Topic 14: Membrane Transport Flashcards
Give some characteristics of transporters
They are variable in size and chemical composition
Some transporters are ubiquitous, others are kingdom-specific
They often serve dual functions as both a transporter and an enzyme/receptor
What are the two modes of membrane transport?
There are two major modes of transport for a molecule to cross the membrane and enter or exit the cell:
Simple (passive) diffusion: Simple diffusion involves movement of molecules across a membrane without the assistance of a membrane protein. Some molecules are able to pass through the membrane because they are lipophillic, meaning they dissolve in the lipid bilayer. Examples of this include steroids and phenyl compounds. Small uncharged molecules, like H2O, O2, CO2 and N2 can also pass through
Mediated diffusion: The movement of molecules that requires the assistance of membrane proteins is called mediated diffusion. This is carried out by proteins, peptides, and small molecular weight carriers. Ions, uncharged organic compounds, peptides and even can be transported. While H2O, urea, and glycerol passively diffuse, they also can be diffused through protein mediated diffusion, and so they fit in both categories
The two major modes of mediated diffusion are passive transport/facilitated diffusion and active transport
Facilitated diffusion is the transport of an ion or molecule down a electrochemical or just chemical concentration gradient, It is also called passive transport because the energy driving the ion movement originates in the ion gradient itself, without any contribution by the transport system
Diffusion of a molecule/ion against its chemical/electrochemical gradient is called active transport, as protein pumps embedded in the membrane use an energy source to move the molecule against its gradient
What are the three main transmembrane proteins involved in membrane transport?
Pumps move substances against their electrochemical gradient, and therefore use ATP or some other energy method to do so
Channels provide a pathway into the cell down the electrochemical gradient. They are often specific to a type of ion. There are two types of channels
Pores: Pores are always open, AQP is an example of a pore
Gated-channels open and close in response to stimuli, ligands or voltages. We have seen previously the 3 types of gated-channels
Carriers have two gates, each facing opposite sides that are never both open. They therefore do not provide continuous access into/out of the cell. They open to let a molecule in the carrier protein, close the open gate, and open the other gate to release the molecule to the side opposite it entered. This occurs through a conformational change. This makes movement into the cell slow. There are three types of carriers:
Uniporters move a single chemical down its electrochemical gradient
Symporters and antiporters both couple the movement of a chemical down its gradient to move another chemical up its concentration gradient
In symporters, both molecules being diffused are going in the same direction, white in antiporters, they travel in opposite directions
What are the two types of active transport
Active transport involves the pumping of solutes against their gradients. This is non-spontaneous and therefore requires an external source of energy. Passive transport did not require energy because the electrochemical gradient favored the movement of the ion. There are two types of active transport:
Primary active transport is powered by a direct source of energy such as ATP, light or electron transport
Secondary active transport is a cellular transport process that uses the energy stored in an electrochemical gradient, which is established by primary active transport, to drive the movement of another substance against its concentration gradient. In other words, the energy derived from the movement of one molecule down its electrochemical gradient is used to transport another molecule against its concentration gradient.
The two types of secondary active transporters are the symporters (ex:Lac- permease and sodium/glucose transporters) and antiporter (ex:Na+/Ca2+ exchanger) we saw
Describe the structure of the the sodium-phosphate pump
The pump consists of an 𝛼-subunit of about 100 kDa. IT contains the ATPase activity and contains all known substrate and inhibitor binding sites
There is also a β-subunit, a small glycopeptide of 30-40kDa
It is required for function, but its role is unknown
It is necessary it seems for proper membrane insertion during synthesis
