5.1 photosynthesis

  • takes place in the chloroplasts of plant cells

Structure of chloroplasts

  • Chloroplasts = flattened organelles surrounded by a double membrane, found in plant cells

  • Thylakoids = fluid filled sacs. Stacked up in the chloroplasts into structures called grana

  • Grana are linked together by bits of thylakoid membrane called lamellae

  • Stroma = a gel like substance which contains enzymes, sugars and organic acids

  • Carbohydrates produced by photosynthesis are not used straight away, they are stored as starch grains in the stroma

Chloroplast pigments + photosystems

  • Contain photosynthetic pigments (chlorophyll a, chlorophyll b and carotene). These are coloured substances that absorb the light energy needed for photosynthesis

  • Pigments are found in the thylakoid membranes, they’re attached to proteins. The protein and pigment is called the photosystem

  • 2 photosystems used by plants to capture light energy, photosystem I (absorbs best- wavelength of 700nm) and photosystem II (absorbs best- wavelength of 680nm)

2 stages of photosynthesis

  • The light dependent reaction

  • The light independent reaction

The light dependent reaction

  1. Needs light energy, takes place in the thylakoid membranes

  2. Light energy is absorbed by chlorophyll in the photosystems. The light energy excites the electrons in the chlorophyll, leading to their eventual release from the molecule

  3. So the chlorophyll has been photoionised

  4. Some of the energy from the released electrons is used to add a phosphate group to ADP to form ATP. Some is used to reduce NADP to form reduced NADP/ NADPH

  5. ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction

  6. During the process H2O is oxidised to O2

The light independent reaction (Calvin cycle)

  1. Doesn’t use light energy, but it does rely on the products of the light dependent reaction

  2. Takes place in the stroma

  3. The ATP and reduced NADP from the light independent reaction supply the energy from hydrogen to make simple sugars from CO2

LDR- photo phosphorylation

— the energy from photoionisation of chlorophyll is used for —

  • making ATP from ADP and inorganic phosphate (photophosphorylation)

  • Making reduced NADP from NADP

  • Splitting water into protons (H+ ions), electrons and oxygen (photolysis)

Non cyclic photophosphorylation

— photo systems in the thylakoid membranes are linked by electron carriers, these are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain, a chain of proteins through which excited electrons flow —

  1. Photoionisation

  • light energy is absorbed by PSII

  • The chlorophyll absorbs light energy, this excites electrons

  • The electrons move to a higher energy level

  • These high energy electrons are released from the chlorophyll and move down the electron transport chain to PSI

  1. Photolysis

  • Electrons that leave the chlorophyll in PSII must be replaced

  • So light energy splits water into protons, electrons and oxygen (photolysis)

  1. makes ATP (chemiosmosis)

  • The excited electrons move along an electron transfer chain by electron carriers, releasing energy

  • This energy is used to actively transport protons from the stroma into the thylakoid

  • This creates an electrochemical gradient

  • So protons move by facilitated diffusion via ATP synthase (which is embedded in thylakoid membrane), from the thylakoid into the stroma down its electrochemical gradient

  • The energy from this movement joins ADP+Pi to make ATP (photophosphoylation)

  1. Generates reduced NADP

  • light energy is absorbed by PSI, which excites the electrons again to an even higher energy level

  • The electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP

Cyclic photophosphorylation only produces ATP

  • only uses PSI

  • The electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI by electron carriers

  • This means the electrons are recycled and can repeatedly flow through PSI

  • This process doesn’t produce any reduced NADP or O2, it only produces small amounts of ATP

Light independent reaction (Calvin cycle)

  1. Carbon dioxide is combined with RuBP to form 2 molecules of glycerate 3-phosphate

  • CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast

  • Here the 1 carbon is combined with RuBP (ribulose biphosphate), a 5-carbon compound

  • This gives an unstable 6-carbon compound, which quickly breaks down into 2 molecules of a 3-carbon compound, called glycerate 3-phosphate

  1. ATP and reduced NADP are required to reduce GP to TP

  • the hydrolysis of ATP (from the LDR) provides energy to turn the 3-carbon compound (GP) into a different 3-carbon compound called triose phosphate (TP)

  • This reaction requires H+ ions which come from reduced NADP (from the LDR), reduced NADP is recycled to NADP

  • Some TP is then converted into useful organic compounds (e.g glucose), some continues in the Calvin cycle to regenerate RuBP

  1. RuBP is regenerated

  • 5 out of 6 TP molecule are used to regenerate RuBP, 1 is used to make hexose sugars

  • Regenerates RuBP uses the rest of the ATP produced by the light-dependent reaction

TP and GP are converted into useful organic substances (e.g glucose)

  • used to make carbohydrates, lipids and amino acids

  • Carbohydrates - hexose sugars (e.g glucose), are made by joining two TP molecules together. Larger carbohydrates (sucrose, starch, cellulose) are made by joining hexose sugars together in different ways

  • Lipids - made by using glycerol, which is synthesised from TP. And fatty acids, which are synthesised from glycerate 3-phosphate (GP)

  • Amino acids - some amino acids are made from glycerate 3-phosphate (GP)

Calvin cycle needs to turn 6 times to make one hexose sugar

  • 3 turns of the cycle produces 6 molecules of TP, because two TP are made from every 1 CO2 molecule used

  • 5 out of 6 are used to regenerate RuBP

  • So only one is used to make a hexose sugar

  • A hexose sugar has six carbons, so 2 TP molecules are needed to form one hexose sugar (TP = triose → 3, phosphate)

  • So the cycle must turn 6x to produce 2 molecules of TP, to make one hexose sugar

  • This requires 18 ATP and 12 reduced NADP from the LDR

Limiting factors in photosynthesis

  1. Light intensity

  • light is needed to provide the energy for the LDR, the higher the intensity, the more energy it provides

  • Only certain wavelengths of light are used for photosynthesis (green light is reflected which is why plants look green)

  1. Temperature

  • low temperature means enzymes become inactive, but if the temp is too high then enzymes may denature

  • At high temperature stomata will close to avoid losing too much water, causing photosynthesis to slow down because less CO2 enters the leaf

  1. Carbon dioxide

  • carbon dioxide makes up 0.04% of the gases in the atmosphere

  • Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata will start to close

— plants also need a constant supply of water. Too little and photosynthesis has to stop, too much and the soil becomes water logged which reduces the uptake of minerals such as magnesium which is needed to make chlorophyll a —

— all level off at some point because something else has become the limiting factor —

Agricultural

  • farmers will try and create an environment where the plant gets the right amount of everything, preventing limiting factors

  • This increases growth and increases yield

  • Carbon is added to their air through burning. Light is provided through lamps. Temperature is kept stable in greenhouses with heaters and cooling systems