3.5 Energy Transfers in and between Organisms

Question 1

Outline the process of photosynthesis

Answer 1

• Light is absorbed by chlorophyll and this is linked to the production of ATP. ATP production occurs when protons diffuse down an electrochemical gradient through molecules of the enzyme ATP synthase, embedded in the membranes of cellular organelles

• Photosynthesis is common to all photoautotrophic organisms and occurs in two stages; the light-dependent reaction and the light-independent reaction

• Energy from ATP and hydrogen from reduced NADP (NADPH) are passed from the light-dependent to the light-independent stage

• The energy and hydrogen are used in the light-independent reactions (the Calvin Cycle) to produce complex organic molecules

Question 2

Identify where the light-dependent reaction of photosynthesis takes place

Answer 2

Thylakoid membrane and thylakoid spaces in the chloroplast

Question 3

Outline the structure and function of a chloroplast

Answer 3

• STRUCTURE:
• Vesicular plastid surrounded by a double membrane envelope, each a phospholipid bilayer
• Filled with a fluid-filled matrix called the stroma
• Contains a series of flattened, fluid-filled sacs called thylakoids (containing photosystems with chlorophyll) that stack to form grana that are connected by membranous channels called stroma lamellae
• Contains small (70s) ribosomes, a loop of DNA and starch grains

• Function: Site of photosynthesis to convert solar energy to chemical energy

Question 4

Identify the function of the stroma in the chloroplast

Answer 4

Contains a separate system of membranes that contains the pigments, enzymes and electron carriers for light dependent reactions

Question 5

Identify the function of the stroma lamellae in chloroplasts

Answer 5

To ensure the stacks of thylakoids (grana) are connected but distanced

Question 6

Identify the function of grana in the chloroplast

Answer 6

• Membranes have a large surface area to increase the number of light dependant reactions that can occur

• Contains a large number of pigment molecules arranged in light-harvesting clusters called photosystems that ensures as much light as necessary is absorbed

Question 7

Identify the function of the loop of DNA in chloroplasts

Answer 7

Codes for some chloroplast ribosomes that aren’t coded for by the plant cell nucleus

Question 8

Identify the function of the starch grains in the chloroplast

Answer 8

Sugars formed during photosynthesis are stored as starch inside the starch grains

Question 9

Identify the four processes make up the light-dependent reaction of photosynthesis

Answer 9

• Photoionisation of chlorophyll
• Chemiosmosis
• Photolysis
• Production of ATP and reduced NADP (NADPH)

Question 10

Describe the process of the light-dependent reaction of photosynthesis

Answer 10

• Chlorophyll absorbs light, leading to photoionisation of chlorophyll. The light excites electrons in the chlorophyll, which becomes positively charged as the electrons are lost

• Electrons are passed down the electron transfer chain (ETC) in a series of redox reactions, gradually releasing energy as they go.

• The production of ATP is explained by the chemiosmotic theory and involves this energy, which is used to actively transport protons (H+ ions) across the thylakoid membrane to the thylakoid lumen through a proton pump

• This creates a proton concentration gradient which causes protons to return to the stroma down this gradient through transmembrane ATP synthase enzymes providing energy for ATP synthesis

• Photolysis is the breakdown of water by light in the lumen, producing protons, electrons and oxygen.

• The electrons produced replace exited electrons on the ETC and combine with the hydrogen ions and are transported out of the thylakoid lumen by ATP synthase. They also combine with carrier molecule NADP to produce NADPH

• ATP and reduced NADP (NADPH) are produced and transferred to the light-independent reactions and oxygens produced as a waste product

Question 11

Identify the equation for photolysis in the light-dependent reaction of photosynthesis

Answer 11

H20 -> 2H+ + 2e- + 1/2 O2

Question 12

Identify the equation for the production of reduced NADP in the light-dependent reaction of photosynthesis

Answer 12

2H+ + 2e- + NADP -> NADPH

Question 13

Identify the equation for photosynthesis

Answer 13

6CO2 + 6H2O -> C6H12O6 + 6O2

Question 14

Identify where the light-independent reaction of photosynthesis takes place

Answer 14

Stroma of the chloroplast

Question 15

Identify the function of the light-independent reaction

Answer 15

• Uses reduced NADP from the light-dependent reaction to form a simple sugar.

• Hydrolyses ATP from the light-dependent reaction to provide additional energy

Question 16

Describe the process of the light-independent reactions (the Calvin Cycle) of photosynthesis

Answer 16

• Carbon dioxide is fixated and reacts with ribulose biphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reactions catalysed by the enzyme rubisco

• ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate

• 5/6 of the time, triose phosphate is used to regenerate RuBP in the Calvin cycle
• 1/6 of the time triose phosphate is converted to useful organic substances

(5+1, 2x3, 2x3, 5)

Question 17

Outline limiting factors in photosynthesis

Answer 17

Photosynthesis can be considered as a number of metabolic pathways. The overall rate depends upon the slowest reaction in the pathway (the limiting factor), this can be:
• Light intensity
• Temperature
• Carbon dioxide concentration

Question 18

Describe how light intensity limits the rate of photosynthesis

Answer 18

• More light leads to more photoionisation so there’s more H+ ions, through chemiosmotic theory, this produces more ATP and reduced NADP for the Calvin cycle (light-independent reactions) which can then occur at a greater rate

• As light intensity increases, more photosynthesis can occur until another factor becomes limiting

Question 19

Describe how temperature limits the rate of photosynthesis

Answer 19

• An optimum temperature will increase the rate of light-dependent reactions and therefore the rate of photosynthesis as enzymes are in their optimal working window, deviation from which can cause them to denature

• Increased temperature causes stomata on a leaf to close in order to reduce water loss, therefore carbon dioxide can’t enter the leaves and therefore can be fixated in the Calvin cycle

• Lower temperature means less kinetic energy and therefore less enzyme-substrate complexes are formed.
- Less rubisco means less GP is formed, so less TP
is formed, so less RuBP and organic molecules
are formed
- Less ATP synthase means less ATP is formed

Question 20

Describe how Carbon dioxide concentration limits the rate of photosynthesis

Answer 20

• The rate of photosynthesis increases as carbon dioxide concentration increases until something else becomes the limiting factor

• It’s required for the light-independent reaction, during carbon fixation so the more carbon dioxide present, the faster this step of the Calvin cycle can occur and the faster overall rate of photosynthesis

Question 21

What organic molecules are produced from the Calvin cycle

Answer 21

• Hexose phosphates (6C) which can be used to produce starch, sucrose or cellulose

• Lipids for cell membranes

• Amino acids for protein synthesis

Question 22

Outline the role of starch, sucrose and cellulose in plants

Answer 22

• Starch; storage of sugars
• Sucrose; translocation around the plant
• Cellulose; making cell walls

Question 23

Define oxidation

Answer 23

The loss of electrons, gain of oxygen or loss of hydrogen in a substance

Question 24

Define reduction

Answer 24

The gain of electrons, loss of oxygen or gain or hydrogen in a substance

Question 25

Outline the process of respiration

Answer 25

• Life depends on continuous transfers of energy. Cellular respiration (common to all organisms) produces ATP and is the process by which the energy contained is made available for all of the active processes within a cell

• In respiration, various substances are used as respiratory substrates. The hydrolysis of these respiratory substrates is linked to the production of ATP

• Anaerobic and aerobic respiration both involve glycolysis, however in aerobic respiration there are three more stages:
1. Glycolysis
2. Link reaction
3. Krebs cycle
4. Oxidative phosphorylation/ETC

Question 26

Describe the process of glycolysis in anaerobic respiration

Answer 26

• Glycolysis is the first stage of aerobic and anaerobic respiration and is an anaerobic process that takes place in the cytoplasm.
• It breaks down two molecules of glucose to pyruvate to produce 2 molecules of ATP and 2 molecules of NADH

• It involves the phosphorylation of glucose to glucose phosphate, using 2 ATP molecules.

• Triose phosphate is produced and then oxidised to pyruvate, which produces 2 molecules of NADH and 4 molecules of ATP, meaning a net gain of 2 ATP molecules.

• If respiration is only anaerobic, oxygen is unavailable so the Kerbs cycle and ETC cannot operate. There’s no way of disposing hydrogen because oxygen isn’t available to act as the final electron acceptor.

• Pyruvate is then converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis

Question 27

Describe the process of glycolysis in aerobic respiration

Answer 27

• Glycolysis is the first stage of aerobic and anaerobic respiration and is an anaerobic process that takes place in the cytoplasm.
• It breaks down two molecules of glucose to pyruvate to produce 2 molecules of ATP and 2 molecules of NADH

• It involves the phosphorylation of glucose to glucose phosphate, using 2 ATP molecules.

• Triose phosphate is produced and then oxidised to pyruvate, which produces 2 molecules of NADH and 4 molecules of ATP, meaning a net gain of 2 ATP molecules.

• If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport.

Question 28

Describe the process of the Link reaction in respiration

Answer 28

• The second stage of aerobic respiration that takes place in the mitochondrial matrix and converts pyruvate into acetyl coenzyme A and carbon dioxide. NADH is also formed.

• If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport. Pyruvate is oxidised to acetate, producing carbon dioxide (via decarboxylation) and NADH in the process.

• Acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A.

• Coenzyme A reacts with a four-carbon molecule (oxaloacetate). This releases coenzyme A to be recycled back into the link reaction and produces a six-carbon molecule (citrate) that enters the Krebs cycle

Question 29

Describe the process of the Krebs cycle in respiration

Answer 29

• Coenzyme A reacts with a four-carbon molecule (oxaloacetate). This releases coenzyme A to be recycled back into the link reaction and produces a six-carbon molecule (citrate) that enters the Krebs cycle

• In a series of oxidation-reduction (redox) reactions catalysed by dehydrogenase enzymes in association with coenzymes, NADH and FADH are produced. The Krebs cycle generates reduced coenzymes and ATP by substrate level phosphorylation and carbon dioxide is lost

Question 30

Explain what dehydrogenase enzymes are

Answer 30

An enzyme that catalyses the removal of hydrogen atoms (oxidation) from substrates. Often work with coenzymes such as NAD/FAD/NADP

Question 31

Identify and define the different types of phosphorylation

Answer 31

• Phosphorylation is the addition of a phosphate molecule

• Substrate level phosphorylation is the addition of a phosphate molecule using enzymes

• Oxidative phosphorylation is the addition of a phosphate molecule using oxygen

• Photophosphorylation is the addition of a phosphate molecule using light

Question 32

Define a coenzyme

Answer 32

Molecules that help enzymes carry out their functions e.g. NAD, FAD, NADP

Question 33

Define decarboxylation

Answer 33

Chemical reaction that removes a carboxyl group and releases a carbon dioxide

Question 34

Describe the process of oxidative phosphorylation/ETC in respiration

Answer 34

• Takes place in the cristae of the inner membranes of mitochondria and is broken into three stages;
1. The Electron Transfer Chain (ETC)
2. Chemiosmosis
3. Oxidative Phosphorylation

• Involves the synthesis of ATP by oxidative phosphorylation associated with the transfer of electrons down the ETC and passage of protons across inner mitochondrial membranes (chemiosmotic theory).

• ATP production occurs when protons diffuse down an electrochemical gradient through molecules of the enzyme ATP synthase, embedded in the membranes of cellular organelles.

• Oxygen acts as the final electron acceptor in the ETC by joining with electrons and protons to produce water. If it didn’t collect electrons, no more could travel down the ETC

Question 35

State the equation for respiration

Answer 35

6O2 + C6H12O6 -> 6Co2 + 6H2O + energy

Question 36

Explain why ATP is a useful energy currency molecules

Answer 36

• Immediate (only have to hydrolyse one bond)
• Small manageable amounts of energy
• Recycled/regenerated quickly
• Cannot leave a cell