AS Biology, Cell membranes and Transport 1 Transport through cell membranes

AS Biology, Cell membranes and AS Biology, Cell membranes and TransportTransport

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Transport through cell Transport through cell membranesmembranes


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Transport through cell Transport through cell membranesmembranes

The phospholipid bilayer is a good barrier around cells, The phospholipid bilayer is a good barrier around cells, especially to water soluble molecules. However, for the especially to water soluble molecules. However, for the cell to survive some materials need to be able to enter cell to survive some materials need to be able to enter and leave the cell.and leave the cell.

There are 4 basic mechanisms:There are 4 basic mechanisms:

1.1. DIFFUSION and FACILITATED DIFFUSIONDIFFUSION and FACILITATED DIFFUSION

2.2. OSMOSISOSMOSIS

3.3. ACTIVE TRANSPORTACTIVE TRANSPORT

4.4. BULK TRANSPORTBULK TRANSPORT


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Diffusion of liquidsDiffusion of liquids


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•Diffusion is the net movement of molecules Diffusion is the net movement of molecules (or ions) from a region of their high (or ions) from a region of their high concentration to a region of their lower concentration to a region of their lower concentration.concentration.

The molecules move down a The molecules move down a concentration gradient.concentration gradient.

Molecules have Molecules have kinetic energykinetic energy, which makes them , which makes them move about randomly. move about randomly.

As a result of diffusion molecules reach an As a result of diffusion molecules reach an equilibrium equilibrium where they are evenly spread out.where they are evenly spread out.This is when there is no net movement of molecules This is when there is no net movement of molecules from either side.from either side.


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DIFFUSIONDIFFUSIONDiffusion is a PASSIVE process which means no energy is used to make the molecules move, they have a natural kinetic energy.


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Diffusion of BromineDiffusion of Bromine


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Diffusion of BromineDiffusion of Bromine


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Diffusion through a membraneDiffusion through a membrane

Cell membrane

Inside cell Outside cell


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Cell membrane


diffusion


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Cell membrane


EQUILIBRIUM


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What determines the rate of diffusion?What determines the rate of diffusion?There 4 factors:There 4 factors:

1.1. The steepness of the concentration gradientThe steepness of the concentration gradient. The bigger the . The bigger the difference between the two sides of the membrane the difference between the two sides of the membrane the quicker the rate of diffusion. quicker the rate of diffusion.

2.2. TemperatureTemperature. Higher temperatures give molecules or ions . Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so more kinetic energy. Molecules move around faster, so diffusion is faster.diffusion is faster.

3.3. The surface areaThe surface area. The greater the surface area the faster the . The greater the surface area the faster the diffusion can take place. This is because the more molecules diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment.or ions can cross the membrane at any one moment.

4.4. The type of molecule or ion diffusingThe type of molecule or ion diffusing. Large molecules need . Large molecules need more energy to get them to move so they tend to diffuse more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar polar molecules because they are soluble in the non polar phospholipid tails.phospholipid tails.


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Molecules that diffuse through cell Molecules that diffuse through cell membranesmembranes

1.1. OxygenOxygen – Non-polar – Non-polar so diffuses very so diffuses very quickly.quickly.

1.1. Carbon dioxideCarbon dioxide – – Polar but very Polar but very small so diffuses small so diffuses quickly.quickly.

2.2. WaterWater – Polar but – Polar but also very small so also very small so diffuses quickly.diffuses quickly.


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Facilitated diffusionFacilitated diffusion Large polar molecules such as Large polar molecules such as

glucose glucose and and amino acidsamino acids, , cannot diffuse across the cannot diffuse across the phospholipid bilayer. Also ions phospholipid bilayer. Also ions such as such as NaNa++ or or ClCl-- cannot cannot pass.pass.

These molecules pass through These molecules pass through protein channelsprotein channels instead. instead. Diffusion through these Diffusion through these channels is called channels is called FACILITATED DIFFUSION.FACILITATED DIFFUSION.

Movement of molecules is still Movement of molecules is still PASSIVEPASSIVE just like ordinary just like ordinary diffusion, the only difference diffusion, the only difference is, the molecules go through a is, the molecules go through a protein channel instead of protein channel instead of passing between the passing between the phospholipids.phospholipids.


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Facilitated Diffusion through a Facilitated Diffusion through a membranemembrane

Cell membrane


Protein channel


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Cell membrane


Protein channel

diffusion


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Cell membrane


Protein channel

diffusion

EQUILIBRIUM


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Facilitated Diffusion:Facilitated Diffusion:Molecules will randomly move through the opening like pore, Molecules will randomly move through the opening like pore, by diffusion. by diffusion. This requires no energy, it is a PASSIVE processThis requires no energy, it is a PASSIVE process. . Molecules move from an area of high concentration to an area Molecules move from an area of high concentration to an area of low conc.of low conc.


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Facilitated diffusionFacilitated diffusion


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OsmosisOsmosis

‘‘The The diffusiondiffusion of water from an of water from an area of high concentration of area of high concentration of water molecules (water molecules (high water high water potentialpotential) to an area of low ) to an area of low concentration of water (concentration of water (low low water potentialwater potential) across a ) across a partially permeable membrane.’partially permeable membrane.’


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OsmosisOsmosis

Cell membrane partially permeable.

Inside cell Outside cellVERY High conc. of water molecules. High water potential.

VERY Low conc. of water molecules. High water potential.

Sugar molecule

DILUTE SOLUTIONCONCENTRATED SOLUTION


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OsmosisOsmosis


Inside cell Outside cellHigh conc. of water molecules. High water potential.

Low conc. of water molecules. High water potential.OSMOSIS


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OsmosisOsmosis



OSMOSIS

EQUILIBRIUM. Equal water concentration on each side. Equal water potential has been reached. There is no net movement of water


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ExocytosisExocytosis The opposite of endocytosis is exocytosis. Large molecules that are The opposite of endocytosis is exocytosis. Large molecules that are

manufactured in the cell are released through the cell membrane.manufactured in the cell are released through the cell membrane.


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EndocytosisEndocytosis is the case when a molecule causes the cell membrane to is the case when a molecule causes the cell membrane to bulge inward, forming a vesicle. bulge inward, forming a vesicle. PhagocytosisPhagocytosis is the type of is the type of

endocytosis where an entire cell is engulfed. Pinocytosis is when the endocytosis where an entire cell is engulfed. Pinocytosis is when the external fluid is engulfed. Receptor-mediated endocytosis occurs when external fluid is engulfed. Receptor-mediated endocytosis occurs when the material to be transported binds to certain specific molecules in the the material to be transported binds to certain specific molecules in the membrane. Examples include the transport of insulin and cholesterol membrane. Examples include the transport of insulin and cholesterol

into animal cells.into animal cells.


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Cotransport also uses the process of diffusion. In this case a molecule Cotransport also uses the process of diffusion. In this case a molecule that is moving naturally into the cell through diffusion is used to drag that is moving naturally into the cell through diffusion is used to drag another molecule into the cell. In this example glucose hitches a ride another molecule into the cell. In this example glucose hitches a ride

with sodium.with sodium.


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Receptor ProteinsReceptor Proteins These proteins are used in intercellular communication. In this These proteins are used in intercellular communication. In this

animation you can see the a hormone binding to the receptor. This animation you can see the a hormone binding to the receptor. This

causes the receptor protein release a signal to perform some action.causes the receptor protein release a signal to perform some action.


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Cotransport also uses the process of diffusion. In this case a molecule Cotransport also uses the process of diffusion. In this case a molecule that is moving naturally into the cell through diffusion is used to drag that is moving naturally into the cell through diffusion is used to drag another molecule into the cell. In this example glucose hitches a ride another molecule into the cell. In this example glucose hitches a ride

with sodium.with sodium.


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These are carrier proteins. They do not extend through the membrane. These are carrier proteins. They do not extend through the membrane. They bond and drag molecules through the bilipid layer and release They bond and drag molecules through the bilipid layer and release

them on the opposite side. them on the opposite side.


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Vesicle-mediated transport Vesicle-mediated transport VesiclesVesicles and and vacuolesvacuoles that fuse with the cell membrane may be that fuse with the cell membrane may be

utilized to release or transport chemicals out of the cell or to allow them utilized to release or transport chemicals out of the cell or to allow them to enter a cell. to enter a cell. ExocytosisExocytosis is the term applied when transport is out of is the term applied when transport is out of

the cell.the cell.


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Cell Membrane - Function - EndocytosisCell Membrane - Function - Endocytosis The cell membrane can also engulf structures that are much too large to fit through the The cell membrane can also engulf structures that are much too large to fit through the

pores in the membrane proteins this process is known as endocytosis. In this process the pores in the membrane proteins this process is known as endocytosis. In this process the membrane itself wraps around the particle and pinches off a vesicle inside the cell. In this membrane itself wraps around the particle and pinches off a vesicle inside the cell. In this

animation an ameba engulfs a food particle.animation an ameba engulfs a food particle.

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AS Biology, Cell membranes and Transport 1 Transport through cell membranes