5.5 ENERGY FOR BIOLOGICAL PROCESSES

Question 1

What is the relationship between heterotrophs and autotrophs?

Answer 1

Autotrophs create complex organic molecules which heterotrophs require. All of these organisms use respiration to release energy from organic molecules.

Question 2

What is photosynthesis?

Answer 2

Photosynthesis is a series of chemical reactions in which light energy is transferred into chemical energy.
- light energy is used to produce complex organic molecules
- produces products essential for life
= glucose- used to produce other organic compounds and a substrate for respiration
= oxygen- released into atmosphere so all aerobes depend on it for respiration

Question 3

Where does photosynthesis occur?

Answer 3

Photosynthesis occurs within chloroplasts which contain chlorophyll. Chlorophyll on the thylakoid membranes absorb light energy. Chloroplasts are found in large quantities in the palisade cells of leaves, which are the main site of photosynthesis.

Question 4

What is an autotroph?

Answer 4

Autotrophs are organisms that use energy to synthesis complex organic molecules from inorganic molecules.

Question 5

What is a chemoautotroph?

Answer 5

Chemoautotrophs are organisms that use energy from chemical reactions to synthesise complex organic molecules from inorganic molecules e.g some bacteria.

Question 6

What is a photoautotroph?

Answer 6

Photoautotrophs are organisms that use light energy and small inorganic molecules to synthesise complex organic molecules e.g algae, plants.

Question 7

What is a heterotroph?

Answer 7

Heterotrophs are organisms that digest complex organic molecules and use the small soluble molecules to synthesis their own organic molecules or to gain energy via respiration e.g animals, fungi, bacteria.

Question 8

What are the adaptations of a leaf?

Answer 8

Adaptations of leaf:
- large surface area = maximum light absorbtion
- veins present in leaves = water transport
- waxy later transparent = light straight to palisade mesophyll
- lots of chloroplasts

Question 9

What are the features of chloroplasts?

Answer 9

Features of chloroplasts:
- disc shaped
- double membrane
- thylakoids arranged as stacks called grana
- stroma

Question 10

What are pigments and photosystems?

Answer 10

Pigments and photosystems:
- photosynthetic pigments absorb light (and therefore energy) of certain wavelengths
- each pigment absorbs light of a particular wavelength and reflects others
- various photosynthetic pigments are arranged into photosystems within the granal membrane

Question 11

What is the structure of photosystems?

Answer 11

Photosystem structure:
- photosystems consist of two main regions
1. light harvesting system (antenna complex) - contains accessory pigments which system absorb the energy of the light then the ‘funnel’ it down to the reaction centre
2. primary pigment reaction centre - contains the primary pigment (a fork of chlorophyll a) which triggers the next stage in the process
- two photo systems = photosystem I and photosystem II

Question 12

What are the pigments in photosynthesis?

Answer 12

Pigments:
- chlorophyll a is a primary pigment
- chlorophyll b and the carotenoids are accessory pigments
- chlorophylls = chlorophyll a (yellow-green), chlorophyll b (blue-green)
- carotenoids = b carotene (orange), xanthophyll (yellow)
- different colour pigments absorb different light wavelengths = allows maximum range of wavelengths to be absorbed
- plants are green as pigments reflect green light

Question 13

What does the absorption spectra show?

Answer 13

The absorption spectra shows the absorption of light of different wavelengths for each pigment.

Question 14

What does the action spectrum show?

Answer 14

The action spectrum shows the relativeness effectiveness of different wavelengths of light on photosynthesis.

Question 15

Chlorophyll summary.

Answer 15

Chlorophyll summary:
- two types = a and b
- absorb red and blue light
- chlorophyll a
1. p680 found in photosystem II - peak absorption of light at wavelength 680nm
2. p700 found in photosystem I - peak absorption of light at 700nm
- chlorophyll a also absorbs light at 450nm (appears yellow-green)
- chlorophyll b absorbs light at 480nm and 640nm (appears blue-green)

Question 16

Carotenoids summary.

Answer 16

Carotenoids summary:
- absorb blue light over range of wavelengths (which are not well-absorbed by the chlorophylls)
- appear yellow (xanthophyll) and organe (carotene)
- pass energy onto chlorophyll a at base of photosystem

Question 17

What is photoexcitation?

Answer 17

Photoexcitation is when light energy is absorbed by a molecule and its electrons gain energy and move to higher energy levels in the molecule.

Question 18

What is photoionisation?

Answer 18

Photoionisation is when sufficient light energy is absorbed by a molecule its electrons gain enough energy to be ‘freed’ and leave the molecule.

Question 19

What are light dependent reactions?

Answer 19

Light dependent reactions:
- light energy captured by pigment molecules and used to split water (photolysis), create ATP (photophosphorylation) and produce NADP
- oxygen also produced

Question 20

What are light independent reactions?

Answer 20

Light independent reactions:
- ATP and reduced NADP used to reduce CO2
- CO2 fixed to a 3-C then a 6-C sugar

Question 21

What is light harvesting?

Answer 21

Light harvesting:
- when a pigment absorbs a photon, the energy from photoexcitation is passed from pigment to pigment in a ‘random walk’ to the reaction centre
- energy can only be transferred to a pigment molecule that absorbs light of a longer wavelength (lower energy) as some energy has been used to excite the electron of the previous pigment molecule

Question 22

What are the differences between photosystems?

Answer 22

Differences between photosystems:
- when the energy hits the primary pigment reaction centre (chlorophyll a molecules) it must have the specific wavelength of the reaction centre to excite the electrons
- e.g ps II p680 and ps I p700
- why photosynthesis is so inefficient

Question 23

What happens in the reaction centre during the light dependent stage of photosynthesis?

Answer 23

In the reaction centre:
- an electron of a chlorophyll a molecules becomes excited and is passed on to the primary electron acceptor (chlorophyll a is photoionised)
- the primary electron acceptor is capable of accepting an electron and becoming reduced
- the electrons are then passed down an electron transport chain, generating ATP in a process called chemiosmosis

Question 24

What happens after electrons in the reaction centre have left chlorophyll a?

Answer 24

After electrons in the reaction centre leave chlorophyll a, photolysis and phosphorylation occur to supply ATP and reduced NADP which are needed in the light independent stage.

Question 25

What is photolysis?

Answer 25

Photolysis is the splitting of water into protons, electrons and oxygen using light.
H2O = 1/2O2 + 2H+ + 2e-
- occurs at ps II only where an enzyme catalyses the reaction

Question 26

What happens during photophosphorylation?

Answer 26

Photophosphorylation:
- electron transport chain (ETC) = as electrons pass from one carrier to the next energy is released to pump protons across the membrane from the stroma into the thylakoid via a proton pump.

Question 27

What is chemiosmosis?

Answer 27

Chemiosmosis creates a gradient that protons flow down into the stroma (via an ATP synthase channel).

Question 28

What is photophosphorylation?

Answer 28

Photophosphorylation is where ATP is formed by the ATP synthase due to electrons excited by light.

Question 29

What happens to electrons at the end of the electron transport chain?

Answer 29

What happens to electrons:
- electrons from ps I are accepted by the coenzyme NADP alongside H+
- this generates reduced NADP and removes hydrogen ions from the stroma, helping to maintain a proton gradient

Question 30

What is non-cyclic photophosphorylation?

Answer 30

Non-cyclic photophosphorylation:
- after leaving ps II then ps I electropns are accepted by NADP = this process is linear

Question 31

What is cyclic photophosphorylation?

Answer 31

Cyclic photophosphorylation:
- photoexcited electrons from ps I are occasionally shunted back to the ETC
- this supplements the formation of ATP by no reduced NADP is produced.

Question 32

What happens during photolysis during the light dependent stage of photosynthesis?

Answer 32

Photolysis:
Means ‘splitting with light’ and occurs at ps II where an enzyme breaks water up into 2H+, 2e- and 1/2O2 using light. Electrons and protons are used during non-cyclic photophosphorylation. Oxygen is used in respiration and excess light diffuses out of the leaf.

Question 33

What happens during photophosphorylation during the light dependent stage of photosynthesis?

Answer 33

Photophosphorylation:
When a photon of light hits a chlorophyll molecules, two electrons are excited. They pass to an electrons acceptor and on to a series of electron carriers in the thylakoid membrane. Each carrier is reduced by receiving the electron and the chlorophyll molecule is oxidised. As the electrons are passed along, energy is released and this is used to pump protons into the thylakoid space. A proton gradient now exists across the thylakoid membrane in which an ATP synthase is embedded. As protons flow down the gradient (chemiosmosis) into the stroma, ATP is synthesised (via photophosphorylation).

Question 34

What happens during cyclic photophosphorylation during the light dependent stage of photosynthesis?

Answer 34

Cyclic photophosphorylation:
Occurs in ps I only. Excited electrons are passed back and forth between chlorophyll and an electron transport chain. ATP is generated in small amounts.

Question 35

What happens during non-cyclic photophosphorylation during the light dependent stage of photosynthesis?

Answer 35

Non-cyclic photophosphorylation:
Uses both ps I and ps II. Excited electrons from photoionisation leave the chlorophyll molecule in ps II and pass along the electrons carriers, generating ATP. Electrons from chlorophyll in ps I and H+ ions from photolysis join NADP to form reduced NADP. Lost electrons in ps I are replaced by those lost from ps II. Lost electrons in ps II are replaced by those from photolysis of water. ATP is generated in larger quantities than in cyclic photophosphorylation via chemiosmosis.

Question 36

What happens during the light independent stage of photosynthesis?

Answer 36

Light independent stage:
- CO2 is converted to carbohydrate
= reduced NADP is the reducing power
= ATP is the energy source
- occurs in the stroma of the chloroplast

Question 37

What are the three phases of the light independent reaction?

Answer 37

Phases of light independent reaction:
1. fixation - CO2 fixed with ribulose biphosphate (RUBP) to organic form = carboxylation, rubisco needed. forms unstable 6C molecule that splits into 2x GP
2. reduction - reduction of glycerate 3-phosphate (GP) to triose phosphate (TP). requires breakdown of 2ATP to 2ADP + Pi and 2 reduced NADP to 2NADP
3. regeneration - re-formation of RUBP from TP. requires breakdown of ATP to ADP + Pi

Question 38

What is triose phosphate?

Answer 38

Triose phosphate (TP) is a carbohydrate and the starting point for the synthesis of many complex biological molecules including glucose.

Question 39

How does the Calvin cycle produce one glucose molecule?

Answer 39

To produce one glucose molecule:
- 6 CO2’s have to enter the cycle
- needs to be 12 full turns of the cycle
- results in 12 TP molecules, 2 of which go towards glucose
- so TP molecules are used to regenerate 6 RUBP molecules (30 carbons in both)

Question 40

Why is rubisco considered a bad enzyme?

Answer 40

Rubisco- bad enzyme:
- oxygen is a competitive inhibitor of rubisco
- if temperatures are high and humidity is low, stomata close and oxygen levels initially rise in air spaces as oxygen is a by product of photosynthesis and it cannot diffuse out into the leaf
- process called photorespiration occurs instead of photosynthesis

Question 41

What happens during photorespiration?

Answer 41

During photorespiration oxygen combines with rubisco instead of CO2. This results in the formation of a 2C compound called phosphoglycolate. Phosphoglycate is toxic and must be removed as it uses up ATP and although some carbon can be salvaged, some is lost as CO2. Overall, no sugar is made and ATP is used up.

Question 42

What are C3 plants?

Answer 42

C3 plants are plants that live in cool, wet climates with average sunshine that carry out C3 photosynthesis (normal plants).

Question 43

What are CAM and C4 plants?

Answer 43

CAM and C4 plants are plants that live in hot and arid environments and are unable to use water efficiently and achieve photosynthesis at high light intensity and high temperatures.

Question 44

C4 plants.

Answer 44

C4 plants (e.g corn):
- adapted to high temperatures and low water
- fix carbon with minimum time spent with stomata open = reduces transpiration
- PEP carboxylase in mesophyll cells fixes CO2 to produce a 4C molecule. this is transported deeper inside the plant where the molecules are decarboxylated (carbon removed) and a 3C molecule can enter the Calvin cycle
- less photorespiration occurs as rubisco is protected from atmospheric oxygen. PEP carboxylase is not inhibited by oxygen

Question 45

CAM plants.

Answer 45

CAM plants (e.g cacti):
- open stomata at night and close them in the day to reduce transpiration
- CO2 is converted into an organic acid an stored up at night
- during the day CO2 is released to enter the Calvin cycle
- in very dry spells, stomata remain closed at night and day and the oxygen from photosynthesis is used in respiration and CO2 is used in photosynthesis.

Question 46

What are the limiting factors in photosynthesis?

Answer 46

The limiting factor is the factor that is in the shortest supply that will limit the rate of photosynthesis. Limiting factors in photosynthesis are light intensity, CO2 concentration and temperature. Water is not considered a limiting factor as if it were low enough to limit photosynthesis the stomata would have already closed and photosynthesis would have already stopped.

Question 47

What happens to photosynthesis when light intensity is high?

Answer 47

If light intensity is high electrons become more excited due to more light energy. There is more photosynthesis so ATP and reduced NADP are produced to go into the Calvin cycle. The hydrogens from these products are used to reduce NADP to reduced NADP. Levels of RUBP, TP and GP remain constant.

Question 48

What happens to photosynthesis when light intensity is low/limited?

Answer 48

If light intensity is low/limited the light dependent stage is affected and may cease to happen. TP and RUBP will no longer be made as there is no ATP or reduced NADP being processed, meaning regeneration of RUBP will not happen. Levels of RUBP and TP will decrease, however levels of GP will increase as there is no ATP and reduced ATP to turn it into TP, but will still be made from RUBP.

Question 49

What do high temperatures cause to happen during photosynthesis?

Answer 49

High temperatures cause:
- rate of enzyme activity increases
- rate of enzyme controlled reactions increase
- rate of photorespiration increases above 25 degrees C

Question 50

What happens to photosynthesis if temperatures are high?

Answer 50

If temperature is high this will have no effect on the light dependent reaction as it requires no enzymes. Above 25 degrees C, the enzyme rubisco (which is required in the light independent reaction to convert RUBP to GP), will start to denature and the rate of reaction will start to slow. Overall, the rate of photorespiration will increase so the rubisco that has not been denatured is inhibited so the rate of photosynthesis decreases.

Question 51

What happens to photosynthesis if CO2 concentration is high?

Answer 51

If CO2 concentration is high more CO2 is introduced to the Calvin cycle. This means more RUBP will be converted to GP and more GP will be converted to TP. This means more amino acids and carbohydrates will be produced and the regeneration of RUBP.

Question 52

What happens to photosynthesis if CO2 concentration is low/limited?

Answer 52

If CO2 concentration is low/limited the conversion of RUBP to GP will not happen. Overtime, levels of RUBP increase as it is not being used to cofix CO2 but is being regenerated from TP, TP production will decrease as there is less GP, and GP levels will decrease as carbon fixation is not taking place.

Question 53

What is the effect of water stress on photosynthesis?

Answer 53

During water stress, the stomata will close to avoid water loss. This decreases the rate of photosynthesis as no gas exchange can occur and oxygen inhibits the light independent reaction.