Q. 18

Draw and label a plant cell. Write the main function of:

Answer :

Plant cell differ from animal cell by the presence of a cell wall outside the cell membrane.

The plant cell is of rectangular shape and is comparatively bigger than the animal cell. Both plant and animal cells are eukaryotic cells and they have few similar cell organelles. When viewed under a microscope we see a plant cell with various components called as cell organelles, which are:


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(i) Chloroplast


A chloroplast is a disc shape organelle within a plant cell which allows plants to capture the Sun energy in energy-rich molecules. Both chloroplast and mitochondria evolved from an independent free-living prokaryote, hence they share similar functions.


Chloroplasts are surrounded by two membranes, i.e. bilayered. It is made of phospholipids. The outer membrane is more permeable to small organic molecules, whereas the inner membrane is less permeable and has transport proteins embedded in it. The innermost matrix of chloroplasts is called stroma. It contains metabolic enzymes and multiple copies of the chloroplast genetic material in the form of a circular DNA.


Chloroplasts also have an internal membrane called the thylakoid membrane which contains the light-harvesting complex, including pigments, as well as the electron transport chains which is used in photosynthesis. Each chloroplast contains a green colored pigment called chlorophyll which is required to carry out the process of photosynthesis. The chlorophyll pigment absorbs sunlight and uses it to transform carbon dioxide (CO2) and water (H2O) into glucose, which will be used up in further processes like respiration.


(ii) Plasma membrane


Every cell has an outer layer which regulates the inflow and outflow of various substances from a cell which is called a cell membrane. It is also called a plasma membrane. Its main function is to surround and protect the cytoplasm of the cell. Everything from outside has to cross the barrier which is cell membrane only then they can go inside of a cell. The plasma membrane is selectively permeable to ions like hydrogen, calcium, sodium, small molecules like oxygen (O2), Carbon-dioxide (CO2) and various larger molecules such as amino acids, glucose, sucrose, etc. Cell membrane performs various functions inside a cell which are osmosis, diffusion, transport of nutrients and molecules into the cell, waste removal, etc. The membrane is made of phospholipids double layer having protein and carbohydrate molecules infused within the lipid bilayer. This fluid-structure provides enough mechanical support and flexibility to allow cells to grow and move.


The Fluid Mosaic Model of the cell membrane was proposed and explained by S.J. Singer and G.L. Nicolson in 1972. The specific proteins present in the double layered membrane are very important for cell-cell recognition and interaction. This interaction allows various cells to acquire oxygen, nutrients, water etc. So basically the cell membrane acts as a gate which keeps the thing outside and inside according to the requirement of the cell.


(iii) Cell wall


Plant cell differ from animal cell by the presence of a cell wall outside the cell membrane. It provides strength, protection against mechanical and osmotic stress to the plant cell and to provide form and structure to the cell. Cell develops turgor pressure against the cell wall. When there is a healthy supply of water, a plant is protected from wilting due to this turgor pressure.


Plant cell walls are made of cellulose, glycoproteins, lignin, pectin, and hemicelluloses. The formation of the cell wall is done by microtubules. It consists of three layers called as primary, secondary and the middle lamella. The primary cell wall is mainly formed by cellulose laid down by various enzymes.


(iv) Lysosomes


Lysosomes are one of the membrane bound organelles which are present in both plant and animal cells. There membrane is also made up of phospholipids. This membrane protects the cell from the hydrolytic enzymes of lysosomes. This organelle inside the plant cell is most important for the process of digestion and excretion. It contains various digestive enzymes which digest the damaged and worn out organelles, bacteria’s, viruses etc. They are also called as “suicidal bags” or “garbage disposable units” of a cell by De Duve.


(v) Chromosomes


Chromosomes are the thread-like entities found in the nucleus of both animal and plant cells. They are made of both protein and nucleic acid, i.e. deoxyribonucleic acid (DNA). These are what makes offspring’s different from their parent while still carrying traits from parents. In humans the parents are two different individuals hence the offspring has the characteristics of both parents, while in plants self fertilization and cross fertilization, both are possible. If we self fertilize a plant, it will be same as its parents because both the parents have the same chromosomes. But if we cross fertilize a plant, then the offspring will be a hybrid having the characteristics of both the plants. Plants have linear chromosomes.


Hence every species of plant have a different set of chromosomes, for example a rice plant has 12 chromosomes while wheat has 42 numbers of chromosomes.


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