(c)+Passive+transport+(Q13-15)

Q13 Carefully define the terms **diffusion** & **osmosis**.

Diffusion – The spontaneous movement of particles down their concentration gradient. A process where a certain concentrated volume of material moves to less concentrated state. []

Osmosis: If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis. Pasted from <[]> []

Q14 As a general rule, which substances can diffuse across a cell membrane and which substances cannot?

a) Plasma membranes control the amount of molecules that come in and out of the cell. If there is too high a concentration of a substance (eg. Glucose) than the membrane will control the amount of glucose that comes in the cell.

b) Since the phospholipid heads are polar and non-polar it is difficult for both polar and non- polar molecules to pass through the membrane.

Q15 What is meant by **facilitated diffusion**? Does it require an energy input? Describe the process.

**Facilitated diffusion** is used when **polar molecules**, called ions, need to cross the membranes. These molecules cannot easily cross without assistance, which is where the term "facilitated", meaning "made easier", is important. There are **two** ways in which diffusion can be facilitated: a. __**Channel Proteins**__: These proteins can be used to form a **hydrophilic "pore"** (hole) in the membrane. The ions can then diffuse through the membrane via this pore. b. __**Transport Proteins**__: Substances like glucose diffuse into the cell more easily with the help of transport proteins.There is a "**binding site**" for the glucose. When it binds, the transport proteins' **structure changes**. This causes it to carry the glucose through the membrane and into the cell. Once it has finished carrying the glucose, the transport protein returns to its regular structure, so that it can carry another glucose molecule through. Facilitated diffusion is a form of **passive transport**, like diffusion, and therefore **does not require any energy input**. It is also the movement of substances **from higher to lower concentration** (lower concentration gradient) Video showing Facilicated Diffusion.



Diagram describing the process: __ [] __