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A2 Biology (Unit 1: F214) > Photosynthesis > Flashcards

Flashcards in Photosynthesis Deck (36)
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What is photosynthesis?

The process whereby light energy from the Sun is transferred to chemical energy from the synthesis of large organic molecules (e.g. sugars) from small inorganic molecules (e.g. carbon dioxide).


What are autotrophs?

Organisms that are able to harness light energy or energy from chemical reactions to synthesise large organic molecules from small inorganic molecules.


What are the 2 types of autotrophs?

1. Chemoautotrophs: Prokaryotes that are able to synthesise organic molecules using energy from exogenic (reactions that release energy) reactions. They usually live in harsh conditions, examples being nitrifying bacteria living in low oxygen soil.
2. Photoautotrophs: Plants and other organisms that contain chloroplast and are able to photosynthesise, using energy from the sun to make complex organic molecules. They make up most of the producers on earth.


What are heterotrophs?

Organisms that are not able to make their own food but instead, rely on breaking down (digesting) pre-made organic compounds from further down the food chain, releasing the potential energy stored in them, or assembling them into new organic molecules.


Why does all life on earth depend on photosynthesis?

- Photosynthesis converts abundant light energy from the sun into chemical form which is accessible by all types of organisms.
- Photosynthesis releases oxygen as a bi-product which is essential for photosynthesis.
- Provides energy to create nutrients and complex sugars which can be broken down by plants/consumers further down the food chain in respiration to release energy.


How are leaves adapted for photosynthesis?

- Leaf blade: Large surface area to trap maximum light.
- Waxy cuticle & upper epidermis: Fully transparent to allow maximum light to pass through to the chloroplast containing palisade cells that carry out most of the photosynthesis.
- Palisade mesophyll cells: Packed full of chloroplasts that are able to move up and down depending on light level, maximising rate of photosynthesis.
- Spongy mesophyll cells: Air pores and high moisture to allow for efficient gas diffusion and exchange.
- Stomata: Opens and closes depending on light intensity and water level to control rate of gas exchange.
- Xylem: Brings water up from the roots for photosynthesis.
Phloem: Takes away products of photosynthesis to be stored/used.


What is the Endosymbiont theory of chloroplast origin?

A very long time ago, the chloroplast was actually a prokaryotic cell that photosynthesised but was one day engulfed by a eukaryotic cell through endocytosis. However, instead of digesting the chloroplast, a permanent symbiotic relationship was formed between the chloroplast and eukaryotic cell which involved the chloroplast providing the cell with energy and sugars while the cell provided the chloroplast with a stable environment and a steady supply of nutrients.


What evidence is there to support the Endosymbiont theory of chloroplast origin?

1. The chloroplast contains a circular ring of DNA, like a prokaryotic cell.
2. Chloroplast contains free-floating 70s ribosomes, like a prokaryotic cell.
3. Inner chloroplast membrane is of similar composition to membrane of prokaryotic cell.
4. Affected by some antibiotics, like prokaryotic cells.


What are the features of a chloroplast?

- Chloroplast envelope: Double membrane surrounding chloroplast. Outer membrane partially permeable to small ions whereas inner membrane is less so and has transport proteins embedded.
- Circular ring of DNA and 70s ribosomes, so that the chloroplast is able to produce some of its own proteins.
- Thylakoid: Inner membrane folded into thin lamellae, acts as site of light absorption and ATP synthesis in the light dependent part of photosynthesis.
- Granum: Thylakoids stacked together into densely packed regions.
- Intergranal lamellae: Thylakoid membranes between grana that connect them together.
- Starch grains and lipid droplets: Forms of storage for the products of photosynthesis.
- Stroma: Fluid part of the chloroplast containing all features of the chloroplast. This is the site of the light independent reactions in photosynthesis which produces carbohydrates and all other nutrients.


How are chloroplasts adapted for photosynthesis?

- Inner membrane is embedded with transport proteins which control which substances enter and exit form the chloroplast., and the flow of chemicals required in photosynthesis.
- Large inner surface area from the grana and stacked thylakoid membranes allows for maximum number of photosynthetic pigments, ATP synthase and other features of the light dependent reaction to be embedded.
- Photosystems allow for the maximum amount of light to be absorbed and used by the plant.
- Stroma contains an abundance of enzymes required for the light independent reactions, which gives maximum rate of reaction.
- Stroma surrounds the grana so that the required light dependent stage products for the light independent stage can readily diffuse into the stroma.
- Chloroplast can manufacture some essential proteins for photosynthesis by itself, using the genetic information it contains.


What are photosystems?

- Funnel-shaped complexes consisting of many coloured pigments that work together to absorb the maximum amount of light energy.
- Consists of the primary pigment at the reaction centre (bottom), then accessory pigments on top which absorb other wavelengths of light and pass to primary pigment.


What is chlorophyll?

- A group of pigments that are all similar in structure, with a hydrocarbon (phytol) tail and porphyrin tail containing a prosthetic Mg group.
- There are 2 different types of chlorophyll.
- Chlorophyll a is a primary pigment and is found in 2 forms, P680 and P700, with peak light absorption at wavelengths 680nm and 700nm respectively.
- Chlorophyll b has a different absorption spectrum and is best at absorbing light of wavelength 500-640nm.


What is an absorption spectrum?

Percentage of light of each wavelength a particular pigment is able to absorb.


What is an action spectrum?

Percentage of light of each wavelength which is used by a plant in photosynthesis.


What are accessory pigments?

- Other photosynthetic pigments that are not directly involved in the light dependent stage of photosynthesis, but absorb light of wavelengths not absorbed by chlorophyll a and pass the energy to it.
- Examples include chlorophyll b and carotenoids which absorb blue light.


Where does the light dependent stage of photosynthesis take place?

- In the thylakoid membranes of the chloroplast.
- PSII is found almost exclusively in the grana whereas PSI is mostly found in the intergranal lamellae.


What are the stages in the light dependent stage of photosynthesis?

1. Light absorbed by photosynthetic pigments in PSII raises pairs of electrons in chlorophyll a to a higher energy level which causes them to be emitted from the photosystem. This is a result of the electrons being hit by energetic photons.
2. Photolysis of water (in presence of enzymes) releases O2 molecule, 2H+ ions and 2e- which replace those lost by PSII.
3. Electrons from PSII pass along a sequence of electron carriers, releasing a bit of energy each transfer, which is used to pump H+ ions from the stroma into the thylakoid lumen as part of chemiosmosis.
4. Chemiosmosis decreases pH in thylakoid lumen and sets up H+ gradient across thylakoid membrane.
5. H+ ions flow down the concentration gradient back into stroma through the enzyme ATP synthase, which provides energy for ADP to be joined with Pi to form ATP. This process ultimately utilises light energy absorbed in the photosystems and is thus called photophosphorylation.
6. Light energy absorbed by photosynthetic pigments in PSI raises the energy level of electron pairs in chlorophyll a, which leads to their emission.
7. These electrons are replaced by electrons from PSII (non-cyclic phosphorylation) or PSI (cyclic phosphorylation).
8. Electrons from PSI are eventually transferred to NADP reductase where they are used, along with H+ ions from the stroma, to convert NADP to reduced NADP.


What is the difference between cyclic and non-cyclic phosphorylation?

- Non-cyclic phosphorylation involves both PSII and PSI whereas cyclic phosphorylation only involves PSI.
- Photolysis in involved in non-cyclic phosphorylation whereas it is not in cyclic phosphorylation.
- Electron from PSI returns to PSI in cyclic phosphorylation whereas electron in PSII ends up being used to produce reduced NADP in non-cyclic phosphorylation.
- Only small amounts of ATP are produced in cyclic phosphorylation whereas ATP, reduced NADP and O2 is produced for non-cyclic phosphorylation.


What is the role of water in photosynthesis?

1. Provides O2 for aerobic respiration.
2. Provides electrons which replaced ones lost from PSII during non-cyclic phosphorylation.
3. Provides H^+ ions which are used to create H^+ gradient in chemiosmosis, which leads to ATP production.
3. Provides H^+ ions for use in making reduced NADP.
4. Keeps turgor pressure in plant cells which prevents wilting and provides maximum surface area for photosynthesis.


Where does the light independent stages of photosynthesis take place?

In the stroma, where the enzymes associated with the light independent stage are located.


What are the stages in the light independent stage of photosynthesis?

1. Carbon dioxide from the air diffuse into the leaves via the stomata on the underside of the leaves.
2. Carbon dioxide diffuse through the air spaces in the spongy mesophyll to reach the palisade mesophyll.
3. Carbon dioxide diffuses through the cell wall, plasma membrane, cytoplasm, and chloroplast envelope to reach the stroma.
4. In stroma, CO2 combines with Ribulose bisphosphate (RuBP) in the presence of rubisco to become carboxylated.
6. 6-carbon compound formed is unstable and breaks down to 2 molecules of glycerate 3-phosphate (GP).
7. In the presence of ATP and reduced NADP (produced in light dependent reaction), GP is phosphorylated to 2 molecules of triose phosphate (TP).
8. 5/6 molecules of TP is phosphorylated by ATP and recycled to make 3 molecules of RuBP.
9. The cycle repeats


How is GP and TP used in plants?

- GP is used to make amino acids and fatty acids.
- 2 molecules of TP can be combined to form hexose sugars like glucose which can be isomerised to form fructose.
- Fructose and glucose can be combined to form the disaccharide sucrose (transport form of sugars in phloem).
- Glucose molecules can be polymerised to form cellulose and starch.
- TP can also be used to make glycerol, which is combined with fatty acids to make triglycerides (lipids).


What is the other name for the light independent stage of photosynthesis?

The Calvin cycle.


What is CO2 used for in the Calvin cycle?

CO2 provides carbon and oxygen which is used by the cycle to generate amino acids, fatty acids, hexose sugars and glycerol.


What is the role of ATP in the calvin cycle?

- Provides energy and phosphate for the phosphorylation of GP to TP.
- Provides phosphate for the phosphorylation of TP to RuBP.


What is the role of reduced NADP in the calvin cycle?

Acts as a reducing agent in the conversion of GP to TP.


What are the environmental limiting factors of photosynthesis?

1. Carbon dioxide concentration.
2. Availability of water.
3. Temperature.
4. Light intensity.


What are the non-environmental limiting factors of photosynthesis?

1. Surface area of leaves.
2. Stomata density.
3. Disease.
4. Number of chloroplast.
5. Range of photosynthetic pigments present.


What is a limiting factor?

The single factor that is present at the lowest or least favourable value and thus determines the maximum rate of reaction.


How does light intensity affect rate of photosynthesis?

- Rate of photosynthesis increases in proportion as light intensity increases up to a maximum when another factor becomes limiting factor, then it plateaus.
- Light intensity determines the rate of electron emission from PSII and PSI which determines the rate of photophosphorylation and thus rate of ATP and reduced NADP production, which limits rate of calvin cycle as these products are required.
- Increasing light intensity causes stomata to open wider, which allows greater volume of CO2 to diffuse into the leaves to be used for the calvin cycle.
- Increasing light intensity increases rate of photolysis and H+ production, which limits rate of chemiosmosis, phosphorylation, ATP and reduced NADP production.