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Transcript
Cells Topic 6 Membranes, Cell structure and Transport Learning Objectives After studying this topic you should be able to: CEB Chapter 4, pages 60 and 61, 69-70 and Chapter 5, pages 74-87 Mastering Biology, Chapters 4 and 5 Membrane Structure Cell Membrane Every cell is encircled by a membrane and most cells contain an extensive intracellular membrane system. Membranes fence off the cell's interior from its surroundings. Membranes let in water, certain ions and substrates and they excrete waste substances. Without a membrane the cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work. Identify (in drawings or micrographs) and describe the structure and function of the cellular membrane. Describe the differences between the following pairs of terms: diffusion versus osmosis, passive transport versus active transport, hypertonic versus hypotonic, endocytosis versus exocytosis, phagocytosis versus pinocytosis. Identify (in drawings or micrographs) and describe the structure and function of the various components of the cytoskeleton. The cell is highly organized with many functional units or organelles inside. Most of these units are limited by one or more membranes. To perform the functions of an organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles. In essence membranes are essential for the integrity and function of the cell. The cell would die! The Cell Membrane Cell membranes What is their structure? Cell Structure and Function 1 Cells We don’t know currently The fluid mosaic model Lipids arranged in bi-layer with proteins embedded or associated with them. There are a number of hypotheses and we will consider the one which is currently accepted The fluid mosaic model - more This proposes that the cell membrane is made up of 2 main layers – lipids and proteins. The phospolipids form themselves into a bi-layer with the water seeking ends facing out and the water hating ends facing in. The proteins are embedded in this layer but can move around or flip over. Special carrier molecules take important elements, like ions, at the cell membrane, using energy supplied by the cell and use the proteins that are embedded in the lipid layer. Fluid Mosaic Model The membrance is a complex 3d Thecell cell membrane is a complex 3 d structure. circular structure Fluid Mosaic Model • • Cell Structure and Function Sometimes the elements bind to the proteins, which flip over, thus transporting the element into the cell. Some proteins form a ‘pore’ through which the element can pass from the outside to the inside of the cell membrane. The movement of the phospholipid and protein components through the plasma membrane permits the membrane to change shape. This flexibility is crucial to many different types of cells. In animal cells, cholesterol also contributes to the fluidity of the plasma membrane. Cholesterol is a small lipid molecule that nestles among the hydrophobic tails of the phospholipids in the interior of the membrane. It prevents phospholipid molecules from packing together too tightly and making the membrane rigid. 2 Cells Structure Composition of the cell membrane Fluid-like composition…like soap bubbles Composed of: Lipids in a bi-layer – what is this? Proteins embedded in lipid layer (called transmembrane proteins) Proteins floating within the lipid sea (called integral proteins) Proteins associated outside the lipid bi-layer (peripheral proteins). The fluid mosaic model of a cell membrane Peripheral Proteins Phospholipids Integral Proteins Cell Membrane Transmembrane proteins Membrane Lipids Composed largely of phospholipids Phospholipids composed of glycerol and two fatty acids + PO4 group Phospholipids are polar molecules - they have a charge. Phospholipid Molecule Model phosphate (hydrophilic) – like water glycerol fatty acids (hydrophobic) – hate water Membrane Lipids The fluid mosaic model form a Bi-layer Outside layer Inside Layer Cell Structure and Function 3 Cells What does the membrane do? = Function Transport Across The Cell Membrane Membrane Permeability Biological membranes are physical barriers, but which allow small uncharged molecules to pass… lipid soluble molecules pass through Big molecules and charged ones do NOT pass through allows for different conditions between inside and outside of cell subdivides cell into compartments with different internal conditions allows release of substances from cell via vesicle fusion with outer membrane: Membrane Permeability 1.) lipid soluble solutes go through faster 2.) smaller molecules go faster 3) uncharged & weakly charged go faster 4) Channels or pores may also exist in the membrane to allow transport 1 2 Its about concentration The concentration of the solution, with respect to other solutions is important Types of solutions Isotonic --- when both solutions have the same concentration of dissolved substances Hypertonic --- a solution with a higher concentration of dissolved substances(solutes) Hypotonic --- a solution with a lower concentration of dissolved substances(solutes) Cell Structure and Function 4 Cells Passive Transport Two types of transport Involves concentration gradients ONLY. NO CELL ENERGY is used. Passive and Active Passive Transport 3 types Diffusion- simple movement from regions of high concentration to low concentration. Osmosis- diffusion of water across a semipermeable membrane. Facilitated diffusion - protein transporters which assist in diffusion. Diffusion • Movement generated by random motion of particles. Movement always from region of high concentration to regions of low concentration. Increased water pressure is caused by water moving to decrease a concentration gradient or concentration difference between two areas. Osmosis Cell Structure and Function The movement of water across a semipermeable membrane. Semi-permeable membrane – allows some particles to cross the membrane, but not others. Water moves from where water concentration is high to where it is low in order to equalize the concentration. 5 Cells Osmosis Cells in distilled water What type of solution? Hypotonic Cells in a salt solution What type of solution? Hypertonic Water in Cells eventually explode Water out Cells shrivel & die Think about it… Based on what you have learnt – why is it bad to drink sea-water if you are thirsty (and lost at sea)??? Transport Proteins Facilitated Diffusion Transport proteins carry specific molecules across the cell membrane Movement is along a concentration gradient (i.e. From higher to lower) Each type of transport protein will carry only one type of molecule. This is how glucose is moved. Cell Structure and Function Move solutes faster across membrane Highly specific to specific solutes Can be inhibited by drugs Also involved in ACTIVE transport 6 Cells Transport protein Transport protein Glucose Concentration gradient Cell membrane Glucose Cell membrane Concentration gradient Glucose binds to the transport protein The transport protein turns over and releases glucose onto the inside of the cell, along the concentration gradient Types of Protein Transporters: Ion Channels Work by facilitated diffusion No E! Deal with small molecules... ions Open pores are “gated”- Can change shape. How? Do a diagram to show how you think this might work. NOW Important in cell communication Transport protein Concentration gradient Carrier molecule Cell membrane The carrier molecule binds to the transport protein, which opens the pore allowing it to move through the cell membrane. The pore closes once the carrier is inside the cell. It is possible to stop the action of transport protein with drugs which will block the pore. Active Transport Cell Energy is used to move substances across the cell membrane The substances are moved against the concentration gradient i.e. from where there is less to where there is more. Transport proteins Substances are moved molecule by molecule. It is similar to facilitated diffusion except that cell energy (ATP) is used in the process. ATP = Adenosine Triphosphate Cell Structure and Function 7 Cells ACTIVE TRANSPORT Salt ion Transport protein Concentration gradient Cell membrane Energy is used Transport protein Salt Ion Concentration gradient Cell membrane Ion binds to the transport protein ENERGY IS USED The transport protein turns over and releases the ion onto the inside of the cell, against the concentration gradient Moving many large molecules at once Endocytosis Transports macromolecules and large particles into the cell. Part of the membrane engulfs the particle and folds inward to “bud off.” The cell membrane envelopes the material If material is liquid the process is called pinocytosis ‘cell drinking’ If material is solid the process is called phagocytosis ‘cell eating’ Exocytosis Material is packaged inside the cell and the package fuses with the cell membrane while the material goes out of the cell. How Endocytosis works Pseudopodia extend to engulf food A food vacuole is formed Pinocytosis works the same, but with no food, only liquid How exocytosis works Vacuole containing particles is moved close to the cell membrane Fuses with the cell membrane to expel the particles Cell Structure and Function 8 Cells Sodium-Potassium Pump Ion Channels Work fast: No conformational changes needed Not simple pores in membrane: specific to different ions (Na, K, Ca...) gates control opening Toxins, drugs may affect channels saxitoxin, tetrodotoxin cystic fibrosis Summary Questions What is the difference between diffusion and osmosis? What is the difference between active and passive transport? What is the difference between Hypotonic and Hypertonic? What is the difference between Endocytosis and Exocytosis? What is the difference between Phagocytosis and Pinocytosis? Key words – will be assessed! Diffusion Osmosis Selectively permeable Passive Transport Active Transport Hypotonic Hypertonic Endocytosis Exocytosis Phagocytosis Pinocytosis Homework Complete Bioflix study sheet: Membrane transport (pge 35 study notes) Complete Primary Functions of membrane proteins table (pge 32 study notes) Complete Topic 6 on Unit Assessment 1 Cell Structure and Function 9