681226125- new curriculum Transport-in-Plants.pptx

Transport in plants new curriculum

1. Transport in Plants

2. Vascular Tissue Functions of Xylem & Phloem Plants contain two types of transport vessel: Xylem vessels  – transport water and minerals from the roots to the stem and leaves Phloem vessels  – transport food materials (mainly sucrose and amino acids ) made by the plant from photosynthesising leaves to non- photosynthesising regions in the roots and stem These vessels are arranged throughout the root, stem and leaves in groups called  vascular bundles   Exam Tip If you are asked to identify the xylem or phloem in a diagram showing a cross-section of a root, stem or leaf just remember that  xylem is always on the inside and phloem is always on the outside .

3. Vascular Bundles Xylem  and  Phloem  tissues are found in groups called  vascular bundles . The position of these bundles varies in different parts of the plant. 1) Root Xylem vessels are tough and strong , so the vascular bundles are in the centre of the root to resist forces that could pull the plant out of the ground. 2) Stem The stem has to resist compression (squashing) and bending forces caused by the plant’s weight and the wind. The vascular bundles are arranged near the edge of the stem, with the phloem on the outside and the xylem on the inside.

4. Position of xylem and phloem Leaf Stem Root

5. Xylem Vessels Mature xylem consists of elongated dead cells whose end walls have disappeared, arranged end to end to form continuous  vessels  (tubes) Mature xylem vessels: contain no  cytoplasm (cells are dead) are  impermeable  to water have tough walls containing a woody material called  lignin which gives support and strength to the plant Water only moves upwards in one direction (from roots to aerial parts)

6. Phloem Vessels Phloem consists of living cells arranged end to end . Unlike xylem, phloem vessels contain cytoplasm , and this goes through holes from one cell to the next. Phloem transports sucrose and amino acids up and down the plant. This is called  translocation . Translocation requires energy as it is an active process. In general, this happens between where these substances are made (the  sources ) and where they are used or stored (the  sinks ). This means, for example, that sucrose is transported: from sources in the root to sinks in the leaves in winter time from sources in the leaves to sinks in the root in the summer Applied chemicals, such as  pesticides , also move through the plant by translocation.

7. Xylem and Phloem Phloem consists of living cells arranged end to end. They contain cytoplasm Phloem transports sucrose and amino acids up and down the plant. This is called  translocation Xylem vessels: are  impermeable  to water have tough walls because of a substance called  lignin which causes the xylem cells to die and allows them to withstand the water movement These cells then become  hollow  (as they lose all their organelles and cytoplasm) and join end-to-end to form a  continuous tube  for water and mineral ions to travel through from the roots

8.  Adaptations Root hair cells  are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption. They also contain lots of  mitochondria , which release energy from  glucose  during  respiration  in order to provide the energy needed for active transport. What process is responsible for uptake of water? What process is responsible for uptake of mineral salts? Guess the structure!

9. Water Uptake Root Hair Cells Root hairs are single-celled extensions of  epidermis cells  in the root They grow between soil particles and absorb water and minerals from the soil

10. How the Large Surface Area of a Root Hair Cell is Useful The root hair increases the surface area of the cells significantly This large surface area is important as it  increases the rate of the absorption of water by osmosis and mineral ions by active transport They are long and thin so they can penetrate in between the soil particles Water passes from the soil water to the root hair cell’s  cytoplasm  by  osmosis . This happens because the soil water has a higher  water potential  than the root hair cell cytoplasm

11. Label the structures

12. Pathway of Water through Root to Leaf Osmosis causes water to pass into the root hair cells, through the  root cortex  and into the xylem vessels:

13. Pathway of Water through Root to Leaf Once the water gets into the  xylem , it is carried up to the leaves where it enters  mesophyll cells The pathway is: root hair cell → root cortex cells → xylem (in root)  xylem in stem  xylem in leaf  leaf mesophyll cells

14. Investigating water movement in plants using a stain The pathway can be investigated by placing a plant (like celery) into a beaker of water that has had a stain added to it (food colouring will work well) After a few hours, you can see the leaves of the celery turning the same colour as the dyed water , proving that water is being taken up by the celery If a cross-section of the celery is cut, only certain areas of the stalk are stained , showing that the water is being carried in specific vessels through the stem – these are the  xylem vessels

15. Transpiration Transpiration  is the loss of water from plant leaves by evaporation  of water at the surfaces of the spongy mesophyll cells in leaves, followed by loss of water vapour through the  stomata by diffusion .

16. Transpiration Water  travels up  xylem  from the roots into the leaves of the plant to replace the water that has been lost due to  transpiration Movement in xylem  only takes place in one direction  – from  roots to leaves  (unlike phloem where movement takes place in different directions)

17. Transpiration Evaporation and Diffusion= Transpiration Evaporation  takes place from the surfaces of spongy mesophyll cells The  many interconnecting air spaces  between these cells and the stomata creates a  large surface area This means evaporation can happen rapidly  when stomata are open

18. Transpiration stream How is the Transpiration Stream Created? Water molecules are attracted to each other by  cohesion  – creating a continuous column of water up the plant As water molecules are held together by  cohesive forces  (each individual molecule ‘pulls’ on the one below it), so water is pulled up through the plant Water moves through the xylem vessels in a continuous  transpiration stream  from roots to leaves via the stem Transpiration produces a  tension  or ‘pull’ on the water in the xylem vessels by the leaves If the rate of transpiration from the leaves  increases , water molecules are pulled up the xylem vessels  quicker The  transpiration stream  is  important  because: It provides leaves with a continuous supply of water (for photosynthesis ). It continually transports minerals from the roots to the leaves and other parts of the plant. It provides plant cells with support from turgor pressure.

19. Importance of transpiration Transpiration has several functions in plants: transporting  mineral ions providing  water to keep cells turgid  in order to support the structure of the plant providing  water  to leaf cells for  photosynthesis keeping the  leaves cool  (the conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The using up of heat to convert water into water vapour helps to cool the plant down)

20. Transpiration stream Water moves through the  xylem vessels  in a continuous  transpiration stream : root → stem → leaf When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyll and palisade mesophyll evaporates and diffuses out of the leaf. Water is drawn from the cells in the  xylem  to replace that which has been lost from the leaves. Water molecules inside the xylem cells are strongly attracted to each other. There is strong cohesion between the molecules because of  hydrogen bonding . A continuous column of water is therefore pulled up the stem in the transpiration stream by evaporation from the leaves. Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves. Water molecules are  cohesive  so water is pulled up through the plant.

21. Transpiration

22. Wilting If more water evaporates from the leaves of a plant than is available in the soil to move into the root by osmosis, then  wilting  will occur Young plant stems and leaves rely on their cells being  turgid  to keep them rigid.  If the amount of  water lost  from the leaves of a plant is > than the amount  taken  into the roots  à  the plant will have a  water   shortage   à  cells become  flaccid  (soft) and eventually plasmolysed and will no longer press against each other  à  Stems and leaves lose their rigidity, and  wilt .  The turgor pressure inside the plant decreases due to which the cell walls cannot support the plant

23. Rate of transpiration If the rate of transpiration increases , the rate of absorption of water by the root increases too. Factors that affect the rate of transpiration also affect water uptake by the plant. If water is scarce, or the roots are damaged, a plant may wilt and eventually die The leaves in hot or dry environments are adapted to reduce  transpiration . For example, their  stomata  may open at night and close at midday. This is the reverse of what usually happens, and it means that the stomata are closed when the rate of transpiration would be greatest.

24. Temperature & Humidity on Transpiration Rate Factors affecting rate of transpiration:

25. Investigating Factors that Affect Transpiration A  potometer / transpirometer , is a device used for measuring the rate of water uptake of a leafy shoot which is almost equal to the water lost through transpiration. The causes of water uptake are photosynthesis and transpiration.

26. Investigating Factors that Affect Transpiration Investigating the role of environmental factors in determining the rate of transpiration from a leafy shoot Cut a shoot underwater to  prevent air entering the xylem  and place in tube Set up the apparatus as shown in the diagram and make sure it is airtight , using vaseline to seal any gaps Dry the leaves of the shoot (wet leaves will affect the results) Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water Set up the environmental factor you are investigating Allow the plant to adapt to the new environment for 5 minutes Record the  starting location  of the air bubble Leave for a set period of time Record the  end location  of air bubble Change the light intensity or wind speed or level of humidity or temperature (only one – whichever factor is being investigated) Reset the bubble by opening the tap below the reservoir Repeat the experiment The  further the bubble travels in the same time period, the faster transpiration is occurring  and vice versa

27. Effect of light on transpiration The opening and closing of the  stomata  is controlled by the  guard cells . In  light , guard cells  take up  water by osmosis and become  turgid . Because their inner walls are rigid they are pulled apart,  opening  the pore.  In  darkness  water is  lost  and the inner walls move together  closing  the pore. The transpiration rate is increased by an increase in light intensity. 

28. Translocation Transport of Food- The transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use, is called translocation The soluble products of photosynthesis are  sugars (mainly sucrose) and amino acids These are transported around the plant in the  phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells) The cells are joined end to end and contain holes in the end cell walls (called  sieve plates ) which allow  easy flow of substances  from one cell to the next Transport in the phloem  goes in many different directions  depending on the stage of development of the plant or the time of year; however dissolved food is always transported from  source  (where it’s made) to  sink  (where it’s stored or used): During  winter , when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue During a  growth period  (e.g. during the spring), the storage organs (roots) would be the source and the many growing areas of the plant would be the sinks After the plant has grown  (usually during the summer), the leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again

29. Leaf Adaptations

30. Comparison between Xylem and Phloem

31. QUESTIONS Q. Why does the rate of transpiration increase if the light intensity is increased? A) The humidity increases, which makes the rate of transpiration increase. B) The stomata open wider to let more carbon dioxide into the leaf for photosynthesis C)The root hair cells become more permeable to water, letting more water into the plant and through the xylem When the light intensity increases, the stomata open wider to let more carbon dioxide into the leaf for photosynthesis. The root hair cells are underground and not affected.

32. QUESTIONS The plastic bag allows the humidity to increase. This reduces the rate of evaporation (transpiration) from the plant. Q. Why does water loss slow down if a plant is put into a plastic bag? A) The humidity increases B) The temperature increases C) The light intensity increases

33. QUESTIONS Q1. State one structural feature of xylem vessels and explain how this is related to the function of water transport. feature ............................................................................................................................... ........................................................................................................................................... explanation ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2]

34. QUESTIONS Q2. Explain the mechanism that is responsible for the movement of water in xylem vessels. [4] Q3. State one role of xylem vessels other than transport. [1] Q4.The rate of transpiration is affected by several factors including the temperature and the humidity of the air. State and explain the effect of an increase in temperature on the rate of transpiration. [3]

35. QUESTIONS Q5. Describe the pathway of water from outside the root to the xylem vessels at the centre of the root. Use the letters in Fig 3.1 in your answer [5]

36. Mark Scheme

37. Mark Scheme

38. Mark Schemes