3.3.5 the transpiration stream

Question 1

adhesion

Answer 1

attraction between water molecules & walls of xylem

Question 2

cohesion

Answer 2

attraction between water molecules caused by hydrogen bonds

Question 3

transpiration stream

Answer 3

movement of water from soil, through the plant, to the air surrounding the leaves

Question 4

main driving force of transpiration stream

Answer 4

water potential gradient between soil & air in leaf spaces

Question 5

what does the outermost layer of cells (endodermis) of a root contain

Answer 5

root hair cells

Question 6

root hair cells

Answer 6

cells with long extension which increases surface area of root

Question 7

what do root hair cells absorb

Answer 7

mineral ions & water from soil

Question 8

steps of water uptake & movement across root (via the root hair cell)

Answer 8

1. mineral ions actively absorbed from soil making water potential of cytoplasm more negative
2. water enters root hair cell by osmosis
3. water moves across root cortex (down water potential gradient) via osmosis & apoplast pathway to endodermis of vascular bundle
4. water must enter medulla/xylem by symplast pathway as apoplast pathway blocked by casparian strip
5. mineral ions actively transported into medulla making water potential more negative so water follows via osmosis

Question 9

what’s the endodermis in root hair cells

Answer 9

layer of cells surrounding medulla & xylem
aka starch sheath (contains granules of starch)

Question 10

role of endodermis in root

Answer 10

• movement of water across root driven by active process occurring at endodermis
• casparian strip blocks apoplast pathway between cortex & medulla
• ensures water & dissolved mineral ions (esp. nitrates) have to pass into cell cytoplasm via membranes
• plasma membranes contain transporter proteins which actively pump mineral ions from cytoplasm of cortex cells into medulla/xylem
• makes water potential of medulla/xylem more negative = water moves from cortex cells into via osmosis
• once water entered medulla, cannot pass back into cortex, as apoplast pathway of endodermal cell is blocked by casparian strip

Question 11

how does water move up stem (xylem)

Answer 11

mass flow

Question 12

name the 3 processes which help move water up the stem

Answer 12

• root pressure
• transpiration pull
• capillary action

Question 13

how does root pressure help move water up the stem

Answer 13

• action of endodermis moving minerals into medulla/xylem by active transport draws water into medulla by osmosis
• pressure in root medulla builds up & forces water into xylem = pushes water up xylem
• can push water few metres up stem
  BUT cannot account for water getting to top of tall trees

Question 14

how does transpiration pull help move water up the stem

Answer 14

• loss of water via evaporation must be replaced
• water molecules attracted to each other via cohesion
• strong enough to hold all water molecules in one column
• as one lost from top, whole chain pulled up & replaces
• cohesion-tension theory = pull from above creates tension in column of water
• xylem vessels strengthened by lignin to stop collapse
• plant must maintain unbroken column up xylem but if broken in 1 vessel, can be maintained in another vessel via bordered pits

Question 15

how does capillary action help move water up the stem

Answer 15

• same forces which holds water molecules together attract water molecules to side of xylem = adhesion
• pull water up sides of xylem vessel

Question 16

steps of transpiration stream

Answer 16

1. minerals actively transported into xylem which lowers water potential in xylem & water follows by osmosis
2. cohesion/adhesion of water molecules enables water to move by mass flow, pulled up via tension
3. movement of water out of xylem creates low hydrostatic pressure & thus, tension
4. osmosis moves water across leaf
5. evaporation of water from cell surface
6. diffusion of water vapour out of leaf

Question 17

how does water leave the leaf

Answer 17

• most exits as vapour through stomata
  –> evaporates from cells lining cavity directly above guard cells (sub-stomatal air space) = lowers water potential in cells so water enters by osmosis from neighbouring cells
  –> thus, water drawn from xylem into leaf by osmosis
  –> water may also reach cells by apoplast pathway from xylem
• tiny amount leaves through waxy cuticle