Biology A2 Chapter 11 - Photosynthesis

Question 1

What are the cellular organelles within the leaf where photosynthesis occurs?

Answer 1

Chloroplasts

Question 2

Name the 9 adaptations of the leaf that aid photosynthesis, and how the adaptation helps them do this

Answer 2

1. Large SA - absorbs sunlight
2. Leaves arranged to minimise overlapping - avoids shadowing of one lead by another
3. Thin - most light is absorbed in the first few micrometres, short diffusion distance
4. Transparent cuticle and epidermis - let light through to the photosynthetic mesophyll cells beneath
5. Long & narrow upper mesophyll cells packed with chloroplasts - collect sunlight
6. Numerous stomata - gaseous exchange so all mesophyll cells are only a short diffusion pathway from one
7. Stomata that open and close - in response to changes in light intensity (save water)
8. Many air spaces in lower mesophyll layer - allow rapid diffusion in the gas phase of carbon dioxide and oxygen
9. Network of xylem and phloem - xylem brings water top leaf cells, and phloem that carries away sugars produced during photosynthesis

Question 3

What is the overall equation for photosynthesis?

Answer 3

6CO2 + 6H2O –> C6H12O6 + 6O2 (carbon dioxide + water –> glucose + oxygen)

Question 4

What are the three main stages of photosynthesis and what do they entail?

Answer 4

1. Capturing of light energy - by chloroplast pigments such as chlorophyll
2. Light dependent reaction - some of the light energy absorbed is conserved in chemical bonds. During the process an electron flow is created by the effect of light on chlorophyll, causing water to split into protons, electrons and oxygen. Produces reduced NAPD, ATP and oxygen
   3.Light independent reaction - these protons (hydrogen ions) are used to produce sugars and other organic molecules

Question 5

What are the 2 distinct regions in chloroplasts, and what happens in them?

Answer 5

1. Grana - stacks of discs called thylakoids. Where the light dependent stage occurs.
2. Stroma - fluid filled matrix where the light independent stage takes place

Question 6

What are in the thylakoids?

Answer 6

Photosynthetic pigment called chlorophyll

Question 7

What are inter-granal lamellae?

Answer 7

Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana

Question 8

What is photolysis?

Answer 8

Splitting of water into hydrogen ions (protons), oxygen and electrons

Question 9

In the light dependent reaction (LDR), what is the energy generated from the light captured used for?

Answer 9

• to add an inorganic phosphate molecule to ADP, thereby making ATP
• photolysis

Question 10

What is oxidation?

Answer 10

• gain of oxygen
• loss of hydrogen
• loss of electrons
• results in energy being given out

Question 11

What is reduction?

Answer 11

• loss of oxygen
• gain of hydrogen
• gain of electrons
• results in energy being taken in

Question 12

What is photoionisation?

Answer 12

Occurs in the LDR - the chlorophyll molecule absorbs the light energy and boosts the energy of a pair of electrons which are now in an excited state. These leave the chlorophyll molecule and it becomes ionised, which is known as photoionisation

Question 13

In the LDR, where do the electrons go once they have left the chlorophyll?

Answer 13

To an electron carrier

Question 14

What happens to the electrons at the electron carrier?

Answer 14

Passed along a number of electron carriers in a series of oxidation-reduction reactions. These form a transfer chain in the membranes of the thylakoids. Each carrier is at a lower energy than the previous, so electrons lose energy at each stage. Some of this energy is used to combine an inorganic phosphate with an ADP to form ATP

Question 15

What is the chemiosmotic theory?

Answer 15

The mechanism that explains how ATP is produced

Question 16

Describe the steps of the chemiosmotic theory

Answer 16

1. Each thylakoid is an enclosed chamber to which protons are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps
2. The energy to drive this process comes from electrons released by photolysis of water
3. The photolysis of water also produces protons which further increases their concentration inside the thylakoid space
4. Overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in the stroma
5. The protons can only cross the thylakoid membrane through ATP synthase channel proteins - the rest of the membrane is impermeable to protons. These channels form small granules on the membrane surface and are so known as stalked granules
6. As the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme which then catalyses the combination of ADP with an inorganic phosphate to form ATP

Question 17

Give the equation for the photolysis of water

Answer 17

2 water –> 4 protons + 4 electrons + 1 oxygen

Question 18

In the LDR, what happens to the protons after they have passed out of the thylakoid space through ATP synthase channels?

Answer 18

Taken up by an electron carrier called NADP, causing it to become reduced. This then enters the light independent reaction (LIR) alongside electrons from the chlorophyll molecules

Question 19

Why is reduced NADP important?

Answer 19

It is a further potential source of chemical energy to the plant

Question 20

What is the oxygen from the photolysis of water used for?

Answer 20

Respiration or diffuses out of the leaf as a waste product of photosynthesis

Question 21

Where does the LDR occur?

Answer 21

In the thylakoids of chloroplasts

Question 22

How are chloroplasts structurally adapted to their function of capturing sunlight and carrying out the LDR?

Answer 22

• thylakoid membranes provide a large SA for attachment of chlorophyll, electron carriers and enzymes that carry out the LDR
• A network of proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorption of light
• The granal membranes have ATP synthase channels within them which catalyse the production of ATP. They are selectively permeable which allows establishment of a proton gradient
• Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the LDR

Question 23

Where does the light independent reaction occur?

Answer 23

In the stroma of the chloroplasts

Question 24

Why does the LIR cease when light is absent?

Answer 24

It requires the products of the LDR and the LDR cannot produce ATP and NADPH when light is absent, so the LIR ceases to occur once the products have been completely used up

Question 25

Describe the 7 stages of the Calvin cycle

Answer 25

1. Carbon dioxide from the atmosphere diffuses into the leaf through stomata and dissolves in water around the walls of the mesophyll cells. It then diffuses through the cell surface membrane, cytoplasm, and chloroplast membranes into the stroma of the chloroplasts
2. In the stroma, the carbon dioxide reacts with the 5 carbon compound (ribulose bisphosphate: RuBP), a reaction catalysed by the enzyme rubisco
3. The reaction between carbon dioxide and RuBP produces 2 molecules of the 3-carbon glycerate 2-phosphate (GP).
4. Reduced NADP from the LDR is used to reduce GP to triose phosphate (TP) using energy supplied by ATP
5. The NADP is reformed and goes back to the LDR to be reduced again by accepting more protons
6. Some TP molecules are converted to organic substances that the plant requires, such as starch, cellulose, lipids, glucose, amino acids, nucleotides…
7. Most TP molecules are used to regenerate RuBP using ATP from the LDR

Question 26

How is the chloroplast adapted to carry out the LIR?

Answer 26

• The fluid filled stroma contains all the enzymes needed to carry out the LIR
• The stroma fluid surrounds the grana and so the products of the LDR in the grana can readily diffuse into the stroma
• It contains both DNA and ribosomes so can quickly and easily manufacture some of the proteins involved in the LIR