Cell Membrane Transport and Root Uptake

Why photosynthetic organisms exchange substances

Plants and other photosynthetic organisms need materials from their surroundings to build new cells and carry out life processes. Carbon dioxide and water are raw materials for photosynthesis, which produces glucose using light energy. Oxygen is also produced and can leave the organism.

Plants also respire. They use oxygen in aerobic respiration to release energy from glucose, producing carbon dioxide. Therefore, oxygen is not simply an unwanted substance: whether there is a net movement of oxygen into or out of a plant depends on the balance between photosynthesis and respiration.

Mineral ions supply elements needed for growth. For example, nitrogen-containing mineral ions are used to make amino acids, which are joined together to make proteins. Plants obtain these ions dissolved in soil water.

Cell membranes regulate exchange

The cell membrane separates a cell’s contents from its surroundings. It is partially permeable: some substances can cross it, while others are restricted. This allows cells to exchange materials without their contents simply mixing with the environment.

Both plant cells and prokaryotic cells, such as bacteria, have cell membranes. A plant cell’s membrane regulates the entry of water and mineral ions and the exchange of gases. A prokaryotic cell also needs to take in materials for its reactions and remove waste products. Its membrane provides the boundary across which this exchange takes place. The small size of prokaryotic cells means substances have only short distances to travel within them.

Diffusion moves particles down a concentration gradient

Particles move randomly. When there is a difference in concentration between two regions, particles move in both directions, but more move from the higher-concentration region towards the lower-concentration region. This overall, or net, movement is diffusion.

The concentration difference is called a concentration gradient. Diffusion moves substances down this gradient and does not require the cell to supply energy from respiration.

In a photosynthesising leaf, carbon dioxide is used up in cells. This helps maintain a lower carbon dioxide concentration inside than outside, so carbon dioxide diffuses into the leaf. Oxygen produced by photosynthesis can diffuse out. Gases enter and leave leaves through pores called stomata and cross cell membranes by diffusion.

Osmosis is the movement of water across a membrane

Osmosis is the net movement of water through a partially permeable membrane from higher water concentration to lower water concentration. Like diffusion, it does not require energy from respiration.

A dilute solution has relatively more water and less dissolved solute than a concentrated solution. If the two solutions are separated by a membrane that lets water through but restricts the solute, water moves overall from the dilute solution towards the concentrated solution.

This explains why plant cells can gain or lose water. When the surrounding solution has a higher water concentration than the cell contents, water enters by osmosis. When it has a lower water concentration, water leaves. Water entering plant cells helps keep them firm, or turgid; excessive water loss reduces this firmness and can cause a plant to wilt.

Active transport allows uptake against a gradient

Diffusion cannot produce a net movement from lower to higher concentration. Cells use active transport when they need to move substances in this direction, against the concentration gradient.

Active transport requires energy from respiration. Root hair cells can use it to absorb mineral ions even when the ions are less concentrated in the soil water than inside the cells. This allows plants to obtain essential minerals from dilute soil solutions.

Root hair cells are adapted for absorption

Root hair cells have long, narrow extensions that provide a large surface area in contact with soil water. This gives more surface across which water and mineral ions can enter.

Water enters root hair cells by osmosis when the soil water has a higher water concentration than the cell contents. Mineral ions can enter by active transport against their concentration gradient. These are different processes, even though both happen at the same root surface.

Root hair cells have many mitochondria, where aerobic respiration releases energy needed for active transport. Their thin boundary also provides a short distance for substances to cross.

Investigating diffusion using agar cubes

Diffusion can be made visible using agar containing an alkali and phenolphthalein indicator. The agar starts pink. When placed in dilute hydrochloric acid, acid diffuses into it and neutralises the alkali, making the indicator colourless. A colourless layer develops at the outside and extends towards the centre.

To investigate how cube size affects the time needed for acid to reach the whole cube, use prepared pink agar, a ruler, a cutting tile and blade, beakers, a measuring cylinder, dilute hydrochloric acid, a timer and a plastic spoon.

1. Cut and measure cubes of different sizes, with several cubes of each size for repeats. Cubes should have flat faces and equal edge lengths.
2. Put equal volumes of acid of the same concentration into labelled beakers. Fully submerge the cubes, keeping them separate, and start timing as they enter the acid.
3. Watch each cube against a white background. Record the time when the last trace of pink disappears from its centre.
4. Calculate a mean time for each cube size and compare the results.

Change the cube size, but keep the agar composition, acid concentration and volume, temperature and any gentle swirling the same. Use the same observer and background to judge the endpoint consistently. Wear eye protection, rinse chemical splashes promptly and cut on a tile with fingers behind the blade under teacher supervision. Do not eat the agar.

Smaller cubes should become colourless sooner because acid has a shorter distance to travel to their centres. They also have more surface area relative to their volume. A larger cube cut in half during the investigation can reveal a colourless outer layer surrounding a pink core: acid has reached the outside but not yet the centre.

This is a model of diffusion into a material, not a living cell. Agar has no cell membrane, so this investigation does not demonstrate selective membrane permeability or active transport.

Investigating osmosis using potato tissue

Potato cells have partially permeable membranes. Placing potato samples in solutions of different sugar concentrations lets us investigate how osmosis changes the mass of plant tissue.

Use fresh potato, a cork borer, a knife and cutting tile, a ruler, a balance, labelled test tubes and rack, a measuring cylinder, distilled water, a range of sucrose solutions, filter paper and a timer.

1. Use the cork borer to make cylinders of the same diameter and cut them to the same length. Use tissue from the same potato to reduce differences between samples.
2. Measure and record each cylinder’s initial mass, making clear which solution it will enter.
3. Add equal volumes of the different solutions to labelled tubes, including distilled water for comparison. Fully immerse a cylinder in each solution.
4. Leave all cylinders for the same set time, for example 15 minutes, at the same temperature. A water bath can help keep temperature constant.
5. Remove the cylinders, blot their surfaces gently and consistently with filter paper, then measure and record their final masses.
6. Repeat at each concentration and calculate the mean percentage mass change.

The independent variable is sucrose concentration; the dependent variable is the percentage change in tissue mass. Keep sample dimensions, tissue source, solution volume, temperature, immersion time and blotting method the same. Use sharp tools on a cutting tile under supervision, keeping fingers away from the blade, and do not eat the laboratory samples.

The measurement period ends after the chosen immersion time; there is no colour-change endpoint. A mass gain indicates that water has entered the cells overall. A mass loss indicates that water has left overall. More concentrated external solutions generally produce less mass gain or greater mass loss because their water concentration is lower.

Using mass changes to assess water uptake

A biological sample that gains water can increase in mass; one that loses water can decrease in mass. Percentage change lets you compare samples with different starting masses fairly.

$$\text{Percentage mass change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$$

Suppose a plant tissue sample initially has a mass of 5.0 g and later has a mass of 5.5 g. Its mass gain is 0.5 g, so its percentage mass change is $(0.5 / 5.0) \times 100 = 10\%$. A second sample starting at 5.0 g and ending at 4.5 g has a percentage mass change of $-10\%$: it has lost 10% of its initial mass.

A positive result indicates net water uptake; a negative result indicates net water loss. No overall mass change is consistent with no net movement of water, rather than water molecules stopping moving.

For the potato investigation, plot solution concentration on the horizontal axis and mean percentage mass change on the vertical axis. Results above zero show net water uptake; those below zero show net water loss. Where a suitable line of best fit crosses zero, the surrounding solution causes no overall gain or loss of water from the tissue.

Materials plants exchange

• Carbon dioxide and water: needed for photosynthesis.
• Oxygen: produced in photosynthesis and used in aerobic respiration.
• Mineral ions: supply elements for growth, including nitrogen for making proteins.

Three transport processes