Xylem transport – Grade 9 Understanding for IGCSE Biology 2.54, 2.55B, 2.56B
The topic of plant transport can appear quite complicated but you will see from your past paper booklets that the questions examiners tend to set on it are much more straightforward.
The key piece of understanding is to realise that there are two transport systems in plants, learn their names and what they transport.
- Xylem vessels move water and mineral ions from the roots to the leaves.
- Phloem sieve tubes move sugars, notably sucrose, and amino acids around the plant. Both of these molecules are made in photosynthesis in the leaves and so can be transported from the leaves to the areas in the plant where they are needed.
Water is needed for photosynthesis of course in the leaves (remember that rain water cannot enter leaves directly because of the waxy cuticle on the surface of the leaf). All the water that is used in photosynthesis is absorbed in the roots from the soil and moved up the plant in the xylem vessels. Minerals such as nitrate, phosphate and magnesium ions are also required in the leaves for making amino acids, DNA and chlorophyll respectively. These minerals are moved up the plant along with the water in the xylem.
How does water enter the roots from the soil?
Water molecules can only enter root hair cells (and indeed can only cross any cell membrane) by one mechanism and that is OSMOSIS. If you understand the mechanism of osmosis that is great but don’t worry too much about it at this stage. You need to know that osmosis is a net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane.
How do mineral ions enter the roots from the soil?
Minerals are pumped into the root hair cells from the soil using ACTIVE TRANSPORT. This a process that uses energy from respiration in the cell to move ions against their concentration gradient (so from a lower concentration in the soil to a higher concentration inside the cell cytoplasm.)
What do we know about xylem vessels?
The cells that water and minerals are transported in are called xylem vessels. They have some interesting specialisations for this function. They are dead cells that are empty with no cytoplasm or nucleus. The end walls of these cells break down to provide a continuous unbroken column of water all the way up the plant. The cell walls of xylem vessels are thick and strengthened and waterproofed with a chemical called lignin.
What causes the water to move up the xylem?
Clearly it will take energy from somewhere to move water against gravity all the way up a plant from the roots to the leaves. The key question here is what provides the energy for this movement? There is no pumping of water up the plant and indeed the plant spends no energy at all on water movement. The answer is that it is the heat energy from the sun that evaporates water in the leaves that provides the energy for water movement. When you combine this with the fact that water molecules are “sticky” – they are attracted to their neighbours by a type of weak bond called a hydrogen bond – you can see that the water evaporating into the air spaces in the leaf can pull water molecules up the continuous column of water found in the xylem. The proper adjective for this stickiness is cohesive and you should know the name for the evaporation of water in the leaves (Transpiration)
PMG Biology 
Why do the end walls break down to form a continuous tube? How does having elongated column like cells benefit the transportation of water and minerals rather than the xylem having many regular length cells?
The end walls break down so that there is nothing blocking the pathway for water and minerals. Xylem vessels are elongated so that fewer cells are needed to make the whole tube. Hope this helps!
Am I right in saying that the continuous flow of water in the xylem occurs because water evaporated out of the leaves and in the spongy mesophyll and in the xylem form bonds so the water in the xylem is drawn up because of the loss of water from the leaves?
Yes that’s basically it. Water molecules are attracted to each other as they form hydrogen bonds between water molecules. So when some water evaporates in the leaf, this can generate a transpiration pull that drags water up in the xylem.?
As the cells that make up the xylem vessel are dead, wouldn’t they just decompose over time?
Decomposition only happens due to the action of bacteria and fungi (decomposers) – there are no decomposers inside the plant so the dead xylem cells persist.
Are the mineral ions in the xylem made to be dissolved in the water for transport? If so, how is it extracted again for making e.g. protein, DNA, chlorophyll.
The minerals are transported in solution and also react in their dissolved ionic form to make the kinds of biological molecules you mention….
How do the dead cells in Xylem vessels brake down their walls if they are dead and have no nucleus?
They are made from living cells and the final stage of their maturation into mature xylem is for the cells to die….. Good question.
If the hydrogen bonds are weak, how are they able to pull up a chain of water molecules?
How to plants found in Mangrove swamps with very salty water uptake water, surely osmotic pressure acts in the opposite direction?
Excellent question – show up! I am not sure the answer to this. I imagine plants with roots dipping into salty water have adaptations that make their cells even more concentrated than the salt water so they can still take up water by osmosis….. But I am not sure – I will ask my colleagues and let you know!
How does the plant keep air bubbles from entering the xylem when it is first created, surely there will be an airlock before water can fill the tube?
Excellent question – well done! A bit of a chicken and egg question….. Air is kept out as the xylem develops but exactly how I’m not sure…
How does the mechanism of osmosis work?
Too detailed to put in an answer – watch the video on my blog (but don’t worry as you study this next year….)
If the xylem vessels are made of dead cells, are these cells recycled from other parts of the plant or were they specifically made to be dead?
Good question – immature xylem vessels are living cells but they die before they become functionally active. The immature living cells are called proto-xylem so look this up if you want more information.
What is the difference in diameter between the phloem and the xylem or are they the same diameter?
Xylem vessels are much larger cells than phloem sieve tubes. Xylem also have cell walls that are much thicker than phloem. Why are these thick walls so important for the efficient functioning of xylem?
How exactly does active transport work?
There are proteins in the cell membrane that can use energy from respiration to pump specific molecules against the concentration gradient either into or indeed out of the cell. This active pumping against the gradient is active transport.
Why does water only enter through the root hair cells and how are they adapted to doing this?
Good question. Water will enter any cell in the plant in contact with water in the soil. But because root hair cells have such a massive surface area, the rate of osmosis in them is much greater….
How has the xylem adapted so that it can do its job more efficiently?
Good question. Xylem cells are dead and empty so there is nothing to impede the flow of water. They have thick cell walls that are waterproofed and strengthened with lignin. They have small openings called bordered pits that link adjacent xylem vessels. The end walls of the cells have broken down to produce a continuous column of water with no gaps.
If the water is moved up the plant by heat energy how come it isn’t evaporated whilst it is moving up?
Archie
Excellent question. The evaporation only happens in the leaf as there are no air spaces for water to evaporate into until the water reaches the leaf tissue. Leaves are also warmer than most of the plant since they are exposed to direct sunlight. Good question though!
How big is the diameter of the xylem vessel compared to the diameter of the stem of a plant?
Each xylem vessel is much smaller than the stem of the plant. Xylem vessels are large cells, up to 0.5mm in diameter. The stem of the plant varies in thickness of course but for non-woody plants may reach 40mm or so.
Are we required to know how phloem moves its sugar molecules about the plant for IGCSE?
Yes but only in a simplified way. No-one really understands phloem transport so if you can remember that it requires both phloem tubes and companion cells to be alive and respiring but that the exact mechanism is not understood, that would be a good start.
What other minerals are transported by xylem besides magnesium and nitrates?
Kieran
Good question – there are a whole load of minerals transported in xylem. As well as the two you mention, I would add phosphates, calcium, sodium, potassium as the next most important…. But there are many more.
How is the continuos column of water created?
Good question. All the water in the plant has either been made as a waste product of respiration or been absorbed through the roots by osmosis. This applies to the water in xylem as much as to any other water in the plant.