Case Study Questions Class 8 Science Chapter 8 Cell – Structure and Functions
CBSE Class 8 Case Study Questions Science Cell – Structure and Functions. Important Case Study Questions for Class 8 Board Exam Students. Here we have arranged some Important Case Base Questions for students who are searching for Paragraph Based Questions Cell – Structure and Functions.
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CBSE Case Study Questions Class 8 Science Cell – Structure and Functions
Case study 1
Discovery of the CellRobert Hooke in 1665 observed slices ofcork under a simple magnifying device.Cork is a part of the bark of atree. He took thin slices of cork andobserved them under a microscope. Henoticedpartitionedboxesorcompartments in the cork slice.These boxes appeared like a honeycomb. He also noticed that one box wasseparated from the other by a wall orpartition. Hooke coined the term ‘cell’for each box. What Hooke observed asboxes or cells in the cork were actuallydead cells.Cells of living organisms could beobserved only after the discovery ofimproved microscopes. Very little wasknown about the cell for the next 150years after Robert Hooke’s observations.Today, we know a lot about cellstructure and its functions because ofimproved microscopes having highmagnification.The CellBoth, bricks in a building and cells inthe living organisms, are basicstructural units. Thebuildings, though built of similar bricks,have different designs, shapes and sizes.Similarly, in the living world, organismsdiffer from one another but all are madeup of cells. Cells in the living organismsare complex.The egg of a hen represents a singlecell and is big enough to be seen by theunaided eye.How do scientists observe and study theliving cells? They use microscopes whichmagnify objects. Stains (dyes) are usedto colour parts of the cell to study thedetailed structure.There are millions of livingorganisms. They are of different shapesand sizes. Their organs also vary inshape, size and number of cells. Let usstudy about some of them.Number of Cells:Can you guess the number of cells in atall tree or in a huge animal like theelephant? The number runs intobillions and trillions. Human body hastrillions of cells which vary in shapesand sizes. Different groups of cellsperform a variety of functions.Organisms made of more than onecell are called multicellular (multi:many; cellular: cell) organisms. Thenumber of cells being less in smallerorganisms does not, in any way, affectthe functioning of the organisms. Youwill be surprised to know that anorganism with billions of cells beginslife as a single cell which is the fertilisedegg. The fertilised egg cell multiplies andthe number of cells increase asdevelopment proceeds.The single-celled organisms are calledunicellular (uni: one; cellular: cell)organisms. A single-celled organismperforms all the necessary functionsthat multicellular organisms perform.A single-celled organism, likeamoeba, captures and digests food,respires,excretes,Growsandreproduces. Similar functions inmulticellular organisms are carried outby groups of specialised cells formingdifferent tissues. Tissues, in turn, formorgans. Youmay say that the shape appearsirregular. In fact, amoeba has nodefinite shape, unlike other organisms.It keeps on changing its shape.Observe the projections of varyinglengths protruding out of itsbody. These are called pseudopodia(pseudo: false; podia: feet). These projectionsappear and disappear as amoebamoves or feeds.A white blood cell (WBC) in humanblood is another example of a single cellwhich can change its shape. But whileWBC is a cell, amoeba is a full-fledgedorganism capable of independentexistence.What shape would you expect inorganisms with millions of cells?As blood, muscle and nerve of humanbeings. The different shapes are relatedto their specific functions.Generally, cells are round, sphericalor elongated. Some cells arelong and pointed at both ends. Theyexhibit a spindle shape.Cells sometimes are quite long. Someare branched like the nerve cell or aneuron. The nerve cellreceives and transfers messages, therebyhelping to control and coordinate theworking of different parts of the body. Components of the cellare enclosed in a membrane. Thismembrane provides shape to the cellsof plants and animals. Cell wall is anadditional covering over the cellmembrane in plant cells. It gives shapeand rigidity to these cells.Bacterial cell also has a cell wall.Size of Cells:The size of cells in living organisms maybe as small as a millionth of a metre(micrometre or micron) or may be as largeas a few centimetres. However, most of thecells are microscopic in size and are notvisible to the unaided eye. They need tobe enlarged or magnified by a microscope.The smallest cell is 0.1 to 0.5 micrometrein bacteria. The largest cell measuring170 mm ×130 mm, is the egg of an ostrich.The size of the cells has no relationwith the size of the body of the animal orplant. It is not necessary that the cells inthe elephant be much bigger than thosein a rat. The size of the cell is related toits function. For example, nerve cells,both in the elephant and rat, are longand branched. They perform the samefunction that of transferring messages.
Que. 1) Which among the following scientists coined the term “cell” for the very first time?
(a) Anton von Leuwenhoek
(b) Robert Brown
(c) Robert Hook
(d) Aristotle
Que. 2) …………………………………………………………………………………. are used to colour the parts of cell to study the detailed structure.
(a) Stains
(b) Microscopes
(c) Slides
(d) Sections
Que. 3) Which among the following cells have a characteristic branched shape?
(a) WBC
(b) Neuron
(c) RBC
(d) Amoeba
Que. 4) Define unicellular organisms and name one example.
Que. 5) Name some of the different shapes of cells that are observed.
Answer Key
Que. 1) (c) Robert Hooke
Que. 2) (a) Stains
Que. 3) (b) Neuron
Que. 4) Answer: Single celled organisms are called unicellular organisms. In unicellular organisms a single cell performs all the necessary functions. Examples of unicellular organisms include amoeba, paramecium.
Que. 5) Answer: Cells exist in different shapes which include: Spherical shape of RBCs, Irregular shape is observed in WBC and amoeba, Nerve cells or neurons have branched shape, some muscles have spindle shaped cells.
Case study 2
You have learnt that each living organism has many organs. You have studied in Class VII about the digestive organs which together constitute the digestive system. Each organ in the system performs different functions such as digestion, assimilation and absorption. Similarly, different organs of a plant perform specific/specialised functions. For example, roots help in the absorption of water and minerals. Leaves, as you have learnt in Class VII, are responsible for synthesis of food. Each organ is further made up of smaller parts called tissues. A tissue is a group of similar cells performing a specific function. Paheli realised that an organ is made up of tissues which in turn, are made up of cells. The cell in a living organism is the basic structural unit.Cell Membrane: The basic components of a cell are cell membrane, cytoplasm and nucleus. The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. The membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and allows themovement of substances or materials both inward and outward.The boundary of the onion cell is the cell membrane covered by another thick covering called the cell wall. The central dense round body in the centre is called the nucleus. The jelly-like substancebetween the nucleus and the cell membrane is called cytoplasm.You have learnt earlier that the cell membrane gives shape to the cell. In addition to the cell membrane, there is an outer thick layer in cells of plants called cell wall. This additional layer surrounding the cell membrane is required by plants for protection. Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture etc. They are exposed to these variations because they cannot move. Cells can be observed in the leaf peel of Tradescantia, Elodea or Rhoeo. You can prepare a slide as in the case of onion.
Que. 1) The jelly like substance between the nucleus and the cell membrane is called the ………………………………………………………………………………………….
(a) Plasma membrane
(b) Vacuole
(c) Mitochondria
(d) Cytoplasm
Que. 2) Which of the following organelle is NOT a basic component of a cell?
(a) Cell membrane
(b) Cytoplasm
(c) Cell wall
(d) Nucleus
Que. 3) A …………………………………………………………………………….. is a group of cells performing a specific function.
(a) Tissue
(b) Organ
(c) Organ system
(d) Organism
Que. 4) Why do only plants cells have cell wall?
Que. 5) Explain some of the characteristics of plasma membrane.
Answer Key
Que. 1) (d) Cytoplasm
Que. 2) (c) Cell wall
Que. 3) (a) Tissue
Que. 4) Answer: Only plant cells have cell wall because they need protection against variations in temperature, high wind speed, and atmospheric moisture. Since they cannot move, they are exposed to these variations and hence need cell wall which is a hard covering.
Que. 5)Answer: Cytoplasm and nucleus are enclosed within the plasma membrane. It separates cells from one another and also from the surroundings. Plasma membrane is porous and allows the transport of substances both inward and outward the cell.
Case study 3
Cytoplasm: It is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, Golgi bodies, ribosomes, etc. You will learn about them in later classes. Nucleus: It is an important component of the living cell. It is generally spherical and located in the centre of the cell. It can be stained and seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus. With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus. In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. This means that your parents pass some of their characteristics on to you. If your father has brown eyes, you may also have brown eyes. If your mother has curly hair, you might also end up having curly hair. However, the different combination of genes from parents results in different characteristics.Nucleus, in addition to its role in inheritance, acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.The nucleus of the bacterial cell is not well-organised like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes (pro: primitive; karyon: nucleus). Examples are bacteria and blue green algae. The cells, like onion cells and cheek cells having well-organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes. (eu: true; karyon: nucleus).While observing the onion cells under the microscope, did you notice any blank-looking structures in the cytoplasm? It is called vacuole. It could be single and big as in an onion cell. Cheek cells have smaller vacuoles. Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller. You might have noticed several small coloured bodies in the cytoplasm of the cells of Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids. They are of different colours. Some of them contain green Pigment called chlorophyll. Green coloured plastids arecalled chloroplasts. They provide green colour to the leaves. You may recall that chlorophyll in the chloroplasts of leaves, is essential for photosynthesis.
Que. 1) The porous membrane present between the nucleus and the cytoplasm is called?
(a) Cell wall
(b) Nuclear membrane
(c) Cell membrane
(d) Golgi body
Que. 2) …………………………………………..………………………………………. is known as the entire content of living cell that include the cytoplasm and nucleus.
(a) Protoplasm
(b) Nucleoplasm
(c) Sarcoplasm
(d) Plastid
Que. 3) Bacteria and blue-green algae are the examples of which of the following category?
(a) Eukaryotes
(b) Archaea
(c) Fungi
(d) Prokaryotes
Que. 4) Define the term “gene”.
Que. 5) What are chloroplasts and what are its functions.
Answer Key
Que. 1) (b) Nuclear membrane
Que. 2) (a) Protoplasm
Que. 3) (d) Prokaryotes
Que. 4) Answer: Gene is a unit of inheritance in a living organism. It controls the transfer of hereditary characteristics from parents to offspring.
Que. 5)Answer: Green coloured plastids are known as chloroplasts. They provide green colour to leaves and are essential for photosynthesis.