Plasma membrane: Plasma membrane or cell membrane is the outer most boundary of cell. However, in most plant cell it is covered by cell wall. It is chemically composed of lipids and proteins: 60 to 80% are proteins while 20 to 40% are lipids. In addition there is small quantity of carbohydrates. Many biologists contributed to establish structural organization of cell membrane. It was proposed earlier that cell membrane is composed of lipid bilayer sandwiched between inner and outer layers of protein. This basic structure is called unit membrane and is present in all cellular organelles. Modern technology has revealed that lipid bilayers are not sandwiched between two protein layers. The protein layers are not continuous and are not confined to the surface of membrane but are embedded in lipid layers in mosaic manner. This discovery lead to proposal of fluid mosaic model. This model at present is most accepted one. Cell membrane also contain charged pores through which movement of materials takes place both by active and passive transport.
Transport of materials is one of the vital roles it plays for cell. It offers a barrier between the cell contents and their environment allowing only selective substances to pass through it thus it is known as differentially permeable or selectively permeable membrane. Substances which are lipid soluble cross it more easily than others therefore it regulates the flow of materials and ions to maintain a definite gradient. Many small gas molecules, water and glucose etc. being neutral can easily cross while ions being charged particles have some difficulty in crossing. Many substances which are not needed constantly enter the cell by passive transport others are taken up against concentration gradient. This uphill movement of materials requires energy and is termed as active transport. Energy used for this movement is provided by ATP. In many animal cells the cell membrane helps to take in materials by infolding in the form of vacuoles. This type of intake is termed as endocytosis which can be either phagocytosis( to engulf solid particles) or pinocytosis( to take in liquid material). In neurons it transmits nerve impulses from one part of the body to other to keep coordination.
Cell wall: The outer most boundary in most of plant cells is cell wall. It is different from that of prokaryotes both in structural and chemical composition. It is secreted by protoplasm of cell. Its thickness varies in different cells of plant. It is composed of three main layers: primary wall, secondary wall and middle lamella. Middle lamella is first to be formed in between primary walls of neighbouring cells. Primary wall is composed of cellulose and some deposition of pectin and hemicellulose. Cellulose molecules are arranged in criss cross arrangement. Primary wall is true wall and develops in newly growing cells. Secondary wall is formed on its inner surface and it is thick and rigid. Chemically it is composed of inorganic salts, silica, waxes, cutin and lignin etc. Prokaryotic cell wall lacks cellulose its strengthening material is peptidoglycan or murein. Fungal cell wall contains chitin. Cell wall is very important. It provides a definite shape to the cell and keeps it rigid. It does not act as barrier to materials passing through it.
Cytoplasm: Living contents of eukaryotic cell are divided into nucleus and cytoplasm the two collectively form protoplasm. Cytoplasm consists of aqueous ground substance containing a variety of cell organelles and other inclusions such as insoluble wastes and storage products. Its soluble part is called cytosol. It forms ground substance of cytoplasm. Chemically it is about 90% water. It forms a solution containing all fundamentals molecules of life. In cytosol small molecules and ions may form true solutions and some large molecules form colloidal solutions. Colloidal solution may be sol or gel. Peripheral parts of cell are often like a gel. Its most important function is to act as a store house of vital chemicals. It is also a site for certain metabolic processes such as glycolysis. In living cells it contains several cell organelles. The free floating cell organelles move about in it due to cytoplasmic streaming movements. This is an active mass movement of cytoplasm.
Endoplasmic reticulum: Under electron microscope a network of channels is seen extending throughout the cytoplasm. These channels are often continuous with plasma membrane and also appear to be in contact with nuclear membrane. This entire system of channels is endoplasmic reticulum. The material present in these channels is separated from the cytoplasmic materials by spherical or tubular membranes called cisternae. There are two morphological forms of endoplasmic reticulum; a rough form with attached ribosomes and a smooth form without ribosomes. Rough surfaced endoplasmic reticulum(RER) is involved in synthesis of proteins. After synthesis proteins are either stored in cytoplasm or exported out of cell through these channels. Smooth surfaced endoplasmic reticulum(SER) helps in metabolism of number of different types of molecules particularly lipids. They also help to detoxify harmful drugs. In some cells SER is responsible for transmission of impulse e.g. muscle cells, nerve cells. In addition SER also play important role in transport of materials from one part of the cell to other. Endoplasmic reticulum also provides mechanical support to cell so that its shape is maintained.
Ribosomes: Cell contains many tiny granular structures known as ribosomes. Palade was the first person to study them. Eukaryotic ribosomes are composed of almost equal amount of RNA and protein hence they are ribonucleo-proteins particles. RNA present in ribosome is called ribosomal RNA. Ribosomes exist in two forms either freely dispersed in cytoplasm or attached with RER as tiny granules. It consists of two subunits. Larger subunit sediments at 60S while smaller subunit sediments at 40S. Two subunits on attachment with each other form 80S particle. They are attached to messenger RNA through small ribosomal subunit. A group of ribosomes attached to mRNA is known as polysome.
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