31 Years NEET Previous Year Questions: Cell: The Unit Of Life - 1

• In some bacteria, (e.g., Bacillus subtilis) the plasma membrane form certain invaginations or in foldings called mesosomes in the cytoplasm.
• The mesosomes have various functions, viz., respiratory, secretory etc. Mesosomes are the infolding of the bacterial cell membrane.
• These structures are thought to be analogous to the eukaryotic mitochondria.

• The cytoskeleton is unique to eukaryotic cells. It is a dynamic three-dimensional structure that fills the cytoplasm. This structure acts as both muscle and skeleton, for movement and stability. The long fibers of the cytoskeleton are polymers of subunits. The primary types of fibers comprising the cytoskeleton are microfilaments, microtubules, and intermediate filaments.
• Microfilaments are fine, thread-like protein fibers, 3-6 nm in diameter. They are composed predominantly of a contractile protein called actin.
• Microtubules are cylindrical tubes, 20-25 nm in diameter. They are composed of subunits of the protein tubulin--these subunits are termed alpha and beta. 
• Intermediate filaments are about 10 nm diameter and provide tensile strength for the cell. Lamin is the protein present in intermediary filaments.

So, the correct answer is 'Microfilaments'.

The vacuole is bound by a single membrane called tonoplast. It also functions as semipermeable membrane. It segregates vacuolar contents from cytoplasm, allows osmotic entry or exit of water, concentration and storage of nutrients as well as wastes.

► Centriole: There are two main functions of centrioles
1. Cell division :The main function of the centriole is to help with cell division in animal cells.  The centrioles help in the formation of the spindle fibers that separate the chromosomes during cell division (mitosis).
2. Celiogenesis :The second function of centrioles that we will focus on is celiogenesis.  Celiogenesis is simply the formation of cilia and flagella on the surface of cells.  Cilia and flagella help the cell move. 

► Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts.

► A crista is a fold in the inner membrane of a mitochondrion.

► Ribosomes comprise a complex macromolecular machine, found within all living cells, that serves as the site of biological protein synthesis  (translation). Ribosomes link amino acids together in the order specified by messenger RNA(mRNA) molecules.

Ribosomes are composed of ribonucleic acid and proteins and are not surrounded by any membrane. These are site for protein synthesis. 

• A nuclear membrane, also known as the nuclear envelope, nucleolemma or karyotheca, is the double lipid bilayer membrane which surrounds the genetic material and nucleolus in eukaryotic cells.
• The nuclear membrane consists of two lipid bilayers - the inner nuclear membrane, and the outer nuclear membrane. The space between the membranes is called the perinuclear space, a region contiguous with the lumen (inside) of the endoplasmic reticulum.
• The nuclear membrane has many small holes called nuclear pores that allow material to move in and out of the nucleus. 
• In late prophase, nuclear envelope disappears and reappears in late telophase from rough endoplasmic reticulum (RER). 

• Prokaryotes are the primitive organism which lacks membrane-bounded organelles and nucleus. Prokaryotic cells have some microscopic structures in their cytoplasm. They serve specific purposes for the cell.
• Inclusions are aggregates of specific chemical compounds and often serve as a reservoir of energy or carbon. Common inclusions are poly--hydroxyalkanoate, sulfur globules, cyanophycin globules, and polyphosphate.
• Polyribosomes (or polysomes) also known as ergosomes are a cluster of ribosomes, bound to the mRNA molecule during translation process found in the eukaryotes.

• Rough endoplasmic reticulum contains ribosomes found attached to it, therefore RER synthesise proteins.
• Smooth endoplasmic reticulum is the major site for synthesis of lipids.
• Phospholipids are different from simple lipids as simple lipids are esters of fatty acid with various alcohols while compound or conjugated lipids like phospholipids are esters of fatty acids with alcohol but contain some other substances also like phosphate group in phospholipid.

Ribosomes are the protein factory of a cell. These are non-membranous organelle which attaches on endoplasmic reticulum by transcolon.

The function of the gap junction is to facilitate communication between adjoining cells by connecting the cytoplasm for rapid transfer of ions, small molecules and some large molecules.

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens.

Protoplasts are cells with the plasma membrane, cytoplasm and nucleus which have their cell wall removed by the action of enzymes. Cellulase enzymes digest the cellulose in plant cell walls while Pectinase enzymes break down the pectin holding cells together. Once the cell wall has been removed , the resulting protoplast is spherical in shape.

RER is frequently observed in the cells actively involved in protein synthesis and secretion. RER is well developed in cells engaged in synthesis of secretory products.

(a) → (iii),

(b) → (iv),

(c) → (i),

(d) → (ii).

• Cellular organelles with membranes are Lysosomes, Golgi apparatus and mitochondria.
• Lysosomes are single membrane-bound organelles which contain acid hydrolases or digestive enzymes that digest excess or worn out organelles, food particles, engulfed viruses or bacteria and macromolecules.
• Golgi apparatus is a single membrane-bound organelle consists of folded and flattened membranes that are involved in secretion and helps in intracellular transport.
• The mitochondria is a double-membraned cell organelle, known as the powerhouse of the cell which is present in all eukaryotic cells.
• So, the correct answer is 'Lysosomes, Golgi apparatus and mitochondria'.

• Satellite DNA is that part of repetitive DNA which has long repetitive nucleotide sequences in tandem that forms a separate fraction on density ultracentrifugation.
• DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA, because in these sequences, a small stretch of DNA is repeated many times.
• These repetitive DNA sequences are separated from bulk genomic DNA as different peaks during density gradient centrifugation.
• The bulk DNA forms a major peak and the other small peaks are referred to as satellite DNA.
• Depending on base composition (A:T rich or G:C rich), length of segment, and number of repetitive units, the satellite DNA is classified into many categories, such as micro-satellites, mini- satellites etc.
• These sequences normally do not code for any proteins, but they form a large portion of human genome.
• These sequences show high degree of polymorphism and form the basis of DNA fingerprinting. Since DNA from every tissue (such as blood, hair-follicle, skin, bone, saliva, sperm etc.) from an individual show the same degree of polymorphism, they become very useful identification tool in forensic applications.
• Satellite DNA is the region which is comprised of repeated sequences. This region does not encode for any protein or enzymes.
• The region shows a high degree of polymorphism in the population or between two different organisms. 
• This property can be used for DNA fingerprinting analysis. The satellite DNA shows the same degree of polymorphism in related individuals.
• This property can be used for paternity testing in which the offspring and parent have the same degree of polymorphism. 

The cell wall is the hard protective covering of all plant, fungal and some bacterial cells. Mycoplasma is a genus of bacteria that lack a cell wall and surrounded by their cell membrane only. 

Mitochondria & chloroplast are semiautonomous cell organelle which are formed by division of pre-existing organelle & contain DNA but they also contain proteinsynthesizing machinery, thus (a) is true & (b) is false.

• The microtubules are found in the cytoplasmic matrix of all eukaryotic cells. They also occur in cilia, flagella, centrioles, basal bodies, and mitotic apparatus.
• Spindle fibers form the protein structure which helps in mitosis and meiosis.
• Centrioles are the two cylindrical structures of a cell organelle called centrosome.
• Cilia and flagella are fine hair-like outgrowths of the membrane.
• While peroxisomes are the microbodies.
• 

So, the correct answer is 'Spindle fibers, centrioles, and cilia'.

Lysosome is enclosed by a single membrane while Mitochondria, Chloroplasts and Nuclei are enclosed by double membrane.