Module 5: Energy cycles and photosynthesis Flashcards
What is photosynthesis?
Light energy is used to split water in a process called photolysis in order to produce a fuel in the form of glucose.
Write the equation for photolysis.
H2O = 2H+ + 2e- + 1/2O2
The oxygen-evolving complex which forms part of PSII is the enzyme that catalyses the breakdown of water.
Describe the structure of a chloroplast.
Contains a stack of thylakoid membranes called grana which contains chlorophyll arranged as photosystems.
It contains stroma which is the fluid surrounding grana.
Adaptations of the chloroplast for photosynthesis?
Thylakoids have a large SA for photosynthetic pigments, electron carriers and ATP synthase.
Stroma contains all enzymes needed for the light-independent reaction.
What is the significance of the photolysis of water in photosynthesis?
It produces oxygen and provides electrons and protons for the light-dependent reactions.
What are the two main stages of photosynthesis?
Light-dependent reactions and the Calvin cycle (light-independent reactions).
What is the role of NADP in photosynthesis?
It acts as an electron carrier, becoming NADPH.
Which of the following is NOT a product of the light-dependent reactions? A) ATP B) NADPH C) Glucose D) Oxygen
C) Glucose.
Which of the following is an end product of the Calvin cycle? A) ATP B) Glucose C) NADPH D) Oxygen
B) Glucose.
What is the role of the electron transport chain in photosynthesis?
It transfers electrons and pumps protons to create a proton gradient used for ATP synthesis.
What is the significance of the proton gradient created during the light-dependent reactions?
It drives ATP synthesis via ATP synthase.
What is the purpose of the light dependent reaction?
It uses light energy to make ATP and the electron carrier reduced NADP.
Where does the light dependent reaction take place?
In plants, in the thylakoid membranes.
What are photosystems?
Large complexes of proteins and pigments that absorb light and transfer the energy onto electrons. They are found in the thylakoid. The pigments include chlorophyll a (primary pigment), chlorophyll b (accessory pigment) and carotene.
Why do plants appear green?
Neither chlorophyll a or b can absorb green light, so reflect it.
What are the 2 types of photosystem?
Photosystem I (PSI) - the primary pigment absorbs light at a wavelength of 700nm. It is present in the middle of the electron transport chain. Contains carotenoids.
Photosystem II (PSII) - primary pigment absorbs light at a wavelength of 680nm. It is present at the beginning of the electron transport chain and is where the photolysis of water takes place. Contains xanothophyll.
It captures light and transfers excited electrons to the ETC.
Outline the role of photosynthetic pigments in photosynthesis.
Pigments absorb and reflect different colours and absorb light. They transfer this light energy to the reaction centre.
Chlorophyll a is located in the reaction centre, which is where reactions involve in photosynthesis take place. Primary pigments lose electrons and these electrons are excited to a higher energy level. These are passed to the ETC for photophosphorylation/light dependent.
Accessory pigments pass energy to the reaction centres/primary pigments.
Carotenoids - accessory pigments that capture light energy and prevent damage to chlorophyll, by dissipating excess light as heat. carotenoids absorb light in blue and green regions, extending the range of light for pht. 2 types:
=> carotene (orange)
=> xanthophyll (yellow)
Define non-cyclic photophosphorylation.
Aka light dependent reaction. Photophosphorylation is the overall process of using light energy and electron transport chain to phosphorylate ADP to ATP.
This process produces ATP and hydrogen from water is used to reduce coenzyme NADP to reduced NADP.
What happens in the light dependent reaction/non-cyclic photophosphorylation? watch video at some point
- Light absorption in PSII. Light passes into pigment until it reaches the reaction centre. Within PSII reaction centre, light excites electrons and causes them to move to a higher energy state (P680). The electrons ate passed onto a series of electron carriers within the electron transport chain to PSI.
- Photolysis of water - light energy causes a water molecule to split apart and release H+ ions, electrons and oxygen. The electrons from water replace the electrons lost from PSII.
- The high energy electron travels down an electron transport chain, losing energy as it goes. Energy lost by electrons is used to pump H+ ions from stroma into the thylakoid. This generates a proton gradient across the thylakoid membrane.
- H+/protons flow down their concentration gradient through the lumen of thylakoids and ATP synthase. The energy from the movement of protons is used to phosphorylate ADP to ATP (photophosphorylation) in a process called chemiosmosis.
- Light is absorbed by PSI. The electron arrives here and joins the P700 special pair of chlorophylls in the reaction centre. An electron becomes excited again and is passed along the rest of the electron transport chain. The special pair’s missing electron is replaced by a new electron from PSII (arriving via the electron transport chain). ATP is produced again by chemiosmosis.
- The electron from the electron transport chain in PSI and H+ ions that have then gone from thylakoid to stroma is passed onto NADP to form reduced NADP (NADPH). NADPH is an electron carrier which transfers electrons from one molecule to another. NADPH also provides the hydrogen for the production of organic molecules in the Calvin cycle.
The energy carried by the ATP is then used during the light-independent reactions of photosynthesis.
Where does the light-independent reaction take place?
The Calvin cycle takes place in the stroma of the chloroplast and uses the products of light dependent (ATP and reduced NADP) to form glucose and other organic molecules.
The Calvin cycle involves photosystem 1 only (PSI)
What happens in the light-independent reaction/cyclic photophosphorylation/Calvin cycle?
CARBON FIXATION => 5C ribulose bisphosphate (RuBP) is combined with CO2 to form a 6-carbon molecule. This reaction is catalysed by the enzyme Rubisco.
The 6C molecule is unstable and immediately breaks down to form two 3C compounds called glycerate-3-phosphate (GP).
REDUCTION => Isomerisation occurs where GP is converted to a different 3C compound called triose phosphate (TP). This reaction requires energy so ATP from light dependent reaction is hydrolysed to ADP (to provide this energy).This reaction also needs electrons from reduced NADP (electron carrier). Reduced NADP transfers electrons to GP, reducing it to triose phosphate. (NADPH = NADP)
Some TP molecules are used to make glucose some to regenerate RuBP. 1 out of 6 TP molecules is used to produce glucose whereas 5 will be used for RuBP regeneration.
REGENERATION => TP is converted to RuBP. This process requires energy which is generated through ATP hydrolysis. The cycle is complete.
Describe the limiting factors of photosynthesis.
Light intensity – if this is in short supply the light dependent reaction will slow therefore
there will be lower amounts of ATP and NADPH created. This will then affect the
Calvin cycle as these are needed to convert GP to TP. So the level of GP will rise and TP
will fall which in turn causes RuBP levels to fall.
* CO2 concentration – if this is in short supply the light independent reaction will slow
* Temperature – if this is low Rubisco and other molecules will have lower levels of
kinetic energy therefore the enzyme-controlled reactions are affected.
How to accurately measure O2 formed?
Oxygen sensor that records O2 conc to reduce human error. If you connect the oxygen sensor to a data logger, a computer would record the results and produce a graph.
