Class 9 Biology — Important Questions with Answers

What it is

Prokaryotes (bacteria) have no true nucleus; eukaryotes (plants, animals) have a nucleus and organelles.

Answer

A prokaryotic cell has no true nucleus - its genetic material lies free in the cytoplasm - and it lacks membrane-bound organelles like mitochondria; bacteria are examples. A eukaryotic cell has a true, membrane-bound nucleus and membrane-bound organelles; plant and animal cells are examples. Eukaryotic cells are usually larger and more complex.

What it is

Osmosis is water moving across a membrane to balance the concentration on both sides.

Answer

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. A cell placed in pure water (a hypotonic solution) gains water by osmosis and swells; an animal cell may even burst, while a plant cell becomes firm (turgid) because of its cell wall.

What it is

Prokaryotic cells lack a true nucleus and membrane-bound organelles; eukaryotic cells have both.

Answer

A prokaryotic cell has no true, membrane-bound nucleus - its genetic material lies free in the cytoplasm as a nucleoid - while a eukaryotic cell has a well-defined nucleus surrounded by a nuclear membrane. Prokaryotic cells lack membrane-bound organelles such as mitochondria, whereas eukaryotic cells possess them. Prokaryotic cells are generally smaller and simpler, for example bacteria, while eukaryotic cells are larger, for example plant and animal cells.

What it is

The plasma membrane is a selectively permeable boundary that controls what enters and leaves the cell; diffusion moves substances from high to low concentration.

Answer

The plasma membrane, or cell membrane, is the outermost living covering of a cell that separates its contents from the surroundings. It is selectively permeable, meaning it allows only certain substances to pass through it. Diffusion is the movement of particles of a substance from a region of higher concentration to a region of lower concentration; gases such as carbon dioxide and oxygen move in and out of the cell by diffusion.

What it is

Osmosis is the movement of water molecules through a selectively permeable membrane from a dilute to a concentrated solution.

Answer

Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a selectively permeable membrane. When a cell is placed in a hypotonic solution (more dilute than the cell), water enters and the cell swells. When placed in a hypertonic solution (more concentrated than the cell), water leaves and the cell shrinks (in plant cells this is plasmolysis). In an isotonic solution there is no net movement of water.

What it is

The cell wall is a rigid outer covering of plant cells made of cellulose; plasmolysis is the shrinking of the cell contents in a hypertonic solution.

Answer

The cell wall is the tough, rigid, non-living outer covering found in plant cells, mainly made of a carbohydrate called cellulose. It gives the cell shape, strength and protection. Plasmolysis is the process in which a plant cell placed in a highly concentrated (hypertonic) solution loses water by osmosis, so its cytoplasm and cell membrane shrink and pull away from the cell wall.

What it is

The nucleus is the control centre of the cell and contains the genetic material (DNA) in chromosomes.

Answer

The nucleus is a large, spherical organelle bounded by a nuclear membrane. It contains the chromosomes, which carry the genetic material DNA in the form of genes. The nucleus controls all the activities of the cell, so it is called the control centre or brain of the cell. It also plays a central role in cell reproduction (cell division) and in the inheritance of characters from parents to offspring.

What it is

Mitochondria release energy from food during respiration and store it as ATP.

Answer

Mitochondria are called the powerhouse of the cell because they release energy during cellular respiration by breaking down glucose in the presence of oxygen. This energy is stored in the form of molecules of ATP (adenosine triphosphate), which is the energy currency of the cell and is used to power all the cell's activities. Mitochondria have their own DNA and ribosomes.

What it is

The endoplasmic reticulum is a network of membranes for transport; ribosomes on the rough ER make proteins.

Answer

The endoplasmic reticulum (ER) is a network of membrane-bound tubes and sheets inside the cell. The rough endoplasmic reticulum (RER) has ribosomes attached to its surface and helps in the synthesis and transport of proteins. The smooth endoplasmic reticulum (SER) has no ribosomes and helps in the synthesis of fats (lipids) and in detoxifying poisons. Ribosomes are the sites where proteins are actually made.

What it is

The Golgi apparatus packages and dispatches materials; lysosomes are the cell's digestive and cleaning bags.

Answer

The Golgi apparatus is a stack of membrane-bound sacs that receives materials from the endoplasmic reticulum, then modifies, packages and dispatches them to various targets inside or outside the cell; it also helps form lysosomes. Lysosomes are membrane-bound sacs filled with digestive enzymes that break down worn-out cell parts and foreign material; because they can burst and digest their own cell when it is damaged, they are called the suicidal bags of the cell.

What it is

Plastids (like chloroplasts) are found in plant cells and help in food-making and colour; vacuoles store water and other substances.

Answer

Plastids are organelles found only in plant cells. The green plastids called chloroplasts contain chlorophyll and carry out photosynthesis, while coloured plastids (chromoplasts) give colour to flowers and fruits and colourless plastids (leucoplasts) store food. The vacuole is a fluid-filled sac that stores water, food, wastes and other substances; in plant cells the large central vacuole also provides turgidity and rigidity to the cell.

What it is

Meristem is a plant's growth tissue - young cells that keep dividing at tips and edges.

Answer

Meristematic tissue is made of small, thin-walled cells that keep dividing to help the plant grow. It is found at the growing regions - apical meristem at the tips of roots and shoots (which increases length) and lateral meristem or cambium (which increases thickness). Because its cells divide actively, they have dense cytoplasm and no large vacuoles.

What it is

Xylem and phloem are the transport tissues of plants - xylem carries water, phloem carries food.

Answer

Xylem and phloem are the two conducting (vascular) tissues in plants. Xylem carries water and dissolved minerals upward from the roots to the leaves. Phloem transports the food (sugar) made in the leaves during photosynthesis to all other parts of the plant. Together they form the plant's transport system.

What it is

Meristematic tissue consists of actively dividing cells responsible for the growth of a plant.

Answer

Meristematic tissue is a plant tissue made of actively dividing, small, thin-walled cells with dense cytoplasm and no vacuoles; it is responsible for growth. Based on location, it is of three types: apical meristem, present at the tips of roots and shoots, which increases length; lateral meristem, present along the sides, which increases the girth (thickness); and intercalary meristem, present at the base of leaves or internodes, which also increases length.

What it is

Parenchyma, collenchyma and sclerenchyma are simple permanent tissues that store food, give flexibility and give strength.

Answer

The three simple permanent tissues are: parenchyma, made of thin-walled living cells that store food and water and carry out photosynthesis (when it contains chlorophyll it is called chlorenchyma); collenchyma, made of cells with unevenly thickened walls that provide flexibility and mechanical support, as in leaf stalks; and sclerenchyma, made of dead cells with thick, lignified walls that provide hardness and strength, as in the husk of a coconut.

What it is

Xylem and phloem are complex tissues that conduct water and food through the plant.

Answer

The two complex permanent tissues are xylem and phloem, together called vascular tissue. Xylem conducts water and dissolved minerals from the roots to the rest of the plant and also provides mechanical support; it is made mostly of dead cells (tracheids, vessels, xylem fibres and xylem parenchyma). Phloem conducts food prepared in the leaves to all parts of the plant; it is made of living cells (sieve tubes, companion cells, phloem fibres and phloem parenchyma).

What it is

Epithelial tissue is the covering or protective tissue that lines the surfaces and cavities of the animal body.

Answer

Epithelial tissue is the animal tissue that covers the outer surface of the body and lines its internal organs and cavities, forming a continuous protective sheet. It protects the underlying parts, and helps in absorption and secretion. Types include squamous epithelium (flat cells that line blood vessels and air sacs), cuboidal epithelium (cube-shaped cells lining kidney tubules), columnar epithelium (tall cells lining the intestine) and glandular epithelium (which secretes substances).

What it is

Connective tissue joins, supports and transports; its cells are loosely spaced in a matrix.

Answer

Connective tissue is an animal tissue whose main function is to connect, support, protect and transport; its cells are loosely spaced and embedded in an intercellular substance called the matrix. Examples include blood, a fluid connective tissue that transports substances; bone, which forms the framework and supports the body; and cartilage, which is a smooth, flexible tissue at joints and in the nose and ears. Ligaments and tendons are also connective tissues.

What it is

Muscular tissue brings about movement; nervous tissue conducts messages as electrical impulses.

Answer

The three types of muscular tissue are: striated (skeletal) muscle, which is voluntary and moves the limbs; smooth (unstriated) muscle, which is involuntary and found in the walls of internal organs such as the stomach; and cardiac muscle, which is involuntary and found only in the heart. Nervous tissue is made of specialised cells called neurons that receive and conduct information rapidly as electrical impulses from one part of the body to another.

What it is

Mixed cropping blends crops with no set pattern; intercropping plants them in definite rows.

Answer

In mixed cropping two or more crops are grown together on the same field at the same time with no fixed pattern, mainly to reduce the risk of total crop failure. In intercropping two or more crops are grown together in a definite row pattern (for example, a few rows of one crop then a few of another) to use nutrients and space efficiently and to help control pests.

What it is

Germination percentage is the fraction of sown seeds that sprout, expressed out of a hundred.

Answer

Germination % = (seeds sprouted / seeds sown) x 100 = (40 / 50) x 100 = 80%. A higher percentage means better quality seeds and a more reliable crop.

What it is

Crop yields are improved by crop variety improvement, crop production management and crop protection management.

Answer

Crop yields can be improved in three broad ways. Crop variety improvement involves developing better varieties by hybridisation and selection for higher yield, disease resistance and drought tolerance. Crop production management involves proper use of nutrients (manures and fertilisers), irrigation and cropping patterns such as mixed and inter-cropping. Crop protection management involves protecting crops from weeds, insect pests and diseases using appropriate and safe methods.

What it is

Animal husbandry is the scientific management and breeding of livestock for food and other products.

Answer

Animal husbandry is the scientific management of farm animals, which includes their proper feeding, breeding, shelter and health care to obtain more and better food and other products. Its branches include cattle farming for milk (dairy) and draught work, poultry farming for eggs and meat, fish farming (pisciculture) for protein-rich food, and bee-keeping (apiculture) for honey and wax. Cross-breeding is used to develop improved varieties with higher yield and disease resistance.

What it is

The atmosphere is Earth's protective blanket of air that supplies gases and steadies temperature.

Answer

The atmosphere is a blanket of air that supports life in several ways: it provides oxygen for breathing and carbon dioxide for photosynthesis, keeps the temperature of the Earth steady by preventing sudden day-to-night changes, and shields us from the Sun's harmful ultraviolet rays through the ozone layer. Winds and the water cycle also depend on it.

What it is

The water cycle is the continuous circulation of water between the land, water bodies, atmosphere and living things.

Answer

The water cycle is the continuous movement of water among the oceans, land, atmosphere and living organisms. Water evaporates from oceans, rivers and lakes, and transpires from plants, forming water vapour. This vapour rises, cools and condenses to form clouds. The water then falls back to the Earth as precipitation (rain, snow or hail), collects in water bodies and on land, and the cycle repeats. The Sun's heat drives the whole process.

What it is

The nitrogen cycle circulates nitrogen between the atmosphere, soil and living organisms.

Answer

In the nitrogen cycle, atmospheric nitrogen gas is converted into usable compounds by nitrogen fixation, carried out by bacteria such as Rhizobium (in legume roots) and by lightning. These compounds are turned into nitrates by nitrifying bacteria, which plants absorb to make proteins. Animals get nitrogen by eating plants. When plants and animals die, decomposers break down their remains and return nitrogen to the soil, and denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.

What it is

The carbon cycle moves carbon between the air, living things and fuels; too much carbon dioxide enhances the greenhouse effect.

Answer

In the carbon cycle, carbon dioxide from the atmosphere is taken up by green plants during photosynthesis and converted into food (carbon compounds). This carbon passes to animals through food, and is returned to the atmosphere as carbon dioxide through respiration, decomposition and the burning of fuels. The greenhouse effect is the trapping of the Sun's heat by gases such as carbon dioxide in the atmosphere; an increase in these gases due to burning fossil fuels enhances this effect and causes global warming.

What it is

The ozone layer in the upper atmosphere absorbs the Sun's harmful ultraviolet radiation.

Answer

The ozone layer is a layer of ozone gas (O3) present in the upper atmosphere (stratosphere). It is important because it absorbs most of the harmful ultraviolet (UV) radiation coming from the Sun, protecting living organisms from its damaging effects such as skin cancer, cataracts and damage to crops. The ozone layer is damaged by man-made chemicals called chlorofluorocarbons (CFCs), which were used in refrigerators and aerosol sprays, leading to a thinning called the ozone hole.

What it is

Living things are sorted into a hierarchy of groups, from Kingdom down to Species.

Answer

Classification is the arrangement of living organisms into groups and sub-groups on the basis of their similarities and differences. It makes the study of a huge variety of organisms easier and shows the evolutionary relationships between them. The main groups, from the largest to the smallest, are Kingdom, Phylum (called Division in plants), Class, Order, Family, Genus and Species. As we move down this hierarchy the number of organisms in a group decreases while their shared characteristics increase.

What it is

Living organisms are classified into groups based on similarities and differences, arranged in a hierarchy of ranks.

Answer

Classification is the arrangement of living organisms into groups and subgroups on the basis of their similarities and differences. This makes their study systematic and easy and shows evolutionary relationships. The hierarchy of classification, from the largest group to the smallest, is: Kingdom, Phylum (or Division in plants), Class, Order, Family, Genus and Species. Species is the basic unit of classification.

What it is

Whittaker classified living organisms into five kingdoms: Monera, Protista, Fungi, Plantae and Animalia.

Answer

R. H. Whittaker classified organisms into five kingdoms based on cell structure, body organisation and mode of nutrition. They are: Monera, prokaryotic organisms such as bacteria; Protista, unicellular eukaryotes such as Amoeba and Paramecium; Fungi, decomposers such as mushrooms and yeast; Plantae, multicellular autotrophs (plants); and Animalia, multicellular heterotrophs (animals).

What it is

Animals without a backbone are invertebrates; animals with a backbone (and a notochord) are vertebrates.

Answer

Invertebrates are animals that do not have a backbone (vertebral column), for example insects, worms and molluscs; they belong to phyla such as Porifera, Coelenterata, Arthropoda and others. Vertebrates are animals that have a backbone and a notochord and belong to the phylum Chordata. The five classes of vertebrates are Pisces (fishes), Amphibia (frogs), Reptilia (snakes, lizards), Aves (birds) and Mammalia (mammals).

What it is

Binomial nomenclature gives every organism a unique two-part scientific name: genus and species.

Answer

Binomial nomenclature is the system, given by Carolus Linnaeus, of naming each organism with two words: the first is the genus and the second is the species. For example, the scientific name of humans is Homo sapiens. By convention, the genus name begins with a capital letter and the species name with a small letter, and the full name is written in italics (or underlined when handwritten). This gives every organism a unique, universally accepted name.