Important Questions Class 8 Science Chapter 8

Important Questions Class 8 Science Chapter 8: Cell – Structure and Functions

 The Science Chapter 8 of Class 8 CBSE syllabus is about ‘Cell – Structure and Functions’. Students will learn about cells as a structural and functional unit of life. All living organisms are made of cells. Cells are similar to bricks that are assembled to make buildings. Similarly, cells are assembled in order to make the body of every organism. Therefore, cells are the building blocks of an organism. Hence understanding cells and their vital functions is essential to comprehending the various aspects of the biological world and its functioning. Class 8 Science Chapter 8 important questions overview cell life and the various elements contributing to cell functioning.

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To help students with Science revision, our expert Science faculty members have prepared a question bank of Important Questions Class 8 Science Chapter 8. It covers all important topics from Chapter 8 such as cells, variety of cells, cell structure and function, shape and size, parts of the cell, plants and animal cells, etc. The questions provided and their solutions help students understand the entire chapter and the related concepts introduced in the upcoming chapter. While preparing for the exams, students can refer to these questions and their step-by-step solutions to clarify all doubts and ace their examinations. 

Important Questions Class 8 Science Chapter 8 – With Solutions

Our list of Important Questions Class 8 Science Chapter 8 has been compiled as per the CBSE syllabus and NCERT guidelines. After preparing from Science Class 8 Chapter 8 Important Questions, students can better understand the question pattern and nature of questions. Several question-answer solutions, including MCQs, short answer questions, long answer questions, etc., have been given. These questions, along with their step-wise solutions, are curated using various sources like the NCERT textbook, NCERT exemplar, CBSE previous years question papers, other sources, etc. 

Below are a few sets of questions and their solutions from our question bank of Chapter 8 Class 8 Science Important Questions.

Q 1. The green colour of leaves is due to the presence of the pigment

• chlorophyll

• ribosomes

• mitochondria

• chloroplast

Answer: The correct option is A

Explanation of the correct option:

1. Chlorophyll is the green photosynthetic pigment
2.  It is found in plants, algae and cyanobacteria.
3. This pigment is present in chloroplasts.
4.  Chlorophyll is primarily responsible for the green colour of leaves.
5. Chlorophyll A is a form of chlorophyll used in oxygenic photosynthesis.
6. Chlorophyll has the ability to absorb energy from the sun and helps in converting it to chemical energy.
7. It absorbs most energy from wavelengths of violet-blue and orange-red light, reflecting green light.
8. Hence, the leaves turn green.

Explanation of the incorrect option:

Options B, C and D did not satisfy the appropriate explanation of the given question.

Final answer: The green colour of leaves is due to the presence of the pigment chlorophyll.

Q 2. Why do plant cells need a cell wall and animal cells do not?

Answer:

The cell wall mainly helps the plant to remain rigid and fixed. Cell walls reduce the flexibility of cells. The animal cell does not have a cell wall because it would affect the animal from moving. Cell walls are observed to reduce the locomotive ability of organisms. As the animal cell walls don’t exist, the membranes are flexible. So animal cells can have various shapes.

Cell walls that are found in plant cells maintain cell shape. This rigidity allows plants to stand upright. It is usually an advantage for plants to stand upright and grow as tall as possible. Plants grow taller in the presence of sunlight. With a rigid cell wall, plants can survive and produce successfully. However, for animals, height does not play a major role. A rigid cell wall would limit the motility of the animal.

Q 3. Write short notes on the following.

(a) Cytoplasm

(b) Nucleus of a cell

Answer:

Cytoplasm

• The cytoplasm is a jelly-like substance between the nucleus and the cell membrane.
• It is a fluid that fills the gaps in the cell around the plasma membrane and nucleus.
• Cell organelles, like ribosomes, mitochondria, Golgi bodies, etc., are suspended inside the cytoplasm.
• The cytoplasm allows the exchange of materials between cell organelles.
• It also helps in storage of substances in the cell.
• Most of the metabolic reactions also occur inside the cytoplasm.

Nucleus

• The spherical round body in the centre of the Cell is called the nucleus.
• The nucleus is known as the master of the cell. It commands the various functioning occurring inside the cell.
• It is an essential component of the living cell.
• It contains different genetic materials like DNA and RNA.
• The nucleus is separated from its cytoplasm by a membrane called the nuclear membrane.
• The nuclear membrane is also porous and helps the movement of materials between the cytoplasm and nucleus.
• The nucleus has a spherically shaped dense mass called the nucleolus.
•  The nucleus also has thread-like structures called chromosomes. 
• Chromosomes have genes and help in the inheritance or transfer of various characters from the parents to the offspring.

Q 4. Define a cell. Explain the variation in cell number, shape and size.

Answer:

A cell is known as the structural and functional unit of an organism. The study of cells, from their basic structure to the functions of every cell organelle, is called Cell Biology. All organisms are made up of cells. These cells may vary according to their cell number, shape or size. Cells are the building blocks of all living beings. Cells are complex, and their components perform several functions in an organism. Cells are at the lowest level of organisation in each life form. From one organism to another organism, the count of cells can vary. Cells are composed of several cell organelles that perform specific specialised functions to carry out life processes. Each organelle has a specific detailed structure. The hereditary material of these organisms is also present in the cells.

Cell Number: The number of cells in the organism may vary from one to millions and trillions. The fact that the smaller creatures have fewer cells has no bearing on the organism’s ability to operate. According to the number of cells, various organisms are mainly classified into Unicellular and Multicellular organisms.

Unicellular organism : Organisms composed of one single cell. A single cell performs all the essential activities required to sustain life, like growth, locomotion, reproduction, excretion, etc. Ex: Paramecium, Amoeba, Chlamydomonas, etc.

Multicellular Organisms : Organisms that have a composition of more than one Cell. The cells of these organisms usually perform specialised functions. Example: plants, animals, etc.

The Shape of Cells: The cell shows a variety of shapes depending on the function they perform. Different shapes of cells are:

• Oval, round or spherical: Human RBC, Ovum.
• Spindle-shaped (long and pointed at both ends): Muscle cell.
• Branched and elongated: Nerve Cells.
• Kidney Shaped: Guard Cells in leaves.
• Some cells keep on changing their shape.

Example: WBC in humans, Amoeba (single-celled) keeps changing its shape while locomotion through pseudopodia also called false feet and digestion of food vacuole.

• The Cell and its various components are enclosed in a cell membrane. These membranes give shape to the cells of plants and animals. 
• Plant cells possess an additional covering over membranes called the cell walls. 
• The cell wall provides shape and rigidity to the cells.

Size of Cells: 

Cells in living things can be as tiny as a millionth of a metre or as big as several centimetres. A majority of the cells are minute and invisible to the human eye. A microscope is required for enlarging or magnifying them. The size of the cells has no bearing on the plant or animal body. The function of the cell is related to its size. For example, the nerve cells of the rat and elephant are lengthy and branching. 

They both do the same thing: they send and receive messages. Cell sizes in living organisms are measured in micrometres. Most cells range from 5-15 micrometres.

• The smallest cell is about 0.1 to 0.5 mm in bacteria.
• The largest cell, 170 mm to 130 mm, is the ostrich egg.
• The longest human cell is Nerve Cells (1-2m).

Q 5. The most important function of the cell membrane is that it:

• Controls the entry and exit of materials from cells.

• Controls only the entry of materials into cells.

• Controls only the exit of materials from cells.

• Allows entry and exit of materials without any control.

Answer:

The correct option is A

Explanation of the correct option:

1. The cell membrane acts as a selective barrier for the entry and exit of the material.
2. The primary function of the cell membrane is to maintain cellular integrity and transport molecules inside and outside the cell.
3.  It is selectively permeable.
4. Many molecules are able to move across the membrane passively, and polar molecules require carrier proteins to facilitate their transport. 
5.  The proteins present in the cell membrane are specific to the material, allowing entry or exit according to the concentration gradient, such as channel and carrier proteins.
6. The lipid bilayer prevents the entry and exit of polar and large-sized materials. Few ions or mitochondria are transported actively against the concentration gradient, e.g. through pump proteins or Na+/K+ pump. 
7. Active transport requires energy and utilises ATP.

Explanation of the incorrect option:

Options B, C and D did not satisfy the appropriate explanation of the given question.

Final answer: The most important function of the cell membrane is that it: controls the entry and exit of materials from cells.

Q 6. Why do plant cells have an additional layer surrounding the cell membrane? What is this layer known as?

Answer:

Plants do not inherit the property of moving. They are immobile in nature, and hence need protection against variations in temperature,atmospheric moisture, high wind speed, etc. These variations can bring damage to the plant cell and can even kill them. For this purpose, plant cells possess a special membrane called the cell wall. This additional membrane acts as an outer covering and gives the plant cell shape. The cell membrane is known to be semi-permeable, flexible and hence, cannot fight these variations on its own. Therefore, a stiff outer covering is present in plant cells, which gives shape, protects and gives rigidity to the cell. The additional layer is called the cell wall. It is made of cellulose.

Q 7. The size of the cells of an organism has no relation to the size of its body. Do you agree? Give the reason for your answer.

Answer:

Yes. I agree with this.

The size of cells of an organism has zero interrelation with the size of its body. The cell organelles, like Golgi complex, mitochondria, ribosomes, nucleus, etc., have specific functions. They carry out specific functions in a cell and act as the structural and functional units of living organisms. However, they cannot act as independent units as they can function only when present inside a living cell, and lose their ability outside the cell. The correlation between the size of cells and the body can be understood by taking the example of a rat and an elephant. The cells in the body of an elephant are not necessarily bigger than those in a rat, and it is not true that the bigger organisms have cells of a bigger size in their body. The size of the cell in an organism is related to the function it performs. For example, the elephant and rat nerve cells are long and branched. The nerve system of elephants and rats has the same function of receiving and transferring messages from body to brain. The nervous system helps coordinate the functions of different body parts. They perform a similar function, that of transferring messages.

Q 8. Give functions.

1. Plasma membrane
2. Cytoplasm
3. Lysosome
4. Vacuole
5. Nucleus

Answer:

1. Functions of the plasma membrane:

• It gives support and shape to the cell.
• It acts as a mechanical barrier to protect the cell’s internal contents.
• It separates all the contents of the cell from its surrounding medium. 
• It performs various physical activities such as diffusion and osmosis. 
• It is selectively permeable to the ions and organic molecules and regulates the movement of substances in and out of cells. 
• The plasma membrane is a base of attachment for the cytoskeleton in some organisms and cell walls in others. 

1. Functions of cytoplasm:

• It is the medium for cellular chemical reactions.
• It is a medium through which the organelles remain suspended.
• It helps in the movement of the different cellular elements. 
• The cytoskeleton of the cytoplasm provides shape to the cell.
• The cytoplasmic organelles are specialised structures with functions like cellular respiration, protein synthesis, etc.
• The cytoplasm is a means of transport for genetic material.
• Cytoplasm acts like a buffer and protects the cell’s genetic material and the cellular organelles from damage caused due to the movement and collision with other cells.

1. Functions of lysosomes:

• Lysosomes destroy the foreign materials that enter the cell, such as bacteria and viruses.
• They are known as the digestive bags of the cell.
• Lysosomes remove dead and worn-out organelles.
• Lysosomes get ruptured, and enzymes are released free in damaged cells, ageing cells, dead cells, or poorly working cell organelles to digest. 
• During these processes, they remove the cell debris.
• Lysosomes autolyse cells, and their contents are released within the cell.

1. Functions of vacuoles:

• The major function of vacuoles is to store food in a cell.
• They also help store and expel wastes and toxic by-products of metabolic reactions.
•  Maintaining the internal hydrostatic pressure and turgor within the cell.
• They also help in maintaining an acidic internal pH. 
• They help in maintaining the rigidity and turgidity of a cell.
• It allows plants to support structures like leaves and flowers due to the pressure of the central vacuole.

1. Functions of the nucleus:

• It is called the control centre of a cell as it controls all the activities of the cell.
• It also contains gene-containing chromosomes, the units of inheritance in an organism.
• It brings about the growth of the cell by directing the synthesis of structural proteins. It also controls all the metabolic activities of the cell.
• The nucleus is the membrane-bound important structure of the eukaryotic cell.
• The nucleus regulates all cellular activities by controlling the enzymes required for cellular processes. 

Benefits of Solving Important Questions Class 8 Science Chapter 5

Important Questions Class 8 Science Chapter 8 covers the main concepts mentioned so students can easily revise and comprehend the chapter. The explanations provided for every answer clear all doubts regarding the chapter. By regular practice of MCQs, short answers, medium answers and long answer questions provided with step-wise solutions, students can get well versed with their question patterns and the marks weightage of the chapter. 

Few additional benefits of referring to Important Questions Class 8 Science Chapter 8 are:

• The questions provided have more probability of being asked in examinations. By solving our Important Questions Class 8 Science Chapter 8, students will understand how the paper will be prepared. 
• The questions-answers given have been curated by the Extramarks subject experts who have been teaching Science and biology-oriented subjects for many years. The step-wise solutions help students revise the chapter’s important theoretical concepts and get good grades.
• Students can rely on the Extramarks solutions as they are prepared and curated carefully per the CBSE syllabus and NCERT guidelines.
• Learning new concepts may assist you in grasping various other related concepts, but going over them again will allow you to remember them. The chances of scoring well in exams increase as you solve more Important Questions Class 8 Science Chapter 8.

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• NCERT books
• CBSE Revision Notes
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• Important formulas 
• CBSE extra questions