Selina Concise Solutions for ICSE Class 6 Biology Chapter 3 Cell The Structure and Functions

Cell - The Structure and Functions

Synopsis

• The single-celled organisms are called unicellular and if the organisms are made up of more than one cell are called multicellular.
• The simple microscope was developed by Antony Von Leeuwenhoek.
• The compound microscope was developed by Robert Hooke.
• The branch of biology which deals with the study of cells is called cytology.
• The living substance of the cell is called protoplasm.

Examples of

• unicellular organisms:

1. bacteria
2. amoeba
3. yeast
4. chlamydomonas etc.

• multicelluar organisms: plants like rose, neem, animals like man, hydra etc.

Examples of different cellular shapes.

1. irregular - amoeba
2. oval - chlamydomonas (slipper organism)
3. oblong - paramecium
4. elongated - striated muscle cells
5. very long or thread like - nerve fibre cells
6. cubical or rectangular - plant cell

The smallest cell - Bacterial cell
The longest cell - Nerve cells
The largest cell - Ostrich egg

• Cell theory explains

1. Every living organism is made up of one or many cells.
2. The structural unit of all the living organisms is the cell.
3. The functional unit of all the living organisms is the cell.
4. All cells arise from the pre-existing cells.

• The three scientists who contributed in the cell theory are:

1. M.J. Schleiden
2. Theodor Schwann
3. Rudolph Virchow

• Animal cells have no cell walls.
• The cell wall is made up of cellulose which is rigid and gives shape to the cell.
• The cell wall is freely permeable while the cell membrane is semi-permeable.
• The supportive framework which helps in the distribution of various product across the cell is endoplasmic reticulum.
• Power house of the cell - Mitochondria
• Synthesise proteins - Ribosomes.
• The organelle found only in the animal cell which initiate and regulate cell division is Centrosome.
• The organelle found only in the plant cell is plastid.
• Green plastids are chloroplasts
• Other plastids are amyloplasts.
• Cell organelles are concerned with specific functions.
• The importance of cell division is.

1. Production of new cells.
2. For growth and repair.
3. Replacement of the dead and worn out cells. -
4. For reproduction.

• The process of fusion of sperm and an egg is called fertilisation.
• The result of fertilisation is Zygote.

Cells are the fundamental building blocks of all living things, performing every function necessary for life. This synopsis introduces the major parts of the cell and the history of how they were discovered.
Teacher's Tip: Imagine the cell as a tiny city where the nucleus is the City Hall and mitochondria are the power stations.
Exam Tip: Memorize the three scientists of the cell theory as they are frequently asked in objective questions.

Review Questions

Multiple Choice Questions:

Put a tick mark (✓) against the correct alternative in the following statements:

Question (i): Identify the part which contain pigment:
(a) cell membrane
(b) plastid
(c) centrosome
(d) cell wall

Answer: (b) plastid
Plastids are organelles found in plant cells that store various chemical compounds, including pigments for color. For example, chloroplasts contain the green pigment chlorophyll used for photosynthesis.
Teacher's Tip: Just think of "Plastic" containers that hold "Paint" (pigments) to remember Plastids.
Exam Tip: Remember that plastids are unique to plant cells; animal cells never have them.

Question  (ii): The organelle that controls all activities in
(a) nucleus
(b) vacoule
(c) plastids
(d) cytoplasm

Answer: (a) nucleus
The nucleus acts as the command center of the cell because it stores the genetic information. It tells the other parts of the cell what to do and when to grow or divide.
Teacher's Tip: Think of the Nucleus as the "Brain" of the cell.
Exam Tip: Always underline the nucleus when drawing a cell diagram as it is the most prominent feature.

Question  (iii): A cell that is spherical in shape is:
(a) white blood cell
(b) nerve cell
(c) red blood cell
(d) amoeba

Answer: (c) red blood cell
Red blood cells in humans are typically circular and biconcave, giving them a spherical appearance under low magnification. This shape helps them squeeze through tiny blood vessels to deliver oxygen.
Teacher's Tip: Think of red blood cells as little round tubes of oxygen flying through your veins.
Exam Tip: Be careful not to confuse $White$ $blood$ $cells$, which can change shape, with the consistently round $Red$ $blood$ $cells$.

Question  (iv): The vacuole contains:
(a) water
(b) cell sap
(c) salts
(d) food

Answer: (a) water
While vacuoles often contain a mixture called cell sap, their primary component in many cells is water. Large vacuoles in plants provide the pressure needed to keep the plant stems upright.
Teacher's Tip: Imagine the vacuole as a water balloon inside a cardboard box (the cell wall).
Exam Tip: In plant cells, the vacuole is usually very large and central, while in animal cells, they are small and temporary.

Short Answer Questions:

Question 1: Name the scientist who invented the first microscope.

Answer: Antony Von Leeuwenhoek.
Leeuwenhoek was a Dutch scientist who designed high-quality lenses that could magnify tiny objects many times. He was the first to observe "animalcules" or tiny living creatures in a drop of water.
Teacher's Tip: Remember the long name "Leeuwenhoek" for the very first "Look" at microbes.
Exam Tip: Spelling is important here; practice writing "Leeuwenhoek" to ensure you get it right.

Question 2: Who coined the term "cell" ?

Answer: The term "cell" was coined by Robert Hooke.
In $1665$, Hooke looked at a thin slice of cork under his microscope and saw little empty compartments. He thought they looked like the small rooms (cells) where monks lived, so he gave them that name.
Teacher's Tip: Think of a "Hook" catching a "Cell" to remember Robert Hooke.
Exam Tip: Don't confuse Leeuwenhoek (microscope) with Hooke (cell name); they are different people!

Question 3: Briefly describe the three essential basic parts of a cell.

Answer: The essential basic parts of a cell are:
1. Cell membrane
2. Cytoplasm
3. Nucleus
1. Cell membrane - It is a very thin, delicate and flexible membrane which surrounds each cell. It is also called plasma membrane. It consists of fine pores which allow only certain molecules to pass through it and prohibit the others and therefore, due to its function, also called selectively permeable membrane.
2. Cytoplasm: This is the living portion of the cell which is a semi-liquid, translucent and colourless liquid. It is the portion of the cell where major functions of the cell are carried out through various finer parts of the cell. The finer structures which are contained in this are called the cell organelles.
3. Nucleus: The small spherical dark coloured body usually located in the centre of the cell. It is the most important part of the cell which regulates and co-ordinates various life processes. Its major role is during cell division. It contains hereditary factors called the genes.
These three parts work in perfect harmony to keep the cell alive and healthy. The membrane protects, the cytoplasm houses the machinery, and the nucleus leads the way.
Teacher's Tip: Think of the cell membrane as the skin, cytoplasm as the blood, and nucleus as the brain.
Exam Tip: If asked for "essential parts," always list all three; leaving one out will cost you marks.

Question 4: The cell membrane is called selectively permeable. Why?

Answer: The cell membrane of the cell is composed of fine pores through which only certain molecules of the different substances can pass into the cell. Since it allows only specific molecules to enter prohibiting the other it is referred to as Selectively Permeable.
This ability is vital because it keeps harmful substances out while letting nutrients like oxygen and water in. It maintains the internal environment of the cell so it can function correctly.
Teacher's Tip: Selective permeability is like a security guard at a gate who only lets people with tickets inside.
Exam Tip: Use the term "selectively permeable" instead of "semi-permeable" for more scientific accuracy in your answers.

Question 5: State the difference between
 

(i) Nucleus and nucleolus

Answer: Nucleus
1. It is a cell organelle.
2. It is the most important part of the cell.
3. It is present in the cytoplasm of the cell.
4. It is bounded by a delicate nuclear membrane
Nucleolus
1. It is a component of the nucleus.
2. It is the most important part of the nucleus.
3. It is present in the nucleoplasm.
4. It does not have any such membrane around it.
The nucleus is the overall control center, while the nucleolus is a small area inside it. Think of the nucleus as the whole office and the nucleolus as the desk where protein instructions are made.
Teacher's Tip: "Nucleolus" is the small "us" inside the big "Nucleus."
Exam Tip: Remember that the Nucleus has a membrane, but the Nucleolus does not.

(ii) Cytoplasm and protoplasm

Answer: Cytoplasm
1. The living portion of the cell inside the cell except the nucleus.
2. It has many finer parts contained in it like golgi bodies, mitochondria etc.
Protoplasm
The living substance of a cell is called the protoplast. It consists of two main parts
1. cytolasm
2. nucleus.
Protoplasm is the total living content of the cell, including the nucleus. Cytoplasm is specifically just the jelly-like fluid and organelles found outside of the nucleus.
Teacher's Tip: Protoplasm = Cytoplasm + Nucleus.
Exam Tip: Be precise; if a question mentions the nucleus, you are usually talking about Protoplasm.

(iii) Cell wall and cell membrane.

Answer: Cell wall
1. It is made up of cellulose.
2. It gives shape and rigidity to the plant cell.
3. It is a non-living structure.
4. It protects the cell from the entry of disease-causing agents, as well the underlying protoplasm against mechanical injuries.
Cell Membrane
1. It is very thin, delicate and flexible.
2. This allows the entry of certain molecules only, while holding back the others.
3. It is living structure.
4. It has fine pores in it, through which only certain substances carfpass in and out, while others cannot.
The cell wall is like a strong outer fence found only in plants, while the cell membrane is the flexible inner skin found in all cells. Plants need the wall because they don't have skeletons to hold them up.
Teacher's Tip: Only plants have the "Wall," but everyone has the "Membrane."
Exam Tip: Use the word "cellulose" when describing the cell wall to get technical marks.

Question 6: List the major differences between a plant cell and an animal cell.

Answer: Plant cell
1. Cell is comparatively larger with distinct outlines.
2. Has a definite and rigid cell wall.
3. Has negligible amount of cytoplasm.
4. Cytoplasm is not very dense.
5. Contains plastids.
6. No centrosome.
7. Have prominent one or more vacuoles.
8. Has simple golgi apparatus composed of units called dictyosomes.
Animal cell
1. Size is small with less distinct outlines.
2. Cell wall absent.
3. Cytoplasm fills almost the entire cell.
4. Cytoplasm is granular and relatively dense.
5. Do not contain plastids
6. Centrosomes are present.
7. Have temporary vacuoles which are small and concerned with secretion or excretion.
8. Have complex and prominent golgi apparatus.
These differences exist because plants and animals live differently. Plants stay still and make their own food using plastids, while animals move and need more flexible, complex organelles.
Teacher's Tip: Remember "PVC": Plants have Plastids, Vacuoles (large), and Cell walls.
Exam Tip: Draw a table for this answer; it makes the differences much easier for the examiner to see.

Question 7: Briefly discuss the importance of chromosomes to an organism.

Answer: The most important feature of a living cell is that it can divide or reproduce of its own kind and this function is regulated by the nucleus of the cell. The nucleus in its nucleoplasm contain a network of dark stained thread like structures called chromatin fibres. These fibres during cell division become thick and ribbon like which are then called chromosomes. The chromosomes are the actual hereditary factors. These are unique for each species both in number and in character. Human body has $46$ chromosomes which occur in pairs. They are further categorised as:
1. Chromosomes specific for determining the sex of the species called the sex chromosomes.
2. Other chromosomes are called autosomes.
Chromosomes carry all the instructions for building and operating your body. They are like the "blueprints" that decide everything from your eye color to how tall you will grow.
Teacher's Tip: Think of chromosomes as the library books that hold all the "recipes" for you.
Exam Tip: Remember the number $46$ for humans; it's a very common exam question.

Question 8: Fill up the blanks with the terms given below in the box.
Pigments, wall, pre-existing, cell, vacoules
1. The cell is the structural unit of all living things.
2. All cells arise from pre-existing cells.
3. Animal cells have no cell wall.
4. Plastids contain pigments.
5. Vacoules are filled with water and dissolved substance

Answer: 1. cell, 2. pre-existing, 3. wall, 4. pigments, 5. vacoules
These sentences summarize the core ideas of biology and the cell theory. Each blank represents a fundamental rule about how life is organized and how it reproduces.
Teacher's Tip: Use the process of elimination; if you are sure about "cell," cross it off the list immediately.
Exam Tip: Double-check your spelling against the words in the box to avoid silly mistakes.

Question 9: Try to find the names of four cell organells hidden in this maze, (hint: The hidden words can appear horizontally or vertically; forwards or backward or even mixed up). Write them in the lines provided.

Answer: 1. Vacuole, 2. Leucoplast, 3. Chromoplast, 4. Nucleus, Plastid
Puzzles like these help you recognize the names of cell parts quickly. Each hidden word is an important structure that performs a specific job inside the living cell.
Teacher's Tip: Start by looking for rare letters like "V" or "X" to find the words faster.
Exam Tip: Practice spelling these long organelle names so you don't struggle in the actual theory exam.

ADDITIONAL QUESTIONS

A. Fill in the blanks.

1: Organisms which are made up of a single cell are called unicellular organisms.
2: Muscles cells are elongated and contractile.
3: The jelly-like fluid that surrounds the nucleus of a cell is called cytoplasm.
4: Plasma membrane is a selectively permeable membrane.
5: The Nucleoplasm contains a network of thread-like structures called chromatin network.
6: Golgi bodies are also called dictyosomes.
7: A plant cell gets shape and support from its cell wall.
8: In a plant cell food is manufactured in chloroplast.
9: Chromoplasts give colour to flowers and fruits.
10: Fluid-filled cell organelles that are usually larger in a plant cell than in an animal cell are called vacuoles.

Answer: (Answers filled in the statements above)
These fill-in-the-blank questions cover the specific functions and names of various organelles. Knowing these details is essential for understanding how a cell operates like a tiny living machine.
Teacher's Tip: Remember that "Chloro" means green, while "Chromo" means color.
Exam Tip: Be careful with the spelling of "dictyosomes"; it's a less common but important term for plant Golgi bodies.

B. Match the name of the organelle given in Column A with its function given in Column B.

Column A (Organelle) - Column B (Function)
1. leucoplast - a. help in cell division
2. nucleus - b. transport of material
3. ribosomes - c. making food
4. mitochondria - d. digesting foreign bodies
5. endoplasmic reticulum - e. storage of starch and proteins
6. Golgi bodies - f. makes the cell turgid
7. chloroplast - g. making proteins
8. centrosome - h. controlling all the vital activities of a cell
9. vacuole - i. releasing energy.
10. lysosomes - j. synthesis of cell wall in a plant cell

Answer:
1. leucoplast - e. storage of starch and proteins
2. nucleus - h. controlling all the vital activities of a cell
3. ribosomes - g. making proteins
4. mitochondria - i. releasing energy.
5. endoplasmic reticulum - b. transport of material
6. Golgi bodies - j. synthesis of cell wall in a plant cell
7. chloroplast - c. making food
8. centrosome - a. help in cell division
9. vacuole - f. makes the cell turgid
10. lysosomes - d. digesting foreign bodies
This matching exercise shows how different organelles in a cell act like specialized departments in a factory to keep the cell running. By identifying these functions, we can better understand how cells perform complex tasks like making energy and managing waste.
Teacher's Tip: Think of the nucleus as the "Brain" and mitochondria as the "Battery" to remember their key roles instantly.
Exam Tip: In matching questions, use a pencil to lightly draw lines first, then write the final answers to ensure your work is neat and accurate for full marks.

C. Write true or false for each statement. Rewrite the false statements correctly.

Question 1: The size of a cell is usually measured in centimetres.

Answer: False. Correct: The size of a cell is usually measured in micrometres.
Cells are far too small to be measured with a normal ruler in centimeters. Scientists use a much smaller unit called a micrometer ($1/1,000,000$ of a meter) to describe their size.
Teacher's Tip: Remember that "micro" means extremely small!
Exam Tip: Always include the correct unit of measurement when correcting a false statement about size.

Question 2: The nuclear membrane encloses a colourless, dense fluid called nucleoplasm.

Answer: True
Just like the cell has cytoplasm, the nucleus has its own specialized fluid called nucleoplasm. This fluid helps support and protect the delicate genetic material inside the nucleus.
Teacher's Tip: "Plasma" means fluid, and "Nucleo" tells you where it is.
Exam Tip: This fluid is sometimes called 'karyoplasm' in higher classes, so keep that in mind.

Question 3: At the time of cell division the chromatin network changes into small thread-like structures called genes.

Answer: False. Correct: At the time of cell division the chromatin network changes into small thread like structures called chromosomes.
Genes are small segments found on chromosomes, but it is the entire chromosome that becomes visible during division. The chromatin coils up into these thick structures so they can be easily separated.
Teacher's Tip: Chromosomes are like "Suitcases" packed for the journey of cell division.
Exam Tip: Don't mix up $genes$ and $chromosomes$; one is a part of the other.

Question 4: Ribosomes are the sites of respiration in a cell.

Answer: False. Correct: Ribosomes are the sites of protein synthesis in a cell.
Mitochondria are the ones responsible for respiration and energy. Ribosomes are the tiny "workshops" where amino acids are put together to build proteins for the cell.
Teacher's Tip: "Ribs" are high in "Protein" - use that to remember Ribosomes make protein.
Exam Tip: Make sure you distinguish between "Protein synthesis" and "Respiration" as they are very different functions.

Question 5: A mitochondrion is bound by two membranes.

Answer: True
Mitochondria have a smooth outer membrane and a folded inner membrane. This double-layer design gives them more surface area to produce energy for the cell.
Teacher's Tip: The inner folds are called "Cristae."
Exam Tip: When drawing a mitochondrion, always show the wavy inner line to represent the second membrane.

Question 6: Golgi bodies form a network of tubes which run throughout the cytoplasm.

Answer: True
Golgi bodies (or apparatus) act as the shipping and receiving center of the cell. They package and process proteins and lipids into vesicles to be sent to their final destinations.
Teacher's Tip: Think of the Golgi body as the "Post Office" of the cell.
Exam Tip: Use the term "vesicles" when explaining how the Golgi bodies transport materials.

Question 7: Every cell is bound by a cell wall.

Answer: False. Correct: Only plant cell is bound by a cell wall.
Animal cells only have a flexible cell membrane, which allows them to move and take on many shapes. If we had cell walls, our bodies would be rigid and we wouldn't be able to walk or run.
Teacher's Tip: Animals are "squishy," and plants are "crunchy" because of cell walls.
Exam Tip: This is a very common trick question; always check if the question mentions "all" cells.

Question 8: Chromoplasts are the kitchen of the cell.

Answer: False. Correct: Chloroplasts are the kitchen of the cell.
Chromoplasts give color, but Chloroplasts are where photosynthesis happens. Since that is where the food (glucose) is prepared, they are called the cell's kitchen.
Teacher's Tip: "Chloro" means green, like most kitchens are full of green veggies!
Exam Tip: Be precise; "plastids" in general are not the kitchen, specifically the $chloroplasts$ are.

Question 9: Vacuoles are filled with a fluid called nucleoplasm.

Answer: False. Correct: Vacuoles are filled with a fluid called cell sap.
Nucleoplasm is only found inside the nucleus. Vacuoles contain a mixture of water, sugars, and salts known as cell sap, which helps maintain the cell's internal pressure.
Teacher's Tip: Sap is like the "Juice" found in the vacuole storage tank.
Exam Tip: Remember that large vacuoles in plants push against the cell wall to keep the plant from wilting.

Question 10: Growth and reproduction are based on cell division.

Answer: True
An organism grows by creating more cells, not by making its existing cells bigger. Cell division also allows life to continue from one generation to the next.
Teacher's Tip: You started as $1$ cell, and now you have $trillions$ because of cell division!
Exam Tip: Mention both "Mitosis" (growth) and "Meiosis" (reproduction) if you want to show advanced knowledge.

D. Write short answers.

Question 1: Why is a cell called the structural unit of a living organism ?

Answer: A cell is called the structural unit of life because all living organism irrespective of their size, shape and complexity are made up of cells and functions performed by organism is the result of the activity of the cells in its body.
Just as a house is built of individual bricks, a living body is built of individual cells. They are the smallest piece of life that can exist on its own.
Teacher's Tip: Think of cells as the "Legos" of life.
Exam Tip: Use the words "building block" to explain this concept clearly.

Question 2: Why is the cell membrane called selectively-permeable membrane ?

Answer: Cell membrane is called selectively permeable membrane as it allows only selected substances to pass through it, and prevents the passage of other substances.
This selection is based on the size and charge of the molecules trying to get through. It ensures that essential nutrients enter the cell while toxins and waste are kept out or pushed out.
Teacher's Tip: It acts like a "Sieve" or "Strainer" that only lets certain things through.
Exam Tip: Always mention both the "allowing" and "preventing" parts of the function.

Question 3: Where are the genes located in a cell ?

Answer: Genes are located on chromosomes in side nucleus of a cell.
Genes are actually long chains of DNA that are tightly packed into chromosomes. Because they are in the nucleus, they are protected like secret documents in a safe.
Teacher's Tip: Genes are the "instructions," chromosomes are the "books," and the nucleus is the "library."
Exam Tip: If asked for the exact location, say "on the chromatin/chromosomes inside the nucleus."

Question 4: Why are mitochondria called the powerhouse of the cell ?

Answer: Mitochondria are the sites of respiration in cell. Mitochondria forms energy in the form of $ATP$ (Adenosine triphosphate) during the process of respiration. Therefore, they are called power house of the cell.
They take in nutrients from the food you eat and turn them into energy molecules that the cell can actually use. Without mitochondria, your cells would have no "fuel" to do their work.
Teacher's Tip: $ATP$ is the "Battery" or "Fuel" for your cells.
Exam Tip: Use the full form of $ATP$ (Adenosine triphosphate) to impress your teacher and get full marks.

Question 5: Define cell division.

Answer: 1. Cell division is a process by which a cell divides, forming two new cells. 2. It is the basis of growth and reproduction of an organism.
This process is how a single fertilized egg grows into a full human being. It also helps your body heal by replacing old or damaged cells with fresh new ones.
Teacher's Tip: In math, division makes things smaller, but in biology, cell division makes things bigger!
Exam Tip: Remember that "Parent cell" and "Daughter cells" are the names used for cells before and after division.

Question 6: What is the relationship between growth and cell division ?

Answer: The life of all multicellular organisms begin with single cell. This cell divided and redivides, there by forming a cluster of many cells. So cell division is necessaiy for growth.
When cells divide, the total number of cells in an organism increases, which causes the organism to get larger. Growth is essentially the result of having more cells working together in a body.
Teacher's Tip: Think of a wall getting taller as you add more and more bricks.
Exam Tip: Clarify that growth happens by adding *more* cells, not by the cells themselves expanding too much.

E. Answer in detail.

Question 1: Why was the invention of the microscope so important to the study of living beings ?

Answer: All living organisms, unicellular or multicellular are made up of cells. Microscope make it possible to see inside the cells that make up all living organisms. Without microscope we could not see or understand, how life works. So the invention of microscope made it possible to see the basic component of life.
Before the microscope, people didn't even know that tiny organisms or cells existed. This invention opened up a whole new world of science and medicine, allowing us to study diseases and how our bodies function at a tiny level.
Teacher's Tip: The microscope is the "Telescope" into the tiny world of biology.
Exam Tip: Link the invention of the microscope to the discovery of the "cell" to provide a more complete historical answer.

Question 2: What is cell theory ? Name the three scientists whose work led to the formulation of the cell theory.

Answer: Cell theory was given by Schleiden and Schwann in the year $1838-1839$. It was refined further by Rudolf Virchow. The main postulates of cell theory are:
1. All living beings are made up of one or more cells.
2. Cells are the basic structural and functional unit of all living beings.
3. All cells are basically alike in chemical composition and metabolic processes.
4. New cells arise by division of pre existing cells. (Rudolf Virchow)
The cell theory is the most fundamental rule in biology because it defines what life is. It proves that despite our differences, all living things share the same tiny building blocks and origins.
Teacher's Tip: Remember "S.S.V.": Schleiden, Schwann, and Virchow.
Exam Tip: Learn all four postulates; many exams specifically ask you to "List the main points of the cell theory."