Lecture 1 BCM PDF

INTRODUCTION TO CELL BIOLOGY Overview of Cell Theory The cell theory is one of the foundational principles of biology and states that: 1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of structure and function in all living organisms. 3. All cell...

INTRODUCTION TO CELL BIOLOGY Overview of Cell Theory The cell theory is one of the foundational principles of biology and states that: 1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of structure and function in all living organisms. 3. All cells arise from pre-existing cells through the process of cell division. The cell theory was developed in the mid-19th century by scientists Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. This theory established that the cell is the fundamental unit of life, whether the organism is unicellular (bacteria) or multicellular (humans). Prokaryotic vs. Eukaryotic Cells Cells are categorized into two types based on their structural complexity: prokaryotic and eukaryotic. Prokaryotic Cells: Found in organisms like bacteria and archaea. Characteristics: Lack a true nucleus; DNA is found in a region called the nucleoid. Generally smaller in size (1-10 µm). Lack membrane-bound organelles. Have a simple structure with components like the plasma membrane, cytoplasm, ribosomes, and sometimes flagella/pili. Many prokaryotes have a rigid cell wall that provides structural support and protection. Eukaryotic Cells: Found in animals, plants, fungi, and protists. Characteristics: Have a true nucleus enclosed by a nuclear membrane. Generally larger in size (10-100 µm). Possess membrane-bound organelles (e.g., mitochondria, Golgi apparatus, endoplasmic reticulum). Eukaryotic cells are highly compartmentalized, allowing for specialized functions within different organelles. Examples include animal cells (no cell wall), plant cells (with cell wall and chloroplasts), fungal cells (cell wall made of chitin). Cell Structure and Function Cells contain various structures, each performing essential functions for the cell’s survival. These structures, called organelles, work in a coordinated manner to maintain cellular function. Nucleus: The nucleus is the control center of the cell. It contains most of the cell’s genetic material (DNA), organized as chromatin. The nucleus is surrounded by a double membrane called the nuclear envelope, which has nuclear pores for the transport of materials. The nucleolus, found within the nucleus, is the site of ribosomal RNA (rRNA) synthesis. Mitochondria: Known as the "powerhouse of the cell," mitochondria are responsible for producing ATP, the energy currency of the cell, through oxidative phosphorylation. Mitochondria have their own DNA and can replicate independently of the cell. The double-membrane structure includes the inner membrane, which folds into cristae to increase surface area for ATP production. Endoplasmic Reticulum (ER): the rough ER is studded with ribosomes; it is the site of protein synthesis and folding. The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification of drugs and toxins, and calcium storage. Golgi Apparatus: Acts as the packaging and distribution center of the cell. It modifies, sorts, and packages proteins and lipids for transport to various destinations (inside or outside the cell). Lysosomes: Contain hydrolytic enzymes for intracellular digestion. It plays a key role in breaking down waste materials, old organelles, and pathogens in a process called autophagy. Cytoskeleton: Composed of protein filaments, the cytoskeleton provides structural support, maintains the cell's shape, and aids in intracellular transport and cellular movement. It includes three main components: actin filaments (microfilaments), microtubules and intermediate filaments. Actin filaments function in cellular movement, and are made of two intertwined strands of actin. They provide rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. Intermediate filaments are made of several strands of fibrous proteins that are wound together. Microtubules are hollow tubes made of 13 polymerized dimers of α-tubulin and β-tubulin. They help the cell resist compression, pull replicated chromosomes to opposite ends of a dividing cell, and are structural elements of flagella and cilia. Intermediate filaments Microfilaments Microtubules Plasma Membrane: The plasma membrane is a selectively permeable barrier that encloses the cell. Composed of a phospholipid bilayer with embedded proteins, it regulates the movement of substances in and out of the cell. The Fluid Mosaic Model of the cell membrane was proposed by S.J. Singer and G.L. Nicolson in 1972 to describe the structure of the plasma membrane. The model portrays the cell membrane as a dynamic and fluid structure composed of a phospholipid bilayer with embedded proteins, where components are free to move laterally within the membrane. Hence, the fluid mosaic model describes the dynamic nature of the membrane, where lipids and proteins can move laterally within the layer. Key Components of the Fluid Mosaic Model: 1. Lipid Bilayer: The fundamental structure of the plasma membrane consists of two layers of phospholipids, arranged tail-to-tail. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. Hydrophilic heads: Composed of glycerol and a phosphate group, these face outward towards the aqueous environments inside and outside the cell. Hydrophobic tails: Composed of fatty acids, these face inward, away from the water, creating a barrier to most water-soluble molecules. The lipid bilayer is fluid because the fatty acid chains of the phospholipids are not rigid, allowing for lateral movement within the layer. 2. Proteins: Integral (Intrinsic) Proteins: These proteins span across the lipid bilayer (transmembrane proteins) or are deeply embedded within it. They play critical roles in transport, signal transduction, and cell communication. Examples include ion channels, transporters, and receptors. Peripheral (Extrinsic) Proteins: These proteins are loosely associated with the outer or inner surfaces of the membrane. They function in signaling, maintaining the cytoskeleton, and cell recognition. Peripheral proteins can be attached to integral proteins or lipid heads. 3. Carbohydrates: Carbohydrate molecules are often attached to proteins or lipids on the extracellular surface of the membrane, forming glycoproteins or glycolipids. These carbohydrate structures are involved in cell recognition, immune responses, and protection. 4. Cholesterol: are interspersed within the phospholipid bilayer. It helps to stabilize membrane fluidity by preventing the fatty acid chains from packing too closely together (which would make the membrane too rigid in cold conditions) and by restraining excessive movement of phospholipids (which would make the membrane too fluid in warm conditions). Ribosomes: These small structures are the sites of protein synthesis. Ribosomes can be found floating freely in the cytoplasm or attached to the rough ER. Ribosomes are involved in translating mRNA into proteins. Cytoplasm: The cytoplasm is the gel-like fluid inside the cell that surrounds the organelles. It is the site of many metabolic reactions, including glycolysis.