Transport of biomolecules across cell membrane
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![ Movement of substances across the cell
membrane
Passive transport
Substances move from [high][low]
No energy input required
Simple Diffusion, Facilitated Diffusion,
Osmosis
Active transport
Substances move from [low][high]
Requires energy input
Protein carriers, Endocytosis, Exocytosis](https://image.slidesharecdn.com/transportacrosscellmembrane-180825101654/75/transport-of-biomolecules-across-cell-membrane-13-2048.jpg?cb=1667385093)
![ Diffusion of water across a differentially
permeable membrane
Water moves from [high] [low]](https://image.slidesharecdn.com/transportacrosscellmembrane-180825101654/75/transport-of-biomolecules-across-cell-membrane-19-2048.jpg?cb=1667385093)
Transport of biomolecules across cell membrane
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The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name: osmosis. This page will examine how ions and small molecules are transported across cell membranes. The transport of macromolecules through membranes is described in Endocytosis.
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Transport of biomolecules across cell membrane
- 3. pancreas produces enzymes to digest proteins & starch stomach kills germs break up food digest proteins store food mouth break up food digest starch kill germs moisten food liver produces bile - stored in gall bladder break up fats
- 4. The plasma membrane “mosaic” is a collection of proteins.The plasma membrane “mosaic” is a collection of proteins.
- 5. Lipids Phospholipid bilayer Cholesterol Proteins Transport proteins Receptor proteins Recognition proteins Carbohydrates Oligosaccharides on glycoproteins
- 6. Lipids Phospholipid bilayer Forms boundary to isolate cell contents from environment Restricts passage of hydrophilic substances across the membrane Cholesterol Increases bilayer strength, flexibility Reduces membrane fluidity Reduces permeability to water-soluble substances
- 7. Proteins Transport proteins Regulate movement of water soluble substances Channel proteins have pores that allow passage of ions and small water-soluble molecules Carrier proteins bind to molecules and change shape for delivery across membrane
- 8. Proteins Receptor proteins Docking site for molecules outside the cell Trigger internal cellular response Recognition proteins Identification tags Oligosaccharides aid in cell-cell recognition Cell-surface attachment sites
- 9. The membrane is permeable to: H2O Gases (O2, CO2, N2) Lipids Small, neutral molecules (such as urea) The membrane is impermeable to: - Small, charged molecules - “large molecules” such as amino acids, glucose and larger these compounds must go through channels present in the membrane in order to enter or exit the cell Factors affecting transport - cell membrane - Chemical gradient - Electrical gradient - Rate of transport
- 10. Compound moves from an area of high concentration to low concentration (or concentration gradient) All compounds permeable to the phospholipid bilayer will move this way
- 11. Figure 4.3 Positive ions are attracted to negative ions and vice versa Ions are repelled by ions of the same charge (+ against + and – against -)
- 12. Both chemical and electrical forces (electrochemical force) drive the movement of compounds across the cell membrane
- 13. Movement of substances across the cell membrane Passive transport Substances move from [high][low] No energy input required Simple Diffusion, Facilitated Diffusion, Osmosis Active transport Substances move from [low][high] Requires energy input Protein carriers, Endocytosis, Exocytosis
- 14. Compounds move toward the area of lower concentration Compounds permeable to the cell membrane will move through diffusion. (Compounds unable to pass through the membrane will only pass if membrane channels open)
- 15. Time 0Time 0 SteepSteep ConcentrationConcentration GradientGradient Time 1Time 1 ReducedReduced ConcentrationConcentration GradientGradient DispersingDispersing Time 2Time 2 NoNo ConcentrationConcentration GradientGradient RandomRandom DispersalDispersal
- 16. (extracellular fluid) (cytoplasm) Some moleculesSome molecules diffuse freelydiffuse freely acrossacross (lipid soluble molecules, CO(lipid soluble molecules, CO22, O, O22, H, H22O )O ) LIPID-SOLUBLELIPID-SOLUBLE WATER-SOLUBLEWATER-SOLUBLE
- 17. In facilitated diffusion small polar molecules and ions diffuse through passive transport proteins. No energy needed Most passive transport proteins are solute specific Example: glucose enter/leaves cells through facilitated diffusion Passive transport protein Higher concentration Lower concentration
- 18. Carrier protein has binding site for molecule Molecule enters binding site Carrier protein changes shape, transporting molecule across membrane Carrier protein resumes original shape (Inside Cell)(Inside Cell)(Inside Cell)(Inside Cell) (Outside(Outside Cell)Cell) (Outside(Outside Cell)Cell) DiffusionDiffusion ChannelChannel ProteinProtein DiffusionDiffusion GradientGradient Molecule inMolecule in TransitTransit
- 19. Diffusion of water across a differentially permeable membrane Water moves from [high] [low]
- 20. Compounds move from area of low concentration toward area of higher concentration ATP (energy) is needed pump
- 21. Transport protein Solute Solute binding 1 Phosphorylation 2 Transport 3 Protein changes shape Protein reversion 4 Phosphate detaches
- 22. Inside CellInside CellInside CellInside Cell Outside CellOutside CellOutside CellOutside Cell EnergyEnergy ProviderProvider Spent EnergySpent Energy ProviderProvider TransportedTransported MoleculeMolecule 1 2 3 4
- 23. The most common: Na/K pumps reestablish membrane potential. Present in all cells. Two K+ ions are exchanged with 3 Na + ions
- 25. (extracellular fluid) Pinocytosis vesicle containing extracellular fluid)(cytoplasm) 1 2 33 cell (extracellular fluid) (cytoplasm) 1 2 33 44 coated pit forming Receptor-mediated Endocytosis Phagocytosis coated vesicle nutrients receptors food particle pseudopod particle enclosed in food vacuole
- 26. Active process for movement of large molecules and organisms Substance is taken in by vesicle formed from cell membrane Phagocytosis: solid substance in vesicle Pinocytosis: liquid droplets in vesicle Receptor-Mediated Endocytosis: incoming substance binds to receptor
- 27. Substance is expelled after being enclosed in a vesicle within the cell Used to move large molecules out of the cell
- 29. Glucose (CARBOHYDRATE) absorption - Na+ dependent glucose transporter. Glucose enters the enterocyte “arm-in- arm” with sodium (Na+) through a special protein transporter (Na+ - glucose co-transporter). Na+ concentration in the cell is low. Na+ therefore moves from the high concentration intestinal lumen into the cell, taking glucose with it. After glucose passes through the cell, it moves out alone through another transporter protein (by facilitated diffusion) at the basal end. The Na+ is pumped out actively. Both enter the capillary blood circulation. Fructose (carbohydrate) absorption Facilitated diffusion Fructose is a simple sugar found in fruits. It enters with the help of a transporter protein embedded into the apical membrane of the enterocyte. After passing through the cell, fructose leaves by a similar mechanism and is taken up by the blood. ABSORPTION OF BIOMOLECULES
- 30. Fat (triglyceride) absorption Transporter proteins This is a complex process. Trigly- cerides are compounds in which 3 fatty acids of variable length are bound to a glycerol (an alcohol). Digestive enzymes split off two fatty acids so glycerol is left with one (monoglyceride). Monoglycerides and all fatty acids use trans-port proteins to enter into the enterocyte. Fatty acids that have a carbon chain of less than 12 carbons (short chain) leave the cells and enter the blood circulation. The fate of long chain fatty acids is more complex. Within the enterocyte, they are attached again to glycerol and form triglycerides (re- esterification). Then they are combined with proteins and form lipoprotein particles. These are transported in vesicles to the basal membrane and ejected from the enterocyte by exocytosis (binds to a cell surface receptor and leaves). Lipoproteins do not enter the blood but are taken up by the lymphatic system that drains later into the blood circulation.
- 31. Protein (amino acids) absorption Na+ dependent amino acid transporters Amino acids are also dragged along by sodium, similarly to glucose. However, there are at least 7 transporter proteins due to the varied chemical structure of different amino acids. Outbound traffic of amino acids goes through 5 known transport systems. Amino acids are also taken up by the blood circulation.
Notas do Editor
- Eukaryotic Cell
- This refers to the types of processes used by cells to get materials in and out across the cell membrane. The membrane is said to be “semi-permeable.” This means that it will allow spontaneous passage of some materials but others must use special processes to get across. A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions. Molecules constantly collide and tend to move according to existing concentration gradients. The net movement of like molecules down a concentration gradient (high to low) is simple diffusion. Gradients in temperature, electric charge, and pressure, can influence movements. Passive transport: Follows a concentration gradient Passive in the sense that the process does not require energy. It just happens naturally. Diffusion is passive transport. Due to the random movement of molecules in liquids & gases (Brownian motion). A concentrated material will tend to become more uniformly dispersed in the space made available to it. Water can diffuse across the cell membrane unassisted. Facilitated transport: Also follows a concentration gradient, but requires gateway to get through. In this case, the particular materials would not spontaneously cross the pure phospholipid membrane, even if encouraged by a concentration gradient. A facilitator molecule is produced in the membrane that serves as a gateway for the material. The material can now cross through the gateway spontaneously whenever a concentration gradient exists. Active transport: Flow up-hill against a concentration gradient; requires energy expenditure to keep going. The required energy is almost always provided directly by ATP. The ATP donates a phosphate to a specific gateway molecule which then “pumps” the desired molecule across the membrane, even if it goes against a concentration gradient. The ATP energy is used to drive the pump.
- Shows successive stages in the dispersal of red dye molecules after being dropped into water.
- Note that the molecules to be transported (little balls) are in higher concentration outside the cell than inside the cell. The molecules are of a type that cannot get to the inside of the cell by going directly through the membrane. The protein channel provides a passage way for the molecules to follow the concentration gradient.
- Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both. Special case of diffusion. Involves the diffusion of water across a membrane that has solutions of differing concentration on opposite sides of the membrane. The water moves more abundantly toward the side with the highest concentration of dissolved materials. The concentration of water is obviously lower on the side with the highest concentration of dissolved materials. So the water is simply diffusing from a higher concentration of water to a lower concentration of water. Example: Pure water on one side; sugar solution on other (ƒ5-5). Sugar molecules can’t cross, but crowd membrane pores on syrupy side. Water crosses faster toward sugar. Net movement of water ====> sugar solution There is a net increase in volume on the side that has the higher concentration of sugar. The side of the membrane with the lower concentration of sugar experiences a loss of volume. This persistent movement can result in a build up of pressure known as osmotic pressure.
- Note that the concentration of the molecules is higher outside the cell than inside, but that the molecules are moving out, against the gradient. This always requires energy, thus the term “active” transport.
- Here are some special adaptations of endocytosis. Pinocytosis is when a vesicle forms around and then surrounds and engulfs the liquid from outside the cell. Phagocytosis is when the vesicle forms around a solid particle like in slide 29. Receptor-mediated Endocytosis uses special receptor molecules that; Bind to specific molecules outside the cell Aggregate together as more and more of the receptors bind target molecules Cause the membrane to warp inwards where they aggregate Eventually pinch off to form a “coated” vesicle.
- This shows the steps in exocytosis. A vesicle on the inside of the cell contains a cell product destined for export. The membrane of the vesicle merges with the plasma membrane, and the contents of the vesicle are now outside the cell.