Photosynthesis means the production of food using light energy. This process takes place in plants and some other organisms such as Algae and cyanobacteria. These organisms are called phototrophs. They utilize water and carbon dioxide and make sugars and starches. Oxygen is released as a byproduct, and then other organisms such as humans use this oxygen for respiration. In this way, photosynthesis also plays an important role in producing and maintaining the oxygen content of Earth.
Plants and their leaves increase the rate of photosynthesis by means of some important adaptations, such as a larger surface area of leaves and the presence of numerous stomata. In this article, first, we will have an overview of the process of photosynthesis. Then we will discuss all the adaptations of plants that help in increasing the rate of photosynthesis. At the end of the article, we will summarize the whole article.
Overview of Photosynthesis
The word “photo” means light, and the word “synthesis” is about making something. In this process, the special structure called chloroplast that is present in the leaves of plants uses carbon dioxide and water and makes carbohydrates. Plants get carbon dioxide from the air and water from the soil. The light energy of the sun is also used in this process, and oxygen is released as a byproduct.
Photosynthesis takes place in two stages. The first is light-dependent, and the second is light-independent. In the light-dependent reactions, chlorophyll present in the leaves absorb sunlight and makes two types of molecules, i.e., ATP and NADPH. NADPH is an electron carrier molecule, and the ATP is called an energy currency molecule. In the second light-independent stage that is also called the Calvin cycle, the energized molecules provide energy to drive the construction of carbohydrate molecules.
Photosynthesis is the opposite of the process of respiration. In respiration, carbohydrates go through an oxidation reaction and are converted into carbon dioxide. The two-process take place in different parts of a cell. The general equation of photosynthesis is written below.
CO2 carbon dioxide + 2H2O water + photons light energy → [CH2O] carbohydrate + O2 oxygen + H2O
Plants adapt to environmental conditions and promote photosynthesis. Now we will discuss what these adaptations are and how they help in promoting and regulating the rate of photosynthesis.
Structure of a Leaf
To understand how plants adapt to environmental conditions to increase the rate of photosynthesis, it is important to understand the structure of a leaf. The leaf has a complex structure consisting of the upper epidermis, mesophyll, lower epidermis, and other structures.
The internal components of a leaf are sandwiched between two layers of the epidermis. The epidermis may be one to several cells thick according to the special type of the plant. There is also a waxy layer present on the epidermis that is released by the epidermis, and it is called the cuticle. The main function of this waxy layer is to reduce water loss from the surface of the leaf. Some special cells called the guard cells are also present among the cells of the epidermis. These cells are in the form of a pair, and they help in gaseous exchange by acting as entrance and exit points of gases.
The mesophyll contains palisaded cells and spongy parenchyma. The palisaded cells are tightly packed, and they help in photosynthesis. On the other side, the spongy parenchyma contains air spaces. These air spaces allow gaseous exchange between the leaf and the outside environment. Both these layers contain many chloroplasts. Chloroplasts are membrane-bound organelles that contain a pigment called chlorophyll. The function of chlorophyll is to absorb light and help in the process of photosynthesis. It also imparts green colour to the leaves because it is a poorer absorber of green light. That’s why the leaves having less chlorophyll appear yellow instead of green.
The middle area of a leaf also contains vascular bundles composed of xylem and phloem. The main function of xylem bundles is to transport water and minerals to the leaves. This water is then used in the process of photosynthesis. The other type of bundle called the phloem transports the photosynthetic products produced in the leaf to the other parts.
The large surface area of Leaves
Leaves have a large surface area that helps in promoting photosynthesis. This is because light energy is needed for photosynthesis, and when the surface area of a leaf is large, more light coming from the sun can enter the leaf. Other parts of the plant, such as stem and internodal areas, do not have large surface areas because they are not concerned with the absorption of light.
When a plant is in a darker area, it needs larger leaves that can absorb more light. On the other side, the plants in sunny areas do not need larger leaves. Experiments that have been done on plants show that when plants are placed in darker areas, their leaves become larger, and the internodal areas (the area of stem between each leaf) grow rapidly and become longer. The rapid growth of internodal areas helps the plant to become longer and rapidly reach the light.
Different species of plants have different sizes of leaves. The banana tree has very large leaves as compared to the leaves of an apple tree. The banana tree leaves may grow 2.7 meters (8.9 ft) and 60 centimetres wide. On the other side, the apple tree leaves are just 1 to 4 inches long.
The leaves of plants are arranged in such a way to increase the absorption of light. When you look at a plant, you do not see leaves staked upon each other. Instead, the branches of a tree or a plant spread so that more leaves would have access to light. If leaves are not arranged in a systematic manner, the upper leaves may get sunlight, but the lower leaves get no sunlight, and the process of photosynthesis stops or slows down in lower leaves.
When a tiny seedling comes out of the soil, its leaves grow on both sides of the stem. Moreover, when a tree becomes larger and longer, its branches spread wide in all directions to cover more area. This allows all the leaves of a tree to have at least some access to light. Nature has provided the plants with this amazing quality which helps them in promoting photosynthesis.
There are different types of leaf arrangements in different species. Some of them are discussed below.
In this arrangement, only one leaf is produced from one node. These leaves are usually on the alternate sides of the stem. Members of the Poaceae family have this type of leaf arrangement.
In this arrangement, two leaves occur at one node in opposite directions. Plants of the mint family (Lamiaceae) and the maple family (Aceraceae) have this type of leaf arrangement.
When three or more leaves are present at one node, this arrangement is called whorled. Rubiaceae family plants show the whorled arrangement of their leaves.
Perfoliate arrangement of leaves refers to a leaf or a pair of connately fused leaves arranged in such a way that the stem is going through the centre. This type of leaf arrangement is seen in the Silphium perfoliatum.
The cuticle is a waxy layer released by the epidermis, and it is present on the outermost side. This serves as a protective covering of the epidermis and other aerial plant organs. The cuticle is composed of lipids, wax, and hydrocarbon polymers. The cuticular membrane is covered with hydrophobic aliphatic compounds that are sometimes called epicuticular waxes.
The main function of the cuticle is to reduce water loss that takes place through evaporation. It acts as a permeability barrier and saves water, especially in those plants which have a very low supply of water from the soil. As we know that water is needed for photosynthesis, so the leaf’s epidermis makes the cuticle retain the water inside the leaf. This helps in promoting the process of photosynthesis.
Moreover, the cuticle possesses special structural properties that help in preventing contamination of plant tissues with noxious agents such as dirt and microorganisms. The cuticle acts as a defensive barrier and resists the penetration of bacteria, viruses, and fungal spores. An example of the water barrier and protective function of the cuticle is the leaves of Nelumbo nucifera that have special hydrophobic and self-cleaning properties due to the presence of the cuticle.
The constant evaporation of water from leaves may also cause dehydration in the plants. That’s why the xerophytic plants have thicker cuticle as compared to the plants of wetter environments. It helps them in the conservation of the water to reduce the risk of dehydration.
The outer layer of the plant leaf is called the epidermis. It also covers other parts of the plant, including roots and stem. The epidermis is usually one cell thick but, in some plants living in extreme climates, it may be several cells thick. In leaves, two layers of the epidermis are present that sandwich the mesophyll of the leaves. The upper layer is called the upper epidermis, and the lower layer is called, the lower epidermis. It forms a barrier between the outer and inner environment of the leaf. There are several functions of the epidermis, such as protection against water loss, secretion of metabolic compounds, and regulation of gaseous exchange.
There are several types of cells present in the epidermis, such as epidermal cells, guard cells, subsidiary cells, and epidermal hairs. The epidermal hairs are also known as trichomes. The trichomes store some toxic or bad-tasting compounds, which help in restricting the movement of insects. Moreover, they also play an important role in reducing the rate of transpiration by blocking airflow across the surface of a leaf.
The epidermis helps in photosynthesis by reducing water loss. It does so by releasing a waxy layer called the cuticle, which has been discussed earlier in this article. There are numerous pores present in the epidermis. These are called stomata, and their function is to regulate the gaseous exchange. The stomata are surrounded by a pair of chloroplast-containing guard cells and two to four non-chloroplast subsidiary cells. The epidermis of the root also helps in the absorption of water from the soil.
The guard cells play an important role in saving the plant from dehydration. When there is a deficiency of water, a plant hormone, abscisic acid (ABA), is released, which causes the stomatal pores of the leaves to close. This hormone causes sudden ionic changes in the plasma membranes of the guard cells. These changes in ionic concentrations cause the guard cells to shrink and close the stomatal opening. This mechanism saves the life of the plant by decreasing the amount of water loss.
Guard cells open the stomata in normal or wetter conditions so that water evaporation and gaseous exchange can happen. Carbon dioxide needed for photosynthesis comes into the leaves from stomata, and guard cells regulate this gaseous exchange by opening and closing of stomata according to the environmental conditions.
As we discussed earlier, stomata (single “stomata”) are tiny openings in the most outer layer of the leaves. They may also present in the stem and other organs of the leaves. The primary function of a stoma is to control the rate of gas exchange. It allows carbon dioxide gas to enter the leaf, which is a necessary step of photosynthesis.
There are a lot of stomata present on the surface of one leaf. According to research, there are almost 121 to 484 stomata present per square millimetre of the leaf. This huge number of stomata efficiently helps in photosynthesis and gaseous exchange. The size of the stomata is very small. According to research, the mean size of an open stoma is 7.7 x 6.7 micrometre. As we described earlier, there are two guard cells present on both sides of the stomata that regulate its opening and closing.
Carbon dioxide enters the leaves through these tiny holes by the process of diffusion, and water evaporates from the stomata. Although the oxygen produced during the process of photosynthesis may also leave from these pores, it may also be used by the plants in the process of cellular respiration. It is also a well-known fact that plants are oxygen bombs and play a very important role in maintaining the oxygen of the environment.
According to various researches, the lower surfaces of leaves have more stomata than the upper epidermis and other organs of the plant. This phenomenon is very important because the lower surfaces of the leaves are protected from direct sunlight. In extreme conditions, the stomata present on the lower surface of the leaves do not get damaged. Moreover, they are also protected from the breeze. This is important because a strong breeze increases the rate of evaporation of water.
Thinness of leaves
Another important adaptation of plants to photosynthesis is the thinness of their leaves. In contrast to the other organs of the plant, the thickness of a leaf is much less. This adaptation helps the leaves get more sunlight. The sunlight easily penetrates the leaves and easily reaches the chloroplasts present in the mesophyll. This tells us that if the leaves of a plant are thicker, there would be less absorption of photosynthesis and, as a consequence, less photosynthesis.
The thinness of leaves helps in gaseous exchange as well because carbon dioxide and oxygen have to cover a short distance to diffuse in and out of the leaves, respectively.
Chloroplasts are the tiny organelles that conduct the process of photosynthesis in phototrophs. They are mostly present in large numbers in the leaves and help in promoting the process of photosynthesis. These structures contain a pigment called chlorophyll that captures the energy of sunlight. Chlorophyll converts the light energy and stores it as ATP and NADPH. In the process of photosynthesis, it also liberates the oxygen from the water, which is then used in cellular respiration.
The size of a chloroplast is about 4 to 6 micrometres in diameter and 1 to 3 micrometres in thickness. There are different parts of the chloroplast, such as inner and outer membranes, thylakoid membranes, intermembranous space, and lamella. There are two regions in the chloroplast. The first is the Grana, which is made up of disc-like thylakoid structures. The second is the stroma, which is a homogenous mixture similar in nature to the cytoplasm of a cell. The stroma contains DNA, ribosomes, enzymes, and other different substances.
The thylakoid system contains green-coloured pigment, the chlorophyll, which is the site for the light-dependent reactions of photosynthesis. In contrast to this, light-independent reactions take place in the stroma, and carbohydrates are produced. The light-independent reactions are also called the Calvin cycle due to their cyclic nature. As we know that the process of photosynthesis depends on the number of chloroplasts, the leaves with more chloroplasts have more photosynthesis.
The extensive system of veins
Photosynthesis needs water, and water gets to the leaves from the soil through a bundle of vessels called the xylem. The leaves of a plant have an extensive system of these vessels. There is a very complex and diverse venation found in the leaves. The structure and distribution of these veins are responsible for the distribution and transport of water, nutrients, and sugars. Moreover, these veins also provide biomechanical and structural support to the leaves.
The environmental conditions may influence the evolution of the leaf venation. The leaf venation influences the rate of photosynthesis. It rapidly transports the water and minerals from the soil to the leaves and food to the different organs of the plant. This rapid system of transportation helps in promoting the rate of photosynthesis because when photosynthesis increase when water is abundantly available.
- Photosynthesis is the production of food using water and carbon dioxide in the presence of sunlight. The energy of sunlight is converted to NADPH and ATP, which are then used to make carbohydrates. The process of photosynthesis is completed in two steps. The first happens in the presence of light, where the ATP and NADPH are produced. This is called a light-dependent reaction. The second step is the creation of sugars using the energy stored in ATP and NADPH in the absence of light. The second reaction is called the light-independent reaction or Calvin cycle.
- The plants have many adaptations which help them in promoting the process of photosynthesis. For example, the larger surface area of leaves helps in getting more sunlight that helps in increasing the rate of photosynthesis. Another adaptation is the arrangement of leaves of a plant. The leaves are arranged in such a way that all the leaves may get some sunlight. The branches of a tree spread in all directions to gain a larger surface area and to ensure the availability of sunlight for all the leaves.
- The process of photosynthesis needs water, and water is conserved in the leaves by the epidermis and cuticle of the leaves. The epidermis is the outermost layer of the leaf, and the cuticle is a waxy layer that is released by the epidermis. A thick cuticle reduces the loss of water and saves the plant from dehydration. Another adaptation is the chloroplast which is a structure present in the mesophyll of the leaf, and it contains a green pigment called chlorophyll. The chlorophyll is the site where light-dependent reactions of photosynthesis take place.
- Another adaptation is the presence of a large number of tiny holes in the epidermis. These holes are called stomata and help in the diffusion of carbon dioxide and oxygen into and out of the leaves, respectively. A pair of two guard cells is responsible for the opening and closing of stomata depending upon the different environmental conditions. The thinness of leaves is also an important adaptation because the sunlight can easily penetrate the leaves and reach the chlorophyll. Lastly, the extensive system of veins in the form of xylem phloem bundles in the leaves helps in rapid transportation. Rapid transportation is very important because it helps in increasing the rate of photosynthesis.
- Bryant DA, Frigaard NU (Nov 2006). “Prokaryotic photosynthesis and phototrophy illuminated”. Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. PMID 16997562.
- Reece J, Urry L, Cain M, Wasserman S, Minorsky P, Jackson R (2011). Biology (International ed.). Upper Saddle River, NJ: Pearson Education. pp. 235, 244. ISBN 978-0-321-73975-9. This initial incorporation of carbon into organic compounds is known as carbon fixation.
- Olson JM (May 2006). “Photosynthesis in the Archean era”. Photosynthesis Research. 88 (2): 109–117. doi:10.1007/s11120-006-9040-5. PMID 16453059. S2CID 20364747