2 Photosynthesis and Respiration

Question 1

What are chloroplasts?

Answer 1

Flattened organelles surrounded by a double membrane.

Found in plant cells.

Question 2

What are thylakoids?

Answer 2

Fluid-filled sacs that are stacked up in the chloroplast into structures called grana.
Grana linked together by lamellae.

Question 3

What is the stroma?

Answer 3

A gel-like substance within the inner membrane of the chloroplast and surrounding the thylakoids.
Contains enzymes, sugars and organic substances.

Question 4

What happens to carbohydrate produced by photosynthesis?

Answer 4

They’re not used straight away but are stored as starch grains in the stroma.

Question 5

Explain what a photosystem is.

Answer 5

Chloroplasts contain photosynthetic pigments found in the thylakoid membranes attached to proteins. The protein and the pigment is called a photosystem.
2 photosystem used by plants to capture light energy. PSI absorbs light best at wavelength of 700nm and PSII absorbs light best at 680nm.

Question 6

What are photosynthetic pigments?

Answer 6

Coloured substances that absorb light energy needed for photosynthesis. Eg. Chlorophyll a, chlorophyll b, and carotene

Question 7

Describe the first stage of the light dependent reaction.

Answer 7

• light energy absorbed by PSII
• light energy excites electrons in the chlorophyll
• electrons move to a higher energy level
• high energy electrons released from chlorophyll and move down electron transport chain to PSI
• chlorophyll has been photoionised

Question 8

Describe the second stage of the light dependent reaction.

Answer 8

• released electrons must be replaced
• light energy splits water into protons, electrons and oxygen (photolysis)
• the reaction: H2O = 2H + 1/2O2

Question 9

Describe the third stage of the light dependent reaction.

Answer 9

• excited electrons lose energy as they move down electron transport chain
• this energy used to transport protons into the thylakoid, so thylakoid has higher concentration of protons than stroma. This forms proton gradient across thylakoid membrane.
• protons move down their concentration gradient into stroma, via enzyme ATP synthase, which is embedded in thylakoid membrane. Energy from this movement combines ADP and Pi to form ATP

Question 10

Describe the fourth stage of the light dependent reaction.

Answer 10

• light energy absorbed by PSI, excites electrons to even higher energy level
• electrons transferred to NADP, along with a proton from the stroma to produce NADPH.

Question 11

How is the light independent reaction linked to the light dependent reaction?

Answer 11

Light independent reaction uses NADPH from light dependent reaction to form a simple sugar. Hydrolysis of ATP, also from light dependent reaction provides additional energy for this reaction.

Question 12

Describe the first stage of the light independent reaction.

Answer 12

• CO2 enters leaf through stomata and diffuses into stroma Of chloroplast
• CO2 combined with RuBP, a 5-carbon compound. Reaction catalysed by enzyme rubisco
• this gives an unstable 6-carbon compound, which quickly breaks down into 2 molecules of a 3-carbon compound called GP.

Question 13

Describe the second stage of the light independent reaction.

Answer 13

• hydrolysis of ATP provides energy to turns GP into TP
• reaction also requires H+ ions, which come from NADPH. NADPH reduced to NADP
• some TP converted into useful organic compounds and some continues in Calvin cycle to regenerate RuBP

Question 14

Describe the third stage of the light independent reaction.

Answer 14

• 5/6 molecules of TP produced in cycle aren’t used to make hexose sugars, but to regenerate RuBP
• regenerating RuBP uses rest of ATP produced by light dependent reaction

Question 15

What is oxidation?

Answer 15

When substance loses hydrogen or electrons or gains oxygen

Question 16

What is reduction?

Answer 16

When substance gains electrons or hydrogen or loses oxygen

Question 17

Describe the chemiosmotic theory.

Answer 17

• protons pumped into each thylakoid from stroma using protein carriers called proton pumps
• energy driving this comes from electrons released in photolysis
• photolysis of water produces protons which increases their conc. inside thylakoid
• creates and maintains conc. gradient of protons across thylakoid membrane with high conc. inside and low conc. in stroma
• protons can only cross thylakoid membrane through ATP synthase channel proteins. Channels form small granules on membrane surface known as stalked granules
• as protons pass through ATP synthase channels they cause changes in structure of enzyme which then catalyses combination of ADP + Pi to form ATP

Question 18

How are carbohydrates made as a product of the light independent reaction

Answer 18

Hexose sugars made by joining 2 TP molecules and larger carbohydrates by joining hexose sugars together in different ways.

Question 19

How are lipids made as a product of the light independent reaction

Answer 19

Made using glycerol, which is synthesised from TP, and fatty acids, which are synthesised from GP.

Question 20

How are amino acids made as a product of the light independent reaction

Answer 20

Some amino acids made from GP.

Question 21

Why does the Calvin cycle need to turn 6 times to make one hexose sugar?

Answer 21

• 3 turns produces 6 molecules of TP, because 2 molecules of TP are made for every 1 CO2 molecule used
• 5/6 of TP molecules used to regenerate RuBP
• means for 3 turns of cycle only 1 TP made that’s used to make a hexose sugar
• Hexose sugar has 6 carbons, so 2 TP molecules needed to form 1 hexose sugar
• means cycle must turn 6 times to produce 2 molecules of TP that can be used to make 1 hexose sugar
• 6 turns of cycle need 18 ATP and 12 NADPH from light dependent reaction

Question 22

What are co-enzymes?

Answer 22

Non protein molecules that help enzymes to work by transferring other molecules around.

Question 23

Explain how high light intensity of a certain wavelength provides an optimum condition for photosynthesis.

Answer 23

• light needed to provide energy for the light-dependent reaction, higher intensity of light=more energy provided
• only certain wavelength of light used for photosynthesis. Chlorophyll a, chlorophyll b and carotene only absorb red and blue light. Green light reflected so plant looks green

Question 24

Explain how temperature of about 25°c provides an optimum condition for photosynthesis.

Answer 24

• photosynthesis involves enzymes, if temp falls below 10°c enzymes become inactive, if temp 45°c+ then may start to denature
• at high temps stomata close to avoid losing too much water, causes photosynthesis to slow down because less CO2 enters leaf when stomata closed

Question 25

Explain how carbon dioxide at 0.4% provides an optimum condition for photosynthesis.

Answer 25

• CO2 makes up 0.04% of gases in atmosphere

- increasing this to 0.4% gives higher rate of photosynthesis, but any higher and stomata start to close

Question 26

How do growers use information about limiting factors to increase plant growth?

Answer 26

They create optimum conditions in glasshouses:

• add CO2 to air by burning small amount of propane in CO2 generator
• light can get through glass, lamps provide light at night time
• glasshouses trap heat energy from sunlight, which warms air. Heaters and cooling systems used to keep optimum temp and air circulation systems make sure temp even throughout glasshouse

Question 27

What are the 2 types of respiration?

Answer 27

Aerobic (requires oxygen) and anaerobic (doesn’t require oxygen). Both produce ATP although anaerobic respiration produces less. Glycolysis is the first stage of anaerobic + aerobic respiration. It occurs in cytoplasm and is an anaerobic process

Question 28

Describe the first stage of glycolysis - phosphorylation.

Answer 28

—glucose phosphorylated using a phosphate from a molecule of ATP this creates 1 molecule of glucose phosphate and 1 molecule of ADP

• ATP used to add another phosphate, forming hexose bisphosphate
• hexose bisphosphate is then split into 2 molecules of triose phosphate

Question 29

Describe stage two of glycolysis - oxidation.

Answer 29

• triose phosphate oxidised, forming 2 molecules of pyruvate
• NAD collects hydrogen the hydrogen ions, forming 2 reduced NAD
• 4 ATP are produced, but 2 were used up in stage 1, so there’s a net gain of 2 ATP

Question 30

What happens to pyruvate if respiration is anaerobic?

Answer 30

• Pyruvate produced in glycolysis converged into ethanol or lactate using reduced NAD
• production of ethanol or lactate regenerates oxidised NAD, means glycolysis can continue even when there isn’t much oxygen around, so small amount of ATP can still be produced to keep some biological processes going

Question 31

What happens to pyruvate if respiration is aerobic?

Answer 31

Pyruvate from glycolysis enters the mitochondrial matrix by active transport.

Question 32

Explain the link reaction.

Answer 32

• pyruvate is decarboxylated
• pyruvate is oxidised to form acetate and NAD is reduced to form NADH
• acetate is combined with coenzyme A to form acetyl coenzyme A
• not ATP produced

Question 33

Explain the first stage of the krebs cycle

Answer 33

• Acetyl CoA form link reaction combined with 4-carbon compound to form a 6-carbon compound.
• Coenzyme A goes back to link reaction to be used again

Question 34

Explain the second stage of Krebs cycle.

Answer 34

• 6C molecule converted to 5C molecule
• decarboxylation occurs, where CO2 is removed
• dehydrogenation occurs, where hydrogen is removed
• hydrogen used to produced NADH from NAD

Question 35

Explain the third stage of Krebs cycle

Answer 35

• 5C molecule converted to 4C molecule.
• decarboxylation and dehydrogenation occur, producing 1 molecule of FADH one 2 of NADH
• ATP is produced by direct transfer of phosphate group from an intermediate compound to ADP.

Question 36

What is substrate -level phosphorylation

Answer 36

When phosphate group is directly transferred from 1 molecule to another it’s called substrate -level phosphorylation

Question 37

What is oxidative phosphorylation?

Answer 37

The process where energy is carried by electrons, from reduced coenzymes, is used to make ATP. It makes lots of ATP and involves the electron transport chain and chemiosmosis.

Question 38

Where do the products of the Krebs cycle go?

Answer 38

1 coenzyme A - reused in next link reaction
Oxaloacetate - regenerated for use in next Krebs cycle
2 CO2 - released as a waste product
1ATP - used for energy
3 NADH - goes to oxidative phosphorylation
1 FADH - to oxidative phosphorylation

Question 39

How does oxidative phosphorylation work?

Answer 39

• hydrogen atoms released from NADH and FADH as they’re oxidised to NAD and FAD. H atoms split into protons (H+) and electrons e-
• e- move down etc, losing energy at each carrier
• energy used by electron carriers to pump protons from mitochondrial matrix into inter embrace space
• conc of protons now higher in inter membrane space than in mitochondrial matrix, forms electrochemical gradient
• protons move down electrochemical gradient via ATP synthase. This movement drives synthesis of ATP from ADP and Pi
• process of ATP production driven by movement of H+ ions across membranes called chemiosmosis
• in mitochondrial matrix, at end of etc protons, electrons and O2 combine to form water. Oxygen said to be final electron acceptor.

Question 40

How many ATP made from one glucose molecule in aerobic respiration?

Answer 40

32

Question 41

How can ATP production be affected by mitochondrial diseases?

Answer 41

• mitochondrial diseases affect functioning of mitochondria, they can affect how proteins involved in oxidative phosphorylation or Krebs cycle function, reducing ATP production
• this may cause anaerobic respiration to increase, to try make up some of ATP shortage
• results in lots of lactate being produced, can cause muscle fatigue + weakness
• some lactate also diffuse into bloodstream, leading to high lactate concentrations in blood.