Photosynthesis Flashcards
Photosynthesis
A complex metabolic pathway that consists of three main stages:
- Capturing of light energy
- Light-dependent reaction
- Light-independent reaction.
Overall, in the presence of light:
Equation for photosynthesis
6CO₂ +6H₂O ⟶CH₁₂O₆ + 6O₂
Site of photosynthesis
Chloroplasts
The 10 adaptations of a leaf:
Name at least 5
- Large surface area
- Arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another
- Thin so this keeps diffusion short
- Transparent cuticle
- Numerous stomata for gases exchange
- Long and narrow upper mesophyll cells packed with chloroplasts that collect sunlight
- Stomata open and close in response to changes in light intensity
- Many air spaces in the lower mesophyll layer to allow rapid diffusion in the gas phase of co2 and o2
- A network of xylem that brings water to the leaf cells and phloem that carries away the sugars produced during photosynthesis
Light-dependent reaction
The 2nd stage of photosynthesis that uses light energy to produce ATP, reduced NADP and oxygen (by-product).
takes place in the thylakoids of the chloroplast.
Light-independent reaction (Calvin cycle)
The third stage of photosynthesis, in which the products of the light-dependent stage and carbon dioxide are used to form a simple sugar.
This stage does not require light energy and takes place in the stroma of the chloroplast.
Limiting Factor
A variable that limits the rate of a particular reaction.
What is the 3 thing included in the structure of chloroplasts
Grana
Thylakoid
Stroma
Where does light independent &light dependant reactions take place in plants.
Light-in dependant: Stroma of chloroplasts
Light dependant: in the thylakoids
Raw material involved in photosynthesis
Water
Carbon dioxide
Products of light-dependent reaction
Reduced NADP, ATP, Oxygen
Products of a light-independent reaction
Sugars and others organic molecules
Explain the role of light in photoionisation
Chlorophyll molecules absorbs energy from photons of light.
This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.
Name the 2 main stages involved in ATP production in light-dependant reaction
- Electron transfer chain
2. Chemiosmosis
What happens in the electron transfer chain (ETC)?
Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions, which releases energy
Oxidation
Loss of electrons of hydrogen or gain of oxygen
Reduction
Gain of electrons or gain of hydrogen or loss oxygen
How is a proton concentration gradient established during chemiosmosis?
Some energy released from the ETC is coupled to the active transport of H+ ions (protons) from the stroma into thylakoid space.
How does chemiosmosis produce ATP in light-dependant stage?
H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase.
Explain the role of light in photolysis
Light energy splits molecules of water
2H₂O → 4H⁺ + 4e⁻ + O₂
What happens to the products of photolysis of water?
- H ⁺ ions: move out of thylakoid space via ATP synthase & are used to reduce the coenzyme NADP.
- e⁻ : Replaces electrons lost from chlorophyll.
- O₂: used for respiration or diffuses out of leaf waste gas.
How and where is reduced NADP produced in the light-dependant reaction?
- NADP + 2H ⁺ + e⁻ → reduced NADP.
- Catalysed by dehydrogenase enzymes.
- Stroma of chloroplasts
Where do the H ions and electrons used to reduce NADP come from?
- H ions: photolysis of water
- Electrons: NADP acts as the final electron acceptor of the electron transfer chain.
Name the 3 main stages of the Calvin cycle.
- Carbon fixation
- Reduction
- Regeneration
What happens during carbon fixation?
- Reaction between CO₂ & ribulose bisphosphate (RuBP) catalysed by rubisco
- Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)
What happes during reduction (in the calvin cycle)?
- 2 X GP are reduced to 2 x triose phosphate (TP)
- Required 2 x reduced NADP & 2 x ATP
- Forms 2 x NADP & 2 x ADP
How does the light-independant reaction result in the production of useful organic substances?
1C leaves the cycle (i.e. some of the TP is converted into useful organic molecules).
What happens during regeneration (in the calvin cycle)?
after
- After 1C leaves the cyle, 5C compound RuP forms
- RuBP is regenerated from RuP using 1x ATP
- Form 1 x ADP
Outline the seqeuence of events in the light-independant reaction (Calvin cycle).
Refer to PMT
State the roles of ATP & (reduced) NADP in the light-indepent reaction
Reduction
transports
ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP.
- (Reduced) NADP: Coenzyme transports electrons needed for reduction of GP to TP.
State the number of Carbon atoms in RuBP, GP & TP.
RuBP: 5
GP : 3
TP : 3
Describe the structure of a chloroplast.
5
Flattened
Intergranal
Fluid F
- Usually disc-shaped
- Doubled membrane (envelope)
- Thylakoids: flattend discs stack to form grana
- Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana.
- Stroma: fluid-filled matrix.
How does the structure of the chloroplast maximise the rate of the light-dependent reaction?
- ATP synthase channels within granal membrane.
- Large SA of thylakoid membrane for ETC.
- photosystems postion chlorophyll to enable maximum absorption of light.
How does the structure of the chloroplasts maximise the rate the light-independent reaction?
- Own DNA & ribosomes for synthesis of enzymes e.g. rubisco.
- Concentration of enzymes & substrates in stroma is high.
Name 4 enviromentl factors that can limit the rate of photosynthesis.
- Light intensity (Light-dependent stage).
- Co₂ levels (light-independent stage).
- Temperature (enzyme-controlled steps).
- Mineral/magnesium levels (maintain normal functioning of chlorophyll).