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Kerala Plus One Botany Notes Chapter 9 Photosynthesis in Higher Plants
What Do We Know?
Role of light, CO2, H2O, and Chlorophyll
Actually, chlorophyll (green pigment of the leaf), light, and CO2 are required for photosynthesis. A variegated leaf ora leaf that was partially covered with black paper, and one that was exposed to light. On testing these leaves for starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light.
Half leaf experiment and the importance of CO2 in photosynthesis
In this, a part of a leaf is enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. The set up is then placed in light for some time. Then conducted the starch test, showed that the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative. This showed that CO2 is required for photosynthesis.
Early Experiments
Historical aspects of photosynthesis
1. Priestley
He observed that a candle burning in a closed space – a bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space.
He concluded that a burning candle or an animal that breathes the air, both damage the air. But when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn.
2. Jan Ingenhousz
He showed that sunlight is essential to the plant process that purifies the air fouled by burning candles or breathing animals. In aquatic habitat, during bright sunlight, small bubbles were formed around the green parts while in the dark they did not. Later he identified these bubbles are oxygen. So the green part of the plants could release oxygen.
3. Julius von Sachs
Glucose is usually stored as starch. He found that the green parts in plants where glucose is made.
4. T.W Engelmann
By using a prism he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum.
An empirical equation for photosynthesis
[CH2O] represent a carbohydrate (e.g., glucose, a six-carbon sugar).
Hydrogen donor in bacteria and green plants
Some organisms do not release O2 during photosynthesis
When H2S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O2. In the green plants, the O2 evolved from H2O, not from carbon dioxide.
The modern equation for photosynthesis
C6H12O6 represents glucose. The O2 released is from water
Where Does Photosynthesis Take Place?
It takes place in the chloroplast of leaves that contain grana, the stroma lamellae, and the fluid stroma.
Where is the energy production site in chloroplast?
The energy-rich molecules like ATP and NADPH are synthesized in grana and stroma lamellae by light reactions.
Where is the Glucose production site in chloroplast?
In stroma by dark reactions, CO2 fixation leading to the synthesis of glucose, which in turn forms starch
Structure of chloroplast
Diagrammatic representation of an electron micrograph of a section of chloroplast
How Many Pigments are Involved in Photosynthesis?
Chromatographic separation of the leaf pigments shows that different types of pigments in leaves i.e
Chlorophyll a (bright or blue-green in the chromatogram)
chlorophyll b (yellow-green)
xanthophylls (yellow)
carotenoids (yellow to yellow-orange)
a) Graph showing the absorption spectrum of chlorophyll a, b, and the carotenoids.
b) Graph showing the action spectrum of photosynthesis.
c) Graph showing action spectrum of photosynthesis superimposed on the absorption spectrum of chlorophyll a.
Wavelengths of light absorbed by pigments
Chlorophyll pigments absorb light, at specific wavelengths of blue and the red regions while carotenoids absorb the blue and green wavelength
Chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls, and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. but also protect chlorophyll a from photo-oxidation.
What is Light Reaction?
It is the photochemical phase include
- light absorption
- water splitting
- oxygen release
- Formation of high-energy rich molecules ATP and NADPH.
The pigments are organised into two light-harvesting complexes(LHC)
- Photosystem I (PS I)/P700
- Photosystem II (PS II)/P680
Each photosystem has single chlorophyll a molecule forms the reaction centre, all the pigments except chlorophyll-a forming a light-harvesting system also called antennae.
In PS I the reaction centre, chlorophyll a has an absorption peak at 700 nm while in PS II it has absorption maxima at 680 nm, and is called P680.
The Electron Transport
How does electron flow in electron carriers that connect two photosystems?
Initially, excitation of chlorophyll molecule occurs due to light, then electrons are emitted from Ps II (uphill) that are accepted by electron acceptor, electron flows through electron carriers cytochromes, (downhill) and (Loss of electrons of PSII is compensated by electrons coming from water and loss of electrons of PS I is compensated by electrons coming from PS II).
PS I is also excited due to light and electrons are emitted (uphill), it transfers an electron to another accepter, and finally down the hill to NADP+ causing it to be reduced to NADPH + H+ is called the Z scheme.
Result of Z-scheme
- production of ATP and NADPH
- O2 evolution
Splitting of Water
Photolysis
It is the splitting of water into H+, [O] and electrons in the presence of light and these electrons are available to PSII.
This process takes place on the inner side of the membrane of the thylakoid.
Oxygen released is one of the net products of photosynthesis.
2H2O → 4H+ + O2 + 4e–
Cyclic and Non-cyclic Photo-phosphorylation
Phosphorylation
The process of which ATP from ADP and inorganic phosphate in the presence of light (in mitochondria and chloroplasts) is named phosphorylation.
Electron in a cyclic process
When only PS I is functional, the cyclic flow of electrons within the photosystem and the phosphorylation occurs in the stroma lamellae.
Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation i.e at 700 nm
Result of cyclic photophosphorylation
ATP is produced.
Where does the light-harvesting complex work for acyclic and noncyclic processes?
The membrane or lamellae of the grana have both PS I and PS II so a noncyclic process occurs,
The stroma lamellae membranes lack PS II as well as NADP reductase enzyme So a cyclic process occurs.
Chemiosmotic Hypothesis
It is the ATP synthesis linked to the development of a proton gradient across a membrane
In chloroplast, the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS.
The proton gradient develops due to,
a. Splitting of the water molecule takes place on the inner side of the membrane, the protons accumulate within the lumen of the thylakoids
b. As electrons move through the photosystems, protons are transported across the membrane moves into the lumen side of the membrane
c. The NADP reductase enzyme is located on the stromal side of the membrane. Along with electrons that come from the accepter of electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH+ H+. These protons are also removed from the stroma.
In chloroplast, protons in the stroma decrease in number, while in the lumen there is an accumulation of protons. This creates a proton gradient across the thylakoid membrane The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0 of the ATP.
ATPase have a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate the ATPase enzyme that catalyses the formation of ATP.
Where are the ATP and NADPH Used?
It is used in the biosynthetic phase of photosynthesis. This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH.
Melvin Calvin studied the algal photosynthesis by using radioactive 14C led to the discovery that the first CO2 fixation product was identified as 3-phosphoglyceric acid or PGA.
The Primary Acceptor of CO2
The studies showed that the accepter molecule was a 5-carbon sugar -ribulose bisphosphate (RuBP) in the Calvin cycle.
The Calvin Cycle
It involves three stages:
- carboxylation
- reduction
- regeneration
1. Carboxylation:
Carboxylation is the fixation of CO2 into a stable compound catalysed by the enzyme RuBisCO that results in the formation of two molecules of 3-PGA.
2. Reduction: These are a series of reactions that lead to the formation of glucose.
The steps involve the utilization of 3 molecules of ATP for phosphorylation and two NADPH for reduction per CO2 molecule fixed. For the fixation of six molecules of CO2, 6 turns of the cycle are required and one molecule of glucose is generated
3. Regeneration: Regeneration of the CO2 acceptor molecule require one ATP for phosphorylation to form RuBP.
Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and molecules of NADPH are required
The Calvin cycle proceeds in three stages:
1. carboxylation, during which CO2 combines with ribulose- 1, 5- bisphosphate
2. reduction, during which carbohydrate is formed at the expenses of the photochemically made ATP and NADPH; and
3. regeneration during which the CO2 acceptor ribulose- 1, 5-bisphosphate has formed again so that the cycle continues
The C4 Pathway (Hatch and Slack Pathway)
Plants that are adapted to dry tropical regions have the C4 pathway, C4 plants have a special type of leaf anatomy. They tolerate higher temperatures. They lack a process called photorespiration and have greater productivity of biomass.
Special leaf anatomy-kranz anatomy
Large cells around the vascular bundles are centripetally arranged bundle sheath cells such anatomy is called ‘Kranz’ anatomy. Eg- maize or sorghum
Primary CO2, accepter, first stable product and Enzyme of C4 Pathway
The primary CO2 acceptor is a 3-carbon molecule- phosphoenolpyruvate (PEP) present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase.
The first stable product C4 acid OAA is formed in the mesophyll cells.
Diagrammatic representation of a Hatch arid Slack Pathway
OAA converted into 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells .which are transported to the bundle sheath cells. In the bundle sheath cells, these C4 acids are broken down to release CO2 and a 3-carbon molecule. The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.
Thus the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4 plants.
Photorespiration
In C3 plants, under high concentration of O2 and low CO2 concentration, RUBP binds with O2 to form one molecule of PGA and phosphoglycolate and a large quantity of CO2 is released.
Can you say photorespiration is a wasteful process?
This process utilise ATP but neither synthesis of sugars, nor of ATP. Hence photorespiration is a wasteful process.
The specialty of C4 plants to avoid Photorespiration
In C4 plants photorespiration does not occur because C4 acid from the mesophyll is broken down in the bundle cells to release CO2 – this results in increasing the intracellular concentration of CO2. Here RuBisCO functions as a carboxylase minimizing the oxygenase activity, productivity, and yields are better in these plants.
Factors Affecting Photosynthesis
Photosynthesis is influenced by several factors, both internal (plant) and external. The plant factors include the number, size, age, and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration, and the amount of chlorophyll. The external factors include the availability of sunlight, temperature, CO2 concentration, and water.
Blackman s Law of Limiting Factors
If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor that directly affects the process if its quantity is changed.
For example, In the green leaf, the light and CO2 conditions are optimum but the plant does not photosynthesize if the temperature is very low.
Light
The availability of light shows a direct relationship with CO2 fixation rates at low light intensities At higher light intensities the rate does not show further increase because other factors are not in optimal amount. The intensity of light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.
Carbon dioxide Concentration
The concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 percent). An increase in concentration up to 0.05 percent can cause an increase in CO2 fixation rates. The C3 and C4 plants respond differently to CO2 concentrations.
Graph of light Intensity on the rate of photosynthesis
C4 plants show saturation at about 360µL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450µL-1 Thus, the current availability of CO2 levels is limiting to the C3 plants.
C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity The above concept is used for some greenhouse crops such as tomatoes and bell pepper.
Temperature
The dark reactions that take place in stoma are enzymatic and temperature controlled. C4 plants respond to higher temperatures and show a higher rate of photosynthesis while C3 plants have a much lower temperature optimum.
Water
Water stress causes the closure of stomata and it is difficult to receive CO2 for photosynthesis. The stress condition also makes leaves wilt and reducing the surface area of the leaves and their metabolic activities.