BI0004 - Lecture 2 - Autotrophs
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What are the sources of energy for chemotrophy?
- Methane (CH4)
- Sulphate (SO42-)
- Nitrate (NO3-)
The habitats of chemoautotrophs are...
- Extreme - Hot springs, anaerobic lakes, deep oceans, volcanic regions and vents, Larsen Ice Shelf in Antarctica.
- Lacking in organic material - methane instead of carbohydrate.
- Anaerobic - e.g. early life on earth.
Methanotrophic Bacteria (methanophiles) use ... instead of ... for respiration.
Methane instead of carbohydrate
Methanotrophic aerobic respiration has the formula...
... O2 + CH4 --> H2CO (Formaldehyde) + H2O
Methanotrophic anaerobic respiration (When using sulphate as an electron donor) oxidizes methane with the formula...
CH4 + SO42- --> HCO3- + HS- + H2O
Syntrophic association is...
- When one species lives off the products of another species.
- e.g Methanotrpohic archaea and sulphate-reducing bacteria
Methanotrophic anaerobic respiration (when using nitrate as an electron donor) oxidizes methane with the formula...
5CH4 + 8NO3- + 8H+ --> 5CO2- + 4N2 + 14H2O
Sulphate-reducing bacteria reduce sulphate to sulphide with the formula...
6H+ + SO42- --> H2S + 2H2O + O2
Photosynthetic plantae ancestors arose through...
endosymbiosis of cyanobacteria - became plastids.
The "Equation" for photosynthesis is...
- ...water and carbon dioxide produce Carbohydrate and Oxygen
- 6H2O + 6CO2 --light, ATP, NADPH--> C6H12O6 + 6O2
All green parts of a plant have...
The major site of photosynthesis in most plants is...
... in the leaves.
how many chloroplasts per square millimeter of leaf surface?
Leaves are green because of...
What drives the synthesis of organic molecules in the chloroplasts?
Light energy absorbed by chlorophyll
Where are Chloroplasts mainly found?
In the cells of the mesophyll (The tissue in the interior of the leaf)
How does Carbon dioxide enter the leaf, and oxygen exit?
Via microscopic pores called stomata
How many chloroplasts does a typical mesophyll cell contain?
30 to 40
What size are chloroplast organelles?
2-4μm by 4-7μm
What is the Stroma, and what encloses it?
- It is the dense fluid within the chloroplast.
- It is enclosed by an envelope of two membranes.
An elaborate system of interconnected membranous sacs called ... segregates the stroma from another compartment, the ... ....
- The Thylakoid space.
Where do chlorophyll reside?
In the thylakoid membranes
Thylakoid sacs are stacked in columns called ... and are connected by ...
Chloroplasts comprise of several membranes, which are?
- Outer envelope membrane
- Inner envelope membrane
- Thylakoid membranes
The two stages of photosynthesis are known as...
- ...the light reactions
- ...the Calvin cycle
The light reactions are the steps of photosynthesis that...
...convert solar energy to chemical energy.
What happens in the light reactions?
- 1. Water is split, providing a source of electrons and protons and giving off O2 as a by-product.
- 2. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+3. The light reactions use solar power to reduce NADH+ to NADPH by adding a pair of electrons along with an H+
- 4. ATP is also generated by using chemiosmosis to power the addition of a phosphate group to ADP in a process called photophosphorylation.
Initially, in the light reactions, light energy is initially converted to chemical energy in the form of which two compounds?
NADPH and ATP
In the Calvin cycle, what is produced?
What happens in the Calvin cycle?
- 1. CO2 from the air is incorporated into organic molecules already present in the chloroplast - called carbon fixation
- 2. The carbon is reduced to carbohydrate by the addition of electrons - reducing power from NADPH and energy from ATP.
Why is the Calvin cycle referred to as the dark reactions?
None of the steps require light directly
Where to the light reactions occur?
In the thylakloids of the chloroplasts
Where does the Calvin cycle occur?
Within the Stroma
Light is a form of energy known as ...
...electromagnetic energy, also called electromagnetic radiation
Why do we see green when we look at a leaf?
Because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light.
Chlorophyll is made of...? Structurally
- A porphyrin ring: light-absorbing "head" of molecule; magnesium atom at center
- A hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylacoid membranes and chloroplasts.
How to Chlorophyll a and Chlorophyll b differ?
- In the functional groups bonded to the porphyrin ring.
- CH3 in chlorophyll a.
- CHO in chlorophyll b.
What happens when Chlorophyll and other pigments absorb a photon of light?
- one of the molecule's electrons is elevated to an orbital where it has more potential energy.
- From a ground state to an excited state.
Why does each pigment have a unique absorption spectrum
Because particular compounds only absorb photos corresponding to a specific wavelength.
What happens once an electron is raised to the exited state?
It is unstable, so drops back down to the group-state orbital releasing their excess energy as heat
Where are the photosystems?
In the membranes of the thylakoids
What is a photosystem composed of?
A protein complex called a reaction-center complex surrounded by several light-harvesting complexes.
The reaction center complex contains...
- A special pair of chlorophyll a molecules.
- A molecule capable of accepting electrons and becoming reduced called the primary electron acceptor.
What do the light harvesting complexes consist of?
What do these allow the photosystem to do
- Various pigment molecules bound to proteins.
- They enable the photosystem to harvest light over a larger surface and a larger portion of the spectrum than a single pigment molecule alone could.
- They act as an antenna for the reaction center complex.
What happens when a pigment molecule in the light harvesting complexes absorbs a photon?
The energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex until it is passed into the reaction center complex.
What do the special pair of Chlorophyll molecules in the reaction center complex do?
They use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to the primary electron acceptor
What are the two photosystems within the thylakoid membrane called?
Photosystem II and Photosystem I
What is the flow of electrons through the photosystems and other molecular components in the thylakoid membrane called?
Linear Electron Flow
What happens in the light reactions? Detailed.
- 1. Photon strikes pigment molecule in PSII light harvesting complex; Energy is transferred to P680 pair of chlorophyll a molecules.
- 2. Electron from P680 pair is transferred to primary electron acceptor. P680+ formed
- 3. Water split by enzyme - 2 electrons, two H+ ions, O atom. Electrons transferred to P680. O combines and forms O2.
- 4. Photoexcited electrons pass to PSI via electron transport chain - Pq, cytochrome complex, Pc.
- 5. Fall of electrons provides energy for synthesis of ATP. pumping of protons builds a proton gradient used in chemiosmosis.
- 6. The electron is accepted by the P700 pair which donated its electron the the primary accpeptor in PSI like in PSII
- 7. Photoexcited electrons passed down a second electron transport chain from PSI to ferredoxin (Fd)
- 8. Electron transferred from Fd to NADP+. two electrons are required for this
What drives ATP synthesis in the thylakoid membrane?
The thylakoid space has a high concentration of H+, whereas the stroma has a low concentration of H+. This concentration gradient drives the phosphorylation of ADP to ATP when H+ ions pass through ATP synthase.
In the Calvin cycle, Carbon enters as ... and leaves in the form of .... The cycle spends ... as an energy source and consumes ... as a reducing power for adding high-energy electrons to make sugar
How many molecules of CO2 Enter the Calvin cycle?
What happens in the three phases of the Calvin cycle?
- 1. Carbon fixation - Each CO2 molecule is added to a 5C sugar named ribulose biphosphate (RuBP) by an enzyme called rubisco. This 6C intermediate is so unstable that it immediately splits, forming 2 3C molecule sof 3-phosphoglycerate
- 2. Reduction - Each 3-phosphoglycerate receives a phosphate group from ATP and is then reduced by NADPH to gain an electron and lose a phosphate group to form G3P. Produces 6 glyceraldehyde-3-phosphates per 3 CO2. One molecule of G3P leaves the cycle to be used by the plant cell. The other 5 molecules are recycled to regenerate the 3 molecules of RuBP.
- 3. Regeneration of the CO2 acceptor (RuBP) - Carbon skeletons of the five G3P molecules are rearranged into three molecules of RuBP, spending 3 more ATP. Cycle can start again.
After the Calvin cycle, Glyceraldehyde-3-Phosphate is used how?
As the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates
What happens to a plant on a hot day?
The plant closes its stomata to prevent water loss, but this also stops CO2 from entering, thereby reducing photosynthetic yield.
What is photorespiration?
It is when, on hot days when a plant's stomata are closed, rubisco adds O2 to the Calvin cycle instaed of CO2. This consumes ATP and produces no sugar. Left over process from evolution.
Why have C4 photosynthesis and CAM evolved?
To minimize photorespiration and optimize the Calvin cycle - even in hot arid climates
What are C4 plants?
What special features do they have?
What do they do?
They are plants which form a four-carbon compound as its product in the Calvin cycle.
They have bundle sheath cells tightly packed around the veins, and mesophyll cells between the bundle sheath and the leaf surface.
The mesophyll cells essentially shield the bundle sheaths from O2 and pump CO2 into the bundle sheath, keeping the CO2 concentration high enough for rubisco to bind carbon dioxide rather than oxygen.
What are CAM plants?
Plants which take up CO2 during the night, when they won't lose water, and incorporate it into a variety of organic acids. Then during the day the CO2 is released to be used in the Calvin cycle.
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