| What is xylem tissue | - Transports water + mineral ions
- Moves up plant from roots to leaves
- Xylem + phloem are mass transport systems - move substances over large distances
- No energy needed |
| Structure of the xylem | - Xylem vessels, long, tube-like
- Made of dead cells joined end-to-end
- No end walls on cells, making an uninterrupted tube
- So allows water to pass easily through middle |
| The cohesion-tension theory of water | - Cohesion + tension help water move up plants against the force of gravity |
| How does cohesion-tension help water move up plants | - Water evaporates from leaves at top of xylem - transpiration
- This creates tension (suction), pulls more water into leaf, water is cohesive
- Water moves from roots, to stem, upwards |
| Transpiration meaning | - The evaporation of water from a plants surface, especially leaves |
| Process of transpiration | - Water evaporates from moist cell walls
- Accumulates in spaces between cells
- When stomata open, it moves out of leaf down concentration gradient
(there's more water inside leaf than the air outside) |
| The main factors that affect transpiration rate | - Light
- Temperature
- Humidity
- Wind |
| Light | - Increase light increases transpiration rate (+ correlation)
- Stomata open when it gets light to let CO2 in for photosynthesis
- When its dark stomata close, little transpiration |
| Temperature | - Increasing temperature increases transpiration (+ correlation)
- Warmer molecules have more energy, evaporate from cells faster
- This increases concentration gradient between inside + outside of leaf, evaporate from leaf faster |
| Humidity | - Increasing humidity decreases transpiration (- correlation)
- Dry air = concentration gradient between inside + outside of leaf decreases, decreasing evaporation |
| Wind | - Increasing wind increases transpiration (+ correlation)
- Blows away water molecules from around stomata
- Increasing concentration gradient between the inside + outside of leaf, increasing evaporation |
| Potometer (pu - tom - eee - ter) | - Used to estimate transpiration rates
- Measures uptake by a plant, based on how far an air-water meniscus moves over time
- But based on assumption that water uptake is directly related to water loss |
| Using a potometer | - Construct potometer underwater, so no air enters
- Cut shoot at slant, increases SA for uptake
- Insert shoot underwater
- Remove apparatus from water
- Dry the leaves, allows time to acclimatise
- Remove end of capillary tube until 1 air bubble has formed
- Put end of tube back into water
- Record the starting position of the air bubble
- Start timer, record distance moved by air bubble per unit time e.g. per hour
- Rate of air movement = estimate of transpiration rate
- Change 1 variable, everything else kept constant |
| Dissecting plants | - Look at phloem and xylem under microscope
- Draw them |
| Dissecting plants method | - Use a scalpel to cut cross-section of the stem
- Cut sections thinly, better under microscope
- Use tweezers to place sections in water, stops drying out
- Transfer each section to dish containing stain e.g toluidine blue (TBO)
- Leave 1 min
- Rinse off sections in water
- Mount each one onto slide |
| Toluidine blue (TBO) (tu - lu - i - deene) | - Stains lignin in the walls of the xylem vessels blue-green |
