll p700 (its reaction center). This cyclic flow generates an electrochemical gradient for ATP synthesis and no electrons are released to make NADPH.