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A2 biology 3
OCR biology - photosynthesis
| Question | Answer |
|---|---|
| WHat is an autotroph + different types? | Synthesises complex organic molecules from simple inorganic molecules + energy source. Photoautotrops synthesise glucose from CO2/H2O using sunlight via photosynthesis. Chemoautotrophs oxidise electron donors in environment |
| What is a heterotroph? | Organism can't synthesise its own organic molecules from simple inorganic ones, so it ingests organic matter, digests it into small soluble molecules then builds it into complex organic molecules |
| What are the type of heterotrophic nutrition? | Holozoic: ingestion, digestion, absorption of soluble molecules, synthesis of complex organic molecules, egestion of unwanted matter. Saprophytic: release enyzmes that digest organic matter into soluble molecuels to be absorbed, external digestion |
| What are parasitic and mutualistic relationships? | Parasitic- non-mutualistic relationship ebwteen2 species where parasite benefits e.g. food, shelter, while host is harmed. Mutualistic - relationship between 2 species where they both benefit |
| What are the adaptations of a leaf for photosynthesis? | Large SA maximises light absorption, waxy cuticle prevents water loss, upper epidermis is transparent to let light penetrate, palisade mesophyll is densely packed with chloroplasts, cytoskeleton can move chlorplasts, palisade cells at 90 degrees... |
| Continued | To leaf surface to minimise light absorption by cell wall, spongy mesophyll air spaces allow rapid gas diffusion, many stomata, stomata open/close in response to water availability +light intensity, vascular tissue transports water to leaf + glucose away |
| What is the structure of a chloroplast? | 2-10μm long. Envelope- 2 phospholipid bilayers separated by intermembrane space (10-20nm). Inner controls movement of substance in/out. Thylakoids- flattened membrane sacs, continuous with inner membrane, site of LDS. Stack of thyalkoids (<100) = granum |
| Continued | GRana are connected by intergranal lamellae. Stroma is fluid-filled matrix inside chloroplast, site of LIS, contains: OIL DROPLETS, starch grains, prokaryote-type ribosomes (70s), chloroplast DNA (circular, naked), enzymes. |
| What is the structure: function of a chloroplast? | Grana provide large SA for photosynthetic pigments, different types of photosynthetic pigments absorb different wavelengths of light, grana are surorudned by stroma so products of LDS easily move into site of LIS, chloroplasts can synthesise enzymes |
| What is the chemical equation for photosynthesis? | 6CO2 + 6H2O -> C6H12O6 + 6O2 |
| What is a photosynthetic pigment? | A pigment in a photosystem (in thylakoid membrane) that absorbs certain wavelengths of light in the visible region of the EM spectrum. |
| What is an absorption spectrum and an action spectrum? | Absorption: graph showing the maount of light absorbed by a particular photosynthetic pigment for different wavelengths of light. Action: graph showing rate of photosynthesis at different wavelengths of light - red and blue are most effective |
| What is a photosystem? | A discrete unit of organisation containing 2 chlorophyll a molecules in primary pigment reaction centre + accessory pigments (carotenoids + chlorophyll b) in antenna complex. Funnel shaped, held in grana membranes by proteins |
| What are the types of chlorophyll a? | P680 - PSII, thylakoid membranes yellow-green, peak absorptions at 680nm (red), 450nm (blue). P700 - PSI, intergranal lamellae, yellow-green, peak absorptions at 700nm (red) and 450nm (blue). |
| What are the accessory pigments? | Antenna complex, absorb wavelengths of light poorly absorbed by chlorophyll a, transfer energy to chlorophyll a by resonance energy transfer. Chlorophyll b- blue-green, 500nm, 640nm. Carotenoids absorb blue light - xanthophyll yellow, carotene orange |
| What is the structure of chlorophyll? | Hydrophobic hydrocarbon tail, hydrophilic head is a porphyrin ring with a central magnesium atom around which electrons can migrate. |
| Describe cyclic photophosphorylation | Light energy is absorbed by chlorophyll a P700/ accessory pigments, resonance energy transfer, PSI. e- in chlorophyll a is promoted, accepted by electron acceptor, passes along chain of electron carriers, redox reactions, ATP synthesis, e- returns to PSI. |
| DEscribe non-cyclic photophosphorylation | Light energy absorbed by P680/ accessory pigmetns in PSII. Electron promoted, accepted, passed along chain of eelctorn carriers, ATP synthesis. Same thing in PSI. NADP reductase catalyses reduction of NADP+ to NADPH, e- from PSI, protons from photolysis. |
| Continued | Electrons from photolysis replace those lsot from PSII, electrons promoted from PSII replace those lost from PSI. |
| What is the photolysis of water? | Catalysed by enzyme in PSII + light! H2O -> 2H+ + 2e- + 0.5 O2. O2 diffuses out of stomata or is used in aerobic respiration, e- reduces PSII, protons are used in chemiosmosis or to reduce NADP+ coenzyme |
| Define chemiosmosis | The ability of thylakoid membranes to pump H+ ions into thylakoid lumen then harness energy stored in H+ electrochemical gradient to drive cellular work e.g. ATP synthesis |
| Explain chemiosmosis | Transfer of e- between electron carriers releases energy (redox), pump H+ into thylakoid lumen, electrochemical gradient, H+ diffuse through channel associated ATP synthase down gradient into stroma, proton motive force, ADP+Pi -> ATP, kinetic -> chemical |
| What is an electron carrier? | A protein containing an Fe3+ ion |
| Why is the H+ gradient called an electrochemical gradient? | It's both a concentration gradient and a potential difference as H+ has a charge |
| Where does the LIS occur and is it independent of light completely? | Stroma, it doesn't directly require photons but uses ATP and NADPH from LDS so wouldn't occur in the absence of light. |
| Explain the Calvin cycle | 1)CO2 + RuBP -> unstable 6C intermediate, RuBisCO, fixation of carbon. 2)6C intermediate splits into 2 GP. 3)some GP is converted to fatty/amino acids. 4)most GP -> TP via phosphorylation (ATP) and reduction (NADPH). 5)5/6 TP recycled into 3 RuBP... |
| Continued | phosphorylation with ATP. 1 of every 6 molecules of TP is converted to useful hexose sugars e.g. glucose, isomerised to fructose, forms sucrose, cellulose, starch. Also: glycerol, which can bond with fatty acids from GP to form lipids |
| What is photorespiration? | Where RuBisCO catalyses O2 + RuBP, forms toxic H2O2, useless phosphoglycolate, reduces efficiency of sugar production, wastes NADPH and ATP from LDS, rate increases with temperature |
| What is a limiting factor? | A variable present at the lowest/ least favourable value that limits the rate of a chemical process, increasing its concentration would increase the rate of reaction until another variable becomes limiting. |
| Draw 2 graphs of light intensity as a limiting factor + explain why high light intensity increases rate of reaction | High light intensity increases rate of LDS (increases rate of electron excitation and photolysis of water) and increases rate of LIS (stomata open wider in response to high LI so more CO2). High -> low: GP accumulates, [TP] decreases, [RuBP] decreases |
| Why does increasing temperature up to the optimum increase the rate of photosynthesis? | Temperature is a limiting factor. Low temp: particles have low KE, few collisions, reaction is slow. Increasing temp increases KE of particles so there are more freqent and high energy collisions. |
| What happens when the temperature is increased above the optimum? Draw graph | Rate of photosynthesis decreases, no longer limiting factor. Rate of LIS> LDS so [ATP]/[NADPH] are limiting. Stomata close as a stress response to excessive water loss by transpiration, [CO2] decrease, O2 accumulates causing photorespiration. Enzymes dena |
| Explain why CO2 is a limting factor + draw 2 graphs. | CO2 only makes up 0.04% of air and becomes a limiting factor when stomata close e.g. stress response to excessive water loss via transpiration. Low [CO2] -> high [CO2] = [RuBP] decreases, [GP] and [TP] increase as more carbon is being fixed |
| What is a light compensation point? | Light intensity at which the rates of respiration and photosynthesis balance exactly, so net input/ output of CO2 is 0. |
| What are the differences between graphs of sun and shade plants for rate of photosynthesis against light intensity? | Shade plants reach max rate of photosynthesis at a lower light intensity and have a lower LCP because they are more adapted to utilising low intensities of light effectively |
| What are the adaptations of shade plants? | Larger SA, thinner leaves, thinner waxy cuticle, larger chloroplasts, chloroplasts have more grana and a higher concentration of chlorophyll, some epidermal cells may contain chloroplasts |
| What are the adaptations of sun plants? | Smaller leaves = less water lost by transpiration. Red anthocyanin pigment in leaves = protection from UV rays |
| How can a photosynthometer be used to measure rate of photosynthesis? | Fill with water, put NaHCO3 + 10cm piece of Cabomba in tube, water bath , 20 degrees, high light intensity. Allow 5 mins to ACCLIMITSE, measure volume of O2 produced in 10 mins, V= πr2h. Repeat with these conditions =mean. Change IV |
| How can the independent variable be changed? | LI: distance between lamp + beaker, light meter/ 1/distance2, only light source. Wavelength: different colour filters, same thickness/ light inteisyt/ cover completely. [CO2]: volume of NaHCO3. Temperature: water bath thermostat (O2 less soluble high T) |
| Draw a photosynthometer | Boiling tube with cabomba and NaHCO3, water bath, fluorescent lamp, capillary tubing, syringe, ruler, stand |
| How can leaf disc density be used to find photosynthesis rate? What property of photosynthesis does it measure? | Rate of O2 production. Cut leaf discs from cress cotyledons with straw, add 5 to syringe of NaHCO3, pull on plunger, replaces air in SPONGY MESOPHYLL AIR SPACES with NaHCO3 so sink, transfer to beaker, bright lamp, measure time taken for 1 to rise |
| What variables must be kept the same? | Same size of leaf discs, from same part of same plant, same [NaHCO3], same light intensity |
| Describe how hydrogencarbonate indicator can be used to measure rate of photosynthesis? | Place hydrogencarbonate indicator + 10cm Cabomba in several boiling tubes + bung, vary conditions, after 10 minutes compare colours (pH 6 = yellow, pH 7 = red, pH > 7 = purple-red) or absorbances with a colorimeter + blue filter |
| Why does the colour change? | CO2 forms carbonic acid in water. High rate of photosynthesis: pH rises as CO2 is used up, red -> purple red. Rate of respiration > rate of photosynthesis: red -> yellow |
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