Chapter 13: Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is the trapping of carbon dioxide and its subsequent reduction to carbohydrate, using hydrogen from water. It takes place inside chloroplasts.
What are the two types of photosynthetic pigments?
Primary and accessory pigments
What are photosystems?
The pigments are arranged in light harvesting clusters called photosystems of which there are two types, i and ii.
What is cyclic photophosphorylation?
This involves only photosystem 1. Light is absorbed by photosystem 1 and is passed to the primary pigment. An electron in the chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule. This is called photoactivation. The excited electron is then captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron carriers. During this process enough energy is released to synthesise ATP from ADP and an inorganic phosphate ion by the process of chemiosmosis. The ATP then passes tot he light independent reactions.
What is non-cyclic photophosphorylation?
It involves both the photosystems. Light is absorbed by both photosystems and excited electrons are emitted from the primary pigments of both the reaction centres. These electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystems positively charged. The primary pigment of photosystem 1 absorbs the electrons from photosystem 2. Its primary pigment receives replacement electrons from the photolysis of water. As in cyclic photophosphorylation, ATP is synthesised as the electrons lose energy while passing along the carrier chain.
What is the photolysis of water equation?
H20= 2H+ + 2e + 0.502
What happens with the hydrogen ions produced through the photolysis of water?
The hydrogen ions combine with electrons from photosystem 1 and the carrier molecule NADP to give reduced NADP.
Give an example of an investigation of the Hill reaction.
The Hill reaction can be used to investigate the effect of light intensity or of light wavelength on the rate of photosynthesis of a suspension of chloroplasts. Hill used Fe3+ ions as his acceptor, but various redox agents, such as the blue dye DCPIP(dichlorophenolindophenol) can substitute for the plant’s NADP in this system. DCPIP become colourless when reduced. The H20 will get converted to 0.5O2.
What happens to the reduced NADP produced by the light dependent reaction?
It passes to the light independent reaction to be used in the synthesis of carbohydrates.
What happens in the light independent reactions of photosynthesis?
Carbon dioxide combines with a 5-carbon sugar, ribulose biphosphate (RuBP) to give two molecules of a GP -glycerate 3-phosphate(3C). GP, in the presence of ATP and reduced NADP from the light dependent reaction, is reduced to two molecules of TP-triose phosphate(3C).
What happens to the triose phosphate molecules produced in the light independent reactions?
5/6 of the triose phosphates are used to regenerate RuBP(ribulose biphosphate), but the remainder (1/6) are used to produce other molecules needed by the plant. Some of these triose phosphates condense to become hexose phosphates which are used to produce starch for storage, sucrose for translocation, or cellulose for making cell walls. Others are converted to glycerol and fatty acids to produce lipids for cellular membranes or to acetyl coenzyme A for use in respiration or in the production of amino acids for protein synthesis.
What is the measurement of as chloroplast?
3-10um
Give a brief overview of the structure of a chloroplast
Under a light microscope, it appears as biconvex discs. It is surrounded by an envelope of two phospholipid bilayer membranes. A system of membranes runs through the stroma consisting of fluid filled sacs known as thylakoids stacked to form grana, held together by lamella. The stroma contains the enzymes of the calvin cycle, sugars, 70S ribosomes, a loop of DNA, lipid droplets, organic acids and starch grains.
Describe the structure of the grana and how it relates to the function.
The membrane system is the site of the light dependent reactions of photosynthesis. The grana provide a large surface area, which holds the pigments, enzymes and electron carriers needed for the light dependent reactions.
The membranes make it possible for a large number of pigment molecules to be arranged so that they can absorb as much light as necessary. In each photosystem, the light harvesting clusters are arranged for efficient light absorption. The pigments are arranged in the thylakoid in funnel-like structures, with each pigment passing energy to the next member of the cluster, reaching the chlorophyll a reaction centre.
The membranes of the grana hold ATP synthase and are the site of ATP synthesis by chemiosmosis.
What is the site of the light independent reactions?
Stroma
What is the site of the light dependent reactions?
The membrane system.
What are limiting factors?
If a process is affected by more than one factor, the rate will be limited by the factor which is nearest its lowest value.
What is the problem that changes whether a plant is C3 or C4?
The problem is with the enzyme rubisco which catalyses the reaction between RuBP and carbon dioxide. But with C4 plants, it can also catalyse the reaction of oxygen with RuBP.
What is photorespiration?
It is where RuBP is being ‘wasted’ and less is available to combine with carbon dioxide due to combining with oxygen.
What are some names of C4 plants?
Maize, sorghum and sugar cane
How is photorespiration avoided?
They keep RuBP and rubisco away from high oxygen concentrations. The cells that contain RuBP and rubisco are arranged around the vascular bundles and are called bundle sheath cells. They have no direct contact with the air inside the leaf.
CO2 is absorbed by another group of cells, the mesophyll cells. The mesophyll cells contain an enzyme called PEP carboxylase which catalyses the combination of carbon dioxide from the air with a three carbon substance known as phosphoenolpyruvate or PEP. The compound formed from this reaction is oxaloacetate. Still inside the mesophyll cells, the oxaloacetate is converted to malate and this is passed on to the bundle sheath cells. In the bundle sheath cells, the carbon dioxide is removed from the malate molecules and delivered to RuBP by rubisco in the same way.
What is an adaptation of C4 plants?
Enzymes in C4 plants generally have a higher optimum temperature than those in C3 plants. This is an adaptation to growing in hot climates.
What is present in the primary pigment?
Chlorophyll
What is present in the accessory pigments?
Carotenoids
What are the colours of the pigments found in chlorophylls?
Chlorophyll a= yellow-green
chlorophyll b= blue-green
What are the colours of the pigments found in carotenoids?
B-carotene= orange Xanthophyll= yellow
Why do plants look green? ref. absorption
Chlorophylls mainly absorb red and blue-violet regions of the light spectrum. They reflect green light which is why the plants look green.
What region of the spectrum do carotenoids absorb?
The carotenoids absorb mainly in the blue-violet region of the spectrum.
What is the absorption spectrum?
It is a graph of the absorbance of different wavelengths of light by a pigment.
What is an action spectrum?
It is a graph of the rate of photosynthesis at different wavelengths of light.
How is the Rf value calculated?
Distance travelled by pigment spot/ distance travelled by solvent
What kind of values is normally seen with:
- Carotenoids
- Chlorophyll b
- Chlorophyll a
- Carotenoids have Rf values close to 1
- Chlorophyll b has a much lower Rf value than carotenoids
- Chlorophyll a has an Rf value between those of carotenoids and chlorophyll b.
