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4.2, 4.3, 4.4, 4.5 Transport in cells (Osmosis (Water potential (Measured…
4.2, 4.3, 4.4, 4.5 Transport in cells
Diffusion
An example of passive transport - the energy comes from natural, inbuilt motion of particles (rather than an external source like ATP)
- all particles are constantly in motion due to the kinetic energy that they possess
- their motion is random
- particles are constantly bouncing off each other and off other objects.
Diffusion = the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed
Facilitated diffusion
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Still a passive process. - relies on inbuilt motion (kinetic energy) of the molecule, no ATP.
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instead of through the phospholipid bilayer, it goes through protein channels and carrier proteins.
Protein Channels
Water-filled hydrophilic channels across the membrane. They allow specific water-soluble ions to pass through. They are selective and only open in the presence of a specific ion. Has control over entry and exit of ions. The ions bind with the proteins, changing its shape in a way that cause it to open on one side of the membrane and close on the other side.
Carrier proteins
Molecules such as glucose (specific to the protein) bind to the protien and cause it to change shape in such a way that the molecule is released to the inside of the membrane. No external energy needed.
Most molecules do not easily pass across the cell-surface membrane, some that can are small, non-polar molecules such as oxygen and carbon dioxide.
Osmosis
Osmosis = the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
Cell surface membranes (and other plasma membranes) are permeable to water molecules and some other small molecules but not to larger molecules.
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Water potential
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- adding a solute to pure water will lower its water potential
- water potential of a solution is always less than zero (always negative)
- the more solute added, the more negative its water potential
- water will move by osmosis from an area of higher water potential to an area of lower water potential
to find the water potential of cells or tissues, place them in a series of solutions of different water potentials. Where there is no net gain or loss of water from them (change in mass) the water potential of the cells must be the same as the solution
A red blood cell in pure water will absorb water by osmosis but because cell membranes cannot stretch that much, it will break and burst the cell (in red blood cells this is called haemolysis). To prevent this, animal cells live n a liquid which has the same water potential as the cells (in blood this is plasma). If a red blood cell is placed in a solution with a water potential lower than its own, water leaves by osmosis and the cell shrinks and becomes shrivelled.
Active transport
Active transport = the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
- metabolic energy in the form of ATP is needed
- substances are moved against a concentration gradient
- carrier proteins are used
- only selective substances are transported
- the carrier proteins bind to the molecule or ion on one side of the plasma membrane using the receptor sites on the protein
- on the inside of the cell/organelle, ATP binds to the protein and splits into ADP and a phosphate molecule. This changes the shape of the protein and opens to the opposite side of the membrane
- the ion or molecule is released to the other side of the membrane
- the phosphate molecule is released from the protein and the protein reverts back to its original shape ready for the process to be repeated. The phosphate recombines with the ADP to form ATP during respiration.
Co-transport
Sodium-potassium pump
- sodium ions are actively transported out of epithelial cells using the sodium potassium pump, and into the blood. Takes place in a specific carrier protein found in the cell-surface membrane of the epithelial cells
- Maintains a higher concentration of sodium ions in the lumen than inside the epithelial cells.
- sodium ions diffuse into the epithelial cells down a concentration gradient through a different carrier protein, and carry an amino acid or glucose molecule into the cell with them.
- The glucose/amino acids pass into the blood plasma by facilitated diffusion using another carrier protein.