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Photosynthesis Notes

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baird swedman

on 21 October 2015

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Transcript of Photosynthesis Notes

What is Photosynthesis?
Calvin Cycle
Leaf Structure
Light is electromagnetic energy.
Electromagnetic energy travels in waves.
Wavelenth is the distance between crests of waves such as those in the electromagnetic spectrum.
They range from less than a nanometer to more than a kilometer. This is the entire range of radiation known as the electromagnetic spectrum.
Visible light is from 380-750 nm.
Visible light is the radiation that drives photosynthesis.
A photon is a quantum, or discrete quantity, of light energy that behaves as if it were a particle.
1. Carbon Fixation- 3 CO2 molecules are attached to 3 molecules of RuBP. Reactions are catalyzed by rubisco and produce an unstable product that splits into 2 3-phosphoglycerates.

2. Reduction- Each molecule of 3-phosphoglycerate receives an additional phosphate group forming 1, 3-bisphosphoglycerate. This is reduce to G3P. 1 G3P leaves the cycle to be used in a plant cell.
3. Regeneration of the CO2 acceptor (RuBP)- RuBP is regenerated as the 5 G3Ps are reworked into 3 of the starting molecules using 3 ATP molecules.
Light Reactions
Light Reactions

Light Reactions
Occur in Thylakoid
Convert Solar to Chemical Energy
Release Oxygen
Produce NO Sugar (Glucose)
The conversion of light energy to chemical energy that is stored in sugars or other organic compounds
The Equation
Plants take in carbon dioxide, water, and light energy to produce glucose, oxygen, and water.
Leaves are made up of a flattened blade and a stalk called the petiole, which joins the leaf to a stem at a node.
Major sites of photosynthesis in most plants.
Leaf Anatomy
- protection
- prevents water evaporation
ta- allows CO2 or O2 to enter and leave the cells on the bottom side of a leaf to reduce water loss
Guard Cells
- regulate the opening and closing of the stomata
have two membranes.
- dense fluid inside the membranes
Suspended within the stroma are sacs called
. These convert light energy to chemical energy of ATP and NADPH.
When they are stacked together, they are called
- green pigment that gives the leaves their color, resides in a thylakoid membranes.
are groups of accessory pigments.
Structure responsible for carrying out photosynthesis.
Found mainly in the cells of the mesophyll.
Typical mesophyll cell has 30-40 chloroplasts.
Chloroplasts - chemical factories powered by the sun.
Chlorophyll in the thylakoid membranes of the chloroplasts capture the sun's energy.
Fate of Reactants
Fate of Products
C6H12O6- plant uses it for food and energy
6 H20- evaporates so that CO2 can enter
6 O2- is released through the stoma
Occurs in:
Certain Prokaryotes
Conversion of Light
Photosystem- A light-capturing unit located in a thykaloid membrane of the chloroplast consisting of a reaction-center complex surrounded by numerous light-harvesting complexes.
They absorb light best at different wavelengths.
They convert solar energy to chemical energy.
Solar Energy- radiant energy emitted by the sun
Chemical Energy- energy available in molecules for release in a chemical reaction; a form of potential energy
Reaction-Center Complex
A complex of proteins associated with chlorophyll a and a primary electron acceptor.
It is located centrally in a photosystem.
It triggers the light reactions of photosynthesis.
It gets excited by light energy. The pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.
Light-Harvesting Complex
A complex of proteins associated with pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reactions that are pigments in a photosystem.
Photosystem 1
A light-capturing unit located in a thykaloid membrane of the chloroplast.
It has 2 molecules of P700 chlorophyll a at it's reaction center.
Photosystem 2
A light-capturing unit located in a thykaloid membrane of the chloroplast.
It has 2 molecules of P680 chlorophyll a at it's reaction center.
Chlorophyll a
A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.
Chlorophyll b
An accessory photosynthetic pigment that transfers energy to chlorophyll a.
The oxygen in sugar comes from the CO2.
The oxygen produced by photosynthesis comes from the water.
The Endosymbiont Theory
States that an early ancestor of eukaryotic cells engulf an oxygen-using nonphotosysnthetic prokaryotic cell.
Eventually the engulfed cell formed a relationship with the host cell, in which it was enclosed becoming endosymbiont.
This is proven because chloroplasts have their own DNA, ribosomes, and they can grow independently.
The two stages of photosynthesis are known as the (1) Light reactions (the photo part) and the (2) Calvin cycle (the synthesis part).
The Nature of Sunlight
The amount of energy is inversely related to the wavelength of light.
The shorter the wavelength, the greater the energy.
A photon of blue light has more energy than a photon of orange light.
Photosynthetic Pigments
When light meets matter, it can be reflected, transmitted, or absorbed.
Substances that absorb light are pigments.
Different pigments absorb different wavelengths. That ability to absorb is measured with a spectrophotometer.
Light can perform work in chloroplasts only if absorbed.
Carotenoid Functions
1. Broaden the spectrum of colors that drive photosynthesis.
2. Photoprotection
Violet blue and red light work best for photosynthesis since they are absorbed.
Green is the least effective.
Chlorophyll absorbs blue best.
Chlorophyll reflects green best.
At the end of PSII, the electrons move down an electron transport chain to replace the electrons from PSI.
NADP+ is an electron acceptor.
It stands for nicotinamide adenine dinuclotide phosphate.
It changes into NADPH.
It is used for temporary storage of hydrogen ions and electrons from water.
Light absorbed by chlorophyll drives a transfer of electrons and ions.
Linear electron flow during light reactions uses both photosystems and produces NADPH, ATP, and Oxygen.
During chemiosmosis in chloroplasts, electron transport chains generate an H+ gradient across the membrane. ATP synthase uses this proton motive force to make ATP.
Light-independent (Dark) reactions
-chemical reactions that convert carbon dioxide and other compounds into glucose. They take place in the stroma.
Cyclic photophosphorylation occurs less commonly in plants than noncyclic photophosphorylation, most likely occurring when there is too little NADP+ available.
Cyclic photophosphorylation involves only Photosystem I and generates ATP but not NADPH or O2.
Noncyclic photophosphorylation is a 2 stage process involving 2 different chlorophyll photosystems.
Being a light reaction, noncyclic photophosphorylation occurs on thylakoid membranes inside chloroplasts.
The Calvin cycle is an anabolic process that builds up carbohydrates and consumes energy.
It occurs in the stroma, where carbon enters in the form of CO2 and leaves in the form of sugar.
It spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar.
It is also known as C3 photosynthesis because the Calvin cycle begins with 3 carbons.
Primary Events in Light Reactions
1. Light is absorbed by chlorophyll

2. Water is split and Oxygen is released.

3. ATP (Energy) is generated

Other Spectrums
A graph plotting a pigment's light absorption vs wavelength is an absorption spectrum.
The absorption spectrum of chlorophyll provides clues to the effectiveness of different wavelengths for driving photosynthesis. This is confirmed by an action spectrum.
An action spectrum for photosynthesis graphs the effectiveness of different wavelengths of light in driving the process of photosynthesis.
Linear (Noncyclic) Electron Flow
The key to the light reactions is a flow of electrons through the phtosystems in the thylakoid membrane, a process called linear (noncyclic) electron flow.
An alternative to linear electron flow is cyclic electron flow.
It uses a short circuit of linear electron flow by cycling the excited electrons back to their original starting point in PSI.
It produces ATP by chemiosmosis.
Cyclic Electron Flow
Ribulose bisphosphate carboxylase (RuBisCO) is the most abundant and most important enzyme on planet earth.
It catalyzes the reaction between RuBP with carbon dioxide.
Reactants: CO2, ATP, NADPH (The last 2 come from light reactions)
Products: ADP, NADP+, Phosphate, G3P, Glucose
A respiratory process by which they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.
Photorespiration can occur when carbon dioxide levels are low.
When the stomata are closed to prevent water loss during drought.
In most plants, photorespiration increases as temperature increases.
Photorespiration produces no ATP and leads to a net loss of carbon and nitrogen, slowing plant growth.
Elements in Chlorophyll
The water in the noncyclic photophoshorylation is split to obtain electrons.
Cyclic Electron Flow
An energy coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work.
The H+ ions pumped to the thylakoid space, diffuse down the ATP synthase BACK to the stroma.
ATP is generated through this cycle.
3 Phases
Alternate modes of carbon fixation have evolved that optimize the Calvin cycle.
The 2 most important photosynthetic adaptations:
C4 photosynthesis
CAM (crassulacean acid metabolism)
C4 Plants
A plant in which the Calvin cycle is preceded by reactions that incorporate CO2 into a 4-carbon compound, the end product of which supplies CO2 for the Calvin cycle.
This is found in sugarcane, corn, and members of the grass family.
It differs from C3 because it starts with 4 carbons instead of 3.
CAM Plants
A plant that uses CAM, an adaptation for photosynthesis in arid conditions. In this process, CO2 entering open stoma during the night is converted to organic acids, which release CO2 for the Calvin cycle during day, when stoma are closed.
This is found in water storing plants like cacti and pineapples.
It differs from C3 because in C3 carbon fixation is done RuBisCo and in CAM carbon fixation is done by CAM.
In CAM plants, the 2 stages of photosynthesis are separated temporally.
Palisade Mesophyll- photosynthesizing cells
Spongy Mesophyll- photosynthesizing cells
Vascular Bundle- transport of nutrients throughout the cells
Xylem- transports water
Phloem- transports glucose
END of Part 1
Photosynthesis Notes
Part 2
Photosynthesis Notes Part 3
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