ISC-12>CONTENT>STRUCTURE & FUNCTION OF PLANTS>5.PHOTOSYNTHESIS
Scope of syllabus
|
|
NUTRITION
- Autotrophic nutrition
- Heterotrophic nutrition
MODES OF NUTRITION
A. PARASITIC
- Plant which obtain their food in ready- made from other living organisms (plant or animals) are known as parasites, and the living organisms from which they obtain their food are said to be hosts.
- Parasitism is common in fungi and bacteria but there are some flowering plants which live on other plants.
- These may be obligate or partial parasite.
A. Obligate or total parasites:
Those plants, which are devoid of chlorophyll and take whole of their food in prepared form from host plants, are said to be obligate or total parasites. There are two types of obligate parasite:
1. Total stem parasites:
Those plants, which are devoid of chlorophyll and take whole of their food in prepared form from host plants, are said to be obligate or total parasites. There are two types of obligate parasite:
1. Total stem parasites:
- Cuscuta (dodder) is the best known example of total stem parasites.
- It is a rootless plant with pale yellow wiry stems and very small scale leaves.
- The stems twine around the branches of the host plant and at intervals send haustoria into the tissue of the host.
- The vascular tissue of haustoria makes connections with the vascular tissue of the host and derives from it water and minerals and prepared food .
- Cuscuta parasitises a large number of plants such as Zizyphus, Duranta, etc.
2. Total root parasites:
- Striga, Orobanche, Balanophora and Rafflesia are common examples of total root parasites.
- They grow parasitically on the roots of other flowering plants and derive their food material from them.
|
|
B.
Partial or semi-parasites
- Those parasites which draw only water and minerals from host plants but manufacture their own food are known as partial parasites.
- These plants, being green, are capable to manufacture their own food with the help of minerals and water absorbed from the host plant.
- Their haustoria communicate only with the xylem of the host to draw water and minerals.
- There are the following two categories of partial parasites.
1. Partial stem parasites:
- Viscum (mistletoe) and Loranthus are two well known examples of partial stem parasites.
- Both these plants have a similar mode of nutrition and dispersal. Viscum is a much-branched shrubby plant with terete and leafy or flattened and leafless dichotomous or trichotomous branches and minute flowers.
- The plant is attached to the host with the help of haustoria which penetrate the bark and sapwood of the host.
- It is commonly found on many rosaceous trees, oaks, walnut, willows and several other forest trees.
- More details
2. Partial root parasite:
- Santalum album (sandal wood tree), well known for its scented heart wood, is the best known example of partial root parasite.
- It is a small evergreen tree with opposite leaves and small in odorous flowers.
- It is indigenous to southern India, particularly Mysore.
- Its roots develop a large number of haustoria which make connections with the roots of the neighbouring plants.
B. SAPROPHYTIC
- Saprophytes are those plants which grow on and obtain their nourishment from dead and decaying organic matter of animals and plants.
- Saprophytes are of great importance since they convert complex organic substances of dead organisms into simple minerals which can be reutilised by plants.
- Neottia (bird’s nest orchid) and Monotropa (indian pipe plant) are two well known flowering plants which belong to this category . Neottia grows on forest soil rich in humus. It has an underground rhizome that gives rise to light brown fleshy aerial stem which bears a few rudimentary leaves and a spike of orange flowers. The roots of Neottia are associated with endotrophic mycorrhiza. The fungus absorbs food from the humus and the plant from the fungus.
- Monotropa grows on humus rich soil in pine forests. It has a white or cream coloured unbranched stem which bears scale leaves and a solitary terminal flower. Absorption of food material takes place from the humus with the help of an ectotrophic mycorrhiza.
- Neottia and Monotropa are generally described as saprophytes living on the humus of the soil but in reality they are parasitic on the investing mycorrhiza. The food material is first absorbed from the humus by the mycorrhiza and then transferred to these plants. They are unable to grow without the fungus member.
C. SYMBIOTIC
- The association of two dissimilar organisms in which both are mutually benefited is said to be symbiosis.
- Each partner of this association is called symbiont.
- Lichens, mycorrhiza and root nodules are some well known examples of symbiosis.
Lichen
|
Mycorrhiza
|
Rhizobium with legumes plants
|
D. INSECTIVOROUS
1. Drosera or Sundew
- The plant is a herb which grows in waterlogged soil.
- It has a rosette of 6-12 radical leaves which lie prostrate on the ground
- Each leaf has a long narrow stalk which is expanded into a more or less circular blade.
- The edges and the surface of the blade bear numerous bright red club-shaped hairs, known as tentacles.
- Each tentacle is a stalked mucilage secreting gland.
- The tip of the gland secretes a sticky juice which shines like a dew drop in bright sunlight, hence the plant derives its popular name sundew,
- The glands act as capturing, digesting and absorbing organs.
- Insects are attracted by the colour, glittering secretion and odour of the gland. When an unwary insect alights on the leaf, it is held by the viscid secretion of glands.
- As glands are sensitive to touch, they bend over the prey. The secretion of glands contains proteolytic enzymes that digest the proteins of the prey, which are absorbed by the plant. Then tentacles return to their original position; the non-digestible parts of the insect are left exposed and are blown away.
2. Nepenthes or Pitcher plants
- Leaves of some insectivorous plants like Nepenthes, Sarracenia and Cephalotus are modified into pitchers to trap insects.
- In Nepenthes, the lamina is modified into a pitcher-like structure, the distal part of the lamina forms a lid which covers the mouth of the pitcher
- The lower part of the petiole becomes flattened and leaf-like, whereas the upper part is coiled like a tendril and holds the pitcher in a vertical position.
- A large number of glands are present on the upper half of the inner wall of the pitcher. These sessile glands are of epidermal origin and secrete proteolytic enzymes.
- On the rim of the pitcher, downwardly and inwardly directed hairs are present which do not allow insects to go out once they enter the pitcher.
- Insects are attracted by the colour of the pitcher and the sweet secretions of the glands. As the insect enters the pitcher, it slips downwards and is drowned in the fluid present in the basal part of the pitcher. T
- his fluid contains water and digestive juices which digest the body of the insect.
3. Utricularia or Bladderwort
- It is a rootless floating aquatic herb with slender straggling branches and very much dissected leaves.
- Some leaf segments are modified into bladder-like structures.
- The bladder is a stalked pear-shaped structure with a hollow cavity having an opening at distal end .
- The opening has a trap door consisting of stiff tapering bristles which can open only inwards.The flap allows minute water flies to pass in but they cannot come out. Once trapped flies die in the bladder.
4. Dionaea or Venus fly trap
- The plant is a herb with a rosette of leaves, modified into fly-traps .
- The broadly flattened petiole is constricted to the mid-rib at its junction with the lamina.
- The roundish lamina is divided into two almost exactly similar halves, separated by the midrib.
- The two halves of the lamina which are like toothed jaw can move up and down along the mid-rib. They close together with a snap and the two sets of teeth are interlocked to prevent the escape of insects.
- On the inner side, in the centre of each half-leaf, numerous rosy glands are present. On the upper surface of each half, about in the centre, there are three trigger hairs which project outwards. If an insect comes in contact with a trigger hair the stimulus moves to the mid-rib, which functions like a hinge, closing the leaf trap .
- When the trapped insect is digested and absorbed, the trap opens again.
5. Pinguicula or Butterwort
- It is a herb with a rosette of large, fleshy and yellowish leaves .
- The edges of the leaves are slightly curled upwards, forming a kind of shallow trough.
- The leaf has two types of glands— stalked and sessile. The stalked gland has a large basal cell, a columnar cell and a head of 2-8 cells which produces mucous material. The sessile gland has a 2-8-celled head which secretes proteolytic enzymes.
- Large drops of mucilage secreted by the stalked glands help in the capture of insects. After the capture of the insect, the sessile glands are stimulated and secrete enzymes acid phosphatase, esterase and ribonuclease, which help in the digestion of the insect. These glands also absorb the digested food .
PHOTOSYNTHESIS
DEFINITION
CHEMICAL EQUATION
The overall process of photosynthesis in eukaryotes can be expressed by the following equation:
- The equation gives no indication as to whether the released oxygen is obtained from the water or the carbon dioxide.
- A milestone contribution to the understanding of photosynthesis was that made by a microbiologist, Cornelius van Niel (1897-1985), who, 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:
- In green plants H2O is the hydrogen donor and is oxidised to O2.
- 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.
- Hence, he inferred that the O2 evolved by the green plant comes from H2O, not from carbon dioxide.
- This was later proved by Ruben and Kamen (1941) using radioisotopic techniques. (Given below)
The correct equation, that would represent the overall process of photosynthesis is therefore:
SITE OF PHOTOSYNTHESIS
Leaf is best suited organ of photosynthesis. Why..?
|
How does the structure of Palisade cells assist the process of photosynthesis..?
Site of photosynthesis : Chloroplast
How does the structure of chloroplast assist its function..?
- DOUBLE MEMBRANE : Contains the grana and stroma, and is permeable to CO2, O2, ATP ,sugars and other products of photosynthesis.
- THYLAKOID MEMBRANE : Provide huge surface area for maximum light absorption.
- THYLAKOID SPACES : Restricted region for accumulation of proton and establishment of the gradient.
- STROMA : Site of all the enzymes require for photosynthesis.
PIGMENTS AND PHOTOSYSTEM
PIGMENTS :
- Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful".
- Flowers, corals, and even animal skin contain pigments which give them their colors.
- More important than their reflection of light is the ability of pigments to absorb certain wavelengths.
- Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs --organisms which make their own food using photosynthesis
SEPARATION OF PIGMENTS BY CHROMATOGRAPHY
- Pigments can be extracted from finely chopped leaves by grinding the tissues in acetone.
- When the pigment extract is subjected to ascending paper chromatography, the pigments can be analysed and identified.
TYPES OF PHOTOSYNTHETIC PIGMENTS
Primary and accessory pigments:
- A pigment that initiates the process of photosynthesis is called primary pigment. Chl a is a primary photosynthetic pigments because this is the only pigment that can act directly to convert light energy to chemical energy.
- Any pigment in plants that can absorb light energy and pass it to the primary pigment (Chl a) which starts the process of photosynthesis. Chl b, carotenoids and phycobilins are categorized as accessory pigments.
- CHLOROPHYLLS:
- Chlorophyll a- blue green is the primary pigment.
- Chlorophyll b- yellow green is the accessory pigment.
- Absorbs violet blue and red light and reflect green light.
- CAROTENOIDS
- Carotenoids –lipid compounds –red, yellow to orange in colour.
- Absorb light in the range of 400 to 500 nm. That is why their colors are red, orange, and yellow.
- Protect chlorophyll from photooxidation.
- They absorb light energy and transfer it to chl-a.
- These are of two types carotenes – orange and red colour and xanthophylls- yellow colour.
- PHYCOBILLINS
- Found in blue green algae and red algae.
- Phycobilins absorb in the range of 550 to 630 nm
- Common phycobillins are phycocyanins, phycoerythrins and allophycocyanin.
- They harvest light energy and transfer it to chl.a.
Difference between Chl a and Chl b
ARRANGEMENTS OF PIGMENTS IN THYLAKOID MEMBRANE (PHOTOSYSTEM)
- PHOTOSYSTEM
- Definition : The region of pigments organization on the membrane of thylakoids is called photosystem.
- Location : Membrane of thylakoids in chloroplast.
- Components : Chlorophyll a molecules, Accessory pigments, a protein matrix.
- Light harvesting complex : Antenna complexes are light-harvesting systems (LHC) which are protein-pigment complexes in or on photosynthetic membranes. LHCs receive radiant energy and transfer it to the reaction centers.
- Reaction centre :
PHOTOSYSTEM (For exams)
- Chl. a, b and carotenoids in the thylakoid membrane are arranged in the form of assemblies.
- Only one chl. a can trigger light reactions by donating electron to the primary electron acceptor.
- Other pigments absorb photons and passes energy from one molecule to the other till it reaches the reaction center.
- This entire apparatus – the antenna complex, with the reaction center, chl. a, and primary electron acceptor is called as photosystem.
- The thylakoid membrane has two kinds of photosystem-
- Absorbs light more than 680nm– PS-I (P700)
- Absorbs light less than 680nm– PS-II (P680)
DIFFERENCE BETWEEN PHOTOSYSTEM
- PSI is located on the outer surface of thylakoid membrane towards stroma whereas PSII is located on the inner surface of the thylakoid membrane.
- PSI is involved in both cyclic and non cyclic photophosphorylation whereas PSII is involved in non cyclic photophosphorylation only.
- In PSI , reaction centre is made of P700, whereas in PSII, the reaction centre is made up of P680.
NATURE OF LIGHT
- The energy necessary comes for photosynthesis comes from light. Light is electromagnetic energy. It travels in rhythmic waves that have characteristic wavelengths. The entire range of radiation is referred to as the electromagnetic spectrum.
- The specific part of this spectrum is that is involved in photosynthesis is the visible light spectrum. This is known as photosynthetically active radiation (PAR).
- The spectrum is visible to the human eye, and its wavelength range from near 400nm to near 740 nm.
- The particle theory proposed by Einstein states that light is composed of particles of energy called photons. The energy contained in an individual photon is referred to quantum.The energy of a photon is not the same for all kinds of light but is inversely proportional to the wavelength.
- The shorter wavelength of visible light have more energy than the longer wavelengths. Blue light, which has higher frequency than red light, has more energy in photon than red light.
- Rate of photosynthesis is measured as the number of oxygen molecules produced per quantum of light absorbed. This is said to be quantum yield.
Absorption and action spectrum
- The various pigments of photosynthesis absorb photon of light from specific wavelength of the visible spectrum. If white light, which contains all the wavelengths of the visible spectrum, passed through the chloroplast of a plant cell, not all wavelengths are absorbed equally. This is because the specific pigments present in the chloroplast of that particular type of plant. A device called spectrophotometer or colorimeter can be used to measure absorption at various wavelengths. This result in a characteristic absorption spectrum for the plant.
- Absorption spectrum- the curve obtained by plotting the amount of absorption of different wavelengths of light by a particular pigment is called absorption spectrum. The absorption spectrum of a plant is the combination of all the absorption spectra of all the pigments in its chloroplasts.
- Action spectrum - Since light provides the energy to drive photosynthesis, the wavelength of the light absorbed by the chloroplasts partly determine the rate of photosynthesis. "The rate of photosynthesis at particular wavelength of visible light is referred to as the action spectrum".
- A common way of determining the rate of photosynthesis in order to produce an action spectrum is to measure oxygen production.High oxygen production indicates a high rate of photosynthesis.
QUANTOSOMES
- Photosynthetic unit can be defines as number of pigment molecules required to affect a photochemical act, that is the release of a molecule of oxygen.
- Park and Biggins coined the term quantosome. They noticed distinct morphological structures on thylakoid membranes and identified them as quantasomes. Each quantosome has 250 -300 chlorophyll, carotenoids, quinone, conjugated lipids and proteins.
Red drop
Emerson enhancement effect
Warburg’s flashing light experiment
- Two plants were kept in different light conditions.
- One was kept in continuous light and other was given alternate light and dark conditions.
- Rate of photosynthesis of the second plant kept in alternate light and dark conditions was found to be higher than the plant kept in continuous light.
- The result of the experiment can be given as- A ------ B------ C
- In continuous light A to B proceeds faster than B to C. Thus B accumulates more.
- In intermittent light B ----- C proceeds well in dark and continues in light also, thus more C is produced in this condition.
- Thus the rate of photosynthesis increases and produces more sugar when the plant receives intermittent or flashing lights than continuous light.
Hill reaction
- He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light.
- The Hill reaction is as follows:
- 2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2 where A is the electron acceptor Therefore, in light, the electron acceptor is reduced and oxygen is evolved.
MECHANISM OF PHOTOSYNTHESIS
OUTLINE OF PHOTOSYNTHESIS MECHANISM
A) LIGHT DEPENDENT REACTION
Cyclic Photophosphorylation
- Simplest pathway that generates ATP
- the excited electron ultimately returns to the original position.
- P700 is in excited state, electron is accepted by primary electron acceptor (PEA)
- PEA passes electron to iron containing ferrodoxin (fd)
- Fd, passes electron to the mobile electron carrier plastoquinone (Pq)
- From Pq they are transferred to a complex of 2 cytochromes.
- From the cytochrome it passes to plastocyanin (Pc) and finally returns to P700.
- At each step electron lose potential energy, a proton gradient is created across the thylakoid membrane.
- This triggers the production of ATP, ATPsynthetase helps in conversion of ADP-ATP.
- This production of ATP during the cyclic pathway of electron flow is referred to as cyclic photophosphorylation.
CYCLIC PHOTOPHOSPHORYLATION : SCHEMATIC
Non-cyclic photophosphorylation
- Occurs in green plants and involves both photosystems.
- Here high energy electrons released from PS-II do not return to PS-II and ATP is produced.
- Thus it is called non cyclic photophosphorylation.
- Light excites electron of P700 in PS-I
- Electron do not return and are stored as high energy NADPH.
- Oxidized chlorophyll has “holes” to fill.
- PS-II supplies electron to P700
- Electron passes through the chain, losing potential energy and generates ATP.
- Non-cyclic electron flow thus restores electron to PS-I
- Now, P680 has “hole” to fill.
- It acquires this by the splitting of water.
- Thus the non-cyclic photophosphorylation is a flow of electron from water (where electron have low potential energy) to NADPH, (where electron are stored at high state of potential energy)
NON CYCLIC PHOTOPHOSPHORYLATION : SCHEMATIC
|
|
Difference between cyclic and non-cyclic photophosphorylation
B) LIGHT INDEPENDENT REACTION (CALVIN CYCLE)
C- fixation or Calvin cycle or C3 cycle or dark phase or biosynthetic phase.
- The process by which CO2 is reduced to carbohydrates is called as C- fixation.
- This process makes use of the ATP produced in the photochemical phase.
- C-fixation occurs in the stroma of chloroplasts, by a series of enzyme catalaysed reactions.
- It consists of four phases-
- 6 molecules of ribulose-bis phosphate reacts with 6 molecules of CO2 in the presence of the enzyme RuBPcarboxylase (Rubisco) to give an unstable 6 C compd.
- This unstable compd breaks into 12 molecules of 3C compd- 3 PGA (phosphoglyceric acid)
- PGA is the first stable compd formed.
- 12 molecules of PGA is converted to 12 molecules of 1,3 diphosphoglycerate and then reduced to 12 molecules of phosphoglyceraldehyde (PGAL).
- This step uses ATP and NADP respectively.
- Two molecules of PGAL are used for synthesis of sugar and then starch.
- RuBP regeneration is essential for the cycle to continue.
- 10 molecules of PGAL by a series of complex reactions are converted to 6 molecules of 5C compound, RuBP. This requires ATP.
- For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P).
PHOTOSYNTHESIS: SUMMARY VIDEO
PHOTORESPIRATION (C2 cycle, glycolate cycle)
- In C3 plants, RuBPcarboxylase (RUBISCO) functions as oxygenase at high temp and O2 conc.
- RuBP oxygenase oxidizes RuBP to phosphoglycerate (3C) and phosphoglycolate (2C).
- Phosphoglycolate is converted into glycolate and transported to peroxisomes.
- In the peroxisomes glycolate is oxidized to glyoxylate.
- This glyoxylate is then converted to an amino acid-glycine.
- Glycine enters the mitochondria and here, two glycine gives rise to serine ,CO2.
- Serine again enters the peroxisome and is converted to glycerate
- Glycerate enters the chloroplast where it is phosphorylated to form PGA.
- PGA now enters the Calvin cycle.
- Since there is loss of photosynthetically fixed carbon and no energy- rich compound is produced, photorespiration is considered as a wasteful process.
- It reduces dry matter production and yield of the plant.
- However, its uses can be- production of glycine and serine that are important for metabolites like proteins and chlorophyll.
C4 pathway or Hatch & Slack pathway
- Occurs in plants like maize, sugarcane, pearl millet etc.
- These plants show presence of two types of photosynthetic cells- mesophyll cells and bundle sheath cells.
- The chloroplast in the mesophyll cells have grana whereas the bundle sheath cells donot have well defined grana.
- CO2 is accepted by phosphoenol pyruvate (PEP) catalysed by enzyme phosphor-enol-pyruvate carboxylase (PEP-case)
- Oxaloacetic acid (OAA) is the first stable compound which is a 4C compound.
- OAA is converted to malic acid and transported to bundle sheath cells.
- Malic acid is decarboxylated to pyruvic acid.
- CO2 liberated is used for Calvin cycle in bundle sheath cells
- Pyruvic acid is transported back to mesophyll, where it is used for regeneration of PEP.
- This is an adaptive mechanism to avoid photorespiration.
- Decarboxylation of malic acid maintains high conc of CO2 so that Rubisco functions as carboxylase and not oxygenase.
- They are found in tropical regions.
- Main feature is Kranz anatomy in leaves.
- Bundle sheath has large green barrel shaped cells with one or two concentric arrangement of mesophyll cells.
- Mesophyll cells lack intercellular spaces.
- Chloroplast in bundle sheath cells are large and donot have well-defined grana.
DIFFERENCES BETWEEN C3 AND C4 PLANTS
KRANZ ANATOMY
- This type of anatomy is found in C4 plants.
- In leaves of such plants, palisade tissues is absent.
- There is bundle sheath around vascular bundles.
- Chloroplast in the bundle sheath cells are large without grana.
- Chloroplast in mesophyll cells are small but with well developed grana.
DIFFERENCES BETWEEN C3 AND C4 PLANTS
FACTORS AFFECTING PHOTOSYNTHESIS
Like all other physiological processes, photosynthesis is also influenced by a number of factors. Factors influencing the process can be grouped in the following two heads:
A. External or Environment Factors : These include:
A. External or Environment Factors : These include:
- Light
- Carbon dioxide
- Temperature
- Water
- Chlorophyll
- Protoplasmic factors
- Accumulation of stored food
- Anatomy of leaf
1. Light : The ultimate source of light for photosynthesis is solar radiation. Light varies in intensity, quality and duration. All the three have direct influence on photosynthesis in the following ways:
(ii) destruction of chlorophyll occurs. The destruction of chlorophyll in the presence of oxygen at very high intensities of light is called photo- oxidation.
- Quality of Light : White light consists of seven colour, each with a different wave length. Light between the wave length of 400 nm and 700 nm is most effective for photosynthesis and this light is called photosynthetically active radiation (PAR). It is due to the fact that different rays of light are not absorbed equally by the chlorophyll.
- Light intensity : It has a direct relationship with the rate of photosynthesis. Under low intensity of light, the rate of photosynthesis is low. As the intensity of light increases, rate of photosynthesis increases. But, at very higher light intensities, the rate of photosynthesis decreases. This is because of two reasons:
(ii) destruction of chlorophyll occurs. The destruction of chlorophyll in the presence of oxygen at very high intensities of light is called photo- oxidation.
- On the basis of requirement of light intensity, plants are recognised as sciophytes or heiophytes. The plants that require low light intensity, for optimum photosynthesis are called sciophytes. On the other hand, certain plants require, high light intensity for Optimum’ photosynthesis. Such plants are called heliophytes.
- Duration of light : Duration of daily light period has a significant effect on the total photosynthetic yield of a plant. A plant will accomplish more photosynthesis when exposed to long periods of light. It has also been found that uninterrupted and continuous photosynthesis for relatively long periods of time may be sustained without any visible damage to the plant.
2.CARBON DI OXIDE
The chief source of CO2 in land plants is the atmosphere, which contains only 0.3% of the gas. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. An increase in concentration of C02 increases the photosyrhesis. The increase in C02 to about 1 % is nIIy advantageous to most of the plants. Higher concentration of the gas has an inhibIty dFcct cin photosynthesis.
The chief source of CO2 in land plants is the atmosphere, which contains only 0.3% of the gas. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. An increase in concentration of C02 increases the photosyrhesis. The increase in C02 to about 1 % is nIIy advantageous to most of the plants. Higher concentration of the gas has an inhibIty dFcct cin photosynthesis.
3. TEMPERATURE
processes, photosynthesis also needs a suitable temperature. In the presence of plenty of light and carbon dioxide, photosynthesis increases with the rise in temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process. The optimum temperature at which the photosynthesis is maximum is 25-30°C, though in certain plants like Opuntia, photosynthesis takes place at as high as 55°C. This is known as the maximum temperature. The temperature at which the process just starts is the minhnum temperature.
For lichens it is as low as —20°C and for certain conifers it is —35°C.
processes, photosynthesis also needs a suitable temperature. In the presence of plenty of light and carbon dioxide, photosynthesis increases with the rise in temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process. The optimum temperature at which the photosynthesis is maximum is 25-30°C, though in certain plants like Opuntia, photosynthesis takes place at as high as 55°C. This is known as the maximum temperature. The temperature at which the process just starts is the minhnum temperature.
For lichens it is as low as —20°C and for certain conifers it is —35°C.
4. Water :
Being one of the raw materials, water is also necessary for the photosynthetic process. An increase in the water content of the leaf results in the corresponding increase in the rate of photosynthesis. Thus, the limiting effect of water is not direct but indirect. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.
Being one of the raw materials, water is also necessary for the photosynthetic process. An increase in the water content of the leaf results in the corresponding increase in the rate of photosynthesis. Thus, the limiting effect of water is not direct but indirect. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.
5. Chlorophyll Content of Leaves :
Though the presence of chlorophyll is essential for photosynthesis but the rate of photosynthesis is not proportional to the quantity of chlorophyll present. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances
Though the presence of chlorophyll is essential for photosynthesis but the rate of photosynthesis is not proportional to the quantity of chlorophyll present. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances
7. Accumulation of Byproducts :
The progress of photosynthesis in the assimilatory cells is maintained as long as the concentration of the proçlucts formed is removed. The final product in the photosynthetic reaction is sugar and its accumulation in the cells slows down the process of photosynthesis.
The progress of photosynthesis in the assimilatory cells is maintained as long as the concentration of the proçlucts formed is removed. The final product in the photosynthetic reaction is sugar and its accumulation in the cells slows down the process of photosynthesis.
8. Internal Structure of Leaf:
The thickness of the cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affect the rate of photosynthesis.
The thickness of the cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affect the rate of photosynthesis.
BLACKMAN'S LAW OF LIMITING FACTORS
- The Blackman’s law of limiting factors states that when a process is conditioned to its rapidity by a number of factors, the rate of process is limited by pace of the “slowest factor” which presents a minimum concentration in relation to others.
- When temperature and CO2 concentration are kept constant.
- Photosynthesis increases with increase in light intensity and then levels.
- When CO2 concentration is increased, photosynthesis rate also increases to a maximum before leveling.
- If temperature is now increased, photosynthesis rate will rise steadily.
COMPENSATION POINT
TRANSLOCATION OF FOOD
MASS FLOW HYPOTHESIS
- The mass flow hypothesis for translocation of organic solvents in plants was given by Munch.
- Accordingly, the transport of food takes place through phloem along a concentration gradient.
- Organic food is manufactured in the mesophyll cells of the leaves.
- This increases the osmotic pressure of these cells.
- Due to the increase in OP, water from the xylem elements and neighbouring cells enters these cells, thus raising the TP.
- This forces some of the dissolved food from these cells into the sieve tubes.
- The cells of the root and storage organs after utilizing the food, will have low TP.
- This creates a turgor pressure gradient between the leaf and the other cells.
- Because of this, movement of water containing dissolved organic food takes place from the upper end to the lower end of the plant through phloem.
- The leaf is described as the source and the root as sink.
- Major criticism against this is that this theory does not explain the side ways translocation that takes place from phloem to the periphery of the stem.
Worksheets and assignments
bqmodesofnutrition.pdf | |
File Size: | 186 kb |
File Type: |