5.Cell Structure and Organization
✅✅✅ Very Very Important Questions (Highly conceptual and often asked in long answers or MCQs):
structure of chloroplast:
C.
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:-
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 (75 Angstorm equivalent to 7.5 nanometers (nm))
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.
Textual exercises solutions:-
Qno.1. Choose correct option
A. Growth of the cell wall during cell elongation take place by...........
a. Apposition b. Intussusception
c. Both a & b d. Super position
B.Cell Membrane is composed of
a. Proteins and cellulose
b. Proteins and Phospholipid
c. Proteins and carbohydrates
d. Proteins, Phospholipid and some carbohydrates
C.Plasma membrane is fluid structure due to presence of-
a. Carbohydrates c. Glycoprotein
b. Lipid d. Polysaccharide
D.Cell Wall is present in -
a. Plant cell b. Prokaryotic cell
c. Algal cell d. All of the above
E. Plasma membrane is -
a. Selectively permeable
b. Permeable
c. Impermeable.
d. Semipermeable
F. Mitochondrial DNA is -
a. Naked.
b. Circular
c. Double stranded
d. each one of the above
G. Which of the following set of organelles
contain DNA?
a. Mitochondria, Peroxisome
b. Plasma membrane, Ribosome
c. Mitochondria, Chloroplast
d. Chloroplast, Golgi body
Qno.2. Answer the following questions
A. Plants have no circulatory system? Then how cells manage intercellular transport?(2marks)
Ans:-
i.Plant cells show presence of plasmodesmata which aru cytoplasmic bridges between mighbouring cells.
ii.This open channel through the cell wall connects the cytoplasm of adjacent plans 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.
B. Is nucleolus covered by member?
Ans:- A nucleolus is not covered by member
C.Fluid mosaic model proposed by Singer and Nicokun replaced Sandwich model proposed by Danielli and Davson? Why?
(3marks)
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-protem sandwich', as the lipid layer was sandwiched between twe 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 proteine 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
D. The RBC surface normally shows glycoprotein molecules. When determining blood group do they play any rule?(2marks)
Ans:-
i. The polysaccharide Parr 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 acts as different type 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 of 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.
E. How cytoplasm differs from nucleoplasm in chemical composition?
(2 Marks)
Ans:-
Based on chemical composition, the cytoplasm differs from nucleoplasm in the following ways:
i.The cytoplasm shows presence of minerals, sugars, amino acids, t-RNA, nucleotides, vitamins, proteins and
enzymes.
ii. Nucleoplasm shows presence of various substances like nucleic acid, protein molecules, minerals, salts and enzymes required for DNA replication.
Qno.3 Answer the following questions
A. Distinguish between smooth and rough endoplasmic reticulum.(2marks)
Ans:-
B. Mitochondria are power House of the cell.Give reasons.
Ans:-
i.Mitochondria possess oxysomes on its inner membrane. These oxysomes take active part in synthesis of ATP molecules.
ii.During cellular respiration, ATP molecules are produced and get accumulated in the mitochondria.
iii.Acerylation of pyruvate and Krebs cycle take place in the mitochondrial matrix, whereas ETS uakes 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".
C. What are the types of plastids?(4marks)
Ans:-
i.Plastids and classified according to the pigments present in it. Three main types of plastids are - leucoplasts, chromoplasts and chloroplasts.
ii.Leucoplasts do not contain any photosynthetic pigments they are of various shapes and sizes. These are meant for storage of nutrients:
a. Amytoplasts store starch
b.Elaioplasts store oils.
c.Aleuroplasts store proteins.
iii.Chromoplasts contain pigments like carotene and xanthophyll etc.
a. They impart yellow, orange or red colour to flowers and fruits.
b. These plastids are found in the coloured parts of flowers and fruits.
iv. Chloroplasts are plastids comaming green pigment chlorophyll along with other enzymes that help in production of sugar by photosynthesis. They are present in plants, algae and few protists like Euglena.
Qno.4. Label the diagrams and write down the details of concept in your words.(4 marks)
A.
Ans:-
Mitochondrion is known as the power house of the cell. It plays significant role in aerobic respiration Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBC)s.
The structure of mitochondrion:
i.Shape of the mitochondria may be oval or spherical or like spiral strip.
ii.It is a double membrane bound organelle.
iii.Outer membrane is permeable to various metabolites due to presence of a protein-Porin or Parson's particles.
iv.Inner membrane is selectively permeable to few substances only.
v.both membranes are separated by intermembrane space.
vi. Inner membrane shows several finger like or plate like folds called as cristae which bears numerous particles oxysomes and cytochromes/electron carriers.
vii. Inner membrane encloses a cavity called inner chamber, containing a fluid-matrix.
viii. Matrix contains few coils of circular DNA, RNA, 70S types of ribosomes, lipids and various enzymes of Krebs cycle and pathways.
B.
Ans:-
Chloroplast plays a significant role in synthesis of starch in plants.
i. In plants.chloroplast is found mainly in mesophyll of leaf.
ii. Chloroplast is lens shaped but it can also be oval, spherical,discord,ribbon like,etc.
iii. A cell may contain single large chloroplast as in Chlamydomonas or there can be 20 to 40 chloroplasts per cell as seen in mesophyll cells.
iv. Chloroplasts contain green pigment called chlorophyll along with other enzymes that help in production of sugar by photosynthesis.
v. Inner membrane of double membraned chloroplast is comparatively less permeable.
vi. Inside the cavity of inner membrane,there is another set of membranous sacs called thylakoids.
vii. Thylakoids are arranged in the form of stacks called grana (Singular: granum).
viii. The grana are connected to each other by means of membranous tubules called stromal lamellae.
ix. Space outside thylakoids is filled with stroma.
x. The stroma and the space inside thylakoids contain various enzymes essential for photosynthesis.
xi. Stroma of chloroplast contains DNA and ribosomes (70s).
C.
Ans:-
Golgi complex or Golgi apparatus or Golgi body act as an assembly, manufacturing cum packaging and transport unit of cell.
i. Structure of Golgi complex:
a. Golgi complex consists of consists of stacks of hollow membranous sacs called cisternae.
b. Diameter of cisternae varied from 0.5 to 1µm.
c. A Golgi complex may have few to several cisternae depending on its function.
d. The thickness and molecular composition of membranes at one end of the stack of a Golgi sac differ from those at the other end.
e. The Golgi sacs show specific orientation in the cell.
f. Each cisterna has a forming or 'cis' face (cis: on the same side) and maturing or 'trans' face (trans: the opposite side).
g. Transport vesicles that pinch off from transitional ER with cis face of Golgi cisterna and add it's contents into the lumen.
ii. Location of Golgi complex: Golgi bodies are usually located near endoplasmic reticulum.
iii. Functions of Golgi complex:
a. Golgi body carries out two types of functions, modification of secretions of ER and production of in own secretions.
b. Cisternae contain specific enzymes for specific functions
c. Refining ( modification) of product takes place in a sequential manner.
d. For example, certain sugar component is added or removed from glycolipids and glycoproteins tha are brought from ER, thus forming a variety of products.
e. Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin.
f. Manufactured or modified, all products of Golgi complex leave cisternae from trans face as transport vesicles.
D.
Ans:-
i. Endoplasmic reticulum is a netwvek present within the cytosol.
ii. It is present in all eukaryotic cells except ova and mature red blood corpuscles.
iii. Under the electron microscope, it appears like network of membranous tubules and sacs called cisternae
iv. This network of ER divides the cytoplasm in two parts viz. one within the lumen of ER called laminal cytoplasm and non-laminal cytoplasm that lies outside ER.
v. Membrane of ER is continuous with nuclear envelope at one end and extends till cell membrane.It thus acts as intracellular supporting framework and helps in maintaining position of various cell organelles in the cytoplasm.
vi. Depending upon the presence of absence of ribosomes, endoplasmic reticulum is called tough endoplasmic reticulum (RER) or Smooth endoplasmic reticulum (SER) respectively.
i. Smooth endoplasmic reticulum (SER):
a. Depending on cell type, it helps in synthesis of lipids for e.g. Steroid secreting cells of cortical region of adrenal gland, testes and ovaries.
b. Smooth endoplasmic reticulum plays a role in detoxification in the liver and storage of calcium (muscle cells).
ii. Rough Endoplasmic Reticulum (RER):
a. Rough ER is primarily involved in protein synthesis. For e.g. Pancreatic cells synthesize the protein insulin in the ER.
b. These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins.These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles.
c. Rough ER is also involved in formation of membrane for the cell. The ER membrane grows in place by addition of membrane proteins and phospholipids to its own membrane. Portions of this expanded membrane are transferred to other components of endomembrane system.
Qno.5. Complete the flow chart.(2 marks)
Ans:-
Qno.6. Label the A, B, C, and D in above diagram and write the functions of organelles A and B.(3marks)
Ans:-
i. A. Mitochondria
B. Endoplasmic Reticulum
C. Golgi complex
D. Amytoplast
ii. Functions of mitochondria:
Mitochondrion is known as the power house of the cell. It plays significant role in aerobic respiration Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBC)s.
The structure of mitochondrion:
i.Shape of the mitochondria may be oval or spherical or like spiral strip.
ii.It is a double membrane bound organelle.
iii.Outer membrane is permeable to various metabolites due to presence of a protein-Porin or Parson's particles.
iv.Inner membrane is selectively permeable to few substances only.
v.both membranes are separated by intermembrane space.
vi. Inner membrane shows several finger like or plate like folds called as cristae which bears numerous particles oxysomes and cytochromes/electron carriers.
vii. Inner membrane encloses a cavity called inner chamber, containing a fluid-matrix.
viii. Matrix contains few coils of circular DNA, RNA, 70S types of ribosomes, lipids and various enzymes of Krebs cycle and pathways.
ii. Functions of Endoplasmic Reticulum:
i. Smooth endoplasmic reticulum (SER):
a. Depending on cell type, it helps in synthesis of lipids for e.g. Steroid secreting cells of cortical region of adrenal gland, testes and ovaries.
b. Smooth endoplasmic reticulum plays a role in detoxification in the liver and storage of calcium (muscle cells).
ii. Rough Endoplasmic Reticulum (RER):
a. Rough ER is primarily involved in protein synthesis. For e.g. Pancreatic cells synthesize the protein insulin in the ER.
b. These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins.These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles.
c. Rough ER is also involved in formation of membrane for the cell. The ER membrane grows in place by addition of membrane proteins and phospholipids to its own membrane. Portions of this expanded membrane are transferred to other components of endomembrane system.
Qno.7. Identify each cell structures or organelle from its description below.(1marks)
i.Manufactures ribosomes
ii. Carries ont photosynthesis
iii. Manufactures ATP in animal and plant cells.
iv. Selectively permeable.
Ans:-
i.Nucleolus
ii.Chloroplast
iii.Mitochondria
iv.Plasma membrane
Qno.8. Onion cells have no chloroplast. How can we tell they are plants?
(2marks)
Ans:-
i. The onion bulb is a modified stem consisting of large number of fleshy leaves.
ii. While photosynthesis takes place in the leaves (present above the ground) of an onion cotaining chloroplast, the little glucose that is produced from this process is converted in to starch (starch ge and stored in the bulb.
iii. Starch act as reserved food material in plants.
iv. Using an iodine solution, we can test for the presence of starch in onion cells. If starch is present, the iodine changes from brown to blue-black or purple.
Hence, we can say that though onion cells have no chloroplast they are considered as plants.
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