The Cell | ICSE Class 6 Biology Notes

This note covers cells as living units, organisms made of one or many cells, microscopic observation, the main parts of plant and animal cells, their structures and functions, comparisons, and labelled cell drawings.

What is a cell, and why is it a unit of life?

Definition: A cell is the basic structural and functional unit of a living organism. An organism is an individual living thing.

All living organisms are made of cells. Calling the cell a structural unit means that living bodies are built from cells. Calling it a functional unit means that the activities which keep an organism alive take place at the cellular level.

A complete cell can carry out the essential functions of life. A single part of a cell is not the same as a complete living unit. The parts work together, so understanding their individual jobs also helps us understand the cell as a whole.

What do unicellular and multicellular mean?

Unicellular means made of a single cell. Bacteria and yeast are examples of unicellular organisms. Their one cell constitutes the organism. It is not merely a small piece taken from the body of a larger organism.

Multicellular means made of many cells. Plants, fish, birds and humans are examples. Their cells work together. A few organisms are unicellular, while the majority of organisms are multicellular.

In a multicellular organism, different cells can perform different jobs. A tissue is a group of similar cells performing similar functions. Different tissues form an organ, a body part with a particular function. Several organs work together in an organ system.

The number of cells is the basis of the unicellular and multicellular distinction. A single human cheek cell is one cell from a multicellular organism. Observing that cell by itself does not make the human it came from unicellular.

Note: Distinguish a cell from an organism. A unicellular organism is a complete organism consisting of one cell; a cheek cell is part of a human body.

How does a microscope help us study cells?

A microscope is an instrument that helps us observe very small structures. A cell is usually too small to be seen by the unaided eye. The word “usually” matters: it does not mean that every cell is invisible without an instrument.

Magnification means making an object appear larger. A light microscope uses visible light and lenses to produce a magnified image. Its eyepiece is the lens through which we look; an objective lens is positioned near the object being studied.

What did early observations reveal?

In 1665, Robert Hooke examined a thin slice of cork using a microscope he had designed. He saw small box-like compartments and called them cells. Microscopes made structures visible that could not be studied in the same detail by unaided sight.

A slide is the glass support on which material is placed for microscopic observation. A coverslip is a thin glass covering placed over the material. A stain is a substance used to colour a specimen so its structures are easier to distinguish.

When observing a slide, distinguish the specimen from its enlarged image. A microscope makes the image appear larger; it does not turn the specimen into a physically larger cell. Looking carefully at boundaries and internal structures is more useful than recording colour alone.

The upper circular photograph shows adjoining, box-like onion peel cells. The lower circular photograph shows blue-stained cheek cells with darker internal regions. These are photographs of specimens, not simplified cell drawings.

See Fig. 2.8 in your NCERT textbook

The onion peel cells are regularly arranged, whereas the cheek cells are irregularly arranged. These observations provide a starting point for comparing cells, but they do not mean that all plant cells or all animal cells have one fixed shape.

How do the main parts fit together in a cell?

Most cells have three basic parts: a cell membrane, the thin boundary around the cell; cytoplasm, a semi-fluid, jelly-like substance inside it; and a nucleus, a membrane-enclosed structure containing information that helps control cellular activities.

Organelles are specialised structures within a cell that perform particular jobs. The cytoplasm contains organelles as well as other substances. A cell is therefore organised internally, rather than being an empty bag surrounded by a boundary.

Which structures should we recognise?

A cell wall is the rigid covering outside the membrane of a plant cell. Plastids are plant-cell organelles involved in food production, storage or colour. A vacuole is a membrane-enclosed storage compartment within a cell.

Genetic information means the inherited instructions associated with an organism's characteristics and cellular activities. At this level, the important connection is that the nucleus contains this information and has a role in controlling the cell.

Position matters when identifying a part. The plant cell wall lies outside the cell membrane. The cytoplasm lies within the membrane, and the nucleus is inside the cell. The membrane surrounding a vacuole encloses that compartment, rather than the entire cell.

When explaining a structure, connect its name with both its position and its function. For example, “cell wall” identifies a part, while “rigid outer support outside the membrane” explains where it is and what it contributes.

How do the cell membrane and cell wall differ?

The cell membrane, also called the plasma membrane, surrounds the cell and protects its contents. It separates the cell contents from their surroundings. Plant and animal cells both have a cell membrane.

The membrane is selectively permeable: it allows some substances to pass through while blocking others. This property helps control exchange between the cell and its surroundings. “Selectively” is essential to the definition; it does not mean that all materials pass freely.

What additional support does a wall provide?

The cell wall provides an additional covering in plant cells. It lies outside the cell membrane and is rigid. It helps plant cells maintain their shape, contributes to firmness in leaves and flowers, and helps plants remain upright.

The plant cell wall is primarily made of cellulose, a structural material formed from many linked units of a sugar called glucose. The word “primarily” should be retained: cellulose is the main material being identified, not a claim that nothing else is present.

The wall is permeable, meaning that water and some dissolved minerals can pass through it. A mineral here is a substance needed by the plant that can be taken up in dissolved form. The wall's support function does not make it a sealed barrier.

Human cheek cells have a membrane but no cell wall. Without a rigid wall, animal cells can change shape more easily. Do not describe the plant wall as replacing the membrane: both structures are present and perform different roles.

What do the cytoplasm and nucleus do?

The cytoplasm is the semi-fluid, jelly-like material inside the cell. It contains organelles and other substances. Many chemical reactions that keep a cell alive take place in it. A chemical reaction is a change in which substances are converted into other substances.

The cytoplasm is therefore an active part of the cell. Describing it merely as “empty space” misses its role. In plant and animal cells, it provides a setting for cellular activities and contains structures with their own particular functions.

How is the nucleus organised?

The nucleus contains genetic information and helps control cellular activities. It has a covering called the nuclear membrane. This covering is double-layered and has pores, or small openings, that allow material to pass between the nucleus and the cytoplasm.

The nuclear membrane and cell membrane are different boundaries. One surrounds the nucleus; the other surrounds the cell contents. A label pointing to the outside edge of an animal cell should therefore identify the cell membrane, not the nuclear membrane.

The nucleus and cytoplasm also differ in function. The nucleus contains the inherited instructions associated with cellular activities. The cytoplasm is the material in which many activities occur and in which other cell structures are situated. Neither term is another name for the whole cell.

Does every animal cell have a nucleus?

Mature red blood cells in humans do not have a nucleus. Red blood cells are blood cells that transport oxygen, a gas needed by body cells. This exception is why “every animal cell has a nucleus” is an incorrect statement.

A typical plant or animal cell drawing includes a nucleus. Such a drawing represents a general pattern; it does not remove the existence of specialised exceptions. When describing the basic parts, retain the statement that most cells have the listed arrangement.

What are plastids, and how do their functions differ?

Plastids are organelles of plant cells associated with food production, food storage and colour. They are absent from animal cells. The different kinds of plastids should be distinguished by their properties and jobs, rather than treated as identical structures.

A pigment is a substance that gives colour. Some plastids contain pigments; others lack them. This difference helps explain why not every plastid is green and why a colourless plastid can still perform a useful function.

What is the connection between chloroplasts and chlorophyll?

Chloroplasts are plastids containing the green pigment chlorophyll. Chlorophyll absorbs sunlight. Photosynthesis is the process by which plants make food using light energy. Chloroplasts are the structures in which this food-making process takes place.

A chloroplast is surrounded by two membranes. For a basic cell drawing, identifying the chloroplast as a structure inside the plant cell is more important than adding detailed internal parts. Do not label the whole organelle “chlorophyll”, because that name belongs to its green pigment.

What do the other plastids do?

Chromoplasts are plastids containing pigments other than chlorophyll. Their pigments may be yellow, orange or red. These pigments give bright colours to flowers and fruits. The word “may” avoids suggesting that every chromoplast has all those colours.

Leucoplasts are colourless plastids that lack pigments and store food materials. These materials include starch, oils or proteins. Starch is a stored food substance; oils and proteins are other kinds of substances that cells can store.

Some leucoplasts in potato cells store starch. This is a useful reminder that storage and food production are different functions. A plastid does not have to contain chlorophyll to contribute to the life of a plant cell.

How do vacuoles provide storage and support?

A vacuole is a compartment surrounded by a membrane. In a mature plant cell, there is usually one large central vacuole. “Mature” refers to a cell that has developed, while “central” describes the position of this large compartment within the cell.

The plant vacuole is surrounded by a single selectively permeable membrane. It contains cell sap, a watery fluid. Materials stored in the vacuole include water, minerals, sugars and waste material. Waste material is material produced by cellular activities that must be managed by the cell.

Why does stored water matter?

By storing large amounts of water, the vacuole helps maintain pressure inside a plant cell. This pressure helps keep the cell firm. The vacuole therefore contributes to support as well as storage, even though it is not the cell wall.

When a plant does not get enough water, the vacuole loses water. The cells become less firm and the plant wilts, meaning that it droops as it loses firmness. This links the water stored inside cells to a change visible in the plant.

The wall and vacuole contribute to support in different ways. The wall provides a rigid outer covering. The water-filled vacuole helps maintain internal pressure. A complete explanation of firmness should distinguish these contributions instead of calling them the same structure.

Are vacuoles present in animal cells?

In animal cells, vacuoles are sometimes present. They are not as large as plant vacuoles and help in temporary storage of materials. Do not change “sometimes present” to either “always present” or “always absent”.

Likewise, retain “usually” when describing one large central vacuole in a mature plant cell. The familiar drawing is a useful general model, but its arrangement should not become an absolute statement about every plant cell.

How can we compare a typical plant cell and animal cell?

Plant and animal cells share a basic organisation. Both have a cell membrane and cytoplasm, and a typical cell of each kind has a nucleus. Recognising these similarities prevents the mistaken idea that the two kinds of cell have entirely unrelated structures.

The main differences concern the wall, plastids and vacuole. A plant cell has a cell wall outside the membrane and has plastids. An animal cell has neither a cell wall nor plastids. Vacuoles also differ in their usual size and occurrence.

Which features should a comparison include?

The drawing shows a green, angular cell with an outer cell wall, a cell membrane, cytoplasm, a nucleus, chloroplasts and a large pale-blue vacuole. The vacuole occupies a prominent central region.

See Fig. 2.10b in your NCERT textbook

The rounded pink cell drawing labels the cell membrane, cytoplasm, nucleus and a small vacuole. Unlike the adjacent plant-cell drawing, it does not show a cell wall or chloroplasts.

See Fig. 2.10c in your NCERT textbook

These drawings use colour to make structures distinguishable. The colour of an illustration is not itself a rule for classifying a cell. Identify the structures shown and compare the same feature on both sides.

Shape can support an observation, as in regularly arranged onion peel cells and irregularly arranged cheek cells. However, use the presence or absence of the relevant structures for the main comparison. Avoid replacing that comparison with “plants are rectangular and animals are round”.

How can onion peel and cheek cells be observed?

Onion peel provides material for observing plant cells, while the inside of the human cheek provides animal cells. Temporary slides are preparations made for observation, while permanent slides are prepared specimens kept for repeated use. Both allow cell structures to be studied.

The key task is to observe, draw and compare the structures seen. A prepared slide of onion peel or cheek cells can be used for this purpose. Preparing a specimen also shows why its thinness, staining and placement under a coverslip matter.

How is an onion peel slide prepared?

1. Take a thin peel from an onion leaf as the specimen for observing plant cells.
2. Place the peel on a glass slide so it can be examined with a microscope.
3. Mount the peel using safranin, a stain used to colour this specimen, and place a coverslip over it.
4. Observe the preparation under a microscope and record the box-shaped, regularly arranged cells.

The onion peel cells have a cell wall outside the membrane. Their adjoining boundaries help reveal the arrangement of cells in the peel. Draw the cells seen, rather than replacing the observation with a detailed general cell diagram.

How is a cheek-cell slide prepared?

1. Gently collect cells from the inside of the cheek using a cotton swab or the blunt end of a toothpick.
2. Spread the collected material on a clean glass slide to prepare it for observation.
3. Add a drop of water, followed by a few drops of methylene blue, a stain used for cheek cells.
4. Carefully place a coverslip over the specimen and observe it under a microscope.
5. Record the appearance of the cheek cells and compare their irregular arrangement with the onion peel cells.

Cheek cells lack a cell wall. Their outer cell boundary is the membrane. The stain helps distinguish structures in the preparation; a blue-stained specimen should not be interpreted as evidence that living cheek cells naturally have that same blue colour.

For either specimen, separate what you directly observe from what a general model shows. A cell structure can be difficult to distinguish in a particular preparation. Do not invent a visible structure merely because its name appears in a list of cell parts.

How can observations become clear labelled cell drawings?

A labelled diagram is a drawing in which names identify particular structures. A cell diagram should make the position of each part clear. Its value comes from the relationship between labels and structures, rather than decoration or an elaborate outline.

First decide whether the task is to record an actual slide or draw a typical cell. An observational drawing records the specimen seen. A typical-cell drawing brings together the main structures in a general model. These tasks use related knowledge but have different starting points.

What sequence makes a comparison systematic?

1. Identify the specimen or model. Establish whether the drawing represents an onion peel cell, a cheek cell, or a typical plant or animal cell.
2. Examine the boundaries. Locate the membrane and, in the plant-cell model, the additional cell wall outside it.
3. Identify the interior. Locate the cytoplasm and nucleus, then identify the vacuole and any plastids shown.
4. Draw and label the structures. Place each name beside a line that reaches the intended part, keeping different boundaries distinguishable.
5. Compare corresponding features. Check wall, membrane, cytoplasm, nucleus, plastids and vacuole in the two drawings, preserving the qualifications about vacuoles.

For a typical plant cell, include the wall outside the membrane, cytoplasm, nucleus, a large vacuole and the plastids represented. A drawing showing chloroplasts should label those structures as chloroplasts. Do not substitute the name of their pigment for the name of the organelle.

For a typical animal cell, include the membrane, cytoplasm and nucleus. If a small vacuole is shown, label it. Do not add a cell wall or plastids. The drawing should be consistent with the comparison, rather than contradicting the features described in words.

How can a drawing be checked?

Read each label while looking at its endpoint. Check that a line for the nucleus reaches the nucleus, and that a line for the membrane reaches the cell boundary. In a plant cell, make sure the wall and membrane are separately identified.

Finally, explain one function for each labelled part. This connects the drawing to the living cell: boundaries protect and regulate exchange, the cytoplasm supports cellular activities, the nucleus contains instructions, and plastids and vacuoles carry out their particular roles.

Glossary

• Cell — The basic structural and functional unit from which living organisms are formed.
• Unicellular organism — A living organism consisting of one cell that performs its essential life functions.
• Multicellular organism — A living organism made of many cells that work together.
• Microscope — An instrument that produces enlarged images to help study very small structures.
• Organelle — A specialised structure within a cell that performs a particular function.
• Cell membrane — The thin boundary surrounding cell contents and controlling the passage of substances.
• Cell wall — The rigid covering outside the plant cell membrane that supports and maintains shape.
• Cytoplasm — The semi-fluid material inside a cell where many cellular activities occur.
• Nucleus — A membrane-enclosed structure containing genetic information and helping control cellular activities.
• Plastid — A plant-cell organelle associated with food production, storage or colour, depending on type.
• Chloroplast — A plastid containing green chlorophyll in which photosynthesis takes place.
• Chlorophyll — The green pigment in chloroplasts that absorbs sunlight for photosynthesis.
• Vacuole — A membrane-enclosed compartment that stores materials and contributes to support in plant cells.
• Cell sap — The watery fluid inside a plant vacuole containing stored dissolved substances.
• Selectively permeable — Allowing some substances to pass through while preventing others from passing.

Common errors and misconceptions

• Misconception: A single cheek cell makes a human unicellular. Correct: The cell is part of a multicellular human; unicellular describes a complete organism consisting of one cell.
• Misconception: A plant cell wall replaces the membrane. Correct: The wall is an additional covering outside the cell membrane.
• Misconception: Cytoplasm is empty space. Correct: It contains cell structures and is a setting for many cellular activities.
• Misconception: Every animal cell has a nucleus. Correct: Mature red blood cells in humans lack a nucleus.
• Misconception: Chloroplast and chlorophyll mean the same thing. Correct: A chloroplast is an organelle; chlorophyll is its green pigment.
• Misconception: All plastids are green. Correct: Chromoplasts contain other pigments, while leucoplasts are colourless.
• Misconception: Animal cells never have vacuoles. Correct: Vacuoles are sometimes present and are smaller than plant vacuoles.
• Misconception: Every plant cell has exactly one large central vacuole. Correct: A mature plant cell usually has one large central vacuole.

Exam-style questions with model answers

Q1. A unicellular organism consists of one cell; a multicellular organism consists of many cells. Yeast consists of one cell, while a human consists of many cells working together. Classify yeast and a human, giving a reason for each. [2 marks]

1. Yeast is unicellular because its complete organism consists of a single cell.
2. A human is multicellular because the body consists of many cells working together.

Q2. A cell membrane is the thin boundary around a cell; cytoplasm is its semi-fluid interior material; the nucleus contains genetic information. An observed cell has these three features. Name the structure matching each description and explain its identification. [3 marks]

1. The thin boundary is the cell membrane. Its position surrounding the cell matches the description of the membrane.
2. The semi-fluid interior material is the cytoplasm. It is the material inside the cell rather than its outer boundary.
3. The structure containing genetic information is the nucleus. This identifies it by its contents rather than by the cell's outline.

Q3. The plant cell wall is rigid, lies outside the membrane, supports cell shape and allows water and some dissolved minerals through. The cell membrane is thin and selectively permeable, controlling exchange by allowing some substances through while blocking others. Compare their position, nature, main function and permeability. [4 marks]

1. The wall lies outside the cell membrane, while the membrane is positioned inside the wall in a plant cell.
2. The wall is a rigid covering, while the membrane is described as a thin boundary surrounding the cell contents.
3. The wall provides structural support, while the membrane controls which substances pass through the cell boundary.
4. The wall allows water and some dissolved minerals through. The membrane is selectively permeable, allowing some substances through while blocking others.

Q4. Typical plant and animal cells both have a membrane, cytoplasm and nucleus. Plant cells have a cell wall outside the membrane and plastids; animal cells lack both. Mature plant cells usually have one large central vacuole; animal vacuoles are sometimes present and smaller. Give five comparisons: wall, plastids, vacuoles, membrane, and cytoplasm with nucleus. [5 marks]

1. A plant cell has a cell wall outside its membrane. An animal cell lacks this additional covering, so the wall is a difference.
2. Plastids are present in plant cells but absent from animal cells. Their occurrence is another structural difference between the two cell types.
3. A mature plant cell usually has one large central vacuole. Animal vacuoles are sometimes present and smaller; these qualifications must be retained.
4. Both kinds of cell have a cell membrane. Its presence is therefore a similarity and cannot alone distinguish the two types.
5. Both typical cells contain cytoplasm and a nucleus. These shared structures show that plant and animal cells have a common basic organisation.

Q5. Chloroplasts contain chlorophyll and make food using light through photosynthesis. Chromoplasts contain other pigments that give colour. Leucoplasts lack pigments and store food. Match these types to a green food-making plastid, a coloured plastid with other pigments, and a colourless food-storing plastid. Give a reason for each. [3 marks]

1. The green food-making plastid is a chloroplast because it contains chlorophyll and carries out photosynthesis, the process of making food using light.
2. The coloured plastid with other pigments is a chromoplast because those pigments provide its colour rather than green chlorophyll.
3. The colourless food-storing plastid is a leucoplast because it lacks pigments and stores food materials inside the cell.

Q6. A plant vacuole is a membrane-enclosed compartment containing watery cell sap. It stores water and dissolved materials, and helps maintain internal pressure. When water is insufficient, the vacuole loses water, cells become less firm and the plant wilts or droops. State what it contains, its storage role, its support role and the effect of water shortage. [4 marks]

1. The vacuole contains cell sap, which is a watery fluid within the compartment enclosed by the vacuole's membrane.
2. Its storage role is to hold water and dissolved materials inside the plant cell.
3. Its support role is to help maintain internal pressure, which keeps the plant cell firm.
4. During water shortage, the vacuole loses water. Cells become less firm, and the plant wilts or droops.

Q7. Most cells have a membrane, cytoplasm and nucleus; mature human red blood cells lack a nucleus. Mature plant cells usually have one large central vacuole, and animal vacuoles are sometimes present. Correct these statements: “Every cell has a nucleus”, “Every plant cell has one large central vacuole” and “Animal cells never have vacuoles”. [3 marks]

1. Most cells have a nucleus, but mature human red blood cells lack one. Therefore, the claim about every cell is too absolute.
2. A mature plant cell usually has one large central vacuole. The words “mature” and “usually” preserve the stated limits of the description.
3. Animal cells sometimes have vacuoles. Therefore, “never” is incorrect, but replacing it with “always” would also change the information supplied.

Q8. For a cheek-cell slide, collect cells gently with a cotton swab or blunt toothpick, spread them on a clean glass slide, add a drop of water and a few drops of methylene blue stain, place a coverslip, then observe microscopically. Onion peel cells are regularly arranged; cheek cells are irregularly arranged. Describe five stages, ending with this comparison. [5 marks]

1. Begin by gently collecting cells from inside the cheek with a cotton swab or the blunt end of a toothpick, as specified.
2. Spread the collected cheek-cell material on a clean glass slide. This places the specimen on the support used for microscopic observation.
3. Add a drop of water and then a few drops of methylene blue stain. Retain this order when describing the preparation.
4. Carefully place a coverslip over the prepared material. The coverslip covers the specimen before it is examined under the microscope.
5. Observe the preparation microscopically. Compare the irregularly arranged cheek cells with the regularly arranged onion peel cells described in the question.

Key takeaways

• The cell is the basic structural and functional unit of life; unicellular organisms consist of one complete living cell.
• Plant and animal cells share a basic organisation, including a membrane, cytoplasm and a nucleus in typical cells.
• The plant cell wall is an additional rigid covering outside the membrane, providing support and helping maintain shape.
• The selectively permeable cell membrane controls passage of substances, while many cellular activities take place in the cytoplasm.
• Chloroplasts contain chlorophyll for photosynthesis; chromoplasts provide colour, while colourless leucoplasts store food materials.
• A mature plant cell usually has one large central vacuole; animal vacuoles are sometimes present and smaller.
• The water stored in a plant vacuole helps maintain internal pressure, linking water supply with cell firmness.
• Use observed structures and accurately placed labels when drawing cells, and preserve exceptions when explaining typical cell features.

Test yourself

What does “structural and functional unit” mean?

Cells build up living bodies and carry out the activities associated with life. Structure concerns what the organism is built from; function concerns what its cells do.

Why is one cheek cell not evidence that humans are unicellular?

A cheek cell is one part of a human body. The complete human organism consists of many cells and is multicellular.

Does a plant cell wall replace the membrane?

No. The wall is an additional rigid covering outside the cell membrane; the plant cell has both structures.

How does chlorophyll differ from a chloroplast?

Chlorophyll is the green pigment that absorbs sunlight. A chloroplast is the organelle containing that pigment and carrying out photosynthesis.

Which plastids store food without containing pigments?

Leucoplasts are colourless plastids that lack pigments and store food materials such as starch, oils or proteins.

How does a vacuole help keep a plant cell firm?

Water stored in the vacuole helps maintain pressure inside the cell. Loss of water makes cells less firm and can result in wilting.

Which human cells provide an exception to the typical nucleus-containing animal cell?

Mature red blood cells in humans lack a nucleus, although a nucleus is present in the typical animal-cell model.

How should animal-cell vacuoles be described?

Vacuoles are sometimes present in animal cells. They are smaller than plant vacuoles and help in temporary storage of materials.