Explain why sodium ions cannot cross phospholipid bilayers by simple diffusion.
Ions and some molecules move across cell surface membranes by facilitated diffusion and active transport.
Compare facilitated diffusion and active transport by stating one way in which they are similar and two ways in which facilitated diffusion is different from active transport.
similarity
difference 1
difference 2
A person who is exposed to tobacco smoke is at greater risk of lung cancer and chronic obstructive pulmonary disease (COPD).
Many people with COPD have both chronic bronchitis and emphysema. These diseases cause changes in the gas exchange system. For example, changes occur in the total lung surface area to volume ratio (SA:V).
A student investigated the effect of SA: V on diffusion.
Agar was prepared with Universal Indicator solution and sodium hydroxide solution. The agar was coloured blue.
Three cubes, A, B, and C, were cut from a solid block of blue agar. Each cube was a different size.
Universal Indicator solution changes to a red colour in the presence of acid.
The student prepared Table 4.1 to show the sizes and SA:V of each cube.
Table 4.1
Complete Table 4.1 by:
- writing the correct units for volume
- calculating the total surface area, and volume, of cube C.
Cubes A, B and C were placed in a small beaker. At time 0 seconds, dilute hydrochloric acid was added to the beaker to cover the cubes.
The student timed how long it took for each cube to change colour completely.
Complete Fig. 4.1 to show the results that were obtained.
Fig. 4.1
A student investigated the effect of changing the surface area to volume ratio on diffusion.
- Two different-sized blocks of agar, X and Y, were made.
- The agar contained Universal Indicator solution.
- Universal Indicator solution changes colour when acid is added.
- The blocks were placed in dilute hydrochloric acid at the same temperature.
- The student timed how long it took for each block to change colour completely.
Blocks X and Y are shown in Fig. 6.1. All dimensions are in cm .
Fig. 6.1
The surface area to volume ratio of block X is 5: 1.
Calculate the surface area to volume ratio of block Y.
Show your working.
The student observed that block X changed colour completely in a much shorter time than block Y.
Explain why.
Suggest how the results of this investigation help to explain why plants need a transport system.