Plant Cell Structure & Function PDF

Structure of Plant Cell Dr. J. Joel Gnanadoss Dept. of Plant Biology and Biotechnology Loyola College, Chennai – 600 034 Plant Cell – Light Microscope Plant Cell – Electron Microscope Plant Cell Diagram  A plant cell consists of three distinct components: (i) Cell wall (ii) P...

Structure of Plant Cell Dr. J. Joel Gnanadoss Dept. of Plant Biology and Biotechnology Loyola College, Chennai – 600 034 Plant Cell – Light Microscope Plant Cell – Electron Microscope Plant Cell Diagram  A plant cell consists of three distinct components: (i) Cell wall (ii) Protoplasm (Cytoplasm + Nucleus) (iii) Vacuole.  The protoplasm is the living part of the cell.  It is externally bounded by cell membrane or plasma membrane.  The cytoplasm contains several cell organelles namely mitochondria, plastids, ribosomes, endoplasmic reticulum, lysosomes etc. (i) Cell wall  Cell wall is the non-living protective layer outside the plasma membrane in the plant cells, bacteria, fungi and algae.  The synthesis of cell wall in controlled by Golgi bodies.  In bacteria the cell wall is composed of protein and non-cellulosic carbohydrates while in most algae, fungi and all plant cells, the cell-wall is formed of cellulose.  Cell wall provides mechanical support and gives a definite shape to the cell.  It protects plasma membrane and helps in imbibition’s of water and movement of solutes towards protoplasm. (ii) Protoplasm  Protoplasm is the living, colourless, elastic, colloidal semi fluid substance present in the cell. Protoplasm with non-living inclusions is called protoplast.  Water is the chief constituent of an active protoplast and normally constitutes 90% of the system.  The remaining parts are organic and inorganic materials.  Each protoplast keeps itself in communication with neighbouring protoplasts through small openings in the cell wall known as plasmodesmata.  Protoplasm consists of cytoplasm and nucleus and is externally bounded by the cell membrane or plasmalemma. (iii) Cell membrane  It is a thin film like pliable membrane, and serves as protective covering of the cell.  Cell membrane mainly consists of proteins and lipids but in certain cases, polysachharides have also been found.  It facilitates the entrance of nutrients into the cells and allows exit of nitrogenous wastes, regulates the passage of materials into and out of the cells.  It controls and maintains differential distribution of ions inside and outside the cell. (iv) Cytoplasm  It is a jelly like fluid mass of protoplasm excluding the nucleus and surrounded by plasma membrane on the outside.  It is semi-permeable in nature.  The cytoplasm is composed of matrix; the membrane bound organelles and non-living inclusions like vacuoles and granules.  The living cytoplasmic organelles are the site of various important metabolic activities such as photosynthesis, respiration, protein synthesis etc.  (v) Plastids Plastids are the largest cytoplasmic organelles bounded by double membranes. There are three types of plastids: Leucoplast: These are colourless plastids found in storage organs where light is not available e.g. underground stem, roots and deeper tissues. Plastids without pigments are called leucoplasts. They store starch, fats or proteins in meristematic, embryonic and germ cells. Chromoplast: Coloured plastids are called chromoplasts. These may contain red, yellow, brown, purple, blue or green pigments. These are mostly found in petals of flowers and fruits. Chloroplast: In a plant cell, chloroplasts are the most prominent forms of plastids that contain chlorophyll, the green pigment. Chloroplast:  In a plant cell, chloroplasts are the most prominent forms of plastids that contain chlorophyll, the green pigment.  The chlorophyll enables the chloroplast to harness kinetic solar energy and trap it in the form of potential energy.  All living organisms directly or indirectly depend on them for energy. Chloroplast in enclosed in two smooth membranes separated by a distinct periplastidial space.  The interior of chloroplast is differentiated into two parts— The Stroma and the Grana.  Stroma is the colourless ground substance that fills the chloroplast. It contains cholorophyll bearing double membranes lamellae that form flattened sac like structures called thylakoides collectively called Grana.  Quantasomes are the smallest units present on the inner surface of thylakoides capable of carrying out photochemical reactions (vi) Ribosomes  Ribosomes are the submicroscopic organelles. These are site of protein synthesis in the cell. These are found in all the cells either attached to the membranes of endoplasmic reticulum or scattered in the cytoplasm. The ribosomes are spheroidal bodies.  Ribosomes from prokaryotes exist as 70S units and ribosomes in eukaryotes exust as 80S units.  A ribosome is formed of two subunits – a large subunit and a small subunit. The small subunit forms a sort of cap on the flat surface of large subunit.  The two units of bacterial ribosome (70S) are represented by 50S and 30S subunits, and eukaryotic ribosomes (80S) are represented by 60S and 40S subunits.  The two subunits of ribosomes usually exist free in the cytoplasm and join only during protein synthesis when a number of ribosomes get attached to mRNA in a linear fashion.  These groups or clusters of ribosomes are known as Polyribosomes. (vii) Mitochondria  Mitochondria are sausage- shaped spherical or thread like organelles present in the cytoplasm.  They break down the complex carbohydrates and sugars into usable forms and supply energy for the cell, they are also called as the powerhouse of the cell.  The mitochondria are surrounded by a double walled membrane known as outer and inner membranes.  The spaces between these two membranes are known as perimitochondrial space.  The outer membrane is smooth but the inner membrane is variously folded into thin cristae.  Inner membrane is covered with special particles called Oxysomes, these are the sites of aerobic respiration. viii) Golgi Body  Golgi body is also referred to as golgi complex or golgi apparatus.  It plays a major role in transporting chemical substances in and out of the cell.  It has three distinct components flattened sac or cisternae, clusters of transition tubules and vesicles and large vesicles or vacuoles.  Golgi is mainly associated with secretory activity of the cell.  It is also associated with the concentration, storage, condensation and packaging of materials for export from the cell across plasmalemma. (ix) Endoplasmic Reticulum  Endoplasmic reticulum (ER) is the connecting link between the nucleus and cytoplasm of the plant cell.  Basically, it is a network of interconnected and convoluted sacs that are located in the cytoplasm.  Based on the presence or absence of ribosomes, ER can be of smooth or rough types.  The former type lacks ribosomes, while the latter is covered with ribosomes.  Overall, endoplasmic reticulum serves as a manufacturing, storing and transporting structure for glycogen, proteins, steroids and other compounds. (x) Lysosomes  Lysosomes are tiny membrane-bound, vesicular structure of cytoplasm which enclose hydrolytic enzymes and perform intracellular digestion.  These are also known as suicidal bags.  These are found in all animal cells but only in few plant cells. (xi) Vacuoles  Vacuoles are sap- filled vesicles in the cytoplasm. These are surrounded by a membrane called tonoplast.  In a plant cell, there can be more than one vacuole; however, the centrally located vacuole is larger than others.  Tonoplast is a semi permeable membrane; it enables the vacuoles to concentrate and store nutrients and waste products.  It facilitates the rapid exchange of solutes aid gases between the cytoplasm and adjoining fluids.