Class 9 Science Chapter - 5th Cell - The Fundamental Unit of Life Notes, Previous Year Questions & NCERT Solutions
Class 9 Science
Chapter - 5th
Cell - The Fundamental Unit of Life
Notes, Previous Year Questions & NCERT Solutions
Quick Review
- All living organisms in this universe are made up of cells.
- They either exist as a single cell or as a combination of multiple cells.
Discoveries about Cells
The Cell Theory
1. A cell is the structural and functional unit of all living organisms.
2. All living organisms are made up of cells.
3. Cells are formed from pre-existing cells.
- Unicellular Organisms – The organisms that consist of a single cell such as Amoeba.
- Multicellular Organisms – The organisms which contain various cells that perform different functions in the organism such as plants fungi and animals
⭐ How can multicellular organisms originate from a single cell?
A cell can divide itself into cells of its own type. Therefore, more cells can generate from an already existing cell.
The Shape of the Cell
The shape of the cell may vary depending upon the type of function they perform in an organism.
Cells are capable of changing their shape. For example, the white blood cells and amoeba can change shapes on their own.
Figure 1 - Cells can have different Shapes and Sizes
⭐ How can cells perform distinct functions in organisms?
Cells are capable of performing multiple functions in an organism. A cell contains specific components which are called Organelles. Each organelle in the cell can perform different functions such as making new cells or clearing the waste of the cell. Thus, organelles allow a cell to perform several kinds of activities in an organism.
The Organisation of a Cell
Figure 2 – The Structure of Cells in Plants and Animals
A cell contains three features –
- The Plasma Membrane
- Nucleus
- Cytoplasm
Plasma Membrane
- It is just like an envelope that covers the whole cell. Therefore, a cell gets separated from the external environment because it has a plasma membrane.
- The plasma membrane can decide which materials should enter or leave the cell and which should not. That is why it is also called a ‘Selectively Permeable Membrane’.
⭐ How can substances move in and out of a cell?
Gaseous Exchange between the Cell and its External Environment –
- Movement of Oxygen and Carbon dioxide to and from the cell is carried out through diffusion.
- Gaseous substances tend to move to areas where their concentration is less from the areas where concentration is higher. This movement is defined as the process of diffusion. Diffusion can take the place of solids, liquids, and gases.
⭐ Movement of Water between the Cell and its External Environment –
It is carried out by the means of osmosis. Osmosis is a process in which water moves from the region of high concentration to one where its concentration is low through a semipermeable membrane. Therefore we can say that Osmosis is just a special case of the process of diffusion.
Hypotonic Solutions
- If the concentration of water outside the cell is higher than the concentration of water inside, then the cell gains water by the process of osmosis.
- Water can move into the cell from the cell membrane, which results in swelling of the cells.
Figure 3 - Hypotonic Solution
Isotonic Solutions
If the cells are put in an environment that has a similar concentration of water as present inside. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side.
Therefore, the size of the cell does not vary in an isotonic solution because there is no net movement of water.
Figure 4 - Isotonic Solution
Hypertonic Solutions
- If the cells are kept in an environment that has a lower concentration of water than what is present inside the cells then due to the process of osmosis water moves out of the cells.
- This results in a decrease in the size of the cells (they shrink) as more water comes out of the cell.
Figure 5 - Hypertonic Solution
The Nucleus
Nucleus is a prominent organelle present in the cell which is the controlling centre of all activities of the cell.
Figure 6 - Nucleus of a Cell
The Structure of the Nucleus
- A nucleus has a nuclear membrane that covers it all around.
- There are pores present on the nuclear membrane that allow the movement of substances in and out of the nucleus.
- There are chromosomes, rod-shaped structures present in the nucleus which contain genetic information.
The chromosomes contain two types of things -
1. DNA - This is responsible for organising and constructing new cells
2. Proteins - These help in the packaging and condensation of DNA.
Chromatin
Chromatin is thread-like material present in a cell. The chromatin organises itself into chromosomes whenever the cell is about to divide.
Figure 7 – Chromosomes and Chromatin
Nucleolus
It is called the Brain of the Nucleus. It comprises 25% of the volume of the nucleus. It consists of proteins and ribonucleic acids (RNA). It helps in the formation of ribosomes which help in the formation of proteins inside the cell.
Figure 8 - Nucleolus inside a Nucleus
⭐ What is a nucleoid?
Sometimes cells do not have a well-defined nucleus because they lack a nuclear membrane. Such a nucleus with no definite nuclear boundaries is called a Nucleoid.
⭐ What are the prokaryotes?
Organisms whose cells do not have a definite cell membrane are called Prokaryotes.
⭐ What are eukaryotes?
Organisms whose cells contain a well-defined nuclear membrane are called Eukaryotes.
⭐ Differences between prokaryotes and eukaryotes:-
Figure 9 - Eukaryotic and Prokaryotic Cells
Cytoplasm
- The plasma membrane has a fluid-like substance in it which is called the cytoplasm.
- The cytoplasm contains several organelles that can perform distinct functions of the cell
Functions of Cytoplasm
- It supports and suspends the cell organelles and molecules.
- The cellular processes occur in the cytoplasm such as the formation of proteins.
- It allows the movement of substances in the cell such as hormones.
- It dissolves cellular wastes.
The Cell Organelles
In the case of Eukaryotic organisms, the cells contain organelles that have their own membranes apart from the overall cell membrane of the cell.
Figure 10 - Different Cell Organelles
⭐ What is the significance of membrane-bound organelles in a cell?
The cells perform several functions. The organelles are useful because they allow the separation of different functions that are being performed by the cell.
Organelles which carry out important activities in a Cell –
1. Endoplasmic Reticulum
2. Golgi Apparatus
3. Lysosomes
4. Mitochondria
5. Plastids
6. Vacuoles
7. Centrioles
8. Ribosomes
9. Peroxisomes
Endoplasmic Reticulum (ER)
Figure 11 - Endoplasmic Reticulum
- The structure of the ER is quite similar to that of the plasma membrane. It is a network-like structure that consists of membrane-bound tubes and sheets.
- Two types of ER –
- Rough ER
- Smooth ER
- Rough ER contains ribosomes that are responsible for the manufacturing of proteins in the cells. They give a rough texture to the cell.
- The smooth ER manufactures fats or lipids in the cell which allow the functioning of the cell.
- What are the functions of lipids and proteins?
- Proteins and lipids synthesised on ER are used for making cell membranes. The process is known as Membrane Biogenesis.
- Proteins can act as an enzyme
- Both protein and lipids can act as hormones
- Functions of ER
- Transportation of material between different parts of the cytoplasm and also between the nucleus and cytoplasm
- Folding of proteins which are synthesised by ribosomes on RER.
- Detoxifying poisons and drugs out of the cell is the function of SER.
Golgi Apparatus
Figure 12 – Golgi Apparatus
- Camillo Golgi discovered the Golgi Apparatus.
- It contains vesicles that are arranged parallel in stacks. These stacks are called Cisterns. These vesicles have their own membranes. These membranes are sometimes connected to those of the ER.
- Functions of Golgi Apparatus
- Golgi apparatus carries materials synthesised by the ER to different parts of the cell. The material is stored and packaged in vesicles.
- Formation of complex sugar
- Formation of lysosomes.
Lysosomes
Figure 13 – Structure of Lysosome
- They are single-membrane vesicles that are responsible for cleaning the cell. They can digest any foreign material such as food or bacteria and even the worn-out cell organelles.
- ⭐ How can lysosomes digest any foreign material that enters the cell?
- Lysosomes are capable of doing so because they have digestive enzymes in them. These enzymes break the materials and digest them. These enzymes are synthesised by RER and packaged into lysosomes by Golgi bodies.
- ⭐ Why lysosomes are called ‘suicide bags’?
- If the cell’s own material gets damaged or dead, there are chances that lysosomes burst out, thus digesting its own cell.
Mitochondria
It is a double membrane organelle that has its own DNA and that is why often called ‘Semi-Autonomous Organelle’
Figure 14 – Structure of Mitochondria
The cell requires energy in order to carry out several activities. This energy is generated by mitochondria which are often called the ‘Powerhouse’ of the Cell. Mitochondria are the site of cellular respiration. They use oxygen from the air to oxidise the carbohydrates and thereby release energy.
⭐ What are the energy currencies of a cell?
The Mitochondria generates ATP (Adenosine Triphosphate) which are energy giving molecules of the cell that are often called their ‘Energy Currency’.
The two membranes of Mitochondria
Outer Membrane – Porous in Nature
Inner Membrane – Deeply Folded
The Inner Membrane of Mitochondria called as Cristae Facilitates Generation of ATP molecules as it has a larger surface area.
Plastids
Just like mitochondria, Plastids are also double membrane organelles that have their own DNA and ribosome.
Plastids exist in plant cells only. Depending upon the type of function they play in the cell they can be classified as –
Figure 15 – Types of Plastids
Classification of Plastids
1. Amyloplast
They are found in tubers, cotyledons and endosperm in plants.
They are used to store starch.
2. Elaioplast
They are found in epidermal cells of the plants
They store oil.
3. Proteinoplast
They are found in seeds and nuts.
They store proteins.
Chloroplasts
Chloroplasts are cell organelles that conduct photosynthesis in plants.
Chloroplast is derived from two Greek words Chloro and Plasts which means green and plants respectively.
Chloroplasts contain photosynthetic pigments called ‘Chlorophyll’ along with lipids, carbohydrates, minerals, DNA, RNA, grana, thylakoids and stroma.
The main functions of chloroplasts are:
Conducting photosynthesis in plants.
Protein synthesis
Releases oxygen
Storage of Starch
Figure 16 – Chloroplast containing thylakoids, stroma and grana
Light-dependent Reactions in Photosynthesis – During photosynthesis chlorophyll absorbs the light energy which is then used for two molecules ATP and NADPH.
Thylakoids – They are pillow-shaped compartments in the chloroplast. The light-dependent reactions in photosynthesis take place in the thylakoids.
Stroma – It is a fluid-filled matrix in the chloroplasts. It is a colourless fluid that contains all the enzymes that are needed for the light-dependent reactions in Photosynthesis.
Grana – Stacks of thylakoids are called Grana. They are found in the stroma. They provide a large surface area so that the reactions of photosynthesis can take place.
Vacuoles
Vacuoles are the places where cells can store liquids and solids. They are present in both plants and animals but the plant vacuoles are bigger in size than the animal vacuoles.
Types of Vacuoles
Sap Vacuoles
Contractile Vacuoles
Food Vacuoles
Sap Vacuoles
Figure 17 - Sap Vacuoles
These vacuoles are filled with a fluid called Vascular Sap. The fluid contains Amino Acids, Salt, Sugar, Proteins, Water, and Waste Materials. Sap vacuoles are separated from the cytoplasm by a semipermeable membrane called Tonoplast. Their main function is to allow rapid exchange between the cytoplasm and the surrounding environment.
Several sap vacuoles are found in young plant cells and animal cells. In mature plants, the small sap vacuoles combine to form a single large central vacuole.
Contractile Vacuoles
Figure 18 – Osmoregulation in Amoeba through Contractile Vacuoles
They are found in protistan and algal cells in freshwater. The membrane of the contractile vacuoles is highly extensible and collapses easily. These vacuoles are responsible for osmoregulation (maintaining the water content of the cells) and excretion in the cells.
Food Vacuoles
Figure 19 – Food Vacuoles and Digestion
They are found in the cells of protozoans and several lower animals. Food vacuoles are responsible for the digestion of food in the cells as they contain food enzymes. The digested food then passes into the cytoplasm. Found in single-celled organisms like Amoeba.
Centrioles
- A centriole is a small set of microtubules arranged in a specific way.
- Their main purpose is to help a cell in cell division.
- They are found near the nucleus but can be seen only during the cell division.
- They are found in pairs and form a special substance called Centrosome which appears near the nucleus.
- When the cell divides, the centrosome divides into two parts and each part moves to opposite sides of the cell.
Figure 20 - Centrioles
Ribosomes
- They are cell organelles responsible for protein synthesis.
- Ribosomes can be found in both prokaryotes and eukaryotes because the synthesis of proteins is important in both of them.
- In prokaryotes, the ribosomes float freely in the cytoplasm.
- In eukaryotes, they can be found floating in the cytoplasm or they are often attached to the endoplasmic reticulum.
- The ribosomes attached to the ER synthesise proteins that are to be exported out of the cell while the ribosomes floating inside the cell synthesise proteins that are used inside the cell.
Peroxisomes
- Peroxisomes are small vesicles found in the cells.
- These enzymes are used to break the toxic materials inside the cell.
- They digest the fatty acids of the cell as well as amino acids by carrying out oxidation reactions in the cell.
- They are also responsible for the digestion of alcohol in the human body. Hence, the liver contains a large number of Peroxisomes.
Figure 21 - Peroxisomes in a cell
-- NCERT Solutions --
In-Text Questions
Page: 59
1. Who discovered cells, and how?
Solution:- In 1665, Robert Hooke discovered cells while examining a thin slice of cork through a self-designed microscope. He observed that the cork resembled the structure of a honeycomb consisting of numerous tiny compartments. The minuscule boxes are referred to as cells.
2. Why is the cell called the structural and functional unit of life?
Solution:- Cells form the structure of an entity. A group of cells form a tissue, further an organ and ultimately an organ system. They perform fundamental functions and life processes such as respiration, digestion, excretion etc., in both unicellular and multicellular entities. They perform all the activities independently. Hence, cells are referred to as structural and fundamental units of life.
Page: 61
3. How do substances like CO2 and water move in and out of the cell? Discuss.
Solution:- CO2 moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO2 in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO2 to diffuse from a region of higher (within the cell) to a lower concentration.
H2O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.
4. Why is the plasma membrane called a selectively permeable membrane?
Solution:- The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.
Page: 63
5. Fill in the gaps in the following table, illustrating the differences between prokaryotic and eukaryotic cells.
Solution:
Page: 65
6. Can you name the two organelles we have studied that contain their own genetic material?
Solution:- The two organelles which have their own genetic material are
1. Mitochondria
2. Plastids
7. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Solution:- In the event of any damage to cells and when the revival of cells is not possible, Lysosomes burst, and enzymes digest such cells. This is why lysosomes are often referred to as ‘suicide bags’.
8. Why are lysosomes known as suicide bags?
Solution:- When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.
9. Where are proteins synthesised inside the cell?
Solution:- Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.
Exercise
1. Make a comparison and write down ways in which plant cells are different from animal cells.
Solution:- The following table depicts the differences between plant cells and animal cells.
2. How is a prokaryotic cell different from a eukaryotic cell?
Solution:- The following are the differences between prokaryotic and eukaryotic cells.
3. What would happen if the plasma membrane ruptures or breaks down?
Solution:- If the plasma membrane ruptures or breaks down, then molecules of some substances will freely move in and out of the cells. As the plasma membrane acts as a mechanical barrier, the exchange of material from its surroundings through osmosis or diffusion in a cell won’t take place. Consequently, the cell would die due to the disappearance of the protoplasmic material.
4. What would happen to the life of a cell if there was no Golgi apparatus?
Solution:- The Golgi apparatus consists of stacks of membrane-bound vesicles whose functions are as follows:
- Storage of substances
- Packaging of substances
- Manufacture of substances
Without the Golgi apparatus, the cells will be disabled from packing and dispatching materials that were produced by the cells. The Golgi apparatus is also involved in the formation of cells. Hence, in the absence of the Golgi apparatus, cells will not be produced.
5. Which organelle is known as the powerhouse of the cell? Why?
Solution:- Mitochondria are known as the powerhouse of the cell. It is because it releases the energy required for different activities of life. Mitochondria releases energy in the form of ATP (Adenosine triphosphate) molecules, essential for numerous chemical activities of life. Hence, ATP is often referred to as the ‘energy currency of the cell’.
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Solution:- Lipids and proteins are synthesised in the ER (Endoplasmic Reticulum).
7. How does an Amoeba obtain its food?
Solution:- Through the process of endocytosis, an Amoeba obtains its food. As its cell membrane is flexible enough, food particles are engulfed, forming a food vacuole girdling it, which is assisted by the pseudopodia. Amoeba secretes digestive enzymes to bring about digestion of the engulfed particle once the food is trapped.
8. What is osmosis?
Solution:- The process of movement of a water molecule from a region of higher concentration to a region of lower concentration through a semipermeable membrane is known as osmosis.
9. Carry out the following osmosis experiment:
Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed-out portions of A and D.
Solution:-
(i) Water accumulates in the hollowed portions of B and C as a difference in the water concentration
is observed. Thereby, endosmosis occurs as the cells act as a semipermeable membrane.
(ii) Potato A is essential in this experiment as it is significant to compare different scenarios seen in
potato cups B, C and D. Potato A in this experiment clearly shows that the potato cavity on its
own cannot bring about water movement.
(iii) Cup in A does not show any change in the water flow concentration for osmosis to occur, which
requires concentration to be higher than the other. Cells in cup D are dead; thus, there is no
existence of a semipermeable membrane for water flow. Consequently, osmosis does not occur.
10. Which type of cell division is required for the growth and repair of the body, and which type is involved in the formation of gametes?
Solution:- There are two ways in which a cell divides:
- Mitosis
- Meiosis
Mitosis is the type of cell division that is involved in the growth and repair of the body, whereas meiosis is a type of cell division which results in the formation of gametes.
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