Passive Transport: Diffusion, Osmosis, and Facilitated Diffusion

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My AP Biology Thoughts

Unit 2 Cell Structure and Function

Welcome to My AP Biology Thoughts podcast, my name is Jacky and I am your host for episode #4 called Unit 2 Cell Structure and Function: Passive Transport. Today we will be discussing the different types of Passive Transport which are Simple Diffusion, Osmosis, and Facilitated Diffusion.

Segment 1: Introduction to Passive Transport.

  • For cells to survive, they must take in or expel certain particles and substances. What monitors their entry or exit is the plasma membrane in a process known as membrane transport.
  • Two types of transport: active and passive.
  • Active requires atp energy, cell is purposely doing it
  • But today focusing on passive transport, where no energy is required to move materials in or out, natural process known as diffusion
  • 3 main types: Simple diffusion, osmosis, and facilitated diffusion
  • In all types, movement is based on a concentration gradient, substances move from areas of high conc. To low, whether this is in or out of cell
  • Seeking equilibrium, a balance in concentration

Segment 2: More About Passive Transport

  • Start with simple diffusion
  • The movement of particles down the concentration gradient across the semipermeable lipid membrane.
  • If there is a greater concentration of a particle on one side of the membrane, simple diffusion will occur and the particle will move to the area of lower concentration.
  • These particles must be small and non-polar, as only these types of particles can make their way through the lipid membrane.
  • For example CO2 and O2, CO2 is often produced and of high conc. Within cells so it will diffuse outwards. O2 is often present in higher conc. Out of cell, so it will typically diffuse inwards.
  • A useful analogy would be to think about the smell of cooking wafting through the house. When you cook, the food molecules are highly concentrated in the air around the stove, so the smell is strongest around there. However, the food molecules will soon diffuse throughout the house into areas of lower concentration, which is why the smell of the food will eventually reach you in a different room.
  • Osmosis: A type of simple diffusion except with water as the specific particle diffusing
  • Specifically, movement of water molecules down the concentration gradient across semipermeable lipid membrane
  • Water in an area with higher conc. Of water vs solute Will diffuse into an area with lower conc of water. Vs solute
  • For example...
  • If placed in a hypertonic solution, where the concentration of solutes is higher than the cell, water will flow out of the cell and into the solution to balance out the lower conc. Of water outside
  • If placed in a hypotonic solution, where conc. Of solutes are lower than the cell, water will flow into the cell to balance out the lower conc. Of water in it
  • Osmosis’s ultimate goal however is to create an isotonic environment, where the concentration of solute and water is equal inside and outside of the cell. In this case, water will still be flowing in and out, and cells will be able to function normally.
  • Facilitated diffusion
  • Sometimes simple diffusion will not work, as certain particles are blocked by the semi-permeable cell membrane whether due to size or polarity
  • Cell still needs these particles, so they must find another way to get them: this way is facilitated diffusion
  • Facilitated diffusion occurs with the help of specialized proteins called channel proteins and carrier proteins. These proteins provide a larger opening for needed molecules to pass through passively.
  • Channel proteins are not specialized for certain molecules, and will like the cell membrane discriminate based on size (only for channel proteins, size allowance is larger)
  • Often carry across ions
  • An example of channel protein is an aquaporin, designed for quick transport of water, quicker than the time needed to cross cell membrane
  • Carrier proteins are often more specialized, usually taking in only one specific type of molecule, and undergo a specific process when transporting the molecule]
  • The carrier channel will mold itself to the shape of the particle before guiding it into or out of the cell
  • This is why it is so selective, as different molecules have distinct shapes that specialized carrier proteins may not accept
  • An example of a molecule transported by carrier proteins is glucose. Glucose is a large polar molecule that cannot undergo simple diffusion, so it must be specifically move into the cell by a carrier protein

Segment 3: Connection to the Course

  • Passive transport and its 3 types are important for many reasons
  • Most significantly, bring in particles/molecules vital to the survival of the cell, such as water and sugars
  • Works in tandem with active transport. Cell cannot wastefully expend ATP on bringing in every type of molecule; passive transport remedies this by allowing many important molecules to pass through naturally without extra energy used. This saved energy can be used to perform many other cell functions.

Passive transport also maintains homeostasis for the cell by adhering to the concentration gradient. When particles diffuse from high to low conc., they are helping to maintain balance so that the cell is not overwhelmed by too much or too little of something.

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com. See you next time!

Music Credits:

  • “Ice Flow” Kevin MacLeod (incompetech.com)
  • Licensed under Creative Commons: By Attribution 4.0 License
  • http://creativecommons.org/licenses/by/4.0/

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