Diffusion and Transport
Question 1A-B (Week 6 Analyzing): What types of transport and diffusion occur in cells? List an example of each, indicate if the listed example requires energy and, if energy is required, where does that energy come from? For each example, be sure to also compare and differentiate the size and chemical properties of the solutes crossing the membrane.
Example 1 Diffusion:
Example 2 Transport:
Question 2A-D (Week 6 Applying): You are working in a lab studying a receptor kinase that is involved in a stress response signaling pathway. Signaling molecule A binds to the receptor kinase causing dimerization and protein B is activated via phosphorylation by an activated Ras signaling protein. Protein B then phosphorylates protein C which translocates to the nucleus and activates (also by phosphorylation) protein D. Protein D is located in the nucleus and interacts with RNA polymerase to express genes involved in stress response.
A. What type (hydrophobic or hydrophilic) of signaling molecule is molecule A? How do you know? Explain your answer.
B. What type(s) of amino acids e.g. nonpolar or polar will interact with signaling molecule A? Explain your answer.
C. What type of protein is protein B? Explain your answer and describe the function of protein B.
D. Do you think there would be a change in the expression of stress response genes if there was a missense mutation in protein D? Explain your answer and the function of protein D.
Question 3 (Week 7 Understanding and Analyzing): Compare and contrast the inputs and outputs of cellular respiration and photosynthesis. Be sure to indicate if the processes are connected in terms of inputs or outputs i.e. do the outputs of photosynthesis input into cellular respiration and vice versa? Inputs and outputs to include in your description: ATP, ADP, glucose, pyruvate, acetyl-CoA, oxygen, water, sunlight, sugar and electron carriers such as NADH and NADPH.
Question 4 (Week 7 Applying): The reactions of the Calvin cycle used to be referred to as the "dark reactions" of photosynthesis and experimental evidence demonstrates that in the absence of light, the Calvin cycle shuts down. Why do you think this happens? Would you predict the Krebs cycle to also shut down in the absence of light? Why or why not?
Sample Solution
- Passive transport: This type of transport does not require energy. Solutes (molecules or ions) move across the cell membrane from an area of high concentration to an area of low concentration. This is called diffusion.
- Facilitated diffusion: This type of transport also does not require energy. However, it does require the help of carrier proteins in the cell membrane. Carrier proteins bind to solutes and then transport them across the membrane.
- Active transport: This type of transport does require energy. It is used to move solutes against their concentration gradient, from an area of low concentration to an area of high concentration. This is done by using ATP, the cell's energy currency.
Full Answer Section
Here are some examples of each type of transport:
- Passive diffusion: Oxygen and carbon dioxide diffuse across the cell membrane. Oxygen is needed for cellular respiration, and carbon dioxide is a waste product of cellular respiration. Both oxygen and carbon dioxide are small, non-polar molecules, which means they can easily diffuse across the cell membrane.
- Facilitated diffusion: Glucose is a solute that is transported into cells by facilitated diffusion. Glucose is a large, polar molecule, which means it cannot easily diffuse across the cell membrane on its own. It needs the help of a carrier protein to transport it across the membrane.
- Active transport: Sodium ions are transported out of cells by active transport. Sodium ions are positively charged, and they tend to accumulate inside cells. This creates an electrical gradient, with the inside of the cell being more positive than the outside of the cell. This electrical gradient is used to power the active transport of sodium ions out of the cell.
The size and chemical properties of the solutes that can cross the cell membrane vary depending on the type of transport. Small, non-polar molecules can easily diffuse across the cell membrane, while large, polar molecules need the help of carrier proteins. Solute charge can also affect the ability of a solute to cross the cell membrane. Positively charged solutes are more likely to be transported out of cells, while negatively charged solutes are more likely to be transported into cells.
The energy for active transport comes from ATP, the cell's energy currency. ATP is hydrolyzed to ADP and phosphate, releasing energy that is used to power the active transport of solutes.