The Fundamental Unit Of Life

Namaste students, welcome to today's science lesson. Today we are going to study one of the most important chapters in your science textbook - Chapter 5: The Fundamental Unit of Life. This is a chapter that will help you understand what you are made of, what every living thing around you is made of, and how life functions at the most basic level. So let's begin our journey into the microscopic world of cells.

So students, let's start by understanding how the concept of the cell was discovered. Imagine it's the year 1665, and a scientist named Robert Hooke is examining a thin slice of cork under a microscope that he designed himself. Cork is a substance that comes from the bark of a tree. As he looks through the lens, what do you think he sees? He observed that the cork resembled the structure of a honeycomb, consisting of many little compartments or boxes. These boxes were separated from each other by walls. Hooke decided to call these boxes "cells" - cell is a Latin word meaning "a little room". This was a moment that would change the entire course of biology forever. This may seem like a small and insignificant incident to us now, but it is extremely important in the history of science. This was the very first time that someone had observed that living things appear to consist of separate units. And the term "cell" that Hooke coined is still used today in biology, more than 350 years later!

Now students, let's do an activity to see what Hooke saw. We are going to prepare a temporary mount of an onion peel and observe it under a microscope. This is Activity 5.1 from your textbook.

First, let us take a small piece from an onion bulb. With the help of a pair of forceps, we can peel off the skin, called the epidermis, from the concave side or inner layer of the onion. This layer can be put immediately in a watch-glass containing water. Why do we do this? This will prevent the peel from getting folded or getting dry. Now, let us take a glass slide, put a drop of water on it, and transfer a small piece of the peel from the watch glass to the slide. Make sure that the peel is perfectly flat on the slide. A thin camel hair paintbrush might be necessary to help transfer the peel. Now we put a drop of safranin solution on this piece followed by a cover slip. We must take care to avoid air bubbles while putting the cover slip with the help of a mounting needle. Ask your teacher for help if needed.

Now, what do we observe as we look through the lens? Can we draw the structures that we are able to see through the microscope on an observation sheet? Does it look like Figure 5.2 in your textbook? We can try preparing temporary mounts of onion peels of different sizes. What do we observe? Do we see similar structures or different structures?

Students, what are these structures that we see? These structures look similar to each other. Together they form a big structure like an onion bulb! We find from this activity that onion bulbs of different sizes have similar small structures visible under a microscope. The cells of the onion peel will all look the same, regardless of the size of the onion they came from. These small structures that we see are the basic building units of the onion bulb. These structures are called cells.

Now students, here's something very important - not only onions, but all organisms that we observe around are made up of cells. However, there are also single cells that live on their own. Can you think of such organisms? Organisms like Chlamydomonas, Paramecium, and bacteria are single-celled organisms. These organisms are called unicellular organisms - "uni" means single. On the other hand, many cells group together in a single body and assume different functions in it to form various body parts in multicellular organisms - "multi" means many - such as some fungi, plants, and animals. Can you think of some more unicellular organisms? You might have heard of Amoeba, which is also a unicellular organism.

Every multi-cellular organism has come from a single cell. How is this possible? Cells divide to produce cells of their own kind. All cells thus come from pre-existing cells. This is a very important concept that we will discuss later in the chapter.

Now let's perform Activity 5.2. We can try preparing temporary mounts of leaf peels, tip of roots of onion, or even peels of onions of different sizes. After performing this activity, let us think about the answers to the following questions:

First, do all cells look alike in terms of shape and size? Second, do all cells look alike in structure? Third, could we find differences among cells from different parts of a plant body? Fourth, what similarities could we find?

Students, the answer to these questions is that cells can look quite different from each other depending on their function. Some organisms can also have cells of different kinds. Look at the picture in your textbook that depicts some cells from the human body. You will see that nerve cells look very different from blood cells, for example. The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes - they can change their shape by extending parts of their body called pseudopodia. In some cases, the cell shape could be more or less fixed and peculiar for a particular type of cell - for example, nerve cells have a typical long, branched shape that helps them transmit signals over long distances in the body.

Now students, let me tell you about the history of cell discovery in more detail. Cells were first discovered by Robert Hooke in 1665 as we discussed. He observed the cells in a cork slice with the help of a primitive microscope. Then, Leeuwenhoek in 1674, with the improved microscope, discovered the free-living cells in pond water for the first time. He was able to see single-celled organisms swimming in water - wasn't that amazing? It was Robert Brown in 1831 who discovered the nucleus in the cell. Purkinje in 1839 coined the term "protoplasm" for the fluid substance of the cell. The cell theory - that all plants and animals are composed of cells and that the cell is the basic unit of life - was presented by two biologists, Schleiden in 1838 and Schwann in 1839. The cell theory was further expanded by Virchow in 1855 by suggesting that all cells arise from pre-existing cells. This is very important - omnis cellula e cellula, which is Latin for "all cells come from cells." With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.

Now students, let's answer the questions from this section.

Question 1: Who discovered cells, and how?

The answer is: Cells were discovered by Robert Hooke in 1665. He observed a thin slice of cork under a self-designed microscope and saw box-like structures that reminded him of the small rooms in a monastery, which are called cells. He called these structures "cells."

Question 2: Why is the cell called the structural and functional unit of life?

The answer is: The cell is called the structural unit because all living organisms are made up of cells - they are the building blocks. It is called the functional unit because all life processes - like nutrition, respiration, excretion, and reproduction - occur at the cellular level. Cells are able to perform all these functions because they have various organelles that carry out specific tasks. Every cell comes from a pre-existing cell, and all cells are similar in basic structure and function.

Now students, let's move to the next section: 5.2 What is a Cell Made Up of?

We saw above that the cell has special components called organelles. How is a cell organised? If we study a cell under a microscope, we would come across three features in almost every cell: plasma membrane, nucleus, and cytoplasm. All activities inside the cell and interactions of the cell with its environment are possible due to these features. Let us see how each of these works.

First, let's talk about the Plasma Membrane or Cell Membrane. This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane. This means that it chooses what can pass through and what cannot.

Now students, how does the movement of substances take place into the cell? How do substances move out of the cell? Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. We have studied the process of diffusion in earlier chapters. We saw that there is spontaneous movement of a substance from a region of high concentration to a region where its concentration is low. This happens on its own, without any energy input.

Something similar to this happens in cells. For example, some substance like CO₂ - which is cellular waste and needs to be excreted out by the cell - accumulates in high concentrations inside the cell. In the cell's external environment, the concentration of CO₂ is low as compared to that inside the cell. As soon as there is a difference of concentration of CO₂ inside and outside a cell, CO₂ moves out of the cell, from a region of high concentration to a region of low concentration outside the cell by the process of diffusion. Similarly, O₂ enters the cell by the process of diffusion when the level or concentration of O₂ inside the cell decreases. Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

Now students, water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.

Now let's understand what happens when we put an animal cell or a plant cell into a solution of sugar or salt in water. One of the following three things could happen:

First, if the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net result is that water enters the cell. The cell is likely to swell up. In plant cells, the cell wall prevents the cell from bursting, but animal cells may burst if they take in too much water.

Second, if the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

Third, if the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink. In animal cells, this is called crenation - the cell becomes shrivelled. In plant cells, the cell membrane pulls away from the cell wall - this is called plasmolysis, which we will discuss later.

Thus, students, osmosis is a special case of diffusion through a selectively permeable membrane. It's specifically the diffusion of water.

Now let's try out Activity 5.3 to understand this better.

Part (a): Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell is mostly calcium carbonate. A thin outer skin now encloses the egg. Put the egg in pure water and observe after 5 minutes. What do we observe? The egg swells because water passes into it by osmosis. This is because the inside of the egg has more dissolved substances than the pure water outside, so water moves in by osmosis.

Part (b): Place a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes. The egg shrinks. Why? Water passes out of the egg solution into the salt solution because the salt solution is more concentrated - it has lower water concentration than the inside of the egg. So water moves out by osmosis, causing the egg to shrink.

We can also try a similar activity with dried raisins or apricots. This is Activity 5.4.

Put dried raisins or apricots in plain water and leave them for some time. Then place them into a concentrated solution of sugar or salt. You will observe the following: first, each gains water and swells when placed in water. However, when placed in the concentrated solution, it loses water and consequently shrinks. This is exactly what happens in osmosis - water moves from a region of higher water concentration to a region of lower water concentration.

Students, what do we infer from this activity? It appears that only living cells, and not dead cells, are able to absorb water by osmosis. This is because the plasma membrane in living cells is intact and can regulate what goes in and out. In dead cells, the membrane is damaged and cannot function properly.

Now, let's talk about another important function of the plasma membrane. The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. However, we can observe the structure of the plasma membrane only through an electron microscope. The flexibility of the cell membrane also enables the cell to engulf food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes. In endocytosis, the cell membrane invaginates or folds inwards to form a pocket, and then it engulfs the material into the cell. This is how Amoeba eats - it surrounds its food particle with its pseudopodia and forms a food vacuole inside.

Now, let's discuss Activity 5.5. Find out about electron microscopes from resources in the school library or through the internet. Discuss it with your teacher. Electron microscopes use beams of electrons instead of light to magnify objects. They can magnify objects up to 500,000 times or more, allowing us to see very tiny structures inside the cell like ribosomes, mitochondria, and other organelles that cannot be seen with a light microscope.

Now students, let's talk about the cell wall. Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants. Cell walls permit the cells of plants, fungi, and bacteria to withstand very dilute or hypotonic external media without bursting. In such media, the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells. This is why plant cells don't burst when placed in pure water, while animal cells might.

Now, let's discuss plasmolysis. When a living plant cell loses water through osmosis, there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis. We can observe this phenomenon by performing Activity 5.6.

Mount the peel of a Rhoeo leaf in water on a slide and examine cells under the high power of a microscope. Note the small green granules called chloroplasts. They contain a green substance called chlorophyll. Now put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe under a microscope. What do we see? The cell contents shrink away from the cell wall - this is plasmolysis.

Now, place some Rhoeo leaves in boiling water for a few minutes. This kills the cells. Then mount one leaf on a slide and observe it under a microscope. Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe it again. What do we find? Did plasmolysis occur now? No, plasmolysis does not occur in dead cells because the cell membrane is no longer living and cannot control the movement of water. The dead cells don't show plasmolysis because the cell membrane is damaged and the cell contents don't shrink away from the wall in the same way.

Now students, let's talk about the Nucleus. Remember the temporary mount of onion peel we prepared? We had put iodine solution on the peel. Why? What would we see if we tried observing the peel without putting the iodine solution? Try it and see what the difference is. Further, when we put iodine solution on the peel, did each cell get evenly coloured?

According to their chemical composition, different regions of cells get coloured differentially. Some regions appear darker than other regions. Apart from iodine solution, we could also use safranin solution or methylene blue solution to stain the cells. Staining helps us see different parts of the cell more clearly.

We have observed cells from an onion; let us now observe cells from our own body. This is Activity 5.7.

Let us take a glass slide with a drop of water on it. Using an ice-cream spoon, gently scrape the inside surface of the cheek. Does any material get stuck on the spoon? With the help of a needle, we can transfer this material and spread it evenly on the glass slide kept ready for this. To colour the material, we can put a drop of methylene blue solution on it. Now the material is ready for observation under microscope. Do not forget to put a cover-slip on it!

What do we observe? What is the shape of the cells we see? Draw it on the observation sheet. Was there a darkly coloured, spherical or oval, dot-like structure near the centre of each cell? This structure is called the nucleus. Were there similar structures in onion peel cells? Yes! The nucleus is present in both plant and animal cells.

The nucleus has a double-layered covering called the nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, that is, to the cytoplasm.

The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of characters from parents to next generation in the form of DNA molecules. DNA stands for Deoxyribo Nucleic Acid. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread-like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.

The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells. It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell. The nucleus is often called the control centre of the cell because it contains the genetic material that determines all the cell's activities.

Now students, let's talk about the Cytoplasm. When we look at the temporary mounts of onion peel as well as human cheek cells, we can see a large region of each cell enclosed by the cell membrane. This region takes up very little stain. It is called the cytoplasm.

The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.

Cell organelles are enclosed by membranes. In prokaryotes, beside the absence of a defined nuclear region, the membrane-bound cell organelles are also absent. On the other hand, the eukaryotic cells have nuclear membrane as well as membrane-enclosed organelles.

The significance of membranes can be illustrated with the example of viruses. Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply. This is why viruses are considered on the borderline of living and non-living things.

Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. Many of the functions of such organelles are also performed by poorly organised parts of the cytoplasm. The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles but not with plastids as in eukaryotic cells.

Now students, let's fill in the table that compares prokaryotic and eukaryotic cells. This is an important question from the textbook.

The table has:

For prokaryotic cells: Size is generally small, 1-10 micrometers. Nuclear region is undefined and known as nucleoid. Chromosome is single. Membrane-bound cell organelles are absent.

For eukaryotic cells: Size is generally large, 5-100 micrometers. Nuclear region is well-defined and surrounded by a nuclear membrane. There is more than one chromosome. Membrane-bound cell organelles are present.

Now let's move to the section on Cell Organelles. Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures, or "organelles," within themselves. This is one of the features of eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope.

We have talked about the nucleus in a previous section. Some important examples of cell organelles which we will discuss now are: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids. They are important because they carry out some very crucial functions in cells.

First, let's talk about the Endoplasmic Reticulum, or ER. The endoplasmic reticulum is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags called vesicles. The ER membrane is similar in structure to the plasma membrane. There are two types of ER - rough endoplasmic reticulum, called RER, and smooth endoplasmic reticulum, called SER. RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on need, using the ER. The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones. Although the ER varies greatly in appearance in different cells, it always forms a network system.

Thus, one function of the ER is to serve as channels for the transport of materials, especially proteins, between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. In the liver cells of vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.

Now let's talk about the Golgi Apparatus. The Golgi apparatus was first described by Camillo Golgi. It consists of a system of membrane-bound vesicles, flattened sacs, arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.

The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include the storage, modification, and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes.

Now let's talk about Lysosomes. Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER. Lysosomes are a kind of waste disposal system of the cell. These help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles, end up in the lysosomes, which break complex substances into simpler substances. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the "suicide bags" of a cell. This is a very important point to remember - lysosomes can actually cause the cell to self-destruct if they burst!

Now let's talk about Mitochondria. Mitochondria are known as the powerhouses of the cell. They have two membrane coverings. The outer membrane is porous while the inner membrane is deeply folded. These folds increase surface area for ATP-generating chemical reactions. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP molecules. ATP stands for Adenosine triphosphate. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work. Whenever you move, think, or do any activity, ATP is being used. Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins. This is why scientists believe that mitochondria were once independent organisms that formed a symbiotic relationship with ancient cells!

Now let's talk about Plastids. Plastids are present only in plant cells. There are two types of plastids - chromoplasts, which are coloured plastids, and leucoplasts, which are white or colourless plastids. Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils, and protein granules are stored.

The internal organisation of the chloroplast consists of numerous membrane layers embedded in a material called the stroma. These are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

Now let's talk about Vacuoles. Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small-sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume. In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids, and some proteins. In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

Each cell thus acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. The cell thus has a basic structural organisation. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins. Thus, students, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.

Now let's discuss cell division. New cells are formed in organisms in order to grow, to replace old, dead, and injured cells, and to form gametes required for reproduction. The process by which new cells are made is called cell division. There are two main types of cell division: mitosis and meiosis.

The process of cell division by which most of the cells divide for growth is called mitosis. In this process, each cell called mother cell divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as mother cell. It helps in growth and repair of tissues in organisms. When you get a cut on your skin, mitosis helps new skin cells to form and heal the wound. When you grow taller, mitosis helps your cells divide and increase in number.

Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions. When a cell divides by meiosis, it produces four new cells instead of just two. The new cells only have half the number of chromosomes than that of the mother cells. Can you think as to why the chromosome number has reduced to half in daughter cells? This is because when the gametes combine during fertilisation, the offspring should have the same number of chromosomes as the parents. If gametes had the full number of chromosomes, the offspring would have double the number! So meiosis ensures that gametes have half the number of chromosomes.

Now students, let's answer the questions from this section.

Question 1: Can you name the two organelles we have studied that contain their own genetic material?

The two organelles that contain their own genetic material are mitochondria and plastids. This is why they can make some of their own proteins.

Question 2: If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

If the organisation of a cell is destroyed, the cell will not be able to carry out its normal functions. The cell may die because the organelles cannot work together properly. For example, if the cell membrane is damaged, the cell cannot regulate what enters and exits, leading to cell death. If the nucleus is damaged, the cell cannot control its activities. In short, the cell will lose its ability to survive and function.

Question 3: Why are lysosomes known as suicide bags?

Lysosomes are known as suicide bags because they contain powerful digestive enzymes. If the cell is damaged or dies, the lysosomes may burst open and release these enzymes, which then digest the cell's own components. This is like the cell destroying itself from within. This can happen during certain diseases or when a cell is old and needs to be replaced.

Question 4: Where are proteins synthesised inside the cell?

Proteins are synthesised at the ribosomes. Ribosomes are small颗粒-like structures that can be found attached to the rough endoplasmic reticulum or floating freely in the cytoplasm. The rough endoplasmic reticulum, or RER, is involved in protein synthesis and transport.

Now let's solve the exercise questions at the end of the chapter.

Exercise Question 1: Make a comparison and write down ways in which plant cells are different from animal cells.

Let me explain this in detail, students.

Plant cells and animal cells are both eukaryotic cells, meaning they have a nucleus and membrane-bound organelles. However, there are several important differences:

First, plant cells have a cell wall outside the plasma membrane, while animal cells do not have a cell wall. The cell wall is made of cellulose and provides structural strength to plants.

Second, plant cells have large vacuoles, while animal cells have small vacuoles. In plant cells, the central vacuole can occupy 50-90% of the cell volume.

Third, plant cells contain plastids, including chloroplasts for photosynthesis. Animal cells do not have plastids.

Fourth, animal cells usually have a more rounded shape, while plant cells are more rectangular or box-like due to the cell wall.

Fifth, plant cells have a large central vacuole that provides turgidity and rigidity to the cell, while animal cells do not have this.

Sixth, animal cells have centrioles involved in cell division, while most plant cells do not have centrioles.

Seventh, in plant cells, during plasmolysis, the cell membrane pulls away from the cell wall, but animal cells do not have a cell wall to pull away from.

Exercise Question 2: How is a prokaryotic cell different from a eukaryotic cell?

Let me explain this, students.

Prokaryotic cells and eukaryotic cells are different in several ways:

First, size: Prokaryotic cells are generally small, 1-10 micrometers, while eukaryotic cells are generally larger, 5-100 micrometers.

Second, nuclear region: Prokaryotic cells have an undefined nuclear region known as nucleoid, not surrounded by a nuclear membrane. Eukaryotic cells have a well-defined nucleus surrounded by a nuclear membrane.

Third, chromosomes: Prokaryotic cells have a single chromosome, while eukaryotic cells have more than one chromosome.

Fourth, membrane-bound organelles: Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells have membrane-bound organelles like mitochondria, ER, Golgi apparatus, etc.

Fifth, cell wall: Prokaryotic cells like bacteria have cell walls, but they are different from plant cell walls. Some eukaryotic cells like plant cells have cell walls, while animal cells do not.

Sixth, examples: Bacteria and blue-green algae are prokaryotes, while plant and animal cells are eukaryotes.

Seventh, ribosomes: Prokaryotic cells have smaller ribosomes compared to eukaryotic cells.

Eighth, organelles: Eukaryotic cells have complex organelles like mitochondria, chloroplasts, etc., while prokaryotic cells do not.

Exercise Question 3: What would happen if the plasma membrane ruptures or breaks down?

If the plasma membrane ruptures or breaks down, the following things would happen:

First, the cell would lose its ability to be selectively permeable. It would no longer be able to control what enters and exits the cell.

Second, the contents of the cell would leak out into the external environment.

Third, harmful substances from outside could enter the cell freely.

Fourth, the cell would not be able to maintain its internal environment properly.

Fifth, ultimately, the cell would die because it cannot function without an intact plasma membrane. The cell would be unable to carry out essential processes like respiration, nutrition, and excretion.

In short, the cell would lose its integrity and die.

Exercise Question 4: What would happen to the life of a cell if there was no Golgi apparatus?

If there was no Golgi apparatus, the following would happen:

First, the cell would not be able to package and transport proteins and lipids properly. The Golgi apparatus is responsible for modifying, packaging, and sending materials to their proper destinations inside and outside the cell.

Second, materials synthesised in the ER would accumulate and not be properly processed.

Third, the formation of lysosomes would be affected because the Golgi apparatus is involved in their formation.

Fourth, the cell's ability to secrete substances would be impaired.

Fifth, overall, the cell would not be able to function properly and would eventually die.

Exercise Question 5: Which organelle is known as the powerhouse of the cell? Why?

Mitochondria are known as the powerhouse of the cell. This is because they are responsible for producing ATP - Adenosine triphosphate - which is the energy currency of the cell. Mitochondria generate most of the cell's supply of ATP through the process of oxidative phosphorylation. The energy released during cellular respiration in mitochondria is used to make ATP. This ATP is then used by the cell for various activities like movement, growth, reproduction, and maintaining homeostasis. Without mitochondria, cells would not have enough energy to carry out their functions.

Exercise Question 6: Where do the lipids and proteins constituting the cell membrane get synthesised?

Lipids and proteins constituting the cell membrane get synthesised in the endoplasmic reticulum. Specifically, proteins are synthesised on the ribosomes attached to the rough endoplasmic reticulum, or RER. Lipids are synthesised in the smooth endoplasmic reticulum, or SER. After synthesis, these proteins and lipids are transported to the Golgi apparatus for modification and packaging, and then they are sent to the cell membrane where they are assembled. This process is called membrane biogenesis.

Exercise Question 7: How does an Amoeba obtain its food?

Amoeba obtains its food through a process called endocytosis. Specifically, Amoeba uses a form of endocytosis called phagocytosis, where it engulfs food particles. Amoeba has a flexible cell membrane that can extend outwards to form pseudopodia, which are finger-like projections. When Amoeba encounters a food particle, it surrounds it with its pseudopodia and engulfs it into a food vacuole. Digestive enzymes from lysosomes then break down the food into simpler substances, which are absorbed into the cytoplasm. The undigested waste is expelled from the cell through exocytosis. This is how Amoeba obtains its food.

Exercise Question 8: What is osmosis?

Osmosis is the net diffusion of water across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. In other words, it is the movement of water molecules through a selectively permeable membrane towards a higher solute concentration. Osmosis is a special case of diffusion that involves only water. It occurs when there is a difference in the concentration of water on either side of a selectively permeable membrane. For example, if a cell is placed in pure water, water will move into the cell by osmosis because the water concentration outside is higher than inside. If placed in a concentrated solution, water will move out of the cell.

Exercise Question 9: Carry out the following osmosis experiment.

This is a practical question about the potato experiment. Let me explain each part:

We 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. We 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.

We keep these for two hours. Then we observe the four potato cups.

Now let's answer the questions:

(i) Explain why water gathers in the hollowed portion of B and C.

In cup B, we have sugar solution, and in cup C, we have salt solution. Both of these are concentrated solutions with lower water concentration compared to the pure water in the trough outside. Therefore, water moves into the potato cups B and C by osmosis. The water from the trough moves into the sugar and salt solutions inside the potato cups because the water concentration is higher in the trough and lower in the concentrated solutions. This is why water gathers in the hollowed portions of B and C.

(ii) Why is potato A necessary for this experiment?

Potato A is the control in this experiment. It is kept empty to show what happens when there is no solute in the potato cup. It helps us compare the results with the other cups. Potato A shows that without any solute, there is no change in water movement due to osmosis. It serves as a reference point to understand the effect of solute concentration on water movement.

(iii) Explain why water does not gather in the hollowed out portions of A and D.

In cup A, there is no solute at all - it's empty. So there is no concentration difference between the inside of the potato cup and the water in the trough. Therefore, there is no net movement of water by osmosis. The water neither enters nor leaves significantly, so no water gathers in the hollowed portion.

In cup D, the potato has been boiled. Boiling kills the cells and damages the plasma membrane. The cell membrane becomes non-functional. Since osmosis requires a living, functional, selectively permeable membrane, it cannot occur in dead cells. Therefore, even though there is sugar solution inside the cup, the dead cells cannot facilitate osmosis. Water does not gather in the hollowed portion of D because the cell membranes are destroyed and cannot control water movement.

Exercise Question 10: Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?

For growth and repair of body, the type of cell division required is mitosis. In mitosis, a mother cell divides to form two identical daughter cells with the same number of chromosomes as the mother cell. This helps in growth by increasing cell number and repair by replacing damaged or dead cells.

For formation of gametes, the type of cell division involved is meiosis. In meiosis, a cell divides twice to produce four daughter cells, each with half the number of chromosomes of the mother cell. This is necessary because gametes (sperm and egg cells) must have half the number of chromosomes so that when they combine during fertilisation, the offspring has the correct number of chromosomes.

Now students, let's review what we have learnt in this chapter.

What you have learnt:

- The fundamental organisational unit of life is the cell. - Cells are enclosed by a plasma membrane composed of lipids and proteins. - The cell membrane is an active part of the cell. It regulates the movement of materials between the ordered interior of the cell and the outer environment. - In plant cells, a cell wall composed mainly of cellulose is located outside the cell membrane. - The presence of the cell wall enables the cells of plants, fungi, and bacteria to exist in hypotonic media without bursting. - The nucleus in eukaryotes is separated from the cytoplasm by a double-layered membrane and it directs the life processes of the cell. - The ER functions both as a passageway for intracellular transport and as a manufacturing surface. - The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification, and packaging of substances manufactured in the cell. - Most plant cells have large membranous organelles called plastids, which are of two types - chromoplasts and leucoplasts. - Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis. - The primary function of leucoplasts is storage. - Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell and stores important substances including wastes. - Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles. - Cells in organisms divide for growth of body, for replacing dead cells, and for forming gametes for reproduction.

Students, this chapter is very important because it forms the foundation for understanding all living things. Every organism, from the smallest bacteria to the largest whale, is made up of cells. The cell is truly the fundamental unit of life. Understanding how cells work helps us understand how our own bodies function, how plants grow, and how all living things survive.

Remember the key points: - Cells were discovered by Robert Hooke in 1665 - The cell is the structural and functional unit of life - The plasma membrane is selectively permeable - Osmosis is the diffusion of water through a selectively permeable membrane - The nucleus contains genetic material - DNA - which controls cell activities - Mitochondria are the powerhouses of the cell, producing ATP - Lysosomes are the suicide bags of the cell - Plant cells have cell walls, large vacuoles, and plastids - Animal cells do not have cell walls but have centrioles - Prokaryotic cells are simpler and smaller than eukaryotic cells - Mitosis is for growth and repair, meiosis is for gamete formation

I hope this lesson has helped you understand the fundamental unit of life - the cell - thoroughly. Keep studying and exploring the amazing world of biology!

Thank you, students. Shikshak ke desh mein, humare liye padhna bahut zaroori hai. Padhai karte raho aur scientist bano!

Goodbye and best of luck with your studies!