Class 8 Biology — Important Questions with Answers

What it is

Plant cells have a cell wall, a big vacuole and chloroplasts; animal cells have none of these.

Answer

A plant cell has a rigid cell wall outside its cell membrane and usually contains a large central vacuole and green chloroplasts for photosynthesis. An animal cell has no cell wall (only a cell membrane), has small or no vacuoles and lacks chloroplasts. This is why plants can make their own food while animals cannot.

What it is

The cell is the smallest structural and functional unit of a living organism.

Answer

The cell is called the basic structural unit of life because the bodies of all living organisms are made up of cells, just as a building is made of bricks. It is called the basic functional unit because all the life processes such as respiration, digestion and growth are carried out inside cells. Organisms may be unicellular (made of one cell, like Amoeba) or multicellular (made of many cells).

What it is

Plant cells have a cell wall, chloroplasts and a large vacuole; animal cells lack these.

Answer

A plant cell has a rigid cell wall outside the cell membrane, whereas an animal cell has only a cell membrane. A plant cell contains chloroplasts that carry out photosynthesis, which animal cells lack. A plant cell usually has one large central vacuole, while an animal cell has small or no vacuoles.

What it is

Each organelle has a special job - the nucleus controls the cell, mitochondria release energy, and chloroplasts make food.

Answer

The nucleus is the control centre of the cell; it directs all cell activities and contains the genetic material. The mitochondria are the powerhouses of the cell; they release energy through respiration. The chloroplast, found in plant cells, contains the green pigment chlorophyll and carries out photosynthesis to make food.

What it is

Prokaryotic cells have no well-defined nucleus; eukaryotic cells have a true, membrane-bound nucleus.

Answer

A prokaryotic cell does not have a well-defined, membrane-bound nucleus; its genetic material lies free in the cytoplasm, for example a bacterium. A eukaryotic cell has a true nucleus surrounded by a nuclear membrane and has membrane-bound organelles, for example a plant cell or an animal cell.

What it is

Microbes are tiny living things that can help us (curd, bread, medicine) or harm us (disease).

Answer

Useful: yeast is used to make bread and in brewing (fermentation), and Lactobacillus bacteria turn milk into curd. Harmful: the bacterium that causes tuberculosis, and the Plasmodium protozoan that causes malaria. So micro-organisms can help or harm us depending on the type.

What it is

Microorganisms are tiny organisms seen only under a microscope; the main groups are bacteria, fungi, protozoa, algae and viruses.

Answer

Microorganisms, or microbes, are living organisms so small that they cannot be seen with the naked eye and are viewed only under a microscope. The major groups are bacteria, fungi, protozoa and some algae. Viruses are also studied with microorganisms, but they reproduce only inside the cells of a host organism.

What it is

Microbes are used to make curd, bread, alcohol, antibiotics and to fix nitrogen in soil.

Answer

Microorganisms are useful in many ways. The bacterium Lactobacillus turns milk into curd. Yeast is used in baking bread and in the fermentation that produces alcohol. Some microorganisms are used to make medicines called antibiotics, such as penicillin. Bacteria such as Rhizobium fix atmospheric nitrogen into the soil, increasing its fertility.

What it is

Some microbes cause diseases in humans, animals and plants and spoil food; disease-causing microbes are called pathogens.

Answer

Pathogens are disease-causing microorganisms. Many diseases in humans are caused by microbes: tuberculosis and cholera are caused by bacteria, while malaria (caused by a protozoan) and dengue are spread by mosquitoes acting as carriers. Such diseases that spread from an infected person to a healthy person through air, water, food or carriers are called communicable diseases.

What it is

The nitrogen cycle keeps the amount of nitrogen in the atmosphere and soil balanced, with bacteria fixing and releasing nitrogen.

Answer

Nitrogen fixation is the process of converting atmospheric nitrogen gas into nitrogen compounds (like nitrates) that plants can absorb. It is carried out by bacteria such as Rhizobium in the roots of leguminous plants, and also by lightning. Plants use these compounds to make proteins, which pass to animals when they eat plants. Decomposers return nitrogen to the soil, and denitrifying bacteria release nitrogen gas back into the atmosphere, keeping the nitrogen cycle balanced.

What it is

Food is preserved by stopping the growth of microorganisms that spoil it.

Answer

Food can be preserved by adding salt (salting) or sugar, which draw out water and prevent microbial growth, as in pickles and jams; by heating and cooling, such as pasteurisation of milk and refrigeration; by drying and dehydration, as with grains; and by using chemical preservatives such as sodium benzoate, or by sealing in airtight cans and vacuum packing.

What it is

Manure is natural and releases nutrients slowly; fertiliser is a chemical that acts fast.

Answer

Manure is a natural substance made by decomposing animal dung and plant waste; it adds humus, improves soil texture and releases nutrients slowly. A fertiliser is a chemical made in factories that is rich in a specific nutrient (like nitrogen, phosphorus or potassium) and acts fast, but its overuse can harm the soil and water. Manure is eco-friendly; fertilisers give a quick nutrient boost.

What it is

Growing crops involves a sequence of steps from preparing the soil to storing the harvest.

Answer

The main agricultural practices, in order, are: preparation of the soil (ploughing, levelling), sowing of seeds, adding manure and fertilisers (nutrient management), irrigation (supplying water), protection from weeds (weeding) and pests, harvesting the mature crop, and finally storage of the produce in a dry, safe place.

What it is

Manure is a natural organic substance; fertiliser is a manufactured chemical rich in specific nutrients.

Answer

Manure is a natural substance prepared by the decomposition of plant and animal wastes; it is rich in humus, improves the texture and water-holding capacity of the soil and is environment friendly. A fertiliser is a chemical substance manufactured in factories that is rich in a particular plant nutrient such as nitrogen, phosphorus or potassium; it gives a good yield but its excessive use harms the soil and causes water pollution.

What it is

Irrigation is the supply of water to crops at the right time; modern methods save water.

Answer

Irrigation is the supply of water to crops at suitable intervals for their proper growth. Two modern water-saving methods are the drip system, in which water falls drop by drop directly at the roots of plants through narrow pipes, and the sprinkler system, in which water is sprayed over the crop like rain through rotating nozzles. Both waste far less water than flooding the field.

What it is

At puberty, hormones bring body changes that mark a child becoming an adult.

Answer

Secondary sexual characters are the physical features that appear at puberty and distinguish males from females - such as growth of facial hair and deepening of the voice in boys, and development of breasts in girls. They are controlled by hormones released by the endocrine glands: testosterone from the testes in boys and oestrogen from the ovaries in girls.

What it is

Adolescence is the period of change from childhood to adulthood; puberty is when the body becomes capable of reproduction.

Answer

Adolescence is the period of life, roughly between the ages of 11 and 19 years, during which a person changes from a child into an adult. The changes occur because of puberty, the stage when the body becomes capable of reproduction. Changes during puberty include a sudden increase in height and body weight, and the development of secondary sexual characters such as a deeper voice and hair growth in boys and development of breasts in girls.

What it is

Endocrine glands are ductless glands that release chemical messengers called hormones directly into the blood.

Answer

Hormones are chemical substances secreted by endocrine (ductless) glands directly into the blood, which carries them to target organs where they control body functions. For example, the thyroid gland secretes thyroxine, which controls the rate of metabolism, and the pancreas secretes insulin, which controls the level of sugar in the blood. The testes secrete testosterone and the ovaries secrete oestrogen.

What it is

The sex of a baby is decided by the sex chromosomes contributed by the father (X or Y).

Answer

Humans have a pair of sex chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The mother's egg always carries an X chromosome, while the father's sperm may carry either an X or a Y chromosome. If a sperm carrying X fertilises the egg, the child is a girl (XX); if a sperm carrying Y fertilises the egg, the child is a boy (XY). Thus the father's chromosome determines the sex of the child.

What it is

Animals either lay eggs (oviparous) or give birth to live young (viviparous).

Answer

Oviparous animals lay eggs that develop and hatch outside the mother's body, for example a hen or a frog. Viviparous animals give birth to live young that develop inside the mother's body, for example a human or a cow.

What it is

Sexual reproduction involves the fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote.

Answer

Fertilisation is the process of fusion of a male gamete (sperm) with a female gamete (egg or ovum). It results in the formation of a single cell called the zygote. The zygote divides repeatedly and develops into a new individual. Since two parents and two types of gametes are involved, this is called sexual reproduction.

What it is

Internal fertilisation occurs inside the female body; external fertilisation occurs outside, in water.

Answer

Internal fertilisation is the fusion of the sperm and egg that takes place inside the body of the female, for example in humans, hens, cows and dogs. External fertilisation is the fusion of the sperm and egg that takes place outside the body of the female, usually in water, for example in frogs and fish, where the female lays eggs and the male deposits sperms over them.

What it is

Viviparous animals give birth to young ones; oviparous animals lay eggs.

Answer

Viviparous animals are those that give birth to live young ones, in which the embryo develops inside the mother's body, for example humans, cows and dogs. Oviparous animals are those that lay eggs, in which the embryo develops inside the egg outside the mother's body, for example hens, frogs and fish.

What it is

Metamorphosis is the transformation of a larva into an adult through distinct stages.

Answer

Metamorphosis is the process by which the larva of an animal undergoes drastic changes in body form to become an adult. For example, a frog develops from an egg into a tadpole (larva) that lives in water, and the tadpole then changes into an adult frog. Similarly, a caterpillar (larva) changes into a pupa and then into an adult butterfly.

What it is

In asexual reproduction a single parent produces offspring without gametes, as in budding (Hydra) and binary fission (Amoeba).

Answer

In asexual reproduction, a new organism is produced from a single parent without the fusion of gametes. In budding, seen in Hydra, a small outgrowth called a bud develops on the parent's body, grows and finally detaches to form a new individual. In binary fission, seen in Amoeba, the parent cell divides into two equal daughter cells, each of which becomes a new individual.

What it is

Biodiversity is the variety of all living things - plants, animals and microbes - in an area.

Answer

Biodiversity is the variety of living organisms - plants, animals and micro-organisms - found in a particular area. It should be conserved because every species has a role in keeping the ecosystem balanced, and the loss of one can affect many others. Biodiversity gives us food, medicines, and clean air and water, and losing it through deforestation and pollution can push species to extinction.

What it is

Biodiversity is the variety of living organisms in an area; species at risk of extinction are endangered.

Answer

Biodiversity, or biological diversity, refers to the variety of plants, animals and micro-organisms found in a particular area. An endangered species is a species whose numbers have become so small that it is at risk of becoming extinct (dying out completely), for example the tiger and the great Indian bustard. Species that have already disappeared, such as the dodo, are called extinct.

What it is

Deforestation is the clearing of forests; it harms climate, soil and wildlife.

Answer

Deforestation is the large-scale cutting down of trees and clearing of forests for farming, houses or industry. Its harmful effects include an increase in the carbon dioxide level of the atmosphere leading to global warming, loss of habitat that endangers wildlife, and soil erosion that lowers soil fertility and can turn land into desert; it also reduces rainfall and increases the chance of floods and droughts.

What it is

Wildlife sanctuaries, national parks and biosphere reserves protect plants and animals in their natural home.

Answer

A wildlife sanctuary is a protected area where wild animals are protected from hunting and disturbance, though some human activities may be allowed. A national park is a larger reserved area that protects the entire ecosystem, that is the plants, animals and landscape, and no human activity is permitted. The Red Data Book is the official record that keeps a list of all endangered species of plants and animals.

What it is

Magnification tells how many times larger an object looks than its real size.

Answer

Magnification = image size / actual size = 60 / 6 = 10. The object looks 10 times larger than it really is. Magnification has no unit because it is a ratio of two lengths.

What it is

A compound microscope's total magnification is the eyepiece magnification times the objective magnification.

Answer

Total magnification = eyepiece x objective = 10 x 40 = 400. The image appears 400 times larger than the real object. Choosing a stronger objective or eyepiece raises the total magnification.

What it is

A plant cell is made of parts (organelles), each with its own job, working together to keep the cell alive.

Answer

The cell wall is a rigid outer layer that gives shape and support. Inside it, the cell membrane controls what enters and leaves. The cytoplasm is the jelly holding the organelles. The nucleus is the control centre with the DNA. The large vacuole stores water and keeps the cell firm, and the chloroplasts contain chlorophyll to carry out photosynthesis.

What it is

Xylem carries water and minerals upward from the roots; phloem carries food made in the leaves to all parts.

Answer

The two conducting (vascular) tissues in plants are xylem and phloem. Xylem conducts water and dissolved minerals absorbed by the roots upward to the stem and leaves. Phloem conducts the food (mainly sugars) prepared in the leaves during photosynthesis to all other parts of the plant, including the roots. This movement of food is called translocation.

What it is

Photosynthesis is the process by which green plants make their own food using sunlight, carbon dioxide and water.

Answer

Photosynthesis is the process by which green plants prepare their own food (glucose) using carbon dioxide and water in the presence of sunlight and the green pigment chlorophyll. The raw materials are carbon dioxide (from the air) and water (from the soil), with sunlight as the energy source. The word equation is: carbon dioxide + water, in the presence of sunlight and chlorophyll, gives glucose + oxygen.