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Bulk Transport – Transport Across Cell Membranes

by Georgina Cornwall, PhD
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    00:00 Moving forward into the bulk transport. We've covered primary active transport, secondary active transport during the active transport section here. And now we'll move in to bulk transport. This means not just moving one thing. It's just like it sounds. Lots of stuff all at once. There are a few different mechanisms. We've kind of covered them already when we discussed the endomembrane system. We have endocytosis which is where we bring in food or water. Specifically we can have phagocytosis. We break that word down. Phag- means to eat, cytosis, so it's cellular eating. And then pinocytosis which is cellular drinking.

    00:48 They pull in a lot of fluid and electrolytes and such to balance the inside of the cell.

    00:54 The opposite of that, that we also explored with the export of things from the endomembrane system whether it was the glucose that we produced inside the cell that was for transport out of the cell, would be exocytosis. So we exocytose stuff out of the cell. The vesicle comes along binds with the cell membrane, pops open and drops all the materials to the external environment.

    01:19 We could get rid of wastes in this manner also. So after we have a lysosome bind with things and break things down then those waste could also be exocytosed. A similar mechanism that is a little bit more complicated is the receptor mediated endocytosis. So endocytosis, yes. We are going to pull some things into the cell. But in this case we want to pull in some very specific things. So the cell actually produces some receptors. Puts the receptors out in the membrane. Once its put the receptors out in the membrane then the molecule of intent can bind to them. Let's use cholesterol as an example. This is how LDLs work in order to move cholesterol into cells. So we can have cholesterol floating around in the blood.

    02:15 We want cholesterol. We'll throw out some receptors for cholesterol. Once cholesterol shows up and binds to receptors in these, they're called clathrin coated pits. Clathrin is just a kind of protein that these receptors exist amongst. And the molecule of interest here, cholesterol jumps on to those receptors and then the cell will endocytose, capture those and draw them inside the cell to eventually have cholesterol hidden out of the blood inside the cell.

    02:47 Now, in hypercholesterolemia, the condition where people are not taking in cholesterol as well, results from having some problems with the actual docking of the little receptors into the clathrin coated pits. So the cells are making receptors for cholesterol however they are not able to actually pick up the cholesterol and bring it into the cell because they don't bind into the pits properly. So what that results in is an individual having much more cholesterol in their blood than you would normally have. And that leads to heart disease and all of those other associated problems. So, receptor mediated endocytosis is different from regular endocytosis because it requires or it's asking for very specific molecules. We just used cholesterol as an example whereas general endocytosis is consuming whatever happens to be there. So in this lesson we have certainly covered some different means of moving things. Passive transport where we require no energy and moving things across the cell membrane. We can move down the cell concentration gradient. So if it's high concentration outside to inside, we move things down. That's passive. If we have to go in the opposite direction, we are having active transport. So you should now be able to describe primary active transport in terms of the sodium potassium pump as well as diagram secondary active transport. Recall that it is dependent on the energy invested in the primary mechanism.

    04:31 So, the sodium potassium pump in this case. And then we spent a moment here with bulk transport.

    04:39 So now you should be able to distinguish the three types of bulk transport.

    04:44 I hope this cleared things up for you. Thanks for joining me. I look forward to seing you again shortly.


    About the Lecture

    The lecture Bulk Transport – Transport Across Cell Membranes by Georgina Cornwall, PhD is from the course Cellular Structure.


    Included Quiz Questions

    1. False
    2. True
    1. Endocytosis ---- To throw away the waste from a cell
    2. Endocytosis ---- Bulk transport
    3. Phagocytosis ---- Cell eating
    4. Pinocytosis --- Cell drinking
    5. Exocytosis ---- Cell vomiting
    1. Exocytosis helps in the transport cell debris and excretory products to the nucleus of the cell to keep cytoplasm free of clutter.
    2. Macrophages participate in innate immune response by engulfing the microbes and cancer cells via phagocytosis process.
    3. Receptor-mediated endocytosis helps in the bulk movement of selective materials inside the cells.
    4. Cholesterol intake occurs via receptor-mediated endocytosis process involving the locking of cholesterol molecules with the specific receptors present in clathrin-coated pits on the cell membrane.
    5. Pinocytosis or cell drinking process helps the cell to maintain homeostasis by taking in the fluids and electrolytes from the exterior.

    Author of lecture Bulk Transport – Transport Across Cell Membranes

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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