5.2.1 - Photosynthesis Flashcards
What is the structure of a chloroplast?
- Thylakoid membranes - Folded membranes which contain photosynthetic proteins (chlorophyll) and electron carrier proteins
- Granum - Stacks of thylakoids
- Stroma - Fluid centre which contains enzymes
- Inner and outer membrane - Control what can enter and leave the organelle
What are the 5 types of photosynthetic pigments?
- Chlorophyll a (blue/green)
- Chlorophyll b (yellow/green)
- Caratenoids (orange)
- Xanthophylls (yellow)
- Phaeophytins (grey)
Why is it an advantage to have different photosynthetic pigments?
Each of the different photosynthetic pigments absorbs a different wavelength of light which maximises the spectrum of visible light absorbed and therefore increases the amount of light energy absorbed.
What is a light harvesting system?
Chlorophyll b, xanthophylls, phaeophytins and carotenoids are all embedded within the thylakoid membrane and form the light harvesting system. This is where light energy of different wavelengths is absorbed and this energy is then transferred to the reaction centre.
What is the reaction centre?
It contains chlorophyll a and is where light dependent reactions occur
What makes up a photosystem?
- Light harvesting system
- Reaction centre
Where does the light-dependent stage take place?
Thylakoid membrane
What are the 2 stages within the light-dependent reaction?
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
What happens during non-cyclic photophosphorylation?
- Chlorophyll in photosystem 2 absorbs light (700nm)
- The light energy is transferred to a pair of electrons within chlorophyll leaving them in an excited state
- The electrons leave chlorophyll and this entire process is known as photoionisation
- To replace these electrons light splits water into electrons, protons and oxygen and this reaction is called photolysis
- The pair of electrons move through a collection of protein complexes and molecules known as the electron transfer chain
- These oxidation-reduction reactions release energy which is transferred to a protein complex allowing it to actively transport protons and maintain a proton gradient between the stroma and thylakoid space
- The electrons at the end of the chain react with NADP and a proton forming NADPH
- As a result of the proton gradient protons in the thylakoid membrane diffuse back into the stroma via ATP synthase which is known as chemiosmosis and supplies ATP synthase with energy that it uses to catalyse the production of ATP
What happens during cyclic photophosphorylation?
- Chlorophyll in photosystem 1 absorbs light (680nm)
- This light energy is transferred to an electron causing it to become excited and leave the chlorophyll
- When it reaches the end of the electron transfer chain it is recycled and passes back through the electron transfer chain to photosystem 1 which provides energy for the active transport of protons meaning ATP can still be synthesised
Where does the light independent stage take place?
Stroma
What 3 molecules are used in the light independent reaction?
- ATP
- NADPH
- CO2
What happens during the light independent stage?
- Carbon dioxide reacts with ribulose bisphosphate (RuBP) forming 2 molecules of glycerate 3-phosphate (GP)
- In this reaction carbon dioxide is said to be fixed in a molecule of glycerate 3-phosphate and this fixation is catalysed by an enzyme of rubisco
- Each glycerate 3-phosphate molecule is converted into a molecule of triose phosphate
- This is a reduction reaction where the hydrogen is supplied by NADPH and it requires ATP
- Around 80% of the triose phosphate produced is converted to ribulose bisphosphate ensuring the cycle can continue and this reaction also requires ATP
- The remaining 20% produce a range of organic substances such as glucose, amino acids and lipids
- After these reactions NADP, ADP and Pi return to the thylakoid membrane to take part in the light-dependent reaction
What are the 3 factors limiting the rate of photosynthesis?
- Light intensity
- CO2 concentration
- Temperature
How does light intensity affect the rate of photosynthesis?
- As light intensity increases the rate of the light dependent reaction increases
- This increases the rate at which ATP and NADPH are produced which in turn increases the rate of the light independent reaction
