If high school or college level chemistry scared you, fear not. Of all the topics in this series, photosynthesis is the most scientific, but I will make every effort to make it the most simple to understand. It is safe to breath now...
Photosynthesis is one of the most remarkable biochemical processes found on Earth and allows plants to create their own food with just water, carbon dioxide and sunlight. Simple experiments carried out by scientists has shown that the rate of photosynthesis is critically dependent upon variables such as temperature, pH and intensity of light. The photosynthetic rate is usually measured indirectly by detecting the amount of carbon dioxide released by plants.
Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed. Don't let this definition confuse you, the goal of this presentation is to explain the photosynthetic process in simple terms. Once understood, you will appreciate how important light is to green-leafed plants, including the orchid.
Let's get the dreaded scientific part out of the way as fast as possible. If you rather, you can use this link to skip the techno mumbo jumbo and land at the "plain English" portion of this presentation.
Photosynthesis can be represented using a chemical equation.
The overall balanced equation is...
6CO2 + 6H2O ------> C6H12O6 + 6O2
CO2 = carbon dioxide
H2O = water
Light energy is required
C6H12O6 = glucose (sugar)
O2 = oxygen
Although the chemical equation appears straight forward the process actually involves several "steps" occurring in two major groups of reactions.
The two main groups of reactions in photosynthesis are classified as 1) the light reactions and 2) the dark reactions
The following diagram represents what occurs in the light reactions. Notice that the sun's light energy is needed to start this portion.
As light energy (in the form of photons) reaches a plant, chlorophyll molecules forming a light harvesting complex absorb that energy, exciting electrons. These electrons move along an electron transport chain, eventually transferring their energy into the bonds of ATP and NADPH.
ATP and NADPH act as highly charged energy carriers ready to provide energy to continue photosynthesis in the dark reactions.
Do the dark reactions have to take place in the dark - at night? NO! The dark reactions are called "dark" because they do NOT use light energy directly in order to occur.
So, what do the dark reactions do? Well, by using the energy of the ATP and NADPH, as well as some other special molecules including CO2 and H2O, a carbohydrate called glucose is able to be formed.
Why is glucose important? Glucose is a special sugar that stores energy for a plant. Humans and other organisms that eat plants receive a portion of that energy as well!
In addition to eating plants and therefore getting some of their stored energy from them, is there any other way photosynthesis is important for humans?
Photosynthesis involves a cycle of give and take on which life as we know it depends!
WOW! We've just shown how the sun is important for biological processes.
The above demonstration and explanation of photosynthesis comes from the Michigan State University and can be found here.
A more technical description of the photosynthetic process from the Royal Society of Chemistry can be found here. I warn you, they get down to the cellular level of the chloroplasts that give green leafs their color. I have included one of their diagrams (below) to further outline photosynthesis.
Summary of stages of photosynthesis
Factors affecting the rate of photosynthesis
The main factors are light intensity, carbon dioxide concentration and temperature, known as limiting factors.
As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately. As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor. Chlorophyll a is used in both photosystems. The wavelength of light is also important. PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis.
An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor.
Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops.
Rate of photosynthesis: limiting factors
The main factors affecting rate of photosynthesis are light intensity, carbon dioxide concentration and temperature.
In any given situation any one of these may become a limiting factor, in other words the factors that directly affects the rate at which photosynthesis can take place masking the effects of the other factors
Light and rate of photosynthesis
At low light intensities, as light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately (straight line relationship). The more photons of light that fall on a leaf, the greater the number of chlorophyll molecules that are ionised and the more ATP and NADPH are generated. Light dependent reactions use light energy and so are not affected by changes in temperature.
As light intensity is increased further, however, the rate of photosynthesis is eventually limited by some other factor. So the rate plateaus. At very high light intensity, chlorophyll may be damaged and the rate drops steeply (not shown in the graph).
Chlorophyll a is used in both photosystems. The wavelength of light is also important. PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a higher proportion of energy concentrated in these wavelengths will produce a higher rate of photosynthesis.
Carbon dioxide and rate of photosynthesis
An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction, and so the rate of photosynthesis generally increases until limited by another factor. As it is normally present in the atmosphere at very low concentrations (about 0.04%), increasing carbon dioxide concentration causes a rapid rise in the rate of photosynthesis, which eventually plateaus when the maximum rate of fixation is reached.
Temperature and rate of photosynthesis
Although the light dependent reactions of photosynthesis are not affected by changes in temperature, the light independent reactions of photosynthesis are dependent on temperature. They are reactions catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. It approximately doubles for every 10°C increase in temperature. Above the optimum temperature the rate begins to decrease, as enzymes are denatured, until it stops.
In 1905, when investigating the factors affecting the rate of photosynthesis, Blackmann formulated the Law of limiting factors. This states that the rate of a physiological process will be limited by the factor which is in shortest supply. Any change in the level of a limiting factor will affect the rate of reaction.
For example, the amount of light will affect the rate of photosynthesis. If there is no light, there will be no photosynthesis. As light intensity increases, the rate of photosynthesis will increase as long as other factors are in adequate supply. As the rate increases, eventually another factor will come into short supply. The graph below shows the effect of low carbon dioxide concentration. It will eventually be insufficient to support a higher rate of photosynthesis, and increasing light intensity will have no effect, so the rate plateaus.
If a higher concentration of carbon dioxide is supplied, light is again a limiting factor and a higher rate can be reached before the rate again plateaus. If carbon dioxide and light levels are high, but temperature is low, increasing temperature will have the greatest effect on reaching a higher rate of photosynthesis.
The Effect of Temperature on the Rate of Photosynthesis
What Is the Relationship Between CO2 & Oxygen in Photosynthesis?
How Does Temperature Affect Metabolism?
How does the intensity of light affect the rate of photosynthesis?
As light intensity increases, the photosynthetic rate increases until a point is reached where the rate begins to level off. At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions.
How does the amount of water affect the rate of photosynthesis?
If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of CO². It is difficult in normal lab conditions to prove that water directly affects photosynthesis unless a heavy isotope is used to trace the path of water. Chlorophyll is needed for photosynthesis.
How does the amount of carbon dioxide affect the rate of photosynthesis?
Gradually the rate falls of and at a certain carbon dioxide concentration the rate of photosynthesis stays constant (from point B to C on the graph). Here a rise in carbon dioxide levels has no affect on the rate of photosynthesis as the other factors such as light intensity become limiting.
Orchids use a photosynthesis strategy called crassulacean acid metabolism (CAM). CAM plants make up approximately 7% of plant species. Other notable CAM plants include cacti (such as the saguaro—a native of my home state, Arizona), agave (where tequila comes from), aloe vera and pineapple. (Source)
Most plants use the C3 metabolic pathway to turn carbon dioxide (CO2) into energy (there is also a third pathway, called C4, used by about 3% of plant species). All plants use sunlight and water to incorporate the carbons from CO2 into sugar, producing oxygen as a byproduct. When it is very hot or dry, C3 plants are at a disadvantage because they cannot efficiently use carbon due to a process called photorespiration. CAM plants are specifically adapted to these extreme environments. Their specialized leaves chemically store the carbon from CO2 acquired during the night and use it for photosynthesis during the day (when their stomata are closed, to prevent water loss). (Source)
So basically the upper surface does the light absorption, so it should face the light, if that is not the case, there is something wrong with the light source, either the orientation or the spectrum. The lower side of many orchid leaves are red in color due to anthocyanins (red coloured pigments) this is to absorb the the high energy blue light radiation that is reflected back from the surfaces on which orchids are growing.
Any green tissue can photosynthesize, though the capacity and efficiency varies. A leaf is going to do a fairly good job of it no matter what the orientation relative to the light, though not necessarily as good as it could be if it maximized the exposure of the top surface.
Most plants, even orchids, will orient new leaves to best catch the available light as they grow. If it doesn't it could mean that there is enough light that the plant doesn't need to maximize it. Or if you move or turn your plants often as they grow you could be causing the twisting as the leaf tries to turn towards the light that keeps changing direction. This might be especially noticeable in a Phal since they have large leaves that grow over a long period of time. Changing the direction of the light will cause twisting of Phal spikes too and for me that is enough reason to avoid turning them, and it usually keeps the leaves all oriented together too. .