Edexcel Biology Gcse - Transport In Cells Part 3

Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist then root hair cells will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.

Plant cells have a strong cellulose cell wall outside the cell membrane. The cell wall is fully permeable to all molecules. It supports the cell and stops it bursting when it gains water by osmosis.

Under normal circumstances, the cells in all organisms live in a safe equilibrium. They have enough water to live but not too much. They also have the correct amount of salts and sugars. If individual cells are placed in different solutions, we can see how important it is that equilibrium is maintained in living organisms.

In pure water, the cell contents push against the cell wall and the cell becomes turgid. Fully turgid cells support the stems of non-woody plants.

In a more concentrated solution, the cell contents lose water by osmosis and the cells shrink. The cell membrance starts to pull away from the cell wall. The cell becomes flaccid.

In a very concentrated solution, the cell membrane pulls away from the cell wall completely; this is plasmolysis.

Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.

In animals, the concentration of body fluids, blood plasma and tissue fluid must be kept within strict limits. If cells lose or gain too much water by osmosis, they do not function efficiently.

eg. Red blood cells lose water and shrink in a concentrated solution. They swell and burst in a solution that is too dilute.

Method:

A 1.0 mol dm^−3 solution of a substance contains one mole of the substance per dm^3.

1) Prepare a range of sucrose solutions eg 0%, 20%, 40% and 100%.

2) Set up a series of boiling tubes with each of these solutions. The 0% sucrose solution will act as the control in the experiment. Make sure each tube is labelled with the concentration.

3) Prepare a blank results table before you begin. Make sure when weighing the potato cylinders, that their masses are not mixed up when recording them. Each cylinder will have a different mass before and after the investigation.

4) Dry a potato strip using a paper towel. Measure the mass of the potato cylinder.

5) Place the potato strip into the 0% solution for 20 minutes.

6) Remove the potato strip, dry it carefully using paper towel. Measure and record the mass of the potato strip.

7) Repeat for each sucrose concentration.

8) For each sucrose concentration, repeat the investigation for several potato cylinders. This allows you to check the precision of your results (results that are close together for the same concentration suggest that the results are precise). The potato cylinders might not all behave in the same way. Making a series of repeat experiments means that any anomalous results can be identified and ignored when a mean is calculated.

Risks:

Make sure that the potato is placed on a ceramic tile when using the cork borer – do not cut the potato cylinders.

Care must be taken when using the scalpel.

Wear eye protection when using chemical solutions.

water uptake in 1 hour = change in mass x ((60 minutes) ÷ (length of experiment in minutes))

Water uptake in 1 hour = 0.30 x (60 ÷ 20) = 0.90 grams

The rate of water uptake is therefore 0.90 g h^−1

change in mass = ((mass at end - mass at start) ÷ (mass at start)) x 100

change in mass = ((2.54 - 2.42) ÷ 2.42) x 100 = 5%

Scientists use percentiles to divide a set of data into 100, and look to see where the data lie within these divisions.

The median is the point in a set of data where 50 per cent of the data fall above this value, and 50 per cent below it. This is the 50th percentile.

The 75th percentile is where 75 percent of the data fall below this value.

There are several methods of finding a percentile. The simplest is the nearest rank method. As with finding the median of a set of data, begin by putting the data into order.

ordered rank = (percentile ÷ 100) x number of entries in in data set

This method will only give percentiles as numbers that exist in the data set.

In other methods, percentiles can be interpolated for values that don't exist in the data set

Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration against a concentration gradient.

Active transport is a process that is required to move molecules against a concentration gradient. The process requires energy.

For plants to take up mineral ions, ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport then occurs to allow the plant to take the nutrients it needs for the soil around it.

In animals, glucose molecules have to be moved across the gut wall into the blood. The glucose molecules in the intestine might be in a higher concentration than in the intestinal cells and blood – for instance, after a sugary meal. At this point it will diffuse from high concentration in the intestine to a lower concentration in the blood. This doesn’t require energy.

However, there will be times when glucose concentration in the intestine might be lower. When this is the case, movement of glucose involves active transport. The process requires energy produced by respiration

diffusion, osmosis and active transport.

Carbon dioxide, oxygen, water, food substances, wastes, eg urea

Water

Mineral ions into plant roots. Glucose from the gut into intestinal cells, from where it moves into the blood