Fundamental Unit Of Life Short Notes | Class 9
Introduction
- Cells are the smallest functional and structural unit of life.
- Invented by Robert Hooke in the year 1665, cells are able to conduct all necessary actions required for the maintenance of life.
Cell Theory
- A cell is the basic and the smallest unit of life responsible for conducting all processes associated with life. Cells are regarded as building blocks of a living organism.
- Every living organism consists of cells.
- Cells are produced by other cells due to cell division.
- Cells contain genetic material transferred to the daughters cells during reproduction.
- The biochemical composition of every cell is essentially the same
>>Type Of Organism
There are two type of organism
- Unicellular Organism
- Multicellular Organism
1. Unicellular organism : Unicellular organisms are comprised of a single cell that is responsible for the entire life activity.
Examples: bacteria, protozoa, algae, and fungi.
Microorganisms are usually not seen to the naked eyes but they provide significant functions within the ecosystem.
2. Multicellular Organism :
Multicellular organisms consist of many cells that work together.
They vary in shape, size, and function.
Examples include plants, animals, and fungi.
<< Types Of cells
There are two types of cells
- Prokaryotes
- Eukaryotes
(i) Prokaryotes :
These cells are primitive and have no defined nucleus.
Examples include bacteria.
Prokaryotes are always unicellular organisms.
(ii) Eukaryotes :
These cells are more advanced and have a defined nucleus.
Eukaryotic organisms are unicellular or multicellular organisms. Examples include amoebae, plants, animals, fungi.
Structural Organisation
Theory Plasma Membrane Or Cell Membrane
- The outermost envelope surrounding the cell and its organelles.
- Comprises a double-layered phospholipid bilayer.
- It is selectively permeable: specific substances can move in and out of the cell.
- It functions as a mechanical barrier, retaining the shape of the cell, and preventing its bursting.
- Protects the cell from pathogenic substances.
- Remember, the plasma membrane is what makes a cell function and talk to other cells!
Cell Wall
A cell wall is a component necessary for plant cells (and other fungi and some prokaryotes).
<< Function:
The cell wall has several crucial functions:
- Maintains the Shape and Provides Support: It acts as a kind of structural scaffolding that prevents the cell from collapsing.
- It acts as a protection to the cell and its organelles by preventing the penetration of external stressors.
- Regulation: Helps regulate water balance through controlling entry and exit of water from the cell.
<<Composition:
- Cellulose is the major constituent of the plant cell wall.
- Hemicelluloses, pectins, and lignin are the other constituents
Nucleus
- Definition: Nucleus is a double membrane-bound eukaryotic cell organelle that carries genetic material. It is derived from a Latin word meaning "kernel of a nut."
- Structure: The nucleus is surrounded by the nuclear envelope and contains chromosomes with DNA-the plan for building cell parts.
- Functions:
- Genetic Information: It carries hereditary information and controls growth and reproduction.
- DNA Replication: This is where DNA is duplicated to prepare for cell division.
- Transcription: Creates various types of RNA from DNA.
Cytoplasm
Definition: The cytoplasm is the fluid-like substance filling up cells. It includes the cytosol (comprising filaments, ions, proteins, and macromolecular structures) along with other organelles suspended within it.
Structure:
- Cytoplasm is a jelly-like semi-liquid substance that connects the nucleus and the cell membrane.
- Cell organelles like the endoplasmic reticulum, mitochondria, ribosomes, and vacuoles are suspended in cytoplasm.
- It can be viewed through a microscope with the help of staining techniques .
Functions:
- The cytoplasm is where the chemical reaction takes place within the cell.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a very extensive network of tubules and sheets, membranous.
• It is shaped like a long tubules or round or oval bags vesicles.
Types of Endoplasmic Reticulum:
- Rough ER (RER): It has small particles called ribosomes attached to its surface, so it appears "rough." This is very important for protein synthesis and folding. It also maintains quality control concerning proper folding of proteins.
- Smooth ER (SER): Smooth ER does not have ribosomes and thus appears "smooth." Some of its functions are:
-Synthesis of essential lipids, such as phospholipids and cholesterol.
-Involved in the synthesis and secretion of steroid hormones.
-Metabolizing carbohydrates.
-Storing and releasing calcium ions, which are crucial for the nervous and muscular systems.
Structure:
- ER comprises a large network of membranous tubes and sheets.
- It can be in three forms; cisternae (closed fluid filled sacs), vesicles, and tubules.
Golgi Apparatus
Golgi apparatus is one of the vital organelles found in eukaryotic cells.
- Structure:
- The Golgi apparatus comprises a series of flattened, stacked pouches known as cisternae
- It is located in the cytoplasm close to the cell nucleus
- Function:
- Transport and Processing: The Golgi apparatus transports, modifies, and packs proteins and lipids.
- It processes the proteins it receives from the ER.
- Packaging: It then packages those modified proteins into vesicles with a membrane and sends them out to the destination within or outside the cell.
Lysosomes
Lysosomes- cool organelles found inside eukaryotic cells. ????
- Lysosomes are small round pouch like organelles that are distributed throughout the cytoplasm with uniformity.
- Each lysosome is a small vesicle with one membrane surrounding it.
- They consist of enzymes which break down the organic material
- Lysosomes are composed of RER-rough endoplasmic reticulum.
Functions:
- Digestive Bags: Lysosomes break down harmful foreign particles inside the cell. They digest unwanted bacteria and viruses.
- Cellular Cleanup: Lysosomes get rid of used up or broken down cellular parts to allow space for new components.
- Suicide Bags: During cell lysis (e.g., damage), lysosomes can rupture, and their enzymes may degrade their own cells.
Mitochondria
- Definition: Double membrane-bounded organelles of eukaryotic cells.
Structure:
- Outer Membrane: That surrounds the mitochondrion and harbors porins.
- Inner Membrane: Complex, folded into cristae to facilitate energy production.
- Matrix: Fluid gel with enzymes, ribosomes, and mitochondrial DNA.
Functions:
- ATP Production or Synthesis.
- Metabolic Control.
- Detoxification (ammonia in hepatocytes).
- Apoptosis or programmed cell death.
Plastids
- Definition: Plastids are double-membrane organelles responsible for manufacturing and storing food within plant cells and algae.
Types of Plastids:
- Chloroplasts: Biconvex, semi-porous organelles where photosynthesis occurs. They synthesize food using sunlight.
- Chromoplasts: Found in flowering plants, ageing leaves, and fruits. They contain carotenoid pigments, giving leaves and fruits their vibrant colors.
- Gerontoplasts: Chloroplasts that transform during leaf aging, converting into other organelles.
- Leucoplasts: Colorless organelles present in non-photosynthetic parts such as roots. They store starch, lipids, and proteins.
Vacuoles
- Definition: Membrane-bound structures that contain cell sap.
Functions:
- Storage: It holds salts, minerals, pigments, proteins, and lipids.
- Turgor Pressure: It helps in maintaining the shape and resilience of the cell.
- Endocytosis and Exocytosis: It regulates the transportation of substances.
- Lysosomes: It facilitates digestion.
Cell Division
Let's talk about the amazing cell division.
- Mitosis: Cells utilize mitosis to make identical copies of themselves. It is performed in nearly all body cells except for the eye cells, skin cells, hair cells, and muscle cells.
- Meiosis: Meiosis results in the formation of gametes (sperm or egg cells) instead of identical daughter cells like in mitosis.
- Binary Fission: In single-celled organisms, such as bacteria, the organism duplicates itself for reproduction.
Stages of the Cell Cycle:
- Interphase: What was once called the resting phase is now understood to be a very active period. It encompasses:
-G1 Phase (Gap 1): Metabolically active growth phase.
S Phase (Synthesis): DNA replication.
-G2 Phase (Gap 2): Protein synthesis.
- M Phase (Mitosis Phase): Actual cell division, comprising:
Prophase
Metaphase
Anaphase
Telophase
Note: Cell division is the basis of growth, repair, and reproduction.