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Cell Structure and Function 1 Cell Structure • In 1655, the English scientist Robert Hooke coined the term “cellulae” for the small box-like structures he saw while examining a thin slice of cork under a microscope. 2 INTRODUCTION • • • • The cell is the basic unit of structure and function in living things. Cells vary in their shape size, and arrangements but all cells have similar components, each with a particular function. Some of the 100 trillion of cells make up human body. All human cell are microscopic in size, shape and function. The diameter range from 7.5 micrometer (RBC) to 150 mm (ovum). ahmad ata 3 3 Generalized Eukaryotic Cell 4 A. Protoplasm 1. Water - 70-85% (0.3 nm) 2. Electrolytes (ions) - K+, Na+, Ca++, Mg++, SO4--, PO4-3, HCO3-, Cl- are most abundant 3. Proteins - 10-20% a. Structural b. Enzymes 4. Lipids - 2-3% (most in membranes) 5. Carbohydrates - 1% (glycogen; energy source) 5 Animal Cell Animal cell anatomy 6 Visualizing Cells 7 Cells • • Basic unit of structure and function of all living things; made of protoplasm Microscopic organisms that carry on all functions of life – – – – – – Take in food and oxygen Produce heat and energy Move and adapt to their environment Eliminate waste Perform special functions Reproduce to create new identical cells 8 Basic cell structure Cytoplasm • • • Semifluid inside the cell membrane but outside the nucleus Contains water (70 to 90 percent), proteins, lipids, carbohydrates, minerals and salts Site for all chemical reactions in cell 9 Basic Structure Organelles • • • Cell structure that help a cell to function Located in cytoplasm Main organelles include the nucleus, mitochondria, ribosome, lysosomes, centrioles, golgi apparatus and endoplasmis reticulum 10 Basic Structure Nucleus • • • • Mass in cytoplasm Separated from cytoplasm by a nuclear membrane and contains pores to allow substances to pass between the nucleus Often called the brains of the cell Controls many cell activities, including the process of mitosis or reproduction 11 Basic Structure Nucleolus • • • • • One or more small round bodies located inside the nucleus Important in reproduction of the cell Ribosomes made of ribonucleic acid (RNA) and protein are manufactured in the nucleolus Ribosomes move to cytoplasm to aid in synthesis (production) of protein Ribosomes can exist freely in the cytoplasm or be attached to the endoplasmic reticulum 12 Basic Structure Chromatin • • • Located inside the nucleus Made up of deoxyribonucleic aid (DNA) and protein Chromatin condenses to from rod-like structure called chromosomes during cell reproduction 13 Chromosomes • • • Human cell has 46 chromosomes or 23 pairs Each chromosome contains between 30,000 and 45,000 genes, the structures that carry inherited characteristics Each gene has a specific and unique sequence of about 1000 base pairs of DNA 14 Basic Structure Mitochrondria • • • • Rod-shaped organelles located throughout the cytoplasm Called powerhouses of the cell Break down carbohydrates, proteins and fats to produce adenoside triphosphate (ATP) which is major energy source of the cell Cell can contain just 1 to over 1000 mitochrondria depending on how much energy the cell requires. 15 Cell Structure Golgi Apparatus • • • Stack of membrane layers located in cytoplasm Produces, stores, and packages secretions for discharge from the cell Cells of salivary, gastric, and pancreatic glands have large numbers of Golgi appartus 16 Cell Structure Endoplasmic reticulum • • • • • Fine network of tubular structures in cytoplasm Allows for transport of materials into and out of the nucleus Also aids in synthesis and storage of proteins Rough endoplasmic reticulum contains ribosomes which are the sites for protein synthesis Smooth endoplasmic reticulum does not contain ribosomes and is not present in all cells; but it does assist with cholesterol synthesis, fat metabolism and detoxification of drugs 17 Cell Structure Vacuoles • • • Pouch like structures found throughout cytoplasm Have a vascuolar membrane with same structure as cell membrane Filled with watery substances, stored food or waste products 18 Cell Structure Pinocytic vesicles • • • • Pocketlike folds in the cell membrane Allow large molecules such as protein and fat to enter the cell When molecule is inside the cell, the pocket closes to form a vacuole, or bubble, in the cytoplasm When cell needs energy, vesicles fuse with lysosomes to allow proteins and fats to be digested and used by mitochondria to produce ATP(energy). 19 Membrane Bound Organelles Nucleus • • • Lysosomes – vesicle containing digestive enzymes that break down food/foreign particles Vacuoles – food storage and water regulation Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms 1 µm Lysosome Lysosome contains active hydrolytic enzymes Food vacuole fuses with lysosome Hydrolytic enzymes digest food particles Digestive enzymes Lysosome Plasma membrane Digestion Food vacuole (a) Phagocytosis: lysosome digesting food 20 Cytoskeleton • • The eukaryotic cytoskeleton is a network of filaments and tubules that extends from the nucleus to the plasma membrane that support cell shape and anchor organelles. Protein fibers – Actin filaments cell movement – Intermediate filaments – Microtubules centrioles 21 Centrioles • • Centrioles are short cylinders with a 9 pattern of microtubule triplets. Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella. 22 Cilia and Flagella • • • Contain specialized arrangements of microtubules Are locomotor appendages of some cells Cilia and flagella share a common ultrastructure Outer microtubule doublet Dynein arms 0.1 µm Central microtubule Outer doublets cross-linking proteins inside Microtubules Radial spoke Plasma membrane Basal body Plasma membrane (b) 0.5 µm (a) 0.1 µm Triplet (c) Cross section of basal body 23 Cilia and Flagella • • • Cilia (small and numerous) and flagella (large and single) have a 9 + 2 pattern of microtubules and are involved in cell movement. Cilia and flagella move when the microtubule doublets slide past one another. Each cilium and flagellum has a basal body at its base. 24 Cilia and Flagella (a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM). Direction of swimming 1 µm (b) Motion of cilia. Cilia have a backand-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM). 15 µm 25 Cell Membrane: Proteins The three types of membrane proteins: integral, peripheral, and lipid-anchored Figure 3-6 26 Cell Membrane Concept Map of cell membrane components Figure 3-9 27 The cell membrane covers cells of various sizes, shapes, and functions Figure 3-10 28 Membrane Function • All cells are surrounded by a plasma membrane. • Cell membranes are composed of a lipid bilayer with globular proteins embedded in the bilayer. • On the external surface, carbohydrate groups join with lipids to form glycolipids, and with proteins to form glycoproteins. These function as cell identity markers. 29 Fluid Mosaic Model • In 1972, S. Singer and G. Nicolson proposed the Fluid Mosaic Model of membrane structure Glycoprotein Extracellular fluid Glycolipid Carbohydrate Cholesterol Transmembrane proteins Peripheral protein Cytoplasm Filaments of cytoskeleton 30 Phospholipids • Glycerol • Two fatty acids • Phosphate group Hydrophilic heads ECF WATER Hydrophobic tails ICF WATER 31 Phospholipid Bilayer • Mainly 2 layers of phospholipids; the non-polar tails point inward and the polar heads are on the surface. • Contains cholesterol in animal cells. • Is fluid, allowing proteins to move around within the bilayer. Polar hydro-philic heads Nonpolar hydro-phobic tails Polar hydro-philic heads 32 Steroid Cholesterol • Effects on membrane fluidity within the animal cell membrane Cholesterol 33 Membrane Proteins • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer • Peripheral proteins are appendages loosely bound to the surface of the membrane Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral protein 34 Integral proteins • Penetrate the hydrophobic core of the lipid bilayer • Are often transmembrane proteins, completely spanning the membrane N-terminus EXTRACELLULAR SIDE C-terminus a Helix CYTOPLASMIC SIDE 35 Types of Membrane Proteins • Structural Proteins • Maintain membrane shape and integrity, movement • Channel Proteins • Pore-like proteins • Enable small ions to pass • Carrier proteins • Shuttle specific substances across membrane Functions of Cell Membranes • Regulate the passage of substance into and out of cells and between cell organelles and cytosol • Detect chemical messengers arriving at the surface • Link adjacent cells together by membrane junctions • Anchor cells to the extracellular matrix 37 6 Major Functions Of Membrane Proteins 1. Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane 2. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. 3. ATP Enzymes Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. Signal Receptor 38 6 Major Functions Of Membrane Proteins 4. Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Glycoprotein 5. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions 6. Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes 39 Functions of Plasma Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme Cell surface identity marker Cell adhesion Cell surface receptor Attachment to the cytoskeleton 40 Chapter Summary Cell Membrane and Associated Structures I. The structure of the cell (plasma) membrane is described by a fluid-mosaic model. A. The membrane is composed predominately of a double layer of phospholipids. B. The membrane also contains proteins most of which span its entire width. II. Some cells move by extending pseudopods; cilia and flagella protrude from the cell membrane of some specialized cells. III. In the process of exocytosis, invaginations of the cell membrane allow the cells to take up molecules from the external environment. A. In phagocytosis, the cell extends pseudopods, which eventually fuse together to create a food vacuole; pinocytosis involves the formation of a narrow furrow in the membrane which eventually fuses. B. Receptor-mediated endocytosis requires the interaction of a specific molecule in the extracellular environment with a specific receptor protein in the cell membrane. C. Exocytosis is the reverse of endocytosis and is a process that allows the cell to secrete its products. Chapter Summary Cytoplasm and Its Organelles I. Microfilaments and microtubules produce a cytoskeleton, which aids movements of organelles within a cell. II. Lysosomes contain digestive enzymes and are responsible for the elimination of structures and molecules within the cell and for digestion of the contents of the phagocytic food vacuoles. III. Mitochondria serve as the major sites for energy production within the cell. They have an outer membrane with a smooth contour and an inner membrane with infoldings called cristae. IV. The endoplasmic reticulum is a system of membranous tubules in the cell. A. The rough endoplasmic reticulum is covered with ribosomes and is involved in protein synthesis. B. The smooth endoplasmic reticulum provides a site for many enzymatic reactions and, in skeletal muscles, serves to store Ca2+. Chapter Summary Cell Nucleus I. The cell nucleus is surrounded by a double-layered nuclear membrane. At some points, the two layers are fused by nuclear pore complexes that allow for the passage of molecules II. Proteins destined for secretion are produced in ribosomes located on the rough endoplasmic reticulum and enter the cisternae of this organelle. III. Secretory proteins move from the rough endoplasmic reticulum to the Golgi complex, which consists of a stack of membranous sac. A. The Golgi complex modifies the proteins it contains, separates different proteins, and packages them in vesicles. B. Secretory vesicles from the Golgi apparatus fuse with the cell membrane and release their products by exocytosis. Membrane Transport 44 Membrane Transport • The plasma membrane is the boundary that separates the living cell from its nonliving surroundings • In order to survive, A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Materials must enter and leave the cell through the plasma membrane. • Membrane structure results in selective permeability, it allows some substances to cross it more easily than others 45 Membrane Transport • The plasma membrane exhibits selective permeability - It allows some substances to cross it more easily than others 46 Passive Transport • Passive transport is diffusion of a substance across a membrane with no energy investment • 4 types • Simple diffusion • Dialysis • Osmosis • Facilitated diffusion 47 Solutions and Transport • Solution – homogeneous mixture of two or more components • Solvent – dissolving medium • Solutes – components in smaller quantities within a solution • Intracellular fluid – nucleoplasm and cytosol • Extracellular fluid • Interstitial fluid – fluid on the exterior of the cell within tissues • Plasma – fluid component of blood 48 Diffusion • • • The net movement of a substance from an area of higher concentration to an area of lower concentration - down a concentration gradient Caused by the constant random motion of all atoms and molecules Movement of individual atoms & molecules is random, but each substance moves down its own concentration gradient. Lump of sugar Random movement leads to net movement down a concentration gradient Water No net movement at equilibrium 49 Diffusion Across a Membrane • • • The membrane has pores large enough for the molecules to pass through. Random movement of the molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Net diffusion Net diffusion Equilibrium 50 Diffusion Across a Membrane • • • Two different solutes are separated by a membrane that is permeable to both Each solute diffuses down its own concentration gradient. There will be a net diffusion of the purple molecules toward the left, even though the total solute concentration was initially greater on the left side Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium 51 The Permeability of the Lipid Bilayer • Permeability Factors • Lipid solubility • Size • Charge • Presence of channels and transporters • Hydrophobic molecules are lipid soluble and can pass through the membrane rapidly • Polar molecules do not cross the membrane rapidly • Transport proteins allow passage of hydrophilic substances across the membrane 52 Passive Transport Processes • 3 special types of diffusion that involve movement of materials across a semipermeable membrane • Dialysis/selective diffusion of solutes • Lipid-soluble materials • Small molecules that can pass through membrane pores unassisted • Facilitated diffusion substances require a protein carrier for passive transport • Osmosis – simple diffusion of water 53 Osmosis • Diffusion of the solvent across a semipermeable membrane. • In living systems the solvent is always water, so biologists generally define osmosis as the diffusion of water across a semipermeable membrane: 54 Osmosis Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis Water moves from an area of higher free water concentration to an area of lower free water concentration 55 Osmotic Pressure • Osmotic pressure of a solution is the pressure needed to keep it in equilibrium with pure H20. • The higher the concentration of solutes in a solution, the higher its osmotic pressure. • Tonicity is the ability of a solution to cause a cell to gain or lose water – based on the concentration of solutes 56 Tonicity • If 2 solutions have equal [solutes], they are called isotonic • If one has a higher [solute], and lower [solvent], is hypertonic • The one with a lower [solute], and higher [solvent], is hypotonic Hypotonic solution H2O Lysed Isotonic solution Hypertonic solution H2O H2O Normal H2O Shriveled 57 Water Balance In Cells With Walls (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H2O Turgid (normal) H2O H2O Flaccid H2O Plasmolyzed 58 My definition of Osmosis • Osmosis is the diffusion of water across a semi-permeable membrane from a hypotonic solution to a hypertonic solution 59 Facilitated Diffusion • • Diffusion of solutes through a semipermeable membrane with the help of special transport proteins i.e. large polar molecules and ions that cannot pass through phospholipid bilayer. Two types of transport proteins can help ions and large polar molecules diffuse through cell membranes: Channel proteins – provide a narrow channel for the substance to pass through. • Carrier proteins – physically bind to the substance on one side of membrane and release it on the other. • EXTRACELLULAR FLUID Channel protein CYTOPLASM Solute Carrier protein Solute 60 Facilitated Diffusion • Specific – each channel or carrier transports certain ions or molecules only • Passive – direction of net movement is always down the concentration gradient • Saturates – once all transport proteins are in use, rate of diffusion cannot be increased further 61 Active Transport • Uses energy (from ATP) to move a substance against its natural tendency e.g. up a concentration gradient. • Requires the use of carrier proteins (transport proteins that physically bind to the substance being transported). • 2 types: • Membrane pump (protein-mediated active transport) • Coupled transport (cotransport). 62 Membrane Pump • A carrier protein uses energy from ATP to move a substance across a membrane, up its concentration gradient: 63 The Sodium-potassium Pump • One type of active transport system [Na+] high [K+] low 1. Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Na+ + NaEXTRACELLULAR FLUID [Na+] low Na+ [K+] high CYTOPLASM 2. Na+ binding stimulates phosphorylation by ATP. Na+ Na+ Na+ Na+ 6. K+ is released and Na+ sites are receptive again; the cycle repeats. ATP P ADP 3. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. Na+ K+ P K+ 5. Loss of the phosphate restores the protein’s original conformation. K+ 4. Extracellular K+ binds to the protein, triggering release of the Phosphate group. K+ K+ K+ Pi P Pi 64 Coupled transport • 2 stages: • Carrier protein uses ATP to move a substance across the membrane against its concentration gradient. Storing energy. • Coupled transport protein allows the substance to move down its concentration gradient using the stored energy to move a second substance up its concentration gradient: 65 Review: Passive And Active Transport Compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. 66 Bulk Transport • Allows small particles, or groups of molecules to enter or leave a cell without actually passing through the membrane. • 2 mechanisms of bulk transport: endocytosis and exocytosis. 67 Endocytosis • The plasma membrane envelops small particles or fluid, then seals on itself to form a vesicle or vacuole which enters the cell: • Phagocytosis • Pinocytosis • Receptor-Mediated Endocytosis - 68 Three Types Of Endocytosis PHAGOCYTOSIS In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. EXTRACELLULAR FLUID Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. 1 µm CYTOPLASM Pseudopodium 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle 69 Process of Phagocytosis 70 Receptor-mediated Endocytosis Coat protein Receptor Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. Coated vesicle Coated pit Ligand Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Plasma membrane 0.25 µm 71 Exocytosis • The reverse of endocytosis • During this process, the membrane of a vesicle fuses with the plasma membrane and its contents are released outside the cell: 72 Cell Junctions • Long-lasting or permanent connections between adjacent cells, 3 types of cell junctions: TIGHT JUNCTIONS Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continuous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells. DESMOSOMES Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm. Tight junctions Intermediate filaments Desmosome Gap junctions Space between Plasma membranes cells of adjacent cells 1 µm Extracellular matrix Gap junction 0.1 µm GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, including heart muscle and animal embryos. 73