An Introduction to Diffusion and Osmosis
Name: __________________
Why do your fingers go wrinkly in the bath? How do moths find a mate in the dead of night? Why
doesn’t jam go off in the cupboard? Plants and animals are always transferring chemicals from one
place to another. In this activity you will investigate two of the ways they do this and discover the
answers to these questions.
Figure 1
Figure 2
1.
Figure 1 shows some red particles moving inside a box. They represent the particles of a gas.
Describe the way the particles of a gas move. In your answer you should use the words random,
thermal energy, collide, and direction.
2.
Figure 2 shows the same number of particles in Fig. 1 except most particles start out on one side
of the box. Explain why after a while, Figure 2 resembles Fig. 1. In your answer you should refer to
the concentration of the particles on either side of the box before and after.
3.
You can smell food when molecules from the food enter your nose.
a) By referring to your answers in questions 1 and 2 above explain how the molecules get from
the food to your nose
b) Would the smell be stronger or weaker if you were stood down wind of the food? Explain your
answer.
c) Why are some foods like hot coffee or fried onions easier to smell than something like ice
cream?
4. Copy and complete the sentence:
Diffusion is the _______________ movement of particles from an area of ________ concentration
to an area of _________ concentration until they are spread out.
5.
Moths emit chemicals called pheromones to attract a mate.
a) Which process is responsible for the distribution of these chemicals through the air?
b)
If a moth detects pheromones, how might it work out which direction they are coming from so it
knows where the potential mate can be found?
c)
What would be the advantage of a moth releasing the pheromones on a night when
there is little wind?
6. The diagram shows a fake “cell.” The cell membrane here has tiny pores (holes) in it. A real
cell’s phosopholipid bilayer may not have actual holes (instead transport proteins help
hydrophilic substances pass), but a regular cell membrane does allow small nonpolar
substances to pass directly through the phospholipid area itself. Water has a harder time
because it is partially charged. However, some water can get through because at least it is still
a very small molecule. Ions, however, that are small but fully charged cannot get through the
hydrophobic part of a real cell membrane because there charge is too great.
Pore
Cell wall
Cytoplasm
Nucleus
So, small molecules, like water, can get
through a membrane slowly. However large
molecules, like starch, or very charged atoms
or molecules like glucose or an ion of sodium
cannot. Because some molecules can get
through the membrane and some cannot,
scientists say the membrane is semipermeable.
Figure 4 shows a semi-permeable membrane
with water molecules moving on either side of
it. Notice that sometimes water molecules
pass through the pores in the membrane. The
purple substances represent a solute, say
starch, dissolved in a solvent of water (notice
the hydration shells that forms because water
is attracting the starch molecules). At the start
there is the same volume of soutions on each
side of the membrane.
a)
Which side (left or right) contains starch solution with the highest concentration (of starch)?
Explain your answer.
b)
Predict what will happen after while….
c)
What happens to the concentration of water particles on the right hand side?
d)
What happens to the concentration of water particles on the left hand side
e)
What happens to the concentration of the solution on the right hand side?
f)
What happens to the concentration of the solution on the left hand side?
g) Explain why the concentrations of solution on either side of the semi-permeable membrane
change?
h) Complete the sentence:
Osmosis is the passage of ___________ molecules from a ___________ concentrated solution
to a ___________ concentrated solution through a __________________________. (Here we
are referring to the concentration of solution not the concentration of water itself!)
8. The roots of a plant have root hair cells, thin plasma membrane
extensions, which allow water to be absorb from the soil into the cell. The
picture shows a root hair cell. The concentration of the solution inside the
cell is higher than the concentration of the solution in the soil.
a) Explain why water moves from the soil into the root hair cell.
b) Suggest why root hair cells are long and thin?
9.
When you take a bath the cells in the skin of your fingers are immersed in water.
a) Which is the more concentrated solution: the solution inside you skin cells or the bath water?
b)
Your skin cells have a semi-permeable membrane. Does osmosis cause water to pass from
the cells in your fingers into the bath, or from the bath into the cells in your finger?
c)
What will happen to the size of the skin cells in your fingers?
d)
Suggest why your fingers get wrinkly in the bath.
e) Would your fingers get more or less wrinkly in the sea? Explain your answer.
10. Jam is a very strong solution of sugar. By contrast the cytoplasm in a microbe is a much weaker
(less concentrated) solution.
a) The cell wall of a microbe is made up of a semi-permeable membrane. Would water flow from
the microbe to the jam, or from the jam to the microbe by osmosis?
b) Suggest a reason why microbes find it difficult to survive in jam.