Photosynthesis, Bio (Pt1)
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Define the term autotroph. (Spec)
An organism that uses light energy or chemical energy and inorganic molecule (eg. CO2 and water) to synthesise complex organic molecules.
2 types of autotrophs introduced to us?
- Chemoautotrophs: prokaryotes that synthesise complex orgnic molecules, using energy derived from exergonic chemical reactions. (eg. nitrifying bacteria)
- Photoautotrophs: Source energy is sunlight and raw materials are inorganic molecules, water and CO2.
Define the term heterotroph. (Spec)
An organism that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them, and form simpler soluble molecules from which they can synthesise complex molecules like lipids, proteins and nucleic acids. (eg. some bacteria, animals, fungi, some protoctists)
Why do plants and animals depend upon the products of photosynthesis?
- They release chemical potential energy in complex organic molecules made during photosynthesis, in respiration.
- Photosynthesis also produce oxygen, which is needed for aerobic respiration.
Equation of photosynthesis:
6CO2 + 6H2O (+light energy) --- C6H12O6 + 6O2
Equation for aerobic respiration:
C6H12O6 + 6O2 --- 6CO2 + 6H2O (+ energy, some as ATP)
Photosynthesis is a ___ ___ proecess that takes place in the ___.
Imagine the structure of a chloroplast and its different parts.
What shape and size is a chloroplast roughly?
- Most are disc-shaped
- between 2-10 micrometres long.
What is characteristic of the outer membrane and inner membrane? What is different?
- The outer membrane is permeable to many small ions
- Inner membrane is less permeable - has transport proteins embedded in it.
Where does the light-dependednt stage occur and where does the light-independent stage occur?
- Thylakoids (flattened membrane compartments) is the site for light-dependent stage.
- Stroma is where reactions of the light-independent stage occur, where the necessary enzmes are located. Stroma also contain starch grains and oil droplets and DNA and ribosomes.
How is the structure of the chloroplast adapted to its role? (7)
- Inner membrane with transport proteins - can control enrty and exit of substances between cytoplasm and stroma.
- Many gana (with up to 100 thylakoid membranes) - provide large surface area for different compounds recquired in light-dependent stage (like photosynthetic pigments, electron carriers and ATP synthase)
- Photosynthetic pigments arranged into special structures called photosystems - allow max absorption of light.
- Proteins embedded in grana hold photosystems in place.
- Fluid-filled stroma contain enzymes needed to catalyse reaction of light-independent stage.
- Grana surrounded by stroma - so products of light-dependent stage can readily pass into stroma for independent stage.
- Chloroplasts able to make some proteins for photosynthesis using chloroplast DNA and ribosomes.
Define the term photosynthetic pigment. (Spec)
- Molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected.
- [Are in thylakoid membranes, arranged in funnel shaped structures alled photosystems, held in place by proteins.]
Brifely describe the structure of a cholophyll molecule.
- A long phytol (hydrocarbon) chain and a porphyrin group.
- Porphyrin group similar to haem group in haemoglobin, but contains a magnesium atom instead of iron.
- Light hitting chlorophyll causes pair of electrons associated with magnesium to become excited.
There are 2 forms of chlorophyll. What are they? What part do they play?
- chlorophyll a: 2 types - each playing part as primary pigment reaction centre) (can also absorp around 450nm)
- chlorophyll b: works as accessory pigment alongside carotenoids. (around 500nm-640nm)
What are the 2 types of chlorophyll a, what wavelength is their peak absorption and what part do each play?
- P680: peak at 680 nm, act as primary pigment reaction centre in centre of photosystem II
- P700: peak at 700nm, act as reaction centre in centre of photosystem I.
Name an accessory pigment and state its function.
- Carotenoids - eg. carotene (orange) and xanthophyll (yellow)
- They absorb light wavelengths that are not well absorbed by chlorophylls and pass the energy assocaited with that light to the chlorophyll a at base of photosystem.
In the light-dependent stage, light energy absorbed is used for 3 things:
- Making ATP from ADP and inorganic phosphate - photophosphorylation.
- Making reduced NADP from NADP
- Splitting water into H+ ions (protons), electrons and oxygen - photolysis.
Give equation for photolysis. Where does it happen?
- H2O --- 2H+ + 2e- + 1/2O2
- In Photosystem II, which contains an enzyme that, in presence of light, split water.
What is the role of water in the light-dependent stage?
- Water is a source of...
- H+ ions: used in chemiosmosis to produce ATP. Then accepted by coenzyme NADP to form reduced NADP, to be used in light-independent stage.
- Electrons: to replace those lost by oxidised chlorophyll. (the orginal pair of electrons go on and reduce NADP in non-cyclic phosphorylation).
- Oxygen produced comes from water.
- Also water makes palisade cells turgid, and makes vacuole push chloroplasts to outer edges of cell to get more sunlight and CO2 can diffuse into them.
Briefly explain what happens in photophosphorylation.
- 1. Light hits chlorophyll and ecites a pair of electrons.
- 2. The 2 electrons move along electron transport chain by being captured by several electron acceptors, which are proteins containing iron atoms which are embedded in thylakoid membrane.
- 3. Energy released as electrons pass along chain. This energy pumps H+ ions into thylakoid space across membrane, creating proton conc gradient.
- 4. Protons (H+) flow down conc gradient, through channels associated with enzyme ATP synthase.
- 5. This flow of protons (chemiosmosis) produce a force that joins ADP + inorganic phosphate (Pi) to form ATP. Kinetic energy from proton flow converted to chemical energy in ATP molecules.
What is photophosphorylation?
- Making of ATP from ADP and Pi in the presence of light.
- 2 types - cyclic (only PSI) and non-cyclic (PSII and PSI).
Outline steps involved in non-cyclic photophosphorylation.
- 1. Light excites electron pair in PSII, which leave chlorophyll molecule in primary pigment reaction centre.
- 2. Electrons pass along electron transport chain, and its energy used to make ATP.
- 3. Light also hits PSI and pair of electrons lost.
- 4. These electrons, along with protons in stroma (through ATP synthase) join to NADP, to make reduced NADP.
- 5. Electrons from oxidised PSII replace electrons lost from PSI.
- 6. Electrons from photolysed water replace those lost by oxidised chlorophyll in PSII.
- 7. H+ from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP, in stroma. With 2 electrons from PSI, produce reduced NADP, used in independent stage.
So what are the products of non-cyclic?
Outline what happens in cyclic photophosphorylation.
- Only uses PSI (P700)
- Cyclic because electrons from chlorophyll aren't passed onto NADP, but are passed back to PSI via electron carriers - electrons recycled and repeatedly flow through PSI.
- No reduced NADP or O2 produced, only small amount of ATP.
- Often in guard cells (where they only contain PSI).
The light-independent stage of photosynthesis takes place in ___ of chloroplasts. It is also called the ___ ___. Products of ___-___ ___ is used.
- Calvin cycle
- light-dependent stage
Explain the role of CO2 in the light-independent stage.
CO2 is the source of carbon and oxygen for the production of all large organic molecules.
CO2 from air diffuse into leaf through open ___, and then diffuses throughout air spaces in the ___ ___ and reaches the ___ ___ layer.
- spongy mesophyll
- palisade mesophyll
Outline the steps of Calvin cycle.
- 1. CO2 diffuses into leaf through open stomata.
- 2. CO2 combines with 5C Ribulose bisphosphate (RuBP), catalysed by rubisco.
- 3. This forms 2 molecules of 3C glycerate 3-phosphate (GP). [carbon now fixed]
- 4. GP is reduced, using reduced NADP from light-dependent stage, and phosphorylated using ATP from dependent stage to form triose phosphate (TP).
- 5. 5/6 molecules of TP are recycled by phosphorylation, using ATP from light-dependent stage, to 3 molecules of RuBP.
- A: CO2
- B: Rubisco (ribulose bisphosphate carboxylase)
- C: 2x Glycerate 3-phosphate (GP)
- D: ATP (2x)
- E: ADP + Pi (2x)
- AB: Reduced NADP (2x)
- AC: NADP (2x)
- AD: 2x Triose phosphate (TP)
- AE: Hexose sugar (needs 2 TP) [or other useful substances like glycerol]
- D2: ATP (only 1)
- E2: ADP (no Pi)
- BC: Ribulose bisphosphate (RuBP)
How are the products of the Calvin cycle used?
- Some GP can be used to make amino acids and fatty acids.
- Pairs of TP molecules combine to form hexose (6C) sugars. eg. glucose and fructose.
- Glucose and fructose can be combined to form sucrose
- Can also be polymerised into other carbohydrates such as cellulose and starch.
- TP can also be converted to glycerol - may be combined with fatty acids formed from GP, to make lipids.
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