Transpiration stream

Transpiration is the loss of water from a plant by evaporation. Water is essential for plants as it is required for photosynthesis to produce glucose; all organisms derive their energy from the oxidation of glucose, minerals and ions are dissolved in water and bring them from the roots to other plant tissue that require the minerals. Also, water keeps the plant cool. The plant undergoes several processes in order for it to lose the amount of water through the lower epidermis of a leaf.

Firstly, soil, is a very dilute solution of ions, which means a lot of water has been dissolved in to solution, and therefore the soil solution has a very high water potential. On the other hand the root hair cells have an insufficient amount of water molecules, resulting in root cells having low water potential. Root cells have a partially permeable membrane, which allows water to travel through them due to osmosis, where water travels from high water potential to low water potential. Root hair cells have a high surface area to volume ratio to efficiently absorb water.

As water enters root hair cells, it has three routes for it travel along symplastic pathway, apoplastic pathway and vacuolar pathway. In the symplastic pathway water crosses the cellular membrane of root hairs and enters the cytoplasm of the cell, it travels between connecting cells through the plasmodesmata, this process occurs due to diffusion of water molecules from high concentration to low concentration. However the process is quite slow. Water travels though intercellular spaces along cell walls in apoplastic pathway, it travels through interconnecting free spaces between the cellulose fibres of the plant. As the water flows, solutes move with the flow or by diffusion. This pathway is very efficient and occurs through mass flow of water. Finally, vacuolar pathway is similar to that of symplastic because it travels from one cell to another crossing the plasmodesmata, however in this type, water travels to vacuoles of cells through osmosis, a benefit of this is that individual cells absorb water for their own use.

Water travels across the cortex and needs to pass through the endodermis (passed the endodermal cells) before entering the xylem vessels. Each cell in the endodermis has a waterproof ‘band’ around it called the casparian strip. This means that water must pass through the cell in some way, rather than around the outside. The purpose of the casparian strip is to remove water travelling through the cell walls across the apoplastic route and redirect the route so water can travel through the symplast or vacuolar pathways. After passing the root hair cells, water enters the xylem.

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The xylem vessels are internally lined with lignin, this substance is waterproof and it also provides structural support to the xylem vessels. Xylem tissue is composed of dead cells joined together to form long tube-like structures, the side cell walls have pits/holes to allow water to travel to other plant tissue which require water. As water enters the xylem, root pressure gives an initial upward force to water. Water travels up the xylem by capillarity, which is the upward movement of a fluid in a narrow tube (xylem). Capillarity occurs because the water molecules have an attraction for each other, this is due to hydrogen bonds, and therefore if one water molecule is forced upwards due to pressure, it will attract other water molecules, resulting in the initial direction of water travelling upwards. This is called cohesion. Also, another factor which contributes to the upward movement of water is adhesion, water molecules attract the sides of the xylem vessels, which pulls the water upwards. This would minimise the tendency for water molecules to fall backwards by gravity. Evaporation of water in the leaves, also pulls water upwards. When this occurs there is a great difference in water potential between the xylem and the leaf (the leaf having the low water potential compared to xylem which will have a high water potential), the water will travel upwards due to this sufficient force that will lifts the column of water up against the pull of gravity.

Water enters the leaf after passing through the xylem to ‘leaf veins’ through osmosis down a concentration gradient. Water finally travels, by a symplast pathway to living cells until it reaches the stomata in the spongy mesophyll where it evaporates. The role of stomata is to allow gaseous exchange (Carbon dioxide entering and oxygen leaving) and also to control the amount of water lost from a leaf. Stomata are openings in the leaf controlled by two very specialised cells called guard cells. When guard cells are turgid (high water content), they change shape depending on their turgidity, which is controlled by osmosis. However, when guard cells are flaccid (low water content) the stomata are closed to reduce the amount of more water being lost.

There are several factors that affect the rate of transpiration light, temperature, and humidity. Light stimulates the stomata to open/close for gaseous exchange and as a result increases the rate of transpiration. If the temperature is high, the rate of evaporation of water in the leaf will increase, this will result in the plant needing more water at a quicker rate, therefore transpiration increases. If the humidity is high, this means there is a smaller difference of water potential between the leaf and the air consequently there will be less evaporation, decreasing the rate of transpiration.

Picture of xylem vessel

Image of different routes taken by water in root hair cells

The Hutchinson Encyclopedia Information on transpiration

Peer Reviews

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groat

8th of Feb 2012

Quality of writing

This essay is structured well, and spelling and grammar are used effectively. If I were answering this question, I would've explained the transpiration stream, explained the factors affecting the rate of transpiration and then commented on its importance. Placing the importance in the introduction doesn't allow any incorporation of evidence to strengthen the argument. For example, I would've mentioned at the end how key it is for xerophytes to reduce transpiration by closing their stomata, which had been mentioned earlier in the essay as reducing the rate of transpiration. Other than this, the essay is strong.

Level of analysis

This essay explains all the relevant points for transpiration, and at times is above GCSE level. I liked how the essay follows the path of water, showing a logical progression. It was nice to see reference to the various pathways - these are easy marks to gain and are reasonably simple concepts to understand when talking about transpiration. Scientific terms are used throughout, and it includes a great explanation of cohesion. If you are struggling to understand this concept, I would recommend reading through this answer, as it puts it quite simply, yet still manages to access the marking points! It was nice to see some awareness of the factors which affect the rate of transpiration. One query would be the use of diagrams: there is no point including a diagram if it is unclear, or of poor quality. A good diagram can make it easier to revise the topic, and shows a full understanding in a piece of coursework.

Response to question

This essay ably explains the transpiration stream in plants. In parts it is slightly unclear. For example, in the introduction they mention the importance of transpiration in plants for photosynthesis, then mentions the oxidation of glucose. At GCSE level, I think it's sufficient enough to say that plants are the producers, so if they cannot photosynthesise then consumers cannot get energy. It's best not to complicate things, and this is not helped by the sentence structure in the introduction.

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