Check your understanding 3
1
What is the difference between passive transport and active transport?
Passive transport is the movement of particles not requiring energy. Movement of particles in
active transport uses energy.
2
A gas tap in the science laboratory is accidentally opened. Explain why students closest to the
tap are the first to notice the smell.
Particles of gas move from where they are in higher concentration, close to the gas tap to
where they are in lower concentration, in the room. It takes time to diffuse out so students
further away from the tap will smell the gas after the students closer to the gas tap.
3
When coloured dye is added to water, diffusion causes the dye to spread throughout the water
until there is an even colour. This is an example of a liquid diffusing in a liquid. Give an everyday
example of a solid diffusing in a liquid. Can you think of any other everyday examples of
diffusion?
Your answer may vary to this but some examples are given here. Adding sugar to a cup of tea,
adding washing powder to a washing machine full of water, adding ground pepper/salt to a
soup.
Many more examples can be given.
4
What gases important for life diffuse easily through the plasma membrane of a cell?
Oxygen and carbon dioxide.
5
Explain what is meant by the concentration gradient.
If the concentration of a substance in one area is different to its concentration in another area,
the difference in concentration is referred to as the concentration gradient.
6
a
Fill in the diagrams in Figure B.3 to show diffusion of particles in this closed system.
Beginning
Midway
End
Figure B.3 Diffusion
b
At which time is equilibrium reached?
Equilibrium is reached at the end.
c
Do particles cease to move at equilibrium? Explain your answer.
Particles will continue to move at equilibrium in a random fashion. They move at equal rates in all directions.
Section B: Movement of substances 29
Check your understanding 3
1
A sugar solution is a mixture of sugar and water.
a
Which one is the solvent and which is the solute?
The solvent is water and the solute is sugar.
b
What would you add to this solution to make it more dilute?
You would add water to make the solution more dilute.
c
How would you make the solution more concentrated?
You would add sugar to make the solution more concentrated.
2
Osmosis is a form of diffusion. In the Venn diagram in Figure B.6, provide details of special features of osmosis and common features of both processes.
Diffusion
Movement of particles from a region
of high concentration to a region of low
concentration. No input of energy
Osmosis
Movement of water through
a semi-permeable membrane
Figure B.6
3
Using red dots for a solute and blue dots for water (solvent), fill in the boxes in Figure B.6 to
illustrate the situations required.
cell
isotonic
solution
cell
hypertonic
solution
cell
hypotonic
solution
Figure B.7
4
Cells are placed in three different solutions. After a period of time, cells in solution X filled with
water. Cells in solution Y remained the same as they were to start with, and cells in solution Z
shrank and lost water.
a
Which solution was hypertonic compared with the cells?
Z
b
Which cells were isotonic compared with the surroundings?
Y
c
Which cells were hypotonic compared with the surroundings?
X.
Section B: Movement of substances 31
Check your understanding 3
1
A red pigment, haemoglobin, is found in the cytoplasm of human red blood cells. Explain why
soaking blood soiled clothes in cold water removes the red colour.
Due to osmosis, when red blood cells are placed in fresh water there is net movement of water
into the cells. This causes them to fill with water and burst. When cells burst, the red pigment
haemoglobin is released into the surroundings, hence removing the colour from the clothes
2
Soaking grass-stained clothes in cold water, however, is not enough to remove the green colour.
Explain.
The same conditions apply in this situation. There is net movement of water into plant cells due
to osmosis. But in this case, even though the cells will fill with water they will not burst due
to the tough cell wall. Chlorophyll, the green pigment, remains trapped within the plant cells
resulting in the green stain remaining.
3
Explain why plants wilt when they lack water.
When water loss is greater than water gain, plant cells will lose volume. As the cytoplasm
shrinks, the plasma membrane is pulled inwards away from the cell wall. This reduces the turgor
of the plants, causing them to wilt.
4
A patient in hospital sometimes needs to have his/her body fluids increased. A saline (salt
solution) drip may be set up. The saline concentration is similar to the patient’s blood plasma
(the liquid part of the blood). Explain why it is important that the saline is neither more nor less
concentrated than blood plasma.
If the saline drip is the same concentration as the normal blood plasma it is likely to be isotonic
in relation to the blood cells. In this case, there will be no net movement of water. If the drip
was more concentrated than blood plasma, water would leave the cells causing them to shrink
and if the drip was more dilute, water would enter the blood cells causing them to burst. In
either of these cases, the patient’s health would be compromised.
5
A cell was placed in three different solutions. Some time later the following observations were
made:
Solution A – cell shrinks Solution B – cell bursts Solution C – no change to cell.
Was the cell from a plant or animal? Explain your answer.
This was an animal cell because even though plant cells will lose water, the cell wall keeps them
much the same size and shape. This cell shrank in solution A. Plant cells become turgid when
filled with water, whereas animal cells burst (solution B).
Section B: Movement of substances 33
Check your understanding 3
1
Explain why there is a limit to the size cells can grow.
As cells get larger, their volume increases proportionally more than their surface area. Because of this,
the amount of diffusion and hence removal of wastes and supply of nutrients decreases. When a cell gets
bigger it reaches a size where the amount of diffusion possible is no longer enough to sustain the cell.
2
a
round cell with a diameter of 2 cm has a greater chance of survival than a round cell
A
with a 5 cm diameter. Explain why this is so.
A 2cm round cell has a greater surface area to volume ratio compared to a 5cm round cell,
therefore diffusion through its plasma membrane is more efficient. This gives it a greater chance
of survival, as requirements are obtained and wastes are removed more quickly.
b
How could the 5 cm cell increase its chances of survival by changing its shape? Explain the
significance of this.
By altering the shape of the 5cm cell, efficiency of movement of substances across the cell
surface can be altered. For instance, if a cell is long and thin or has increased folding its surface
3
area is greatly increased allowing for more efficient diffusion.
A plant cell has a roughly cubic shape.
See Figure B.9.
a
Complete the following table.
height (h )
width (w )
Figure B.10
length ( l )
Cube size
(length)
Surface area (SA) mm2
l × h × no. of faces
Volume (V ) mm3
l ×w×h
Surface area to
Volume ratio (SA : V )
10 mm
600
1000
0.6 : 1
20 mm
2400
8000
0.3 : 1
30 mm
5400
27000
0.2 : 1
40 mm
9600
64000
0.15 : 1
4
b
On graph paper, draw a graph based on data from your table, showing the relationship
between the SA : V ratio and the increase in size of the cubes. Put the cube size on the
horizontal axis and the SA : V ratio on the vertical axis.
c
What happens to the SA : V ratio as the cubes get bigger?
The graph will show that as the size of the cube increases, the SA:V decreases.
How does flattening a cell improve the efficiency of moving substances across its surface? To help answer this, calculate the SA : V ratio of three shapes of equal volume in Figure B.10.
Cell A
h = 4 mm
Cell B
w = 4 mm
I = 4 mm
Cell C
h = 2 mm
h = 1 mm
w = 4 mm
w = 4 mm
I = 8 mm
I = 16 mm
Figure B.11
Cell
Surface area
Volume
A
l × w × no. of sides
l ×w×h
4x4x4=
64mm3
l ×w×h
16 x 4 x 1 =
64mm3
4 x 4 x 6 = 96mm2
C
2(l × w + l × h + w × h)
2(16 x 4 + 16 x 1 + 4 x 1) =
168mm2
SA : V
96 ÷ 64 =
1.50 : 1
Cell
Surface area
Volume
SA : V
B
2(l × w + l × h + w × h)
2(8 x 4 + 8 x 2 + 4 x 2) =
112mm2
l ×w×h
8x4x2=
64mm3
112 ÷ 64 =
1.75 : 1
168 ÷ 64
= 2.63 : 1
Flattening the cell increases its SA:V
Section B: Movement of substances 35
Check your understanding 3
1
List the four membrane proteins and briefly describe their functions.
Membrane protein
Function
Transport proteins Span the plasma membrane from one side to the other and thus allow certain
substances but not others to pass through
Receptor proteins Bind hormones and other substances to the plasma membrane, and therefore
cause changes to the cell’s activities
Recognition
proteins
Attach to carbohydrate molecules on the surface of the plasma membrane and
act as markers, called antigens which allow the immune system to distinguish
between the body’s self cells and non-self cells
Adhesion proteins Link cells together in multicellular organisms
2
a
List examples of substances that pass through the plasma membrane easily.
Water, oxygen, carbon dioxide and other small molecules as well as lipid-soluble
molecules pass through the plasma membrane easily.
b
List examples of substances that do not pass through the plasma membrane easily.
Water-soluble molecules, charged molecules and atoms and many large molecules do not
pass easily through the plasma membrane.
3
Explain why membranes are permeable to lipid-soluble molecules, but impermeable to watersoluble molecules.
Lipid-soluble molecules pass easily through the plasma membrane because it is composed of a
phospholipid bilayer. The lipids in the membrane form a barrier to water-soluble molecules.
Section B: Movement of substances 37
Check your understanding 3
1
Compare and contrast channel proteins and carrier proteins.
Both form passageways for movement of substances across plasma membranes. Both types
of proteins are selective in the particles they allow through. Carrier proteins bind to specific
molecules or ions on one side of the membrane, change shape and release them on the other side
whereas channel proteins form narrow passageways for small ions to diffuse rapidly through. Some
carrier proteins move particles against a concentration gradient, channel proteins do not.
2
List four features of facilitated diffusion.
No energy is required.
Particles move from high concentration to low concentration.
Proteins spanning the plasma membrane are involved.
Transport is selective.
3
What differences are there between diffusion and facilitated diffusion?
Particles move through the phospholipid bilayer by diffusion whereas facilitated diffusion relies on
carrier and channel proteins embedded throughout the membrane.
4
Give two reasons why simple diffusion and osmosis are different from the process of active
transport.
No energy is required for simple diffusion and osmosis and particles move from a region of high
concentration to a region of lower concentration. Active transport needs an input of energy and
particles move from a region of low concentration to a region of higher concentration.
5
Show the similarities and differences between facilitated diffusion and active transport by
completing the Venn diagram in Figure B.16.
Facilitated diffusion
no energy
required
movement from
high to low
concentration
Active transport
transport
proteins
across
membrane
energy
required
movement from
low to high
concentration
Figure B.16
Section B: Movement of substances 39
Check your understanding 3
1
Name a type of cell where:
a
exocytosis takes place
Secretory cells such as those lining the stomach, and nerve cells.
b
phagocytosis takes place
White blood cells called macrophages.
c
pinocytosis takes place.
Cells lining the small intestine.
2
Explain how the plasma membrane is involved in the processes of endocytosis and exocytosis.
In exocytosis, for instance in a cell that secretes enzymes, membrane-bound vesicles form around
the enzymes and move through the cytoplasm to the plasma membrane, fusing with it and releasing
the enzymes to the exterior of the cell. In endocytosis, the plasma membrane extends around a
substance, engulfing it and forming an endocytic vesicle. This is then drawn into the cytoplasm.
3
Pinocytosis is sometimes called ‘cell drinking’ and phagocytosis can be referred to as ‘cell
eating’. Considering these processes, explain why these are appropriate descriptions.
Pinocytosis involves intake of fluid, similar to drinking. When food is eaten solids are taken in, just
like phagocytosis, which is when a cell takes in solids.
4
Fill in the appropriate links in the concept map in Figure B.19 illustrating movement of
substances in and out of cells. Use the letters from the labels in the following list.
a
Large molecules enter the cell
b
Movement of water molecules across a semi-permeable membrane
c
Net movement of particles from a region of high concentration to an area of low
concentration
d
Energy required
e
Transport of fluid into the cell
f
Large molecules within the cell are transported out
g
Proteins in cell membranes help substances move through
h
No energy required
i
Transport of large, solid materials into the cell
movement
in and out
of cells
h
d
active
transport
passive
transport
f
c
a
exocytosis
endocytosis
diffusion
i
b
osmosis
Figure B.19
g
phagocytosis
e
pinocytosis
facilitated
diffusion
Section B: Movement of substances 41