Episode 53 brings biology, physics and chemistry all together to answer this age old question - why did the fart cross the room?
Episode 53 brings biology, physics and chemistry all together to answer this age old question - why did the fart cross the room? Melanie begins by recapping the fluid mosaic model and kinetic energy (2:19). To get across the plasma membrane without the use of energy there are two options - sneak in between the phospholipids themselves (3:40) or use a transport protein (6:50). It's true - whoever smelt it most likely dealt it as solutes diffuse from high to low concentration.
The Question of the Day asks (9:08) Describe the cellular condition of a red blood cell placed in a hypertonic solution.
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap passive transport.
Zoom out:
Unit 2 - Cell Structure and Function
Topics 2.5 - 2.9
Big idea - Energetics
Whoever smelt it dealt it - a common phrase when trying to pin the passing of gas on an unassuming friend. But there may be some truth to it. After all, molecules move from high to low concentration. So the most likely person to smell the odorant initially is the one who produced it - with greatest concentration. Bringing biology, physics and chemistry all together to answer this age old question - why did the fart cross the room? Time to concentrate.
Let’s Zoom in:
Starting off as we often do - with a nerdy word moment. The prefix trans- comes from Latin meaning across. And port? Just like the location for ships to dock in the harbor, also from Latin - porto, portare, meaning to carry. So - transport - to carry across. And so, passive transport is the method of moving solutes across a membrane without the use of cellular energy.
Let’s take a moment to remind ourselves about the structure of cellular membranes - as this will be the barrier that must be overcome for anything attempting to cross. We refer to the plasma membrane as a “fluid mosaic model.” It is composed of a bilayer of amphipathic phospholipids with embedded proteins, cholesterol, and carbohydrate chains - some continuously shifting around the surface of the cell membrane. It is very dynamic. And because each of the plasma membrane components has unique chemistry (specifically hydrophobic or hydrophilic) the molecules that it interacts with for transport will also be distinct.
First - we need to remember that molecules are always in motion due to kinetic energy. They bump into each other often, even more often in warmer temperatures or in areas where they are crowded, also known as concentrated. Passive transport is the net movement of molecules from an area where they are more highly concentrated (like 2 M) to an area where they are less concentrated (as in 1 M) without the input of metabolic energy, or ATP. We say net movement because molecules don’t have an agenda. And even though it is more likely that molecules will bump into each other and spread out, a few will also bounce over to the more concentrated area. Passive transport has a direct role in the import of materials and the export of wastes.
To get across the plasma membrane without the use of energy there are two options - sneak in between the phospholipids themselves or use a transport protein. Small, non-polar molecules like oxygen and carbon dioxide can pass freely directly through the phospholipid membrane since the fatty acid tails are also nonpolar. This movement of molecules from high to low through a membrane is called simple diffusion and is a type of passive transport. The diffusion of oxygen and carbon dioxide across membranes is crucial to the process of cellular respiration and external respiration from your bloodstream to the alveoli of your lungs!
Small polar molecules like water can also pass through, but in smaller amounts. Osmosis is the term for the passive diffusion of water. Same rules apply however - water will flow from high to low, down its concentration gradient without the input of ATP. Areas that have a greater concentration of solutes (such as ions, sugars etc.) are hypertonic compared to areas with lesser solutes. The prefixes hyper and hypo refer to the relative amount of dissolved solutes in a solution. Since hypertonic solutions have more solutes, they have a lesser water concentration. Assuming that two solutions are separated by a semipermeable membrane, water will flow from a hypotonic area to a hypertonic area until the water concentrations are equal, or isotonic. Water is still moving between the isotonic areas, but at an equal rate and in both directions. Be careful - equal concentrations does not mean equal amounts. This means that water will enter or exit an area, or cell whether there is room for the water volume or not. Vacuoles, both contractile and storage, are cellular organelles which contribute to osmoregulation. You’ll need to dive deeper into water potential for the full picture on osmosis.
So we covered movement through the phospholipids - now let's talk about movement through a membrane protein. 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 term for easy. Anything large or charged needs the use of a transport protein - either carrier or channel. Since proteins are composed of amino acids with distinct R groups and folding patterns, it is safe to assume that these transport proteins will be chemically picky about the types of molecules that use them. Carrier proteins are integral glycoproteins that bind with specific solutes and undergo a physical conformational change to move the molecule across the membrane. This interaction is similar in nature to an enzyme/substrate interaction. In contrast, channel proteins are lipoproteins that have a pore (which is sometimes gated) allowing specific ions to cross. Membranes may become polarized from the movement of ions. When water moves in larger quantities, it passes through a channel protein known as aquaporins. Channel proteins have a much greater rate of transport than carrier proteins.
Time for unit connections. You’ll find passive transport concepts in Unit 3 with cellular respiration and photosynthesis and Unit 8 with energy - after all, the mechanisms of transport support energy conservation. Alright - what about the exam? You may be asked to make transport predictions based upon membrane models, apply graphical representation to transport rate, analyze experimental results or contractile vacuole rates. Follow the concentration gradient, and you can apply it to any unknown situation.
To recap……
Passive transport is the net movement of molecules from high to low concentration without ATP. It's true - whoever smelt it most likely dealt it as solutes diffuse from high to low concentration. Small, uncharged, non-polar molecules diffuse directly through the membrane while large or charged move through a carrier or channel protein.
Coming up next on the Apsolute RecAP Biology Edition: Active Transport
Today’s question of the day is about tonicity
Question of the day: Describe the cellular condition of a red blood cell placed in a hypertonic solution.