Chapter 8: Transport in plants Flashcards
(17 cards)
What are the functions of xylem and phloem?
Xylem: transports water and mineral ions, and helps support the plant
Phloem: transports sucrose and amino acids
How are xylem adapted to their function? (3)
- Thick walls made of lignin to not become soggy
- No cell contents
- Cells are joined end to end, with no cross walls, to form a long, continuous tube
What do vascular bundles look like in roots, stems, and leaves?
Roots: found in the centre, xylem in a star shape, with phloem filling in the spaces
Stems: arranged in a ring near the outside edge, xylem on the inside, phloem on the outside
Leaves: Xylem is on top, phloem on the bottom
What is the function of root hair cells?
Absorb water and mineral ions from the soil
What is the path taken by water through the root, stem, and leaf?
Root hair cells, root cortex cells, xylem, mesophyll cells
How do you investigate, using a suitable stain, the pathway of water through the above-ground parts of a plant?
Cut off the end of a stick of celery, then stand it in coloured water and leave for a couple of hours. Remove the celery, cut off a thin cross-section of the celery and observe the dye present
Define ‘transpiration’
The loss of water vapour from leaves
How does transpiration occur?
The water evaporates from the surface of mesophyll cells into the air spaces, then diffuses out of the leaves through stomata as water vapour
How is water vapour loss related to the large internal surface area provided by the interconnecting air spaces between mesophyll cells, and the size and number of stomata?
Large internal surface area: allows for a greater amount of water to evaporate into air spaces, increasing concentration inside the leaf and leading to a higher transpiration rate
Size and number of stomata: rate of transpiration is generally greater in leaves with many open stomata
What is the mechanism by which water moves upwards in the xylem?
A transpiration pull (a force produced by the loss of water vapour from a leaf, which reduces the pressure at the top of xylem vessels) draws up a column of water molecules, held together by forces of attraction between water molecules
How do you investigate the effects of variation of temperature and wind speed on transpiration rate?
Set up a potometer and attach a plant to it. Introduce an air bubble into the capillary tubing and ensure it is at zero on the scale. Set a timer for 2 minutes and record the distance the air bubble has travelled. Repeat the experiment, but blow it with a fan or place it in a refrigerator. The air bubble will have travelled further when blown with a fan, and travelled less in the refrigerator
What are the factors that affect transpiration?
Temperature, wind speed and humidity
What are the effects of varying temperature, wind speed and humidity on the rate of transpiration?
Temperature: the higher the temperature, the greater the kinetic energy of water molecules, the higher the rate of transpiration
Wind speed: air around the leaf, which contains water vapour that has just diffused out of the leaf, is quickly moved away, higher rate of transpiration
Humidity: the higher the humidity, the lower the rate of transpiration due to the lack of diffusion gradient
Why does wilting occur?
The plant cells lose so much water that they become flaccid, and the tissues are no longer supported by the turgid cells pushing outwards against one another, causing the leaves to become soft and floppy
Define ‘translocation’
The movement of sucrose and amino acids in the phloem from sources to sinks
Define ‘sources’ and ‘sinks’
Source: the parts of plants that release sucrose or amino acids
Sink: the parts of plants that use or store sucrose or amino acids
Why do some parts of a plant act as a source and a sink at different times?
Sucrose may be converted into starch and stored in the roots, which are the sinks originally, but during winter, stored starch is transported to other parts of the plant, turning the roots into the source instead
