3.5 Energy transfers in and between organisms

What are the 3 types of phosphorylation? (forming ATP from the addition of a phosphate group to ADP)

1. Photophosphorylation- takes place in chlorophyll-containing plant cells during photosynthesis

2. Oxidative phosphorylation- occurs in the mitochondria of plant and animal cells during the process of electron transport

3. Substrate-level phosphorylation- occurs in plant and animal cells when phosphate groups are transferred from donor molecules to ADP to make ATP

What are the word and chemical equations for photosynthesis?

• carbon dioxide + water → glucose + oxygen

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(‘light’ above arrows)

What are components of the chloroplast?

• Thylakoid- contains chlorophyll

• Stroma- fluid-filled space that is the site of the light-independent reaction and contains starch grains (immediate energy store for products of photosynthesis)

• Thylakoid membrane- site of light-dependent reaction

• Granum- stack of thylakoids (increased surface area)

Where does the light-dependent reaction occur?

Thylakoid

Describe the light-dependent reaction

1. Photoionisation: Photosystem II/ chlorophyll absorbs light so electrons are raised to a higher energy level. Some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP

2. Chemiosmotic theory: Electrons are transferred along electron carriers down the electron transfer chain. Protons are transported into the thylakoid to form a proton gradient. They then move down their concentration gradient into the stroma via ATP synthase embedded in the chloroplast membrane. ATP synthase catalyses the production of ATP from ADP and Pi

3. Electrons are passed to photosystem II and photoionisation occurs again. Electrons are transferred along another set of carriers then are used to reduce NADP which synthesises NADPH

4. Photolysis of water produces protons, electrons and oxygen. Electrons replace those lost by chlorophyll in photoionisation

Where does the light-independent reaction occur?

Stroma

Describe the light-independent reaction

1. Fixation of carbon dioxide: CO₂ reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco

2. ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate (TP). Hydrolysis of ATP provides the additional energy

3. Some of the triose phosphate is used to regenerate RuBP in the Calvin cycle (also uses ATP)

4. Some of the triose phosphate is converted to useful organic substances eg hexose sugars such as glucose

Explain the factors which affect the rate of photosynthesis

1. Light intensity- increasing light intensity increases the rate of photosynthesis until it is no longer the limiting factor

2. Carbon dioxide concentration- increasing CO₂ concentration increases the rate of photosynthesis until it is no longer the limiting factor

3. Temperature- increasing temperature increases the rate of photosynthesis until a point where enzymes eg rubisco denature (also increases rate of respiration which uses glucose therefore net growth of plant is reduced. Oxygen is a competitive inhibitor for rubisco)

*not humidity or water availability as the plant would be dead before one of those became a limiting factor

How can the light compensation point be useful to a commercial plant grower?

If temperature if increased but light intensity is less than or equal to the light compensation point, the plant will respire more than photosynthesise so it will have to use stored glucose and could die over time

How can the effect of environmental variables on the rate of photosynthesis be investigated?

Use aquatic plants, algae or immobilised algal beads

What are the word and chemical equations for aerobic respiration?

• Glucose + oxygen → carbon dioxide + water + ATP

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

What components make up the mitochondrion?

• outer membrane

• inner membrane

• cristae (large SA)

• matrix (contains enzymes involved in oxidative decarboxylation and the Krebs cycle

• ATP synthase/ stalked particle

• ribosomes (70S)

• DNA

What are similarities and differences between mitochondria and chloroplasts?

Similarities

• Have outer envelope, own DNA, ribosomes, electron transfer chain

• Have membranes which increase surface area

• Both fluid-filled

Differences

• Matrix in mitochondria vs stroma in chloroplasts

• Chloroplasts have chlorophyll but mitochondria don’t

• Cristae in mitochondria vs thylakoids in chloroplasts

• Chloroplasts have starch granules but mitochondria don’t

Where does glycolysis occur and is it an aerobic or anaerobic process?

In the cytoplasm

Anaerobic

Describe glycolysis

1. 1x glucose phosphorylated to 1x glucose phosphate using 2x ATP (converted to ADP)

2. 1x glucose phosphate produces 2x triose phosphate (TP)

3. 2x triose phosphate oxidised to 2x pyruvate. Net gain of 2x ATP and 1x reduced NAD each

Overall products= 2 ATP, 2 NADH, 2 pyruvate

Where does the link reaction occur?

In the matrix of the mitochondrion

Describe the link reaction

1. If respiration is aerobic, 2x pyruvate from glycolysis enters the mitochondrial matrix by active transport

2. Oxidative decarboxylation to 2x acetate (each pyruvate loses 1C in the form of CO₂ and is oxidised- reduced NAD produced in the process)

3. 2x acetate combines with 2x coenzyme A in the link reaction to produce 2x acetylcoenzyme A

Overall products= 2x acetyl CoA, 2x CO₂, 2x NADH

Where does the Krebs cycle occur?

Matrix of mitochondrion

Describe the Krebs cycle

1 turn of cycle: (1 molecule of glucose= 2 turns)

1. Acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle

2. 6C molecule undergoes oxidative decarboxylation. NAD reduced to

   NADH

3. 5C molecule undergoes oxidative decarboxylation. NAD reduced to NADH and ATP generated by substrate-level phosphorylation (Pi added to ADP)

4. 4C molecule converted to acceptor molecule. NAD reduced to NADH and FAD reduced to FADH₂- both reduced coenzymes

Overall products (per 1 molecule of glucose)= 4x CO₂, 6x NADH, 2x ATP, 2x FADH₂

How can methylene blue be used to investigate coenzymes in respiration?

Methylene blue is a redox indicator- it can accept hydrogen atoms and become reduced which turns it colourless.

• A test tube with yeast, glucose and methylene blue turns colourless because the indicator accepts hydrogen atoms from the respiring yeast (using the glucose to respire) and becomes reduced.

• A test tube with distilled water, glucose and methylene blue stays blue because there is no yeast to accept hydrogen atoms from, therefore the indicator is not reduced. *this test tube is a control to show that the yeast is having an effect, not glucose on its own

• A test tube with yeast, distilled water and methylene blue turns pale blue because there is yeast which can respire other respiratory substrates eg amino acids and lipids, causing some indicator to be reduced, however there is no glucose for the yeast to use to continue respiring.

Where does oxidative phosphorylation occur?

Cristae of mitochondria

Describe oxidative phosphorylation

1. NADH and FADH₂ from the first 3 stages release H atoms which split into protons and electrons

2. Electrons are transferred down the electron transfer chain and lose energy at each level

3. Energy released is used to pump protons into intermembrane space

4. Chemiosmotic theory- protons diffuse down electrochemical gradient through ATP synthase which forms ATP from ADP and Pi (3 per NADH and 2 per FADH₂)

5. Oxygen (final electron acceptor) combines with protons to form water

What are other respiratory substrates?

• lipids (triglycerides) are hydrolysed into glycerol and fatty acids. Glycerol is phosphorylated and converted to triose phosphate in glycolysis and fatty acids are broken down to 2-carbon fragments then converted to acetyl CoA which enters the Krebs cycle

• amino acids are deaminated. 3-carbon compounds are converted to triose phosphate in glycolysis and 4- or 5-carbon compounds are converted to intermediates in the Krebs cycle

What happens if respiration is only anaerobic?

• plants and microorganisms eg yeast: pyruvate + NADH → ethanol + CO₂ + NAD

• animals: pyruvate + NADH → lactate + NAD

  The oxidised NAD produced in these ways can be used in further glycolysis

What is respiratory quotient (RQ)?

Volume of CO₂ evolved/ volume of O₂ absorbed

How does RQ vary for different respiratory substrates?

• carbohydrates= 1

• fats= 0.7 (contain less oxygen than carbohydrates so require more oxygen for oxidation)

• organic acids= >1 (contain more oxygen than carbohydrates so require less oxygen for their oxidation)

How is RQ determined?

Using a respirometer

As respiration occurs, the volume of the air in the test tube containing woodlice will decrease due to oxygen consumed. CO₂ produced is absorbed by potassium hydroxide. Pressure in the test tube will decrease and cause the coloured fluid in the capillary tube to move towards it.