Transport In Plants PDF

TRANSPORT IN PLANTS 1 6.1 Transport in the plants Learning outcomes: To relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions To describe how transpiration can occur in the plant. To explain how leaves of xerophytic...

TRANSPORT IN PLANTS 1 6.1 Transport in the plants Learning outcomes: To relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions To describe how transpiration can occur in the plant. To explain how leaves of xerophytic plants are adapted to reduce water loss by transpiration 2  STRUCTURE OF XYLEM Xylem is a mixed tissue with different types of cells (xylem vessels, fibres, parenchyma cells, tracheids) Xylem vessels (found in leaves, stem & root)  Continuous- an unbroken column of water is possible due to cohesion  Narrow- capillarity increases adhesion  Lignified- lignin provides strength & makes xylem waterproof, so water does not seep out  Pits- these are holes in the lignin through which water can pass laterally  no living contents- water flow is not restricted, the lumen is empty 3 4 TYPES OF LIGNIFICATION IN XYLEM VESSELS In young stems, lignin is deposited in rings & spiral flexible and further elongation of stems are possible Older stem, lignin forms an complete layer more rigid preventing further growth 5 TRANSVERSE SECTION OF LEAF, ROOT & STEM OF DICOT Stem Root 6 Leaf STRUCTURE OF PHLOEM Phloem is a mixed tissue containing many types of cell; living tissue (sieve tube elements, companion cells, fibres, parenchyma cells) 7 Phloem sieve tube elements Elongated, with pores at each end in the cross walls allowing longitudinal flow of materials These pores+ polysaccharice callose sieve plate The sieve tube cells have cytoplasm with a few organelles (plastids, SER, mitochondria but no nucleus) pushed to the edge Most of them do not contain cytoplasmic organelles and is called lumen If damaged, callose quickly seals & blocks sieve plates prevent loss of valuable solutes 8 9 Companion cells Found next to sieve tube cells Cytoplasm dense in companion cells, contains large nucleus, numerous mitochondria (metabolically active) & RER (protein synthesized & transported) Plasmodesmata allow communication between companion cells and phloem sieve tube cells 10  TRANSPIRATIO N Transpiration: loss of water in the form of vapor from plants through leaves Water is lost from 3 main areas of plant: a) Stomata- mainly on lower surface of leaves, lose ~90% water as vapor b) Cuticle- small amount of water lost through waxy layer by diffusion; thicker the cuticle, less water lost c) Lenticels- woody stems have loosely packed cork cells on the outer surface through which gas exchange occurs Transpiration In Plants.flv 11 Spongy mesophyll (gas exchange surface in plants); thin, moist & large surface area for diffusion of gases As a result of transpiration, water moves through the plant from roots to leaves & lost into air as water vapour (transpiration stream) 12  TRANSPIRATION MECHANISM 1. Stoma opens. Water evaporates from the surface of mesophyll cells surrounding the sub-stomatal air space & accumulate in air chambers as water vapor. 2. Water diffuse from adjacent mesophyll cells into mesophyll cells surrounding sub-stomatal air space 3. Subsequently, water potential gradient is established from xylem vessel to sub-stomatal air spaces; water flow continuously along this potential gradient 13 4. Accumulation of water vapour causes the [water vapour] in sub-stoma air chamber> outside the leaf; water vapour diffuses out. 5. The steeper the diffusion gradient of water vapour between stoma and surroundings, the faster the rate of transpiration 14 15 EXTERNAL FACTORS AFFECTING TRANSPIRATION RATE Air movements. Windy surroundings, water vapor outside stoma swept away quickly Humidity. Low humidity of surrounding, step diffusion gradient  increase transpiration rate Temperature. High temperature, facilitate water evaporation. Lower relative humidity outside leaf, steeper gradient Water supply. Drier soil, less water root will absorb. Water lost through transpiration reduce, increase survival in dry Light. During day, transpiration rate high as stomas open 16 INTERNAL FACTORS AFFECTING TRANSPIRATION Leaf surface area. Flat and large leaves, larger surface area, > stomata exposed, increase rate of transpiration Number and distribution of stomata. Most stomata are found at lower leaf surface, do not face sun and wind; reduce transpiration, yet allow efficient gas exchange Thickness of cuticle. Thinner the layer of cuticles on leaf and stem, higher the transpiration rate 17  FUNCTIONS OF TRANSPIRATION Cooling the plant -exposing photosynthesis structure to sunlight increases temperature of plant -higher rate of transpiration, higher cooling rate -vapour evaporation from mesophyll cells uses latent heat cooling effect Transporting water to shoots and leaves of the plant Absorption and translocation of minerals in plants 18 XEROPHYTES Adapted to live in dry conditions (low rainfall/ excessive wind) To cope with dry conditions, xerophytes have: a) Specialized leaves- reduce water loss (sunken stomata, curled leaves) b) Extensive root system- to increase water uptake c) Swollen stem- to store water d) Thick waxy cuticle 19 FEATURES OF XEROPHYTES Features Functions Smaller leaves Smaller surface area for water to be lost from (needle-shaped) Densely-packed Less surface area exposed to air spaces inside the leaf, spongy mesophyll less water evaporates into these air spaces layer Thicker waxy cuticle Impermeable to water, control evaporation Trichomes Trap humid air, keep water vapour potential low Rolled leaves Maintain humid air around stomata Extensive roots Maximises uptake of water 20 21 22 Why this leaf section belongs to xerophytes? 23 6.2 Transport mechanisms  To describe the pathways and explain the mechanisms by which water and mineral ions are transported from soil to xylem and from roots to leaves (include reference to the symplastic pathway and apoplastic pathway and Casparian strip) To state that assimilates, such as sucrose and amino acids, move between sources and sinks in phloem sieve tubes To explain how sucrose is loaded into phloem sieve tubes by companion cells using proton pumping and the co-transporter mechanism in their cell surface membranes To explain mass flow in phloem sap down a hydrostatic pressure gradient from source to sink 24 WATER UPTAKE BY ROOTS Plants need to replace the water lost in transpiration & provide water for photosynthesis, turgidity and other metabolic processes. Water passes from the soil water which has higher water potential into the root hair cells (increases surface area for minerals & water absorption) which have lower water potential Once in root hair cells, water passes across parenchyma cells making up the cortex, to xylem Water transported to all parts of plant in xylem vessel As water moves through root, more water is drawn in, to form continuous stream 25 26 PATHWAYS OF WATER UPTAKES Water can pass through root in 3 ways by osmosis: I. Through cell wall (apoplast pathway) II. Through cytoplasm (symplast pathway) III. Through vacuole (vacuolar pathway) 27 I. APOPLAST PATHWAY Apoplast is the intercellular space + spaces in the cell wall of plants In apoplast pathway, water moves through the apoplast from root hairs (soil) to xylem vessels of the root Water moves in apoplast by diffusion/transpiration flow until it reaches endodermis Water movement stopped by casparian strips (water proof barrier made of suberin); diverted into cytoplasm Along symplast pathway, water reaches xylem vessel 28 Water encounter the least resistance in apoplast pathway 90% of water taken in moves across root by this pathway If plant are treated with chemicals that causes precipitation in apoplast, transpiration flow blocked apoplast is the main pathway for water transfer across the root 29 II. SYMPLAST PATHWAY Cytoplasm of neighboring plant cells is linked by plasmodesmata (symplast) Water flow down the water potential gradient that exists across the roots through a partially permeable membrane Water continue to pass along cytoplasm of parenchyma cells by osmosis until xylem vessel is reached 30 III. VACUOLAR PATHWAY Water moves from vacuole of 1 cell into vacuole of another cell (via plasmodesmata) because of water potential that exists between the cells Salt is actively pumped into the xylem vessels from the endodermis; lower water potential in xylem vessel This draw in water from endodermis & pericycle, creating high pressure of water in xylem causes root pressure, pushed water up xylem More water is now attracted from parenchyma cells into endodermis & pericycle Cohesion forces maintain a steady stream of water through root hair 31  Transport of water in xylem Cohesion-tension theory Sap ascent in xylem is caused by transpiration in leaves; transpiration pull i. Xylem vessel of all plants have continuous column of water ii. Water molecules stick together by cohesion (water column hard to break; water molecules also stick to vessel walls by adhesion; narrow vessel produces strong adhesion force iii. Water lost from leaf during transpiration lower ѱ of mesophyll cells 32 iv. Difference in ѱ cause water from xylem sap enter mesophyll cell by osmosis v. This create tension in water column of xylem vessel in leaves, tension created is transferred along water column in xylem vessel until it reaches roots vi. Powerful force (transpiration pull) pulls whole water column up xylem vessel vii. Water from nearby root cells enters xylem vessel in the roots; followed by entering of water from soil 33  Concept check 1. What is transpiration? Name 3 environment factors which will increase the rate of transpiration. 2. What evidence is there that root hairs take in ions by active transport? 3. In summer, the diameter of a branch is smaller at mid-day than at mid-night. Explain this observation. 4. Land plants have most stomata on the lower leaf surface. Floating aquatic plants have many stomata on the upper surface of their leaves. Why is this not a problem for them? Suggest some advantages of this arrangement. 34