CELLS

Zeri Plant Provide the detailed Structure of a Generalized Eukaryotic Cell an its Organelles stating their Functions.
A eukaryotic cell may compared to a factory where different machines and people have different jobs, all working togather for achieving one goal. Efficiency is improved by "division of labour", the sharing out of jobs. In the cell different organelles have their own specific roles, involving their specialized structure and chemistry, Nucleus, mitochondria,Golgi bodies, endoplasmic reticulum and vacuoles are found in plant cells, while plastids are found in plant cells.
In order to know the detailed structure and function of a generalized cell, it is studies with the help of light and electron microscope.
Following is the detailed structure of a generalized eukaryotic cell and its organelles.

Cell Wall:

It is the outer semirigid and non-Living boundary of a plant cell.

Chemical Composition:

In its chemical composition cellulose is the most prominent substance, where, the cell wall of fungi is made up of chitin.

Structure:

Thickness of a cell wall varies in different cells e.g. plants; xylem, vessel elements and tracheids have thick walls while parenchyma cells have thin walls.

The primary layer of the cell wall is called primary wall, which is further strengthened by an additional layer called secondary wall, especially in xylem vessels. It is comparatively thicker than primary wall. The cell walls of neighbouring cells are connected by cytoplasmic bridges which are called Plasmodesmata. Through these connections, cells transfer chemicals among each other.

Function:

•          i.            Shape & rigidity: Cell wall provides a definite shape to the cell and keeps it rigid.
•        ii.            Permeable:It does not act as a barrier to the materials passing through it.
•       iii.            Support & Strength: Cell wall provides mechanical strength and skeletal support for the individual cell and to the plant as a whole.
•      iv.            Movement of water & minerals: Water and minerals can move through an interconnected system of cell wall (i.e. the apoplast).
•        v.            Protection:Cell wall protects the cells from osmotic lysis.

Cell Membrane:

Plasma membrane or cell membrane is the outer most boundaries of the animal cells but in plant cells, it is covered by a cell wall.

Cell membrane is chemically composed of:

       a.        60-80 % Protein.
 b.      20-40 % Lipids.

In addition there is a small quantity (2-10%) of carbohydrates.

Structure:

Structurally, cell membrane is composed of layers of different materials and has extremely small pores in it through which only small molecules can pass. There are different theories about the structure of plasma membrane.

               i.            In 1935, Danielle and Davson proposed that cell membrane consists of three layers.

            ii.            In 1938, Harvey and Danielle suggested that plasma membrane is composed of liquid bilayer sandwiched between two protein layers.

iii.            Fluid Mosaic Model: In i972, Singer and Nicolson that lipid bilayer is not sandwiched between two protein layers. Instead proteins are embedded in the lipid in a mosaic manner. This discovery led to the proposed of “Fluid Mosaic Model”. This model is most accepted model. Cell membrane also contains charged pores through which movement of materials takes place both by active and passive transport.

Some proteins are embedded and some are present on the outside of cell. Small amounts of carbohydrates are also found. These may be joined with proteins (glycoprotein) or with lipids (glycolipids). Both these forms act as fingerprints of the cell.

Functions:

•         i.            Plasma membrane provides mechanical support and external form to the protoplasm (living matter inside the cell).
•       ii.            Plasma membrane offers a barrier between the cell contains and their environment, allowing only selective substance to pass through it, thus it is known as differentially permeable or selectively permeable membrane.

    iii.            Transport of Materials: Transport of materials is one of the vital roles it plays of the cell.

Cytoplasm:

The material present between the plasma membrane (cell membrane) and the nuclear membrane is called cytoplasm.

It is composed of cytosol (suitable part or the liquid matrix of cytoplasm), Inclusions (Insoluble wastes and storage products) and organelles (Mitochondria, Ribosome etc).

Chemical Composition:

It is about 90% water. It forms a solution containing all the fundamental molecules of life. In the cytosol, small molecules and ions may form “true solution,” and some large molecules form “colloidal solution.” Colloidal solution may be a sol (non-viscous) or a gel (viscous). Peripheral parts of the cell are often like a gel.

Function:

The cytoplasm of the cell provides space for the proper functioning of the organelles and also acts as the site for various biochemical (metabolic) reactions for example Glycolysis (breakdown of glucose during cellular respiration).

Cytoskeleton:

An important part of cell which is invisible under light microscope is cytoskeleton. It is made up of many types of filaments but most important are microtubules, microfilaments and Microtrabaculac.

                      i.            Microtubules:

Structure: Microtubules are long, unbranched and cylindrical structures with an average diameter of about 25nm. The structures are made primarily by the self. Assembly of tubulin protein.

Function

One very important function of microtubules in their role in the assembly and disassembly of the spindle structure during mitosis. They also provide internal structure to the cell.

                     ii.            Microfilaments:

Structure: Microfilaments are considerable more slender cylinders made up of contractile protein, action and linked to the inner face of the plasma membrane.

Function

They are involved internal cell motion.

    iii.            Microtrabaculae:

Microtrabaculae appear to be very fragile tubes that form a transient network in the cytosol.

Functions

Cytoskeleton maintains the cell’s shape, anchors organelles in place and moves parts of the cell in processes of growth and motility.

Nucleus:

The most important and visible part of a cell is nucleus. It is present in the centre of the animal cell but due to presence of large central vacuole, it is pushed to one side in the plant cell.

Structure

Its structure contain various component parts e.g.

                      i.            Nuclear Membrane:

The outer most covering of nucleus is called nucleus membrane. It is a double membrane and also pores are present in it.

                     ii.            Nucleoplasm:

The granular matrix present inside cell is called Nucleoplasm.

                    iii.            Nucleoli:

In the Nucleoplasm, one or more nucleoli are present. These are usually visible as dark spots and are responsible for the formation of ribosomal RNA.

                   iv.            Chromosomes:

Chromosomes or genetic material is also present in Nucleoplasm in the form of chromatin (thread like structures).

Chromosomes are made up of Deoxyribonucleic acid (DNA) and histone proteins.

DNA contains the message for the synthesis of cellular proteins. This message is read by messenger RNA which carries it to ribosome for the protein synthesis.

Ribosomes:

These are small particles which occur freely in cytoplasm on the outside of the endoplasmic reticulum and in the nucleus.

Chemical Composition:

Ribosomes are made up of almost equal amounts of proteins and ribosomal RNA (rRNA).

Eukaryotic Ribosome are slightly larger than prokaryotic ones.

Number

Ribosomes are present in extremely large amounts in cell.

Functions

Ribosomes carry out the function of protein synthesis.

Mitochondria:

These are the organelles which provide energy to the cell during respiration. Because of this property, these are also called “power house of the cell”.

Shapes

These are spherical or may be rod like in shape.

Structure

These are bounded by double membrane. Outer membrane is smooth while inner membrane forms infolding called cristae. Small rounded bodies called elementary particles ranging up to one million in number are parent on inner membrane. These particles involve in respiration. Inside the inner membrane, mitochondria are filled with mitochondrial matrix. Mitochondria have their own DNA and their own Ribosomes.

Functions

Mitohondria extract energy from different components of food and convert it in the form of ATP which is the useable form of energy for cell.

Plastids:

Plastids are organelle characteristics of plants lying greenly in cytoplasm. These are also present in photosynthesis protists (algae).

Types

   ·         Chloroplasts        Chromoplasts      and    Leucoplasts.

      1.       Chloroplasts:

These are present in the green parts of the plant particularly in leaves.

Structure of Chloroplasts

Each chloroplast is bounded by a double membrane. The outer membrane is smooth. Inner membrane gives rise to stacked membrane system composed of sacs which are called thylakoids. Stacks of thylakoids are called Grana. Chloroplasts are filled with semi fluid matrix called stoma. Thylakoids contain green pigments (chlorophyll)

Function of Chloroplast

Due to the chlorophyll, photosynthesis takes place in them.

2.       Chromoplasts

These give various colours to the plant and plant parts (other than green). These are present in the cells of flowers petals and fruits. Their function is to give colours so that insects attract towards them and so help in pollination and dispersal of fruits.

3.       Leucoplasts

These are present in the cells or those parts of the plants where food is stored because their function is to store starch, proteins and lipids. These are colourless.

Endoplasmic Reticulum (ER):

This is a complex membranous system, which consists of two unit-membranes enclosing a narrow apace about 4mm wide. These form a network of interconnected channels that extends from cell membrane to the nuclear membrane.

Types

                         i.            Rough Endoplasmic Reticulum.

                          ii.            Smooth Endoplasmic Reticulum.

      1.       Rough Endoplasmic Reticulum

Numerous Ribosomes are present on this type of Endoplasmic Reticulum giving it a rough texture.

      2.       Smooth Endoplasmic Reticulum

These are free from Ribosomes.

Functions

     a.       Rough Endoplasmic Reticulum helps in protein synthesis.

     b.      Smooth Endoplasmic Reticulum is involved in lipid metabolism.

c.       These also help in transport of materials from one part of the cell to the other part.

d.      These also help to detoxify harmful chemicals that have entered the cell.

Golgi Apparatus:

Golgi apparatus or Golgi complex is an assemblage of flat or curved sacs (cisternae) lying one above the other.

Discovery

These were discovered by “Camillo Golgi” in 1898 in the neural tissue of the animals.

Functions

                      i.            These connected with cell secretions, which are products formed within the cell on Ribosomes and then passed to the outside through endoplasmic reticulum and Golgi apparatus. These secretions are converted into finished product.

                    ii.            These also modify the proteins and lipids by adding carbohydrates and convert them into glycoprotein or glycolipids.

                   iii.            These are also involved in lysosome synthesis.

Lysosomes:

Lysosomes are cytoplasmic organelles and are different from others due to their morphology. Lysosomes (Lyso = splitting; soma = body) are found in most eukaryotic cells. They are most abundant in those animal cells, which exhibit phagocytic activity. They are bound by a single membrane and are simple sacs rich in acid phosphatise and several other hydrolytic enzymes.

These enzymes are synthesized on RER and are further processed in the Golgi apparatus. The processed enzymes are budded off as Golgi vesicles and are called as primary Lysosomes.

Discovery

These were discovered in the mid-twentieth century by a Belgian Scientist “de Duve”.

Functions

Because lysosomes contain strong digestive enzymes so these work for the break-down of food and waste materials inside the cell.

Centrioles

Location

Animal cells and cells of some microorganisms and lower plants contain two Centrioles located near the exterior surface of the nucleus. They are absent in higher plants.

Structure

In cross section each Centrioles consists of a cylindrical array of nine microtubules. However, each of the nine microtubules is further composed of tree tubules. The two Centrioles are usually placed at right angle to each other.

Functions

Just before a cell divides, its Centrioles duplicate and one pair migrates to the opposite side of the nucleus. The spindle then forms between them.

Vacuole:

These are fluid filled single-membrane bound organelles.

Number

Animal cells have large number of small sized vacuoles in them but plant cells have single large vacuole in them.

Types of vacuole and their functions

Food Vacuole

Many cells take in materials from outside in the form of food vacuole.

Contractile Vacuole

Sometimes, vacuoles are associated with elimination of wastes from the cells; such vacuoles are called contractile vacuoles.