1.4: Membrane Transport
Mission 1: In Da Club, Part II
Mission Objectives. You should be able to
1. Describe and explain the structure and function of the Na+/K+ pump.
2. Define "osmolarity."
3. Compare and contrast diffusion, facilitated diffusion and osmosis.
4. Compare and contrast passive and active transport.
5. Explain the difference between cells that are hypotonic, isotonic and hypertonic.
6. Estimate osmolarity in tissues by soaking samples in different solutions (LAB).
7. Compare and contrast endocytosis and exocytosis.
Mission Objectives. You should be able to
1. Describe and explain the structure and function of the Na+/K+ pump.
2. Define "osmolarity."
3. Compare and contrast diffusion, facilitated diffusion and osmosis.
4. Compare and contrast passive and active transport.
5. Explain the difference between cells that are hypotonic, isotonic and hypertonic.
6. Estimate osmolarity in tissues by soaking samples in different solutions (LAB).
7. Compare and contrast endocytosis and exocytosis.
Understandings:
Diffusion: This is the net movement of particles from an area of high concentration to an area of low concentration. Think of how scents and fragrances make their way to your nose. If someone sprays perfume, the scent is strongest near the bottle, but in time, the scent particle will make their way to your nose. At some point, there will be an even distribution of scent particles in the air. This is diffusion.
In terms of living systems, diffusion involves a membrane. For example, oxygen gas moves from outside of the cell to inside to be used by the mighty mitochondria to carry out respiration. This creates a lower concentration of oxygen inside the cell, so oxygen will diffuse into the cell as a result. Carbon dioxide, a waste product of mitochondrial respiration, diffuses out of the cell in much the same way. Facilitated diffusion: When substances move through the plasma membrane via a water-filled transport protein (called a carrier protein). Carrier proteins change shape by opening and closing to allow necessary substances to diffuse through the membrane. Energy is not required for facilitated diffusion. Passive & Active Transport
Passive transport is basically movement of substances that does not require energy. Diffusion, osmosis, and facilitated diffusion are forms of passive transport. By contrast, active transport requires energy to move substances into and out of the cell. The proteins embedded in the plasma membrane are positioned so that half lies inside the bilayer and half outside the bilayer. They must work against a concentration gradient. This means instead of substances (usually ions) moving from areas of high concentration to areas of low concentration, the proteins must work in reverse: moving ions from low concentration to high concentration. This is especially true of neurons; they must pump K+, Na+, and Cl- in and out in order to fire (or synapse). Because they are working against the concentration gradient, they require a form of chemical energy called adenosine triphosphate (ATP) to assist in the process. Animal cells have a higher concentration of potassium ions than their exterior environment, whereas sodium ions are more concentrated in the extracellular environment than inside the cells. The cell maintains these conditions by pumping K ions into the cell and pumping Na ions out of it. A membrane protein is required for this to occur.
The slideshow to the right comes from pgs. 35-38 in your text and describes the steps of the Na/K pump. Below is the sequence. 1. A protein binds to three intracellular Na ions. 2. When the Na ions bind, ATP (adenosine triphosphate) loses a phosphate ion and turns into ADP (adenosine diphosphate). 3. This loss of a phosphate causes the protein to change shape and push the Na ions out of the cell. 4. Random K atoms hanging outside of the cell bind to the protein which releases the phosphate group that was captured in step 2. 5. The loss of the phosphate group restores the protein's original shape and allows the release of the K ions into the cell. |
Applications & Skills:
Osmosis: This is a form of specialized diffusion of water (moving passively) across a selectively permeable membrane. This is the result of a difference between solute concentration on either side of the selectively permeable membrane.
Usually the water is mixed with a solute (sometimes sugar or salt). The water can pass through the membrane but the solute cannot. The water will diffuse through the membrane towards the side with the greater concentration of solute particles until dynamic equilibrium is reached (when the concentration of the solution is the same on both sides of the membrane). For a cell to survive, ion concentrations need to be the same on both sides of the cell membrane. If the cell does not pump out all of its extra ions to even out the concentrations, the water is going to move in. This can be very bad. The cell can swell up and explode. The classic example of this type of swelling happens when red blood cells are placed in water. The water rushes in to the cells, they expand and eventually rupture (Biology4kids.com). Tonicity
Tonicity is a measure of the osmotic pressure gradient. Cells can be immersed in tonic solutions. There are three variations: hypotonic, hypertonic, and isotonic. Hypotonic (Hypo-osmotic): The solution the cell is in has a lower concentration of solutes (ions, sugars, proteins, etc). There is more water outside of the cell than inside, so it flows into the cell. See the image to the left. Too much water flowing in causes the cells to burst. Hypertonic (Hyperosmotic): The solution the cell is in has a higher concentration of solutes (ions, sugars, proteins, etc). There is more water inside of the cell than outside, so it flows out of the cell. See the image to the left. Too much water flowing out causes the cells to shrink. Water essentially moves from a hypotonic solution to a hypertonic solution across a selectively permeable membrane. Isotonic: The solution has an equal concentration of water and solutes (ions, sugars, proteins, etc.) inside and outside of the cell, and moves in and out at an even rate. The cell retains its size. Most cells are in isotonic solutions. |
Size and Charge: How easily a substances can move across a membrane passively depends on two factors: size and charge. Small, nonpolar substances will move easily across the membrane. Substances that are polar (water excepted), large, or both, do not cross membranes easily. Examples of substances that move with ease are oxygen, nitrogen and carbon dioxide. Substances that do not move easily are ions (potassium, sodium and chloride), glucose, and sucrose.
Endocytosis is the active transport process by which a cell encloses a substance in a portion of the plasma membrane. This process is dependent of the fluidity and the orientation of the plasma membrane to enclose particles or macromolecules to form a vesicle that then enters the cytoplasm of the cell. The membrane then reattaches itself with the vesicle enclosed.
Exocytosis is the opposite process. Materials are secreted or expelled from the cell at the plasma membrane. Protein exocytosis begins in the ribosomes of rough ER and progresses through a series of steps (p. 37-38) until the substance is secreted to the extracellular environment. Both processes require ATP and are used for the transport of large particles. See the below image (Courtesy of WW Norton & Company).
Exocytosis is the opposite process. Materials are secreted or expelled from the cell at the plasma membrane. Protein exocytosis begins in the ribosomes of rough ER and progresses through a series of steps (p. 37-38) until the substance is secreted to the extracellular environment. Both processes require ATP and are used for the transport of large particles. See the below image (Courtesy of WW Norton & Company).
Corresponding workbook pages: #18 - #21, #23 - 26
18: Max
19: Brian
20: Na Young
21: Efa
23: Felicia
24: Yujin
25: Ica
26: Glen
18: Max
19: Brian
20: Na Young
21: Efa
23: Felicia
24: Yujin
25: Ica
26: Glen