Lecture 1: Chloroplasts Flashcards
(44 cards)
Which organisms have chloroplasts in their cells?
Plants and algae
How is it thought that chloroplasts became part of cells?
It is thought that chloroplasts were originally bacterial which gained a symbiotic relationship with cells. Eventually, the chloroplasts transferred most of its genes into the nuclear DNA of the cell and its own DNA now only contains a few of the genes.
What is a plastid organelle?
A small organelle with two membranes and its own small genome commonly found in plants and algae.
Give the permeability of the inner and outer membranes of the chloroplast?
The outer membrane is impermeable and the inner membrane is permeable.
What is the consequence of having a large surface area of folded membrane?
There are many enzymes present on/in the membrane.
Which is smaller: a chloroplast or a mitochondria?
A mitochondria is smaller than a chloroplast
Give 3 characteristics about chloroplasts and mitochondria which are different from each other, e.g. organism the organelle is in.
- Bacterial origin
- ATPase orientation
- Enzyme series
Why are starch granules in chloroplasts an important area of research?
If more starch can be packed into chloroplasts, plants would have more calories, which would help to prevent starvation.
Packing lots of calories into a small amount of food is very useful for space travel.
Describe the main function of chloroplasts.
The chloroplast’s main function is to use energy from the sun to fix carbon from CO2 during photosynthesis to build up complex macromolecules.
Give 3 enzymes/enzyme sets required on the thylakoid membranes for photosynthesis to occur.
- ETC
- Photosynthetic complexes (PSI, PSII)
- ATPsynthase
Compare the functions of chloroplasts and mitochondria and how they carry these out.
Both produce high energy electrons and use the energy from these to produce a transmembrane proton motive force. This in turn leads to ATP synthesis.
Chloroplasts get these high energy electrons when a normal electron is excited by a photon of light.
Mitochondria get these high energy electrons from oxidation of glucose.
Describe the process of photosynthesis.
1) Photosystem II (PSII) absorbs light and splits water to generate high energy electrons.
2) An ETC is used to generate a proton motive force using the energy from the high energy electrons. This proton motive force is used to synthesise ATP by ATPase.
3) More light hits Photosystem I (PSI), raising more electrons to an excited (high-energy) state, which reduces NADP+ to NADPH.
4) The NADPH and ATP are used to convert CO2 into carbohydrates. This is carbon fixation.
Steps 1 to 3 occur in the thylakoid membranes and are light-dependent, whereas step 4 occurs in the stroma/cytosol and is light-independent.
What is special about Photosystem II?
It is the only enzyme which splits water.
How energetically favourable is carbon fixation?
Very energetically unfavourable
Where does carbon fixation occur?
In the stroma
Which enzymes is responsible for fixing carbon?
Ribulose biphosphate carboxylase (RuBisCo)
It is the most abundant protein on earth.
How many molecules of ATP and NADPH does each molecule of CO2 use in the Calvin cycle?
3 ATP and 2 NADPH (reducing power)
Give the overall reaction of one turn of the Calvin cycle, i.e. what goes in and what comes out.
3 CO2 + 9 ATP + 6 NADPH + water —> glyceraldehyde-3-phosphate + 8 Pi + 9 ADP + 6 NADP+. The last three are recycled.
Describe the Calvin cycle, naming the all the substrates.
CO2 combines with ribulose-1,5-bisphosphate (5C) to give two molecules of 3-phosphoglycerate (3C). This reaction is catalysed by ribulose bisphosphate carboxylase (RuBisCo). Then each 3-phosphoglycerate is phosphorylated to 1,3-bisphosphoglycerate, with ATP donating the Pi, releasing ADP. Then each of the 1,3-bisphosphoglycerate molecules is converted to glyceraldehyde-3-phosphate, with the release of Pi and the oxidation of NADPH to NADP+. For every 6 molecules of glyceraldehyde-3-phosphate (requires the fixation of 3 CO2 molecules), 1 is used in the synthesis of sugars, fatty acids and amino acids, and 5 are used to regenerate ribulose-1,5-bisphosphate. This occurs in two steps: first Pi is lost, giving ribulose-5-phosphate, which is then converted to ribulose-1,5-bisphosphate with ATP becoming ADP.
What happens to the glyceraldehyde-3-phosphate produced by the Calvin cycle?
Most is exported to the cytosol and converted to sucrose. Some remains in the chloroplast, where it is made into a starch store.
What are the two main components of a photosystem?
An antenna complex and a photochemical reaction centre
Where is a chloroplast are the photosystems?
In the thylakoid membranes
Describe what happens in the antenna complex.
When a photon of light hits a chlorophyll molecule in the antenna complex, an electron in the chlorophyll moves from one molecular orbital to another of higher energy. This energy is passed onto a neighbouring chlorophyll molecule, raising one of its electrons to a higher energy orbital as the electron in the first chlorophyll moves back to its original lower energy orbital. This process is called ‘resonance energy transfer’. The energy is transferred to the photochemical reaction centre, where it moves an electron (from the splitting of water) to a higher-energy orbital.
What is the function of carotenoids in the antenna complex?
They protect chlorophyll from oxidation and collect light from other wavelengths.
