In Episode 9, Melanie reviews all things transport - active and passive. Do you know who is invited to the party?
In Episode 9, Melanie reviews all things transport - active and passive. Do you know who is invited to the party? (0:45). Molecular chemistry will greatly influence the method of transport (1:40). She distinguishes between active and passive transport, with multiple examples (2:20). Sounds like a great time to take a deep breath and make a cup of tea (2:48). Moving big things all at once will require vesicles (4:19) with endo and exocytosis. Melanie warns you of a common misconception! (4:39).
The Question of the Day (5:34) asks “What is the effect of cholesterol in the phospholipid membrane?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Transport
Lets Zoom out:
Unit 2 - Cell Structure and function
Topic - 2.4 -2.7
Big idea - Energetics
I have some bad news: Not everybody is invited to the party. The truth of the matter is, some molecules can enter the cell, others need passwords or special keys, while still others will have to wait in line for a tunnel. It's important that cell membranes establish and maintain their internal environments through selective permeability.
Lets Zoom in: The fluid mosaic model – sounds so fancy! Essentially it implies that the phospholipids which comprise the majority of the bilayer, along with the embedded proteins, steroids, and carbohydrate chains are continuously swimming, moving, and shifting around the surface of the cell within the membrane. It is dynamic. And because each of the plasma membrane components has unique chemistry, the molecules that it interacts with for transport will be unique as well.
Ok so who can come and go to the party freely? Take a deep - breath and think about molecules that are small, unassuming, but critical to cellular function. Yes - deep breath – oxygen and carbon dioxide for sure. They are both small and non polar and can pass freely directly through the phospholipid membrane. Small polar molecules like water can also pass through, but it in smaller amounts. What about large or charged? You are going to need permission. Molecules like ions, amino acids or glucose will need the assistance of a transport protein or vesicle.
There are two types of cellular transport – passive, and active. These categories are divided by the use of energy and direction of molecular movement. Passive transport is the net movement of molecules from an area where they are more highly concentrated to an area where they are less concentrated without the input of metabolic energy, or ATP. Molecules are always in motion and bumping into each other. Think of making a cup of tea. When you first submerge the bag of tea into hot water, the tea is most concentrated, or crowded inside the bag. But over time, small tea particles begin to bump into each other, crossing the membrane of the bag and eventually are evenly distributed in the cup of water. This movement of molecules from high to low through a membrane is called diffusion and is a type of passive transport. But, the larger tea leaves stay in the bag. However, if we poked a hole in the bag, the larger tea leaves might move into the cup too, single file, through that pore. The movement of molecules from high to low concentration through a transport protein embedded in the membrane is called facilitated diffusion – or literally diffusion made facil – the Spanish word for easy.
OK – Flip it and reverse it. How does the cell move molecules against the concentration gradient? What if I want to make the molecules more crowded? The movement of molecules from regions of low concentration to high concentration is known as Active transport and requires the spending of cellular energy, or ATP. The use of transport proteins for either active transport or facilitated diffusion are often solute specific.
Membrane proteins can have a variety of other functions, such as enzymatic activity cytoskeleton attachment, and signal transduction – We will cover many of these in future units.
You want to move many things at once? This will require vesicle formation. Bringing molecules into the cell, we will borrow from the plasma membrane to form a vesicle through endocytosis. Having molecules leave the cell, the exciting vesicle fuses with the plasma membrane, emptying its contents into the extracellular fluid.
Students beware! Don’t fall in this trap! Remember, there is no up, down, left, right, in 3D space. Make sure you describe the movement of molecules either into or out of the cell. Molecules don’t just move “up the page” because you are looking at a diagram from that perspective.
Don’t worry – we didn’t forget about osmosis – more to come on future episodes.
To recap….
The structure and chemical properties of molecules affects their ability to pass through the plasma membrane. Small and non polar – diffuse on through. Large and Charged? Assistance is needed. Active and passive transport are both essential cellular functions to take in nutrients and get rid of wastes for metabolism.
Today’s Question of the day is about membrane structure
Question: What is the effect of cholesterol in the phospholipid membrane?