Block of Life
It is widely accepted amongst the scientific community that life originated in water. Some researchers believe that life may have originated in small water pools with changing environmental conditions rather than in the oceans. Hot springs are examples of such environments. In India, the hot springs of Puga Valley in Ladakh maintain very high temperatures (nearly at the boiling point of water) even in a cold climate. These environmental conditions seem to be similar to those on the early Earth, about 3.5 billion years ago. The organisms living in these hot springs are mostly heat-loving bacteria called thermophiles, which are unicellular.
Think It Over
• Where does a cell come from? • How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye? • How is the cell structural and functional unit of life? • How does a cell multiply?
Scientists from the Birbal Sahni Institute of Palaeosciences, Lucknow, studied these hot springs and found that calcium carbonate formed rapidly around them. These deposits may have protected early organic molecules from harmful radiation and extreme conditions, and they may have also helped in the formation of the first protective membrane — the barrier that defines a cell.
All living organisms are made up of cells. The cell represents the basic level at which life exists. Some organisms, such as bacteria or yeast consist of only one cell (unicellular), while others like plants, fish, birds or humans are made up of millions of cells (multicellular) that work together. A group of similar cells performing similar
functions forms tissues. Different tissues are organised to form an organ and several organs work together to form organ systems. Such as, nasal pores, nasal cavity, trachea and lungs form respiratory system.
1 km
100 m
Even when cells are organised into tissues, organs, and organ systems the cell remains the fundamental unit of structure and function in all living organisms. This makes us wonder how such tiny cells perform so many different activities. What are the different components of a cell? How do cells in our body communicate with each other? Do cells live forever, or do they die? In this chapter, we will explore the answers to these questions as we enter the fascinating world of cells!
Rocket
Neem Tree
10 m
Unaided eye
Human height
1 m
Great Indian Bustard
0.1 m
Chicken egg
1 cm
2.1 How to Study Cells?
What do we call the ability of the human eye to see two very close objects as separate and distinct? Imagine two tiny dots drawn on a piece of paper. As the dots are moved closer, there comes a point at which they can no longer be seen as separate. When viewed from about 25 cm (the near point of human eye), two points separated by about 0.1 mm can be seen as distinct; otherwise, they appear as a single point. This is called the limit of resolution of the human eye, which is 0.1 mm.
Fish egg
1 mm
Amoeba
Light microscope
100 µm
Most plant andanimal cells
10 µm
Nucleus
Most bacteria
Mitochondrion
1 µm
Electron microscope
A cell is usually too small to be seen by the unaided eye (Fig. 2.1). This raises an important question — how have cell biologists studied the structure and function of cells that are much smaller than the limit of resolution of the human eye?
Smallest bacteria
100 nm
Viruses
Ribosomes
10 nm
Proteins
You have learnt about convex lens. A convex lens or a combination of lenses, i.e., an objective lens and an eyepiece are used for the magnification of an object (Fig. 2.1) to make it appear larger.
Grade 8CuriosityChapter 10
Lipids
1 nm
Small molecules
Atoms
0.1 nm
Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope (capable of about 200–300X magnification). While examining a thin slice of cork, he observed small box-like compartments and named them ‘cells’. In school laboratories, light microscopes are used to observe objects using
Grade 8CuriosityChapter 2
Fig. 2.1: Size of the objects and its visibility through
unaided to aided eye
Eye piece
Course adjustment
different objective lenses (e.g., 10X, 40X) to achieve better magnification and resolution under visible light. Explore different parts of a microscope (Fig. 2.2) in your school laboratory and use it to observe fine structures of various materials. Under the microscope, you will see a magnified image of an object. Can you estimate its actual size?
knob
Body tube
Grade 8CuriosityChapter 9
Objective
Fine adjustment
lens
knob
Handle
Stage
Activity 2.1: Let us estimate the size of a cell
Mirror
Base
1. Take a transparent ruler with millimetre (mm)
markings.2. Place the ruler on the stage of microscope, focus
Fig. 2.2: Structure of a light microscope
on it using the adjustment knob and observe the diameter of the circular field of view through the eyepiece and measure it in mm. 3. Convert the diameter from mm to micrometre
Ready to Go Beyond
(µm). Suppose the diameter of the visible field is 5 mm, meaning 5 × 1000 = 5000 µm.4. Remove the ruler and place an onion peel slide
on the stage of the microscope.5. Focus on the slide and count the number of cells
present along the diameter of the field of view in one straight line.6. Estimate the real size of the cell using the
Apart from light microscopes, scientists also use powerful electron microscopes that reveal the fine details of a cell structure. These instruments use a beam of electrons instead of light to produce highly magnified images, allowing us to see cell structure at the nanometre scale with remarkable clarity (a nanometre is one-billionth of a metre).
formula:
Unit conversion:1 millimetre (mm) = 1000 micrometre (µm)
Diameter of the visible field in micrometre
Estimated size of the onion peel cell =
Number of cells along the diameter
Suppose, 25 cells are seen along the diameter. In that case, the size of one onion cell would be 5000 µm/25 = 200 µm.
If the estimated size of an onion peel cell is 200 µm, how much does a light microscope magnify this cell? The total magnification of a microscope depends on its magnifying power of the eyepiece and the objective lens. If both the eyepiece and the objective lens have the magnifying power of 10X, then the total magnification will be 100X. This means that a cell with an estimated size of 200 µm will appear 100 times larger.
Thus, a microscope allows us to see very small structures clearly and is an essential tool for studying the cell structure. Over the years, scientists have improved the microscope by improving its three main features — resolution (measure of clarity), contrast (the difference in brightness between various
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Exploration|Grade 9
Stomata
parts of an object), and magnification. These improvements have turned microscope into a powerful tool for studying cells.
You can see a picture of the lower surface of Colocasia leaf observed under a Scanning Electron Microscope (Fig. 2.4).
2.2 Structure of a Cell
You have learnt that cells are organised into specialised tissues and organs, and collectively perform specific function. For these cells to function as units, they must be able to interact with one another and with their surroundings. These interactions occur at the cell boundary, where substances move between the cells and their external environment. Even single-celled organisms exchange materials and respond to their environment through cell membrane.
Fig. 2.4: Electron micrograph of lower surface of a Colocasia leaf showing stomata
2.2.1 Cell membrane — The universal feature of a cell
The cell membrane is a thin boundary that surrounds a cell and protects its contents. It defines the individuality of a cell and is also called the plasma membrane. The cell membrane is selectively permeable, which means it allows some substances to pass through it while blocking others. You have learnt how oxygen and carbon dioxide move across the membranes of alveoli in the lungs. How does the structure of the cell membrane in the cells of alveoli control the movement of substances across it?
Grade 7CuriosityChapter 9
Activity 2.2: Let us experiment
Experimental set-up
Beaker BBeaker A
1. With the help of a kitchen knife,
carefully cut a potato into two pieces of roughly equal size (Fig. 2.5). 2. Measure and record the initial weight
of both the pieces using a weighing balance. 3. Put one piece of the potato in Beaker
A with plain water.4. Put the other piece of the potato in
Initial(b)(a)
Beaker B with 20 per cent salt or sugar solution.5. Leave them undisturbed for about
an hour or until you notice a visible change in the size of the pieces.6. Measure and record the final weight
Final(b)(a)
of each piece.7. Calculate the difference between the
initial and their final weights.What do you observe? You may observe that —Beaker A — The potato piece swells.Beaker B — The potato piece shrinks.
Fig. 2.5: Experimental set-up, and initial and final states of potato pieces in (a) plain
water, and (b) 20 per cent salt solution
What if ...
What do you infer? What do you expect in terms of changes in their weight? The weight of the potato piece in Beaker A has increased, while the weight of the potato piece in Beaker B has decreased. This happens because the cell membrane allows water to move in and out of the cell but not the sugar or salt molecules. Water moves from an area with more water and less solute (dilute solution) to an area with less water and more solute (concentrated solution) until the concentrations in the two areas become equal. This movement of water through a selectively permeable membrane is called osmosis. You have learnt in Activities 7.8 (dye spreading in water) and 7.9 (fragrance spreading in air) in Grade 8 that particles of matter intermix, due to a difference in their concentrations, also called a concentration gradient. Diffusion is the net movement of particles from a higher to a lower concentration (which occurs even without a membrane). Osmosis is the diffusion of water across a selectively permeable membrane. In plants, water from the soil enters root cells by this process of osmosis.
mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?
Grade 8CuriosityChapter 7
What if ...
a cell is kept in salt or sugar solutions of different concentrations?
Solute concentration of extracellular medium = Solute concentration of intracellular medium — Isotonic solution
Isotonic
solution
Solute concentration of extracellular medium < Solute concentration of intracellular medium — Hypotonic solution
Hypotonic
solution
Solute concentration of extracellular medium > Solute concentration of intracellular medium — Hypertonic solution
Hypertonic
solution
Fig. 2.6: Effect of solutions of different concentrations on a cell
Structurally, the cell membrane is extremely thin, about 7 to 10 nanometres (nm) thick (1 nanometre = 0.000001 mm). It is made up of lipids (fats) and proteins. The fluid-mosaic model explains its structure (Fig. 2.7) :
Various types of
proteins
Lipid bilayer
y
The membrane has a lipid bilayer (two
layers of special fat molecules with water attracting heads outwards and water repelling tails inwards) with proteins embedded in them. • The molecules can move sideways, flip and rotate within the membrane.
Fig. 2.7: Structure of a cell membrane
Hence, it is fluid. • Proteins in the membrane act like gatekeepers in helping substances
pass through.
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Exploration|Grade 9
y
Since the molecules are arranged like tiles in a mosaic, it is called the
‘mosaic’ model.All living cells communicate with their surroundings and their neighbouring cells through the cell membrane. However, cells of plant, fungi, and bacteria have an additional layer around the cell membrane, called the cell wall. What do you think is the necessity of the cell wall in these cells?
Activity 2.3: Let us investigate
1. Prepare temporary slides of a thin peel of an onion leaf or a Rhoeo
(Cradle lily) leaf and mount it with safranin using cover slip to observe plant cells under a microscope. 2. Similarly, prepare a temporary slide of cheek cells by gently scraping
(a)
the inner side of your cheek with a cotton swab or the blunt end of a toothpick. 3. Spread the cheek cells on a clean glass slide.
4. Add a drop of water followed by a few drops of methylene blue stain and
carefully place a coverslip. 5. Observe both the slides under a microscope.What do you observe? Onion peel cells (Fig. 2.8a) or Rhoeo leaf peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged (Fig. 2.8b). Why do you think this difference exists?
(b)
Fig. 2.8: (a) The onion peel cells, and (b) human cheek cells
Prepare two slides of a Rhoeo leaf peel and human cheek cells again, and put 20 per cent sugar solution on them. Observe them under a microscope after half an hour. What do you observe? You must have observed that the boundaries of the plant cells remain the same but their inner content shrinks, and the space between the inner and outer boundaries increases (Fig. 2.9a and b). You may observe that the cheek cells, on the other hand, have shrunk considerably.
2.2.2 Cell wall — The outer covering of cells
In general, plants cannot move from place to place, so they need a rigid structure to withstand environmental stresses like wind and rain. Therefore, plant cells have an additional covering outside the cell membrane called a cell wall. The cell wall also helps leaves and flowers remain firm, and maintain their shapes and help plants stay upright. Although rigid the cell wall is permeable, which means water and some dissolved minerals can pass through it. Along with the selective permeability of the cell membrane, the permeability of the cell wall helps plant roots absorb water and nutrients from the soil.
(a)
When we place a Rhoeo leaf or an onion peel in a concentrated sugar solution the plant cells lose water due to osmosis. However, the cells do not shrink in size because their rigid cell wall maintains their shape. The inner content of a cell shrinks as the cell membrane pulls away from the cell wall. This shows that the cell wall helps plant cells remain firm in their original shape.
(b)
Fig. 2.9: Cradle lily leaf peel cells (a) in water, and
(b) in 20 per cent sugar
Animal cells, such as cheek cells do not have a cell wall. Therefore, when placed in a concentrated sugar solution, they lose water and shrink.
solution
Without a rigid cell wall, animal cells can change shape easily. This cellular flexibility supports the overall movement and functioning of animal tissues.
The plant cell wall is primarily made of cellulose, a type of carbohydrate formed by many glucose units linked together. Cellulose in our diet acts as roughage, helping in digestion. Some microorganisms, like fungi and bacteria also have a cell wall to provide protection and structural support to their cells.
Pause and Ponder
1. What argument would you give for the necessity of a cell wall in plants
usually fixed in one place versus in animals usually moving from one place to the other?
2. What consequences would you predict for a plant cell if its cell wall were
to become as flexible as a cell membrane?
3. Why is it important to cut the two potato pieces in roughly equal size and
measure their initial weight before placing them in different liquids?
2.3 The Cell Interior — A Coordinated Working System
You have learnt that most cells have three basic parts:
Grade 7CuriosityChapter 9
y
a selectively permeable membrane called the plasma
membrane, • a semi-fluid, jelly-like substance called the cytoplasm, and • a prominent nucleus.
In addition to the nucleus, the cytoplasm contains several sub-cellular components called organelles, along with other substances present in it, most of which are only visible with an electron microscope.
Activity 2.4: Let us study
Lysosome
Vacuole
Cell membrane
Nucleus
Vacuole
Nucleolus
Endoplasmic
Chloroplast
reticulum
Cell wallMitochondria
CytoplasmCytoplasm
RibosomeRibosome
NucleoidAppendage for
(Genetic material)
locomotion
Ribosome
Golgi body
(a)(b)(c)
Fig. 2.10: (a) A typical bacterial cell, (b) a typical plant cell, and (c) a typical animal cell
1. Study the given diagrams of a bacterial cell, a plant cell, and an animal
cell (Fig. 2.10a, b and c). 2. Observe the different structures present in each of them. 3. Record your observations in Table 2.1.
A bacterial cell lacks a well-defined nucleus and membrane-bound organelles (organelles surrounded by their own membranes). Such cells are called prokaryotic cells (pro means primitive and karyon means
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Exploration|Grade 9
nucleus). In prokaryotic cells, most cellular activities take place directly in the cytoplasm. In contrast, plant and animal cells have a well-defined nucleus and several membrane-bound organelles. Such cells are called eukaryotic cells (eu means true, and karyon means nucleus). More details of the characterstics of prokaryotic and eukaryotic cells is given in Table 2.2.
Ready to Go Beyond
S. No.Cell structuresBacterial cellPlant cellAnimal cell
Viruses, viroids, and prions are acellular (no cells) infectious agents that are too small to be seen under a light microscope. Viruses are composed of some genetic material with a protein coat. Viroids lack protein coat around its genetic material, while prions are misfolded proteins which lack genetic material.
1.Cell membrane
2.Cell wall
3.Cytoplasm
4.Well-defined nucleus (genetic material enclosed by a membrane)
5.Primitive nucleus or nucleoid (genetic material without membrane around it)
6.Membrane-bound organelles
CharacteristicsProkaryotic cellEukaryotic cell
Primitive nucleusPresentAbsent
Diameter of a typical cell1 to 10 µm10 to 100 µm
Number of cells in an organismUsually unicellularCan be unicellular or multicellular
Membrane-bound organellesAbsentPresent
Membrane-bound nucleusAbsentPresent
Which of the cells given in Fig. 2.10 fall under the categories of prokayotic and eukaryotic cells?
Ready to Go Beyond
In eukaryotic cells, a network of fine fibres forms the cytoskeleton, which provides structural support, maintains cell shape, and enables cell movement and internal transport. It is visible only under an electron microscope as a separate entity. The cytoplasm may also store starch (in plant cells), or crystals of calcium oxalate or silica (in some plant cells). These are known as cell inclusions.
2.3.1 Why do eukaryotic cells need these organelles?
Eukaryotic cells carry out various life processes in different cell organelles independently at the same time.
Cell organelles help in building new materials, removing waste, and providing energy to the cell. They work together to perform all functions of a cell. In other words, a cell is like a tiny living factory, with each of its part doing a specific job.
Let us explore these specialised structures present inside a cell.
Nucleus
Nucleus — House of coded instructionsThe nucleus has a double-layered covering called the nuclear membrane, which has pores that allow the transfer of material between the nucleus and the cytoplasm. The nucleolus is the dense round body in the nucleus, where synthesis of ribosomal subunits take place. These subunits then exit the nucleus to cytoplasm where one large and one small subunits assemble to form ribosome (Fig. 2.11).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 the next generation in the form of DNA (Deoxyribonucleic acid) molecules. Chromosomes are composed of DNA and specific proteins. DNA molecules contain the genetic information. The functional segments of DNA are called genes. In a non-dividing cell, this DNA is present as part of chromatin material. Chromatin material is visible as an entangled mass of thread-like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes (Fig. 2.12).
Double-layered nuclear membrane
Nucleolus
Nuclear pore
Nucleus
Chromatin
Chromatin
Chromosome
DNA
to DNA
Threads of Curiosity
Some cells are specialised to perform specific functions. For example, mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). The absence of a nucleus provides more space for haemoglobin, allowing it to transport a larger amount of oxygen to all cells of the body. Since they lack a nucleus, they cannot repair or divide themselves. As a result, their lifespan is short and they survive approximately for 120 days. Do you know any other cells without nucleus?
Prokaryotic cells do not have a well-defined nucleus. Their DNA is present as a single circular molecule associated with specific proteins. The region containing this genetic material is called the nucleoid.
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Exploration|Grade 9
Ribosomes — The protein factories
These are tiny structures may be present either freely in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are the sites of protein synthesis.
Smooth endoplasmic
Endoplasmic Reticulum (ER) — Manufacturing factory
The Endoplasmic Reticulum (ER) is a large organelle that spreads like a network within the cytoplasm of the cell. The ER is continuous with the outer membrane of nuclear envelop. The ER plays a key role in the synthesis and transport of proteins, fats (lipids), and some hormones in some of the specialised cell. The structure of the ER in a cell varies depending on its function. There are two types of ER:
reticulum
Rough endoplasmic
reticulum
Ribosome
apparatus
Golgi
Plasma membrane
y
Rough Endoplasmic Reticulum (RER): It looks
rough under an electron microscope because it has ribosomes attached to its surface, and is mainly involved in protein synthesis and protein secretion (for example, in gland cells, such as pancreatic cells). • Smooth Endoplasmic Reticulum (SER): It does
Vesicles
Fig. 2.13: Endoplasmic reticulum and Golgi apparatus — pathway for protein
processing and secretion
not have ribosomes on its surface, and therefore, looks smooth. It is involved in the synthesis, and storage of fats and hormones (Fig. 2.13).
Meet a Scientist
The Golgi apparatus was first observed in 1898 by an Italian scientist, Camillo Golgi, in the nerve cells of a barn owl. Using special staining techniques, he observed a thread-like network. Early microscopes could not resolve it clearly, therefore, many doubted its existence. However, electron microscope observations confirmed it decades later.
Golgi apparatus — The packaging and shipping centres
If you carefully look at the diagram of a cell (Fig. 2.10b and c), you will notice stacks of flattened, sac-like structures. Together, these stacks form the Golgi apparatus (Fig. 2.13). It is functionally linked to the ER, the cell membrane and the other cell organelles. The Golgi apparatus acts like the cell’s post office. It modifies, sorts, and packages proteins and/or lipids into vesicles for transport, secretion, or lysosome formation.
Lysosomes — The clean-up system
Cells produce waste materials and damaged, worn-out organelles during their activities. How does the cell prevent these wastes from accumulating inside it?
When the structure was clearly seen and confirmed, it was named the ‘Golgi apparatus’ in his honour.
Lysosomes are single membrane-bound sacs filled with enzymes, which can break down unwanted proteins, carbohydrates, fats, and even
damaged parts of the cell, keeping it clean and healthy. The products formed by the breakdown are released into the cytoplasm, where they may be reused in other cellular processes.
Threads of Curiosity
Human sperm cells contain lysosomal enzymes. When a sperm meets an egg, these enzymes help break down the outer layer of the egg, allowing fertilisation to take place. You will learn about the human sperm cells later in Chapter 11.
Mitochondria — The powerhouse of the cell
Mitochondria are often called the ‘powerhouses of the cell’ because they supply the energy needed for most cellular activities. Each mitochondrion (Fig. 2.14) is surrounded by two membranes :
Ribosomes
DNA
Intermembrane
space
Grade 7CuriosityChapter 9
y
The outer membrane is smooth and
Cristae
porous. • The inner membrane is folded into finger-like
Inner membrane
projections called cristae, which increase the surface area for chemical reactions and facilitate energy production. In mitochondria, the glucose and other molecules are broken down to release energy during a process called cellular respiration. You have already studied about it. The energy released is stored in the form of a molecule called Adenosine Triphosphate (ATP), which acts as the energy currency and is used for most of the cellular activities.
Outer membrane
Plastids — Centre for food synthesis in the plant cells and beyond
You have learnt that mitochondria provide energy to the cell that comes from food. Animals can obtain food from their surroundings, however, plants synthesise food in the presence of sunlight. But where do plants
synthesise their food and obtain energy for cellular activities? Plants use special organelles called plastids for food synthesis and storage.
Inner membrane
As you have learnt, plants prepare their food by the process of photosynthesis in the presence of sunlight. A green pigment called chlorophyll, which is present in the chloroplast (a type of plastid) absorbs sunlight. Chloroplasts are double-membrane-bound organelles, like mitochondria. Inside the chloroplast (Fig. 2.15) there is a semi-fluid substance called the stroma. Within the stroma are disc-shaped membrane structures that contain chlorophyll. Light energy is absorbed
Grade 7CuriosityChapter 9
Outer membrane
Stroma
DNA
Ribosomes
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Exploration|Grade 9
by them during photosynthesis. The sugars synthesised in this process are stored in stroma, along with the starch granules.
Mitochondria and plastids have some features that are similar to certain bacteria. For example, they both have their own DNA and ribosomes, and thus, they can make some of their own proteins. These characteristics suggest that both mitochondria and plastids share an evolutionary history with these single-celled organisms.
Are there any other plastids in plant cells that contain any pigments other than the green pigments?
How do flowers, fruits, and vegetables acquire varied colours? In flower petals and fruits, plastids contain pigments other than chlorophyll. These plastids are called chromoplasts (in Greek, chroma means colour). Their pigments (may be yellow, orange or red) are the source of bright colours in such flowers and fruits. Such bright colours help in attracting pollinators for pollination and fruit-eating animals that help in seed dispersal.
Some plastids lack pigments and are thus, colourless. These are called leucoplasts (in Greek, leukos means white). Leucoplasts store food material, such as starch, oils or proteins and are classified based on the type of food they store. For example, some leucoplasts in potato and taro (Colocasia) cells store starch.
Vacuoles — The organelles for storage and support
Plastids, such as chloroplasts help plants produce food and temporarily store it. Other plastids, such as leucoplasts help in storing food. But where are water, minerals, and waste materials stored in the cell? Why do plants look wilted when they do not get enough water?
In a mature plant cell, there is usually one large central vacuole surrounded by a single selectively permeable membrane. The vacuole is filled with a watery fluid called cell sap. The vacuole stores water, minerals, sugars and waste material. By storing large amounts of water, the vacuole helps maintain pressure inside the cell, which keeps a plant cell firm. When a plant does not get enough water, the vacuole loses water, the cells become less firm, and the plant gets wilted.
In animal cells, vacuoles are sometimes present. Although they are not as large as plant vacuoles, they help in the temporary storage of materials.
Pause and Ponder
4. Do white flowers contain any pigment? Give reasons.5. Draw a well-labelled schematic diagram of a plant or an animal cell using
these clues —(i) Nucleus appears as a dark and round body inside the cell.(ii) ER spreads like a network of extended nuclear envelope. (iii) Mitochondria and chloroplasts are rod shaped.You may refer to Fig. 2.10.
Threads of Curiosity
In 2010, scientist J. Craig Venter and his team made an important discovery in the field of synthetic biology. They first studied the complete DNA sequence of a simple bacterium called Mycoplasma mycoides using a computer programming. Then, they chemically synthesised (in the laboratory) an exact copy of this DNA.
Next, they took another closely related bacterium and removed its DNA, but kept the rest of the cell (such as the cytoplasm and the cell membrane) intact. They inserted the synthetic DNA into this cell. After this, the cell started to grow and divide, following the instructions from the newly inserted synthetic DNA. This experiment showed that DNA controls the structure and activities of a cell.
However, scientists did not create a completely new cell from scratch. Only the DNA was synthetic. The other parts of the cell were taken from an already existing living cell.
2.4 How do Normal Cells Grow and Divide?
When you get a small cut on your skin, it heals after a few days. When hair fall out, new hair grow back. How does this happen? It happens because cells in our body can grow and divide to replace the old, dead, or damaged cells. When our body grows, it is not just because cells get bigger. Cells can grow only up to a certain size but growth happens because cells divide to form new cells. Let us study freshly growing root tips of an onion to understand how cells divide (Fig. 2.16).
Activity 2.5: Let us enhance our skills
1. Take a jar and fill it up with plain water.2. Now, place an onion bulb over the jar in such a way that its base
Fig. 2.16: Growing roots of an onion in a
jar containing water
bearing roots, is immersed in the water. 3. Leave the setup for 5–6 days and observe. Do you observe the roots
growing? Cut 2–3 cm of the freshly grown roots and transfer them to freshly prepared aceto-alcohol (glacial acetic acid:ethanol :: 1:3). Keep the root tips in aceto-alcohol for 24 hours and then transfer them to 70 per cent ethanol (for preservation).4. Take one or two preserved roots, wash them in water and then, place
them on a clean slide. 5. Put one drop of dilute Hydrochloric acid (HCl) on the root tips to
soften the tissue. Rinse the roots after 10–15 minutes. Then add 2–3 drops of aceto-carmine stain on them. 6. Leave the slide for 5–10 minutes and then, gently warm it (with
caution) over a spirit lamp.7. Cut the tip portion of the root on the slide and put a coverslip.
Gently squash the coverslip with your thumb to spread the cells on the slide.8. Observe the slide under a microscope.
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Exploration|Grade 9
What do you observe? Do you observe the cells of the onion root tip? Are they similar in structure? Do you find any structural differences in these cells? If yes, why is it so?
This is because the cells of a growing tip of root of onion divide continuously. This process is called cell division. Fig. 2.17 shows various stages of cell division. Therefore, these cells exhibit different structures corresponding to different stages of cell division. Can you identify which stage comes first during cell division?
Every day, an estimated hundreds of billions of cells in our body are replaced, which is almost 1 per cent of the total number of cells in our body. Both prokaryotic and eukaryotic cells divide, but eukaryotic cells divide in a more controlled and orderly manner by a process called the cell cycle. You will learn about the stages of cell division in higher grades.
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Fig. 2.17: Different stages of cell division in onion root tip cells
2.4.1 Cell division
Cell division is the process by which new cells are formed from pre‑existing cells. It allows living organisms to grow, repair damaged tissues and reproduce. Some cells, such as skin cells divide continuously to replace cells that are lost regularly. There are two major types of cell division — mitosis and meiosis. Mitosis is important for normal growth, repair, maintenance and asexual reproduction, while meiosis is important for sexual reproduction for creation of genetic diversity.
Mitosis
Every human begins life as a single fertilised egg. This one cell divides repeatedly to form trillions of cells in the body. Cells increase in number through mitosis, which is the most common type of cell division (Fig. 2.18).
Mitosis produces two genetically identical daughter cells from one parent cell. Each new cell gets the same DNA and the same number of chromosomes as the parent cell. This ensures that genetic information is largely maintained across body cells.
that produces two genetically
identical daughter cells
Meiosis
Meiosis is a type of cell division that produces gametes and occurs only in the cells of reproductive organs. Gametes produced for sexual reproduction create variations and diversity among living organisms. Therefore, children resemble their parents but are not exactly the same. In animals including humans, meiosis occurs only in the cells of testes of males to produce sperm and ovaries of females to produce eggs. In plants, meiosis occurs in the anthers (male parts) to form pollen grains (that later produce sperm cells) and the ovaries (female parts) to produces egg cells. In meiosis, the parent cell divides twice, one after the other, to form four daughter cells (Fig. 2.19). During the first division, the cells divide into two daughter cells and the number of chromosomes in each daughter cell is reduced to half. The second division is similar to mitosis where each daughter cell divides into two thus, forming four daughter cells with half the number of chromosomes. As a result, each gamete has half the number of DNA compared to the parent cell. During fertilisation, when gametes from two individuals combine, the original chromosome number is restored. You will learn about this process in detail in Chapter 11.
Fig. 2.19: Meiosis is a two-step process
that produces four gametes
Meet a Scientist
Arun Kumar Sharma was a famous Indian scientist known for his work on chromosomes. As a botanist, he is known for his work in the area of plant taxonomy, evolution and development. He also invented many useful lab methods to study chromosomes in plants. For his contributions to botany, he received many honours, including the Shanti Swarup Bhatnagar award and Padma Bhushan.
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Bridging Science and Society
Scientists have developed methods to grow plant and animal cells outside the body in special conditions. This is called cell culture. In this process, cells are taken from an organism and placed in a nutrient-rich medium that allows them to grow and multiply. To keep the culture safe, the right temperature, acidic or alkaline conditions and moisture under sterile conditions are maintained. Cell culture is crucial for studying how cells work and for the production of biochemicals, food, medicines, vaccines, and more.
The processes of mitosis and meiosis must occur in a proper and controlled manner. What happens if meiosis and mitosis do not happen properly? If there is any error in these processes, it can lead to various problems in the body of an organism.
Pause and Ponder
6. Instead of many small ones, why
does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?7. If the skin cells start dividing by
Errors in mitosis lead to uncontrolled cell divisions. This can lead to the formation of tumours and abnormal number of chromosomes in body cells.
meiosis instead of mitosis, what do you think will happen to a cut on the skin?
Errors in meiosis may result in genetic disorders, which may be associated with developmental problems or distinctive physical features. Faulty meiosis may also cause early pregnancy loss or reduced fertility.
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Exploration|Grade 9
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2.5 Cell Theory — The Unifying Principle of Biology
An important observation about living organisms is that all organisms are made up of cells. In 1838, a German botanist named Matthias Schleiden reported that all plants are made up of cells. In 1839, German zoologist Theodor Schwann, found that all animals are also made up of cells. Later, in 1855, a German scientist named Rudolf Virchow, further expanded the Cell Theory by stating that new cells are formed only from pre-existing cells. Together, their work led to the formulation of the Cell Theory.
In 1902, the Austrian botanist, Gottlieb Haberlandt, proposed that any living plant cell, even a fully mature cell from a permanent tissue, can develop into a complete plant if it is provided with suitable nutrients and favourable conditions. He suggested that plant cells have the ability to form different types of cells and also change them. This special ability of plant cells is called totipotency. Haberlandt’s idea laid the foundation for a new branch of biology known as Plant Tissue Culture Technology.
According to the classical Cell Theory:
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All living organisms are made up of one or more cells. • The cell is the basic unit of structure and function in living
beings. • All cells arise from pre-existing cells.This unifies all biology, from bacteria to humans, and explains life’s continuity through cell division.
2.5.1 Do cells grow and reproduce forever?
Cells grow and divide in a controlled way, stay in the right place, carry out their functions, and eventually die when they are no longer needed. Dead cells are replaced by new cells that carry out the same function. Thus, every cell has a definite life span. If cells do not die when they should or if they die too early problems can arise in the body. In many animal cells, cell division usually stops when cells come in contact with neighbouring cells. This process is called contact inhibition. However, cancer cells lose this control and keep dividing uncontrollably, leading to the formation of tumours. Plant cells grow differently. Due to their rigid cell walls plant cells do not show contact inhibition and follow a different pattern of growth.
Threads of Curiosity
How do cancer cells grow and spread? Normal cells grow, age and die in a controlled manner. Sometimes, this system breaks down, and abnormal cells start growing and dividing uncontrollably. This results in the formation of tumours, which may be benign or malignant. Cancerous tumours can invade nearby tissues and even spread to other parts of the body to form new tumours.
Even though cells are extremely tiny they perform remarkable functions. They produce energy, synthesise and secrete substances, divide to form new cells, and work together to maintain the proper functioning of the body. Every living being from a tiny bacterium to a giant tree is made up of cells. You are one of the products of these tiny hardworking cells.
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How do cells monitor their growth to maintain a balance?
Cells also have natural ways of dying to maintain a balance. Programmed Cell Death (PCD) is a genetically regulated and organised process of selective cell destruction. It is essential for normal development, cellular quality control and immune function. When an embryo develops, the PCD helps form fingers by eliminating cells between digits, without it we would have webbed hands.
Explore different ways by which cells maintain themselves.
At a Glance
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The cell is the basic structural and functional unit of all living
organisms. • Prokaryotic cells do not have a well-defined nucleus, instead their
genetic material is present in a region called the nucleoid. They also lack membrane-bound organelles. • Eukaryotic cells are larger and more complex. They have a well-
defined nucleus and several membrane-bound organelles. • All cells are surrounded by a cell membrane. In addition, cells
of plants, fungi and bacteria have a cell wall outside the cell membrane. • The nucleus of eukaryotic cells contains chromosomes, which are
composed of DNA and associated proteins, which carry genetic information. • All cells are filled with cytoplasm. In eukaryotic cells, cytoplasm
contains several cell organelles, each performing a specific function. • Important cell organelles include the nucleus, endoplasmic reticulum,
mitochondria, golgi apparatus, ribosomes and lysosomes. • Plant cells also contain special organelles called plastids, such as
chloroplasts, leucoplasts and chromoplasts. • Mitosis produces two daughter cells identical to the parent cell. • Meiosis is a two-step division process which produces four daughter
cells, each having half the number of chromosomes. • Normal cells grow in a controlled manner, perform their functions
and die naturally, while cancer cells lose control and keep dividing uncontrollably leading to the formation of tumours.