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Xylem: Evapo-transpiration

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1 Xylem: Evapo-transpiration
Transport in plants Xylem: Evapo-transpiration

2 Why do plants need a transport system?
To transport food and water throughout the plant from roots to leaves at the top of the plant. These substances are used by the plant to aid in photosynthesis. There are two structures used to transport material, the xylem vessels and phloem tubes. Vascular bundles are xylem and phloem tubes in close proximity to each other.

3 The position and orientation of vascular bundles vary throughout the plant.
The phloem however is always found on the outer part of the bundle

4 Xylem Xylem vessels transport water and dissolved minerals from the root to the shoot of plants.

5 Direction of movement of substances in vascular tissue.

6 Xylem tissue

7 MONOCOTS Root pattern (xylem and phloem) – ring

8 DICOTS Stem pattern – Ring-shaped xylem and phloem.
LARGE CELLS ARE XYLEM Stem pattern – Ring-shaped xylem and phloem. SMALL CELLS ON OUTSIDE ARE PHLOEM

9 DICOTS Root pattern (xylem and phloem) – X-shape

10 Mineral and water uptake

11 Casparian Strip The Casparian strip controls water movement into the vascular cylinder of the root. Water cannot move between cells. It must move through the cells by osmosis. Why is this important?

12 Thinking question: What would happen in a root that had no Casparian strip? Why would this be a problem?

13 Capillary action Cohesion and adhesion cause water to “crawl” up narrow tubes. The narrower the tube the higher the same mass of water can climb. Maximum height: 32 feet.

14 Step 2: Capillary action
Cohesion: polar water molecules tend to stick together with hydrogen bonds. Adhesion: water molecules tend to stick to polar surfaces.

15 Cohesion-tension theory
Cohesion between water molecules creates a “water chain” effect. As molecules are removed from the column by evaporation in the leaf, more are drawn up.

16 Part 3: Evaporation Evaporation at the surface of the leaf keeps the water column moving. This is the strongest force involved in transpiration.

17 Stomata control Guard cells around the stomata are sensitive to light, CO2, and water loss. Cells expand in response to light and low CO2 levels, and collapse in response to water loss.

18 Stomata When stomata are open, evaporation draws water out of the leaf. Gas exchange can also occur to keep photosynthesis and respiration running. When stomata are closed, evaporation cannot occur, nor can gas exchange. What happens to photosynthesis and transpiration?

19 HOW DOES XYLEM WORK? Cohesion of water molecules – Water is “sticky”


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