Brief Summary
This lecture provides a foundational understanding of cells, starting from the basic differences between living and non-living things to the detailed structure of a prokaryotic cell. It covers the history of cell discovery, cell theory, and an overview of cell types, sizes, and components. The lecture is designed for students with little to no prior knowledge, aiming to build a strong base for understanding more complex topics in biology.
- Cell is the basic structural and functional unit of life.
- Living organisms can be unicellular or multicellular.
- Prokaryotic cells lack membrane-bound organelles and a nucleus, while eukaryotic cells possess them.
Introduction to Cells
The lecture begins by emphasizing the fundamental nature of the cell as the building block of life. Understanding the cell is crucial for grasping concepts in plant and human physiology, genetics, and reproduction. The instructor encourages active participation through answering questions in the chat to ensure an engaging learning environment.
Living vs. Non-Living Things
Everything around us can be divided into living and non-living things. The key difference is that living things are made up of cells. The instructor uses the analogy of a house being made of bricks to explain that our bodies are made of cells. Atoms are smaller than cells; atoms form molecules, which then make up cells. Multiple cells form tissues, tissues form organs, organs form organ systems, and organ systems form an organism.
Unicellular vs. Multicellular Organisms
All life on Earth starts with a single cell, called a zygote in humans, which divides to form trillions of cells. Organisms can be unicellular, made of a single cell, or multicellular, made of many cells. Examples of unicellular organisms include bacteria, amoeba, and paramecium, while multicellular organisms include humans, plants, animals, and most fungi (except yeast).
Definition of a Cell
A cell is the basic structural and functional unit of life. It is the smallest unit capable of independent existence, performing functions like respiration, reproduction, and excretion. This is especially true for unicellular organisms, where one cell performs all functions without division of labor. A cell must be complete to function; an incomplete cell cannot sustain life.
Discovery of the Cell
Robert Hook discovered the dead cell by observing cork cells, which are the cell walls remaining after the cell dies. He observed hollow, honeycomb-like structures and called them "cellulae," which translates to small compartments or rooms, and this term evolved into "cell." Antonie van Leeuwenhoek observed living cells, including bacteria, protozoa, spermatozoa, and red blood cells, which he called "animalcules," or little animals. Robert Brown discovered the nucleus in orchid roots.
Understanding Protoplasm
Protoplasm is the living part of the cell, consisting of the cytoplasm, nucleus, and other organelles within the cell membrane. Robert Hook did not observe the protoplasm because he examined dead cork cells, which only contained the cell wall.
Cell Theory
Matthias Schleiden, a botanist, stated that all plants are made up of different kinds of cells that form tissues. Theodor Schwann, a zoologist, observed that animal cells have a covering called the cell membrane (or plasma membrane) and that plant cells have a unique cell wall. Both concluded that all plants and animals are made up of cells and their products. Rudolf Virchow added that cells arise from pre-existing cells, summarized by "omnis cellula e cellula."
Modern Cell Theory and Exceptions
The modern cell theory includes that all living organisms are composed of cells and their products, cells arise from pre-existing cells, each cell is made up of a small mass of protoplasm containing a nucleus inside and a plasma membrane with or without a cell wall on the outside, all cells are basically alike in chemistry and physiology, and activities of organisms are the sum total of activities of and interaction of their cell constituents. Viruses are an exception to the cell theory because they are acellular or non-cellular.
Cell Overview: Unicellular vs. Multicellular, Plant vs. Animal
Living organisms can be unicellular or multicellular, and cells can be plant or animal cells. Plant cells have a cell wall, while animal cells do not. Inside the nucleus, genetic material is found in the form of chromatin or chromosomes. Cells are broadly divided into prokaryotic and eukaryotic cells. Prokaryotic cells lack membrane-bound organelles and a nucleus, while eukaryotic cells have membrane-bound organelles and a nucleus. Ribosomes and centrosomes are two organelles in eukaryotes that do not have a membrane. Prokaryotic cells have 70S ribosomes, while eukaryotic cells have 80S ribosomes. Both cell types have a cell membrane, cytoplasm, DNA, and ribosomes.
Cell Size and Shape
The size of an animal depends on the number of cells, not the size of the cells. The smallest cell is mycoplasma (0.1 to 0.3 micrometers), the largest is an ostrich egg, bacterial cells range from 3 to 5 micrometers, human red blood cells are 7 micrometers, and the longest cell is a nerve cell.
Prokaryotic Cells: Kingdom Monera
Prokaryotic cells are found in the kingdom Monera, which includes bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLO. Bacteria come in different shapes: cocci (spherical), bacilli (rod-shaped), vibrio (comma-shaped), and spirilla (spiral-shaped).
Structure of a Prokaryotic Cell
A typical prokaryotic cell has a flagellum, three layers (glycocalyx, cell wall, and cell membrane), ribosomes, and genetic material scattered in the nucleoid. The outermost layer, the glycocalyx, can be a loose slime layer or a thick capsule. The second layer is the cell wall, made of peptidoglycan (nag and nam). The innermost layer is the selectively permeable cell membrane.
Cell Membrane Extensions: Mesosomes and Chromatophores
The cell membrane has extensions called mesosomes, which are involved in DNA replication, cell wall formation, respiration, increasing surface area, and distribution of daughter cells. Chromatophores are extensions found in blue-green algae and contain pigments.
External Structures: Flagella, Pili, and Fimbriae
Flagella are extensions from the cell wall that aid in motility. Pili and fimbriae are tube-like structures that help in attachment to a substrate; pili also aid in conjugation (exchange of genetic material).
Internal Components: Nucleoid, Ribosomes, and Plasmids
The genetic material in a prokaryotic cell is present as a nucleoid, without a nucleus. Ribosomes (70S) are present for protein synthesis. Plasmids are extra-chromosomal, self-replicating DNA. Inclusion bodies are storage units for materials like phosphate granules, cyanophycean granules, and glycogen granules. Gas vacuoles are found in blue-green and purple sulfur bacteria, aiding in buoyancy.
Mesosomes: Structure and Function
Mesosomes are extensions of the cell membrane that can be vesicular, tubular, or lamellar. Their functions include DNA replication, cell wall formation, respiration, increasing surface area, distribution of daughter cells, secretion, and enzymatic activity.
Flagella Structure and Function
Flagella have three main parts: the basal body, hook, and filament. They function like a motor, rotating to provide motility.
Ribosomes: Structure and Function
Ribosomes (70S in prokaryotes) are involved in translation and protein production. They consist of a large subunit (50S) and a small subunit (30S), both made of RNA and proteins. The "S" in 70S stands for Svedberg unit, a measure of sedimentation coefficient.
Polysomes and Inclusion Bodies
A polysome (or polyribosome) consists of a single mRNA molecule with multiple ribosomes attached. Inclusion bodies are storage units within the cell, containing phosphate granules, cyanophycean granules, glycogen granules, and gas vacuoles. Gas vacuoles, found in blue-green and purple sulfur bacteria, provide buoyancy.
NCERT Recap
The lecture concludes with a recap of the key points from the NCERT textbook, including the definition of biology, cell theory, cell structure, and the functions of various cell components. It emphasizes the importance of understanding the physicochemical approach to studying living organisms, known as reductionist biology. The recap covers cell theory, cell overview, prokaryotic cell structure, cell envelope, mesosomes, flagella, pili, fimbriae, ribosomes, polysomes, and inclusion bodies.
