Cambium immature cell (could become either a xylem or phloem cell)
| Tissue | Xylem | Phloem |
| Transport | Water an minerals from roots to shoots and leaves | Sucrose and amino acids produced in leaves during photosynthesis to every part of the plant |
| Process | Transpiration | Translocation |
| Structure | Cylindrical cells arranged end to end, in which the cytoplasm dies and the cell walls between adjoining cells breaks down leaving a dead empty tube with strengthened cell walls | Phloem tubes are made up of columns of living cylindrical cells. The cell walls between adjoining cells develop holes like a sieve allowing transport through the tube |
XYLEM VESSELS: Why does the water move up? What is the “transpiration stream”?
“Water is lost through the leaf of the plant by evaporation in a process called transpiration. This results in a pull on a column of water molecules which extends right down through the stem to the roots.
Water molecules are held together by hydrogen bonds. As a water molecule evaporates from the leaf, it pulls on the water molecule next to it, which in turn exerts a pull on the water molecule next to it and so on. This transmits a pull all the way down the column of water to the root system. In addition, the water molecules also adhere to the walls of the xylem vessel, preventing gravity from pulling the molecules back down.
So as each water molecule exits the leaf, it pulls the column of water molecules upwards from the root of the plant up the xylem and into the leaf. This upward movement of water resulting from the evaporation of water from the leaf is called the transpiration stream.” (from Pass My Exams)
24/10/13
Translocation
Sucrose is glucose molecules bonded together (in a condensation reaction). Sucrose can get broken down into glucose which is used for
- releasing energy by respiration
- producing proteins, such as enzymes and chlorophyll
- producing cellulose, which strengthens the cell wall
*NOTE: Plants also need nitrates (absorbed from the soil as nitrate ions)
- Nitrates are used to make amino acids…
- Amino acids are needed to make proteins…
- Proteins are used to help plant grow
ALL THE STEPS ABOVE ARE NEEDED FOR MAX. MARKS IN EXAM QS. “NITRATES HELP PLANT GROW” IS REJECTED.
Sucrose is transported to:
- shoot tip
- root tip
- fruits
- flowers (pollen production)
- etc
Transpiration the evaporation of water at the surfaces of the mesophyll cells followed by loss of water vapour from plant leaves, through the stomata
- Q: Why does more transpiration happen during the day than at night?
- A: Higher light intensity triggers stomata to open + higher temperature decreases the concentration of water vapor particles outside the leaf because heated particles have more kinetic energy and spread out away from the leaf
- Q: Suggest how plants cope with hot, dry conditions
- A: long and narrow leaf/spines/less stomata/hairs (water vapor particles passing out of the leaf clump together into droplets which attach to the hairs – this balances out the concentration gradient because the hairs are outside the leaf)
Why are there more stomata on the bottom of the leaf?
General idea: MINIMISES WATER LOSS: THE PLANT DOES NOT WANT TO LOSE WATER.
- Gas exchange would occur faster if there were more stomata on top because more sunlight hits the top (light causes stomata to open and the stomata are the site of gas exchange). If there were more stomata on top, a lot more water vapor would be lost. The plant does not want this!
Annissa's IGCSE & IB Notes 