Question

1. Briefly describe the function of each of the organelles listed below
   1. cell wall:
   2. plasma membrane:
   3. nucleus:
   4. nucleolus:
   5. ribosomes:
   6. endoplasmic reticulum (smooth and rough):
2. 7. Golgi apparatus:
   8. Peroxisomes:
   9. Mitochondria:
   10. Chloroplasts:
   11. large central vacuole:
   12. lysosomes:
   13. extra cellular matrix:

Answer 1

a) CELL WALL

It is the non-living rigid structure which forms the outer covering for the plasma membrane of fungi and plants.Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection.It also help helps in cell-to-cell interaction and provides barrier to undesirable macromolecules. The cell wall of a young plant cell which is the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner side of the cell.The middle lamella is a layer mainly of calcium pectate which holds the different neighboring cells together. The cell wall and the middle lamella may be traversed by plasmodesmata which connects the cytoplasm of neighboring cells.

b)PLASMA MEMBRANE:

Plasma membrane is made up of lipids that are arranged in a bilayer. Also the lipids are arranged within the membrane with the polar head towards outside and the Hydrophilic tail towards the inner part.One of the most important function of the plasma membrane is the transport of molecules across it. The membrane is selectively permeable to some of the molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called passive transport. If some kind of molecules is to move against concentration gradient ,it requires energy this is the active transport in the plasma membrane.

c)NUCLEUS:

It was first described by Robert Brown. The nucleus has highly extended and elaborate nucleoprotein fibres called chromatin,nuclear matrix and one or more spherical bodies called nucleoli.Nuclear envelope consists of two parallel membranes with a space between called perinuclear space. It forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The nuclear matrix of the nucleoplasm contains nucleolus and chromatin.The nucleoli is a spherical structure present in the nucleoplasm. The cell division starts in the nucleus.

d) NUCLEOLUS:

It is the 25% of nucleus and it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis .

e)RIBOSOMES:

Ribosomes are the granular structures . They are composed of ribonucleic acid and proteins and are not surrounded by any membrane. The eukaryotic ribosomes are 80s while the prokaryotic ribosomes are 70s .These have mainly two functions, that are formation of peptide bonds and decoding of messages.

f) ENDOPLASMIC RETICULUM:

It is a network or reticulum of tiny tubular structures scattered in the cytoplasm. The ER divides the intracellular space into two distinct compartments, that is luminal(inside ER) and extra luminal(cytoplasm) compartments. ER often shows ribosome attached to their outer surface. The ER bearing ribosomes on their surface is called rough endoplasmic reticulum(RER) and in the absence of ribosome they appear smooth and are called smooth endoplasmic reticulum(SER). RER is actively involved in protein synthesis and secretion. SER is the major site for synthesis of lipid.

g) GOLGI APPARATUS:

These are the flat, disc shaped sacs or cisternae. They are stacked parallel to each other.It have cis and trans faces which are entirely different.It mainly perform the function of packaging the materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the Golgi apparatus and move towards the maturing face.A number of proteins synthesized by ribosomes on the ER are modified by the cisternae of the Golgi apparatus before they are released from the trans face.Golgi proteins are the important site of formation of glycoproteins and glycolipids.

h)PEROXISOMES:

They are single membrane organelles which contain enzymes.It helps in the breakdown of very long chain fatty acids.

i)MITOCHONDRIA:

It is the double membrane bound structure with the outer membrane and inner membrane dividing its lumen distinctly into two aqueous compartments. The inner compartment is called matrix .The outer membrane forms the continues limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae towards the matrix.The cristae increases the surface area.The two membranes have their own specific enzymes associated with the mitochondrial function.These are the site of aerobic respiration.They produce ATP and is known as the power house of the cell.

j)CHLOROPLASTS:

Plastids are found in all plant cells and euglenoids. They bear some specific pigments,thus imparting specific colours to the plant. Chloroplasts are one of them. These contain Chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

k)LARGE CENTRAL VACUOLES:

The vacuole is the membrane bound space found in the cytoplasm.It contains water,sap,excretory product and other materials not useful for the cell.

l)LYSOSOMES:

These are membrane bound vesicular structures formed by the packaging in the Golgi apparatus. These are very rich in all types of hydrolytic enzymes (hydrolases-lipases,proteases,carbohydrases) optimally active at the acidic pH.These are capable of digesting Carbohydrates,proteins,lipids and nucleic acids and known as the suicidal bag of cell.

m)EXTRACELLULAR MATRIX:

It is an extensive molecular network which have many functions like providing support,segregating tissues and regulating the intracellular communication.