Important cell organelles which are present in the plant cell are listed below:
Cell is the basic unit of structure and function in living organisms. This is known as cell theory, and was proposed by Schleiden and Schwann. A cell can be thought of as a bag of chemicals which is capable of surviving and replicating itself. All living cells are surrounded by a membrane. A brief description of various cells organelles is given here.
1. Cell wall:
It is present in plants, prokaryotes and fungi. It is a rigid structure and secreted by the living cell within. In plants primary and secondary cell walls are present.
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Primary cell wall-consists of hemicelluloses (53%), Cellulose (30%), Pectin (5), and protein (5%). Secondary cell wall-this is the innermost layer of the wall and primarily consists of cellulose.
Basically cellulose fibrils run through a matrix of other polysaccharides. Cell walls are hydrated and 60-70% of their wall mass is usually water. In fungi cell walls are made up of chitin where as in bacteria the walls contain muramic acid.
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Cell wall gives the mechanical support to the cell. Walls are fairly rigid and are resistant to the expansion and therefore allow development of turgidity when water enters the cell by osmosis.
2. Middle lamella:
In plants, the walls of immediate neighbor cells are joined by middle lamella, which is composed mainly of pectin in the form of calcium and magnesium salts. Cellulose is characteristically absent from middle lamella.
3. Plasma membrane:
It is the most important of the biological membranes. It consists of two layers of lipid molecules (lipid bilayer) with the protein molecules embedded either fully or partially in the lipid layer. (Fluid mosaic model-Danielle-Daveson). It is a partially permeable barrier controlling the exchange between the cell and its environment.
4. Endoplasmic reticulum (ER):
It is a system of flattened membrane bound sacs called cisternae, forming tubes and sheets. It is continuous with the outer membrane of the nuclear membrane. It is absent in prokaryotes.
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If ribosomes are found on its surface, it is called rough ER and transports proteins made by ribosomes through the cisternae. Smooth ER (without ribosomes) is a site of lipid and steroid synthesis. It also acts the endo skeleton of the cell giving it structural stability.
5. Golgi bodies:
Function as packing organs of the cell. It is a stack of flattened membrane bound sacs called cisternae continuously being formed at the end of the stack and budded of as vesicles at the other. Processing of many cell materials takes place in the cisternae. Golgi vesicles transport the materials to other parts of the cell or to the cell surface membrane for secretion. It also makes Lysosomes.
6. Lysosome:
It is a simple spherical structure bound by a single membrane and contains hydrolytic enzymes. They help in digestion of food particles and microorganisms ingested by a cell.
7. Mitochondria:
These are the power houses of the cells concerned with respiration. It is enclosed by two unit membranes. The inner membrane is highly folded inside and these folding are called christae. It also contains 70s ribosomes and DNA fragments.
8. Ribosomes:
These are very small organelles consisting of a large and a small subunit. They are made of roughly equal parts of protein and RNA. Slightly smaller ribosomes are found in mitochondria and chloroplasts.
These are of two type-80 S size in eukaryotes (with sub units of 60S and 40S) and 70S size in prokaryotes, chloroplast and mitochondria (with subunits of 50S and 30S).
These are the sites of protein synthesise. They are either bound to ER or lie free in the cytoplasm. They may form polyribosomes which are collections of ribosomes strung along the messenger RNA.
9. Nucleus:
It is the largest cell organelle enclosed by an envelope of two membranes that is perforated by nucjear pores. It contains chromatin which is the extended form taken by chromosomes during interphase. It also contains nucleolus.
Chromosomes contain DNA, the molecule of inheritance. DNA is organized into genes which control the activities of the cell. Nuclear division is the basis of cell replication and hence reproduction. The nucleolus manufactures ribosomes.
10. Chloroplast:
It is an ovular structure containing the green pigment chlorophyll. It is surrounded by an envelope of two membranes and contains a gel like stroma through which runs a system of membranes that are stacked in places to form grana.
It may store starch. The stroma also contains ribosomes, a circular DNA molecule and lipid dropletes. It is the unique structure present in plants and algae where photosynthesis takes place.
11. Vacuole:
It is a sac bound by single membranes such as mineral salts, sugars, pigments, organic acids and enzymes. These are typically large in mature cells. It makes an important contribution to the osmotic properties of the cell.