Cell Structure - Biology PDF

LEBANESE UNIVERSITY Faculty of Medical Sciences Hadath Campus 1 Cell BIOLOGY 2nd year Medicine Academic year: 2024 - 2025 Course illustrated by Dr. Ghenwa NASR About This Course: ▪ Cell biology (also cellular biology or cytology) is a branch o...

LEBANESE UNIVERSITY Faculty of Medical Sciences Hadath Campus 1 Cell BIOLOGY 2nd year Medicine Academic year: 2024 - 2025 Course illustrated by Dr. Ghenwa NASR About This Course: ▪ Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. ▪ In this course, the following topics will be covered: ❑ Cell Structure ❑ Cell Signaling ❑ Cell Cycle ❑ Cellular Communication and Tissue Organization ❑ Integration and Advanced topics in Cell Biology 2 3 Chapter 1: Cell Structure Outline 1. The Cell 1.1. Definition 1.2. A historical overview 1.3. The Cell Theory 2. The diversity of Cells 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells 3.2. Eukaryotic Cells 4. Techniques in Cell Biology 5. Cellular Organelles and their Functions 6. The plasma membrane: structure, properties and transport mechanisms. 4 1. The Cell 1.1. Definition ▪ Cell is the basic structural and functional unit of living organisms. ▪ Cells are referred to as “the building blocks” of all living organisms. ▪ The term “cell” comes from the Latin word cellula meaning “small room”. ▪ Cells are small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals constituting the cytoplasm. Many cells contain organelles, each with a specific function. ▪ Cells make up living things and carry out activities that keep a living thing alive. In other words, the continuity of life depends on the growth and division of cells. ▪ Cells are complex and their components perform various functions in an organism. 5 1.2. The Cell: A Historical Overview the discovery of the microscope allowed the Robert Hooke used a microscope to examine cork and coined the discovery of the term "cell," describing the small, cell box-like structures he observed. Pre-17th Century 1590s 1830 1665 The concept of the cell Hans and Zacharias Janssen The development of the wasn't understood. created the first compound cell theory. microscope, allowing for closer observation of small structures. 5 1.3. The Cell Theory ▪ Until microscopes became powerful enough to view individual cells, no-one knew for certain what living organisms were made from. ▪ The scientist Robert Hooke is thought to be the first person to view cells (including single-celled microorganisms) and Hooke also came up with the term "cells" to describe these newly discovered structures. ▪ Matthias Schleiden and Theodor Schwann were two other scientists who studied animal and plant cells: ✓ In 1837, they came up with the idea that all living organisms are made of cells ✓ This idea is known as “cell theory” ✓ The cell theory is a unifying concept in biology (it is universally accepted). What is the cell theory ? It is that all living organisms are made up of cells Dates are not required 7 1.3. The Cell Theory ▪ According to the conclusions made by Schleiden and Schwann in 1838, the traditional cell theory includes three main statements: ❑ All living organisms are made up of one or more cells ❑ Cells are the basic functional unit (the basic unit of structure and organization) in living organisms ❑ New cells are produced from pre-existing cells. 8 1.3. The Cell Theory 9 1.3. The Cell Theory ▪ Further studies on cells with the advancement of microscope have led to the formation of the modern cell theory, which has four main additions to the principles of the original theory: ❑ The cells contain hereditary information or genetic material (DNA) which is passed on from one cell to another during cell division. ❑ All cells have the same basic chemical composition ❑ Energy flow (metabolism and biochemistry) occurs within cells ❑ Cell activity depends on the activities of sub-cellular structures (organelles) within the cell. 10 2. The Diversity of Cells ▪ Cells differ vastly in form and function. ▪ Unicellular organisms differ from multicellular organisms. ▪ Animal cells differ from those in a plant, and even cells within a single multicellular organism can differ wildly in appearance and activity. ▪ Despite these differences, all cells share a fundamental chemistry and other common features. ▪ In this section, we will highlight some of the similarities and differences among cells that account for their diversity. 11 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Size ▪ Shape ▪ Chemical requirements ▪ Function ▪ Internal organization 12 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Size ❑ Cells vary in size. ❑ Most cells are very small (microscopic), some may be very large (macroscopic). The smallest cell The largest cell Mycoplasma Ostrich egg Size: 0.1 μm Size: 18 cm 13 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Size of Cells in Humans Smallest cell Largest cell Longest cell Sperm cell Ovum cell Nerve cell Size: 5 μm Size: 120 μm Size: 1 m 14 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Size Biological size and cell diversity 15 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Shape There is a big correlation between the function And the shape of the cell ❑ Cells vary in shape. ❑ Most cells are roughly spherical or cuboidal. ❑ Shape variation depends mainly upon the function of the cell. ❑ Some cells like Euglena and Amoeba change their shape, but most cells have a fixed shape. 16 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Shape Human RBCs are Nerve cells are Human WBCs can circular biconcave for branched to conduct change their shape to easy passage through impulses from one engulf the human capillaries point to another microorganisms that enter the body. 17 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Chemical requirements ❑ Cells are also enormously diverse in their chemical requirements. ❑ Some require oxygen to live; for others the gas is deadly. ❑ Some cells consume carbon dioxide (CO2), sunlight, and water as their raw materials; others need a complex mixture of molecules produced by other cells. 18 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Function ❑ These differences in size, shape, and chemical requirements often reflect differences in cell function. ❑ Cell shape and structure is correlated to cell function. ❑ Some cells are specialized factories for the production of particular substances, such as hormones, starch, fat, latex, or pigments. Others, like muscle cells, are engines that burn fuel to do mechanical work. 19 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Internal organization ❑ Depending on the internal organization of cells, they are mainly categorized into two types: prokaryotic cells and eukaryotic cells. ❑ Eukaryotic cells possess a nucleus and membrane-bound organelles. ❑ Prokaryotic cells lack nuclei and membrane-bound organelles. ❑ Nucleus: contains the DNA which directs the activity of the cell. ❑ Organelle: a cell component that performs specific functions in the cell. How I can tell if the cell is eukaryotic or prokaryotic? By looking at its internal oganization 20 2. The Diversity of Cells Cells Vary Enormously in Appearance and Function ▪ Internal organization 21 2. The Diversity of Cells Cells Similarities ▪ Living Cells All Have a Similar Basic Chemistry ✓ Although the cells of all living things are enormously varied when viewed from the outside, they are fundamentally similar inside. ✓ They are composed of the same sorts of molecules, which participate in the same types of chemical reactions. ✓ In all organisms, the genetic information (in the form of genes) is carried in DNA molecules. ✓ This information is written in the same chemical code, constructed out of the same chemical building blocks, interpreted by essentially the same chemical machinery, and replicated in the same way when a cell or an organism reproduces. ✓ In all living cells, the sequence of nucleotides in a particular segment of DNA (a gene) is transcribed into an RNA molecule, which can then be translated into the linear sequence of amino acids of a protein. 22 2. The Diversity of Cells Cells Similarities ▪ Living Cells are self-replicating collections of catalysts. ▪ All Living Cells have apparently evolved from the same Ancestral Cell. (Cell theory) ▪ Genes provide instructions for the Form, Function, and Behavior of Cells and Organisms. 23 3. Prokaryotic and Eukaryotic Cells ▪ Based on complexity in structure and parts, all cells are divided into prokaryotic and eukaryotic. 24 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells ▪ A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. ▪ The term “prokaryote” is derived from the Greek word “pro“, (meaning: before) and “karyon” (meaning: kernel). It translates to “before nuclei.“ ▪ Prokaryotic cells are single-celled microorganisms known to be the earliest on earth. Prokaryotes include two domains: Bacteria and Archaea. The photosynthetic prokaryotes include cyanobacteria that perform photosynthesis. ▪ Typically, prokaryotic cell sizes range from 0.1 to 5.0 μm in diameter. ▪ Prokaryotes are typically spherical, rodlike, or spiral-shaped. 25 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells ▪ Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a region within the cell where the genetic material is freely suspended. This region is called the nucleoid. ▪ The hereditary material can either be DNA or RNA. ▪ Prokaryotes often have a tough protective coat, or cell wall, surrounding the plasma membrane, which encloses a single compartment containing the cytoplasm and the DNA. ▪ The genetic material is present on a single (circular) chromosome. ▪ Histone proteins are also lacking in prokaryotes. ▪ Prokaryotes generally reproduce by binary fission, a form of asexual reproduction. They are also known to use conjugation which is often seen as the prokaryotic equivalent to sexual reproduction. ▪ Some are aerobic, using oxygen to oxidize food molecules; some are strictly anaerobic and are killed by the slightest exposure to oxygen. 26 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells ▪ Structure of a prokaryotic cell: ❑ Capsule: It is an outer protective covering found in the bacterial cells, in addition to the cell wall. It is made of polysaccharides helps in moisture retention, protects the cell when engulfed, and helps in the attachment of cells to nutrients and surfaces. ❑ Cell Wall: It is the outermost layer of the cell which gives shape to the cell. It consists of carbohydrates and amino acids and is found below the capsule. In bacteria, it is a covering made of carbohydrate, and lipid polymer termed peptidoglycan. However, the archaeal cell wall contains no peptidoglycan and is made up of proteins and other polymers. ❑ Cell Membrane: It is found underneath the cell wall and is made of the phospholipid bilayer. The cell membrane protects the cell while allowing the transport of essential molecules across it. ❑ Pili: These are hair-like outgrowths that help the cell to attach to the surface of other bacterial cells. ❑ Cytoplasm: The cytoplasm is mainly composed of enzymes, salts, cell organelles and is a gel-like component. 27 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells ▪ Structure of a prokaryotic cell: ❑ Flagella: These long structures in the form of a whip (a tail-like structure) help in the locomotion of a cell. ❑ Ribosomes: These are involved in protein synthesis. ❑ Plasmids: Plasmids are non-chromosomal DNA structures. These are not involved in reproduction. ❑ Nucleoid Region: It is the region in the cytoplasm where the genetic material is present. ❑ Membrane-bound organelles such as mitochondria, chloroplast, Golgi bodies, and lysosomes are absent. 28 3. Prokaryotic and Eukaryotic Cells 3.1. Prokaryotic Cells ▪ Examples of prokaryotic cells: Bacterial Cells Archaeal Cells These are unicellular organisms found They are found in extreme everywhere on earth from soil to the environments such as hot springs and human body. other places such as soil, marshes, and even inside humans. The cell wall is composed of They have a cell wall and flagella. The peptidoglycan that provides structure cell wall of archaea does not contain to the cell wall. peptidoglycan. Bacteria have some unique structures Just like bacteria, archaea have one such as pili, flagella and capsule. circular chromosome. They also possess plasmids. They also possess extrachromosomal DNA known as plasmids. 29 3. Prokaryotic and Eukaryotic Cells 3.2. Eukaryotic Cells ▪ Eukaryotes are advanced organisms with a well-defined nucleus and membrane-bound organelles. ▪ The term ‘eukaryotes’ is derived from the Greek words ‘eu’, meaning ‘good’ and ‘karyon’, meaning ‘kernel’, translating to “good or true nuclei”. ▪ The eukaryotes are thought to have originated from the prokaryotes about 2.7 billion years ago. ▪ Eukaryotes may be either unicellular or multicellular. ▪ Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. ▪ Eukaryotes are categorized into 4 groups or domains: protozoa (or protists), fungi, plants, and animals. Protists and fungi are usually unicellular, while animals and plants are multicellular. 30 3. Prokaryotic and Eukaryotic Cells 3.2. Eukaryotic Cells ▪ Eukaryotic cells are cells that contain a nucleus and organelles, and are enclosed by a plasma membrane. ▪ They have a more advanced structural organization that is large and more complex than a prokaryotic cell. ▪ The genetic material is DNA, which is linear and has multiple origins of replication. The DNA is complexed with histone proteins forming rod-shaped chromosomes that are enclosed within the nucleus. ▪ Eukaryotic cells comprise organelles (such as mitochondria, endoplasmic reticulum, ribosome, Golgi apparatus, etc.), each assigned for a particular function. ▪ They contain cytoskeletal structural elements (microtubules, microfilaments, and intermediate filaments) that provide structural support to the cell. ▪ Cilia and flagella are the locomotory organs for eukaryotic cells. ▪ Their cell wall consists of cellulose and some other carbohydrates. 31 3. Prokaryotic and Eukaryotic Cells 3.2. Eukaryotic Cells ▪ Their size is significantly larger than prokaryotic cells, with an average of 10 to 100 µm in diameter. ▪ The shape of eukaryotic cells varies significantly with the type of cell. Some common shapes include spheroid, ovoid, cuboidal, lenticular, cylindrical, flat, fusiform, discoidal, and polygonal. 32 3. Prokaryotic and Eukaryotic Cells Prokaryotic vs Eukaryotic Cells 33 3. Prokaryotic and Eukaryotic Cells 34 4. Techniques in Cell Biology ▪ Techniques used to visualize and study the cells: ❑ Microscopy: light microscopy and electron microscopy ❑ Cell Fractionation and imaging 35 4. Techniques in Cell Biology Microscopy ▪ Cells are very tiny. Most cells are too small to see with the naked eye. ▪ The invention of the Light Microscope Led to the discovery of Cells in the seventeenth century. ▪ Scientists use microscopes to visualize cells. Different types of microscopy have been invented; it is possible to group them into two major categories: ✓ Light microscopy ✓ Electron microscopy ▪ Magnification: refers to the microscope’s power to increase an object’s apparent size ▪ Resolution: refers to the microscope’s power to show detail clearly 36 4. Techniques in Cell Biology Light Microscopy ▪ A light microscope uses light and lenses to magnify an object. ▪ In a light microscope (LM), visible light passes through a specimen and then through glass lenses, which magnify the image. ▪ The quality of an image depends on: ✓ Magnification: the ratio of an object’s image size to its real size ✓ Resolution: the measure of the clarity of the image, or the minimum distance of two distinguishable points ✓ Contrast: visible differences in parts of the sample. ▪ LMs can magnify effectively to about 1,000 times the size of the actual specimen. ▪ Various techniques enhance contrast and enable cell components to be stained or labeled. ▪ Most subcellular structures, including organelles (membrane-enclosed compartments), are too small to be resolved by an LM. 37 4. Techniques in Cell Biology Light Microscopy ▪ Several LM techniques were developed including: ❑ Brightfield microscopy ❑ Phase-contrast microscopy ❑ Differential-interference-contrast microscopy ❑ Fluorescence microscopy ❑ Confocal microcopy 38 4. Techniques in Cell Biology Light Microscopy 39 4. Techniques in Cell Biology Light Microscopy 40 4. Techniques in Cell Biology Light Microscopy 41 4. Techniques in Cell Biology Light Microscopy ▪ Elodea - Aquatic Plant visualized using a light microscope. 40 x 400 x 42 4. Techniques in Cell Biology Electron Microscopy ▪ Electron microscopy works in a very similar way to light microscopy, except that it uses a beam of electrons instead of light to image the sample. ▪ Electron microscopy allows the visualization of subcellular components or structures such as organelles. ▪ Two basic types of electron microscopes (EMs) are used to study subcellular structures: ✓ Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a specimen, providing images that look 3-D ✓ Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen ▪ TEMs are used mainly to study the internal structure of cells 43 4. Techniques in Cell Biology Electron Microscopy 44 4. Techniques in Cell Biology Electron Microscopy ▪ Transmission Electron Microscope (TEM) 45 4. Techniques in Cell Biology Electron Microscopy ▪ Transmission Electron Microscope (TEM) Herpes Virus Plant root cell 46 4. Techniques in Cell Biology Electron Microscopy ▪ Scanning Electron Microscope (SEM) 47 4. Techniques in Cell Biology Electron Microscopy ▪ Scanning Electron Microscope (SEM) Neuron The surface of tongue The inside surface of a stomach 48 4. Techniques in Cell Biology Electron Microscopy ▪ Scanning Electron Microscope (SEM) RBC, platelet and Yeast Pollen WBC 49 4. Techniques in Cell Biology Cell Fractionation ▪ In cell biology, cell fractionation is the process used to separate cellular components while preserving individual functions of each component. ▪ Cell fractionation takes cells apart and separates the major organelles from one another. ▪ Ultracentrifuges fractionate cells into their component parts. ▪ Cell fractionation enables scientists to determine the functions of organelles. ▪ Biochemistry and cytology help correlate cell function with structure. 50 4. Techniques in Cell Biology Cell Fractionation 51 5. Cellular Organelles and Their Functions Structure of the Cell ▪ Three main components: 1. Plasma membrane 2. Nucleus 3. Cytoplasm A. Cytosol o Ribosomes o Cytoskeleton B. Cell organelles o Endoplasmic Reticulum o Golgi Body o Lysosomes o Vacuoles o Mitochondria o Plastids 52 5. Cellular Organelles and Their Functions Structure of the Cell ▪ Cell organelles: ❑ It is a specialized cellular subunit or structure that performs a specific function within the cell. ❑ The name “organelle” comes from the idea that these structures are to cells what an organ is to the body. organBody organellecell ❑ They are involved in many processes including: energy production, proteins synthesis and secretions, destroying toxins, and responding to external signals. ❑ Some organelles are common to most cell types while others are specific to a certain cell type; like plastids in plant cells. ❑ Organelles are identified by microscopy, and can also be purified by cell fractionation. ❑ Organelles can be classified into two main categories: ✓ membrane-bound organelles: nucleus, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondria, etc. ✓ non-membrane-bound organelles: ribosomes and cytoskeleton. 53 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Nucleus: ❑ A membrane-bound organelle found in eukaryotic cells. ❑ It is the largest organelle that stores genetic material and coordinates cellular activities. ❑ Present in all the cells except the red blood cells and sieve tube cells (in plants). ❑ Absent in prokaryotic cells. ❑ Most of the cells are uninucleated (having only one nucleus) while few types of cells contain multiple nuclei, such as skeletal muscle cells (osteoclasts). ❑ Anatomically, the nucleus is made of : nuclear envelope, nucleoplasm, nucleolus and the chromatin. 54 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Nucleus: ❑ The nuclear envelope: ✓ A double-layered covering consisting of two membranes: an inner and an outer nuclear membrane, perforated by nuclear pores. ✓ It is selectively permeable in nature. ✓ Together, these membranes serve to separate the cell's genetic material from the rest of the cell contents. ✓ Nuclear pores regulate the flow of molecules into and out of the nucleus. ✓ The inner face of the nuclear envelope is associated with the nuclear lamina whereas the outer face of the nuclear lamina is associated with the endoplasmic reticulum. ❑ The nuclear lamina: ✓ A dense fibrillar network composed of intermediate filaments known as lamins and membrane associated proteins. ✓ It supports the nuclear envelope, maintaining the overall shape and structure of the nucleus. It regulates important cellular events 55 such as DNA replication and cell division. 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Nucleus: ❑ The nucleoplasm: ✓ Also known as karyoplasm, it is found inside the nucleus, and is a gelatinous substance similar to the cytoplasm. ✓ Composed mainly of water with dissolved salts, enzymes, and suspended organic molecules. ✓ It protects the nuclear content by providing a cushion around the nucleolus and the chromosome. ✓ It serves as a suspension substance for the structures inside the nucleus. ✓ It maintains the structure and shape of the nucleus. ✓ It helps in the transportation of materials that are vital to cell metabolism. ✓ It acts as the site for various nuclear events and activities, for instance, gene expression, DNA replication, and DNA repair. ✓ It synthesizes ribosomal RNA (rRNA). 56 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Nucleus: ❑ The nucleolus: ✓ A dense structure within the nucleus composed of RNA, proteins, granules, and fibers. ✓ Its primary roles are rRNA synthesis and the biogenesis of ribosomes. ✓ The nucleolus disappears when a cell undergoes division and is reformed after the completion of cell division. ❑ The chromatin: ✓ A complex of genetic material (DNA or RNA) and proteins (histone) found in in a resting or non-dividing cell nucleus. ✓ The chromatin is classified into two types, heterochromatin and euchromatin, based on functions. The heterochromatin is a functionally inactive form of chromatin, found near the nuclear envelope. On the contrary, euchromatin is a mild, less condensed form that is in functionally active state. ✓ It contains the hereditary information and instructions necessary for cellular processes and help in gene regulation. 57 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Endoplasmic Reticulum: ❑ A network of tubular and vesicular structures that are interconnected with each other. ❑ ER packages and transports proteins to the Golgi apparatus. ❑ Some parts of the ER are connected to the nuclear membrane while others are connected to the cell membrane. ❑ Two types of ER: 1) Rough endoplasmic reticulum (RER): studded with ribosomes, synthesizes secretory proteins and membrane proteins. 2) Smooth endoplasmic reticulum (SER): it lacks ribosomes, performs lipid synthesis, carries out protein-folding and transports them to Golgi apparatus. In the liver, it helps detoxify or remove several drugs from the body. 58 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Golgi Apparatus: ❑ A membrane-bound organelle comprising 5 to 8 flattened, disk- shaped membranous sacs known as cisternae or dictyosomes. ❑ It has two ends: the cis face located near the endoplasmic reticulum and the trans face situated near the cell membrane. ❑ It modifies, sorts and packs materials synthesized in the cell. ❑ It delivers synthesized materials to various targets inside and outside the cell. ❑ It produces vacuoles and secretory vesicles. ❑ It forms lysosomes. 59 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Golgi body at work 60 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Mitochondria: ❑ A rod-shaped, double-membrane bound organelle. ❑ It comprises two membranes: the outer membrane is smooth, while the inner one is folded into tubule structures called cristae. ❑ The space between the outer and inner membranes is referred to as the intermembrane space and the matrix is the space inside the inner membrane and contains many enzymes. ❑ Mitochondria are unique in that they contain small amounts of DNA containing the genes for the synthesis of some mitochondrial proteins. The DNA is inherited solely from the mother. ❑ Cells with greater activity have more mitochondria, while those that are less active have less need for energy- 61 producing mitochondria. 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Mitochondria: ❑ Mitochondria are the powerhouses of the cell. Cellular respiration, the generation of energy from sugars and fats, occurs in these organelles. ❑ It is also involved in various cellular activities like cellular differentiation, cell signaling, cell senescence, controlling the cell cycle and also in cell growth. 62 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Lysosomes: ❑ A single membrane-bound organelle found in many animal cells. ❑ They are spherical vesicles that contain hydrolytic enzymes that digest many kinds of biomolecules. ❑ Lysosomes are degradative organelles that act as the waste disposal system of the cell by digesting used materials in the cytoplasm, from both inside and outside the cell. Material from outside the cell is taken up through endocytosis, while material from the inside of the cell is digested through autophagy. ❑ The lysosome is involved in various cellular processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism. ❑ It performs autolysis in dead cells. 63 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Lysosomes: ❑ They are formed in the Golgi body and are called “the suicidal bags”. ❑ They prevent the entry of foreign particles such as bacteria and viruses and destroy them once they enter the cell through phagocytosis. 64 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ Peroxisomes: ❑ Single membrane-bound small, round-shaped structures. ❑ They contain digestive and oxidative enzymes. ❑ Peroxisomes are oxidative organelles: they are involved in the production and elimination of hydrogen peroxide during biochemical processes. ❑ Peroxisomes oversee reactions that neutralize free radicals, which cause cellular damage and cell death. ❑ Peroxisomes chemically neutralize poisons through a process that produces large amounts of toxic H₂O₂, which is then converted into water and oxygen (in the liver, metabolism of drugs and alcohol). ❑ Peroxisomes are responsible for oxidation reactions that break down fatty acids and amino acids. 65 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ A complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells. ❑ In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. ❑ It is composed of three main components: microfilaments, intermediate filaments, and microtubules. ❑ Each type is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution. 66 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ The cytoskeleton matrix is composed of different types of proteins that can divide rapidly or disassemble depending on the requirement of the cells. ❑ The primary functions include providing the shape and mechanical resistance to the cell against deformation, the contractile nature of the filaments helps in motility during cytokinesis. 67 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microfilaments ✓ The thinnest and most abundant of the cytoskeleton proteins. ✓ They are composed of polymers of actin, a contractile protein, and can be assembled and disassembled quickly according to the needs of the cell or organelle structure. ✓ When first produced by the cell, the actin monomers join together to form two parallel polymers of globular-(G)-actin. Once they are joined, the elongated strands twist around each other into a helical orientation having a diameter of about 6-7 nm and are called filamentous-(F)-actin. ✓ All the subunits of microfilaments are connected in the same orientation, thus exhibiting polarity having a positive or plus (+) end and a negative or minus (-) end. ✓ They are involved in cell movement ✓ They help to maintain the cell shape 68 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments ✓ A structural component of the cytoskeleton along with the microfilaments and the microtubules. ✓ Made of multiple strands of fibrous proteins wound together, each consisting of amino acids arranged in a chain. ✓ They have a diameter of about 10 nm. They are thicker than actin filaments (about 7 nm) and thinner than microtubules (about 25 nm). ✓ Animal intermediate filaments are subcategorized into six types based on similarities in amino acid sequence and protein structure. Most types are cytoplasmic, but one type, Type V is a nuclear lamin. ✓ They form an extensive network in the cytoplasm of most animal cells, extending from a ring surrounding the nucleus to the plasma membrane. ✓ IFs provide structural support to the cell. They also act as a mechanical stress absorber for the entire cell cytoskeleton. 69 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments ✓ A diverse family of intermediate filament proteins: more than 50 different intermediate filament proteins have been identified so far. They are classified into different groups: 1. Type I and II: They consist of two groups of keratins, expressed in epithelial cells. 2. Type III: They include vimentin and desmin. Vimentin is found in different cell types, such as fibroblasts, smooth muscle cells, and white blood cells, whereas desmin expresses specifically in muscle cells. 3. Type IV: They include the three neurofilaments (NF) proteins found in neurons. 4. Type V: They are lamins found in most eukaryotic cells. They are not part of the cytoskeleton. Instead, they are components of the nuclear envelope. 5. Type VI: They include beaded filaments such as filensin and phakinin. 70 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments Assembly 1. It starts with the formation of dimers. The central rod domains of two polypeptide chains are wound around each other in a coiled-coil structure, similar to the assembly of myosin II heavy chains. 2. The dimers then associate in a staggered antiparallel fashion to form tetramers, which can assemble end to end, forming protofilaments. 3. The final intermediate filament contains approximately eight protofilaments wound around each other in a ropelike structure. ❑ In contrast to actin filaments and microtubules, intermediate filaments do not have distinct plus and 71 minus ends and thus lack polarity. 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microtubules ✓ A component of the cytoskeleton ✓ Hollow cylinders, round tubes with a diameter of about 24 nanometers. ✓ They consist of a single type of cytoplasmic, globular protein subunits called tubulin. ✓ They are arranged as heterodimers of α-tubulin and β-tubulin. ✓ These help in transporting cellular materials and dividing chromosomes during cell division. ✓ They are found throughout the cytoplasm in eukaryotic plant and animal cells. 72 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microtubules assembly ✓ Each microtubule subunit comprises two closely related polypeptides: α-tubulin, and β-tubulin, forming heterodimers. ✓ Multiple units of these dimers polymerize to form a chain called the protofilament. ✓ Then, 13 protofilaments arrange into a cylindrical pattern to form a microtubule. ✓ In a microtubule, the subunits are organized such that they all face the same direction to form 13 parallel protofilaments. Thus, the microtubule is polar with the alpha-tubulin exposed at one end and beta-tubulin at the other. ✓ Since microtubules are polar, they have a positively charged or (+) end that grows relatively fast and a negatively charged or (-) end that grows relatively slowly. 73 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Ribosomes: ❑ A non-membrane bound organelle ❑ It is composed of (1/3rd) proteins and (2/3rd) RNA ❑ It serves as the site of protein synthesis: it is used to translate genetic code into chains of amino acids (translation process in the cytoplasm). ❑ Ribosomes occur both as free particles in prokaryotic and eukaryotic cells and as particles attached to the membranes of the endoplasmic reticulum (RER) in eukaryotic cells. ❑ Each ribosome is composed of two subunits: a large subunit and a small subunit, each of which has a characteristic shape. In eukaryotes, ribosomal subunits are formed in the nucleolus of the cell’s nucleus. 74 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Endoplasmic Reticulum: ❑ A network of tubular and vesicular structures that are interconnected with each other. ❑ ER packages and transports proteins to the Golgi apparatus. ❑ Some parts of the ER are connected to the nuclear membrane while others are connected to the cell membrane. ❑ Two types of ER: 1) Rough endoplasmic reticulum (RER): studded with ribosomes, synthesizes secretory proteins and membrane proteins. 2) Smooth endoplasmic reticulum (SER): it lacks ribosomes, performs lipid synthesis, carries out protein-folding and transports them to Golgi apparatus. In the liver, it helps detoxify or remove several drugs from the body. 58 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Golgi Apparatus: ❑ A membrane-bound organelle comprising 5 to 8 flattened, disk- shaped membranous sacs known as cisternae or dictyosomes. ❑ It has two ends: the cis face located near the endoplasmic reticulum and the trans face situated near the cell membrane. ❑ It modifies, sorts and packs materials synthesized in the cell. ❑ It delivers synthesized materials to various targets inside and outside the cell. ❑ It produces vacuoles and secretory vesicles. ❑ It forms lysosomes. 59 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Golgi body at work 60 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Mitochondria: ❑ A rod-shaped, double-membrane bound organelle. ❑ It comprises two membranes: the outer membrane is smooth, while the inner one is folded into tubule structures called cristae. ❑ The space between the outer and inner membranes is referred to as the intermembrane space and the matrix is the space inside the inner membrane and contains many enzymes. ❑ Mitochondria are unique in that they contain small amounts of DNA containing the genes for the synthesis of some mitochondrial proteins. The DNA is inherited solely from the mother. ❑ Cells with greater activity have more mitochondria, while those that are less active have less need for energy- 61 producing mitochondria. 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Mitochondria: ❑ Mitochondria are the powerhouses of the cell. Cellular respiration, the generation of energy from sugars and fats, occurs in these organelles. ❑ It is also involved in various cellular activities like cellular differentiation, cell signaling, cell senescence, controlling the cell cycle and also in cell growth. 62 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Lysosomes: ❑ A single membrane-bound organelle found in many animal cells. ❑ They are spherical vesicles that contain hydrolytic enzymes that digest many kinds of biomolecules. ❑ Lysosomes are degradative organelles that act as the waste disposal system of the cell by digesting used materials in the cytoplasm, from both inside and outside the cell. Material from outside the cell is taken up through endocytosis, while material from the inside of the cell is digested through autophagy. ❑ The lysosome is involved in various cellular processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism. ❑ It performs autolysis in dead cells. 63 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Lysosomes: ❑ They are formed in the Golgi body and are called “the suicidal bags”. ❑ They prevent the entry of foreign particles such as bacteria and viruses and destroy them once they enter the cell through phagocytosis. 64 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ Peroxisomes: ❑ Single membrane-bound small, round-shaped structures. ❑ They contain digestive and oxidative enzymes. ❑ Peroxisomes are oxidative organelles: they are involved in the production and elimination of hydrogen peroxide during biochemical processes. ❑ Peroxisomes oversee reactions that neutralize free radicals, which cause cellular damage and cell death. ❑ Peroxisomes chemically neutralize poisons through a process that produces large amounts of toxic H₂O₂, which is then converted into water and oxygen (in the liver, metabolism of drugs and alcohol). ❑ Peroxisomes are responsible for oxidation reactions that break down fatty acids and amino acids. 65 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ A complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells. ❑ In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. ❑ It is composed of three main components: microfilaments, intermediate filaments, and microtubules. ❑ Each type is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution. 66 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ The cytoskeleton matrix is composed of different types of proteins that can divide rapidly or disassemble depending on the requirement of the cells. ❑ The primary functions include providing the shape and mechanical resistance to the cell against deformation, the contractile nature of the filaments helps in motility during cytokinesis. 67 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microfilaments ✓ The thinnest and most abundant of the cytoskeleton proteins. ✓ They are composed of polymers of actin, a contractile protein, and can be assembled and disassembled quickly according to the needs of the cell or organelle structure. ✓ When first produced by the cell, the actin monomers join together to form two parallel polymers of globular-(G)-actin. Once they are joined, the elongated strands twist around each other into a helical orientation having a diameter of about 6-7 nm and are called filamentous-(F)-actin. ✓ All the subunits of microfilaments are connected in the same orientation, thus exhibiting polarity having a positive or plus (+) end and a negative or minus (-) end. ✓ They are involved in cell movement ✓ They help to maintain the cell shape 68 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments ✓ A structural component of the cytoskeleton along with the microfilaments and the microtubules. ✓ Made of multiple strands of fibrous proteins wound together, each consisting of amino acids arranged in a chain. ✓ They have a diameter of about 10 nm. They are thicker than actin filaments (about 7 nm) and thinner than microtubules (about 25 nm). ✓ Animal intermediate filaments are subcategorized into six types based on similarities in amino acid sequence and protein structure. Most types are cytoplasmic, but one type, Type V is a nuclear lamin. ✓ They form an extensive network in the cytoplasm of most animal cells, extending from a ring surrounding the nucleus to the plasma membrane. ✓ IFs provide structural support to the cell. They also act as a mechanical stress absorber for the entire cell cytoskeleton. 69 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments ✓ A diverse family of intermediate filament proteins: more than 50 different intermediate filament proteins have been identified so far. They are classified into different groups: 1. Type I and II: They consist of two groups of keratins, expressed in epithelial cells. 2. Type III: They include vimentin and desmin. Vimentin is found in different cell types, such as fibroblasts, smooth muscle cells, and white blood cells, whereas desmin expresses specifically in muscle cells. 3. Type IV: They include the three neurofilaments (NF) proteins found in neurons. 4. Type V: They are lamins found in most eukaryotic cells. They are not part of the cytoskeleton. Instead, they are components of the nuclear envelope. 5. Type VI: They include beaded filaments such as filensin and phakinin. 70 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Intermediate Filaments Assembly 1. It starts with the formation of dimers. The central rod domains of two polypeptide chains are wound around each other in a coiled-coil structure, similar to the assembly of myosin II heavy chains. 2. The dimers then associate in a staggered antiparallel fashion to form tetramers, which can assemble end to end, forming protofilaments. 3. The final intermediate filament contains approximately eight protofilaments wound around each other in a ropelike structure. ❑ In contrast to actin filaments and microtubules, intermediate filaments do not have distinct plus and 71 minus ends and thus lack polarity. 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microtubules ✓ A component of the cytoskeleton ✓ Hollow cylinders, round tubes with a diameter of about 24 nanometers. ✓ They consist of a single type of cytoplasmic, globular protein subunits called tubulin. ✓ They are arranged as heterodimers of α-tubulin and β-tubulin. ✓ These help in transporting cellular materials and dividing chromosomes during cell division. ✓ They are found throughout the cytoplasm in eukaryotic plant and animal cells. 72 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Cytoskeleton: ❑ Microtubules assembly ✓ Each microtubule subunit comprises two closely related polypeptides: α-tubulin, and β-tubulin, forming heterodimers. ✓ Multiple units of these dimers polymerize to form a chain called the protofilament. ✓ Then, 13 protofilaments arrange into a cylindrical pattern to form a microtubule. ✓ In a microtubule, the subunits are organized such that they all face the same direction to form 13 parallel protofilaments. Thus, the microtubule is polar with the alpha-tubulin exposed at one end and beta-tubulin at the other. ✓ Since microtubules are polar, they have a positively charged or (+) end that grows relatively fast and a negatively charged or (-) end that grows relatively slowly. 73 5. Cellular Organelles and Their Functions Cellular Organelles and their Functions ▪ The Ribosomes: ❑ A non-membrane bound organelle ❑ It is composed of (1/3rd) proteins and (2/3rd) RNA ❑ It serves as the site of protein synthesis: it is used to translate genetic code into chains of amino acids (translation process in the cytoplasm). ❑ Ribosomes occur both as free particles in prokaryotic and eukaryotic cells and as particles attached to the membranes of the endoplasmic reticulum (RER) in eukaryotic cells. ❑ Each ribosome is composed of two subunits: a large subunit and a small subunit, each of which has a characteristic shape. In eukaryotes, ribosomal subunits are formed in the nucleolus of the cell’s nucleus. 74 6. The Plasma Membrane 6.1. Structure and Composition ▪ The plasma membrane is also termed as a Cell Membrane or Cytoplasmic Membrane. ▪ It surrounds the cell to create a barrier between the cytosol and the extracellular matrix; allowing certain substances inside the cell while preventing others to pass through. ▪ The membrane is composed of lipids arranged into a lipid bilayer, with proteins and carbohydrates. ▪ It also serves as the site of attachment for the cytoskeleton that helps to provide shape and support to the cell. ▪ The most widely accepted model of the cell membrane was given by S.J. Singer and Garth L. Nicolson in 1972, popularly known as the fluid mosaic model. According to this model, the membrane consists of a lipid bilayer in which proteins are embedded and randomly distributed. 75 6. The Plasma Membrane 6.1. Structure and Composition ▪ Membrane Lipids ❑ Phospholipids: ✓ A major component of the cell membrane forming a bilayer structure. The building blocks of cell membranes. ✓ 55% of membrane lipids ✓ Amphiphilic molecules possessing hydrophilic and hydrophobic regions. ✓ Each phospholipid molecule has a glycerol backbone with two fatty-acid molecules and a phosphate group attached to it. The phosphate group is linked to a polar entity such as ethanolamine, serine, etc. ✓ The hydrophilic (water-loving) head of phospholipids points towards the inner cytoplasmic side and outer extracellular fluid while the hydrophobic (water-hating) tail faces each other. This arrangement allows phospholipid molecules to form a bilayer structure that separates the cell’s interior from the exterior. 76 6. The Plasma Membrane 6.1. Structure and Composition ▪ Membrane Lipids ❑ Cholesterol: ✓ An amphiphilic molecule inserted between phospholipids ✓ Abundant in animal cell membranes (30 to 50% of membrane lipids) ✓ Absent in plant cells and prokaryotic cells membrane. ✓ Cholesterol affects the membrane physicochemical properties including membrane fluidity and permeability. ❑ Glycolipids ✓ 5% of membrane lipids in the cell. ✓ A component where carbohydrates are combined with simple lipids (fatty acids), forming a glycolipid. ✓ These components are oriented toward the extracellular medium. 77 6. The Plasma Membrane 6.1. Structure and Composition ▪ Membrane Proteins ❑ It is the second major part of the cell membrane. The two main categories of membrane proteins are: 1. Integral Membrane Proteins: ✓ Also called intrinsic proteins, ✓ They are permanently embedded within the cell membrane ✓ the integral proteins are hydrophobic in nature that penetrates the phospholipid bilayer, thus anchoring the protein to the membrane. 2. Peripheral Membrane Proteins: ✓ Also called extrinsic proteins, ✓ They are only temporarily associated with the membrane. ✓ Most peripheral membrane proteins are hydrophilic, so they are usually attached to integral membrane proteins or are loosely bound to the phospholipid head group. ✓ They help in cell signaling and are often associated with ion channels and transmembrane receptors. 78 6. The Plasma Membrane 6.1. Structure and Composition ▪ Membrane Carbohydrates ❑ It is the least abundant component of the cell membrane. Carbohydrates are found on the outside surface of cells that exists two forms: 1. Glycoproteins: ✓ Proteins having carbohydrate chains attached to them. ✓ They are embedded within the cell membrane and are important in cell-to-cell communications and transport of substances across the membrane. 2. Glycolipids: ✓ Lipids having carbohydrate chains attached to them. ✓ They are located on the surface of the cell membrane, extending from the phospholipid bilayer into the extracellular environment. ✓ Glycolipids help to maintain membrane stability and to facilitate cellular recognition and cell-to-cell communication. 79 6. The Plasma Membrane 6.2. Properties ❑ Asymmetry: ✓ The type of phospholipids on both sides of the membrane is not the same; & ✓ The external monolayer is rich in phosphatidylcholine while phosphatidylserine and phosphatidylethanolamine are predominant in the internal monolayer. ✓ The oligosaccharide chains of glycolipids and glycoproteins are located on the external face of the plasma membrane. ❑ Fluidity: ✓ The membrane is fluid in nature ✓ The fluidity of a membrane is affected by the composition of fatty acids within the phospholipid bilayer: Unsaturated fatty acids have double bonds in their lipid chain which results in a kinked hydrocarbon tail making the membrane more fluid and flexible. Saturated fatty acids have no double bonds in their lipid chain which results in a straight hydrocarbon tail making the membrane less fluid and more rigid. ✓ Increasing the length of hydrocarbon chains reduce the membrane fluidity. ✓ The fluidity of a membrane depends on the temperature: increasing the temperature above the melting temperature of phospholipids increases the fluidity of the membrane. ✓ Cholesterol increases the fluidity of the membrane at low temperatures but reduces it at higher temperatures 80 6. The Plasma Membrane 6.2. Properties ▪ Effect of fatty acid composition on membrane fluidity 81 6. The Plasma Membrane 6.2. Properties ▪ Effect of temperature on membrane fluidity ❑ Below Tm: Gel or solid state ❑ Above Tm: Liquid state 82 6. The Plasma Membrane 6.2. Properties ❑ Permeability: ✓ A selectively permeable membrane. ✓ It allows the diffusion of small uncharged molecules such as oxygen, carbon dioxide, and water as well as hydrophobic substances such as lipids across the membrane. 83 6. The Plasma Membrane 6.3. Functions ❑ Maintaining Cell Shape and Morphology: Acting as the base of attachment for the cytoskeleton that helps in cell movement wh help of cytoskeleton ❑ Protection and Cell Defense: Insulates the interior of the cell and provides mechanical support from outside shock and harmful agents ❑ Maintaining Homeostasis: Determines the internal milieu of the cell, the physiological conditions such as temperature and osmotic pressure, etc. ❑ Maintaining Concentration Gradient: Maintains the differences in concentration of substances inside and outside the cell thus helping in their transport eg. Nat/K + pump ❑ Signal Transduction: Receives and processes the extracellular signals by receptor molecules present in the cell membrane and relay them inside the cell for necessary actions ❑ Catalysis of Chemical Reactions: Stimulates chemical reactions that help in the growth and metabolism of the cell using enzymes ❑ Cell Communication: Allows exchange (receiving and sending) of messages between adjacent cells helping them to function in a coordinated manner 84 6. The Plasma Membrane & 6.4. Transport Mechanisms ▪ The plasma membrane is selectively permeable: it allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, (a transport protein) and occasionally, even energy investment for crossing. ▪ The selective permeability of the membrane is mainly due to its amphiphilic character or property; the hydrophobic core of the membrane sets as a barrier for polar and charged molecules hindering their passage across the membrane. 85 6. The Plasma Membrane 6.4. Transport Mechanisms drophobic ▪ Lipid Bilayers are impermeable to ions and most uncharged polar molecules Zhe ❑ The rate at which a molecule diffuses across a lipid bilayer varies enormously depending on the size of the molecule and its solubility properties. ❑ In general, the smaller the molecule and the more hydrophobic, or nonpolar, the more rapidly it will diffuse across the lipid bilayer. highrate ❑ The relative ease with which a variety of solutes can cross a lipid bilayer that lacks membrane transport proteins is shown here (fig →): can't diffuse I ✓ Small, nonpolar molecules, such as molecular oxygen (O2) and carbon dioxide (CO2), dissolve readily in lipid bilayers and therefore diffuse rapidly across them; ✓ Uncharged polar molecules (those with an uneven distribution of electric charge) also diffuse readily across a bilayer, but only if they are small enough. ↑size = drate until they ✓ Lipid bilayers are highly impermeable to all charged substances, including all inorganic ions, no matter how small. on't 86 diffuse 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ The transport of molecules across the cell membrane can be achieved either by a passive transport, an active transport or a vesicular transport. voenergy ▪ Passive transport: ✓ Simple Diffusion a ✓ Facilitated Diffusion ▪ Active transport: ✓ Primary active transport permit ✓ Secondary active transport ▪ Vesicular transport ✓ Endocytosis venergy ✓ Exocytosis 87 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ A type of membrane transport that does not require energy to move substances across cell membranes. ❑ It describes the movement of substances across a cell membrane down their concentration gradients: molecules that are passively transported across the membrane will move from an area of high concentration to an area of low concentration. ❑ The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. ❑ Cell membranes are selectively permeable, so only certain substances can passively diffuse directly across the membrane. ❑ Passive transport systems include: simple diffusion and facilitated diffusion. 88 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Simple Diffusion: ✓ A passive process of transport ✓ It describes the passive movement of a solute from an area of high concentration to an area with lower concentration. ✓ It is described as moving solutes "down the concentration gradient". ✓ Diffusion continues until the gradient is eliminated when the concentration of the solute is the same on both sides of the membrane; the equilibrium is then reached. ✓ Diffusion expends no energy. ✓ It occurs without the help of any protein molecule. Passive diffusion on a cell membrane. Examples: The movement of water, oxygen, carbon dioxide, ethanol, and urea. ❖ One of the most important factors that determines how rapidly a substance diffuses through 89 the lipid bilayer is the lipid solubility of the substance. 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Simple Diffusion: Factors affecting the diffusion ✓ “Steepness” of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes. kI m n2areb 31 = rated eq ✓ Mass or molecular weight of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. ✓ Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. ✓ Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials. 90 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Facilitated Diffusion: ✓ A passive transport process. ✓ Movement of solutes down the concentration gradient. ✓ It occurs with the help of a transmembrane protein molecule. ✓ Facilitated diffusion can occur either through a channel protein or through a carrier protein. only certain mics can use it ✓ It is a selective process, which means the membrane allows only selective molecules and ions to pass through it, denying passage to others. ✓ Examples: Transport of glucose, sodium ions, and potassium ions. 91 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Facilitated Diffusion: What is the difference between a channel and a carrier protein? ❑ A channel protein: ✓ A transmembrane protein that span the membrane ✓ It allows the passive transport of ions- mainly ✓ Mainly gated-channels open due to a certain signal ✓ No binding is required to allow the channel to open or transport the ion ❑ A carrier protein ✓ It possesses a specific binding site to the transported solute ✓ It carries the molecules or ions across the membrane by changing their shape after binding (conformational change) S Bind - cont change 92 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Facilitated Diffusion: Biological Examples of Facilitated Diffusion ✓ Transport of glucose across the cell membrane with the help of carrier proteins called glucose transporter. ✓ Passage of water across the lipid bilayer of the cell membrane using specific transmembrane channel proteins called aquaporins ✓ Selective transport of ions and solutes in and out of the cell using membrane proteins called ion channels. 93 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Passive Transport: ❑ Facilitated Diffusion: ins s 94 6. The Plasma Membrane Facilitated diff 6.4. Transport Mechanisms ret in some Passive a transport types ↳ passivfe 95 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Active Transport: ❑ A type of membrane transport that requires energy to move substances across cell membranes with the help of a membrane protein. + require transporter ❑ It describes the movement of substances across a cell membrane against their concentration gradients: molecules that are actively transported across the membrane will move from an area of low concentration to an area of high concentration using cellular energy. ↳ chemical ❑ Active transport is highly selective and regulated, with different transporters specific to different molecules or ions. ❑ It is essential for various physiological processes, such as nutrient uptake, hormone secretion, and nerve impulse transmission. ❑ Depending on the type of energy used to achieve the movement of solutes across the membrane, active transport is classified into : primary active transport and secondary active transport. 96 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Active Transport: ❑ Concentration gradient vs Electrochemical gradient: ✓ A concentration gradient or a chemical gradient refers to the different concentrations of a substance across a space or a membrane. ✓ Concentration gradients are the driving force of the passive diffusion of uncharged solutes. ✓ In living systems, gradients are more complex. Due to the presence of charged entities (ions and charged solutes such as proteins, etc.), there is also an electrical gradient, a difference of charge, across the plasma membrane. ✓ The driving force for the passive transport of charged solutes such as ions across the membrane is the electrochemical gradient which is the combined gradient of concentration and electrical charge that affects an ion or any charged solute. 97 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Active Transport: ❑ Concentration gradient vs Electrochemical gradient: high ✓ The active transport moves the solute against its ↑ electrochemical gradient. low ↑ ↑ rate 98 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Active Transport: ❑ Primary Active Transport: ✓ Also called direct active transport ✓ It directly uses metabolic energy (ATP) to transport molecules across a membrane against their concentration or electrochemical gradients. ✓ Substances that are transported across the cell membrane by primary active transport include metal ions, such as Na+, K+, Mg2+, and Ca2+. ✓ This type of active transport requires protein transporters called pumps to cross membranes. Ito allow this transport 99 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Active Transport: ❑ Primary Active Transport: Example: The Sodium-Potassium Pump ✓ Also called Na+/K+-ATPase ✓ It is a ATP-dependent transporter protein pump found in animals’ cell (plasma) membrane. ✓ Its fundamental purpose is to transport sodium and potassium ions across the cell in the ratio of 3: 2. ✓ During one cycle, It exchanges three sodium ions (exports out of the cell) for every two potassium ions (import into the cell) against the electrochemical gradient. ✓ Thesodium-potassium pump helps to maintain the membrane potential. laulehon kn 3na 31 may Hein 100 eq 6. The Plasma Membrane used for the I 6.4. Transport Mechanisms passive energy against gradientto - - ▪ Active Transport: ❑ Secondary Active Transport: ✓ Also called coupled transport or cotransport ✓ It involves the movement of substances across the cell membrane against their electrochemical gradient utilizing energy in other forms than ATP. ✓ It couples the passive movement of a solute down its concentration gradient to the movement of another solute against its concentration gradient. ✓ This energy comes from the electrochemical gradient created by pumping ions out of the cell, which powers the movement of another ion or molecule in the same A symporter moves two types of direction (symport) or opposite direction (antiport) molecules in the same direction of the membrane. while an antiporter moves two ✓ Secondary active transport relies on gradients types of molecules in opposite established by primary active transporters to move directions. molecules across the membrane. 101 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Secondary Active Transport: Example: Cotransporting glucose and sodium in the small intestine using sodium-glucose transporter sympot facilitated diffusion 102 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Transporters used in Passive and Active Transport: a for kindrea a o A uniporter carries one molecule or ion. It can be used as a passive or a primary active transporter o A symporter carries two different molecules or ions, both in the same direction. o An antiporter also carries two different molecules or ions, but in different directions. o Symporters and antiporters are commonly secondary active transporters or cotransporters. 103 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles. ❑A large particle, however, cannot pass through the membrane, even with energy supplied by the cell. ❑ Macromolecules are too large to move with membrane proteins and must be transported across membranes in vesicles. (A) During exocytosis, a vesicle fuses with ❑ Vesicular transport allows materials to exit or enter the the plasma membrane, releasing its cell. content to the cell’s surroundings. (B) During endocytosis, extracellular ❑ Two transport mechanisms can take place: endocytosis materials are captured by vesicles that and exocytosis. bud inward from the plasma membrane and are carried into the cell. 104 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytosis: ✓ Exocytosis is the process of moving materials from within a cell to the exterior of the cell. ✓ This process requires energy and is therefore a type of active transport. ✓ In exocytosis, membrane-bound vesicles containing cellular molecules are transported to the cell membrane. The vesicles fuse with the cell membrane and expel their contents to the exterior of the cell. ✓ Exocytosis serves several important functions as it allows cells to secrete waste substances and molecules, such as hormones and proteins. Exocytosis is also important for chemical signal messaging and cell to cell communication. In addition, exocytosis is used to rebuild the cell membrane by fusing lipids and proteins removed through endocytosis back into the membrane. 105 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytotic Vesicles: two types ✓ Exocytotic vesicles containing protein products are typically derived from the Golgi apparatus. ✓ Proteins and lipids synthesized in the endoplasmic reticulum are sent to Golgi complexes for modification and sorting. Once processed, the products are contained within secretory vesicles, which bud from the trans face of the Golgi apparatus. 106 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytotic Vesicles: ✓ Other vesicles that fuse with the cell membrane do not come directly from the Golgi apparatus. ✓ Some vesicles are formed from early endosomes, which are membrane sacs found in the cytoplasm. Early endosomes fuse with vesicles internalized by endocytosis of the cell membrane. ✓ These endosomes sort the internalized material (proteins, lipids, microbes, etc.) and direct the substances to their proper destinations. Transport vesicles bud off from early endosomes sending waste material on to lysosomes for degradation, while returning proteins and lipids to the cell membrane. ✓ Vesicles located at synaptic terminals in neurons are also examples of vesicles that are not derived from Golgi complexes. 107 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Types of Exocytosis: 1. Constitutive exocytosis: involves the regular secretion of molecules. This action is performed by all cells. Constitutive exocytosis functions to deliver membrane proteins and lipids to the cell's surface and to expel substances to the cell's exterior. 2. Regulated exocytosis: relies on the presence of extracellular signals for the expulsion of materials within vesicles. Regulated exocytosis occurs commonly in secretory cells and not in all cell types (mainly in neurons). Secretory cells store products such as hormones, neurotransmitters, and digestive enzymes that are released only when triggered by extracellular signals or an increase of calcium ions. That’s why, regulated exocytosis is also called the Ca2+ triggered non-constitutive exocytosis. Secretory vesicles are not incorporated into the cell membrane but fuse only long enough to release their contents. Once the delivery has been made, the vesicles reform and return to the cytoplasm. co 3. Vesicular exocytosis: involves the fusion of vesicles with lysosomes. These organelles contain hydrolytic enzymes that break down waste materials, microbes, and cellular debris. Lysosomes carry their digested material to the cell membrane where they fuse with the membrane and release their contents into the extracellular matrix. It is found only in bacteria. no paral 108 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytosis Steps: ✓ Exocytosis occurs in four steps in constitutive exocytosis and in five steps in regulated exocytosis. These steps include vesicle trafficking, tethering, docking, priming, and fusing. 1. Vesicle trafficking: Vesicles containing molecules are transported from within the cell to the cell membrane along microtubules of the cytoskeleton. Movement of the vesicles is powered by the motor proteins kinesins, dyneins, and myosins. 2. Tethering: The vesicles reaches the cell membrane and becomes in contact with the cell membrane. 3. Docking: The vesicle membrane attaches to the cell membrane. The phospholipid bilayers of the vesicle membrane and cell membrane begin to merge. 4. Priming: it occurs in regulated exocytosis and not in constitutive exocytosis. This step involves specific modifications (molecular rearrangements) that must happen in certain cell membrane one molecules for exocytosis to occur. These modifications are required for signaling processes that trigger exocytosis to take place. 5. Fusion: Fusion of the vesicle membrane with the cell membrane releases the vesicle contents outside the cell. 109 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytosis Steps: ✓ There are two types of fusion that can take place in exocytosis. 1) Complete fusion: o In complete fusion, the vesicle membrane fully fuses with the cell membrane. o The energy required to separate and fuse the lipid membranes comes from ATP. o The fusion of the membranes creates a fusion pore, which allows the contents of the vesicle to be expelled as the vesicle becomes part of the cell membrane. 2) kiss-and-run fusion: o In kiss-and-run fusion, the vesicle temporarily fuses with the cell membrane long enough to create a fusion pore and release its contents to the exterior of the cell. o The vesicle then pulls away from the cell membrane and reforms before returning to the interior of the cell. 110 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Exocytosis Steps: 111 6. The Plasma Membrane 6.4. Transport Mechanisms Read ▪ Vesicular Transport: ❑ Examples on exocytosis in our body: ✓ In the pancreas, a cluster of cells called islets of Langerhans secretes the hormones glucagon and insulin. These hormones are stored within secretory granules and are released into the blood by exocytosis, thus maintaining blood sugar level. ✓ Exocytosis from other cells in the pancreas releases digestive enzymes into the gut. ✓ Communication between neurons through the transmission of chemical signals called neurotransmitters occurs through exocytosis. The neurotransmitters are stored in vesicles and lie next to the cytoplasmic face of the plasma membrane. When the appropriate signal is given, the vesicles having the neurotransmitters make contact with the cell membrane and secrete their contents into the space between the two neurons. This allows the adjacent neuron to receive those neurotransmitters. ✓ Cellular waste products or toxins are regularly removed from the cell by exocytosis. The byproducts of aerobic respiration, such as water and carbon dioxide, are also removed from the cell by exocytosis. This helps to maintain homeostasis. ✓ Immune cells such as macrophages engulf foreign microorganisms such as viruses, which are then destroyed and removed through exocytosis. This prevents body cells from being infected. 112 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Endocytosis: ✓ Endocytosis is a cellular process in which substances are brought into the cell from their external environment. ✓ The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested materials. ✓ It is how cells get the nutrients they need to grow and develop. Substances internalized by endocytosis include fluids, electrolytes, proteins, and other macromolecules. 113 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Basic Steps of Endocytosis: 1. The cell membrane folds inward, forming a cavity that contains fluid, dissolved substances, food materials, foreign matter, microorganisms, and some other substances. This process is known as invagination. 2. The cell membrane then folds back on itself until it forms a uniformly enclosed membrane around the trapped molecules, forming a vesicle. 3. The vesicle gets detached from the cell membrane, which is then processed by the cell. 114 6. The Plasma Membrane 6.4. Transport Mechanisms ▪ Vesicular Transport: ❑ Types of Endocytosis: There are three primary types of endocytosis: 1. Phagocytosis: also called "cell eating" and involves the ingestion of large particles, such as microorganisms and cell debris, via large vesicles called phagosomes (generally >250 nm in diameter). 2. Pinocytosis: also called "cell drinking", involves the ingestion of fluid and molecules via small pinocytic vesicles (