A plant cell consists of three distinct components:
(i) Cell wall
(ii) Protoplasm, and
(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. (Fig. 2.1).
(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. 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 (Fig. 2.2).
(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. In prokaryotic cells (Bacteria) the ribosomes are abour 15 nm and in eukaryotic cells about 25 nm.
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. The larger subunits (i.e. 60S and 50S) are attached to the membrane of endoplasmic reticulum and the smaller subunits are then bound to larger subunits. The cleft separating the two subunits lies parallel to the membrane. The messenger RNA is held by the smaller subunit, while tRNA molecule is bound to the larger subunit.
(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. (Fig. 2.2).
(viii) Nucleus:
The nucleus is the most important part of the cell which regulates all metabolic and hereditary activities within the cell It is more or less spherical, lying in the cytoplasm and occupying about two-thirds of the cell space. A typical nucleus is composed of the following structures (Fig. 2.2).
Nuclear membrane:
It is a selectively permeable envelope-like structure surrounding the nucleolus and nucleoplasm. It is formed of two layers separated by a fluid-filled perinuclear space. The nuclear membrane disappears during prophase stage of nuclear division and reorganizes during telophase. It regulates the passage of ions, small molecules and macromolecules of ribosomal subunits, mRNA, t RNA etc.
Nucleoplasm:
Nucleoplasm is the transparent semifluid ground substance formed of a mixture of proteins, phosphorus and some nucleic acids. Chromatin fibres or the chromonemata remain suspended in the nucleoplasm.
Chromatin network:
Chromatin fibres form a network in the nucleoplasm called chromatin net work or nuclear reticulum. Chromatin fibres are the sites of main genetic material which controls all the activities of the cell, metabolism and heredity. During cell division, the chromatin threads condense and form thick chromosomes.
Nucleolus:
The nucleolus is a spherical body lying in the nucleoplasm closely associated to the nucleolar organizer region of the chromosome. It was first described by Schleiden in 1838. It contains large amount of RNA though DNA is also present. Its main function is the synthesis of ribosomal RNA (rRNA), which helps in synthesis of ribosomes.
(ix) 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.
(x) 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.
(xi) 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.
Types of Lysosomes:
Based on function or stage of digestion lysosomes are of following four types:
(i) Primary lysosomes are newly formed lysosomes with hydrolytic enzymes.
(ii) Secondary lysosomes are newly formed by the fusion of phagosomes and primary lysosomes. Here contents of phagosome are digested or hydrolysed.
(iii) Residual bodies are exhausted secondary lysosomes. These contain undigested remains.
(iv) Autophagic vacuoles are formed by the fusion of primary lysosomes with cell organelles from cell’s own cytoplasm. This brings about autodigestion of cell or its organelles.
Functions:
Lysosomes bring about digestion of extracellular and intracellular materials. Because of this basic character lysosomes perform following functions:
(i) Lysosomes of granular leucocytes or macrophages devour foreign substances and microbes which enter the cell and guard our body against infection.
(ii) Lysosomes remove worn out cell organelles, dead cell and provide energy during starvation by controlled breakdown of stored food substances.
(iii) During dedifferentiation of tissues and cells, the lysosomes dissolve the specialized parts of the cells. This helps in regeneration of damaged tissue or damaged part and formation of bone from cartilage.
(iv) The lytic enzymes of sperm acrosome help in the penetration of sperm into the ovum.
(v) During metamorphosis, reabsorption of various larval structures like external gills of tadpole, the tail of tadpole in frog or the larval organs in the pupa of various insects is brought about by autolytic action of lysosomes.
(vi) Lysosomes bring about cellular breakdown associated with ageing.
(vii) Lysosomes may cause cancer by breaking down chromosomes.
(xii) Peroxisomes:
Peroxisomes occur widely both in plant and animal cells. They are spherical or ovoid bodies surrounded by a single membrane. It contains certain oxidative enzymes, used for the metabolic breakdown of fatty acids into simple sugar forms. In green plants, peroxisomes help in undergoing photorespiration.
(xiii) 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.
(xiv) Cilia and Flagella:
Cilia and flagella are motile hair -like appendages on the free surfaces of the cells. These are cytoplasmic processes and create water currents, food currents, act as sensory organs and perform several other functions of the cell. Cilia and flagella can be differentiated on the basis of their size, however, other physiological and morphological characteristics are almost the same.
The main difference between cilia and flagella are as follows:
Cilia and flagella are cylindrical processes projecting from the free surface of the cell. These originate from their basal bodies embedded in the cytoplasm. The basal bodies form their kinetic centres. A ciliam or flagellum consists of a longitudinal axoneme enclosed in a spiral sheath of cytoplasm and a plasma membrane continuous with the cell membrane.