Plus One Botany Notes Chapter 9 Photosynthesis in Higher Plants is part of Plus One Botany Notes. Here we have given Kerala Plus One Botany Notes Chapter 9 Photosynthesis in Higher Plants.
|Text Book||NCERT Based|
|Chapter Name||Photosynthesis in Higher Plants|
|Category||Plus One Kerala|
Kerala Plus One Botany Notes Chapter 9 Photosynthesis in Higher Plants
All animals including human beings depend on plants for their food. Green plants synthesise the food they need, and for all other organisms depend on them for their needs. They carry out photosynthesis, a physico chemical process by which they use light energy to drive the synthesis of organic compounds. The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth. Photosynthesis is the primary source of all food on earth. It is also responsible for the release of oxygen into the atmosphere by green plants.
On the basis of the knowledge several experiments might be performed indicating that chlorophyll, light and carbon dioxide are essential components for photosynthesis to take place.
- A variegated leaf or a 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.
- 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 setup is then placed in light for some time. The exposed part of the leaf tested positive for starch. This showed that CO2 was required for photosynthesis.
- Joseph Priestley performed a series of experiments that revealed the essential role of air in the growth of green plants.
- Priestley 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 aburning candle or an animal that breathe the air, both somehow, damage the air.
- When he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn.
- Priestley hypothesised as follows plants re-store to the air whatever breathing animals and burning candles remove.
- Jan Ingenhousz concluded that sunlight is essential for plant that purifies the foul air produced by burning candles or by breathing of animals.
- Julius Von Sachs came to the conclusion that green parts are the place in the plants where production of glucose takes place and the same is stored in the form of starch.
- T.W Engelmann 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. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b.
- The empirical equation representing the total process of photosynthesis for oxygen evolving organisms was then understood as,
[CH2O]n is a carbohydrate here like glucose.
- Cornelius Van Niel, based on his studies of purple and green bacteria, demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates. This can be expressed by,
The correct equation, that would represent the overall process of photosynthesis is therefore,
Photosynthesis does take place in the green leaves of plants because chloroplasts are abundantly present in the mesophyll cell of the leaves. Within the chloroplast there is the membranous system consisting of grana, the stroma lamellae, and the fluid stroma.
The former set of reactions, since they are directly light driven are called light reactions. The latter are not directly light driven but are dependent on the products of light reactions (ATP and NADPH). Hence, to distinguish the latter they are called, by convention, as dark reactions.
Pigments involved in Photosynthesis
The pigments involved in the process of photo-synthesis are called photosynthetic pigments. These pigments provides different shades of green in the leaves in different plants or in the leaves of same plant. Separate the pigments through paper chromatography.
On the basis of their significance, the photosynthetic pigments are of two types,
- Primary pigments. The pigment forms the main molecule of photosystem,
e.g., chlorophyll a, b.
- Accessory pigments. These support the function of primary pigments,
e.g., xanthophyll and caratenoids.
Different pigments present in leaf described below,
- Chlorophyll a (bright or blue green ).
It is known to be the chief plant pigment associated with photosynthesis.
- Chlorophyll b (yellow green)
- Xanthophylls (yellow).
These pigments are oxidised carotenoids.
- Carotenoids (yellow to yellow-orange).
They are also known as 1 antenna pigment’.
Pigments are substances that have an ability to absorb light, at specific wavelengths.
It is curve that shows the amount of different wavelength of lights absorbed by a substance. Chlorophyll-a shows the maximum absorption peak at 450 nm and also shows another peak at 650 nm.
It is the curve that depicts the relative rates of photosynthesis at different wavelengths of light.
Below graph showing action spectrum of photosynthesis.
Below graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a.
- Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.
- The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II).The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II).
- Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae.
- The single chlorophyll a molecule forms the reaction centre.
In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.
The Electron Transport
In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes.
The whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor,down the electron transport chain to PS I, excitation of electrons, transfer to another accepter, and finally down hill to NADP causing it to be reduced to NADPH + H+ is called the Z scheme.
Splitting Of water
The electrons that were moved from photo-system II must be replaced. This is achieved by electrons available due to splitting of water. Electrons obtained by the splitting of water are needed to replace those electrons which are removed from the photosystems-1 thus, are provided by photosystem-ll.
2H2O → 4H+ + O2 + 4e–
It is the process through which, ATP is synthesised from ADP and inorganic phosphate by cell organelles with the help of energy from solar radiation. The process of photophosphorylation is of two types,
i. Non-cyclic photophosphorylation.
When the two photosystems work in a series, first PS II and then the PS I, a process called non- cyclic photo-phosphorylation occurs.
ii. Cyclic photophosphorylation.
It is the type of photophosphorylation in which only PS-1 is taking part and the electron released from the reaction centre P700 returns to it after passing through a series of carrier i.e., circulation takes within the photosystem and the phosphorylation occurs due to cyclic flow of electrons
Through the cyclic flow only the synthesis of ATP takes place.
The chemiosmotic hypothesis has been put forward to explain the mechanism. Like respiration, in photosynthesis.Breakdown of proton gradient is leads to the release of energy (ATP).
Dark reaction (biosynthetic phase).
This process does not directly depend on the presence of light but is dependent on the products of the light reaction. This takes place in the stroma of chloroplast. This process is known as carbon- fixation cycle. Thus, assimilation of CO2 during photosynthesis is of two main types.
i. Calvin cycle (C3 pathway)
This is a cycle biochemical pathway of reduction of CO2 or photosynthetic carbon cycle, which was discovered by Calvin. The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3 or C4 (or any other) pathways.
Calvin cycle consists of three satges, they are given below.
These are a series of reactions that lead to the formation of glucose.
of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
In Out Six CO2 One glucose 18 ATP 18 ADP 12 NADPH 12 NADP
ii. C4 pathway
Plants that are adapted to dry tropical regions have the C pathway mentioned earlier. Though these plants have the C4 oxaloacetic acid as the first CO2 fixation product they use the C3 pathway or the Calvin cycle as the main biosynthetic pathway. This pathway that has been named the Hatch and Slack Pathway, is again a cyclic process.
It is a light dependent cyclic process of oxygenation of RuBP and release of CO2 by the photosynthetic organs of a plant. The site of photorespiration is chloroplast. Mitochondria and peroxisome are also required for completing the process.
In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process. As C4– plants lacks the process of photorespiration the productivity and yields in these plants are better than those in C3 and other plants.
Different between c3 and c4 plants
Factors affecting photosynthesis
Photosynthesis is under the influence of several factors, both internal (plant) and external. These are as follows,
1. Internal factors.
It include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll. The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.
2. External factors.
It include the availability of sunlight, temperature, CO2 concentration and water.
Blackman’s Law of Limiting Factors states the following:
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 which directly affects the process if its quantity is changed.
There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
At higher light intensities, gradually the rate does not show further increase.
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