Photosynthesis Descriptive Essay
Nowadays one can hardly find a man who does not know how we get oxygen on the Earth. One of the most simple explanations tell us that plants convert sunlight into chemical energy stored in two stages: first, they capture energy from sunlight, and then use it to bind carbon to form organic molecules. Plants absorb carbon dioxide formed during respiration, and release oxygen – a waste product of plants. In addition, photosynthesis plays a crucial role in the carbon cycle in nature.
Green plants – biologists call them autotrophs – are the basis of life on the planet. They convert the energy falling on them in the form of sunlight into energy stored in carbohydrates. This energy conversion process is called photosynthesis. Other organisms have access to this energy by eating plants. This creates a food chain that supports the planetary ecosystem. Now let us look back at the time when people did not know anything about photosynthesis.
Invention of Photosynthesis
For centuries, people have believed that plants exist due to their roots absorbing all the necessary substances from the soil through them. The first scientist who started suspecting this theory to be not true was Dutch naturalist Jan van Helmont. That is why he decided to check this view in the early nineteenth century. He grew a potted tree in the ground, which he watered only by rainwater. After five years he noted that the tree has grown to a large size, although the amount of land in the pot almost not decreased. Van Helmont, naturally concluded that the material from which the tree was formed came from the water used for irrigation.
In 1777, the English botanist Stephen Hales published a book in which it was reported that as a nutrient necessary for growth, plants use mainly air.
At the same period, the famous English chemist Joseph Priestley (he was one of the discoverers of oxygen) conducted a series of experiments on combustion and respiration, and came to the conclusion that green plants are able to perform all the respiratory processes, which have been found in animal tissues. Priestley burned a candle in an enclosed container of air, and found that the air could not support combustion. The mouse placed in a container died. However, a leaf of mint continued to live in the air for weeks. In conclusion, Priestley found that that leaf could let a candle burn and a mouse live. Now we know that burning candle consumes oxygen from a closed volume of air, but then the air was filled up with oxygen through photosynthesis again.
Several years later, a Dutch physician Ingenhauz found that plants oxidize oxygen only in sunlight and that their green parts provide oxygen evolution.
In 1817, two French chemists, Pelletier and Kavantu, separated some green stuff from the leaves and called it chlorophyll.
The next milestone in the history of the study of photosynthesis was made in 1845 by the German physicist Robert Mayer assertion that green plants convert energy of sunlight into chemical energy.
According to Regina Bailey,
“In plants, photosynthesis occurs mainly within the leaves. Since photosynthesis requires carbon dioxide, water, and sunlight, all of these substances must be obtained by or transported to the leaves. Carbon dioxide is obtained through tiny pores in plant leaves called stomata. Oxygen is also released through the stomata. Water is obtained by the plant through the roots and delivered to the leaves through vascular plant tissue systems. Sunlight is absorbed by chlorophyll, a green pigment located in plant cell structures called chloroplasts. Chloroplasts are the sites of photosynthesis.”
Chloroplasts consist of some structures which differ in their functions:
- Outer and inner membranes:represent a sort of protective covering which is responsible for keeping chloroplast structures safe and enclosed.
- Stroma:site of conversion of carbon dioxide to sugar. It is a dense fluid within the chloroplast.
- Thylakoid:site of conversion of light energy to chemical energy (flattened sac-like membrane structure).
- Chlorophyll:represents a green pigment within the chloroplast. Absorbs light energy.
- Grana:sites of conversion of light energy to chemical energy.
Light for photosynthesis is captured more fully thanks to a flat form of a leaf, providing a large surface. Water is delivered to the root through the extensive net of blood vessels (leaf veins). Carbon dioxide is supplied partly by diffusion through the cuticle epidermis, but most of it diffuses into the leaf through the stomata and leaf on the intercellular space.
Leaf carries out three very important processes – photosynthesis, evaporation and gas exchange. In the process of photosynthesis leaves that are using water and carbon dioxide under the influence of solar rays synthesize the organic substances. In the afternoon, as a result of photosynthesis and respiration, the plant produces oxygen and carbon dioxide, and at night – only carbon dioxide is produced during respiration.
Scientists proved that the source of energy for photosynthesis is predominantly red rays of the spectrum. This is indicated by the absorption spectrum of chlorophyll, where the most intense absorption is observed in the red, and less intense – in the blue-violet.
Factors Affecting the Rate of Photosynthesis
The intensity of the process of photosynthesis in plants depends on a number of internal and external factors. Among the internal factors, the most important are the leaf structure and content of chlorophyll, the rate of accumulation of photosynthesis products in chloroplasts, the effect of enzymes, as well as the presence of inorganic substances in low concentrations. The external factors include the quantity and quality of light reaching the leaves, temperature, the concentration of carbon dioxide and oxygen in the atmosphere near the plant.
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Most plants are able to synthesize chlorophyll in low light. In direct sunlight chlorophyll is synthesized faster. The less time the leaf is in dark, the more energy it can absorb. That is why, the plants in the process of evolution developed the ability to rotate leaves to light so that they get more sunlight. Leaves on the plant are located so as not to make any inconveniences to each other.
The rate of photosynthesis increases linearly, or is directly proportional to the intensity of light. Further increase in light intensity makes an increase in the rate of photosynthesis less and less pronounced, and finally stops when the illumination reaches a certain level of 10,000 lux. Further increase in the intensity of light has no influence on the rate of photosynthesis. If it is necessary to increase the rate of photosynthesis having provided the maximum intensity of light, one should not change the light intensity, and try to provide changes in any other factors. The intensity of sunlight on a clear summer day on the ground in many places on our planet is about 100,000 lux. Consequently, for plants, except for those that grow in dense forests and in the shadow, sunlight is enough to saturate their photosynthetic activity.
At low light intensities, the rate of photosynthesis at 15 and 25 ° C is the same. Reactions occurring at such intensities of light that corresponds to the limitation level of light, like photochemical reactions are not sensitive to temperature. However, at higher intensities, the rate of photosynthesis at 25 ° C is much higher than at 15 ° C. Consequently, photosynthesis depends not only on the photon absorption but also on other factors. Most of the plants in a temperate climate are used to the temperature that ranges from 10 to 35 ° C, the most favorable conditions are provided when the temperature is about 25 ° C.
At the average intensity of light the rate of photosynthesis is not changed by decreasing the concentration of CO2. This lets us conclude that C02 is directly involved in the photochemical reaction. At the same time, at the higher intensities of illumination exceeding the limitation, photosynthesis can be significantly increased by increasing concentration of CO2. High values of the rate of photosynthesis can be observed when the concentration of CO2 reaches about 0.1 %. The average concentration of carbon dioxide in the atmosphere is 0.03 %. Therefore, under normal conditions the concentration of CO2 in the air is not enough for plants to maximize the use from the sunlight. If a plant is placed in an enclosed container with the necessary light intensity, the concentration of CO2 in the air will gradually decrease and reach a constant level, known as “a degree of compensation”. At this point, the appearance of CO2 in photosynthesis is balanced by the release of O2.
The rate of photosynthesis also depends on the phase of plant development. The maximum rate of photosynthesis is observed in the flowering stage. Typical level of carbon dioxide in the air is 0.03%. Reduction of carbon dioxide in the air reduces the rate of photosynthesis. Increased level of carbon dioxide proportionally increases the rate of photosynthesis. However, with further increase of carbon dioxide in the air, photosynthetic rate does not increase. When it reaches the level of 1% – plants suffer.
What Do We Know about Photosynthesis Today?
Nowadays it is known that the process of photosynthesis consists of two stages, one of them depends on light. The proof for this fact was first obtained in 1905 by the English plant scientists who investigated the effect of light and temperature on the rate of photosynthesis.
Today, based on the experiments, we have the following conclusions:
- There is one group of light-dependent reactions that do not depend on temperature. The rate of these reactions can increase with increasing illumination, but never with increasing temperature.
- There is a second group of reactions dependent on the temperature, and not on light. Both groups of reactions are necessary for photosynthesis. Increase of only one group of reactions increases the rate of the entire process, but only until the point when the second group of reactions begins to hold the first. After this it is necessary to accelerate the reactions of the second group so that the first can pass without limitations.
Thus, it was shown that both stages are light-dependent. It is important to remember that the dark reactions are normal in light and need the conditions of the light stage. The term “dark reactions” simply means that light itself is not involved in them.
The rate of the dark reactions increases with temperature, but only to 30 °, and then begins to decrease. Basing on this fact scientists suggested that dark reactions were catalyzed by enzymes, since the exchange of enzymatic reactions depends on the temperature. In consequence it was found that this conclusion was wrong.
In the first stage of photosynthesis (light reaction) light energy is used to produce ATP (adenosine triphosphate molecule). In the second stage of photosynthesis (dark reaction) energy products formed in the light reactions are used for CO2 recovery to a simple sugar that is called glucose.