Question

1. why can certain particles pass through a semi-permeable membrane and others not?

2. what is a positive control test

Answer 1

1.

Semi permeable membranes allows some substances to cross through easily, but not others. Small molecules that are nonpolar (have no charge) can cross the membrane easily through diffusion, but ions (charged molecules) and larger molecules typically cannot. Depending on the membrane and the solute, permeability of substances may depend on solute size, solubility, properties, or chemistry.Some of the factors that helps to determine if a particle will pass through a semi permeable membrane or not are:

• Concentration is a very important factor that determines the permeability.Diffusion of molecules occur from higher concentration to lower concentration through passive transport usually.In order to move against the concentration gradiant,particles must undergo active transport.During active transport, the membrane needs to put forth energy to move molecules across the membrane.
• Size is another very important factor.If the molecules are too large to fit in the semi permeable layers/pores then they undergo facilitated diffusion using a carrier protein
• Charged molecules also cannot diffuse by themselves and require a carrier.
• Large uncharged molecules such as glucose, also cannot easily permeate the cell membrane. Sometimes they manage to slip across the membrane through diffusion, but the process is extremely slow due to the size of the molecules.

2.

Controlled experiments perfoms the same experiment in two different ways, one being the "control test " and the other being the "experimental test".

Positive control is used to check the validity of a test.Suppose you have a medicine that might cure a certain disease,administering it to a group of people with the disease maynot guarantee that it was the medicine that cured the disease.So we try treating the same group of people with an already working medicine .Now we have something to compare our medicine to.This is positive control.If positive control doesnot produce the expected result,then check your experimental setup again and repeat the experiment.