Introduction to photosynthesis
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Every living creature needs food or energy to survive. Some depend on others for food and energy, whiles others can produce their own food.
Plants make their own food, glucose, in a process called photosynthesis. We say that plants can photosynthesise.
Photosynthesis and respiration go hand in hand. The result of photosynthesis is glucose, which is stored as chemical energy in the plant cells. This stored chemical energy comes from the conversion of inorganic carbon (carbon dioxide) into organic carbon. Respiration releases the stored chemical energy.
NOTES:
* Living things that depend on others for food are called heterotrophs. Herbivores like cows and other plant eating insects are examples of heterotrophs.
* Living things that produce their own food are called autotrophs. Green plants and algae are good examples of autotrophs.
Apart from the food they manufacture, plants also need carbon, hydrogen and oxygen to survive. Water absorbed from the soil provides the hydrogen and oxygen. During photosynthesis, carbon and water are used to synthesize food. They also need nitrate to make amino acids (Amino acid an ingredient for making protein). In addition to that, they need magnesium to make chlorophyl.
In this lesson, we shall learn more about how plants manufacture food (photosynthesis), and learn more about the conditions that must be present for this to happen.
What is photosynthesis?
Photosynthesis is a chemical process through which plants, some bacteria and algae, produce glucose and oxygen from carbon dioxide and water, using only light as a source of energy.
This process is extremely important for life on earth as it provides the oxygen that all other life depend on.
What does the plant need glucose (food) for? Just like humans and other living things, plants also need this food for many things. Le’s see a few:
* Glucose resulting from photosynthesis is used during respiration to release energy that the plant needs for other life processes.
* The plant cells also convert some of the glucose into starch for storage. This can then be used when the plant needs them. This is why dead plants are a used as biomass, because they have stored chemical energy in them)
* Glucose is also needed to make other chemicals such as proteins, fats and plant sugars that are all need for the plant to carry out essential growth and other life processes.
Photosynthesis is divided in two parts:
Light and dark reactions in photosynthesis
- Light-dependent reactions (light reactions)
- Light-independent reactions (dark reactions)
Light reactions need light to produce organic energy molecules (ATP and NADPH). They are initiated by colored pigments, mainly green colored chlorophylls.
Dark reactions make use of these organic energy molecules (ATP and NADPH). This reaction cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy, while NADPH provides the electrons required to fix the CO2(carbon dioxide) into carbohydrates.
NOTES This means Dark reactions will fail to continue if the plants are deprived of light for too long, since they use the output of the initial light-dependent reactions.
The basic structure of a leaf
We cannot fully study photosynthesis without knowing more about the structure of a leaf. The leaf is adapted to play a vital role in the process of photosynthesis.
* Wide surface area Mosts green plants have leaves that are broad, flat and exposed to capture as much of the suns energy (sunlight) need for photosynthesis.
* Veins The network of veins in the leaf also carry water from the stems to the leaves. Glucose produced is also sent to the other parts of the plant from the leaves through the veins. Additionally, the veins support and holds the leaf flat to capture sunlight
* Pores (holes) The stomata (tiny holes underneath the leaf) allows air in and out of the leaf. Stomata (single is called stoma) is usually at the bottom surface of the leaf but some plant species have them on the upper surface whiles other have them on both sides. The stomata closes in the night to retain gases and moisture in the leaf cells, and opens during the day for gaseous exchange to continue.
Below is a close diagram of the leaf structure:
The structure of a leaf Take a good look at the diagram and the various parts of the leaf structure. It will be very useful on the next page.
Conditions for photosynthesis
Below are things that plants need for photosynthesis:
* Carbon dioxide (A colorless, naturally occuring odorless gas found in the air we breathe. It has a scientific symbol CO2. CO2 is produced by burning carbon and organic compounds. It is also produced when plants and animals breathe out during respiration)
* Water
* Light (Even though both natural and artificial light is OK for plants, natural sunlight is usually great for photosynthesis because they have other natural UV properties that help the plant)
* Chlorophyll (This is the green pigment found in the leaves of plants)
* Nutrients and minerals (Chemicals and organic compounds which the plant-roots absorb from the soil)
Below are things that plants make by photosynthesis:
* Glucose
* Oxygen
NOTES
* Plants get CO2 from the air through their leaves, and water from the ground through their roots. Light energy comes from the sun.
* The oxygen produced is released into the air from the leaves. The glucose produced can be turned into other substances, such as starch, which is used as a store of energy. This energy can be released by respiration.
If factors that aid in photosynthesis are absent or less, it can negatively affect the fruits of the plant. For example, less light, insects that chew on leaves, less water can make plants such as (tomato plant) suffer and produce less yield.
Where does photosynthesis take place?
Photosynthesis takes place inside plant cells in small things called chloroplasts. Chloroplasts (mostly found in the mesophyll layer) contain a green substance called chlorophyll. Below are the other parts of the cell that work with the chloroplast to make photosynthesis happen.
Structure of a mesophyll cell
What role do these parts play? Cell walls: provide structural and mechanical support, protect cells against pathogens, maintain and determine cell shape, control the rate and direction of growth and generally provide form to the plant.
Cytoplasm: provides the platform for most chemical processes, controlled by enzymes. Cell membrane: acts as a barrier, controlling the movement of substances into and out of the cell.
Chloroplasts: As described above, simply contain chlorophyll, a green substance which absorbs light energy for photosynthesis.
Vacuole: the container that hold moisture, and keeps the plant turgid.
Nucleus: this contains genetic make (the DNA), which controls the activities of the cell. Chlorophyll absorbs the light energy needed to make photosynthesis happen. It is important to note that not all the colour wavelengths of light are absorbed. Plants mostly absorb red and blue wavelengths — they do not absorb light from the green range.
Carbon dioxide in photosynthesis
Plants get carbon dioxide from the air through their leaves. The carbon dioxide diffuses through small holes in the underside of the leaf called stomata. (singular: stoma. plural: stomata)
The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to reach the other cells in the leaf. This also allow the oxygen produced in photosynthesis to leave the leaf easily.
Carbon dioxide is present in the air we breathe, at very low concentrations. Even though it forms about .04% of the air, it is a needed factor in light-independent photosynthesis.
In higher concentrations, more carbon is incorporated into carbohydrate, therefore increasing the rate of photosynthesis in light-independent reactions.
Light for photosynthesis
A leaf usually has a large surface area, so that it can absorb a lot of light. Its’ top surface is protected from water loss, disease and weather damage by a waxy layer. The upper part of the leaf is where the light falls, and it contains a type of cell called a palisade cell. This is adapted to absorb a lot of light. It has lots of chloroplasts.
In light dependent reactions (as explained in light and dark reactions), photosynthesis increases with more light. More chlorophyl molecules are ionised and more ATP and NADPH are generated as more light photons are focussed on a green leaf. Even though light is extremely important in light dependent reactions, it is important to note that excessive light can damage chlorophyl and photosynthesis can reduce.
Light dependant reactions do not rely too much on temeperature, water or carbon dioxide, even though they are all neccesary for the process to complete. This means cold or hot, the reactions will occur as long as there is enough light.
Water for Photosynthesis
Plants get the water they need for photosynthesis through their roots. The roots have a type of cell called a root hair cell – these project out from the root into the soil. Roots have a big surface area and thin walls, which allow water to pass into them easily.
NOTES Root cells do not contain chloroplasts, as they are normally in the dark and cannot photosynthesise.
Plants need water for other important things such as:
* provide dissolved minerals that keep the plants healthy
* provide a medium for transporting minerals
* keep the plant firm and upright
* keep the plant cool and hydrated
* allow other chemical reactions to occur in plant cells
The Structure of the Roots The water absorbed by the root hair cells passes through the plant in xylem tubes, and eventually reaches the leaves. Energy from the sun is eventually used to convert water into hydrogen and oxygen.
Structure of the roothairs
This hydrogen is combined with the carbon dioxide in order to produce the food (glucose) for the plant, whereas the oxygen, which is a by-product of the entire process, is let out through the stomata.
If a plant does not absorb enough water, it will wilt or go floppy. Without water it may also not photosynthesise quickly enough, and it may die. Roots Roots systems and root hairs are adapted to play a special role for the plant. The root network spreads out to to absorb water (and mineral salts) from a large amount of soil. It is also adapted to hold the plant firmly and provide support (anchor) to the plant in the soil. Hope you enjoyed the lesson and found it useful. We would like to hear from you with feedback. Send us a mail: teacher@eschooltoday.com Thank you.
