BIOL1000-2024-Lecture3-2 PDF - Cell Biology Lecture 3

Content – Cell Biology - Lecture 3 Introduction to cells Anatomy of a cell Membranes Osmoregulation Membrane transport Video 1 1  Basic concepts in cells Internal membranes compartmentalize functions Nucleus containing genetic instr...

Content – Cell Biology - Lecture 3 Introduction to cells Anatomy of a cell Membranes Osmoregulation Membrane transport Video 1 1  Basic concepts in cells Internal membranes compartmentalize functions Nucleus containing genetic instructions Metabolic functions: Endomembrane system regulates protein traffic and performs metabolic functions in the cell Energy: Mitochondria change energy Organization: The cytoskeleton is a network of fibers organizing structures and activities of the cell Communications: Extracellular components and connections between cells help coordinate cellular activities Video 1 2  The cell Basic structural and functional unit of every organism Two types: – Prokaryotic – Bacteria and Archaea – Eukaryotic – fungi, animals, plants etc. Electron microscopy Super- Light microscopy resolution microscopy Unaided eye Length of some nerve Nucleus Most Smallest and Plant and Most Small bacteria Proteins muscle animal bacteria Viruses molecules Human cells cells Ribo- Human Chicken Frog egg Mitochondrion height egg egg somes Lipids Atoms 10 m 1m 0.1 m 1 cm 1 mm 100 μm 10 μm 1 μm 100 nm 10 nm 1 nm 0.1 nm Video 1 3  Basic features of all cells ENDOPLASMIC Plasma membrane RETICULUM (ER) Semifluid substance Rough ER Smooth ER Nuclear envelope called cytosol Flagellum Nucleolus Chromatin NUCLEUS Chromosomes (carry Centrosome genes) Plasma membrane Ribosomes (make CYTOSKELETON: proteins) Microfilaments Intermediate filaments Microtubules Ribosomes Microvilli Golgi apparatus Peroxisome Lysosome Mitochondrion Video 1 4 Why are cells so small? Metabolic requirements of cell set upper limit on size Small cells have a greater surface area to volume ratio – more surface to allow passage of materials in and out of cell Video 1 5 High surface area to volume ratio Larger organisms are not made of larger cells, just more cells Maintains high surface area-to- volume ratio Video 1 6  Prokaryotic cells No membrane-bound organelles Cytoplasm bound by plasma membrane No nucleus DNA in an unbound region called nucleoid Video 1 7  Eukaryotic cells Membrane-bound organelles DNA in a nucleus bounded by a membranous nuclear envelope GENERALLY MUCH LARGER THAN PROKARYOTIC CELLS Video 1 8 Video of cell biology 9 Eukaryotic cell Video 1 10 Tour of an animal cell Video 1 11 Nucleus: Information centre Nucleus is usually most conspicuous organelle Nuclear membrane is a double layer (outer and inner membrane) – Each membrane is a lipid bilayer Video 2 12 Nucleus: Pore complex Pores in membrane regulate entry and exit of molecules from nucleus – lined by a pore complex of proteins Shape of the nucleus is maintained by nuclear lamina – composed of protein Video 2 13  Nucleus: Information centre In nucleus, DNA is organized into discrete units called chromosomes A chromosome is composed of a single DNA molecule associated with proteins DNA and proteins of chromosome together are called chromatin (diffuse mass when not dividing) Nucleolus within nucleus is the site of ribosomal RNA (rRNA) synthesis, and construction of small and large subunits of ribosomes Video 2 14  Ribosomes: Protein Factories Ribosomes are particles made of ribosomal RNA and protein Ribosomes carry out protein synthesis – In the cytosol (free ribosomes) – On the outside of endoplasmic reticulum (bound ribosomes) Video 2 15  Endomembrane system Regulates protein traffic and performs metabolic functions – Nuclear envelope – Endoplasmic reticulum – Golgi apparatus – Lysosomes – Vacuoles – Plasma membrane Video 2 16  Endoplasmic reticulum: Biosynthetic factory ER accounts for more than half of total membrane in many eukaryotic cells ER membrane is continuous with nuclear envelope Components are either continuous or connected via transfer by vesicles Video 2 17 Endoplasmic reticulum: Biosynthetic factory Two distinct regions of ER – Smooth ER, which lacks ribosomes – Rough ER, surface is studded with ribosomes Video 2 18  Smooth ER Synthesizes lipids (oils, phospholipids, steroids/hormones) – Testes, ovaries rich in smooth ER Metabolizes carbohydrates Detoxifies drugs and poisons – Liver cells rich in smooth ER – Some drugs induce proliferation of more smooth ER, leading to drug tolerance Stores calcium ions Video 2 19  Rough ER Has bound ribosomes which synthesize many secretory proteins Most are glycoproteins (proteins covalently bonded to carbohydrates) Transports proteins via transport vesicles – Proteins wrapped in membrane Is a membrane factory for the cell Video 2 20 Video: Staining of Endoplasmic Reticulum Video 2 21  Golgi apparatus: Shipping and receiving Modifies ER products Manufactures certain molecules – Polysaccharides, pectins Sorts and packages materials into transport vesicles Consists of flattened membranous sacs called cisternae Video 2 22 Video: Golgi Complex in 3-D Video 2 23  Lysosomes: Digestive compartments Membranous sac of hydrolytic enzymes that can digest macromolecules Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids Lysosomal enzymes work best in acidic environment inside lysosome Video 3 24  Lysosomes: Digestive compartments Some cells can engulf another cell by phagocytosis; forming a food vacuole A lysosome fuses with food vacuole and digests the molecules Video 3 25 Animation: Lysosome Formation Video 3 26  Lysosomes: Digestive compartments Lysosomes also recycle cell’s own organelles and macromolecules, a process called autophagy Video 3 27 Video: Phagocytosis in Action Video 3 28  Evolutionary of eukaryotic cells Mitochondria and chloroplasts have similarities with bacteria (prokaryotes) – Enveloped by a double membrane – Contain free ribosomes and circular DNA molecules – Grow and reproduce somewhat independently in cells Video 3 29  Endosymbiont Theory Prokaryotic ancestor of eukaryotic cells engulfed energy producing prokaryotes Video 3 30  Mitochondria: Chemical energy conversion Have a smooth outer membrane and an inner membrane folded into cristae Inner membrane creates two compartments: intermembrane space and mitochondrial matrix Video 3 31  Mitochondria: Chemical energy conversion Mitochondria are in nearly all eukaryotic cells Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix Cristae present a large surface area for enzymes that synthesize ATP Video 3 32 Video: Mitochondria in 3-D Video 3 33  Cytoskeleton organizes structures and activities in cells Network of fibres extending throughout cytoplasm It organizes the cell’s structures and activities, anchoring many organelles Video 4 34  Cytoskeleton organizes structures and activities in cells The cytoskeleton composed of three types of molecular structures – Microtubules (thickest) – Microfilaments (thinnest) – Intermediate filaments (intermediate..) It helps to support the cell and maintain its shape Video 4 35  Cytoskeleton: Support and motility Cytoskeleton interacts with motor proteins to produce motility Inside the cell, vesicles can travel along “monorails” (microtubules or microfilaments) Video 4 36 Video: The Cytoskeleton in Neuron Growth Cone Video 4 37  Cytoskeleton: Microtubules Hollow tube made of globular protein called tubulin Tubulin dimer made of 2 subunits: α-tubulin, β- tubulin – Serve as tracks for motor proteins – Guide vesicles from ER to Golgi to plasma membrane – Cell division Video 4 38 Video: Interphase Microtubule Dynamics Video 4 39  Microtubules and Centrosomes Important microtubule function is chromosome separation during cell division In many cells, microtubules grow out from a centrosome near the nucleus Centrosome is “microtubule- organizing centre” Video 4 40  Centrosome containing a pair of centrioles In animal cells, centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Video 4 41  Cilia and flagella Microtubules control the beating of cilia and flagella, locomotor appendages of some cells Cilia and flagella share a common structure “ 9 + 2” microtubules Video 5 42  Cilia and Flagella Move by dynein “walking” Dynein arms alternately grab, move, and release outer microtubules – Protein cross-links limit sliding – Forces exerted by dynein arms cause doublets to curve, bending cilium or flagellum Video 5 43 Video: Movement of Organelles In Vitro Video 5 44  Cytoskeleton: Microfilaments Twisted double chain of actin subunits – SOLID ROD Bear tension, resist pulling forces Form a 3-D network called cortex just inside the plasma membrane to help support the cell’s shape Video 5 45 Microfilaments (Actin filaments)  Bundles make up core of microvilli of intestinal cells Video 5 46 Microfilaments (Actin filaments) Microfilaments that function in cellular motility contain the protein myosin in addition to actin Thicker myosin filaments walk along thinner actin fibres to pull them together, contracting muscle cells Video 5 47  Cytoskeleton: Intermediate filaments Range in diameter from 8–12 nm, larger than microfilaments but smaller than microtubules Support cell shape and fix organelles in place Intermediate filaments are more permanent cytoskeleton fixtures Video 5 48  Extracellular components Connect and coordinate cellular activities Most cells synthesize and secrete materials external to plasma membrane These extracellular structures include – Cell walls of plants – Extracellular matrix (ECM) of animal cells – Intercellular junctions Video 6 49  Extracellular matrix (ECM) of animal cells Animal cells lack cell walls but have an elaborate extracellular matrix (ECM) Functions of the ECM – Support – Adhesion – Movement – Regulation Video 6 50  Extracellular matrix (ECM) of animal cells Made of glycoproteins such as collagen, proteoglycans, and fibronectin Polysaccharide Collagen Fibronectin Integrins Cell Proteoglycans membrane Proteoglycan Microfilaments Complex ECM proteins bind to receptor proteins in plasma membrane Video called 6 integrins 51 Extracellular matrix (ECM) Collagen! Tonkotsu Ramen…pork bone broth 52  Cell junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Intercellular junctions facilitate this contact Video 6 53  Cell junctions There are several types of intercellular junctions – Plasmodesmata (Plants) – Tight junctions – Desmosomes – Gap junctions Video 6 54 Tight junctions in animal cells At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Video 6 55 Desmosomes in animal cells Desmosomes (anchoring junctions) fasten cells together into strong sheets Video 6 56 Gap junctions in animal cells Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells Video 6 57 Cellular membrane Selective barrier Allows passage of oxygen, nutrients, waste General structure: phospholipid layer – hydrophilic exterior – hydrophobic interior Video 7 58 Transport through cellular membranes Cellular membranes are fluid mosaics of lipids and proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance across a membrane with no energy investment Active transport uses energy to move solutes against their gradients Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Video 7 59  Cellular membranes: fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions Video 7 60  Fluid mosaic model a membrane is a fluid structure with a “mosaic” of proteins embedded in it Video 7 61  Membrane fluidity Phospholipids in plasma membrane can move within bilayer Most lipids and some proteins drift laterally Rarely a molecule flip-flops transversely Video 7 62  Membrane fluidity As temperatures cool, membranes go from a fluid state to a solid state Temperature when membrane solidifies depends on types of lipids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Video 7 63  Membrane fluidity Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Video 7 64  Membrane fluidity and Cholesterol Cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures, it restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing Video 7 65  Membrane proteins Membrane is a collage of different proteins, often grouped together, embedded in fluid matrix of lipid bilayer Proteins determine most of membrane’s specific functions – Peripheral proteins are bound to the surface of the membrane – Integral proteins penetrate the hydrophobic core Video 7 66 Membrane proteins Integral proteins that span membrane are called transmembrane proteins Hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices Video 7 67  Membrane proteins Six major functions of membrane proteins – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Video 7 68 Membrane proteins Video 7 69 Membrane proteins Video 7 70 IMPACT: HIV HIV requires CD4 receptor AND CCR5 co- receptor to enter cells Immune patients lack the CCR5 co-receptor Drugs then target CCR5 as CD4 has too many other roles Video 7 71  Synthesis and sidedness of membranes Membranes have distinct inside and outside faces Asymmetrical distribution of plasma membrane components determined when membrane is built by ER and Golgi apparatus Video 7 72 Synthesis and sidedness of membranes Video 7 73  Membrane structure and selective permeability Cells must exchange materials with surroundings, a process controlled by plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic – Hydrophobic (non-polar) molecules can dissolve in lipid bilayer and pass through membrane rapidly – Polar (hydrophilic) molecules do not cross membrane easily – need help Video 7 74  Transport proteins Transport proteins allow hydrophilic substances to pass across membrane Some have hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate passage of water Video 8 75  Transport proteins Others bind to molecules and change shape to shuttle them across membrane A transport protein is specific for the substance it moves Video 8 76  Passive transport: diffusion of substances across membranes Diffusion- tendency for molecules to spread out evenly into available space Each molecule moves randomly, but diffusion of a population of molecules may be directional Dynamic equilibrium- as many molecules cross membrane in one direction as in the other Video 8 77 Diffusion of one solute Video 8 78 Diffusion of two solutes Video 8 79  Passive transport Substances diffuse down their concentration gradient Diffusion is a spontaneous process Diffusion of a substance across a biological membrane is passive transport; no energy is expended by the cell to make it happen Video 8 80 Osmosis and water balance Osmosis- diffusion of water across a selectively permeable membrane Video 8 81  Water balance of cells Tonicity is ability of surrounding solution to cause cells to gain or lose water Video 8 82  Osmoregulation Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, control of solute concentrations and water balance, is a necessary adaptation for life in such environments Video 8 83  Osmoregulation Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole (acts as a pump) Video 8 84 Facilitated diffusion: passive transport aided by proteins Facilitated diffusion, transport proteins aid passive movement of molecules across plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross Channel proteins include – Aquaporins, facilitated diffusion of water – Ion channels open or close in response to a stimulus (gated channels) Video 8 85  Facilitated diffusion: passive transport aided by proteins Carrier proteins undergo a subtle change in shape that translocates the solute- binding site across the membrane Video 8 86 Active transport uses energy to move solutes against gradients Facilitated diffusion- the solute moves down its concentration gradient, and transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients Video 9 87  Energy in active transport Active transport moves substances against their concentration gradients Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in membranes Active transport allows cells to maintain concentration gradients that differ from their surroundings Video 9 88  Sodium-potassium pump Cytoplasmic Na+ binds to the sodium- potassium pump Affinity for Na+ is high when the protein has this shape Video 9 89  Sodium-potassium pump Na+ binding stimulates phosphorylation by ATP Video 9 90 Sodium-potassium pump Phosphorylation leads to a change in protein shape reducing affinity for Na+ Video 9 91  Sodium-potassium pump New shape has high affinity for K+ K+ binds and triggers release of phosphate group Video 9 92  Sodium-potassium pump Loss of phosphate changes shape resulting in low affinity for K+ Video 9 93  Sodium-potassium pump K+ is released Affinity for Na+ is restored Cycle starts again Video 9 94 Review: passive and active transport Video 9 95  How ion pumps maintain membrane potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive (outer) and negative (inner) ions across a membrane Two combined forces, collectively called electrochemical gradient, drive diffusion of ions across a membrane – A chemical force (the ion’s concentration gradient) – An electrical force (effect of membrane potential on ion’s movement) Video 9 96  How ion pumps maintain membrane potential An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells Video 9 97  Bulk transport across a membrane Small molecules and water enter or leave cell through lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins, cross membrane via vesicles (Bulk transport) Bulk transport requires energy Video 9 98  Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Video 9 99  Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis Video 9 100  Phagocytosis  Phagocytosis cell engulfs a particle in vacuole  Vacuole fuses with lysosome to digest particle Video 9 101  Pinocytosis Pinocytosis, extracellular fluid is “gulped” into tiny vesicles (drinking) Video 9 102  Receptor-mediated endocytosis Receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation Video 9 103 Summary – Membrane transport Video 9 104