Movement of Substances : Plant Transport

Question 1

what does the transport system in plants enable leaves to do?

Answer 1

• receive water for photosynthesis and mineral ions needed to convert the products of photosynthesis into useful substances.
• once formed, these are then transported to other parts of the plants like growing tips, flowers and storage areas.

Question 2

what are xylem made of?

Answer 2

• thick walled dead cells which contain no cytoplasm- they are completely hollow and arranged end to end

Question 3

what does the cell wall of xylem contain?

Answer 3

• lignin, which is waterproof

Question 4

how do the xylem form a transport system throughout the plant?

Answer 4

• through the end walls having broken down

Question 5

what does the xylem transport and through which direction ?

Answer 5

• water and mineral ions from the roots to the leaves

Question 6

why are nitrate ions needed in the xylem needed?

Answer 6

• to make amino acids and proteins

Question 7

why are magnesium ions in the xylem needed?

Answer 7

• to make chlorophyll

Question 8

what is the phloem made of?

Answer 8

• consists of living cells
• tubes are formed by cells arranged end to end

Question 9

what is the cell wall in phloem made of?

Answer 9

• cellulose

Question 10

what happens to the sugar made in photosynthesis in the phloem?

Answer 10

• converted to sucrose

Question 11

what does the phloem transport?

Answer 11

• sucrose and amino acids from the leaves (where they are made) to the growing points (e.g. the tips of the shoot and flowers) or storage areas (such as the roots or bulbs).

Question 12

what is translocation?

Answer 12

• when transportation processes require energy
• for example, moving sucrose and amino acids requires energy

Question 13

where and how do plants take in water from the soil?

Answer 13

• through their root hairs
• these are the main site of water absorption by the roots
• they are thin walled (short distance) and have hair-like extensions that greatly increase the surface area of the root epidermis
• the function of a root hair is to absorb water and minerals from the soil
• the water has some solutes dissolved in it but their concentration is much lower than the concentrations of solutes inside the root hair cells
• mineral ions are actively transported into root hair cells and the soil water has a higher concentration of water molecules and water will enter the cell by osmosis
• water continues to move down a water concentration gradient from where there are more water molecules to where there are fewer water molecules
• this gradient is maintained as water is continually being taken up by the xylem in the middle of the root. the water is carried in the xylem vessels to all parts of the plant

Question 14

what is transpiration?

Answer 14

• the evaporation of water from the surface of a plant

Question 15

how is osmosis involved in the movement of water through the leaves?

Answer 15

• the epidermis is covered by a waxy cuticle so is impermeable to water
• most water passes out of the stomata as water vapour
• water leaves the cells of the mesophyll and evaporates into the air spaces between the spongy mesophyll. the water vapour then diffuses out through the stomata
• loss of water from the mesophyll cells makes the cells have a lower concentration of water molecules. so water moves into them by osmosis from the surrounding mesophyll cells. water leaves the xylem to replace the water lost from nearby cells
• the loss of water vapour from the leaves is called transpiration
• transpiration causes water to be pulled up the xylem in the stem and roots in a continuous flow known as the transpiration stream

Question 16

what are the functions of the transpiration stream?

Answer 16

• to carry mineral ions to the leaves for the synthesis of amino acids and chlorophyll
• to keep turgor pressure high in leaf cells, holding the leaves up
• evaporation cools the leaves
• supplies water for photosynthesis

Question 17

which conditions affect transpiration?

Answer 17

• temperature
• humidity
• wind speed
• light intensity
• water supply

Question 18

how does temperature affect transpiration?

Answer 18

• on a hot day, water will evaporate quickly from the leaves of a plant as the water molecules will have more kinetic energy
• therefore transpiration will increase as temperature increases

Question 19

how does humidity affect transpiration?

Answer 19

• very humid air contains a great deal of water vapour - there is a smaller concentration gradient, so transpiration slows down
• in dry air, the diffusion of water vapour from the leaf to the atmosphere will be rapid
• therefore transpiration increases if humidity decreases

Question 20

how does wind speed affect transpiration?

Answer 20

• in still air, the region around a transpiring lead will become saturated (full) with water vapour so that no more can escape from the leaf- causing transpiration to slow down
• in moving air, the water vapour will be blown away from the leaf as fast as it diffuses out, which will speed up transpiration
• therefore, transpiration increases as wind speed increases

Question 21

how does light intensity affect transpiration?

Answer 21

• light itself doesn’t affect evaporation, but in daylight, the stomata of leaves are open to supply carbon dioxide for photosynthesis
• this allows more water to diffuse out of the leaves and into the atmosphere

Question 22

how does water supply affect transpiration?

Answer 22

• if water is in short supply, and the plant is losing water by transpiration faster than it is being taken up by the roots, then the plant may start to wilt
• before this happens, the guard cells become flaccid and stomata start to close, therefore reducing transpiration and delaying wilting

Question 23

what are stomata, where are they found and describe the structure?

Answer 23

• they are holes in the surface of the leaf
• they are mainly found on the bottom surface
• there are two guard cells on either side of the stomata - these are sausage shaped cells which control the opening and closing of stomata
• each guard cell has a thickened, inflexible inner cell wall (on the stomatal side)
• when the stomata are open, the guard cells are swollen and when the stomata are closed, the guard cells are shrunken

Question 24

describe what happens to the guard cells and stomata in the light vs in the dark.

Answer 24

• in the light the guard cells photosynthesise. the concentration of the sugars increases, the water potential in the guard cells falls and so water moves into the guard cells by osmosis
  -they become turgid (swollen), which causes the guard cells to become banana shaped, due to the inflexible inner cell wall, and opens the stomata
• photosynthesis stops in the dark. as the sugar concentration falls (due to the respiration), water potential increases and water moves out of the guard cells . they become flaccid and the stomata close.

Question 25

describe the process of the translocation of sugars.

Answer 25

• the phloem move sugars from the leaves to other parts of the plant, where they are needed for respiration or growth
• the sieve tube is living, it has a thin layer of cytoplasm at the edge of the cell - but is mainly filled with sap (a concentrated solution of sucrose and amino acids). the sap flows through the sieve plates and through the phloem.
• the sieve tube does not have a nucleus - it is controlled and supported by the neighbouring companion cells
• these have lots of mitochondria and actively transport sucrose into the phloem
• this produces a low water potential (due to the high sucrose concentration) and so water moves into the sieve tube by osmosis from nearly cells
• this creates a high pressure and pushes the sap towards areas where sucrose (and water) are removed from the phloem

Question 26

what is a potometer?

Answer 26

• used to measure the rate of water uptake from a leafy shoot - as 90% of this water is transpired, we use this as a measure of the rate of transpiration

Question 27

how do you set up potometer?

Answer 27

1. the potometer must be set up under water - this prevents any air bubbles from entering the system and blocking the xylem
2. cut the stem of the shoot
3. put the shoot stem into the bung, grease the joint with plenty of petroleum jelly- this prevents water loss and air entry
4. put the bung into the potometer
5. make sure the tap is closed and it is full of water (no bubbles). then lift the potometer out of the water.
6. leave the end of the capillary tube out of the water until an air bubble forms then put the end into a beaker of water.
7. you can measure the transpiration rate as distance the bubble travels in five minutes (or the time taken for the bubble to travel a set distance). you should take a number of readings and calculate a mean rate

Question 28

how can you use a potometer to measure the rate of transpiration in different conditions?

Answer 28

• a potometer can be used to collect readings in normal air, windy conditions (e.g. using a hairdryer on cold), increased temperature, increased humidity (e.g. using a clear plastic bag) and darkness. this stimulates all the different conditions which affect the rate of transpiration.
• you could also remove the leaves (or cover them in Vaseline), to demonstrate the importance of the stomata