MEMBRANE TRANSPORT : Controlling the Flow of Materials in the Cell
PASSIVE TRANSPORT : Diffusion Across Membranes <ul><li>WHAT IS DIFFUSION? </li></ul><ul><li>Diffusion is the tendency for molecules of any substance to spread out into the available space.Substances diffuse from where it is more concentrated to where it is less concentrated, in other words,substances diffuse down their concentration gradient </li></ul>
<ul><li>WHAT IS PASSIVE TRANSPORT? </li></ul><ul><li>Passive Transport is the diffusion across a membrane. It is “passive” because the cell does not expend any energy during the spread of molecules on a selectively permeable membrane. </li></ul><ul><li>Example: </li></ul><ul><li>In the lungs: Oxygen (O 2 ), essential for metabolism, enters red blood cells and Carbon Dioxide (CO 2 ), a metabolic waste, passes out of them. </li></ul>
OSMOSIS AND WATER BALANCE IN CELLS <ul><li>WHAT IS OSMOSIS? </li></ul><ul><li>Osmosis is the passive transport of water across a selectively permeable membrane.In osmosis, the kind of solution involved is important. Solutions with a higher concentration of solute is hypertonic. Solutions with a lower concentration of solute is hypotonic. And solutions of equal solute concentration is isotonic . </li></ul>
<ul><li>Example of Osmosis: </li></ul><ul><li>A membrane separates two solutions, one hypotonic and one hypertonic. The membrane is permeable to water, not to the solute. The hypotonic solution contains a lower solute concentration, therefore, it contains more water. And since the hypertonic solution has more solute, it has lesser amount of water. Water will then diffuse across the membrane along its concentration gradient from the hypotonic sol.(with more water) to the hypertonic sol.(with less water).Once the total solute concentration become equal on both sides of the membrane, water molecules will move at the same rate in both directions and the resulting solutions will be isotonic. </li></ul>
WATER BALANCE IN ANIMAL CELLS <ul><li>Animal cells survive because of their ability to balance water uptake and loss. The control of water balance is called osmoregulation . </li></ul><ul><li>WHAT WILL HAPPEN TO THE CELL WITHOUT OSMOREGULATION? </li></ul><ul><li>- If the cell is placed in a hypotonic environment,the cell will gain water, it will swell and burst (lyse). </li></ul><ul><li>- If it is placed in a hypertonic solution, the cell will die from water loss </li></ul>
WATER BALANCE IN PLANT CELLS <ul><li>Water balance in plant cells is different because of their rigid cell walls. </li></ul><ul><li>a)In an isotonic solution, plant cell is flaccid (floppy), and a plant wilts. </li></ul><ul><li>b)In a hypotonic environment,it is turgid (firm) and healthiest with a net inflow of water. </li></ul><ul><li>c)In a hypertonic solution, plant cell loses water and shrivels. Its plasma membrane pulls away from the cell wall (plasmolysis) . </li></ul><ul><li> The back pressure exerted by the elastic cell wall of plant cells prevents itself from taking in too much water and bursting. </li></ul><ul><li> Turgor , the normal fullness or tension produced by the fluid content of plant cells,is needed to retain the upright upright posture and extended state of the leaves of plants </li></ul>
ACTIVE TRANSPORT: THE PUMPING OF MOLECULES ACROSS A MEMBRANE <ul><li>Active Transport uses a specific transport protein to pump a solute across a membrane against its concentration gradient-that is towards the portion having higher concentration or away from that which has a low concentration.This process consumes energy unlike the passive transport that has no expenditure of energy.Membrane proteins usually use ATP as energy source for this. </li></ul>
<ul><li>Active Transport enables cells to maintain internal internal concentrations of small molecules that differ from environmental concentrations. </li></ul><ul><li>Example: </li></ul><ul><li>Sodium(Na)-Potassium(K) Pump :Cells maintain a much higher concentration of K ions and a much lower concentration of Na ions through the plasma membrane which pumps Na out of the cell and K into the cell. </li></ul>
EXOCYTOSIS AND ENDOCYTOSIS: TRAFFIC OF LARGE MOLECULES <ul><li> For large molecules like protein , its passage into and out of the cell is through package in vesicles. </li></ul><ul><li>I. EXOCYTOSIS -the exit of molecules from the cell through transport vesicles that fuse with the plasma membrane,spilling the contents outside of the cell </li></ul><ul><li>II. ENDOCYTOSIS - takes materials into the cell within vesicles that bud inward form the plasma membrane. </li></ul>
<ul><li>THREE TYPES: </li></ul><ul><li>1. Phagocytosis (cellular eating) - cell engulfs a particle and packages it within a food vacoule </li></ul><ul><li>2. Pinocytosis(cellular drinking) - cell gulps droplets of fluid by forming tiny vesicles; this is unspecific in the substances it transports </li></ul><ul><li>3. Receptor-mediated endocytosis - similar to pinocytosis but specific in the substanse it transports. This is possible through the binding of molecules to specific receptor proteins built into the plasma membrane. </li></ul><ul><li>example: </li></ul><ul><li>Liver cells remove excess cholesterol from the blood to prevent the excessive cholesterol levels in the blood that contributes to cardiovascular disease. </li></ul>
THE ROLE OF MEMBRANE IN CELL SIGNALING <ul><li>STEPS IN CELL SIGNALING </li></ul><ul><li>Reception of an extra cellular signal(hormone) by a specific receptor protein </li></ul><ul><li> </li></ul><ul><li>Signal triggers a chain reaction in the molecules that function in transduction (passing the signal along) </li></ul><ul><li> </li></ul><ul><li>Molecules of the signal transduction pathway relay signal and convert it to chemical forms </li></ul><ul><li> </li></ul><ul><li>Chemical Responses (ex: activation of metabolic function) </li></ul>
Evolution Connection: EVOLVING ENZYMES <ul><li>There is a great variety of enzymes within an organisms body. This condition is brought about by evolution.Most enzymes are proteins and proteins are encoded by genes. Genes undergone molecular evolution.Through genetic mutation,one gene becomes two distinct genes that produce two distinct enzymes. </li></ul><ul><li>Enzyme evolution can be accelerated in the laboratory. In a process called directed evolution, many copies of a gene for the original enzyme were randomly mutated. The mutated enzymes that functions the best were artificially selected using a screening test. Those enzymes are again subjected to duplication, mutation, and selection. After seven rounds, this process produced a novel enzyme that recognized a new substrate. </li></ul>
<ul><li>Natural selection and directed evolution both result in the production of new enzymes. However, there are differences between the two. In natural selection, it takes years before evolution, while in directed evolution, only weeks are needed. But the most important difference is that laboratory experiments are directed-the researchers engineered the evolutionary outcome to suit their chosen purpose;natural selection selects enzyme variants that work best in the organism’s natural environment. </li></ul>
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