3.5 Energy transfers in and between organisms Flashcards
3.5.1 Photosynthesis
What is the equation for photosynthesis?
Light
6CO₂ + 6H₂O —–> C₆H₁₂O₆ + 6O₂
Chlorophyll
Carbon dioxide + water -> Glucose + Oxygen
What makes leaves good at photosynthesis?
Large surface area - Absorbs maximum light. Leaf Arrangement - Minimises leaves overlapping. Thin - Most light absorbed by the first few micrometres. - Efficient gas exchange Transparent cuticle and epidermis - Allows light into upper mesophyll. Palisade cells are long, thin, and tightly packed. - Max light absorption Many stomata - Efficient gas exchange, short diffusion distance Stomata open and close in response to light intensity - Reduces transpiration. Many air spaces in lower mesophyll - Allows rapid gas exchange. Xylem and Phloem - Allow transpiration and translocation
What is the law of limiting factors?
When a process depends on two or more factors, the rate of that process is limited by the factor which is in shortest supply
What are the limiting factors of photosynthesis?
Light intensity Availability of water CO2 Chlorophyll Temperature
How are chloroplasts adapted to their function?
Granal membranes
- Provide a large surface area for chlorophyll, electron carriers and enzymes to attach.
Network of proteins in grana
- Hold chlorophyll in specific way to allow maximum light absorption.
Fluid in stroma contains enzymes.
- Enzymes are needed for light-independent stage of photosynthesis.
- As the chloroplast is membrane-bound it allows it to maintain the optimum enzyme concentration
Chloroplasts contain DNA and ribosomes.
- They can quickly and easily manufacture proteins needed for photosynthesis.
Granal membranes have ATP synthase channels.
- Catalyse the production of ATP.
- Also, selectively permeable
Stroma surrounds the grana.
- Products from the LDR easily diffuse into the stroma for the LDR
Where does the light dependent reaction of photosynthesis take place?
The thylakoid membranes of chloroplasts
Describe the first stage of light dependent reaction (Photoionisation)
Light energy is absorbed by Photosystem II.
The light energy excites electrons in the chlorophyll.
The electrons move to a higher energy level and are released from the chlorophyll and move down the electron transport chain to Photosystem I
Describe the second stage of light dependent reaction (Photolysis of water)
As the excited electrons move from chlorophyll leave PSII to move down the electron transport chain, they must be replaced.
Light energy splits water into protons (H+ ions), electrons and oxygen
Reaction is:
2H₂O O₂ + 4H⁺ + 4e⁻
Describe the third stage of light dependent reaction (chemiosmotic theory)
The excited electrons lose energy as they move down the electron transport chain.
This energy is used to transport protons into the thylakoid, so that the thylakoid has a higher concentration of protons than the stroma.
This forms a proton gradient across the thylakoid membrane.
Protons move down their concentration gradient, into the stroma, via the enzyme ATP synthase, which is embedded in the thylakoid membrane.
The energy from this movement combines ADP and inorganic phosphate to form ATP
Describe the final stage of light dependent reaction (reduced NADP)
Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level.
Finally, the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP (NADPH)
What are the products of non-cyclic photophosphorylation?
ATP
Reduced NADP
Oxygen
What is cyclic photophosphorylation?
Only uses PSI.
The electrons from the chlorophyll are not passed onto NADP but are passed back to PSI via electron carriers.
This means the electrons are recycled and can repeatedly flow through PSI.
This process does not produce any reduced NADP or Oxygen.
It only produces small amounts of ATP
Where does the light independent reaction take place?
In the stroma of chloroplasts
What is needed for the light independent reaction?
Products of LDR
- NADPH (protons and electrons) to form simple sugar
- ATP as the hydrolysis of ATP provides the energy for the reaction
What are the three sections of the Calvin Cycle?
- Carbon Fixation
- CO2 combines with RuBP to make 2x GPs (glycerate 3-phosphate) - Reduction
- ATP and NADPH used to reduce GP to TP (triose phosphate) - Regeneration
- RuBP is regenerated
Describe the steps of the Calvin cycle
- CO2 diffuses through the stomata and into the stroma of the chloroplast
- The CO2 (1 carbon compound) reacts with a 5-carbon compound ribulose bisphosphate (RuBP)
- This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (Rubisco)
- This reaction between CO2 and RuBP forms a very unstable 6-carbon compound
- The 6-carbon compound is immediately broken down into 2x 3C GPs
- NADPH (from LDR) reduces 3 carbon TP using ATP (from LDR)
- NADPH is reformed (OILRIG) and returns to the LDR
- TP has 2 uses
- 5/6 carbons are used to regenerate RuBP using ATP (from LDR)
- 1/6 carbons form organic substances such as carbohydrates (glucose), lipids and amino acids
How many times must the calvin cycle turn to make 1 glucose molecule?
6 times as requires 6 carbons since only one carbon released from TP each cycle
- It requires 18 ATP, 12 NADPH and 6 CO2
3.5.2 respiration
What is the equation for respiration?
C6H12O6 + 6O2 –> 6CO2 + 6H2O
Glucose + Oxygen –> Carbon dioxide + water
What is respiration?
A series of reactions in which energy is transferred from organic compounds, such as carbohydrates, to the temporary energy store ATP
What are the four stages of respiration?
- Glycolysis
- Splitting 6C glucose into 2x 3C pyruvates - Link Reaction
- Converts 3C pyruvate into CO2 and 2C acetyl coenzyme A - Krebs Cycle
- Acetyl coenzyme A goes through redox reactions that produces ATP and reduced NAD and reduced FAD - Oxidative Phosphorylation
- Uses electrons associated with rFAD to synthesise ATP (and waste water)
Where do each of the four stages of respiration happen?
Glycolysis = Cytoplasm of cell
Link Reaction = Matrix of mitochondria
Krebs Cycle = Also in matrix
Oxidative Phosphorylation = Utilises proteins found in the membrane of the crista
What are the stages of glycolysis?
- Activation of glucose by phosphorylation
- Before splitting glucose is made more reactive by adding 2 phosphate molecules (phosphorylation)
- Phosphate molecules come from hydrolysis of 2 ATP molecules to ADP
- This provides energy to activate glucose and lowers activation energy for enzyme-controlled reactions - Splitting of phosphorylated glucose
- Each glucose molecule split into 2x 3C molecules (TP) - Oxidation of TP to Pyruvate
- Hydrogen removes from each of 2 x TP molecules
- Hydrogen transferred to NAD (hydrogen carrier)
- This forms reduced NAD - Production of ATP
- Enzyme-controlled reactions convert each TP into another 3C molecule called pyruvate
- 2 ATPs regenerated from ADP
What is the total yield of glycolysis?
2 ATP
2 NADH
2 Pyruvate
What does glycolysis being anaerobic prove?
As the process is carried out by all living things it provides evidence for common ancestry
What are the stages of the link reaction?
- Pyruvate produced in glycolysis actively transported into matrix of mitochondria
- Pyruvate is oxidised to acetate. To do this it is decarboxylated (loses 1 CO2 molecule) and is dehydrogenated (2 hydrogen molecules removed). This produces acetate
- The 2 hydrogens removed are accepted by NAD to form reduced NAD which is later used to produce ATP
- Coenzyme A combines with acetate to form acetyl coenzyme A
What is the overall equation for link reaction?
Pyruvate + NAD + CoA-> acetyl CoA + reduced NAD + CO2
How many times does the link reaction have to occur for every glucose molecule?
Twice
- For each glucose molecule used in glycolysis, 2 pyruvate molecules are made
- Link reaction only uses one pyruvate molecule so link reaction happens twice for every glucose molecule going through glycolysis
What are the products of the link reaction for each glucose molecule?
2 acetyl coenzyme A (go into Krebs cycle) 2 CO2 (released as waste products) 2 reduced NAD (go to oxidative phosphorylation)
What are the stages of the Krebs Cycle?
- Acetyl CoA from link reaction joins with oxaloacetate (4C) to form citric acid/citrate
- Coenzyme A goes back to link reaction to be reused
- 6C citrate loses CO2 (decarboxylation) and hydrogen (dehydrogenation)
- Hydrogen reduces NAD
- 5C compound is formed which is then converted into 4C molecule
- Decarboxylation and dehydrogenation occur which forms 1 reduced FAD and 2 reduced NAD
- ATP is produced (substrate level phosphorylation)
- 4C oxaloacetate then joins with acetyl CoA (whole cycle starts again)
How many times does Krebs cycle go around per glucose molecule?
Twice
From one glucose molecule you get 2 pyruvate molecules so krebs cycle twice
What are the products of the Krebs Cycle for each acetyl CoA entering?
2 CO2 molecules (plus one from link reaction)
1 ATP molecule
Reduced coenzymes NAD and FAD (later used in oxidative phosphorylation to produce ATP)
What is substrate level phosphorylation?
Direct phosphorylation of ADP with a phosphate group using energy obtained from a coupled reaction
What is oxidative phosphorylation?
Production of ATP from oxidised NADH and FADH
What are the stages of oxidative phosphorylation?
- As the electrons pass along the chain they release energy which causes the active transport of protons across the inner mitochondrial membrane into the inter-membranal space
- The protons gather in the area between the mitochondrial membranes
- The electrons pass along the ETC in a series of redox reactions
- Hydrogen atoms from glycolysis and Krebs join with coenzymes NAD and FAD
- Reduced NAD and FAD donate electrons of the hydrogen atoms they are carrying to the first electron transport molecule
- They then diffuse back into the mitochondrial matrix through ATP synthase channels in the inner mitochondrial membrane
- At the end of the chain the electrons combine with the protons and oxygen making water
- Oxygen is the final acceptor of the electrons in the transport chain
What is the importance of oxygen in respiration?
Oxygen is crucial in respiration as it is the final acceptor
Also important so hydrogen atoms produced in glycolysis and Krebs cycle can be converted to water and drive production of ATP
What would happen if there was no oxygen?
Without its presence to remove protons and electrons, they would block the flow along the chain and respiration would stop
Krebs and ETC can’t take place so pyruvate biulds up in cell
Why is oxidative phosphorylation a step by step approach?
If a lot of energy was released in one step. a lot is lost as heat and therefore energy is wasted
However if it is released slowly over number of steps, more energy is available for use of the organism
This is why NAD and FAD transfer their electrons gradually
What other substances aside from glucose are able to undergo oxidation to release energy?
Lipids
Proteins
