B5-1 Photosynthesis Flashcards
Upper Epidermis
One cell thick
Light is allowed through to the photosynthetic tissue below
Palisade Mesophyll
Cytoplasm of palisade cells is full of chloroplasts
Cell shape, number and arrangement of chloroplasts allows maximum light capture
Spongy Mesophyll
Contains fewer cells than the palisade mesophyll
Each cell has fewer chloroplasts and so is not as active in photosynthesis as palisade
Large intercellular spaces for diffusion of CO2
Lower Epidermis
Has stomata which are pores surrounded by a pair of guard cells
Only cells with chloroplasts in the lower epidermis are the guard cells
Chloroplast
Made up of a double membrane
The outer membrane is selectively permeable to some solutes
The inner membrane is highly permeable. Substances pass through with the aid of transporters.
Stroma
Site of light-independent reactions
A gel-like matrix enclosed by the chloroplast envelope
Thylakoids
Site of light dependent reactions
A third membrane system within the stroma consisting of flattened sacs or pouches
Photosynthetic pigments and electron carriers are embedded within the membrane.
The space enclosed within the thylakoid is known as the thylakoid lumen or thylakoid space
This compartmentalisation allows chemiosmosis to take place and for ATP to be produced by photophosphorylation.
Granum
A stack of thylakoids
This increases the surface area and the amount of pigments available for the light-dependent reactions of photosynthesis.
Connecting the grana are flattened tubular thylakoids known as intergranal lamellae (singular: lamella). These lamellae connect the thylakoid compartments into a single, continuous compartment within the stroma
Light
Light is a form of electromagnetic energy. Light with different amounts of energy have different wavelengths. The entire range is called the electromagnetic spectrum.
Visible light is the range of wavelengths that can be detected by the human eye.
When light meets matter, it may be reflected, transmitted, or absorbed.
Pigments are chemical compounds which reflect and absorb only certain wavelengths of visible light.
The pigment molecules act like energy-receiving antennas to capture light energy for photosynthesis.
Each pigment absorbs wavelengths of a narrow range within the spectrum. Hence, plants usually need to use several kinds of pigments to effectively increase the range of wavelengths from which they can obtain energy.
There are two basic classes of photosynthetic pigments in plants: (1) chlorophyll (the main photosynthetic and most abundant pigment) and (2) carotenoids (a type of accessory pigment includes both carotenes and xanthophylls).
Both classes of pigments are found in the thylakoid membrane of chloroplasts.
Chlorophyll (General)
Chlorophyll absorbs mainly red and blue-violet light. It reflects green light and therefore gives most plants their characteristic green colour.
Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in the thylakoid membrane.
Chlorophyll (Structure)
Each molecule of chlorophyll consists of two parts 9):
A hydrophilic porphyrin ring that functions in light absorption.
The porphyrin ring has a flat, light-absorbing hydrophilic head which contains a magnesium atom at its center.
A hydrophobic hydrocarbon tail that projects into the thylakoid membrane to keep the chlorophyll embedded in the thylakoid membrane.
Chlorophyll a and b are the primary gatherers of light energy for photosynthesis to occur. The conjugated system (alternating single and double bonds) is largely responsible for the absorption of visible light.
Chlorophyll (Different types)
Different chlorophylls have different side chains on their hydrophilic head and this modifies the absorption spectra, increasing the range of wavelengths of light absorbed.
Chlorophyll a is the major in photoautotrophs and it absorbs blue and red light. Only chlorophyll a can participate directly in the light-dependent reaction, which converts light energy to chemical energy. The other pigments in the thylakoid membrane can absorb light and transfer the energy to chlorophyll a, which initiates the light-dependent reaction.
Any pigment molecule that absorbs light energy and transfers it to chlorophyll a of the reaction centre is known as an accessory pigment. - Most common – chlorophyll b
Carotenoids
Carotenoids are accessory pigments, as they pass the light energy they absorb onto chlorophyll a of the reaction centre
They are yellow, orange, red or brown pigments that absorb strongly in the blue-violet range.
The two main types of carotenoids: carotene and xanthophylls, both of which absorb light in the 460 to 550nm range of the visible light spectrum.
Hence, by using both chlorophylls and carotenoids for absorption of light, efficiency of light-harvesting is increased as the range of wavelengths that can be captured is increased. Carotenoids peak at a slightly higher wavelength than chlorophylls
These pigments are normally masked by chlorophyll in photosynthetic tissues. However, they can be seen in autumn leaves, prior to leaf-fall, when the chlorophyll has been broken down.
Purpose of Carotenoids
Broadening spectrum of light for photosynthesis
Accessory pigments absorb the intermediate wavelengths of light which chlorophyll cannot, thus broadening the spectrum of colours that can drive photosynthesis
However, carotenoids are not very effective as a photosynthetic pigment and transfer only about 10% of their absorbed energy
Photoprotection
Carotenoids are more important in absorbing excessive light and preventing auto-oxidation of chlorophyll and hence, preventing photobleaching. This function is known as photoprotection.
Excessive light intensity can damage the chlorophyll pigments, so instead of transmitting energy to the chlorophyll, some carotenoids absorb and dissipate excess light energy from chlorophyll, thereby protecting them from destruction by light.
Absorption and Action Spectrums
Phototrophs only capture wavelengths in the visible light range.
The absorption spectrum of a photosynthetic pigment is a graph of the amount of light absorbed at different wavelengths by a pigment.
The action spectrum for photosynthesis is a graph of the effectiveness of different wavelengths of light in driving photosynthesis.
Both spectrums show that:
The wavelengths of light absorbed by chlorophyll, namely red and blue light, are very similar to the wavelengths that drive photosynthesis, hence, chlorophyll is mainly responsible for the absorption of light in photosynthesis
The wavelengths optimally absorbed are the ones that provide most energy for photosynthesis; hence, both blue and red light are used by green plants as the energy source for photosynthesis.
General Summary of Photosynthesis
Photosynthesis has 3 main stages:
Light Harvesting Stage
Light energy is captured by the plant using a mixture of pigments including chlorophyll.
Light-dependent / Light Reaction
Light energy is harnessed to excite and displace an electron from chlorophyll.
Light energy is converted to chemical energy through a flow of electrons that is coupled to ATP synthesis.
NADPH is produced.
Photolysis of water – light is involved in the splitting of water into hydrogen ions and oxygen.
The first two stages require light and occur in the thylakoid membranes of chloroplasts.
Light-independent Reaction
Chemical energy of ATP and NADPH (produced from the light-dependent stage) is used in the reduction of carbon dioxide and hence, the manufacture of sugars.
The third stage does not require light and takes place in the stroma of chloroplasts.
Excitation of Chlorophyll by Light
When a molecule of chlorophyll or some other photosynthetic pigment absorbs light, it changes from its ground state to excited state.
The excited molecule is unstable and tends to return to its original, ground state in one of three ways:
Decay by resonance energy transfer - By transferring the energy - but not the electron – directly to a neighbouring chlorophyll molecule by a process called resonance energy transfer - this occurs during light harvesting.
Decay by successive electron transfers - By boosting an electron to a higher energy level and then transferring it to a nearby molecule capable of accepting electrons, the electron acceptor. The molecule returns to its original state by taking up a low-energy electron from another molecule, an electron donor (i.e. electron transfer) - this occurs in light-dependent reaction.
In the chloroplasts, water serves as a weak electron donor. When water is oxidised this way, oxygen is released along with two protons.
Decay by Giving off light and heat - Energy is lost in the process. This is achieved by converting the excess energy into heat (molecular motions) or to a combination of heat and light of a longer wavelength (fluorescence). This occurs when light energy is absorbed by an isolated chlorophyll molecule in solution.
Mechanisms (1) and (2) are used in photosynthesis.
Photosystems
In the thylakoid membrane, photosynthetic pigments that trap light energy are arranged into photosystems.
These multiprotein complexes convert the captured light energy into useful forms.
A photosystem consists of three closely-linked components:
Light-harvesting complexes (LHCs) - Proteins that hold pigments together, but ultimately pigments are bound by hydrocarbon tails
Light is collected by the 200 to 300 pigment molecules that are bound to them.
They are important in capturing light. They absorb light energy and transfer the light energy to the reaction centre.
Reaction Centre
Contains a pair of special chlorophyll a molecules which act as irreversible trap for energy. An excited electron is immediately passed to an adjacent chain of electron acceptors in the same complex.
A primary electron acceptor
It is found in the reaction centre and is involved in electron transfer.
PSII VS PSI
Two functionally and spatially distinct photosystems are present that differ in the waveIengths that they absorb. Photosystem II (PSII) and Photosystem I (PSI).
In PSII, the special chlorophyll a is known as P680 as it absorbs light maximally at a wavelength of 680nm.
In PSI, the special chlorophyll a is known as P700 as it absorbs light maximally at a wavelength of 700nm.
The two photosystems, P680 and P700:
are identical in their special chlorophyll a molecule.
but differ In their light-absorbing properties because of association with different accessory pigments and proteins in the thylakoid membrane, hence affecting the electron distribution.
ETC
ETCs are found at the thylakoid membranes, in between the photosystems.
At the ETC, electrons are passed down the carriers by a series of redox reactions. Each carrier molecule receives an electron (reduction), and in turn donates it (oxidation) to the next carrier down the chain.
An ETC allows the transfer of electrons to be done in several energy-releasing steps instead of one.
An electron progressively loses energy as it is transferred from one carrier to another.
Some of the energy released is used to make ATP.
Coenzyme carriers
Coenzymes electron carriers are:
FAD, flavin adenine dinucleotide (in mitochondrion)
NAD, nicotinamide adenine dinucleotide (in mitochondrion)
NADP, nicotinamide adenine dinucleotide phosphate (in chloroplast)
These coenzymes can up a pair of electrons and a proton thereby becoming reduced to FADH2, NADH and NADPH respectively.
Light Harvesting Stage
Light of appropriate wavelength strikes any pigment molecule within a photosystem.
Light energy is absorbed by that pigment molecule and the pigment molecule becomes excited.
The excitation energy is then transferred from one molecule to another within the cluster of pigment molecules.
This is known as resonance energy transfer.
Light-Dependent Reaction
Also known as the light reaction
The reaction occurs in the thylakoid membrane of chloroplasts.
The role of this stage is to synthesise reduced nicotinamide dinucleotide phosphate (NADPH) and ATP using captured light energy from the light-harvesting stage. Chemical energy is trapped in ATP and NADPH.
The NADPH and ATP produced are used in the light-independent reaction to fix carbon dioxide and finally trap energy in glucose.
There are two ways by which the light dependent reaction can proceed:
Non-cyclic photophosphorylation
The two photosystems and ETC work cooperatively to build a chemiosmotic gradient for ATP synthesis and reduce NADP to NADPH.
Cyclic photophosphorylation
Photosystem I can act alone to build the chemiosmotic gradient for ATP synthesis.
Finally, the ATP is synthesised through chemiosmosis.
Non-cyclic Photophosphorylation
Non-cyclic photophosphorylation involves both Photosystems I and II in the electron flow. ATP and NADPH are produced
Steps:
A photon of light strikes a pigment molecule in a LHC and energy is relayed via resonance energy transfer until it reaches one of the two special chlorophyll a molecules in the PSII reaction centre. It excites one of the electrons in P680 to a higher energy state.
This photoexcited electron from P680 is captured by the primary electron acceptor in the reaction centre. Now each P680 is missing an electron.
An enzyme splits a water molecule into two electrons, two hydrogen ions and an oxygen atom. This process involves light and is known as photolysis of water. The electrons released is used to replenish the deficit of electrons from the reaction centre of PSII. The oxygen atom immediately combines with another oxygen atom, releasing O2 as a by-product.
H20 -> 2e- + 2H+ + ½ O2
From the primary electron acceptor, the energised electron passes from PSII to PSI via a first electron transport chain (ETC) consisting of the following electron carrier molecules: from plastoquinone (Pq) down a cytochrome (b-f) complex then to plastocyanin (Pc) through a series of oxidation-reduction reactions. These electron carrier molecules are arranged in increasing electron affinity so that transport of electrons down the ETC is unidirectional.
As electrons flow from molecule to molecule it drops to lower energy levels. Free energy released from this exergonic reaction is used to pump protons against concentration gradient from the stroma into the thylakoid space. A proton gradient will be generated across the thylakoid membrane, which is used to drive ATP synthesis. This synthesis of ATP is called photophosphorylation because it uses light energy to phosphorylate (addition of phosphate) ADP.
Meanwhile, light energy from another photon of light strikes a pigment molecule in a light-harvesting complex of the PSI, exciting an electron of one of the two special chlorophyll a in the PSI reaction centre. The excited electron is then captured by the PSI’s primary electron acceptor, creating an electron deficit in the P700. This electron deficit is replenished by the electron from PSII that reaches the last electron acceptor of the first ETC.
The excited electron is passed from PSl’s primary electron acceptor down a second ETC through ferredoxin (Fd) .
The enzyme NADP reductase transfers electrons from Fd to NADP. Two electrons are required for its reduction to NADPH.
Cyclic Photophosphorylation
Cyclic photophosphorylation involves Photosystem I only. No NADPH is produced.
Light is absorbed by the LHC and passed on to chlorophyll a (P700) in the reaction centre of PSI.
This causes the P700 molecule to emit an energised electron which is raised to a higher energy level and picked up by the primary electron acceptor in the reaction centre.
The energised electrons from PSl are passed to ferredoxin (Fd), cycled back to cytochrome (b-f) complex on the first ETC and from there, back to PSI.
As these electrons are passed along the first ETC, enough energy is released to synthesise ATP from ADP and Pi. Consequently, ATP is produced.
The ATP is needed in the light-independent stage of photosynthesis.
Chemiosmosis (ETC, Proton gradient)
Chemiosmosis is the process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP.
The thylakoid membrane is impermeable to H+, thus as the light reaction proceeds, accumulation of hydrogen Ions occurs in the thylakoid space.
In the ‘Z-scheme’ of electron flow in non-cyclic photophosphorylation, electrons flow down energy levels along the electron transport chain from PSII to PSI, and release free energy. Using this free energy, the cytochrome complex pumps hydrogen ions against the concentration gradient from the stroma, across the thylakoid membrane, into the thylakoid space.
Photolysis of water produces H+ which also contributes to the proton concentration in the thylakoid space.
This results in an electrochemical and concentration gradient, known as a proton gradient, where there are more hydrogen ions inside the thylakoid space than there are in the stroma.
Chemiosmosis (ATP Synthesis)
Energy-coupling
The proton gradient drives the synthesis of ATP by ATP synthase complex (enzyme embedded in thylakoid membrane).
Hydrogen ions diffuse down this gradient from the thylakoid space across the thylakoid membrane into the stroma through the ATP synthase complex.
This drives the formation of ATP catalysed by the enzyme ATP synthase, where one ATP is synthesised for every 2 H+ that return to the stroma through the ATP synthase complex.
ATP synthesised is used for active transport of protons into thylakoid (endogonic). Diffusion of H+ is energy-releasing (exergonic).
Calvin Cycle / Light-independent
The reaction occurs in the stroma of chloroplasts.
Its purpose is to reduce carbon dioxide using ATP (energy source) and NADPH (reducing power), produced in the light-dependent reaction.
The reactions of the Calvin cycle for a C3 plant can be classified into three phases: (1) carbon dioxide uptake and fixation, (2) reduction of phosphoglyceric acid (PCA) and (3) regeneration of carbon dioxide acceptor (RuBP), which ultimately leads to product synthesis and sugar formation.
Carbon Dioxide Fixation
Carbon dioxide diffuses through the stomata and into the cytoplasm of the mesophyll cells and into the chloroplasts.
Carbon dioxide is fixed when it combines with a five-carbon CO2 acceptor, ribulose bisphosphate (RuBP), to form an unstable six-carbon intermediate. This reaction (carboxylation of RuBP) is catalysed by an enzyme, ribulose bisphosphate carboxylase oxygenase (rubisco).
The unstable six-carbon intermediate breaks down spontaneously Into two molecules of a three-carbon compound called phosphoglyceric acid (PGA) / 3-phosphoglycerate / glycerate-3-phosphate (GP).
RuBP (5C) + CO2 (1C) + H20 —rubisco–> 2PGA (2 x 3C)
Reduction of PGA
Each molecule of PGA is phosphorylated by ATP (i.e. receives an additional phosphate group from ATP) forming 1,3-bisphosphoglycerate.
A pair of electrons donated from NADPH further reduces 1,3-bisphosphoglycerate to form glyceraldehyde-3-phosphate (GALP or G3P) or triose phosphate (TP). The hydrogen for the reduction comes from the NADPH (which is oxidised) while the energy for this step comes from ATP.
Regeneration of Carbon Dioxide Acceptor (RuBP)
For every three molecules of carbon dioxide that enters the Calvin cycle, three molecules of RuBP are carboxylated and a total of six molecules of TP are formed.
Only one molecule of TP can be counted as a net gain of carbohydrate. The other five molecules of TP must be used to regenerate the three molecules of RuBP used in the fixation step 1 (see above). To accomplish this, three more molecules of ATP are invested.
RuBP is regenerated and the Calvin cycle continues.
Production Synthesis and Sugar Formation
The TP spun off from the light-independent reaction becomes the starting material for metabolic pathways that synthesise other organic compounds, including glucose and other carbohydrates.
Two molecules of TP are utilised to synthesise one molecule of hexose sugar. Hence, the formation of one molecule of hexose sugar requires six turns of the Calvin cycle.
Note: The carbon and oxygen atoms of hexose sugars (C6H1206) come from carbon dioxide as well as RuBP while the hydrogen atoms come from NADPH.
Fate of Photosynthetic Products
Although TP is the end product of the light-independent reaction, it does not accumulate in large quantities. Both PGA and TP are also intermediates in glycolysis.
Both glycolysis and the Krebs cycle occupy a central role in metabolism.
TP can be used by the cell outside of the chloroplasts in the synthesis of all other forms of carbon-containing substances including other carbohydrates, lipids, proteins. nucleic acids and chlorophyll.
Synthesis of Carbohydrates
A large proportion of TP is converted to hexose sugars, particularly glucose and fructose which are respiratory substrates for the ATP production.
For storage purposes, many glucose molecules are in turn covalently linked together to form starch molecules that are stored as starch granules.
Glucose molecules are also polymerised to form cellulose which is needed for the structural growth of plants (i.e. in the formation of new cell walls)
Synthesis of Lipids
PGA can enter the glycolytic pathway and is converted to an acetyl group, which is added to coenzyme A to form acetyl CoA. Acetyl CoA is converted to fatty acids in both the cytoplasm and chloroplasts.
Glycerol is made from TP. Fatty acids and glycerol combine to form triglycerides (for storage) and phospholipids (for cellular membranes).
Synthesis of Proteins
PGA and TP are important precursors in the synthesis of amino acids (and hence proteins).
In the process, nitrogen is incorporated into the product molecules.
Plants in turn use these molecules to make other nitrogen-containing compounds, including nucleotides for DNA and RNA synthesis.
Light saturation point
The point beyond which an increase in light intensity will cause no further increase in the rate of photosynthesis.
Light Intensity
Measured in lux (lx), a measure of luminosity as perceived by the human eye.
As seen in 27, at low light intensities, the rate of photosynthesis increases linearly with light intensity.
Photosynthesis is no longer limited by light - it is said to be light-saturated. Other factors become limiting.
Except for in shaded plants, light is not normally a major limiting factor.
Very high light intensities may damage the chlorophyll (i.e. photobleaching) and decrease the rate of photosynthesis. However, plants that have been naturally exposed to such conditions are usually protected by thick cuticles and hairy leaves.
Compensation Point
Point at which the rate of photosynthesis is equal to the rate of respiration.
At the compensation point, all the carbon dioxide produced during respiration is used for photosynthesis and all the oxygen produced during photosynthesis is used for respiration. Thus, there is no NET gaseous exchange between the plant and its environment at compensation point.
The compensation point is reached at quite low light intensities, usually at sunrise and at sunset.
Below the compensation point, the rate of photosynthesis is less than the rate of respiration so there is a net release of carbon dioxide and absorption of oxygen from the atmosphere.
Above the compensation point, the rate of photosynthesis is greater than the rate of respiration and there is net absorption of carbon dioxide and release of oxygen into the atmosphere.
Sun VS Shade Plants
Shade plants have a lower rate of respiration than that of sun plants.
Shade leaves are thin with fewer palisade mesophyll layers.
Have fewer cells so require less energy for maintenance.
Thus, shade plants reach compensation point at a lower light intensity and much sooner than sun plants.
Sun plants have a higher level of respiration and a much higher rate of photosynthesis than shade plants.
Sun leaves are thick with more palisade mesophyll layers.
Have more cells so require more energy invested into maintenance.
However, they are also able to absorb higher light intensities.
By producing more palisade layers, the sun plants can trap more light energy for the production of carbohydrates.
Carbon Dioxide Concentration
Carbon dioxide is needed in the light-independent reaction where it is used to make sugar.
As the concentration of carbon dioxide increases, the rate of photosynthesis also increases, until carbon dioxide becomes saturated and is no longer limiting.
Under normal conditions, carbon dioxide is the major limiting factor in photosynthesis; its concentration in the atmosphere varies between 0.03% and 0.04%, but increased photosynthetic rate can be achieved by increasing this percentage.
Temperature
The light-independent reactions are enzyme-controlied and thus temperature-sensitive.
The rate of reaction doubles for every 10°C rise up to about 35°C. As temperature rises towards the optimum, the rate of reaction increases because the molecules Involved move more rapidly and have a greater chance of colliding.
The rate of photosynthesis decreases at higher temperatures as enzymes start to denature.
Chlorophyll Concentration
Chlorophyll concentration is not normally a limiting factor, but reduction in chlorophyll levels can be induced by several factors, including: diseases, mineral deficiency, normal ageing process, lack of light because light is needed for the final stage of chlorophyll synthesis.
If the leaf turns yellow, it is termed to be chlorotic and this yellowing process is called chlorosis
Specific Inhibitors
An obvious way of killing a plant is to inhibit photosynthesis, and various herbicides have been used to do this.
Water
Periods of temporary wilting can lead to severe losses in crop yield. Plants usually close their stomata in response to wilting, and this would prevent access of carbon dioxide for photosynthesis.
Even slight water deficiency, with no visible effects, might significantly reduce crop yield.
Oxygen
The concentration of oxygen in the Earth’s atmosphere is 21%, as compared to 0.03-0.04% carbon dioxide. At any one time, there is much more oxygen than carbon dioxide
In many plants, the initial fixation of carbon in the Calvin cycle (light-independent reaction) is catalysed by the enzyme rubisco.
However, oxygen competes with carbon dioxide for the active site in rubisco, Less carbon dioxide will be fixed, and previously fixed carbon in RuBP will be lost as carbon dioxide i.e. there is a net loss of carbon.
Less PGA will be synthesized and photosynthetic output is decreased
This process is known as photorespiration (photo: occurs in the light, respiration: consumes oxygen while producing carbon dioxide)
Pollution
Low levels of certain gases of industrial origin, notably ozone and sulfur dioxide, are very damaging to the leaves of some plants.
It is estimated, that cereal crop losses as high as 15% may occur in badly polluted areas.
Soot can block stomata and reduce the transparency of the leaf epidermis.
