Cell structure and organisation Important Questions

✅✅✅ Very Very Important Questions (Highly conceptual and often asked in long answers or MCQs):

1)Fluid mosaic model proposed by Singer and Nicolson replaced Sandwich model proposed by Danielli and Davson? Why?

Ans)

i. The Davson–Danielli model of the plasma membrane of a cell, was proposed in 1935 by Hugh Davson and James Danielli.

ii. The model describes a phospholipid bilayer that lies between two layers of globular proteins.

iii. This model was also known as a ‘lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers.

iv. But through experimental studies, membrane proteins were discovered to be insoluble in water (representing hydrophobic surfaces) and varied in size. Such type of proteins would not be able to form an even and continuous layer around the outer surface of a cell membrane.

v. In case of Fluid-mosaic model, the experimental evidence from research supports every major hypothesis proposed by Singer and Nicolson.

This hypothesis stated that membrane lipids are arranged in a bilayer; the lipid bilayer is fluid; proteins are suspended individually in the bilayer; and the arrangement of both membrane lipids and proteins is asymmetric.

Therefore, Fluid mosaic model proposed by Singer and Nicolson replaced Sandwich model proposed by Danielli and Davson.

2)Mitochondria are power house of cell. Give reason.

Ans)

a. Mitochondria possess oxysomes on its inner membrane. These oxysomes take active part in synthesis of ATP molecules.

b. During cellular respiration, ATP molecules are produced and get accumulated in the mitochondria.

c. Acetylation of pyruvate and Krebs cycle take place in the mitochondrial matrix, whereas ETS takes place in the inner mitochondrial membrane. These processes result in the synthesis of 36 ATP molecules in the mitochondria.

Therefore, mitochondria are called ‘power house of the cell’.

✅✅ Very Important Questions (Frequently asked in exams):

3)Plants have no circulatory system? Then how cells manage intercellular transport?

Ans)

1) Plants have no circulatory system? Then how cells manage intercellular transport?

Ans:

i. Plant cells show presence of plasmodesmata which are cytoplasmic bridges between neighbouring cells.

ii. This open channel through the cell wall connects the cytoplasm of adjacent plant cells and allows water, small solutes, and some larger molecules to pass between the cells.

In this way, though plants have no circulatory system, plant cells manage intercellular transport.

4)The RBC surface normally shows glycoprotein molecules. When determining blood group do they play any role?

Ans)

i. The polysaccharide part of glycoproteins located on the surfaces of red blood cells acts as the antigen responsible for determining the blood group of an individual.

ii. Different polysaccharide parts of glycoproteins act as different types of antigens that determine the blood groups.

iii. Four types of blood groups A, B, AB, and O are recognized on the basis of presence or absence of these antigens.

iv. Individuals with blood group A, B, and AB have antigens A, B, and AB on the surface of their RBCs respectively. Individuals with blood group O lack antigens on their RBC surface.

5.)Give an account of eukaryotic plasma membrane.  [3 Marks]

Ans: Eukaryotic plasma membrane/ Cell membrane/ Biomembrane:

i. It is thin, quasi-fluid structure present both extracellularly and intracellularly.

ii. Extracellularly, it is present around protoplast and intracellularly, it is present around most of the cell organelles in eukaryotic cell. It separates cell organelles from cytosol.

iii. Thickness of bio-membrane is about 75Å.

iv. Cell membrane appears trilaminar (made up of three layers) when observed under electron microscope. It shows presence of lipids (mostly phospholipids) arranged in bilayer.

V. Lipids possess one hydrophilic polar head and two hydrophobic non-polar tails. Therefore, phospholipids are amphipathic.

vi. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwiched in between heads. Due to this, tails never come in direct contact with aqueous surrounding.

vii. Cell membrane also shows presence of proteins and carbohydrates.

viii. Ratio of proteins and lipids varies in different cells. For example, in human beings, RBCs show approximately 52% protein and 40% lipids.

6). Write a note on cell wall in Eukaryotic cells.

Ans:

i. The rigid, protective and supportive covering, outside the cell membrane is called cell wall. It is present in

plant cells, fungi and some protists. Algae show presence of cellulose, galactans, mannans and minerals like calcium carbonate in cell wall. ii.

In other plants, it is made up of hemicelluloses, pectin, lipids and protein.

iv. Microfibrils of plant cell wall show presence of cellulose which is responsible for rigidity.

v. Some of the depositions of cell wall are silica (grass stem), cutin (epidermal walls of land plants), suberin (endodermal cells of root), wax, lignin.

vi. Function:

Provides support, rigidity and shape to the cell.

Protects the protoplasm against mechanical injury and infections.

7.) Explain Structure of Plant Cell wall

Ans.:

i. Middle lamella:

It is thin and present between two adjacent cells. It is the first structure formed from cell plate during cytokinesis. It is mainly made up of pectin, calcium and magnesium pectate. Softening of ripe fruit is due to solubilization of pectin.

ii. Primary wall:

In young plant cell, it is capable of growth. It is laid inner to middle lamella. It is the only wall seen in cells meristematic tissue, mesophyll, pith, etc.

iii. Secondary wall:

It is present inner to primary wall. Once the growth of primary wall stops, secondary wall is laid. At some places thickening is absent which leads to formation of pits.