1. Energy and living Organisms
1.1 Important Concepts
Energy is defined as the capacity to do work. All living organisms may be regarded as working machines, which requires continuous supplies of energy in order to keep working and so, stay alive. Energy can exist as different forms: heat, light, electrical, magnetic, chemical, atomic, mechanical and sound. The laws that apply to energy conversions are the laws of thermodynamics.
The 1st law of thermodynamics states that energy cannot be created or destroyed but may be converted from one form to another. Living organisms cannot generate or destroy energy. Living organisms can only transform one form of energy to another. They can also store energy. However, some energy is lost at each transition e.g. heat.
The 2nd law of thermodynamics states that all natural processes tend to proceed in a direction that increases the randomness or disorder of a system. The degree of randomness is called entropy. Entropy and free energy (available for work) are inversely related. Living organisms represent phenomena in which particles become more orderly. These particles are organized into organic molecules, cells, tissues and organs. To accomplish this, “useful” energy must be supplied into the living system. After an organism’s death, decay results in the organism’s molecules becoming more disordered and scattered, thus increasing their entropy.
There are 3 stages to the flow of energy through living systems. The conversion of sun’s energy to chemical energy (which is stored in the form of organic molecules such as carbohydrates, ATP, etc) by plants during photosynthesis. The conversion of chemical energy acquired during photosynthesis into ATP during respiration. The utilization of ATP by cells in order to perform useful work.
Some common examples of the use of energy in living organisms are (1) chemical synthesis of substances for growth and repair, (2) active transport of substances into and out of cells, (3) electrical transmission of nerve impulses, (4) mechanical contraction of muscles, (5) maintenance of a constant body temperature in birds and mammals, (6) bioluminescence and (7) electrical discharge as in the electric eel.
1.2 Nutrition
The process by which organisms obtain the basic materials and energy necessary for all live processes such as growth, respiration, reproduction, movement etc. Living organisms can be classified according to their principal sources of carbon and energy.
Autotrophs are organisms which are able to use simple inorganic substances as starting materials for the synthesis of complex organic compounds, using either sunlight energy (photoautotrophs) or chemical energy (chemoautotrophs).
Heterotrophs are organisms which cannot synthesize their own food but rely on organic molecules, which they require as nutrients from their environment in the form of food. Some of these organisms obtain chemical energy through the degradation of the highly-reduced, energy-rich organic molecules (chemoheterotrophs).
Energy is stored in the form of chemicals within organisms, such as starch or glucose in plants; fats and glycogen in animals.
1.3 Adenosine Triphosphate (ATP)
At the cellular level, all forms of life temporarily store energy in chemical bonds of ATP, the universal energy currency. This implies that energy-consuming processes need only 1 system that can accept energy form ATP. Hence, a great economy of mechanism is achieved.
ATP comprises an adenine molecule linked via a ribose sugar to 3 phosphate units. The triphosphate tail of ATP is the chemical equivalent of a loaded spring – the 3 negatively charged phosphate groups are in an unstable energy-charged arrangements.
The high-energy phosphate bonds carry enough energy to force almost any reaction to proceed and ATP can enter almost any reaction for which energy is required. The chemical energy is readily released to the organism / cell by only one hydrolytic reaction, which breaks the phosphoanhydride bond and converts ATP to ADP. This reaction reduces large amounts of energy to allow other reactions to occur.
Conversely, a phosphate can be reattached to ADP with the absorption of large amounts of energy, yet again forming the high energy phosphate bonds in ATP. ADP can be phosphorylated to form ATP by (1) photophosphorylation (using light energy trapped by photosynthesis), (2) substrate level phosphorylation (respiring some of the consumed food to produce high-energy phosphate groups, which are then forced onto ADP making ATP) and (3) oxidative phosphorylation (using energy released during the final steps of respiration which occurs in the presence of oxygen).
1.4 Coupled Reactions
Chemical reactions of life are neither isolated nor independent. Instead, they are more likely to be part of a complex and interacting chains of events.
When a fuel molecule such as glucose is oxidized in a cell, enzyme catalyzed reactions ensure that a large part of the free energy that is released by oxidation is captured in a chemically useful form.
This is achieved by means of a coupled reaction in which an energetically favourable reaction is used to drive an energetically unfavourable reaction that produces an activated carrier molecule or some other useful energy store. Coupling mechanisms require enzymes and are fundamental to all the energy transactions of the cell.
The phosphorylation of ADP to form ATP involves an exergonic process which releases energy and an endergonic process which involves the uptake and storage of energy.
1.5 Redox Reactions
Many biochemical reactions involve both oxidation and reduction. Reduction reactions involve (1) addition of hydrogen, (2) addition of electrons and (3) removal of oxygen. Oxidation reactions involve (1) removal of hydrogen, (2) removal of electrons and (3) addition of oxygen.
1.5.1 Electron Transport Chains (ETC)
Many redox reactions are mediated by electron carriers (coenzymes and proteins). Electrons are often passed down a series of electron carriers, collectively known as the electron transport chain, through a series of reduction and oxidation steps. Each carrier molecule receives an electron and in turn donates it to a carrier down the chain.
Electron carriers are often found embedded on the membrane of organelles, such as on the inner convoluted membrane of mitochondria and the thylakoid membranes in the chloroplast.
Several important electron and hydrogen carriers work in conjunction with enzymes such as dehydrogenases e.g. FAD, NAD, and NADP. These coenzymes can pick up a pair of electrons and a proton, thereby becoming reduced.
2. Autotrophic Nutrition
The type of feeding employed by plants and other non-heterotrophic organisms involve the synthesis of organic compounds from inorganic compounds. It is called autotrophic nutrition.
2.1 Overview of Photosynthesis
Green plants are able to trap sunlight and then use these trapped energy to convert simple inorganic substances into complex organic compounds. These organisms are called photoautotrophs.
The process usually involves the synthesis of hexose sugars from carbon dioxide and water using sunlight as the source of energy and the green pigment, chlorophyll, for trapping the light energy. This process is called photosynthesis.
Carbon Dioxide + Water + Sunlight + Chlorophyll à Glucose + Oxygen + Water
From the above equation, photosynthesis is seen to be principally important because it is a means by which the sun’s energy is captured by plants for use by all organisms. It provides the source of complex organic molecules for heterotrophic organisms. It releases oxygen for use by aerobic organisms.
3. Leaf as a Photosynthetic Organ
In higher plants, the leaf is the major photosynthetic organ and its structure is closely linked to its function.
3.1 Adaptation of Leaf for Photosynthesis
3.1.1 External Shape and Structure
Leaves with a large surface area allows maximum absorption of sunlight and the leaves are thin so carbon dioxide has to diffuse across only short distances to reach the mesophyll cells.
3.1.2 Stoma
Many stoma are found in the lower epidermis, Stoma allows carbon dioxide to enter the leaf. At the same time, loss of water vapour in transpiration is minimized. Changes in the turgidity of the guard cells cause the stoma to open or close. This regulates the opening of stoma and ensures that the stomata open only in light when photosynthesis occurs.
3.1.3 Palisade Mesophyll
These cells lie just below the upper epidermis and contain numerous chloroplasts. This allows maximum absorption of sunlight. The chloroplasts are located near the periphery of cell for easier gaseous exchange with intercellular air spaces. Chloroplasts may be phototactic as they move within cells towards the light.
3.1.4 Spongy Mesophyll and Air Spaces
These cells are oval and loosely packed, with large intercellular sir spaces. This provides a passage through which carbon dioxide can diffuse.
3.1.5 Veins containing Vascular Bundles
This is a network of small vascular bundles continuous with those of stem and root. It is this that supports the leaf so that the leaf blade is often held at right angles to incident light. The vascular bundles contain xylem tissue which transports water and mineral salts to leaf cells and the water prevents wilting. The vascular bundles also contain phloem tissue which transports products of photosynthesis away from the leaf.
3.2 Chloroplasts – the organelle for photosynthesis
In eukaryotic cells, both light dependent and light independent phases of photosynthesis occur in the chloroplasts. In higher plants, chloroplasts are elongated and lens shaped. Average size of chloroplasts is about 4 to 7 microns. A typical plant mesophyll cell contains about 20 – 80 chloroplasts.
3.2.1 Ultrastructure of a Chloroplast
It is surrounded by 2 membranes which form the chloroplast envelope.
3.2.1.1 Thylakoids
They are flattened, fluid-filled sacs. Photosynthetic pigments and electron carriers are embedded on the thylakoid membrane. They are the site of light reaction. The thylakoid membrane surrounds and encloses a continuous internal / thylakoid space known as the lumen. Prominent stacks of thylakoids are known as granna.
3.2.1.2 Stroma
It is a protein-rich, gel-like matrix surrounding the thylakoid membranes. It contains the enzymes of dark reactions, organic acids, sugars, lipid droplets, chloroplast DNA etc.
3.2.2 Chloroplast as the site of Photosynthesis
Photosynthesis as indicated by the evolution of oxygen was associated primarily with chloroplasts. Certain wavelengths of light were more effective in bringing about photosynthesis than others.
4. Light – energy for Photosynthesis
To be of use as an energy source for living organisms, light energy must first be converted to chemical energy, such as in the form of complex organic molecules. Light has a wave nature. It forms a part of electromagnetic spectrum. Light also comes in discrete packets called quanta. A single quantum of light is called a photon. The shorter the wavelength, the more energy a photon of light possesses.
There are 3 features of light that make it biologically important. (1) Spectral quality (2) intensity and (3) its duration.
4.1 Spectral Quality
Visible light represents part of the electromagnetic spectrum which has a wavelength of between 400 nm to 700 nm. The amount of energy associated with a particular type of colour of light is inversely proportional to the wavelength. Blue light (400 nm) has more energy than red light (700 nm).
4.2 Photosynthetic Pigments as Light Receptors
Substances that absorb visible light are called pigments. Different pigments absorb light of different wavelengths. Most pigments absorb certain wavelengths of light more than other wavelengths.
Photosynthetic pigments transfer absorbed light energy to electrons which then enter chemical reactions. The role of pigments is to absorb light energy, thereby transducing it to chemical energy. A variety of pigments are involved in photosynthesis, with chlorophyll as the most important. The second groups of pigments are called caroteniods.
4.2.1 Chlorophyll
It absorbs light maximally in the blue-violet and red regions of the visible spectrum. Green light is reflected giving chlorophyll its characteristic colour. All chlorophylls comprise a complex ring system caller the porphyrin ring with a magnesium atom in the centre and a long hydrocarbon tail called the phytol tail.
The phytol tail is hydrophobic and so is anchored in the thylakoid membrane. The porphyrin ring is hydrophilic and so generally lies in the surface of the membrane next to the aqueous solution of the stroma.
The flat head is parallel to the membrane surface for light absorption. Modifications of side groups on the head cause changes in the absorption spectrum so that different energies of light are absorbed. The porphyrin ring structure contains some loosely bounded electrons involved in the electronic transitions ad redox reactions.
There are a number of different chlorophylls with chlorophyll a (chl a) and b (chl b) being the most common.
The primary photosynthetic pigment in green plants is chl a, which is found universally in all photosynthetic organisms. Chl a exists in several forms, depending on its arrangement in the thylakoid membrane and the type of pigments associated with it. Each form differs slightly in its red absorption peak i.e. at 670 nm, 680 nm, 690 nm, 700 nm.
Chlorophyll participates in the transfer of energy by resonance transfer or sentitised fluorescence (i.e. the direct transfer of energy) and electron transfer (i.e. redox reaction, the simultaneous gain and loss of electrons).
4.2.2 Carotenoids
They absorb light strongly in the blue-violet regions of the visible spectrum. They have the characteristic colours of yellow, orange, red or brown. The colours are usually masked by green chlorophylls but can be seen in leaves prior to leaf fall since chlorophylls break down first.
The basic structure comprises of 2 small rings linked by a long hydrocarbon chain. There are 2 main types of carotenoids: the carotenes and the xanthophylls. Carotenoids absorb wavelengths of light that chlorophyll cannot, thus broadening the spectrum of colours that can drive photosynthesis.
Carotenoids are not very effective as photosynthetic pigments. They transfer only about 10% of their absorbed energy. They are more important in absorbing excessive light and preventing auto-oxidation of chlorophyll. Photobleaching of chlorophyll is thus prevented. This is also known as photoprotection.
Excessive light intensity can damage the chlorophyll pigments. Instead of transmitting their absorbed light energy to chlorophylls, some carotenoids use photoprotection to accept excess energy from chlorophylls, thus preventing the chlorophylls from light destruction.
4.3 Absorption and Action Spectra
If a pigment, such as chlorophyll, is subjected to different wavelengths of light, it absorbs some wavelengths more than others. An absorption spectrum plots the degree of absorption of each wavelength of light by a particular pigment.
An action spectrum plots the effectiveness of different wavelengths of light on a photochemical process such as photosynthesis. Plants produce oxygen at a rate proportional to photosynthesis. Groups of plants are subjected to different wavelengths of visible light. The volume of oxygen produced under these varied wavelengths can be used to determine the most effective wavelength in photosynthesis.
The action spectrum for photosynthesis is closely related to the combination of absorption spectra for chl a and b and carotenoids. These photosynthetic pigments are responsible for the absorption of light used in photosynthesis.
5. Biochemistry of Photosynthesis
Photosynthesis is the process of energy transduction. Light energy is firstly converted into electrical energy and finally into chemical energy. The light harvesting phase is the stage where by light is captured by the plant using a mixture of pigments including chlorophyll. The light dependant stage is the stage where harnessed light energy is used to excite and displace an electron from chlorophyll, resulting in electron flow that is coupled to ATP synthesis. NADPH is also synthesized. Photolysis of water results in hydrogen ion formation and oxygen evolution. The dark stage is when hydrogen ions are used in the reduction of carbon dioxide and hence the manufacture of sugars.
The 1st two stages require light and take place in the thylakoid membranes of the chloroplasts. The 3rd stage does not require light and takes place in the stroma.
5.1 Light Harvesting Stage
Within the thylakoid membranes of the chloroplast, chlorophyll molecules are arranged along with their accessory pigments and proteins into groups of several hundred molecules. Each group is called a photosystem.
Photosystem contains hundreds of accessory pigments (chlorophylls and caroteniods) which act as light-harvesting antenna. It also contains a reaction centre which contains a specialized chl a molecule that can act as an energy trap. Lastly, it contains a primary electron acceptor and donor.
Accessory pigments are molecules that strongly absorb wavelengths not absorbed be reaction centre chl a. They serve to broaden the action spectrum of photosynthesis with the most common accessory pigment being chl b and carotenoids.
Accessory pigments work together. They act like a funnel to absorb light energy and pass it to the specialized reaction centre chl a, which absorbs light at a longer wavelength (light having a lower amount of energy).
When a photon of light strikes a pigment molecule, the energy is passed on to the neighbouring molecules by resonance (direct transfer of energy) and finally to the reaction centre for use in chemical reactions. In this transfer of energy, some energy is lost as heat. The energy that is passed on to the reaction centre excites an electron in its orbit. This electron is boosted to a higher energy level. This electron from the reaction centre is, in turn, accepted by a membrane-bound primary electron acceptor, thereby initiating a flow of electrons.
Whilst many chlorophyll molecules are involved in the capture of light, only very few special chl a molecules make up the reaction centre. There are 2 photosystems in plants: photosystem I (PSI) and photosystem II (PSII). In PSI, the reaction centre chl a is known as P700 as it absorbs light maximally at the wavelength if 700 nm. The reaction centre chl a of PSII is known as P680. P680 and P700 are identical chl a molecules, however, they are associated with different proteins in the thylakoid membrane. Electron distribution in the chlorophyll molecule varies as a result thus there are slight differences in the light absorption properties.
5.1.1 Presence of 2 Photosystems: Emmerson Red Drop and Enhancement Effect
Emmerson measures the yield of photosynthesis with different wavelength at which chlorophyll absorbs light. The values are constant, indicating that light absorbed at any of these wavelengths are as effective in driving photosynthesis, except at wavelengths above 685 nm. This effect is since known as the Emmerson Red Drop. Emmerson found that the efficiency of red light at 700 nm could be increased by adding a shorter wavelength of light (650 nm). The rate of photosynthesis in light of the wavelengths together was greater than the added rates if photosynthesis in either alone. Evidently, light is absorbed separately by 2 different pigment systems, one of which absorbs light maximally at 700 nm and the other at 680 nm.
5.2 Light Dependent Stage
This occurs in the thylakoid of the chloroplasts and involves photolysis of water. The purpose is to synthesize reduced nicotinamide adenine dinucleotide phosphate (NADPH + H+) and adenosine triphosphate (ATP).
When energy form photons of light is transferred to the reaction centre chl a, an electron in its orbit is raised to a higher energy level, initiating a flow of electrons. The electron released by the reaction centre chl a is passed on to the electron carriers of the ETC in the thylakoid membrane. Reaction centre chl a (P680 or P700) is now positively charged and needs to be neutralized by electrons.
As the electron passes down the ETC, energy released during the sequential reduction and oxidation of cytochromes creates a hydrogen ion concentration gradient, which drives the synthesis of ATP. In this process, ADP is converted to ATP. This addition of phosphate is termed phosphorylation. Since light is involved, this is called photophosphorylation. 2 main phosphorylation pathways in the light dependant stage are as follows: non-cyclic photophosphorylation (Z-scheme) and cyclic phosphorylation.
5.2.1 Non-cyclic Photophosphorylation
The photosystems involved are PSI and PSII. There has to be water to provide the huge energy electrons and carbon dioxide to be reduced by NADPH / H+. The processes involved include photolysis of water with the evolution of oxygen and the production of ATP and HADPH/H+.
The excited electron of P680 (PSII) is passed to an unknown electron acceptor ‘Q’, which then passes the electron to plastoquinone (pQ) and from there, down an ETC (cytochromes b to f) to another electron acceptor plastocyanine (pC). As each electron cascades down the ETC, its exergonic ‘fall’ to a lower energy level is harnessed by the thylakoid membrane to produce ATP from ADP and Pi.
The electron continues to descend down the ETC to P700 of PSI, where another photon of light provides the energy to boost the electron to another higher level. In the second series of reactions, the electron is passed to a different primary electron acceptor and from there to ferredoxin (Fd). NADP+ reductase transfers the electrons from Fd to NADP+. NADP+ is thus reduced to NADPH and H+ when it takes up a H+ from the stroma side of the thylakoid membrane (not lumen).
In addition, enzymatic photolysis of water occurs on the lumenal side of the thylakoid membrane. The electrons from the water molecule (2e-) replace the electrons from P680, which had been excited and passed down the ETC. 2 hydrogen ions from water accumulate in the thylakoid lumen and oxygen evolved as the oxygen molecule.
5.2.2 Cyclic Photophosphorylation
Only PSI is involved. There must be a lack of water and lack of carbon dioxide. ATP is produced. Photolysis of water with the evolution of oxygen does not occur. Reduced NADP is not produced.
Electrons from PSI return to the same chlorophyll via ferredoxin, through a series of redox reactions along the ETC in the thylakoid membrane, forming ATP in the process.
5.2.3 ATP formation – Chemiosmotic Theory
It is often known as the chemiosmotic photophosphorylation theory. Energy released as a result of electron flow down the ETC from PSII to PSI in the thylakoid membrane is used to move hydrogen ions form the stroma side into the lumen of the thylakoid. Also, photolysis of water, which produces hydrogen ions, occurs on the thylakoid membrane facing the thylakoid lumen.
The thylakoid membrane is impermeable to hydrogen ions. As the light reaction proceeds, hydrogen ions accumulate in the lumen of the thylakoid, which functions as the hydrogen ion reservoir. A concentration gradient develops between the thylakoid lumen and the stroma. The hydrogen ions diffuse out of the thylakoid lumen to the stroma through special channels in the thylakoid membrane. This movement is along the proton gradient and the energy released is coupled to ATP synthase (CF0-CF1 Complex).
ATP synthase catalyzes the phosphorylation of ADP to ATP on the stroma side of the thylakoid membrane. ATP formed in the stroma is used to help drive sugar synthesis during the Calvin cycle.
5.3 Light Independent Stage / Dark Stage / Calvin cycle
It occurs in the stroma of chloroplasts. It involves the following 4 stages.
5.3.1 Carbon Dioxide Fixation (acceptance of carbon dioxide)
Carbon dioxide is initially fixed by the combination with a 5C sugar (which acts as the carbon dioxide acceptor) – ribulose bisphosphate (RuBP) – to yield an unstable 6C intermediate that breaks down to form 2 molecules of phosphoglyceric acid (PGA) a.k.a. glycerate-3-phosphate (GP) a.k.a. 3-phosphoglycerate. Carboxylation is catalyzed by ribulose bisphosphate carboxylase oxygenase (RUBISCO)
5.3.2 Reduction of GP
Each molecule of GP receives an additional phosphate group from ATP to form 1,3-bisphosphoglycerate (same as step 6 of glycolysis). A pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (GALP; same compound as step 5 of glycolysis) which is a triose phosphate. Specifically, the electrons from NADPH reduce the carboxyl group of GP to the carbonyl group of GALP, which stores more potential energy.
5.3.3 Regeneration of RuBP
For every 3 molecules of carbon dioxide (3C) that enters the dark stage, 3 molecules of RuBP (15C) are carboxylated, generating 6 molecules of GP (18C). Only one molecule of GP can be counted as a net gain of carbohydrate. The other 5 molecules of GP must first be recycled to regenerate the 3 molecules of RuBP used in the fixation step. To accomplish this, the cycle spends 3 more molecules of ATP. RuBP is regenerated and the cycle continues.
5.3.4 Product Synthesis
The GALP /TP spun off from the dark stage become the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.
5.4 Fate of Photosynthetic Products
Although triose phosphate (TP) is the end product of the dark reaction, it does not accumulate in large quantities. Both GP and TP are also intermediates in glycolysis. Glycolysis and Krebs cycle occupy a central role in metabolism. TP can be taken out of the chloroplast and used by the cell in the synthesis of all other forms of carbon-containing substances. These include other carbohydrates, lipids, proteins, nucleic acids and chlorophyll.
5.4.1 Synthesis of Carbohydrates
A large proportion of TP is converted to hexose sugars, especially glucose and fructose. These may be used in respiration and for the production of ATP. For storage purposes, many glucose molecules are in turn linked together to form starch that are stored as starch granules. Glucose is also used in the synthesis of cellulose needed for the structural growth of plants (formation of the cell wall).
5.4.2 Synthesis of Lipids
GP enters the glycolytic pathway and is converted to an acetyl group, which is added to coenzyme A or form acetyl CoA. This is then converted to fatty acids in both cytoplasm and chloroplasts. Glycerol is made from TP. Fatty acids and glycerol combines to form triacylglycerides (for storage) and phospholipids (for cell membrane).
5.4.3 Synthesis of Proteins
GP and TP are important precursors in the synthesis of amino acids. In the process, nitrogen is incorporated into the molecules. Plants can, in turn, use these molecules to make other nitrogen-containing compounds including nucleotides (RNA and DNA).
5.4.4 Anabolic Metabolism
Most biological molecules are larger than TP, and so must be rearranged to build larger molecules. Such constructive metabolism is known as anabolism and it comprises anabolic reactions.
2 important anabolic pathways with regards to energy metabolism are the production of reduced NADP and ATP during photosynthesis and the synthetic pathways of polysaccharides and fats which are storage forms of energy and carbon.
Several types of storage compounds have evolved for different purposes.
For short term storage, ATP and reduced NADP are excellent sources of energy. They are very reactive and unstable and cannot be stored even for a very short time. A plant cannot stockpile such compounds to survive times when photosynthesis is not available. Neither can these compounds be transported over long distances.
For intermediate storage, there are glucose and sucrose. These are stable enough to be transported from one region to another (especially sucrose). However, they can affect osmotic pressure even at low quantities.
For long term storage, there are starch and lipids. These cannot affect osmotic pressure. They are stable and can last for years.
6. Factors Affecting Photosynthesis
The rate of photosynthesis is an important factor in crop production since it affects yields. An understanding of those factors affecting the rate is therefore likely to lead to an improvement in crop production.
6.1 Concept of Limiting Factors
When a chemical process is affected by more than 1 factor, the rate is affected by that factor that is nearest its minimum value (limiting factor). Limiting factor is any factor that directly alters the rate of a process when its quantity is changed. Changes in the levels of other factors have no effect on the reaction.
In photosynthesis, provided with all other factors, the process cannot proceed in the dark because the absence of light limits the process. The supply of light will alter the rate of photosynthesis, more light, the higher rate of photosynthesis. Any increase in the amount of water or carbon dioxide will not change the rate of the reaction as light is the limiting factor. However, if the amount of light is increased further, the rate of reaction will increase up to a point when other factors, such as concentration of carbon dioxide or temperature becomes limiting.
6.2 Effect of Light on the Rate of Photosynthesis
When considering the effect of light on photosynthesis, it is important to distinguish between the effects of light intensity, light quality and the duration of exposure to light.
6.2.1 Light Intensity
In low light intensity, the rate of photosynthesis increases linearly with increasing light intensity. Gradually, the rate of increase falls as some other factors becomes limiting.
Illumination on a clear summer’s day is about 100 000 lux, whereas light saturation for C3 plants is reached at about 10 000 lux, which is 10% of the full sunlight. Thus, except in early morning or early evening, and for shaded plants, light intensity is not normally a major limiting factor for photosynthesis.
Very high light intensities may damage chlorophyll and reduce the rate of photosynthesis. However, plants exposed to such conditions are usually protected by thick cuticles and hairy leaves.
The rate of photosynthesis is often measured by the amount of carbon dioxide absorbed or oxygen evolved by the plant. This, however, does not give an absolute measure of photosynthesis because oxygen is absorbed and carbon dioxide is evolved as a result of cellular respiration.
As light intensity is increased, photosynthesis begins. Some carbon dioxide form respiration is utilized in photosynthesis and so, less is evolved from respiration.
With continuing increase in light intensity, a point is reached where carbon dioxide is neither evolved nor absorbed. At this point, the amount of carbon dioxide produced in respiration exactly balances that being used in photosynthesis. It is known as the compensation point.
Further increase in light intensity results in a proportional increase in the rate of photosynthesis until light saturation is reached. Beyond this point, further increase in light intensity has no effect on the rate of photosynthesis. It is limited by other factors.
6.2.1.1 Compensation Point of Sun and Shade Plants
Some plants are adapted to positions of full sunlight and others to permanently shaded positions, Plants of these types are referred to as sun and shade plants respectively.
Shade plants have a lower rate of respiration than that of sun plants. One reason is that shade plants build thinner leaves with fewer palisade mesophyll layers. The smaller number of cells requires less energy for their production and maintenance. Consequently, the shade plants reaches its compensation point at a rather low light intensity and much sooner than the sun plants does.
Plants adapted to positions of full sunlight have a higher level of respiration and much higher compensation points. The benefit of this investment is the ability to absorb higher light intensities available. By producing more palisade layers, the sun plant can trap more light energy for the production of carbohydrates, at the expense of this higher investment.
6.2.2 Light Quality
Red light (linger wavelength) has less energy than blue light (shorter wavelength), so one photon of blue light provides more energy for photosynthesis than one photon of red light.
6.3 Effect of Carbon Dioxide Concentration on the Rate of Photosynthesis
The normally low atmospheric carbon dioxide concentration of 0.04% is a major limiting factor to photosynthesis. An increase in the concentration of carbon dioxide results in a proportional increase in the rate of photosynthesis, Greenhouse crops like tomatoes re grown in carbon dioxide enriched environment to provide greater yield.
6.4 Effect of Temperature on the Rate of Photosynthesis
The light stage of photosynthesis is unaffected by temperature. The dark stage is however, temperature dependent, as the reactions are enzyme catalyzed. Provided the other factors are not limiting, the rate of photosynthesis increases with an increase in temperature.
The rate of photosynthesis approximately doubles for each rise of 10oC from a minimum of 0oC up to an optimum temperature, which varies from species to species. Above the optimum temperature, the rate of increase is reduced until a point is reached above which there is no increase in photosynthetic rate.
The optimum temperature for most plants is around 25oC. Above this, the rate of reaction levels off, followed by a fall in the level of photosynthesis. This fall occurs at a temperature that is too low to be entirely accounted for by the denaturation of enzymes.
6.5 Effect of Oxygen Concentration on the Rate of Photosynthesis
As a product of photosynthesis, it is perhaps surprising that oxygen concentration can affect the rate of photosynthesis. Oxygen competes with carbon dioxide for the active site of the enzyme RUBISCO, which combines carbon dioxide with RuBP during the dark stage of the reaction. Therefore, high concentration of oxygen lowers the rate of photosynthesis.
6.6 Effect of Inorganic Ions on the Rate of Photosynthesis
Chlorophyll concentration is not normally a limiting factor. Certain inorganic ions are important for the synthesis of chlorophyll. Deficiency in any of these leads to the reduction of chlorophyll concentration and the leaves become yellow, a condition called chlorosis. This reduces the rate of photosynthesis significantly.
6.7 Other factors affecting the Rate of Photosynthesis
Water, as a substrate in the photosynthetic process, if it is deficient, will reduce the rate of photosynthesis and the amount of organic matter synthesized. Plants usually close their stomata in response to wilting and this would prevent the entry of carbon dioxide into the plant for photosynthesis.
Enzyme inhibitors are also a factor e.g. cyanide and dichlorophenyl dimethyl urea (DCMU).
Other factors include sulphur dioxide as a pollutant.
