Passive Transport: Diffusion, Osmosis, and Facilitated Diffusion

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...