THE FUNDAMENTAL UNIT OF LIFE in 1 Shot | FULL Chapter Animation | Class 9th Biology | NCERT Science

Cells are the fundamental units of life, first discovered by Robert Hooke in 1665 when he observed cork under a microscope and identified small box-like structures he named cells. All living organisms consist of cells; unicellular organisms like amoeba and bacteria have only one cell, while multicellular organisms develop from a single cell into various types such as muscle or blood cells. The diversity in size and shape among these different cell types is determined by their specific functions within the organism's systems, which exemplifies division of labor at both cellular and systemic levels. Each cell contains organelles that perform essential tasks to maintain life, including producing materials and eliminating waste.

Cytoplasm, a jelly-like substance within the cell membrane, is composed of water, minerals, enzymes, and organelles where biochemical reactions occur. Key organelles include rough endoplasmic reticulum (protein synthesis), smooth endoplasmic reticulum (fat production), Golgi apparatus (packaging and distribution center), mitochondria (energy production from glucose), vacuoles (storage units), and lysosomes (digestive enzyme containers). Plant cells also contain plastids for pigment storage and photosynthesis. Organelles are essential as they compartmentalize various chemical activities in the cell to ensure efficiency; different processes require distinct environments that cannot all coexist in cytoplasm alone. While not visible under a compound microscope, these structures can be observed with an electron microscope.

The plasma membrane, or cell membrane, serves as a protective barrier for the inner components of cells and regulates material entry and exit. It functions like a checkpoint, allowing only selected materials to pass through due to its selectively permeable nature. Gas exchange occurs via diffusion—oxygen moves from areas of high concentration outside the cell into lower concentrations inside it. Water movement is facilitated by osmosis through this semi-permeable membrane while other substances require energy for active transport into the cell. The plasma membrane's structure consists mainly of lipids and proteins but can only be observed with an electron microscope; processes such as endocytosis allow organisms like amoeba to engulf food.

Plasmolysis occurs when the cell wall and nucleus are affected, highlighting the structure of the nucleus. It is encased in a double-layered nuclear membrane with pores that regulate material exchange. Inside, chromatin appears as tangled threads but condenses into chromosomes during cell division; these structures contain DNA and proteins essential for cellular function. Unlike complex cells, some single-celled organisms like bacteria lack a defined nuclear membrane and possess nucleoid regions instead—these simpler forms are classified as prokaryotes.

Eukaryotic cells are characterized by a clear nucleus and the presence of membrane-bound organelles, distinguishing them from prokaryotic organisms. Prokaryotes lack both a nuclear membrane and organized cell structures, resulting in poorly defined cellular components. This fundamental difference highlights the complexity of eukaryotic cells compared to their prokaryotic counterparts.

The endoplasmic reticulum (ER) is a network of membranes surrounding the nucleus in eukaryotic cells, consisting of rough and smooth types. Rough ER has ribosomes attached, giving it a rough appearance; its primary function is to synthesize proteins. Smooth ER lacks ribosomes and specializes in producing lipids or fats. Both protein and lipid molecules produced by the ER are essential for constructing cell membranes during division, as well as forming enzymes and hormones that perform various cellular functions. Additionally, the ER facilitates material transport within cytoplasm regions and between cytoplasm and nucleus while also hosting critical biochemical reactions.

The Golgi apparatus consists of flattened bags or vesicles arranged in stacks known as cisterns. It functions similarly to a factory, where proteins manufactured in the endoplasmic reticulum are sent for modification and packaging. The Golgi apparatus performs three key roles: storage, modification, and dispatching of materials. It combines simple molecules into complex ones and packages them into vesicles for delivery within or outside the cell. Additionally, it is responsible for preparing lysosomes.

Lysosomes are membrane-bound organelles filled with digestive enzymes that serve as the cell's waste disposal system. They eliminate unwanted materials such as bacteria, viruses, and damaged cellular components. If a cell is irreparably harmed, lysosomes can trigger self-destruction by releasing their enzymes to digest the cell’s contents—hence they are known as 'suicide bags.' Mitochondria have a double membrane structure; the outer layer is porous while the inner layer has deep folds to increase surface area for ATP production. ATP (adenosine triphosphate) acts as energy currency in cells, powering various reactions necessary for growth and maintenance.

Plastids are unique organelles found only in plant cells, categorized into two types: chromoplasts and leukoplasts. Chromoplasts contain pigments that give color to plants; chloroplasts, a type of chromoplast, are green due to chlorophyll and play a crucial role in photosynthesis. In contrast, leukoplasts are colorless plastids primarily responsible for storing starches, oils, and proteins. Both plastid types possess their own DNA and ribosomes for protein synthesis. Vacuoles serve as storage sacs within cells; they vary significantly between animal and plant cells—small in animals but large (50-90% cell volume) in plants. These larger vacuoles maintain turgidity by holding cell sap while also storing essential materials like sugars and amino acids. Additionally, vacuoles perform vital functions in unicellular organisms such as amoeba by digesting food or excreting excess water through contractile mechanisms.

Cell division is essential for growth, repair, and reproduction in living organisms. It occurs through two main processes: mitosis and meiosis. Mitosis facilitates the formation of new cells necessary for body growth and healing; it results in two identical daughter cells with the same chromosome number as the parent cell. In contrast, meiosis produces reproductive cells or gametes that contain half the chromosomes of their parent cell—leading to four unique daughter cells after a series of divisions. This reduction allows male and female gametes to combine during fertilization, restoring full chromosomal count in zygotes which then develop into new individuals.