Biology Unit 4.3.1- Photosynthesis

importance of photosynthesis
1. photosynthesis
  1. the process where light transformed to chemical substance/energy and used to synthesise organic and inorganic molecules
  2. first evolved in prokaryotes (2500 million years ago)
  3. most important biochemical process
  4. light --> chemical potential energy --> available to consumers and decomposers
  5. release oxygen from water into atmosphere therefore all aerobes depend on it for respiration
2. Autotrophs- use light/chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules (carbs, protein)
3. chemoautotrophs- prokaryotes that synthesise complex organic molecules using energy from exergonic chemical reactions. many bacteria (nitryifying) obtain energy from oxidising ammonia to nitrate or nitrite to nitrate
4. phototrophs- organisms that can photosynthesise. source of energy = sun, raw materials = inorganic. majority producers are photoautotrophs
5. Heterotrophs- cannot make own food but digest complex organic molecules into simple soluble where they can synthesise complex
6. why does respiration in autotrophs and heterotrophs depend on photosynthesis
  1. release chemical potential energy in complex organic molecules (made during photosynthesis)= respiration
  2. use oxygen for aerobic respiration
  3. photosynthetic equation: 6CO2 + 6H2O (+light)---> C6H12O6
  4. organisms evolved so that they could use the O2 for aerobic respiration: C6H12O6 + H20 ---> 6CO2 + 6H2O and energy/ATP
7. Radioactive isotopes. O2 is produced during photosynthesis = radioactive. when plants given radioactive O2, O2 produced wasnt radioactive. shows oxygen released from water
How does the structure of chloroplast enable them to carry out their new function
1. Structure of chloroplast
  1. disc shaped, 2-10 um long- usually
  2. chloroplast surrounded by double membrane envelope
    1. inter membrane space= 10-20 nm wide
    2. outer= permeable to small ions
    3. inner= less permeable, transport proteins, folded into lamellae (thin plates), stacked in piles. each stack - Granum
  3. between Grana
  4. 2 regions in a chloroplast:
    1. 1. Stroma- fluid filled matrix (light dependent stage occurs here)- necessary enzymes are located (starch grains, oil depiosits, DNA
    2. 2. Grana- thylakoids (stack of flattened membrane sacks). light dependent stage. only sen under electron microscope
2. How chloroplasts are adapted for their role:
  1. 1. Inner membrane: control entry and exit of substance between cytoplasm and stroma inside chloroplast
  2. 2. Grana (stack of 100 thylakoids) - increases SA for photosynthetic pigments electron carriers, ATP Synthase (light dependent)
  3. 3. photosystems- arrangements of photosynthetic pigments allows max light absorption
  4. 4. proteins- hold photosystem in place
  5. 5. stroma- contains enzymes to catalyse light independent stage
  6. 6. Grana- surrounded by stroma so products from dependent --> stroma. needed for independent
  7. 7. Chloroplast- makes some proteins by using genetic instructions in DNA and ribosomes to make proteins
3. Photosynthetic pigments
  1. substances that absorb certain wavelengths of light and reflect others
  2. appear to us as colour of light wavelength that they are reflecting
  3. many different pigments that act together to capture as much light as possible
  4. Thlakoid membranes- arranged in funnel shapes held in place with proteins
4. chlorophylls- mixture of pigments that have a similar molecular structure that consists of a long phytol (hydrocarbon) chain and a porphyrin group which is similar to haem but contains Mg
  1. 1. light--> chlorophyll --> pair of electrons associated with Mg are excited
  2. 2. two forms of chlorophyll: a-P680 and P700 (yellowy green). both absorb red light at different wave lengths (absorption peak)
  3. 3. found at centre of photosystem, known as primary pigment reaction centre
  4. 4. P680 (photosystem 2 and peak absorption = 680 nm)
  5. 5. P700 (photosystem 1 and peak absorption = 700 nm
  6. 6. chlorophyll a (blue light at 450 nm), chlorophyll b- (blue/green light at 500-640nm)
5. accessory pigments
  1. caroteroids- reflect orange and yellow. absorb blue
  2. no prophyrin group and not directly involved in light dependent
  3. absorbs wavelengths not absorbed by chlorophyll and pass the energy associated with that light to chlorophyll at base
  4. carotene(orange and xanthophyll (yellow) are main carotenoid pigments
the light dependent stage
1. light dependent stage
  1. first stage of photosynthesis
  2. Occurs in Thylakoid membranes of chloroplasts. Involves using light to make ATP and other products (reduced NADP and Oxygen)
  3. 2 Photosystems- embedded in membrane
    1. Photosystem 1- intergranal lamellae (mainly)
    2. Photosystem 2- granal lamellae (almost exclusively)
2. Role of water
  1. photolysis- splitting of water into H+ in presence of light and special enzyme. 2H20--> 4H+ +4e- + O2
    1. O2 used by plants for aerobic respiration- diffuses out leaves through stomata
  2. Water is a source of...
    1. H+ used in chemiosmosis to produce ATP. protons accepted by coenzyme NADP ---> reduced NADP used in independent, lowers CO2
    2. Electrons replace those lost by oxididsing chlorophyll
    3. Keeps plant cells trugid
3. Photophosphorylation
  1. Making of ATP from ADP= Pi in the presence of light
  2. chemiosmosis- flow of protons described in the process of photophosphorylation
  3. Photoon hits chlorophyll moeculeenergy of protons (2) cause an excitation. Captured by electron acceptors and electron carriers (thylakoid membranes). Energy released as electrons pass carrier chain. Protons pumped across thylakoid membrane into Thylakoid. Space accumulates and proton gradient formed across membrane. Protons flow DOWN conc gradient through channels with ATP Synthase which produces the force for the reaction. Kinetic energy produces CHEM energy in ATP used for independent stage.
    1. 2 types
      1. 1. Non Cyclic
        Photosystems 1 and 2
        1. Light strikes PS2, excites electron pair, leaves chlorophyll from primary pigment reaction centre, electron pass along chain of electron carriers, energy released sythesises ATP.
        2. Light strikes PS1, pair of electrons lost, electrons + protons(photolysis) + NADP = reduced NADP
        3. electrons from oxidised PS2 replace electrons lost from PS1
        4. electrons from phtolysed water replace those lost from oxidised chlorophyll in PS2
        5. Protons from photolysed water take part in chemiosmosis to produce ATP, which is captured by NADP in stroma and used in independent
      2. 2. Cyclic
        Photosystem 1
        1. excited electrons enter electron acceptor which enter the chlorophyll molecule from where they were lost
        2. no photolysis of water and no generation of reduced NADP lowers ATP amount made. used in light independent or in gaurd cells to bring in K+, lowers WP therefore causing water to follow via osmosis, causes gaurd cells to swell and open stomata
The light independent stages
1. consists of...
  1. Second stage of photosynthesis
  2. CO2 fixed and used to build complex organic molecules (6 carbon sugars)
  3. take place in stroma of chloroplast
  4. calvin cycle. Light not directly used, products of light dependent used and light independent ceases if no lights.
2. Role of CO2
  1. source of carbon for production of organic molecules. Used as structures and act as energy stores/sources for carbon based life forms.
3. Calvin cycle
  1. 1. CO2 diffuses into leaf from stomata into air spaces in spongey mesophyll to the palisade layer, into the thin cellulose walls, through the surface membranes, iinto the cytoplasm, through the chloroplast envelope and into the stroma
  2. 2. in stroma: CO2 and ribulose biophosphate (RuBP)- CO2 acceptor. reaction catalysed by Ribulose Biophosphate Carboxylase Oxygenase (Rubisco). RuBP becomes carboxylated
  3. 3. products of reaction = two molecules of a 3 carbon compound Glycerate 3- phosphate (GP)
  4. 4. GP reduced and phosphorylated to Triose Phosphate (TP). ATP + Reduced NADP from light dependent used in process
  5. 5. 5 out of 6 molecules of TP (3C) = recycled via phosphorylation using ATP from light dependent to 3 molecules of RuBP(5C)
4. How the products of the calvin cycle are used
  1. 6. GP--> Amino and Fatty Acids
  2. 7. 2TP--> Hexose
  3. 8. Glucose is isomerised and converted into hexose sugars
  4. 9. Glucose + Fructose --> disaccharide sucrose
  5. 10. Hexose Polymerises into carbohydrates(Polysaccharides) such as cellulose and starch
  6. 11a- TP--> Glycerol
  7. 11b- TP + Fatty Acids(From GP) --> Lipids
Limiting factors
1. Photosynthetic equation: 6CO2 + 6H2O---> C6H12O6 + 6O2. in the presence of light energy and chlorophyll
2. supplies of CO2 and H2O influence rate of photosynthesis and the rate food is produced
3. Limiting Factors - for metabolic processes, it is the factor that is present at the lowest or least favourable value
  1. Effects of varying light intensity and temperature on the rate of photosynthesis. Constant temperature rate varies according to light. Zero light intensity = no photosynthesis
    1. CO2 avaliability
      1. high CO2 = high photosynthesis rate
      2. warmer temperatures= high photosynthesis rate. If too hot, enzymes may denature
    2. light intensity increases, rate of photosynthesis increases. light intensity = limiting process
    3. High light intensity, photosynthesis plateus. light intensity isnt a limiting process , therefore, doesnt alter the rate
  2. Effects of CO2 on photosynthesis rate
    1. 0.03%/0.06% of earths atmosphere. 0.039% volume, 0.058% mass
    2. 500million years ago, 20x this amount in the atmosphere
    3. Fell in carboniferous period- fossil fuels were made. Rose in Triassic and Jurassic periods then gradually lowered till inidustrialisation till it rose due to the burning of fossil fuels
    4. ocean acts as carbon sink- absorbs 1/3 of CO2
    5. growing forests absorbs CO2 but mature forests produce (via respiration and decomposition) dead leaves and wood as much as they take in from photosynthesis
    6. Greenhouse- CO2 decreases to 0.02%. intoduction to CO2 by burning methane or oil filled fire heaters. higher CO2 lelevs = higher photosynthesis rates
4. Effects of light intensity
  1. high light intensity= higher photosynthesis rates
  2. effects of light
    1. opens stomata- allows CO2 in
    2. trapped by chlorophyll where it excites electrons
    3. splits water moleculrs into protons
  3. photophosphorylation- electrons and protons used in producing ATP for CO2 fixation
5. Effect of temperature on rate of photosynthesis
  1. enzymes depend on temp in Calvin Cycle
  2. 0 degrees- 25 degrees, rate of photosynthesis doubles for every 10 degree rise.
  3. 25 degrees- photosynthesis levels off, as it lowers, enzymes work less efficiently and as O2 move more successfully, competes for the active site of rubisco and prevents thr acceptance of CO2
  4. higher temps = water loss through stomata = stress response (stomata close = limiting availability of CO2
Investigating the factors that affect the rate of photosynthesis
1. measuring photosynthesis
  1. uptake of substances and the appearance of products
  2. if we measure photosynthesis happening per unit time, we have measured the rate.
  3. measure the...
    1. vol of O2 produced
    2. rate of CO2 uptake
    3. rate of increase in dry mass of plants
  4. ate of photosynthesis usually found by measuring vol of O2 produced per min by aquatic plants. Limitations include:
    1. some O2 produced for respiration
    2. dissolved Nitrogen gas is also collected
2. Light intensity on the rate of photosynthesis
  1. photosynthometer- measures rate by collecting and measuring volume of O2 produced in a certain time
    1. Audus microburette
    2. airtight, no air bubbles in tubing. Gas given off by plant over known period of time. Collects in flared end of capillary tubing.
    3. Volume of air calculated, if radius of capillary tube is known.
      1. length of bubble x Pi x r2
    4. Apparatus set up
      1. 1. remove plunger from syringe slowly and allow barrel to fill with tap water. once full, let water out flared end of capillary tube.
      2. 2. Cut pond weed (7cm) and ensures bubbles of gas emerge from cut stem. Place cut end into test tube and add hydrogen carbonate solution. stand test tube in water bath (20 degrees)
      3. 3. place light close to beaker, measure distance (d) from pondweed and light. light intensity equation: l=1/d2. leave for 5-10 mins
      4. 4. read and note length of bubbles. push plunger to expell bubbles. repeat 2 more times, move light further each time
3. Temperature
  1. keep other factors constant
  2. increases intensity
  3. alter temp using waterbath and measure vol of gas produced
  4. warmer= less soluble to oxygen gas
4. CO2 conc
  1. vary number of drops of sodium hydrogen carbonate. measure vol of gas produced at each CO2
5. rate of photosynthesis using changes in density of leaf discs
  1. 1. use drinking straw- cut discs from cress cotyledons
  2. 2. 5/6 discs- 10cm3 syringe and half fill with dilute sodium hydrogen carbonate
  3. 3. hold syringe upright and place finger over end of syringe and pull on plunger. pulls air out of air spaces in spongey mesophyll. air replaced by sodium hydrogen carbonate solution. density of discs increase, therefore sink to bottom.
  4. 4. syringe in beaker. illuminate from above using light and time how long it takes for discs to rise to top. rate = 1/t
  5. 5. repeat twice and at different intensities
Limiting factors and the Calvin Cycle
1. Light intensity
  1. gives measure of how much energy is associated with light.
  2. light from sources spread therefore if distance is doubled, intensity is quatered. flows inverse law and : L=1/d2
  3. increase in light intensity = altered rate of light dependent reactions.
    1. creased light intensity= more excited elctrons.
    2. photophosphorylation- more light=more ATP=more reduced NADP
    3. used in indemendent stages as source of H2 and energy. ATP also used to phosphorylate 5/6 TP tp regenerate RuBP
    4. or little light --> dependent stage will cease therefore, stop independent as needs products of dependent
    5. cant be converted to TP, lowers amount of RuBP. fixation of CO2 and more GP formed
2. CO2 concentration
  1. more CO2 = more CO2 fixation in calvin (if light intensity isnt a limiting factor)
  2. more CO2 fixation = more GP (Amino and Fatty) therefore more TP (Sugars) and RuBP
  3. No. of Stomata open allowing exchange = more transcription = more writing. stress response = release of plant growth regulator and stomata close. Low CO2 = Low Photosynthesis
3. Temperature
  1. increase has little or no effect on dependent stage apart from photolysis. not dependent on enzymes.
  2. Alter independent because its of biochemical steps each catalysed by enzyme therefore, high temp increases rate
  3. more that 25 degrees, oxygenase activity of rubisco increases more than carboxylase activity. therefore, photoresiration exceeds photosynthesis therefore, ATP and reduced NADP from dependent are dissipitated and wasted. reduces photosynthesis rate
  4. higher temps damage proteins in photosynthesis. higher temp means more water loss therefore closed stomata and lowers photosynthesis rate

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	Created by Sarah Pirbhai over 11 years ago