CHAPTER- 5 THE FUNDAMENTAL UNIT OF LIFE
NOTES
1. In 1665, Robert Hooke discovered the cell.- In 1674, Leeuwenhoek was the first to observe freely living cells in pond water using an advanced microscope.
- In 1831, Robert Brown discovered the nucleus in the cell.
- In 1839, J.E. Purkinje named the fluid biological substance present in the cell as protoplasm.
- Two biologists, M. Schleiden (1838) and T. Schwann (1839), proposed the cell theory. According to this theory, all plants and animals are made up of cells, which are the fundamental units of life. Virchow (1855) further advanced the cell theory, stating that all cells arise from pre-existing cells.
- After the invention of the electron microscope in 1940, it became possible to understand the complex structure of the cell and many of its organelles.
A. Unicellular Organisms- Organisms made up of a single cell are called unicellular organisms, such as Amoeba, Chlamydomonas, Paramecium, and bacteria.B. Multicellular Organisms- Multicellular organisms consist of many cells that organize to form different organs to perform various functions. Examples- fungi, plants, and animals.
2. Cell- The shape and size of cells are adapted to their specific functions. Some cells can change their shape, such as Amoeba. In some organisms, the cell shape remains constant, such as nerve cells.Each living cell has the ability to perform certain fundamental functions, which is a characteristic of all living beings.- In multicellular organisms, there is division of labor, where different organs of the body perform different functions. For example, the heart pumps blood, and the stomach digests food. Even in unicellular organisms, there is division of labor. Each such cell contains specific components, which are called organelles.
- Due to the organelles, a cell remains alive and performs all its functions. These organelles together form a fundamental unit called the cell.
3. Structural Organization of the Cell- Every cell shows three characteristics: plasma membrane, nucleus, and cytoplasm. All the activities occurring inside the cell and its interactions with the external environment are made possible due to these characteristics.
i) Plasma Membrane/Cell Membrane- This is the outermost layer of the cell, which separates the cell's components from the external environment. The plasma membrane allows certain substances to move in and out, while it also restricts the movement of other substances. The cell membrane is therefore called a selectively permeable membrane.- Some substances, such as carbon dioxide or oxygen, can pass through the cell membrane by the process of diffusion.
A. Diffusion- The process in which substances move from an area of higher concentration to an area of lower concentration is called diffusion. Diffusion plays an important role in the exchange of gases.B. Osmosis- The movement of water molecules through a selectively permeable membrane is called osmosis. The movement of water through the plasma membrane is also influenced by the concentration of dissolved substances in the water.Types of Osmosis- 1. Hypo-osmosis, 2. Iso-osmosis, 3. Hyper-osmosis.
1. Hypotonic solution- If a cell is placed in a dilute solution, water will move into the cell through osmosis. Such a solution is called a hypotonic solution.2. Isotonic solution- If a cell is placed in a medium solution where the concentration of water outside the cell is exactly equal to the concentration of water inside the cell, there will be no net movement of water across the cell membrane. Such a solution is called an isotonic solution.3. Hypertonic solution- If the solution outside the cell is more concentrated than the solution inside the cell, water will move out of the cell through osmosis. Such a solution is called a hypertonic solution.
- Unicellular freshwater organisms and most plant cells absorb water through osmosis.
- The absorption of water by the roots of plants is an example of osmosis.
- The plasma membrane is flexible and is made up of organic molecules such as lipids and proteins.
C. Endocytosis- The flexibility of the cell membrane helps unicellular organisms to intake food and other substances from their external environment. This process is called endocytosis. Amoeba obtains its food through this process.
ii) Cell wall- In plant cells, in addition to the plasma membrane, there is also a cell wall. The plant cell wall is made of cellulose, and it provides structural rigidity to the plants.- The cell wall keeps the cells of plants, fungi, and bacteria intact in relatively hypotonic solutions without bursting.
- Due to the cell wall, plant cells can tolerate changing environments more easily than animal cells.
iii) Nucleus- The nucleus is surrounded by a double-layered membrane called the nuclear membrane. The nuclear membrane has small pores. Through these pores, the cytoplasm inside the nucleus can move out to the area outside the nucleus.- The nucleus contains chromosomes that appear rod-shaped during cell division. The genetic information contained in the chromosomes is passed from the parents to the next generation in the form of DNA molecules. Chromosomes are made of DNA and proteins. The DNA molecule contains all the necessary information for the construction and organization of the cell. The functional segments of DNA are called genes.
- In cells that are not dividing, the DNA exists in the form of chromatin material. Chromatin material is a mass of thread-like structures arranged in a network.
- In some organisms, the nucleus plays an important role in cellular reproduction. In this process, a single cell divides to form two new cells.
Prokaryotic organisms- Organisms whose cells do not have a nuclear membrane are called prokaryotes.Eukaryotic organisms- Organisms whose cells have a nuclear membrane are called eukaryotes.- Prokaryotic cells do not have most of the other organelles that are present in eukaryotic cells.
- In photosynthetic bacteria, chlorophyll is found in membrane-bound vesicles, whereas in eukaryotic cells, chlorophyll is found in plastids.
iv. Cytoplasm- The cytoplasm is surrounded by the cell membrane. It contains many specialized cell components, which are called organelles.- The cytoplasm and the nucleus together are called protoplasm.
- Cell organelles are membrane-bound.
v. Cell organelles-
A. Endoplasmic Reticulum (ER)- The endoplasmic reticulum is a vast network of membrane-bound tubes and sheets.- The structure of the endoplasmic reticulum is similar to that of the plasma membrane. It is of two types: 1. Rough Endoplasmic Reticulum (RER), 2. Smooth Endoplasmic Reticulum (SER).
1. Rough Endoplasmic Reticulum (RER)- Ribosomes are present on it.2. Smooth Endoplasmic Reticulum (SER)- Ribosomes are absent.- Proteins are synthesized on ribosomes. The RER sends the synthesized proteins to other parts of the cell as needed through the ER.
- The SER helps in the formation of fats or lipid molecules. It is very important for cellular functions.
- Some proteins and fats help in the formation of the cell membrane. This process is called membrane biogenesis. The ER always forms a network of tubules.
- In the liver cells of vertebrates, the SER plays an important role in detoxifying toxins and drugs.
B. Golgi apparatus- Described by Camillo Golgi, the Golgi apparatus is a system of membrane-bound flat vesicles that are arranged parallel to each other, which are called cisternae.- These membranes are in contact with the ER membranes and therefore form another part of the complex cellular membrane system.
- The substances synthesized in the ER are packaged in the Golgi apparatus and are sent to various regions both inside and outside the cell.
- The Golgi apparatus also creates lysosomes.
C. Lysosome- It is a membrane-bound structure that contains digestive enzymes. The RER produces these enzymes. The lysosome is the cell's waste disposal system.- Lysosomes contain very powerful digestive enzymes that break down complex substances into simpler ones. They are also called suicide bags.
D. Mitochondria- Mitochondria are the powerhouses of the cell. They are made up of a double membrane. The outer membrane is porous, while the inner membrane is highly folded. These folds increase the surface area for the chemical reactions that produce ATP.- Mitochondria provide energy in the form of ATP.
- The body uses the energy stored in ATP to create new chemical compounds and for mechanical work.
- Mitochondria have their own DNA and ribosomes. Therefore, they produce some of their own proteins.
E. Plastids- Plastids are found only in plant cells. They are of two types: chromoplasts and leukoplasts.I. Chromoplast (Colored Plastid)- The chromoplast that contains the pigment chlorophyll is called a chloroplast. It is essential for photosynthesis in plants.II. Leukoplast (Colorless and White Plastid)- Leukoplasts are primarily organelles in which substances such as starch, oil, and proteins are stored.- The internal structure of the chloroplast contains many membrane layers that are located in the stroma.
- Like mitochondria, plastids also have their own DNA and ribosomes.
F. Vacuoles- Vacuoles are storage sacs for solid or liquid substances. They are small in animal cells, while in plant cells, vacuoles are much larger.- In some plant cells, the central vacuole can occupy 50% to 90% of the cell's volume.
- The vacuoles in plant cells are filled with cytoplasm, and they provide turgidity and rigidity to the cells.
- Many substances essential for plants are located in the vacuole. These include amino acids, sugars, various organic acids, and some proteins.
- In some unicellular organisms, specialized vacuoles play important roles in expelling excess water and some waste products from the body.
4. Cell division- It creates new cells for the growth of organisms, replacing old, dead, and damaged cells, and forms gametes for reproduction.- The process of forming new cells is called cell division. It is of two types: 1. Mitosis, 2. Meiosis.
1. Mitosis- The process of cell division through which most cells divide for growth is called mitosis. In this process, each cell, also known as the mother cell, divides to form two identical daughter cells. The number of chromosomes in these daughter cells is the same as in the mother cell. This helps in the growth of organisms and the repair of tissues.
2. Meiosis- Specialized cells in the reproductive organs or tissues of animals and plants divide to form gametes, which, after fertilization, create offspring.- In meiosis, there are two successive divisions. This division results in the formation of 4 new cells. The number of chromosomes in these new cells is half of that in the mother cell.
- In 1674, Leeuwenhoek was the first to observe freely living cells in pond water using an advanced microscope.
- In 1831, Robert Brown discovered the nucleus in the cell.
- In 1839, J.E. Purkinje named the fluid biological substance present in the cell as protoplasm.
- Two biologists, M. Schleiden (1838) and T. Schwann (1839), proposed the cell theory. According to this theory, all plants and animals are made up of cells, which are the fundamental units of life. Virchow (1855) further advanced the cell theory, stating that all cells arise from pre-existing cells.
- After the invention of the electron microscope in 1940, it became possible to understand the complex structure of the cell and many of its organelles.
A. Unicellular Organisms- Organisms made up of a single cell are called unicellular organisms, such as Amoeba, Chlamydomonas, Paramecium, and bacteria.
- In multicellular organisms, there is division of labor, where different organs of the body perform different functions. For example, the heart pumps blood, and the stomach digests food. Even in unicellular organisms, there is division of labor. Each such cell contains specific components, which are called organelles.
- Due to the organelles, a cell remains alive and performs all its functions. These organelles together form a fundamental unit called the cell.
3. Structural Organization of the Cell- Every cell shows three characteristics: plasma membrane, nucleus, and cytoplasm. All the activities occurring inside the cell and its interactions with the external environment are made possible due to these characteristics.
i) Plasma Membrane/Cell Membrane- This is the outermost layer of the cell, which separates the cell's components from the external environment. The plasma membrane allows certain substances to move in and out, while it also restricts the movement of other substances. The cell membrane is therefore called a selectively permeable membrane.
- Some substances, such as carbon dioxide or oxygen, can pass through the cell membrane by the process of diffusion.
A. Diffusion- The process in which substances move from an area of higher concentration to an area of lower concentration is called diffusion. Diffusion plays an important role in the exchange of gases.
B. Osmosis- The movement of water molecules through a selectively permeable membrane is called osmosis. The movement of water through the plasma membrane is also influenced by the concentration of dissolved substances in the water.
- Unicellular freshwater organisms and most plant cells absorb water through osmosis.
- The absorption of water by the roots of plants is an example of osmosis.
- The plasma membrane is flexible and is made up of organic molecules such as lipids and proteins.
- The cell wall keeps the cells of plants, fungi, and bacteria intact in relatively hypotonic solutions without bursting.
- Due to the cell wall, plant cells can tolerate changing environments more easily than animal cells.
- The nucleus contains chromosomes that appear rod-shaped during cell division. The genetic information contained in the chromosomes is passed from the parents to the next generation in the form of DNA molecules. Chromosomes are made of DNA and proteins. The DNA molecule contains all the necessary information for the construction and organization of the cell. The functional segments of DNA are called genes.
- In cells that are not dividing, the DNA exists in the form of chromatin material. Chromatin material is a mass of thread-like structures arranged in a network.
- In some organisms, the nucleus plays an important role in cellular reproduction. In this process, a single cell divides to form two new cells.
Prokaryotic organisms- Organisms whose cells do not have a nuclear membrane are called prokaryotes.
- Prokaryotic cells do not have most of the other organelles that are present in eukaryotic cells.
- In photosynthetic bacteria, chlorophyll is found in membrane-bound vesicles, whereas in eukaryotic cells, chlorophyll is found in plastids.
iv. Cytoplasm- The cytoplasm is surrounded by the cell membrane. It contains many specialized cell components, which are called organelles.
- The cytoplasm and the nucleus together are called protoplasm.
- Cell organelles are membrane-bound.
v. Cell organelles-
A. Endoplasmic Reticulum (ER)- The endoplasmic reticulum is a vast network of membrane-bound tubes and sheets.
- The structure of the endoplasmic reticulum is similar to that of the plasma membrane. It is of two types: 1. Rough Endoplasmic Reticulum (RER), 2. Smooth Endoplasmic Reticulum (SER).
- Proteins are synthesized on ribosomes. The RER sends the synthesized proteins to other parts of the cell as needed through the ER.
- The SER helps in the formation of fats or lipid molecules. It is very important for cellular functions.
- Some proteins and fats help in the formation of the cell membrane. This process is called membrane biogenesis. The ER always forms a network of tubules.
- In the liver cells of vertebrates, the SER plays an important role in detoxifying toxins and drugs.
B. Golgi apparatus- Described by Camillo Golgi, the Golgi apparatus is a system of membrane-bound flat vesicles that are arranged parallel to each other, which are called cisternae.
- These membranes are in contact with the ER membranes and therefore form another part of the complex cellular membrane system.
- The substances synthesized in the ER are packaged in the Golgi apparatus and are sent to various regions both inside and outside the cell.
- The Golgi apparatus also creates lysosomes.
C. Lysosome- It is a membrane-bound structure that contains digestive enzymes. The RER produces these enzymes. The lysosome is the cell's waste disposal system.
- Lysosomes contain very powerful digestive enzymes that break down complex substances into simpler ones. They are also called suicide bags.
- Mitochondria provide energy in the form of ATP.
- The body uses the energy stored in ATP to create new chemical compounds and for mechanical work.
- Mitochondria have their own DNA and ribosomes. Therefore, they produce some of their own proteins.
E. Plastids- Plastids are found only in plant cells. They are of two types: chromoplasts and leukoplasts.
- The internal structure of the chloroplast contains many membrane layers that are located in the stroma.
- Like mitochondria, plastids also have their own DNA and ribosomes.
- In some plant cells, the central vacuole can occupy 50% to 90% of the cell's volume.
- The vacuoles in plant cells are filled with cytoplasm, and they provide turgidity and rigidity to the cells.
- Many substances essential for plants are located in the vacuole. These include amino acids, sugars, various organic acids, and some proteins.
- In some unicellular organisms, specialized vacuoles play important roles in expelling excess water and some waste products from the body.
4. Cell division- It creates new cells for the growth of organisms, replacing old, dead, and damaged cells, and forms gametes for reproduction.
- The process of forming new cells is called cell division. It is of two types: 1. Mitosis, 2. Meiosis.
1. Mitosis- The process of cell division through which most cells divide for growth is called mitosis. In this process, each cell, also known as the mother cell, divides to form two identical daughter cells. The number of chromosomes in these daughter cells is the same as in the mother cell. This helps in the growth of organisms and the repair of tissues.
- In meiosis, there are two successive divisions. This division results in the formation of 4 new cells. The number of chromosomes in these new cells is half of that in the mother cell.
