PHOTOSYNTHESIS IN HIGHER PLANTS- INTRODUCTION
- Photosynthesis in higher plants, Green plants synthesise the food they need, by photosynthesis and all other organisms depend on them for their needs.
- Photosynthesis ia a physico-chemical process by which plants 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 in higher plants is important due to two reasons : (a) It is the primary source of all food on earth and (b) It is also responsible for the release of oxygen into the atmosphere
WHAT DO WE KNOW?
- Experiment for starch formations on variegated leaf or a leaf that was partially covered with black paper & exposed to light showed that photosynthesis occurred only in green part of leaves in the presence of light.
- Experiment where a part of leaf is enclosed in a test-tube with some KOH soaked cotton (which absorbs CO2), while other half is exposed to air and set-up kept in light proved that CO2 is needed for photosynthesis.
PHOTOSYNTHESIS IN HIGHER PLANTS- EARLY EXPERIMENTS
(1) Joseph Priestley
Using a burning candle, a mouse, mint plant and a bell jar for closed space, hypothesised that plants restore to the air whatever burning candles or brathing animals remove.
(2) Jan Ingenhousz
In an elegant experiment with an aquatic plant , showed that in bright sunlight plants produce oxygen.
(3) Julius von Sachs
Found that glucose is made in green plant parts and stored as starch.
(4) T.W. Engelmann
Using prism, green alga Cladophora and aerobic bacteria, described the action spectrum of photosynthesis which roughly resembles the absorption spectrum of chlorophyll- a and b.
- Demonstrated thet photosynthesis is essentially a light dependent reaction in which hydrogen from suitable pxidisable compound reduces CO2 ti carbohydrates.
- H2 S is hydrogen donor for purple and green sulphur bacteria. H2 O, The hydrogen donor in green plants is oxidised to O2.
- The oxidation product is sulphur or sulphate in purple and green sulphur bacteria and not O2. Hence it was inferred that O2 evolved by green plants comes from H2 O and not from CO2 . This was later proved by using radioisotopic techniques. The correct equation, for the overall process: 6CO2+12H2O →light→ C6H12O6+6H2O+6O2.
WHERE DOES PHOTOSYNTHESIS IN HIGHER PLANTS TAKE PLACE
- Photosynthesis in higher plants, in green parts of the plants, mainly in the mesophyll cells in the leaves, which have large number of chloroplasts.
- Usually the chloroplast align themselves along the walls of mesophyll cells to get optimum quality of the inident light.
CHLOROPLAST ALIGNMENT
PARALLEl :- In low or optimum light intensity to get maximum incident.
PERPENDICULAR:- In extremely high light intensity to avoid photo-oxidation.
There is a clear DIVISION OF LABOUR with in the chloroplast.
CHLOROPLAST
MEMBRANOUS SYSTEM
- (Grana+Stroma lamellae)
- Responsible for trapping light & synthesis of ATP and NADPH.
- Directly light driven,called LIGHT REACTION (photochemical reactios)
STROMA
- Enzymatic reactions to synthesise sugar, which in turn forms starch, takes place
- Dependent on products of light reactions (ATP & NADPH)
- By convention called DARK REACTIONS (Carbon reactions)
Note:- However, this should not be construed to mean that the dark reaction occur in darkness or that they are not light-dependent.
HOW MANY TYPES OF PIGMENT ARE INVOLVED IN PHOTOSYNTHESIS
- Leaf-pigment of any green plant can be separated through paper chromatography.
- The colour in leaves is due to four pigments, that have the ability to absorb light, at specific wavelengths.
COLOUR OF THE PIGMENTS IN THE CHROMATOGRAM
i. Chlorophyll-a= Bright or blue green
ii. Chlorophyll-b= Yellow-green
iii. Xanthophyll= Yellow
iv. Carotenoids= Yellow to yellow orange.
- The wavelength of light at which there is maximum absorption by chlorophyll-a i.e; in blue and red photosynthesis
- Hence, we can conclude that Chl-a is the chief pigment associated with photosynthesis.
- Chl-b carotenoids and xanthophyll are accessory pigments.
- They absorb light and transfer the energy to Chl-a. They enable a wider range of wavelength of incoming light to be utilised for photosynthesis and also protect chlorophyll-a from photo-oxidation.
WHAT IS LIGHT REACTION?
- Light reactions of the photochemical phase include:-
(a) Light absorption
(b) Water splitting
(c) Oxygen release, and
(d) ATP and NADPH formation.
- Several protein complexes are involved in the process.
- The pigments are organised into two photosystems
THE ELECTRON TRANSPORT
- The whole scheme of transfer of electrons starting from PS-II → uphill to the acceptor→ down the ETC to PS-I → Excitation of electrons → transfer to another acceptor→ finally downhill→ to NADP’ → reducing it to NADPH+ His called the z-scheme, due to its characteristic shape.
- This shape is formed when all the carries are placed in a sequence on a redox potential scale.
SPLITTING OF WATER
- PS-II continuously supplies electrons which becomes available by splitting of water.
- Water splitting complex is associated with PS-II which itself is physically located on inner side of membrane of thylakoid.
- Water split into 2H+, [O] & electrons.
- This creates oxygen, one of the net products of photosynthesis.
CYCLIC AND NON-CYCLIC PHOTO-PHOSPHORYLATION
- When both PS-I and PS-II are involved, the process is non-cyclic, producing ATP, NADPH + H+ and oxygen.
- When only PS-I is functional, cyclic flow takes place to produce only ATP.
- A possible location for cyclic flow is the stroma lamellae membranes which lack PS-II and NADP reductase enzyme.
- Cyclic photo-phosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.
- The membrane or lamellae of the grana have both PS-I and PS-II.
CHEMIOSMOTIC HYPOTHESIS
- ATP sysnthesis is photosynthesis is linked to the development of a proton gradient across the membranes of thylakoid and protons accumulate in the lumen of thylakoids.
- The proton gradient is caused by:-
(a) Protons or hydrogen ions produced by splitting of water, accumulate in the lumen of the thylakoids.
(b) The primary acceptor of electron located towards outer side of membrane transfers its electron to an H carrier, which removes a proton from stroma while transporting an electron to thylakoid lumen.
(c) The NADP reductase enzyme located on stroma side of membrane, removes protons from stroma, while reducing NADP’ to NADPH+ H+.
- Within chloroplast, protons decrease in stroma and accumulate in lumen. This creates a proton-gardient across thylakoid membrane as wellas measurable decrease in pH in the lumen.
- Breakdown of this gradient leads to synthesis of ATP, when protons move across the membrane to the stroma through transmembrane channel of the CF0 of the ATP synthase.
ATP Synthsse (Two parts):-
CF0 = Embedded in the thylakoid membrane. A transmembrane channel for facilitated diffusion of protons
CF1 = Protrudes on outer surface of thylakoid membrane on the side that faces stroma. It synthesise ATP.
- Chemiosmosis requires- a membrane, a proton pump, a proton gradient and ATP synthase.
WHERE ARE THE ATP AND NADPH USED?
- Of the products of light-ATP, NADPH and O2, O2 diffuses out of chloroplast while ATP and NADPH are used to synthesise sugars in the biosynthetic phase of photosynthesis. Melvin Calvin used radioactive 14C in algae photosynthesis studies to discover the CO2 fixation product. The 3-C organic acid (3-PGA) (C3-pathway).
- In another group of plants, the first stable product was 4 carbon, oxaloacetic acid OAA (C4-pathway).
PHOTOSYNTHESIS IN HIGHER PLANTS-THE CALVIN CYCLE
- The calvin cycle occurs in all photosynthetic plants, weather they have C3 or C4 (or any other) pathways.
- Calvin cycle can be described under three stages_-
(1) CARBOXYLATION:- Most crucial step RuBP→RuBisCO→2 x 3 -PGA
(5C) (3C)
CO2 + H2 O
(2) REDUCTION:- A series of reactions that lead to formation of glucose. Utilises 2 ATP and 2 NADPH per CO2 , (The fixation of 6CO2 and 6 turns of the cycle are needed to form one molecule of glucose from the pathway).
(3) REGENERATION:- Regeneration of RUBP is crucial for the cycle to continue. This step require one ATP.So, to produce one molecule of glucose in calvin cycle an input of 6CO2 18 ATP & 12 NADPH are required.
THE C4-PATHWAY
- Plants adapted to dry tropical regions have the C4- pathway.
- C4- plants are special: They have special type of leaf anatomy, tolerate higher temperatures, show response to high light intensities, lack photorespiration and have great biomass productivity.
- C4- plants have leaves showing KRANZ ANATOMY the particularly large cells around the vascular bundles, which may form several layers and are called bundle sheath cells, characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
- The pathway is cyclic & called the Hatch and Slack Pathway. It is partly completed in mesophyll & partly in bundle sheath cell.
PHOTOSYNTHESIS IN HIGHER PLANTS-PHOTORESPIRATION
- RuBisCo, the most abundant enzyme in the world, has active site that can bind to both CO2 and O2 . This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind ti the enzyme.
- RuBisCo, has a much greater affinity for CO2 when the CO2, : O2 nearly equal than for O2.
- In C3- plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased, due to the following reaction.
RuBP + O2 →RuBis CO→ 3 PGA (3C)+2 phosphoglycolate (2C)
This is called photo-respiration
- In photo- respiration there is neither synthesis of sugars, nor of ATP. It results in release of CO2 with utilisation of ATP.
- The biological functions of photorespiration is not known yet.
- In C4-plants photo-respiration does not occur, as they a mechanism that increase the concentration of CO2 at the enzyme site. This ensures that the RuBisCO functions as acarboxylase minimising the oxygenase activity.
FACTORS AFFECTING PHOTOSYNTHESIS IN HIGHER PLANTS
- Photosynthesis is under the influence of several factors, both internal (plant) & external.
Internal Factor:-
- The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration & the amount of chlorophyll.
- The plant or internal factors are dependent on the genetic predisposition and growth of the plant.
- External factors:- include availability of sunlight, temperature, CO2 concentration and water.
- Blackman’s Law of limiting Factor:- If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is neares to its minimal value. It is the factor which directly affects the process if its quality is change.
(i) Light:- Light saturation occurs at 10% of the full sunlight. Except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
- There is a linear relationship between incident light & CO2 fixation rates at low light intensities. At higher light intensities, gradually the rate does not show further, increase as other factors become limiting.
(ii) CO2 concentration: Major limiting factor. The concentration of CO2 is very low in the atmosphere (0.03 & 0.04 %), so increase in concentrationupto 0.05% can cause increase in CO2 fixationrates, beyond the levels it can become demanding over longer periods.
- At low light conditions neither group responds to high CO2 conditionsC4-plants show saturation at 360μ1L -1C3-saturation is seen at 450μ1L-1. Some greenhouse crops like tomatoes and bell pepper show higher yields in CO2 enriched atmosphere.
(iii) Temperature: Dark reactions being enzymatic are temperature controlled . Light reactions are also temperature sensitive . C4 – plants show higher yields at high temperature while C3- plants have a much lower temperature optimum.
(iv) Water:- Effect of water as a factor is more through its effect on the plant rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing CO2 availability. Water stress also makes leaves wilt, thus, reducing the area of leaves and their metabolic activity as well.
FAQ’s
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose (sugar). It occurs in chloroplasts and is vital for the production of oxygen and organic compounds essential for life on Earth.
The main inputs of photosynthesis are carbon dioxide (CO2), water (H2O), and light energy. The outputs are oxygen (O2) and glucose (C6H12O6), a simple sugar that serves as a source of energy for the plant and other organisms.
Photosynthesis primarily occurs in the green tissues of higher plants, such as leaves, where chloroplasts are abundant. Chloroplasts contain chlorophyll, the green pigment that captures light energy for photosynthesis.
Chlorophyll is the primary pigment involved in photosynthesis. It absorbs light energy from the sun, particularly in the blue and red regions of the electromagnetic spectrum, and transfers this energy to chloroplasts, where it drives the photosynthetic reactions.
Photosynthesis consists of two main stages: the light-dependent reactions (also known as the light reactions) and the light-independent reactions (also known as the Calvin cycle or dark reactions).
PHOTOSYNTHESIS IN HIGHER PLANTS-FUNFACTS
The process of photosynthesis in higher plants is so efficient that, on a global scale, plants convert approximately 100 billion tons of carbon dioxide into oxygen each year, replenishing the Earth’s atmospheric oxygen levels.
While chlorophyll is the most well-known pigment involved in photosynthesis, plants also contain accessory pigments such as carotenoids and xanthophylls, which help broaden the range of light wavelengths that can be absorbed for photosynthesis in higher plants.
Photosynthesis in higher plants doesn’t just occur in leaves; it also takes place in other green parts of plants, such as stems and even some roots, where chloroplasts are present.
Photosynthesis in higher plants, C4 plants, such as corn and sugarcane, have a specialized pathway for carbon fixation that allows them to thrive in hot and dry environments by minimizing water loss and increasing efficiency in photosynthesis in higher plants.
Some plants, called CAM (Crassulacean Acid Metabolism) plants, exhibit a unique photosynthetic in higher plants adaptation where they open their stomata at night to take in carbon dioxide and store it in the form of organic acids, which are then used during the day to carry out photosynthesis in higher plants while minimizing water loss.
The rate of photosynthesis in higher plants can be influenced by factors such as the angle and intensity of sunlight, the availability of water and nutrients, and even the concentration of atmospheric pollutants.
Photosynthesis in higher plants isn’t limited to plants alone; some bacteria, algae, and even some fungi are capable of photosynthesis in higher plants, utilizing similar processes to convert light energy into chemical energy.
The Sahara Desert, one of the driest and hottest regions on Earth, experiences occasional blooms of phytoplankton in the nearby Atlantic Ocean, demonstrating the resilience and adaptability of photosynthetic organisms in diverse environments.
Scientists are exploring ways to harness the principles of photosynthesis in higher plants to develop artificial systems that can efficiently convert sunlight into energy, potentially offering sustainable solutions for renewable energy production.
The discovery of photosynthesis in higher plants paved the way for significant advancements in various fields, including agriculture, biochemistry, and environmental science, revolutionizing our understanding of life processes and our relationship with the natural world.
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