Download 9.1 Transport in the Xylem of Plants

9.1 TRANSPORT IN THE
XYLEM OF PLANTS
Plant Leaves
• Absorb radiant energy
• Optimize the rate of photosynthesis
• Cross Section of a leaf
Leaf Part
Leaf Function
Waxy Cuticle
Water conservation
Stomata
Opening in leaf/gas exchange
Guard cells
Control stomata opening
Vascular Tissue
Xylem: transports water and minerals
from the roots to the shoot by passive
transport
Phloem: transports nutrients
throughout the plant by active
transport
Collenchyma cells
Chloroplasts (mainly in the mesophyll,
spongy and pallisade layers)
Support vascular bundles (xylem and
phloem) and give leaf structure
Carry out photosynthesis
Spongy layer: filled with air spaces to
help facilitate gas exchange
Pallisade layer: packed with
chloroplasts
Transpiration
• The passive movement and evaporation of water from the
stems and leaves of a plant
• Made of two types of cells
• Tracheid cells
• Vessel elements
• Pits in the sides of the cells allow water to flow
between the cells of the xylem
Cohesive & Adhesive Properties
• Cohesive properties: (hydrogen bonding) hold water
molecules together as they travel up the xylem
• Evaporation creates a “pull” causing them to move up
xylem
• Water is also entering the plant causing a “push” of
water
• Adhesive properties: water is attracted to the sides of
they xylem cells
• Water molecules adhere to sides of vessels and resist
gravitation pull downward
Stomata and Guard Cells
• opening and closing of stomata controlled by guard cells
1. stomata open and close due to changes in turgor
pressure
2. gain and loss of water in the guard cells is largely
due to the transport of potassium ions
3. A hormone abscicis acid causes potassium ions to
rapidly diffuse across guard cells, causing them to
close (due to lack of available water)
4. other factors like carbon dioxide levels and
circadian rhythms affect opening and closing
Stomata & Guard cells
Abiotic Factors and Transpiration Rates
Environmental Factor
Effect
Light
Speeds up transpiration by warming
leaf and opening stomata
Humidity
Decreasing humidity increases
transpiration because of the greater
difference in water concentration
Wind
Increases the rate of transpiration
because humid air near the stomata is
carried away
Temperature
Increasing temperature causes greater
transpiration because more water
evaporates
Soil water
If the intake of water at the roots
doesn’t keep up with transpiration ,
turgor loss occurs causing the stomata
to close decreasing transpiration
Carbon Dioxide
High CO2 levels in the air around the
plant usually causes guard cells to
lose turgor and stomata close
Root System and Water Uptake
Water and mineral uptake
• Occurs in the roots which have adapted in several
ways:
1. roots are covered in tiny hairs that increase the
surface area for greater water absorption
2. Branching allows roots to cover more area and
increase uptake
3. Cortex cell walls are permeable for osmosis
Uptake of Mineral Ions
1. Diffusion of mineral ions
a. Requires concentration gradient (not common
because ions are usually in low conc. In the soil)
2. Mutualistic relationships with fungal hyphae
a. Fungi have long multinucleated cells called
hyphae
b. Form dense networks called mycelium that grow
around roots
c. Plant root provides fungi with nitrates in exchange
for needed minerals
3. Mass flow of water in the soil carrying ions
Active Transport of Minerals
Mineral ions are taken into the plant by active transport
• a. root hairs secrete hydrogen ions into soil particles,
exchanging them for mineral ions (calcium,
magnesium, iron etc.)
• b. ATP is hydrolyzed in order to provide the energy
for this process
Adaptions of Xerophytes
• Xerophytes live in arid (dry) environments
• They have developed adaptations to help with
transpiration
• Reduced leaves sometimes spines
• Rolled leaves: block the wind from reaching stomata
• Deep roots for more surface area
• Thickened waxy cuticle
• Reduced number of stomata
• Hairs to help trap water
• Water storage organ
• Low growth
• CAM and C4 plants: fix CO2 at very low concentrations, stomata
open less frequently