PASSIVE TRANSPORT PROCESSES

Transcription

1 BIOZONE Assignment #2 Cell Membrane Transport PASSIVE TRANSPORT PROCESSES 1. Describe two properties of an exchange surface that would facilitate rapid diffusion rates*: (a) thin membrane (b) porous membrane 2. Describe two biologically important features of diffusion*: (a) Glucose transport into red blood cell (b) Gas exchange in alveoli of lungs *Multiple answers possible! 3. Describe how facilitated diffusion is achieved for: (a) Small polar molecules and ions: Channel-mediated (ions diffuse through a channel) (b) Glucose: Carrier-mediated (glucose binds to the carrier protein, causing a conformational change, and it is released to the other side of the membrane) 4. How are concentration gradients maintained across membranes? Molecules move from high to low concentrations 5. Describe the role of aquaporins in the rapid movement of water through some cells: Since water cannot move through the hydrophobic regions easily, the aquaporin creates a favorable environment for the water to enter due to the polar amino acid side chains on the inside of the aquaporin. 6. (a) What happens if a cell takes up sucrose by active transport? If sucrose enters the cell via active transport, then water will follow via osmosis until concentrations of water equalize. (b) Describe a situation where this occurs in plants: Sucrose would be transported via active transport into a plant cell since it is a nutrient needed for cellular respiration (can break down into glucose). Some plant cells will get their sucrose from the leaves and other photosynthetic parts of the plant. 7. Describe what would happen to an animal cell (e.g. a red blood cell) if it was placed into (a) Pure water: The cell would lyse due to too much water entering the cell and placing pressure on the cell membrane ( hemolysis, for RBCs) (b) A hypertonic solution: The cell would lose water due to water moving out of cell ( crenation, for RBCs) (c) A hypotonic solution: The cell would gain water due to water moving into cell

2 8. Paramecium is a freshwater protozoan. Describe the problem it has in controlling the amount of water inside the cell: Since freshwater will have a very high water potential, the paramecium must have an efficient way of balancing its internal water content with that of the environment. If not, the paramecium may lyse due to too much water entering its internal structure. 9. Fluid replacements are usually provided for heavily perspiring athletes after endurance events. (a) Identify the preferable tonicity of these replacement drinks (isotonic, hypertonic, or hypotonic): After an endurance event, an athlete would want to drink a slightly hypotonic solution. (b) Give a reason for your answer: A hypotonic (pure water) solution will help replenish the water and lost from perspiration. A hypertonic solution would cause cells to lose even more water. An isotonic drink (like coconut water!) would also be fine, but not as effective as hydrating as the hypotonic solution. 10. The malarial parasite lives in human blood. Relative to the tonicity of the blood, the parasites cell contents would be: hypertonic / isotonic / hypotonic (circle the correct answer). 11. (a) Explain the role of cell wall pressure in generating cell turgor in plants: Because of turgor pressure building up inside due to the cell wall, water will eventually be unable to flow into the plant cell. This prevents lysing, which could happen in animal cells (b) Discuss the role of cell turgor in plants: Turgor not only prevents lysis, but it also helps the plant get its structure. If plant cells were not turgid, the plant itself would be wilted and flaccid. ION PUMPS 1. Why is ATP required for membrane pump systems to operate? ATP is required because these membrane pumps move nutrients against the concentration (low to high). 2. (a) Explain what is meant by cotransport: Cotransport is a type of membrane protein that is dependent on other protein pumps to create a concentration gradient. The concentration gradient will then cause these proteins to facilitate the diffusion of other molecules down their gradient. (b) How is cotransport used to move glucose into the intestinal epithelial cells?

3 A sodium-potassium pump will use ATP to pump sodium across the membrane against its concentration gradient. Since there is now more sodium in the extracellular space, the sodium ions will naturally diffuse into the cell (down its concentration gradient). They do this by moving through a symport protein that moves both Na+ and glucose through at the same time. (c) What happens to the glucose that is transported into the intestinal epithelial cells? The glucose will most likely be used to generate ATP via cellular respiration in the mitochondria. 3. Describe two consequences of the extracellular accumulation of sodium ions: Increased amounts of sodium ions outside the cell could create a hypertonic environment, causing intracellular water to diffuse out. Since glucose and sodium can be transported together in a symport protein, it may also cause an excess amount of glucose to enter the cell, leaving less glucose in the extracellular fluid for other cells. (In addition, excess extracellular sodium causes antidiuretic hormones to be released, decreasing the water loss during filtration in kidneys. So watch your sodium intake!!) DISTURBANCES TO ION TRANSPORT 1. How does the cholera toxin cause a loss of water into the gut lumen? Cholera is a toxic molecule that activates the production of cyclic AMP (camp*) inside the epithelial cells* of the intestine. The camp will keep chloride ion channels open, which causes Cl- to flow into the gut lumen*. This then causes Na+ to also diffuse out of the epithelial cell (due to trying to balance out its electrochemical gradient*). This causes a solute imbalance, so now water will leave the intestinal cells too. So ultimately, you have a lot of water inside your intestine in other words, diarrhea!!! L *Vocab: - camp: a nucleotide that sends messages inside cells - epithelial cells: the cells along the inside of your intestines; surrounds the gut lumen - gut lumen: the space inside your intestines - electrochemical gradient: the difference in electric potential (+/ ) across a membrane; similar to concentration gradient in that charged particles (ions) will move in a way they balance out the electrochemical potential. 2. Oral rehydration salts are used to treat cholera and contain high concentrations of glucose and sodium. Explain how these are able to replace the water and electrolytes lost from a cell: Because oral rehydration salts contain high amounts of glucose and sodium, when these enter the gut lumen, they will try to diffuse down their concentration gradient, into the intestinal epithelial cells. This will cause water to diffuse into the cell as well, balancing out the water content.

4 3. Why was it important to understand ion transport in the intestine when devising a treatment for cholera? It is important to understand ion transport in treating cholera. Cholera affects the cystic fibrosis transmembrane regulator ion channels (CFTR). Even though CFTR only directly affects the chloride ion, it will also cause changes to the concentration gradients of other ions, due to the changes in the electrochemical gradient. EXOCYTOSIS and ENDOCYTOSIS 1. Distinguish between phagocytosis and pinocytosis: Phagocytosis and pinocytosis are both types of endocytosis. However, they differ in what they transport into the cell. Phagocytosis transports solid materials into the cell, while pinocytosis transports liquids. 2. Describe an example of phagocytosis and identify the cell type involved: Lymphocytes (white blood cell), such as neutrophils or macrophages, can ingest foreign materials, such as bacterial pathogens. 3. Describe an example of exocytosis and identify the cell type involved: Lymphocytes, such as B-cells, can produce antibodies and other proteins, which need to be released into the extracellular space via exocytosis. 4. Why is cytosis affected by changes in oxygen level, whereas diffusion is not? Cytosis is a type of active transport, which means it requires ATP. ATP is generated by the mitochondria using the available oxygen in the cell. 5. How does each of the following substances enter a living macrophage (for help, see Passive Transport Processes)'. (a) Oxygen: simple diffusion (c) Water: facilitated diffusion (b) Cellular debris: endocytosis (phagocytosis) (d) Glucose: facilitated diffusion ACTIVE AND PASSIVE TRANSPORT SUMMARY 1. Identify each of the processes (A-G) described in the diagram above in the spaces provided. Passive Transport Active Transport [ A: SIMPLE DIFFUSION ] [ D: COTRANSPORT ] [ B: OSMOSIS ] [ E: PINOCYTOSIS ] [ C: FACILITATED DIFFUSION ] [ F: EXOCYTOSIS ] [ G: PHAGOCYTOSIS ] 2. In general terms, describe the energy requirements of passive and active transport. Passive transport does not require energy, while active transport does require ATP (even in cases of cotransport, the first part has to occur with ATP).

5 3. Name two gases that move into or out of our bodies by diffusion*: Oxygen, Carbon Dioxide *Multiple answers possible! 4. Identify the transport mechanism involved in each of the following processes in cells: (a) Uptake of extracellular fluid by liver cells: pinocytosis (endocytosis) (b) Capture and destruction of a bacterial cell by a WBC: phagocytosis (endocytosis) (c) Movement of water into the cell: osmosis (facilitated diffusion) (d) Secretion of digestive enzymes from cells of the pancreas: exocytosis (e) Uptake of lipoproteins in the blood by mammalian cells: endocytosis (f) Ingestion of a food particle by a protozoan: phagocytosis (endocytosis) (g) Transport of chloride ions into a cell: facilitated diffusion (h) Uptake of glucose into RBCs: facilitated diffusion (i) Establishment of potential difference across the membrane of nerve cell: Na+/K+ pump