Ch 9.1 Flashcards
What is transpiration?
- transpiration is the inevitable consequence of gas exchange in the leaf
- loss of water vapor from the leaves of a plant
Transport in the xylem of plants
- plant leaves are the primary organ of photosynthesis (CO2 is vital to this process, and O2 is produced as well)
- exchange of CO2 and O2 must occur in order to sustain photosynthesis
- absorption of CO2 is essential for photosynthesis and the waxy cuticle of the top of the leaf has low permeability to it
- pores in the epidermis are needed- stomata
- if stomata are open to absorb CO2, then water will be lost from the leaf to the atmosphere
- thus, transpiration is the inevitable consequence of gas exchange in the leaf
Structure of water
- H20 is a covalent molecule
- the oxygen and hydrogens share electrons, but not equally
- because oxygen has a larger nucleus, it has a stronger pull on the negatively charged electrons. The electrons thus spend more time around the oxygen than the hydrogens
- the oxygen end then becomes slightly negative and the hydrogens slightly positive
- water is thus a POLAR molecule
- because water is polar, it is attracted to other water molecules in a specific way
- water is thus COHESIVE, it is attracted to other water molecules and forms weak hydrogen bonds
- this makes water “sticky,” it has high surface tension and does not change temperature easily
- water is also ADHESIVE, it sticks to other charged particles
- in plants, it sticks to the sides of xylem
- this helps prevent gravity from pulling water back down to the roots
Transpiration
- plants transport water from the roots to the leaves to replace water losses from transportation
- the cohesive property of water and the structure of xylem vessels allow transport under tension
- the adhesive property of water and evaporation generate tension forces in leaf cell walls
- active uptake of mineral ions in the root causes absorption of water by osmosis
Active uptake
- energy is needed for active transport
- used to absorb minerals
- water will passively follow
- some minerals are dissolved in water and enter with it
- fungae hyphae on roots absorb minerals
What are the different paths of water absorption?
apoplast (most)
symplast (some)
vacuolar (few)
What are the steps in transpiration?
- water forms a continuous stream from the leaves to the roots because of the cohesion of the water molecules (TRANSPIRATION STREAM)
- stomata in leaves open to allow CO2 in for photosynthesis
- this creates a “gap” in the transpiration stream in cells by the stomata
- water moves by osmosis into the “gap” in order to replace water lost by transpiration
- because water is cohesive, when the top molecules move into the gap, they pull the entire transpiration stream up with them
- adhesion of water molecules to the xylem walls helps to prevent gravity from pulling the transpiration stream back down the xylem
- at the roots, minerals are being actively transported from the soil into the root. This keeps the root high in mineral concentration and low in water.
- water moves into the roots by osmosis, following the movement of the minerals
Is water movement a passive or an active process?
- passive process (no energy is required)
- it moves by osmosis from areas of high water content to areas of low water content
How do leaves help to maintain the transpiration stream?
- tension in leaf cell walls maintains the transpiration stream
- the adhesive property of water and evaporation generate tension forces in leaf cell walls
transpiration stream
-a continuous stream of water from the leaves to the roots that forms because of the cohesion of the water molecules
xerophytes
- low water
- specialized roots, stems or leaves for storage of water
- leaves reduced in size to needles to reduce loss of water through transpiration
- thick waxy cuticles to reduce water loss
- stomata open at night to let in CO2, which is stored until the day when it is used in photosynthesis (CAM physiology)
- leaves rolled to reduce surface area
- stomata in pits to prevent water loss
- hairs on leaves to reduce air flow over stomata and thus decrease transpiration
- stores water in stem
halophytes
- high salt
- leaves reduced in size to spines or small scaly structures to reduce loss of water through transpiration
- leaves shed when water is scarce and stem takes over photosynthesis
- water storage in leaves
- thick cuticle and thick epidermis
- sunken stomata
- long roots
- structures to remove salt build up
water transport models - three cups + paper towel model
-water is cohesive and adhesive, so will move up paper toweling. Red and blue combine to make purple in the cup between them. Demonstrates how cohesion and adhesion of water play a role in transpiration
water transport models - capillary tubes
- water is polar so cohesion and adhesion are shown
- oil does not have this same effect
water transport models - porous pot
-water will move by cohesion and adhesion up the tube into the pot. The pot has pores like the stomata on a leaf and water will evaporate out, modelling transpiration in a leaf
