Biology 102
Fall 2001
R. Brundage
Lecture 7: Part 2
Plant Nutrition and Transport
.I.Soil and Its Nutrients
A.Properties of Soil
1.Soil consists of particles of minerals mixed with humus (dead organisms and their
litter).
a.Particles come in three sizes: sand, silt, and clay.
b.Clay, the smallest, holds onto nutrients as water percolates through the
soil.
c.Plants do best in loams, soils with nearly equal proportions of the three
particle types.
2.Layers of soil can be classified by profile properties; topsoil, the uppermost, is the
most essential layer for plant growth.
B.Nutrients Essential for Plant Growth
1.Nutrients are elements needed for growth and survival.
a.The "big three" elements are oxygen, hydrogen, and carbon.
b.Thirteen others are taken in water through the roots.
2.Six elements have deemed macronutrients (e.g. N, K, Ca); seven others are
micronutrients (e.g. Cl, Fe, Zn).
C.Leaching and Erosion
1.Leaching refers to the removal of some of the nutrients in soil as water percolates
through it.
2.Erosion is the movement of land under the force of wind, running water, and ice.
II.Absorption of Water and Mineral Ions into Roots
A.Absorption Routes
1.Water moves from the soil across the root epidermis to the vascular cylinder, a
column of vascular tissue in the center of the root.
a.A sheetlike layer of cells, the endodermis, surrounds the column.
b.The water-repellent Casparian strip forces water to move through the
cytoplasm of the cells of the endodermis.
c.Therefore, membrane transport proteins help control the types of
absorbed solutes that will become distributed throughout the plant.
2.Most flowering plants also have an exodermis, a layer of cells just inside the roots
which also has a Casparian strip that functions just like the one next to the root
vascular cylinder.
B.Specialized Absorptive Structures
1.Root hairs, extensions of the root epidermal cells, greatly increase the absorptive
surface.
2.Bacteria residing in root nodules of legumes convert gaseous nitrogen to forms
useful in the growth of the plants; this is a form of mutualism.
3.Mycorrhizae (fungi growing around plant roots) aid in absorbing minerals that are
supplied to the plant in exchange for sugars, a symbiotic relationship that is
beneficial to both.
III.A Theory of Water Transport Through Plants
A.Transpiration Defined
1.Water moves from roots to stems and then to leaves.
2.Some water is used for growth and metabolism, but most evaporates into the air
by transpiration.
B.Cohesion-Tension Theory of Water Transport
1.Water moves through pipelines called xylem, composed of cells (dead at maturity)
called tracheids and vessel members.
2.The cohesion-tension theory of water transport explains water movement in
plants:
a.The drying power of air causes transpiration, which puts the water in the
xylem in a state of tension leading from leaves to stems to roots.
b.Unbroken, fluid columns of water show cohesion (aided by the hydrogen
bonds); they resist rupturing as they are pulled upward under tension.
c.As long as water molecules escape from the plant, molecules are pulled
up to replace them.
IV.Conservation of Water in Stems and Leaves
A.Of the water that moves through the leaf, 90 percent is lost by transpiration; only about 2
percent is used during photosynthesis.
B.The Water-Conserving Cuticle
1.The cuticle is a translucent, water-impermeable layer secreted from epidermal
cells.
2.It coats the outer walls, which are exposed to air.
a.Waxes are embedded in a matrix of cutin, a lipid polymer.
b.The cuticle does not bar the entry of light rays, but does restrict water
loss, the inward diffusion of CO2 and the outward diffusion of oxygen.
C.Controlled Water Loss at Stomata
1.Stomata regulate the passage of water, carbon dioxide, and oxygen.
2.A pair of guard cells defines each opening.
a.In sunlight, a drop in carbon dioxide levels in the guard cells causes
potassium and water to move into the guard cells causing them to swell
(turgor pressure); this creates an opening for carbon dioxide entry (a
benefit) and water loss (a detriment).
b.At night, potassium and water move out and the guard cells collapse to
close the gap and conserve water.
3.In CAM plants (cacti, for example), the stomata open at night when cells of these
plants fix carbon dioxide; the stomata close during the day to conserve water in the
arid habitats where these plants live.
V.Distribution of Organic Compounds Through the Plant
A.Phloem distributes organic products of photosynthesis throughout the plant.
1.Sieve tube cells are alive at maturity and are interconnected from leaf to root.
2.Companion cells also participate in a supportive role
3.Storage forms of organic molecules (examples: starch, fats, proteins) are
unsuitable for transport throughout the plant body.
4.They are therefore converted to more soluble forms, such as sucrose.
B.Translocation
1.The term translocation is most often used to signify the transport of sucrose and
other compounds through phloem.
2.Observations of aphids feeding show that sugars inside the sieve tubes are being
moved under pressure.
C.Pressure Flow Theory
1.Movement of molecules through phloem is from sources (mostly leaves) to sinks
(flowers and fruits).
2.According to the pressure flow theory, translocation depends on pressure
gradients.
a.Solutes are loaded by active transport into the phloem from a source;
water follows.
b.As pressure builds in the tubes it pushes the sucrose-laden fluid out of the
leaf, into the stem, and on to the sink.