BIOLOGY 5 Marks Questions

Centromere is a constriction present on the chromosomes where the chromatids are held together. Chromosomes are divided into four types based on the position of the centromere.

1. Metacentric chromosome

style="margin-left: 40px;">The chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome.

• Sub-metacentric chromosome

The chromosome in which the centromere is slightly away from the middle region is known as a sub-metacentric chromosome. In this, one arm is slightly longer than the other.

1. Acrocentric chromosome

The chromosome in which the centromere is located close to one of the terminal ends is known as an acrocentric chromosome. In this, one arm is extremely long and the other is extremely short.

1. Telocentric chromosome

The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome.

Nucleus: The nucleus is a double membrane structure which contains the genetic material. It is generally round or oval shaped. It has five parts - nuclear envelope, nucleoplasm, nuclear matrix, chromatin and nucleolus.

1. Nuclear envelope: The nucleus is bounded by a double membrane nuclear envelope with minute pores called nuclear pores in the membrane. The pores work like channels for the passage of substances into and outside the nucleus. The outer membrane is connected with the endoplasmic reticulum which also bears ribosomes on it.
2. Nucleoplasm: The fluid filled in the nucleus is called the nucleoplasm. It contains nucleosides, enzymes, proteins and factors for the functioning of genetic material. Chromatin fibres and nucleolus are found embedded in the nucleoplasm.
3. Chromatin: The chromatin is a fine network of thread-like structures found inside the nucleus. It contains DNA and some basic proteins called histones, some non-histone proteins and RNA. During cell division, chromatin fibres condense to form chromosomes.
4. Nucleolus: It is a round, slightly irregular, naked structure attached to chromatin which synthesises ribosome subunits. It synthesises proteins.

Centrosome: The centrosome is an organelle containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole which is connected to the triplets via radial spokes. The centrosome plays an important role during cell division by organising the spindle fibres and astral rays. They form the basal body of cilia and flagella.

1. In prokaryotic cells, the nuclear membrane is absent. The genetic material is not enclosed by an envelope. The naked DNA lies coiled in the cytoplasm and is called a nucleoid or genophore.
2. The cell wall is present in bacteria and cyanobacteria and is absent in mycoplasma.
3. Cell lumen is filled with a fluid called the cytoplasm.
4. Prokaryotic cells lack membrane-bound cell organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules, microfilaments and centrioles.
5. There are no true, fluid-filled vacuoles. Some prokaryotic cells contain gas vacuoles.
6. Ribosomes are the only cytoplasmic organelles found in prokaryotic cells. They are small (70S) ribosomes.

Plasma membrane is the outermost covering of the cell that separates it from the environment. It regulates the movement of substances into the cell and out from it. It allows the entry of only some substances and prevents the movement of other materials. Hence, the membrane is selectively-permeable. Movement of neutral solutes across the cell membrane – Neutral molecules move across the plasma membrane by simple passive diffusion. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Movement of polar molecules across the cell membrane – The cell membrane is made up of a phospholipid bilayer and proteins. The movement of polar molecules across the non-polar lipid bilayer requires carrier-proteins. Carrier-proteins are integral protein particles having certain affinity for specific solutes. As a result, they facilitate the transport of molecules across the membrane.

Prokaryotic Cells	Eukaryotic Cells
These cells lack a nuclear membrane and other membrane bound organelles.	These cells have a nuclear membrane and other membrane-bound organelles.
They have 70 S ribosomes.	They have 80 S ribosomes.
Gas vacuoles are present.	Sap/ food/ contractile vacuoles are present.
Cell wall is always present and made of peptidoglycan (murein).	Cell wall is present in plant cells (cellulose), fungal cells (chitin) and some protists (cellulose) but absent in animal cells.
DNA is not complexed with histones.	DNA is complexed with histones.
Flagella lack (9 + 2) organisation.	Flagella show (9 + 2) organisation.

To be called as a living cell, a cell must possess the following features: Structure: It must contain the genetic blueprint and machinery to use it. This means the cells should contain cytoplasm, plasma membrane and hereditary material or the genetic material (DNA or RNA) and the proteins. Also, it should contain various internal organelles carrying out various functions necessary for its metabolic activity and survival. Functions:

1. The cell should be able to perform various chemical reactions like transforming simple organic molecules into complex molecules, called anabolism, as well as be able to breakdown complex molecules to release energy, called catabolism. This comprises the metabolism of a cell.
2. The cells should be able to perform mechanical activities like responding to stimuli, for example, chemotaxis, phototaxis, etc.
3. They should be able to regulate important activities like DNA synthesis, cell division or gene regulation, etc.

Plastids are small organelles. They contain food pigments and are found in the cytoplasm of the cell of plant. They can be both region specific (occur according to their function) or species specific. There are three types of plastids: Chromoplasts, Chloroplasts and Leucoplasts.

1. Leucoplasts: Leucoplasts are found near the nucleus and are colorless in nature. They have lamella whereas Grana and photosynthetic pigments are absent. They occur majorly in non green plants.
2. Chromoplasts: Chromoplast is found to be reddish or yellow in colour. This is because of the fact that they contain carotenoid pigment. The fascinating colours of many flowers are because of this pigment or plastid.
3. Chloroplast: Chloroplast is green in colour.

They occur in mesophyll cells which are photosynthetic. They also take part in food synthesis. Apart from being region specific, plastids are also species specific. For example, Chloroplasts that are there in Euglenoids have three layers present.

Mitochondria are cell organelles of aerobic eukaryotes which take part in oxidative phosphorylation and Krebs cycle of aerobic respiration. They are called power houses of cell because they are the major centers of release of energy in the aerobic respiration. Mitochondria are miniature biochemical factories where food stuffs or respiratory substances are completely oxidized to carbon dioxide and water. The energy liberated in the process is initially stored in the form of reduced coenzymes and reduced prosthetic groups. The latter soon undergo oxidation and form energy rich ATR ATP comes out of mitochondria and helps to perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement etc. Because of the formation of ATP, the mitochondria are called power houses of the cell.

Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement. Structure:

1. The electron microscopic study of a cilium or the flagellum shows that they are covered with plasma membrane.
2. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules.
3. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
4. The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral couplet by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers.
5. Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

Difference Between Flagella of Prokryotes and Eukaryotes:

1. Flagella of prokryotes are arising from the plasma membrane, while those of eukaryotes are arising from centrioles.
2. Flagella of prokaryotes are not membrane bound, while those of eukaryotes are membrane bound.
3. Flagella of prokaryotes are simple in structures, while those of eukaryotes are complex in structure.

The genomic DNA content of nucleus for a species is constant. In a given species, the haploid organisms have half the DNA content to that of their parents. The amount of DNA content remains constant and is maintained same throughout their life. The DNA content and chromosome number is same in the nucleus of all the members of the species. Whereas the extra chromosomal DNA mainly found in the cell organelles like mitochondria, chloroplasts are variable in the individuals of population. This is due to the reason that highly active organisms and cells have more chloroplasts and mitochondria and hence extra chromosomal DNA as compared to the less active organisms or cells.

Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials both the centriole in centrosome lie perpendicular to each other in which each has an organization like that of an 'cartwheel'. A centriole possesses a whorl of. 9. peripheral fibrils. There fibrils are absent in the centre, hence the arrangement is called 9 + 0. Each fibril is made of 3 sub-fibres. Therefore, called triplet fibril. The centrioles form the basal body of cilia and flagella. It also forms spindle fibres that gives rise to spindle apparatus during cell division in animals.

Golgi apparatus It is the important site for the formation of glycoprotein and glycolipids. It is also involved in the synthesis of cell wall materials and also play an important role in formation of cell plate during cell division.

Smooth ER It helps in synthesis of lipids, metabolism of carbohydrates, and regulation of calcium concentration, drug detoxification and attachment of receptor on cell membrane proteins. The smooth ER also contains enzymes-glucose 6 phosphatase, which converts glucose6 phosphate to glycogen, essential in glucose metabolism.

Plant Cells	Animal Cells
There is a cell wall of cellulose outer to plasma membrane.	There is no cell wall and cell membrane is the outermost layer.
Plastids, containing pigments are present.	No plastid of any kind is present.
Centrioles are absent, hence mitosis is anastral.	Centrioles are present and mitosis is astral.
Vacuoles are large in size and many in number	Vacuoles are either absent or very small and few in number.
Golgi bodies occur as distinct units, called dictyosomes.	Golgi bodies are elaborate and inter connected.

1. Ribosomes are present in association with the plasma membrane in prokaryotic cells:

• In prokaryotes, ribosomes are associated with the plasma membrane of the cell.
• They are about 15 nm × 20 nm in size.
• Each ribosome consists of two subunits- 50 S and 30 S and together they form 70 S ribosomes.
• They are the sites of protein synthesis.

2. The two groups of bacteria are:

• Gram positive bacteria, which take up gram's stain.
• Gram-negative bacteria, which do not take up gram's stain.

3. Contractile vacuoles perform excretion and osmoregulation.

• They are found protists like Amoeba, Paramoecium, etc.

• The cell membrane is made of lipids mainly phosphoglycerides, that are arranged as a bilayer, proteins and carbohydrates.
• The lipids are arranged within the protein layers with the polar head towards the outer surface and the hydrophobic tails towards the inner surface.
• The cell membrane also possesses some carbohydrates.
• The membrane proteins may lie on the surface of membrane (peripheral proteins) or partially or totally buried in the membrane proteins.
• The ratio of lipids to proteins varies in different types of cells.

This is true that structure and function are correlated in living organisms. Following aspects of structure and function of plasma membrane justify this fact.

• Plasma membrane provides a boundary to the cell contents. This is made possible because of its ability to segregate the inside of the cell with the external environment.
• Plasma membrane is a semi-permeable membrane. This attribute helps the plasma membrane to allow exchange of some selected materials between the cell and its external environment.
• We know that plasma membrane is composed of a lipid bilayer which has proteinintersperse in a mosaic like pattern. The protein molecules serve as channels for those substances which need active transport mechanism to enter or exit a cell.
• The fluid layer of lipid allows those molecules to pass through the membrane which can easily travel through osmosis.

An animal cell has following cell structures:

1. Plasma membrane.
2. Endoplasmic reticulum.
3. Mitochondria.
4. Golgi body.
5. Lysosomes.
6. Ribosomes.
7. Vacuoles.
8. Nucleus.
9. Centriole.

Plasma Membrane: This is also known as cell membrane. Plasma membrane is made up of lipid and protein. It is semi-permeable in nature. Certain substances are transported through plasma membrane by passive transport. Some substances get transported by osmosis and some by active transport. Active transport involves use of some carrier to facilitate transport. Apart from transport of materials, plasma membrane gives a shape and size to the animal cell. Endoplasmic Reticulum: These are networks of fine tubules extending from plasma membrane to nucleus. They work like pipelines and facilitate transport of substances from outside the cell to nucleus and cytoplasm. Depending on presence or absence of ribosomes ER can be either rough or smooth. Golgi Body: This is composed of many sack like structures stacked one over another. The function of golgi body is to package different materials, like carbohydrate, protein and lipid. Lysosome: Lysosome is small spherical structure filled with digestive enzymes. The digestive enzyme helps in digesting foreign materials and waste products. Sometimes the lysosome digests the contents of cytoplasm which in turn kills the cell itself. That is why lysosome is also known as 'suicide bag of the cell.’ Ribosome: Ribosomes are small dot like structures. They are made of two subunits. The function of ribosome is to synthesize protein. Vacuoles: These are small fluid filled structures. Vacuoles help in maintaining osmotic pressure inside the cell. Mitochondria: Mitochondria is double membrane structure. The inner membrane is projected in finger like structures, called cristae. The presence of cristae helps in increasing the inner surface are of mitochondria. Aerobic respiration takes place in the mitochondria and energy released is stored in the form of ATP (Adenosine triphosphate). Nucleus: Nucleus is covered by nuclear membrane. Nucleus contains chromosomes which are genetic materials. Nucleus also controls various functions of the cell. Centriole. These are spindle like structures. During cell division they form spindle fibres.

Chemical composition of plasma membrane

Component	Composition
Lipids	(20-79%)
Proteins	(20-70%)
Carbohydrate	(1-5%)
Water	20

Lipids are the major components of the cell membrane as they form the continuous structural frame of the cell membrane. Lipids sucas phospholipids, glycolip.ds, and steroids are found in membranes. The lipid molecule possess both polar hydrophilic (water loving) and non-polar hydrophobia (water repelling) ends. The hydrophilicregion is in the form of a head, while the hydrophobic part contains two tails of fatty acid. Hydrophobic tail is present towardsthe centre of the membrane. This structures result is the formation of lipid bilayer known as unit membrane/biologicalmembrane/cell membrane.

Yes, extra genomic DNA is present in both prokaryotes and eukaryotes. In prokaryotes, extra genomic DNA is present in form of plasmids and as mitochondria and plastids in eukaryotes. In Prokaryotes plasmids are self replicating, extra chromosomal segments of double stranded, circular, naked DNA. Plasmids provide unique phenotypic characters to bacteria. They are independent of main nucleoid. Some of them contain important genes like fertility factor, nif genes and resistance factors. Plasmids are used as vectors in genetic engineering. In Eukaryotes: mtDNA (mitochondrial DNA) and cpDNA (chloroplast DNA) are present as a extra genomic DNA in mitochondria and chloroplast respectively. Mitochondria have their own naked, circular or linear DNA which can replicate independently. Mitochondrial DNA produces its own mRNA, fRNA and rRNA. Plastids are other semi-autonomous organelles having DNA and double membrane envelope which store or synthesize various types of organic compounds. Chloroplast DNA is naked, circular or occasionally linear.

Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large. 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. Chloroplasts: The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis. Chromoplasts: In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour. Leucoplasts: 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. Shape and Size of Chloroplasts: Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10nm) and width (2-4nm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Structure of Chloroplasts: Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

The Cell theory was proposed by M.J. Schleiden and Theodore Schwann. It states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organisms. Modern cell theory has the following postulates:

1. All organisms are composed of cells and their products.
2. Cell consists of nucleus, cytoplasm, cell organelles.
3. All cells arise from pre-existing cells i.e. no cell originates spontaneously but comes into being only by division of cells.
4. Every organism starts its life as a cell.
5. All cells show similarity in structure, chemical composition and function.
6. Genetic information is stored and expressed inside cells.
7. Cells contain hereditary material.
8. Cells are responsible for carrying out metabolic activities within living organisms.
9. The function of an organism is the result of the coordinated activities and interaction of the cell constituents.
10. Cell is the structural and functional unit of life. All vital functions of an organism occur within cells.