The Cell
Class 6 | Science — Biology | Unit 5
Class 6 Unit 5 Science
Unit 5. The Cell
An explanation of the above flow chart begins
Types of Cell
There are two types of cells
1. Prokaryotic cells
2. Eukaryotic cells
The Prokaryotic cells does not have true nucleus
Examples of Prokaryotic cells Bacteria and Cyanobacteria
Eukaryotic cell has a true nucleus
Examples of plant cell and animal cell
An explanation of the above flow chart ended here.
Learning Objectives
To know that all living things are made up of cells
To observe the cell structure using microscope
To understand the structure of a cell
To explain the components of a cell
To understand the structural difference between animal and plant cell
Introduction
Observe the two pictures given above. Do you observe any similarity between them? Close your eyes and imagine a brick wall. What is the basic building block of the wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks, and are named as "Cells". The cell is the basic structural and functional unit of every living organism. The cell is self - sufficient to carry out all the fundamental and essential functions of an organism.
5.1 The Cell
All living things are made of one or more cells. There are variety of cell types however, they all have some common characteristic features.
More to Know
Can you see a cell with your naked eye? Cells are very minute and said to be microscopic and cannot be seen with our naked eyes. They can be observed only through a specialized scientific instrument called "microscope". Now a days an electron microscope is used to magnify and observe the cells.
5.1.1 Discovery of the cell
The Englishman Robert Hooke was a scientist, mathematician, and inventor. He improved microscope which was used in those days, and built a compound microscope. He placed water-lens beside the microscope to focus the light from an oil-lamp on specimens to illuminate them brightly. So that he was able to see the minute parts of the objects clearly. One day Hooke made thin sections of the cork and observed them through his microscope. He observed many small identical chambers which were hexagonal in shape. He was surprised. After that he observed many objects like Butterfly's wings, Bee's compound eyes etc., Based on this observation, Hooke published a book named Micrographia in the year 1665, where he first used the term Cell. He described the structure of tissue using the term cell. In Latin the word 'cellua' means a small chamber. The branch of science that deals with the study of cells is called 'Cell Biology'.
5.2 The Structural Organization of The Cell
A typical cell consists of three major parts:
1. An outer cell membrane.
2. A liquid cytoplasm.
3. A nucleus.
Analogous to the body's internal organ, like eyes, heart, lungs, etc., organelles are specialized structures and perform valuable functions necessary for normal cellular operation. Many distinct structures called Organelles lie within the cell.
5.2.1 Size of the cell
The size of cells may vary from a micrometer (a million of a meter) to a few centimeters. Most cells are microscopic and cannot be seen with the naked eye. They can be observed only through the Microscope. Smallest size of the cell is present in Bacteria. The size of the bacterial cell ranges from 0.01 micrometer to 0.5 micro meter.
Activity 1
Aim: To observe the structure of a single cell (Hen's egg).
Materials Needed: A Hen's egg and a plate.
Method: Crack the shell and break open the egg in a plate.
Observation: The egg has a yellow part and a transparent part surrounding it. The white transparent part (albumin) is jelly-like and represents the cell's cytoplasm, while the yellow part (yolk) is thicker and represents the cell's nucleus. On the internal side of the shell can be seen a thin membranelike structure, which represents the cell membrane.
On the other hand, the largest cell is the egg of an ostrich with 170-millimeter width. We can see this with the naked eye. In Human body the nerve cells are believed to be the longest cells.
Do You Know
Cell size has no relation to the size of an organism. It is not necessary that the cells of, say an Elephant be much larger than those of a Mouse.
5.2.2 Shapes
Cells are of different shapes. For example, some shapes are given in the below pictures.
5.2.3 Number
The number of cells present in different organisms may vary. Organisms may be either unicellular (single cell) or multicellular. Organisms such as Bacteria, Amoeba, Chlamydomonas, and Yeast are unicellular. On the other hand, organisms such as Spirogyra, Mango, and Human beings are multicellular. (that is) made up of a few hundreds to millions of cells.
Do You Know
Approximate number of cells in the Human body is 3.7 Multiply 1013 or 37,000,000,000,000. Ranges of cell size (From small to Large)
5.3 Types of cell
Generally, cells are classified into two types. First one is Prokaryotic cell. It has no true nucleus and no nuclear membrane. Another one is Eukaryotic cell. It has true nucleus consisting of nuclear membrane.
5.3.1 Prokaryotic cell
The unicellular organisms like Bacteria has Prokaryotic cells. It has no true nucleus. This type of nucleus is called as nucleoid. No nuclear membrane is around this nucleoid. These cells were the first form of life on Earth. It is ranging from 0.003 to 2.0 micro meter in diameter. Example. Escherichia coli bacteria.
5.3.2 Eukaryotic cell
Cell which has true nucleus is called as eukaryotic cell. It is bigger than prokaryotic cell. Its organelles are bounded by membrane. Example. Plants, animals, most of the fungi and algae.
Activity 2
Aim: To observe onion peel cells under a microscope
Materials Required: Glass slide, cover slip, onion, iodine solution, knife and microscope.
Procedure: Take an onion and cut it into two halves along its length. Take out one of its fleshy leaves. With the help of a pair of forceps, remove a transparent, thin peel from the inner surface of the leaf. Take a glass slide and put a drop of water at the centre. Place the peel on the drop of water. Pour a drop of iodine solution on the peel. Now place a cover slip over the material. Observe under the microscope.
Observation: You will be able to see rectangular cells of the onion peel, with a nucleus in each of them.
Differences between Prokaryotic cell and Eukaryotic cell
| Prokaryotic cell | Eukaryotic cell |
|---|
| It's diameter ranges from 1 to 2 micron | It's diameter ranges from 10 t0 100 micron |
| Absence of membrane bound organelles | Presence of membrane bound organelles |
| Nucleus is not surrounded by nuclear membrane | True nucleus is surrounded by nuclear membrane |
| Absence of nucleoli | Presence of nucleoli |
5.3.3 Plant cell and Animal cell
Both plant and animals are made up of cells. Both cells are eukaryotic in nature, having a well-defined membrane – bound nucleus.
| Plant cell | Animal cell |
|---|
| It is usually larger in size. | Animal cells are generally smaller than plant cells. |
| It is hard in nature. | It is not so hard as plant cell. |
| Plant cells have a cell wall in addition to their cell membrane. | Cell wall is absent. |
| Plant cells have chloroplast which contain chlorophyll | Chloroplast is absent. |
| Plant cells have large vacuoles. | An animal cell may have many small vacuoles. |
| Centrioles are absent. | Centrioles are found in animal cells. |
3 Dimension - cell structure
1. How does a cell look like?
2. What is its shape and size?
We can see the three sides of the cell structure. You can also view the size, shape and location on the organelles of the cell. 3-D view is appealing because it is more like reality. In 3-D, view we can see the entire view of the cell. It exposes the accurate size and shape and shows the correct location of the cell organelles.
Activity 3
Aim: To rectify the variation between 2-D shape and 3-D shape.
Materials required: Polythene bag, water, marble ball (golli gundu)
Procedure: Take a polythene bag with water. Put a marble ball into the polythene bag. Then draw a picture in your note book about this task. If you draw a picture in round shape. It will be called 2-Dimensional picture. If you draw a picture in spherical shape it is called 3-Dimensional.
Result: Now you can realize your misconception. So, the animal cells are spherical in shape and structure, not in a round shape.
5.3.4 Cell components and their functions
| Serial number | Cell Components | Main Functions | Special Name |
|---|
| 1 | Cell wall | Surrounds and protects the cell Make the cell stiff and strong | Supporter and protector |
| 2 | Cell membrane | Holds and protects the cell Controls the movement of materials in and out of the cell | Gate of the cell |
| 3 | Cytoplasm | A watery, gel-like material in which cell parts move | Area of movement |
| 4 | Mitochondria | Produce and supply most of the energy for the cell | Power house of the cell |
| 5 | Chloroplasts | Contain green pigment chlorophyll Capture the energy of sunlight and use it to produce food for the cell by photosynthesis. | Food producers for the cell (Plant cell) |
| 6 | Vacuoles | Store food, water, and chemicals | Storage tanks |
| 7 | Nucleus | Acts as 'brain' of the cell Regulates and controls all the cell activities | Control centre |
| 8 | Nucleus membrane | Surrounds and protects the nucleus control the movement of materials in and out of the nucleus | Gate of the nucleus |
Points to Remember
Cells are the basic units of all living organisms.
There are two major cell types such as prokaryotic and eukaryotic cell.
Both plant and animal cells have unique organelles which are capable of carrying out specialized functions.
Plant cells have two unique components such as cell wall and chloroplasts compared to animal cells.
ICT Corner
The Cell
Through this activity you will be able to understand the differences between Plant and Animal Cell, their organelles and their functions.
Step 1: Use the given URL in the browser. 'What do Cells do? Page will open. Click the Start Button to begin the activity.
Step 2: Click continue to proceed to the activity, a column with cell organelles is given. Your task is to build a plant cell and animal cell. Roll the mouse over each organelle to learn about it.
Step 3: Use the mouse to drag the appropriate organelles to build the cell.
Step 4: After finishing the animal cell, continue the same process to finish the plant cell.
What do Cells do?
URL: http://sepuplhs.org/high/sgi/teachers/cell_sim.html
*Pictures are indicative only
Evaluation
Exercise I. Choose the appropriate answer
1. The unit of measurement used for expressing dimension (size) of cell is ____
a. centimeter b. millimeter c. micrometer d. meter
2. Under the microscope Priya observes a cell that has a cell wall and distinct nucleus. The cell that she observed is
a. a plant cell b. an animal cell c. a nerve cell d. a bacteria cell
3. The 'control centre' of the eukaryotic cell is
a. Cell wall b. Nucleus c. Vacuoles d. Chloroplast
4. Which one of the following is not an unicellular organism?
a. Yeast b. Amoeba c. Spirogyra d. Bacteria
5. Most organelles in an eukaryotic cell is found in the
a. Cell wall b. cytoplasm c. nucleus d. Vacuole Exercise II. Fill in the Blanks
1. The instrument used to observe the cell is .
2. I take part in food production of a cell. Who am I? .
3. I am like a policeman. Who am I. .
4. The Term "cell" was coined by .
5. The egg of an Ostrich is the single cell.
Exercise III. True or False. If False, give the correct answer. 1. A cell is the smallest unit of life. 2. Nerve cell is the longest cell 3. Prokaryotes were the first form of life on earth. 4. The organelles of both plants and animals are made up of cells. 5. New cells are produced from the existing cells. Exercise IV. Match the following
| 1. Control center | |
| 2. Food producer (Plant cell) | |
| 3. Gate of the nucleus | |
| 4. Gate of the cell | |
| 5. Energy producer | |
Options: Cell membrane, Chloroplasts, Nucleus, Mitochondria, Nuclear membrane
Exercise V. Arrange in a correct sequence
1. Elephant, Cow, Bacteria, Mango, Rose plant.
_______ _______ _______ _______ _______
2. Hen's Egg, Ostrich's egg, Insect's egg.
_______ _______ _______ Exercise VI. Analogy
1. Prokaryote is Bacteria : Eukaryote is
2. Spirogyra is Plant cell : Amoeba is
3. Food producer is Chloroplasts : Power house is
Exercise VII. Give very short answer
1. Who discovered the cell in 1665?
2. What type of cells do we have?
3. What are the essential components of a cell?
4. What are the organelles found only in plant cell?
5. Give any three examples of eukaryotic cell?
6. Which one is called as "Area of movement"?
7. Shiva said "Bigger onion has larger cells when compared to the cells of smaller onion"! Do you agree with his statement or not? Explain Why?
Exercise VIII. Give short answer
1. Why cells are called building blocks of life?
2. Identify any four parts of the Plant cell.
3. Distinguish between prokaryotic and eukaryotic cells
4. Make sketches of animal and plant cells which you observe under microscope.
5. Write about the contribution of Robert Hooke in cell biology.
Exercise IX. Answer in detail
1. Tabulate any five cell organelles and their function.
2. Draw a neat labelled diagram of a prokaryotic cell.
Exercise X. Project
1. Use your imagination and create 3-D model of a plant cell?
2. You can use numerous food materials such as jelly and some cake to make a cell body. Cell organelles can be made using nuts and dry fruits. You can display the model in your class room and invite teachers or students from other classes. If they rise questions on the project try to give answer.
📝 Summary
- All living things are made up of cells, which are the basic structural and functional units of every living organism and can only be observed through microscopes due to their microscopic size.
- There are two main types of cells: prokaryotic cells (without true nucleus, like bacteria and cyanobacteria) and eukaryotic cells (with true nucleus, like plant and animal cells).
- Robert Hooke discovered cells in 1665 when he observed cork sections through his microscope, seeing hexagonal chambers which he called "cells" (from Latin 'cellua' meaning small chamber) and published his findings in a book called Micrographia.
- A typical cell consists of three major parts: an outer cell membrane, liquid cytoplasm, and a nucleus, along with specialized structures called organelles that perform various cellular functions.
- Cell sizes vary greatly, from bacteria measuring 0.01 to 0.5 micrometers (smallest) to ostrich eggs with 170-millimeter width (largest), with most cells being microscopic and invisible to the naked eye.