CBSE Class 9 Science Chapter-2 Notes || Fundamental Units of Life
1. Discovery of Cell
- Robert Hooke (in 1665) examined a thin slice of cork under the primitive microscope. He observed that cork consists of small box-like structures resembling a honeycomb. He called these boxes cells.
- The cell is a Latin word that means ‘a little room. The substance called cork comes from the bark of a tree.
Major Landmarks Related to Cell Discovery
- Leeuwenhoek 1674 Discovered free-living cells in pond water using an improved microscope.
- Robert Brown 1831 Discovered nucleus in the cell.
- Schleiden and Schwann 1838-1839 Presented cell theory, which states that all plants and animals are composed of cells and are the basic unit of life.
- Purkinje:- 1839 Coined the term ‘protoplasm’ for the fluid substance of the cell.
- Virchow:- 1855 Expanded the cell theory by suggesting that all cells arise from pre-existing cells.
2. Features of Cells
- Small organisms like Amoeba, Chlamydomonas, Paramecium and bactería are made up of single cells called unicellular organisms.
- In multicellular animals, such as some fungi, plants and animals, many cells group together in a single body and perform different functions to form various body parts.
- The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changed shapes, while nerve cells have a typical shape.
- Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms besides some special functions.
- Division of labour is also present in each cell. It contains certain specific components within it known as cell organelles.
- Each kind of cell organelle performs a special function, such as making new material in the cell.
3. Structural Organisation of a Cell
- Each cell has at least three features, i.e. plasma membrane, nucleus and cytoplasm.
- All the activities inside the cell and interactions of the cell with its environment are possible due to these features.
Plasma Membrane or Cell Membrane
- It is the outermost covering of the cell that separates the contents of the cell from its external environment.
- It is extremely thin, elastic and selectively permeable.
- It is made up of lipids and proteins. Gases like CO, and O, move across by diffusion process.
- Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration.
- The water molecules move from the region of high water concentration to the region of low water concentration through the semipermeable membrane by the process of osmosis.
- lf a cell is placed in a hypotonic solution, i.e. the solution having high water concentration than the cell, it will swell up as water enters from the hypotonic solution to inside the cell.
- lt a cell is placed in a hypertonic solution, i.e. the solution having low water concentration than the cell, it will shrink because the cell will lose water.
- If the cell is placed in an isotonic solution, i.e. the medium has exactly the same water concentration as the cell, then it will stay the same size.
- Endocytosis is the process by which cell engulf food and other material from its external environment. This is possible due to the flexibility of the cell membrane, e.g. Amoeba.
(i) Cell Wall
- Plant cells have a rigid outer covering called the cell wall.
- It is composed of cellulose, which is a complex substance that provides structural strength to the plants.
- Plasmolysis is the phenomenon in which a living plant cell loses water through Osmosis. During this, there is shrinkage or contraction of the contents of the cell away from the cell wall.
- Plants, fungi and bacteria can withstand very dilute external media, without bursting due to the presence of a cell wall.
- It has a double-layered covering called a nuclear membrane.
- The nuclear membrane has pores, which allow the transfer of material from inside to outside.
- It contains chromosomes that contain information for the inheritance of features from parents to the next generation in the form of DNA molecules.
- Chromosomes are made up of DNA and protein.
- DNA Contain hereditary information necessary for constructing and organising cells.
- Genes are functional segments of DNA.
- A cell that is not dividing contains elongated thread-like material known as chromatin, which gets organised into chromosomes during cell division.
- The nucleus plays an important role in cell reproduction and cell development.
- Bacteria contain a less developed nuclear region due to the absence of a nuclear membrane. this region contained genetic material as nucleoids.
- Such organisms, whose cell wall lack a nuclear membrane, are called prokaryotes. They also lack other cytoplasmic organelles.
- Organisms with nuclear membranes are known as eukaryotes.
- It is a fluid inside a cell membrane that contains specialised cell organelles. Cell organelles are enclosed by the plasma membrane, but in prokaryotes membrane-bound organelles are absent, while eukaryotes have membrane-bound organelles.
- Viruses lack any membrane and do not show characteristics of life until they enter a living body and use their cell machinery to multiply.
4. Cell Organelles
- The cellular components of the cell are called cell organelles. These include both membrane and non-membrane organelles within the cells and are distinct in their functions and structures.
- The important cell organelles are the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.
a) Endoplasmic Reticulum (ER)
- It is a large network of membrane-bound tubes and sheets.
- It is of two types, 1.e. Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic
- Rough ER looks rough because of the presence of ribosomes, which are sites of protein manufacture.
- Smooth ER helps in the manufacture of fat molecules and lipids.
- .These proteins and lipids helps in the building. the cell membrane. This proces is known as membrane biogenesis.
- ER also serves as channels for the sport of materials from the cytoplasm to the nucleus and vice-versa.
- It also provides a framework for biochemical activities.
- In the liver, ER plays a crucial role in detoxifying many poisons and drugs.
b) Golgi Apparatus
- Golgi apparatus or Golgi complex was first described by Camillo Golgi. It consists of membrane-bound, fluid-filled vesicles, vacuoles and a stack of many flattened closed sacs (cisternae).
- It helps in the storage, modification and packaging of products in vesicles.
- It also forms complex sugars from simple sugar.
- It is also involved in the formation of lysosomes
- These are small-sized vesicles containing hydrolytic enzymes (digestive enzymes, are bounded by a single membrane. these are also known as ‘suicidal bags’ or ‘dig bags.
- They help to keep the cell clean by digesting any foreign material as well as worn-out organelles.
- It is known as the powerhouse of the cell’ as it releases ATP molecules.
- ATP is known as ‘energy currency of the cell and body uses it for making new chemical compounds and mechanical work.
- It is a double membrane-bound organelle whose outer membrane is porous and the inner membrane is deeply folded.
- These folds create a large surface area for ATP generating chemical reactions.
- They have their own DNA and ribosomes and can make their own proteins.
- These are present only in plant cells. The internal organisation of plastids consists of numerous membrane layers embedded in a material called the stroma.
- Like the mitochondria, they also has their own DNA and ribosomes.
- The plastids are of the following three types
1) Leucoplasts are colourless and store food in the form of starch, protein and fats.
2) Chromoplasts They are colourful plastids like red, orange, yellow, etc., except green.
3) Chloroplasts They are green coloured plastids because of the presence of chlorophyll. They trap the solar energy and perform the function of photosynthesis.
- They are storage sacs for solids or liquids such as amino acids sugars, organic acid and some protein.
- Plant cells have large vacuoles compared to animal cells.
- They provide turgidity and rigidity to the cells.
- In Amoeba, the vacuole contains food items known as food vacuoles.
- It also play important role in expelling excess water and waste from the cell.