5.3 Chloroplast Flashcards
What are the similarities between mito and chloro?
primitive DNA produced by endosymbiosis double membrane binary fussion ATP synthase ETC is present in both
What is the difference between mito and chloro in structure?
mito has cristae and chloro does not. instead chloro has third internal membrane aka thylakoid membrane. Chloro also has a stroma (space between membrane and thylakoid space
What is believed about the difference between cristae and thylakoid membrane?
it is likely that the thylakoid membrane represents cristae that have fully separated from ancient inner membrane
What supports the view between the diff between cristae and thylakoid membrane?
thylakoid membrane houses an ETC and ATP synthase, They also have division cycle like mito
Where are chloroplast located in cells? How many?
chloro are found along the perimeter edges of cells so they can have better access to light.
a cell can have about a dozen to hundreds of chloroplast
What is the structure of the thylakoid membrane? Why is it believed to have this structure?
resembles stacks of pancakes connected by narrow membrane tubules.
presumably this config. allows for highly effective capture of light energy w/in stacks
Chloroplast belong to what larger family of organelles?
plastids
What do plastids do?
form chloroplast
form organelles that are specialized for storage of things like oils, fats or starch
form chromoplast which give plant tissue coloration
How does the chloroplast import most of their protein needed for their function?
protein complexes (2) involved in translocating proteins across outer to inner membrane
1) TOC - translocase at the outer chloroplast membrane
2) TIC - translocase at the inner chloroplast membrane
Does chloroplast have an electrochemical gradient across inner membrane?
no - ATP/GTP is used instead
What are the functions of chloroplast?
1) photosynthesis
2) carbon fixation
what is photosynthesis and where does it take place?
takes place on thylakoid membrane
photosyntesis uses energy from light in the light reaction
rxn: catalyzes the removal of electron from water and eventually creates high energy electron carrier NADPH
(the opposite of what happens in mitochondria)
What is carbon fixation aka the calvin cycle aka dark rxn and where does it take place?
rxn performed in stroma
rxn involves the conversion of CO2 to organic carbohydrates
this rxn requires energy from ATP and NADPH. It doesn’t use light energy and this is why it’s called the dark rxn
How are 6 molecules of glyceraldehyde 3 phosphate formed in the calvin cycle?
addition of ATP to 6 (3-phosphoglycerate)
After 6 molecules of glyceraldehyde 3 phosphate is produced one of them leave the cycle. why is this important?
this molecule is used downstream process that produces cells of the organic molecules in the worlds. Sugars, fatty acids, and amino acids
What happens to the other 5 molecules of glyceraldehyde 3-phosphate?
they undergo further modification that requires energy and result in the formation of 3 molecules of the 5carbon containing sugar ribulose 1,5 bisphosphate
The calvin cycle aka dark rxn produces how many molecules of ribulose 1,5 bisphosphate?
3 molecules of ribulose 1,5-bisphosphate in the 1st step of the calvin cycle
then it produces 3 molecules of ribulose 1,5 bisphosphate at the end
what is significat about this is the cycle does not consume any of these molecules for energy. only energy in the form of ATP and NADPH
How many ATP molecules and NADPH molecules are needed for a complete turn on the calvin cycle?
9 ATP molecules
6 NADPH molecules
What is the initial rxn in the calvin cycle?
CO2 is added to 5carbon molecule ribulose 1,5-bisphosphate, which results in direct formation of six molecules 3 phosphoglycerate (3 carbons)
Plant that perform the first rxn in the calvin cycle are called what? Why?
known as 3C plants b/c the carbon dioxide molecule directly contributes to the formation of molecules containing three carbons
What is the initial reaction in the calvin cycle catalyzed by?
ca560kDa enzyne complex (ribulose bis-phosphate carboxylase/oxygenase)
AKA RUBISCO
How does RUBISCO enzyme act under normal circumstances?
as a carboxylase - it adds carbon dioxide to ribulose bisphosphate as in rxn 1 of calvin cycle
why do plants/ calvin cycle not use RUBISCO again in the cycle?
the last part of the name RUBISCO “oxygenase” implies that under some circumstances the enzyme will add oxygen to ribulose bisphosphate . the result molecule cannot be used futher by the calvin cycle
this is harmful to plants which then have to create more ribulose bisphosphate through other mechanisms for use in the calvin cycle
Dark rxns occur where in chloroplast?
thykaloid STROMA
why is the dark rxn know as dark rxn?
it does not need light to proceed
What is the dark rxn catalyzed by?
giant and really slow enzyme RUBISCO
Under normal conditions what happens in the 1st reaction of the dark rxn or calvin cycle?
takes 3CO2 and 3 sugar molecules (ribulose 1,5-bisphosphate
requires 9 ATP and 6 NADPH
generates a 3 carbon molecule (glyceraldehyde 3 phosphate) and regenerates 3 sugar molecules (ribulose 1,5-bisphospahte)
What happens when CO2 is low in the initial rxn of the calvin cycle?
RUBISCO oxidizes (destroys) ribulose 1,5-bisphosphate
what are the pores in leaves of plants made of? What is their purpose?
made from guard cells (2 of them)
the pores take up co2 from the air and are called stoma
What happens to guard cells when leaves are hydrated?
- they swell up w/water
- this opens the pore and allows air to enter and leave the leaves easily
What happens to guard cells under dry conditions?
guard cells wilt and collapse which closes the stoma/pore in an effort to minimize evaporation of water from leaves
What happens to calvin cycle when guard cells collapes/wild under dry conditions?
atmosphere w/in leaf is rapidly depleted of CO2 since CO2 makes a msall percentage of air (about 0.03%). in contrast air is 21% oxygen and so plants w/this organization of leaves is going to have oxygen added by RUBISCO to ribulose 1,5-bisphosphate which will destroy it for the calvin cycle
Do C3 plants high high drought tolerance?
no they don’t. they grow inefficeintly under dry conditions b/c CO2 levels cause RUBISCO to add oxygen to ribulose 1,5-bisphosphate, rendering useless in the calvin cycle
ex of 3C plants: wheat and rice
What type of plant develop a strategy for dealing w/problems created by RUBISCO? How do they deal w/it?
C4 plants like corn and sugar cane
-C4 plants compartmentalized carbon fixation to enhance carbohydrate synthesis at low co2 concentrations
–calvin cycle is restricted to vascular bundle sheaths and these sheaths are protected by mesophyll cells
What do mesophyll cells do?
they surround cells that conduct the calvin cycle in C4 plants, aka vascular bundle sheaths. They also use light energy to add CO2 to malate (a 4 carbon molecule). Malate is transferred to bundle sheath cells and it is used to generate CO2 (to use for calvin). Does not use RUBISCO
What is the purpose of vascular bundles in C4 plants?
allows plant to transfer carbohydrate molecules to use for the calvin cycle (check this answer)
Why do plants that use malate (C4 plants) to generate CO2 for calvin cycle known as C4 plants?
b/c malate has 4 carbons and these plants are much more drought resistant than C3 plant
Why does closure of stoma cause loss of CO2 but not O2?
b/c O2 is so much more abundant in air than CO2
What happens in C3 plants when CO2 is unavailable?
They use RUBISCO to oxidizes ribulose 1,5-bisphosphate rendering it useless for calvin cycle.
Why are C4 plants more drought resistant than C3 plants?
b//c they find a way to deal w/RUBISCO which includes:
1) compartmentalization of calvin cycle
2) mesophyll cells to generate 4 carbon malate which is transported to vascular bundle sheath cells
3) bundle sheath cells break down malate to release CO2
What is the light rxn?
process plants use to absorb energy from photons and convert it to chemical energy that is more useful for plants.
What molecule catalyses the initial conversion of light into useful chemical energy?
chlorophyll
What type of molecule is chlorophyll and why is this important?
- Fluorescent molecule
- Fluorescent molecules are compounds that can absorb the energy of light and store it in the form of excited electrons for some period of time.
-energy given off from fluorescent molecules may be in the form of a photon of light and can be transferred to a chemical.
What is important about the structure of chlorophyll?
- the heme ring, which is the portion of the molecule that absorbs light energy in the form of an excited electron.
- chlorophyll also has a manganese ion in the center which is going to be used as an electron carrier
- it also has a hydrophobic tail which embeds molecule in thylakoid membrane
What is forced to resonance energy transfer (FRET)?
- vibration of excited electrons can repel an electron in a nearby molecule and transfer some of its energy to the neighboring molecule
- light energy is captured in plants in a structure called antenna complex and that energy is transferred towards the center of the antenna complex using FRET
Apart from FRET, what can happen to excited electrons?
- energy of electrons can be large enough to cause electrons to physically move from one molecule to another, Therefore the electron itself is moving not just the energy.
- loss of electrons is called oxidation
What happens to a molecule when it is oxidized?
=leaves molecule w/a net positive charge, which in turn can remove an electron from a neighboring molecule
In plants, how is a loss of an electron replaced?
-from water, which is what splits the water in the chemical rxn, 2H20 = 4H+ + 4e- + O2
This reaction produces hydrogen and oxygen
What is the role of the antenna complex? How does the antenna complex accomplish this ?
to funnel the energy of photons into the center of the complex using FRET
What is special about the center of the antenna complex?
it splits water. and this is where the photochemical rxn center.
-there is a unique pair of chlorophyll molecules that receive all of this energy and they use the energy to physically move an electron from one chlorophyll molecule to another and then to a difuusable electron carrier called quinone.
Once electrons make it to quinone, there is a pause in the photochem rxn, why?
the rxn wants to receive an electron from a weak electron donor aka water
what is the state of the center during the pauase?
overall electrically neutral but it is polarized w/a positive charge on the chlorophyl molecule and a negative charge on the quinone
Why don’t electrons move backward along photochemical rxn chain?
b/c quinone is going to hold onto its electron more tightly than phophytin. Quinone has greater affinity for electrons and electron affinity increases as it moves through the process.
After H20 splits and chlorophyll gains an electron, what happens to the state of the complex?
generates a total net negative charge.
-at this point the electron bound to fixed quinone is transferred to a moveable quinone in the thylakoid membrane. this returns the whole complex back to an electrically neutral state ready to start the process over again.
The electron transport chain in chloro has how many antenna complex? why?
TWO. to separate an electron from water and then boost the energy of that electron to a high enough level to add to NADP+ which produces high energy molecule NADPH
What is the name of the first photosystem in ETC? What does it do?
PHOTOSYSTEM II. this is a proton pump and it transfers an electron via a diffusible quinone to plastoquinone and then to an intermediate complex called the cytochrome b6-f complex, which is also a proton pump.
What is the pH gradient created by photosystem II and cytochrome b6-f used for?
to create ATP by an ATP synthase
After the electron moves from cytochrome b6-f complex to diffusible carrier plastocyanin, it enters a second photosystem. What is the name of the second photosystem and what happens here
Photosystem I.
an additional photon of light energy is absorbed and added to the electron. the electron then moves to a final carrier called ferredoxin then to the last complex called ferredoxin-NADP reductase
What happens to the electron at ferredoxin-NADP reductase complex?
electron is added to NADP+ to produce high energy NADPH
What are the carriers and complexes involved in photosynthesis (name them in order)
Antenna complex Photosystem II (proton pump) Quinone (carrier) plastoquinone (carrier) cytochrome b6-f (proton pump) plastocyanin (e carrier) Photosystem I ferredoxin ferredoxin-NADP reductase complex
In photosynthesis electron in water are at what energy level?
lower energy levels
What happens to electron energy as it moves from photosystem to diffusible electron carriers and intermediates and back to photosystem I?
high energy at photosytem but then electron travels downward losing energy as it travels through carriers and intermediates.
Why is there two photosystems?
the energy gained by the first photosystem is not enough to allow the electron to move to ferrodoxin
-2 systems needed to complete the whole process.
How did modern photosystem II and I evolve?
evolved from more primitive systems that used something other than water as the initial electron donor. Like purple bacteria only have one system and the electron donor is hydrogen sulfide
Why can purple plants have one photosystem and use hydrogen sulfide as their donor?
b/c electron in hydrogen sulfide are already at higher energy levels than those found in water so not much additional energy is required to get the electron to add to NADP+
How do clorophyle molecules in antenna complex transfer light energy to photochem rxn center?
by resonance energy transfer
How is a proton moved across the thylakoid membrane?
by cytochrome b6 complex
