6a. Plant Transport Systems
What you need to know...
Water is required for transporting materials and for photosynthesis.
Structures and processes involved in water movement to include root hairs, guard cells, stomata, epidermis, mesophyll cells and transpiration.
Water and minerals are transported up through the stem in xylem.
Xylem cells are lignified.
Sugar is transported up and down the plant in living phloem cells.
The theme for this unit is all about how multicellular organisms develop and the issues multicellular organisms need to overcome to survive. One of the biggest potential problems for a multicellular organism is the need for transport. A unicellular organism has no need for a transport system. The resources it needs to survive can just be transported through the cell membrane, and the waste materials likewise. Think of a cell deep inside one of your tissues though. Without some sort of transport system, how would it get the oxygen, glucose, amino acids and everything it needs to survive? How would it get rid of the waste materials produced from its chemical reactions without killing the cells around it and itself? It is for this reason that as organisms became multicellular they had to evolve ever more complex transport mechanisms to connect all of the cells with each other and the external environment. For your course, you need to know about a number of such transport systems. We start in this topic with the transport systems of plants, and in the next we'll learn about the transport systems of animals, with a particular focus on mammals (i.e. you).
Before we go on to learn about the transport systems in plants, we need to first remind ourselves of the tissues of the leaf. These were mentioned in the first topic of the unit, but let's quickly go over them again.
The leaf consists of a number of tissues which include...
Epidermis: This is a layer of cells at the top and bottom of the leaf which protects the leaf.
Palisade Mesophyll: These cells carry out the majority of photosynthesis in the leaf. They have a number of features to maximise the absorption of light such as their location at the top of the leaf, their tall and thin shape and the fact that they are tightly packed together. They also contain a large number of chloroplasts.
Spongy Mesophyll: These mesophyll cells also photosynthesise, but not to the same extent as the palisade cells. They are spaced out to allow a large surface area for the absorption of carbon dioxide and the evaporation of water.
Guard Cells: These cells surround the stomatal pores on the underside of the leaf. These create holes in the leaf to allow for the exchange of gases and evaporation of water. The guard cells close the stomatal pores at night as photosynthesis is not possible at night so less gas exchange is required and closing the pores prevents unnecessary water loss.
Much of the transport systems of plants can be viewed from the perspective of photosynthesis. I'm sure you'll remember what photosynthesis and where it takes place, if not look back at the last unit now! Photosynthesis requires a supply of carbon dioxide and water. The carbon dioxide simply diffuses through holes in the leaf called stomata and into the photosynthesising leaf cells. Water on the other hand is more complex. For many plants (especially ones which live on land) their leaves are as far from the soil as possible in order to maximise light absorption, but their water supply is from the soil. These types of plants therefore have a transport system which brings water up from the roots to the leaves. This water also carries other materials to the plant cells from the soil, such as minerals.
So, how does this happen? Water enters the plant in the root via the root hairs. These increase the surface area of the roots to maximise the absorption of water and minerals. Once in the plant's root, the water and dissolved minerals travel up a network of tubes called xylem. Xylem tissue grows as all tissues, in the form of cells, but once the xylem has grown their cell walls become lignified. This involves the addition of the chemical lignin to the walls. This has the dual effect of making the cells waterproof, and killing the xylem cells. So, the xylem tissue consists of long, dead lignified tubes which the water and minerals can be transported up through the plant.
Detailed Xylem diagram from WikiMedia Commons
But, you might already be thinking...how on earth is the water pumped up the plant in the opposite direction to gravity? Especially if you think of a large tree. It's all to do with transpiration. Although we've started our water transport story in the roots, in actual fact we should probably start at the leaves, as it's evaporation from the leaves that provides much of the force to move water up through the plant. Water is continuously evaporating from the leaf, particularly in daylight. This is because the small holes in the leaf, primarily on the underside, are surrounded by guard cells. These open the stomatal pore during the day to allow the carbon dioxide to diffuse in for photosynthesis and the waste oxygen to diffuse out. These stomatal pores also all the evaporation of water from the leaf. Water has a unique property for a liquid, it holds together remarkably well. If you pull a column of water, the water molecules bond with each other and pull each other along behind. Therefore the water evaporating from the leaf pulls more water out of the mesophyll cells to replace it. The water moving out of the mesophyll results in water being pulled out of the xylem in the leaf into the mesophyll cells to replace it. The water leaving the xylem in the leaf causes the water behind it in the xylem vessels to be pulled up towards the leaf to replace it. The water at the bottom of the xylem in the roots being pulled up the column causes more water to be pulled into the xylem from the root hairs to replace it and so on. This process is called transpiration and allows water to be transported up through the plant to the cells which need it with remarkable efficiency.
Includes an image adapted from WikiMedia Commons
Plant leaves need less water at night as they aren't photosynthesising and so they try to reduce water loss at this time. The guard cells close the stomatal pores and the plant has specialised epidermis cells at the top and bottom of the leaf to minimise water loss. Many plants also have a waxy cuticle on their leaves to reduce water loss even further.
Stomata by Yersinia
The image above shows the underside of a leaf under a microscope. You'll notice that in amongst the cells there are a few stomata. These are the pores which allow the exchange of oxygen and carbon dioxide and the evaporation of water. Each stoma is surrounded by two guard cells. These cells bend and straighten to open and close the stomata pores. The stomata of most plants are open in daylight and close at night.
As well as transporting water and minerals, plants need to transport something else - sugar. The sugar produced from photosynthesis in the leaves is needed by all the cells in the plant for energy, growth and storage. As this is largely moving in the opposite direction to the water and minerals plants have a separate transport system for the movement of sugar: phloem. The biggest difference between the xylem and the phloem is that whilst xylem is dead, phloem tissue is alive. Phloem cells have reduced cytoplasms to allow the movement sugar through, but they have companion cells which carry out many of the functions required to keep the phloem cells alive.