9.2 Transport in angiospermophytes

1. 9.2 Transport in angiospermophytes

2. Assessment Statements • 9.2.1 Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs. • 9.2.2 List ways in which mineral ions in the soil move to the root. • 9.2.3 Explain the process of mineral ion absorption from the soil into roots by active transport. • 9.2.4 State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem. • 9.2.5 Define transpiration. • 9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation. • 9.2.7 State that guard cells can regulate transpiration by opening and closing stomata. • 9.2.8 State that the plant hormone abscisic acid causes the closing of stomata. • 9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant. • 9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration. • 9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

3. Transport in angiospermophytes • Transport in flowering plants occurs on three levels: • the uptake and loss of water and solutes by individual cells • short-distance transport of substances from cell to cell at the level of tissues or organs • long-distance transport of sap within xylem and phloem at the level of the whole plant

4. Variety of physical processes involved in the different types of transport

6. Root system • Functions of roots; • absorb water • absorb minerals ions • support and anchor • sometimes used for food storage • Fibrous roots Tap root

8. How the root system provides a large surface area for mineral ion and water uptake • the root system of a plant must supply sufficient water &mineral ions • for this reason, it has developed a large surface area due to; • branching • presence of root hairs near the tip

9. Ways in which mineral ions in the soil move to the root • mineral ions are absorbed by root hairs on the epidermis • root hairs increase the surface area for absorption • mineral ions enter the root hairs trough active transport whichuses energy in form of ATP • active transport uses of proteins pumps to move ions across membrane • against concentration gradient i.e. from low concentration in the soil into the root cells where they are in high concentration • the rate of absorption of mineral ions is limited by the rate at which the ions move through the soil to the roots • there are three ways in which the ions move to the root: • through facilitated diffusion • through mass flow of water containing dissolved ions • through mutualistic fungal hyphae growing around the root

10. Adaptations of plant roots for absorption of mineral ions from the soil • mineral ions are absorbed by active transport • large surface area is required • branching of the root & presence of root hairs increases surface area • root hair cells have carrier protein (ion pumps) in their plasma membrane • many mitochondria are present in root hair cells to provide ATP for active transport • connections with fungi in the soil (fungal hyphae)

11. How terrestrial plants support themselves • terrestrial plants support their tissues through: • thickening of the cellulose cell wall • lignified xylem vessels • cell turgidity, turgor pressure provide mechanical support to the plant tissue

12. Define transpiration • transpiration is water loss from plant by evaporation • excess water loss may harm the plant • transpiration is the driving force that pulls water up from the roots to the leaves to supply photosynthesizing tissue • thus, transpiration is a necessary evil

14. How water is carried by the transpiration stream • transpiration is water loss from plant by evaporation • flow of water through xylem from roots to leaves is the transpiration stream • water enters roots through the root hairs by osmosis • root hairs provide an extended surface area for active transport & osmosis • active transport of ions from soil into the roots enhances osmotic pressure • osmotic pressure moves water into the xylem • water is carried in a transpiration stream in the xylem • adhesion of water to the inside of the xylem helps move water up • cohesion of water to itself enhances water movement up the xylem • water vapour diffuses into air spaces in spongy mesophyll of leaves • it passes out through the stomata by evaporation i.e. transpiration • evaporation of water vapour sets up a transpiration pull that keeps the water moving • guard cells control the rate of transpiration pull by controlling evaporation • xylem vessels are tubes with helical rings to enhance water movement by resisting low pressure

15. How guard cells regulate transpiration guard cells gain water & open stoma is large, rate of transpiration is high • stomata are pores usually in the lower epidermis • each stomata is formed by two specialised guard Cells • the epidermis &its waxy cuticle is impermeable to carbon dioxide & water • during the day the pore opens to allow carbon dioxide to enter for photosynthesis • however, the plant will experience water loss, if the water loss is too severe the stoma will close • dehydration, low water potential, of the mesophyll cell causes them to release the hormone abscisic acid • abscisic acid stimulates the stoma to close • during the night plants cannot photosynthesis, so the plant closes the pores thereby conserving water guard cells lose water & close stoma is small, rate of transpiration is low

16. Hormone abscisic acid causes the closing of stomata guard cells gain water & open stoma is large, rate of transpiration is high guard cells lose water & close stoma is small, rate of transpiration is low

17. How the abiotic factors affect the rate of transpiration in terrestrial plant • transpiration is loss of water vapour from the stomata of leaves & stems of plants • temperature, humidity, light intensity & wind all affect rate of transpiration • humidity, less transpiration as atmospheric humidity rises due to smaller concentration gradient of water vapour • relatively high temperatures, more transpiration as temperature rises due to faster diffusion as a result of more kinetic energy of water molecules • faster evaporation due to more latent heat available • windy conditions, more transpiration as wind speed increases as water vapour blown away from the leaf • increasing the concentration gradient of water vapour • high light intensity, more transpiration in the light due to light causing stomata to open • wider opening of stomata with brighter light hence more transpiration • CAM plants opposite, narrower stomata with high carbon dioxide concentration hence less transpiration • low air pressure, low levels of carbon dioxide

18. Adaptations of xerophytes that help to reduce transpiration • xerophytes are plants that live in dry conditions • xerophytes are adapted in the following ways to reduce water loss: • reduced leaves (spines or needle like) to reduce the surface area for transpiration • rolled leaves with stomata on the inside to prevent water loss by transpiration • sunken stomataallows layer of humidity to build up reducing water loss by evaporation • thick waxy cuticleon leaves epidermis to prevent water loss by transpiration • hairsallow water vapour to be retained • reduced stomata / stomata on under side of the leaf to prevent water loss by transpiration • special water storage tissue, • wide-spreading network of shallow rootsobtain more water • deep roots to absorb water from deep sources • vertical stems to avoid mid-day sun • reversed stomata rhythm, take in carbon dioxide at night to prevent water loss during the day

19. Role of phloem in active translocation of sugars (sucrose) &amino acids • phloem is a living tissue composed of companion cells & sieve tube membranes • companion cells involved in ATP production • assimilate products of photosynthesis, sucrose & amino acids transported in phloem • translocation is a bi-directional transport • from the source; leaves to the sinks; fruits, roots, the storage organs such as stem tubers, roots • pressure flow hypothesis;- movement of water into phloem causes transport

20. 1. Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. 2. This uptake of water generates a positive pressure that forces the sap to flow along the tube. 3. The pressure is relieved by the unloading of sugar and the consequent loss of water from the tubeat the sink. 4. In the case of leaf-to-root translocation, xylem recycles water from sink to source.

21. How glucose is transported & stored • glucose transformed to sucrose • sucrose is translocation of sugars by phloem • translocation is an active process which requires energy • it occurs from source to sink • the source is photosynthetic tissue in the leaves • sink is fruits, seeds, roots & other storage organs • sucrose is converted to starch & stored in storage organs such as roots, tubers, stem etc.

23. Revision Questions • Outline the adaptations of plant roots for absorption of mineral ions from the soil. [5] • Describe the process of mineral ion uptake into roots. [5] • Describe how water is carried by the transpiration stream. [7 ] • Explain how abiotic factors affect the rate of transpiration in a terrestrial plant. [8] • List four abiotic factors which affect the rate of transpiration in a typical mesophytic plant. [4] • Explain how wind affects the rate of transpiration from a leaf. [5] • Outline adaptations of xerophytes that help to reduce transpiration [8] • Outline the role of the phloem in the active translocation of biochemicals. [5]