Transport across the cell membrane II

Left Damion Douglas a student of Dint Hill High School holds a Sextant used by Sailors to find their position if lost at Sea while Jason Massiah Antigua Deck Student Class Three at the Caribbean Maritime Institute explains the correct positioning of the instrument at the World Maritime Week expo at the Caribbean Maritime Institute on September 29. - Ricardo Makyn/Staff Photographer

Last week, we took a look at the mechanisms of transport across the cell membrane. As young biologists, you will realise that many biological phenomena can be explained using these principles. Today, we will work some questions based on transport across the cell membrane.

Question 1

Plants absorb water in their root hairs by pumping in mineral ions against their concentration gradient from the soil into the root hair cells.

a) Define osmosis. [2 marks]

b) Explain why the entry of ions into the root hair cell causes water molecules to move from the soil into the root. [4 marks]

c) What is the mechanism by which the ions are transported into the root hair cell?
[1 mark]

d) Cyanide is a poison which inhibits cell respiration. Suggest why the absorption of water by root cells decreases when the cyanide ion is present. [3 marks]

Answers

a) Osmosis is the movement of water molecules from an area where they are in high concentration to one where they are in low concentration across a selectively permeable membrane. Full marks are awarded for mentioning all elements of the process, none being given if any part is missing. Examiners prefer the term selectively permeable, as it is more scientifically correct than semi-permeable.

b) Entry of the ions causes an increase in the concentration of the solution inside the root hair cells. The concentration of water inside the cells is therefore much lower than that in the soil, thus the concentration gradient between the two is steeper. Water then moves by osmosis from the soil where it is in high concentration to inside the root hairs where it is in lower concentration. Again, full marks are awarded for mentioning all key steps in the process. In questions such as these, clarity of expression becomes very important. Also, always remember to use your technical terms.

c) Active Transport. A very straightforward response is required. Do not waste time by writing more words than necessary. For example, do not answer this question by writing, "The process by which these ... is active transport."

d) Active transport requires ATP produced by respiration. Cyanide inhibits respiration, thus no ATP is available, and active transport is not possible. As a result, less water is absorbed by the roots. This question tests your analysis and application skills. Again, make sure that your language is clear and precise.

Let's look at another question.

Question 2

Give two similarities and two differences between the processes of osmosis and diffusion. [4 marks]

Answer

Both processes are passive, thus do not require energy.

In both cases, materials are moving from an area of high concentration to one of low concentration.

Osmosis is the movement of water, while diffusion is the movement of solute particles.

Osmosis requires the presence of a membrane, while diffusion does not necessarily take place across a membrane.

It is often easier to place these comparison questions in point form, as it will help both you and the examiner to keep track of your points. When showing a difference, ensure that the properties being contrasted are similar. Confused? For example, it would be incorrect to say that object A is heavy while object B is green. Your differences must be clearly stated. Students sometimes describe one concept and neglect to show how it differs from the other. NO MARKS are given for incomplete comparisons.

Let's look at one final question:

Question 3

Three cylinders of identical length and diameter were cut from a potato. The cylinders were placed into salt-water solutions of known concentration for 30 minutes. They were then removed from the solutions, patted with a paper towel to remove excess water and their lengths measured. The following results were obtained:

Solution Concentration/mol dm-30.251.001.65 Length Before Treatment/mm40.040.040.0 Length After Treatment/mm43.040.037.5

a) Using the change in length of each cylinder, give a full explanation of the results obtained. You do not need to show your calculations. [7 marks]

Answers

a) The potato cylinder placed in the 0.25 mol dm-3 solution increased in length by 3 mm. This suggests that the solution inside the potato cells is more concentrated than the salt solution. Water enters the cells by osmosis, moving down its osmotic gradient.

The cylinders in the 1.00 mol dm-3 solution showed no change in length. This is because the solution inside the cells is isotonic to the salt solution, thus there is no net movement of water across the cell membrane

The cylinder placed in the 1.65 mol dm-3 solution decreased in length by 2.50mm, suggesting that the solution inside the cell is more dilute than the salt-water solution. As a result, water moves by osmosis out of the cell down its concentration gradient.

Only one mark was awarded for correctly calculating the change in length, so it was not necessary to show this calculation. However, in general it is necessary to show all working. Also, remember your units. Often students can lose marks by forgetting to place the proper unit with their numerical answers. Omitting any value for the change in length would mean that you lose this one mark. Two marks were awarded for each explanation, one for stating the difference in concentration between the intracellular and extracellular environments, the other for stating the direction of movement of the water. It is always necessary to mention your processes, so the word osmosis must be somewhere in your answer.