IGCSE Biology Lesson: Transport in Plants


Transport is the means by which we organisms transport the things we need to the parts of us that need it – energy for instance, or water. Our leg muscles need energy to move and our whole bodies need to remain hydrated, yet they are not near our stomachs. Hence we have a complex circulatory system, gas exchange and everything else. Plants have slightly different systems to get sugars formed in the leaves around the plant or water from the roots. 

Flowering plants 

In simple terms Xylem and Phloem are something like our veins and arteries. Xylem are the larger, more sturdy transport system for water and Phloem the system for nutrients. 

Xylem 

Xylem are the vessels that deliver the majority of water and minerals to the leaf. They form a continuous transport system much like blood vessels etc. in animals. Their diameter can be up to 0.7mm and their walls are thick, strengthened with cellulose and usually lignin. Water initially enters the plant through the roots, travels around this circulatory system and enters the leaf where it is used by the two layers of mesophyll cells. Xylem also contains tracheids, long, overlapping tapered vessels with perforations meaning water can flow between them.

Phloem (Paper 2) 

The phloem transport the products of photosynthesis and distribute them to the rest of the plant as required. This is another circulatory system and, like the xylem, is critical to the survival of the plant. In this way all parts of the plant are supplied with the products of photosynthesis. The veins in a leaf contain both phloem and xylem and they circulate around the entire plant branching off as they go to reach all parts of the plant. 

Transpiration

Transpiration is the loss of water from plant leaves via diffusion. In hot, dry and windy conditions the rate of transpiration will increase. This will create a shortage of water in the leaf. During a growing season a leaf will transpire far more water than its own weight. An acre of corn can transpire 11000 – 15000 litres of water each day a large tree can transpire 150000 litres in a year.

To combat this, more water will be drawn up from the roots through the plant’s xylem tube (within plant structure the xylem tube transports water from the roots to the top of the plant, the phloem transports food from the sites of photosynthesis to the roots). This constant stream of water from the ground to the leaves, keeps the plant cool. The water also aids with the turgor of the plant’s cells (the firmness). This constant movement of water also aids in the uptake of minerals from the soil.

Transpiration rate is affected by four main elements. These are: 

  • Light intensity. The more intense the light, the greater the rate of transpiration. This is because during hours of darkness, the stomata are closed. This is due to the fact that photosynthesis cannot happen in the dark. The additional photosynthesis caused by greater light intensity causes stomata to open which also increases loss of water by transpiration. 
  • Temperature. Transpiration increases with temperature. This is due to the fact that the higher the temperature, the more energy the water molecules have, making it more likely that they will evaporate. Also higher temperatures cause the stomata to open. 
  • Air movement. In high wind, transpiration occurs faster. This is due to the fact that if air is moving, the air above the leaf will not become saturated with water molecules. Instead these water molecules will blow away. 
  • Relative humidity. The more humid the atmosphere, the slower transpiration will take place because the concentration of water molecules in the air is very high in humid conditions. This, in turn means, that the concentration gradient between the inside of the leaf and the atmosphere is low, thus retarding diffusion 

Some adaptations plants have adopted to reduce water loss are: 

  • A waxy, waterproof cuticle covering the upper surface of the leaves. This waxy covering reduces water loss 
  • The stomata of the leaf are found on a lower surface where it is cooler and darker. This helps to reduce the diffusion of water out of the leaf 
  • Plants in hot, dry countries have a lot less stomata than those in cooler climates

Watch This Video:

Root hairs

Plants obtain nutrients and water from the soil that they are planted in. The root system of a plant is designed to have the maximum surface area possible. Roots branch and then branch again and each root is covered in fine projections known as root hairs. These root hairs are fine, single cells, with a thin cell membrane easing the passage of substances in and out of the plant. Water moves into the roots by osmosis and mineral ions move into the root by diffusion. At times of higher concentration of mineral ions within the plant, mineral ions will be moved by active transport. See the diagrams below: 

Experiment

You can measure how certain factors affect the rate of transpiration experimentally quite easily. You will need the following apparatus: 

  • A plant cutting
  • A potometer
  • Calibrated pipette
  • A stand and clamp 

Set up the apparatus as indicated in the diagram below.

  1. Take care to ensure that there is an ait tight seal between the plant cutting and the water filled tube. If this is not the case then some water will evaporate and the experiments results will be flawed.
  2. Fill the photometer with water so that it touches the cut off end of the plant cutting. The water should also extend up into the pipette. Measure the starting level of the water in the pipette. 
  3. In the first experiment simply leave the potometer in a light room at room temperature for a given period of time. Measure the level of water in the pipette after this time. The difference should be recorded; this is how much water was lost due to transpiration through the plant cutting. 
  4. Refill the photometer with water, record the level and restart the experiment using different parameters. For example, conduct the experiment in a darkened room, or a warmer room and record the water level after the same period of time. This will illustrate how changing environmental conditions will impact on the rate of transpiration. 

Activity

List and describe the factors that affect the rate of transpiration in plants.

Last modified: Wednesday, 6 September 2017, 11:59 PM