Cell: The unit of living organisms
Class 9 & 10
Key Learning Objectives
1 Hour (Approximately)
- THE MISSION
There is a crisis happening in your locality where there is a destruction of cells present in all plants and animals that were preserved due to widespread chemical leak in a factory which was later fixed. You are a scientist designated by a research company to solve the crisis by rejuvenating cells by developing a prototype of a plant and animal cell. The prototype of the cells will be taken for research and various other purposes.
Before delving deeper into the structure of the cell and its function, let’s first understand a cell, its discovery, types of organisms on the number of cells, shape and size as well as the various differences between prokaryotic, eukaryotic cells as well as plant and animal cells.
It is the structural and functional unit of life. Cell is termed as the structural unit of life as it provides structure to our body. Cell is considered as the functional unit of life as all the functions of the body take place at cell level.
- The history of discovery of the cell
Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time. 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 the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing 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.
- Types of organisms on the basis of number of cells
There are two kinds of organisms on the basis of cells:
- Unicellular Organisms: The organisms that are made up of single cell and may constitute a whole organism, are named as unicellular organisms. For example: Amoeba, Paramecium, bacteria, etc.
- Multicellular Organisms: The organisms which are composed of a collection of cells that assume function in a coordinated manner, with different cells specialized to perform particular tasks in the body, are named as multicellular organisms. For example: Plants, human beings, animals, etc.
Shape and Size of Cells
- Cells vary in shape and size. They may be oval, spherical, rectangular, spindle shaped, or totally irregular like the nerve cell.
- The size of cells also varies in different organisms. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells.
Types of Cells
The cells can be categorized in two types:
- Prokaryotic Cell 2. Eukaryotic Cell
- Prokaryotic cell
Prokaryotic cells are cells in which the true nucleus is absent. They are primitive and incomplete cells. Prokaryotes are always unicellular organisms. For example, archaebacteria, bacteria, blue green algae are all prokaryotes.
Eukaryotic cells are the cells in which the true nucleus is present. They are advanced and complete cells. Eukaryotes include all living organisms (both unicellular and multicellular organisms) except bacteria and blue green algae.
Difference Between Prokaryotic and Eukaryotic Cells:
Size of the cell is generally small (1-10 mm).
Size of the cell is generally large (5-100 mm).
Nucleus is absent.
Nucleus is present.
It contains a single chromosome.
It contains more than one chromosome.
Nucleolus is absent.
Nucleolus is present.
Membrane bound cell organelles are absent.
Membrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus, lysosomes, etc., are present.
Cell division takes place by fission or budding.
Cell division takes place by mitotic or meiotic cell division.
Structure of Cell
Cell is generally composed of three basic components:
(i) Cell wall and cell membrane
(i) Cell membrane or Plasma membrane:
Plasma membrane is the covering of the cell that separates the contents of the cell from its external environment.
It is a living part of the cell and is present in cells of plants, animals and microorganisms.
It is a very thin, delicate, elastic and selectively permeable membrane.
It is composed of lipid and protein.
As it is a selectively permeable membrane, it allows the flow of limited substances in and out of the cell.
(ii) Cell wall:
The Cell wall is non-living, thick and freely permeable covering made up of cellulose.
It is present in eukaryotic plant cells and in prokaryotic cells.
- It determines the shape and rigidity to the plant cell.
- It protects the plasma membrane.
- It prevents desiccation or dryness in the cell.
- It helps in the transport of various substances in and out of the cell.
Nucleus is dense and spherical organelle.
Nucleus is bounded by two membranes, both forming a nuclear envelope. Nuclear envelope contains many pores known as nuclear pores.
The fluid which is present inside the nucleus is called nucleoplasm.
Nucleus contains chromosomes and chromosomes contain genes which are the centres of genetic information.
- Nucleus controls all the metabolic activities of the cell.
- It regulates the cell cycle.
- Nucleus is the storehouse of genes.It is concerned with the transmission of hereditary traits from the parent to offspring.
It is a jelly-like, viscous, colourless semi-fluid substance that occurs between the plasma membrane and the nuclear membrane.
The aqueous ground substance of cytoplasm is called cytosol that contains a variety of cell organelles and other insoluble waste products and storage products, like starch, glycogen, lipid, etc.
- Protoplasm acts as a store of vital chemicals like amino acids, proteins, sugars, vitamins, etc.
- It is the site of certain metabolic reactions, like glycolysis, synthesis of fatty acids, nucleotides, etc.
Inside the cell there are different parts performing different activities to keep the cell alive and function able. These parts are called Cell organelles. They are explained below:
- Golgi Apparatus:
Golgi apparatus consists of a set of membrane bound, fluid filled vesicles, vacuoles and flattened cisternae (closed sacks).
Cisternae are usually arranged parallel to each other.
- Its main function is to store, modify, package and dispatch the substances.
- It is also involved in the synthesis of cell walls, plasma membrane and lysosomes.
- Endoplasmic Reticulum:
It is a membranous network of tube-like structures extending from nuclear membrane to plasma membrane.
It is absent in prokaryotic cells and matured RBCs of mammals.
There are two types of endoplasmic reticulum:
(i) Rough Endoplasmic Reticulum (RER): Here ribosomes are present on the surface for the synthesis of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): Here ribosomes are absent and are meant for secreting lipids.
- It gives internal support to cells.
- It helps in transport of various substances from nuclear membrane to plasma membrane or vice versa.
- RER helps in synthesis and transportation of proteins.
- SER helps in synthesis and transportation of lipids.
These are extremely small, dense and spherical bodies which occur freely in the matrix (cytosol) or remain attached to the endoplasmic reticulum.
These are made up of ribonucleic acid (RNA) and proteins.
They play a major role in the synthesis of proteins.
They are small rod-shaped organelles.
It is a double membrane structure with outer membrane being smooth and porous whereas inner membrane being thrown into a number of folds called cristae.
They contain their own DNA and ribosomes.
They are absent in bacteria and red blood cells of mammals.
- They are the sites of cellular respiration, hence provide energy for the vital activities of living cells.
- They store energy releases during reactions, in the form of ATP (Energy currency of the cell). Therefore, they are also called ‘power houses’ of the cell.
- Centrosome and Centrioles:
Centrosome is found only in eukaryotic animal cells. It is not bounded by any membrane but consists of centrioles.
Centrioles are hollow cylindrical structures arranged at right angles to each other and made up of microtubules.
Centrioles help in cell division and also help in the formation of cilia and flagella.
Plastids are present in most of the plant cells and absent in animal cells.
They are usually spherical or discoidal in shaped and double membrane bound organelles.
They also have their own DNA and ribosomes.
Plastids are of three types:
(a) Chloroplasts: These are the green coloured plastids containing chlorophyll. Chloroplasts aid in the manufacture of food by the process of photosynthesis.
(b) Chromoplasts: These are the colourful plastids (except green colour).
(c) Leucoplasts: These are the colourless plastids.
- Chloroplasts trap solar energy and utilise it to manufacture food for the plant.
- Chromoplasts impart various colours to flowers to attract insects for pollination.
- Leucoplasts help in the storage of food in the form of starch, proteins and fats.
Lysosomes are small, spherical, sac like structures which contain several digestive enzymes enclosed in a membrane.
They are found in eukaryotic cells mostly in animals.
- Lysosomes help in digestion of foreign substances and worn-out cell organelles.
- They provide protection against bacteria and viruses.
- They help to keep the cell clean.
- 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 suicide bags of a cell
Vacuoles are liquid/solid filled and membrane bound organelles.
In plant cells, vacuoles are large and permanent. In animal cells, vacuoles are small In size and temporary.
In mature plant cells, It occupies 90% space of cell volume.
Due to its size, other organelles, including nucleus shift towards plasma membrane.
- They help to maintain the osmotic pressure in a cell.
- They provide turgidity and rigidity to the plant cell.
They are small and spherical organelles containing powerful oxidative enzymes.
They are bounded by a single membrane.
They are found in kidney and liver cells.
- They are specialized to carry out some oxidative reactions, such as detoxification or removal of toxic substances from cells.
Difference Between Animal Cell and Plant Cell:
Animal cells are generally small in size.
Plant cells are larger than animal cells.
Cell wall is absent.
Plasma membrane of a plant cell is surrounded by a rigid cell wall of cellulose.
Plastids are absent except in the case of protozoan Euglena.
Plastids are present.
Here vacuoles are many, small and temporary.
They have a permanent and large central sap vacuole.
They have centrosomes and centrioles.
They lack centrosomes and centrioles.
Structure of Plant cell and Animal cell:
- Problem statement
The student plays the role of a researcher who will have to design prototypes of a plant and animal cell as there has been a chemical in a locality leading to the death of cells of animals and plants stored in a research facility. The prototypes of the cells prepared will be used for further use in other forms of research. The plant and animal cell should have all the organelles needed for proper functioning to give the other researchers exact clarity on the workings of the cell.
- How to design the game?
- Step I
There is a chemical leak in a locality leading to the death of cells of animals and plants stored in the research facility the student works at. The student is now tasked with the responsibility of preparing a prototype of a plant and animal cell with its differences and various organelles should be constructed and designed.
Hint: Go through the structure and functions of the cells as well as the difference between a plant and animal cell.
- Step II
On completion of the first round of completing the prototype of a plant cell, the gamer receives 100 points. The plant cell should have all the organelles mentioned with clarity and a clear difference between an animal cell should also be highlighted.
- Step III
On completion of structuring the plant cell, the animal cell should be designed and on completion the gamer attains another 100 points, winning the game.
Learning Objective Based Evaluation (0-1)
Higher Order Thinking Skills
Learning Objective Based Evaluation Total Score
Higher Order Thinking Skills Total Score
Learning Objective Based Evaluation
1 point Questions:
Q1. What is the type of cell in which a true nucleus is present?
- Prokaryotic cell
- Eukaryotic cell
- Neither of the tw0
Q2. Which organelle is known as the suicide bag of the cell?
- Endoplasmic reticulum
- None of the above
Q3. What is the main site of cellular respiration?
- None of the above
Q4. Which of the following statements is false about an animal cell?
- Cell wall is absent
- They lack centrosomes and centrioles.
- Animal cells are generally small in size.
Evaluation of HOTS (Higher Order Thinking Skills)
Identification of the Problem Statement:
Q5. What is the goal of the game that you have designed? (1 point)
Q6. Does the game designed solve the problem? Justify your answer. (1 point)
Solution of the Problem Statement:
Q7. What are the reasons that we should understand the structure of the cell? (1 point)
Q8. Why do you think a cell is called the functional unit of an organism? (1 point)
Innovation in the Problem Statement:
Q9. What are the innovative aspects of the solution for which the game is designed? (2 points)
Creativity in the Game Design:
Q10. Describe and explain the unique design elements used in the game. (2 points)
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