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Transcript
Part 3: Homeostasis and Cell Transport (Chapter 5) Passive Transport •Involves the movement of molecules across the cell membrane without an input of energy by the cell. • NO ENERGY REQUIRED to move substances across membrane -- water, lipids, and other lipid soluble substances. • Types: 1. Diffusion 2. Osmosis 3. Facilitated Diffusion 4. Diffusion through Ion Channels 1. Diffusion • Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration (with the concentration gradient*) until equilibrium** is reached • *Concentration gradient: the difference in concentration across space. • **Equilibrium: molecules are eventually evenly dispersed OR the concentration of molecules is the same throughout the entire space. Simple Diffusion: diffusion across a membrane • Cell Membrane = semi-permeable • Some materials can pass through, some cannot • Molecules will move from high to low concentration • The membrane determine what molecules can pass through freely and what molecules need “assistance” Diffusion 2. Osmosis Osmosis is the diffusion of water across a membrane. Direction of Osmosis The net direction of osmosis is determined by the relative solute concentrations on the two sides of the membrane. • Direction of Osmosis – When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic (more solute, less water) to the cytosol, and water will diffuse out of the cell. – Plasmolysis = cell’s shrinking due to water loss • Direction of Osmosis – When the solute concentration outside the cell is lower than that in the cytosol, the solution outside is hypotonic (less solute, more water) to the cytosol, and water will diffuse into the cell. – Cytolysis = cell’s bursting due to water gain • Direction of Osmosis – When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic (same solute, same water), and there will be no net movement of water. Review of Osmosis terms Hypotonic Isotonic Hypertonic Hypertonic, Hypotonic, Isotonic Solutions • How Cells Deal With Osmosis – Cells must compensate for the water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments. – Multicellular organisms – cells respond to hypotonic environments by pumping solutes out of the cytosol – *RBCs cannot compensate for changes in solute concentration – Unicellular organisms – May have Contractile vacuoles: organelles that regulate water levels Real cells 3. Facilitated Diffusion • Diffusion of molecules across a membrane when they are not soluble in lipids or are too large (e.g. glucose) to pass through pores in membrane • A molecule binds to a carrier protein on one side of the cell membrane. • Carrier Protein • specific for one type of molecule • changes its shape and transports the molecule Facilitated Diffusion 4. Diffusion Through Ion Channels • Ion channels: proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse. • Some are always open • Some are “gated” • Ions: • Charged particles like Na+, Ca2+, Cl• important in cell function • include sodium, potassium, calcium, and chloride Ion Channels Active Transport Cell Membrane Pumps and Vesicles • Active transport • moves molecules across the cell membrane from an area of lower concentration to an area of higher concentration (against concentration gradient) • requires cells to expend ENERGY • Some types of active transport are performed by carrier proteins called cell membrane pumps. Cell Membrane Pumps • Sodium-Potassium Pump • moves three Na+ ions into the cell’s external environment for every two K+ ions it moves into the cytosol. • most animal cells must have a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell • The Na+/K- pump allows for the gradient • ATP supplies the energy that drives the pump. • More on the Sodium-Potassium The exchange of three Na+ ions for two K+ Pump ions creates an electrical gradient across the cell membrane • Outside becomes positively charged relative to the inside, which becomes negative • Difference in electrical charge is important for the conduction of electrical impulses along nerve cells Sodium-Potassium Pump Sodium-Potassium Pump Movement in Vesicles • Endocytosis • • • cells ingest external fluid, macromolecules, and large particles, including cells by folding around them and forming a pouch. The pouch then pinches off and becomes a membranebound organelle called a vesicle. • Some vesicles fuse with lysosomes, and their contents are digested by lysosomal enzymes Two main types • Pinocytosis: vesicle contains solutes or fluids • Phagocytosis: vesicle contains large particles or whole cells. Endocytosis Movement in Vesicles • Exocytosis • Vesicles made by the cell fuse with the cell membrane, releasing their contents into the external environment. • Used to release large molecules, such as proteins, waste products, or toxins that would damage the cell if they were released within the cytosol Exocytosis Endocytosis and Exocytosis