Answer the following questions about molecule movement across a membrane. a. How is osmosis similar to simple diffusion of CO_2 across a membrane? How is it different? If an aquaporin is involved, how does this change your answer? b. Compare and contrast carrier proteins and channel proteins. Include information about both structure and mechanism of transport. c. Compare and contrast indirect and direct active transport. Give a specific example for each type of transport. Solution 5a: Osmosis is the movement of water molecules down the concentration gradiant through a semipermeable membrane. Both osmosis and simple diffusion of CO2 across a membrane occur down the concentration gradiant and are passive transport. However, osmosis transports hydrophilic water molecules while simple diffusion transports hydrophobic non polar CO2 molecules. If an aquaporin is involved, the proces would be called as facililated diffusion which refers to transport of polar, charged, hydrophilic molecules down the concentration gradiant with the help of membrane transport proteins. 5b: Carrier proteins are the transporter proteins that are actually the integral membrane proteins serving in transport of ions across the membrane. These proteins have ion binding sites and exhibit changes in shape to transport the ions. Channel proteins are the integral membrane proteins that form the hydrophilic channels for transport of polar, charged and hydrophilic substances across the membrane. They do not undergo the shape change. 5c: Direct active transport couples the exergonic chemical reaction for increasing the solute/ion concentration on one side of the membrane. The indirect active transport includes coupling of favorable downhill movement of one solute to the unfavorable uphill movement of other substance. For ex: sodium potassium pump couples ATP hydrolysis with uphill movement of sodium (direct active transport). Downhill movement of sodium coupled with uphill movement of glucose (indirect active transport)..

1. Answer the following questions about molecule movement across a membrane. a. How is
osmosis similar to simple diffusion of CO_2 across a membrane? How is it different? If an
aquaporin is involved, how does this change your answer? b. Compare and contrast carrier
proteins and channel proteins. Include information about both structure and mechanism of
transport. c. Compare and contrast indirect and direct active transport. Give a specific example
for each type of transport.
Solution
5a: Osmosis is the movement of water molecules down the concentration gradiant through a
semipermeable membrane. Both osmosis and simple diffusion of CO2 across a membrane occur
down the concentration gradiant and are passive transport. However, osmosis transports
hydrophilic water molecules while simple diffusion transports hydrophobic non polar CO2
molecules. If an aquaporin is involved, the proces would be called as facililated diffusion which
refers to transport of polar, charged, hydrophilic molecules down the concentration gradiant with
the help of membrane transport proteins.
5b: Carrier proteins are the transporter proteins that are actually the integral membrane proteins
serving in transport of ions across the membrane. These proteins have ion binding sites and
exhibit changes in shape to transport the ions. Channel proteins are the integral membrane
proteins that form the hydrophilic channels for transport of polar, charged and hydrophilic
substances across the membrane. They do not undergo the shape change.
5c: Direct active transport couples the exergonic chemical reaction for increasing the solute/ion
concentration on one side of the membrane. The indirect active transport includes coupling of
favorable downhill movement of one solute to the unfavorable uphill movement of other
substance. For ex: sodium potassium pump couples ATP hydrolysis with uphill movement of
sodium (direct active transport). Downhill movement of sodium coupled with uphill movement
of glucose (indirect active transport).