Cell : The Unit Of Life

Absence of nucleus

Explanation:

Seive tubes are a component of phloem and erythrocytes are RBCs.

Mitochondria are present in sieve tube cells but absent in RBCs.

Haemoglobin is absent in sieve tube cells but present in erythrocytes.

Plant cells have cell wall so sieve tube cells have cell wall but animal cells don’t have cell wall so RBCs don’t have cell wall.

Both sieve tube cells and RBCs have no nucleus.

Have no role in protein synthesis

Explanation:

Ribosomes are formed of two subunits. In prokaryotes, it’s 70S (50S and 80S) and in eukaryotes it’s 80S (60S and 40S).

Several ribosomes attach to a single mRNA to form polysomes or polyribosomes.

They play an important role in protein synthesis.

Anabena

Explanation:

Anabena has absence of membrane bound organelles and undefined nucleus. But Euglena, Spirogyra, Agaricus have membrane bound organelles and well defined nucleus.

Safranin

Explanation:

Safranin is used as the classic counterstain in both Gram stains, and endospore staining while the rest of the options are used to stain chromosomes.

i, iv, iii & ii

Explanation:

Mycoplasmas are the smallest cells. The size of bacteria is ranging from 0.5–5.0 micrometres in length. The diameter of human RBC ranges from 6 - 8 micrometres. Ostrich egg is the largest single cell in the world with size 15 cm x 13 cm.

Cell wall present

Explanation:

Chromatin material is present in eukaryotes only.

Cell wall is present in bacteria (prokaryotes) and in plants (eukaryotes) .

The nuclear membrane is present in eukaryotes but not in prokaryotes.

Sub-cellular organelles in prokaryotes are not well defined and are not membrane bound while eukaryotes have membranous organelles.

Singer and Nicolson

Explanation:

Carl Benda coined the term ‘mitochondria’.

Schleiden and Schwann gave the first cell theory.

Singer and Nicolson proposed the fluid mosaic model of plasma membrane.

Robert Brown discovered the cell.

Rough Endoplasmic Reticulum (RER) is easily observed in the cell

Explanation:

Rough Endoplasmic Reticulum (RER) is observed in the cells that are actively involved in the protein synthesis and secretin. SER is also present in secretory cells. SER is involved in lipid synthesis.

Golgi apparatus is present in secretary cells.

Secretory granules are not formed in nucleus.

Membrane boundary of the vacuole of plant cells

A tonoplast is actually the membrane that surrounds the large vacuole in a mature plant cell.

It is also known as a vacuole membrane.

Cell wall is made up of peptidoglycans

Explanation:

The cell wall in eukaryotic cell is made up of cellulose. In prokaryotes, Cell wall is made up of peptidoglycans.

The rest of the statements are true.

Carbohydrate is never found in it.

Explanation:

Carbohydrates are found in the plasma membrane. Plasma membrane is composed of water, proteins, lipids and carbohydrates.

It is present in both plant and animal cell.

Lipid is present as a bilayer in it.

Proteins are present integrated as well as loosely associated with the lipid bilayer.

Presence of thylakoids

Explanation:

Thylakoids are present in plastids but not in mitochondria.

Both mitochondria and plastids have double membrane, ribosomes and DNA.

Intracellular transport

Explanation:

Cytoskeleton in a cell maintains cell shape and structure, support the organelles, help in cell motility but not in intracellular support.

Janus Green

Explanation:

Janus green is used to stain mitochondria.

Safranin is used to stain nuclei and lignified walls of cell.

Acetocarmine is used for staining nucleic acid and chromosomes.

Fast green is used for staining histones at alkaline pH after acid extraction from DNA.

Vacuole is a membrane-bound space found in the cytoplasm. It is found in both plant and animal cells but it is much larger in plants (occupies 90% of the space in plant cells).

Vacuole in a plant cell helps in

• the storage

• waste disposal

• cell elongation

• cell protection

Ribosomes are the site of protein synthesis. The ‘S’ refers to Svedberg’s unit or sedimentation coefficient. It is indirectly a measure of density and size. In cells heavier the structure, higher is the sedimentation coefficient.

Lysosomes are the single membrane bound organelle which is rich in hydrolytic enzymes.They are formed by Golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH.

Gas vacuoles are aggregates of hollow cylindrical structures called gas vesicles. They are found in blue-green and purple and green photosynthetic bacteria.

Functions of gas vacuoles are storing metabolic gases and take part in buoyancy.

Several ribososmes attached to a single mRNA form a chain is called polysome. It is also called polyribosome.

The function of a polysome is to translate an mRNA into proteins.

The metacentric chromosome has middle centromere forming two equal arms of chromosome.

Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite. Such chromosomes are called satellite chromosomes.

• The nucleoplasma contains small spherical bodies which are known as the nucleolus. It is not a membrane bound structure.

• It is a site for active ribosomal RNA synthesis.

• It also receives and stores ribosomal proteins formed in the cytoplasm.

Carbohydrates form glycoproteins and glycolipids.

• Centromere is an organelle usually containing two cylindrical structures called centrioles.

• Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.

• They are made of nine evenly spaced peripheral fibrils of tubulin protein.

• Each of the peripheral fibril is a triplet.

• The adjacent triplets are also linked.

• The central part of the proximal region of the centriole is also proteinaceous and called the hub.

• The hub is connected with the tubules of the peripheral triplets by radial spokes made up of protein.

Cell theory was given by Schleiden and Schwann which stated –

a) Bodies of all the plants and animals are made up of cells.

b) The cells are structural and functional units of life.

But it failed to state the origin of life.

Few years later, Rudolf Virchow stated that cells divide to produce new cells. So the modern cell theory is –

a) all living organisms are composed of cells and products of cells.

b) all cells arise from pre-existing cells.

Plasma membrane is composed of proteins, lipids, water and carbohydrates. The ratio of lipids and proteins varies considerably in different types cells.

The major lipids are phospholipids that are arranged in a bilayer. In addition to phospholipids membrane also contains cholesterol

Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic (water-repellent) tails towards the inner part. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.

In bacteria, there’s single chromosome/circular DNA which is called genomic DNA but many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.

Role of plasmids in bacteria –

The plasmid DNA grants certain phenotypic characters like resistance to antibiotics to such bacteria. DNA is used to monitor bacterial transformation with foreign DNA.

In eukaryotes, chromatin contains DNA and some basic proteins called histones, non-histone proteins and also RNA.

Histones are highly alkaline proteins that are found in nuclei of eukaryotes. They package and order DNA into structural units. Since DNA wraps around histones, they also play role in gene regulation (increase or decrease the production of specific gene products).

A cell must have the following structural and functional attributes to be a called a living cell -

a) Cell membrane – It is the outer membrane of the cell which is made up of phospholipids primarily. This membrane is selectively permeable as it allows only selected material like water, food to pass through it.

b) Cytoplasm – The liquid containing the controlled concentration of organic and inorganic compounds. It functions as a site for metabolism and provide energy and material for growth and reproduction.

c) Nucleus – It contains DNA, the nuclear matrix or the nucleoplasm containing nucleolus and chromatin. It is also called the brain of the cell as it controls all the activities carried out by the cell. It also transfers the genetic material required for reproduction and growth. In eukaryotic cells, it is membrane bound but not in prokaryotic cells

The nucleus, cell membrane, cytoplasm are common in both prokaryotic (primitive) cells and eukaryotic cells. But eukaryotic cells have other things like golgi apparatus, endoplasmic reticulum, plastids, mitochondria, etc. which are all membrane bound. The prokaryotes don’t have membrane bound organelles.

a. Rudolf Virchow first explained that the cells divide and new cells are formed from the pre-existing cells. He gave the modern cell theory by modifying the old one given by Schleiden and Schwann –

• all living organisms are composed of cells and products of cells.

• all cells arise from pre-existing cells.

b. • In 1838, Schleiden conducted tests on different plants and concluded that all the plant are made up of different types of cells which together form plant tissues.

• In 1839, Schwann studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’.

• He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.

• So the cell theory with the contribution of the two stated that the bodies of animals and plants are composed of cells and products of cells.

Yes, extra genomic DNA is present in prokaryotes and eukaryotes.

• In eukaryotes, the extra genomic DNA is present in mitochondria and plastids.

• In many prokaryotic bacteria, small circular DNA called plasmids are present outside genomic DNA which helps the cell develop some phenotypic characteristics like resistance to anti-biotics.

Structure and function are correlatable in living organisms.

• Taking a look at the structure of RBCs-

It is called biconcave shape. The function of RBCs is to pass through capillaries and carry more oxygen and this shape helps it to absorb more oxygen.

• Taking a look at the structure of plasma membrane –

It is made up of proteins, carbohydrates and lipids. The lipids arranged with their hydrophilic polar head directed outwards and non-polar hydrophobic tails directed inwards providing fluidity to the membrane. This provides the cell membrane its selective permeability.

a. Non-membrane bound cell organelles – Ribosomes, Centriole, nucleolus, Cytoskeletal structures

b. Single membrane bound cell organelles – Endoplasmic reticulum, Golgi bodies, lycosomes, vacuoles, Thylakoid.

c. Double membrane bound cell organelles – Plastids, Mitochondrias and nucleus.

• The genomic content of the nucleus is constant for a given species where as the extra chromosomal DNA is found to be variable among the members of a population.

• For example, all the human cells have 46 chromosomes. This is constant in all the humans. But mitochondria in different humans contain different amount of extra chromosomal DNA.

• Similarly, in bacteria, the amount of genomic DNA is same but the extra genomic DNA contained in plastids and mitochondria are different.

• Each mitochondrion has 2 membranes – the outer and the inner membrane.

• These membranes have their own specific enzymes associated with the mitochondrial functions.

• Mitochondria are the sites of aerobic respiration.

• They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell

• Plastids are species specific or region specific. They bear some specific pigments, thus imparting specific colours to the plants.

• Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

• The chloroplasts impart green color as they contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

• The chromoplasts contain fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour.

• The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.

a. Centromere - Centromere holds two chromatids or sister chromatids of a chromosome.

b. Cell wall – It is present only in plant cells. It gives the cell a shape and protects it from mechanical damage and from undesirable macromolecules.

c. Smooth ER – It is a major site for lipid synthesis. In animal cells, lipid-like steroidal hormones are synthesised at Smooth ER.

d. Golgi apparatus – It is an important site of formation of glycolipids and glycoproteins.

e. Centrioles - They form the base body of cilia and flagella and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Yes, plastids are interchangeable in their form.

There are three types of plastids - Chloroplasts (green colour), Chromoplasts (red, yellow, orange colour), Leucoplasts (Colourless). Depending upon different circumstances, these plastids interchange.

For example -

• The colour of green tomatoes changes to red due to replacement of chloroplasts with chromoplasts.

• The stem tuber of potato has leucoplasts but when exposed to sunlight, the leucoplasts are replaced by the chromoplasts.