8-2: How Plants Feed

Question 1

What nutrition do plants need?

Answer 1

• CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur)
• carbon via CO2 thru leaves
• H via H2O thru roots
• N,P, S thru roots

Question 2

Transport Within Plant

Answer 2

• roots absorb water and minerals
• water and minerals transported up from roots to shoots as xylem sap
• water is lost thru stomata = transpiration, main force pulling xylem sap upwards
• stomata let CO2 in, O2 out
• when enough sunlight, sugars produced in leaves
• sugars translocated as phloem sap to roots and other parts
• roots take in O2 and release CO2 to air spaces in soil

Question 3

Water Potential

Answer 3

• indicates potential energy that water has in an environment vs in standard pressure and temperature
• water flows from high water potential to low water potential
• measured in megapascals

Question 4

Water Potential Application In Plants

Answer 4

-Atmosphere Y is -95.2 MPa;
changes with humidity, but
generally remains very low.
-Leaf y is -0.8 MPa; depends on
transpiration rate, and is low when stomata are open. The higher y in leaves than in the atmosphere, allows water to leave the leaves and move
into the atmosphere.
-Root Y is -0.6 MPa; considered
medium high
-Soil y is -0.3 MPa; when moist, this value is high. When extremely dry this value is low.

Question 5

Capillarity

Answer 5

• movement of water up a narrow tube
• occurs thru a combination of 3 forces:
  1) Surface Tension
  2) Cohesion
  3) Adhesion
• as water molecules cohere to each other adhere to side of capillary tube, they’re pulled upwards

Question 6

Surface Tensions

Answer 6

• pull that exists on water molecules at an air-water

interface. This results in formation of meniscus a concave boundary layer

Question 7

Cohesion

Answer 7

• mutual attraction among like molecules

Question 8

Adhesion

Answer 8

-is the attraction of unlike molecules

Question 9

Lateral Transport In Roots

Answer 9

-apoplastic route
-symplastic route
-transmembrane route
-endodermis
-transport in xylem
-at level of endodermis, casparian strip blocks apoplastic route and only minerals in symplastic or transmembrane routes may go thru
-Endodermal and parenchyma cells take water and minerals in. The xylem vessels then
transport water and minerals upward into the shoot.
-Endodermis with the Casparian strip is found only in the roots

Question 10

Apoplastic route

Answer 10

• uses only the cell walls to travel through the root, and does not cross the semipermeable plasma membranes
• opposite of symplastic route

Question 11

Symplastic route

Answer 11

-travels between cells via
plasmodesmata and does not travel within the cell walls
-opposite of apoplastic route

Question 12

Transmembrane route

Answer 12

• uses both, cell walls and plasma membrane to travel through the root cells in the epidermal and cortex cells

Question 13

Functions of Casparian Strip

Answer 13

1) to ensure no minerals reach vascular tissue of root without crossing selectively permeable plasma membrane (endodermal cells act as filters)
2) prevent solutes that have been accumulated i n xylem sap from leaking back into soil (endodermal cells act as a barrier)

Question 14

Availability of Soil Water

Answer 14

• water held tightly by hydrophilic soil particles is not available to a plant
• roots can only absorb water that is less tightly bound to soil

Question 15

Availability of Minerals

Answer 15

• elements required for plant growth behave depending on their charge:
  1) Anions (negatively charged) usually dissolve in soil water, and are readily available to plants for absorption.
  2) Cations (positively charged) are usually much less readily available than anions, because they interact with soil particles.
• most plants grow in neutral pH (6-7)
• plants facilitate cation exchange when they need cations

Question 16

Cation Exchange

Answer 16

-During cation exchange,
roots exchange H+ ions for
other cations found in soil
-Plants contribute H+ directly by
secretion from the root hair
-Plants contribute H+ ions indirectly, by cellular respiration,
which releases CO2 into the soil
-In the soil, CO2 reacts with water forming carbonic acid, which dissociates adding more H+ available for cation exchange. These hydrogen ions displace cations previously attached to negatively charged soil particles, so that the cations are available for uptake by the root.

Question 17

Xylem Transport

Answer 17

-Xylem transports inorganic material dissolved in water, and it moves unidirectionally: from roots to the top of the shoots.
-Water moves up a plant by:
1. Root pressure
2. Capillary action (cohesion and adhesion)
- long-distance water movement
in plants is the cohesion-tension theory –> water is pulled from
roots to tops of trees by transpirational pull at leaf surfaces (transpiration creates tension that is transmitted from leaves to roots).

Question 18

Water Movement via Root Pressure

Answer 18

-root pressure = pushing force but can drive the xylem sap up for only a few meters
-occurs at night when evap is low and when root cells continue pumping minerals into the xylem of the vascular cylinder
-The root cells accumulate minerals in the xylem, which lowers the water potential in the root => water flows in, generating root pressure
-Root pressure is not strong enough
to push the water over long
distances, such as in a big tree
-Guttation: the exudation of water droplets seen in the morning on the leaf edges; particularly true of
the low-growing plants

Question 19

Water Movement via Capillarity

Answer 19

-Surface tension creates a pull at the surface; adhesion (between water molecules and xylem cells) resists the download pull of cohesion; cohesion (between water molecules) transmits both
forces to the water bellow.
-The result is capillarity.
- It is all possible because the xylem vessels are very narrow.

Question 20

Water Movement via Transpirational Pull (cohesion-tension)

Answer 20

1) Water diffuses down its
potential gradient (the water
potential in the atmosphere is even lower than in the leaf).
2) Water evaporates from the
menisci that exist at the air water interphases.
3) Water is pulled from
the surrounding mesophyll
cells, and further out of
the xylem.
4) Tension is further
transmitted from water in
leaf xylem through stem all
the way to root xylem by
cohesion (continuous
hydrogen bonding).
5) Tension pulls water from root cortex cells into root xylem.
6) Tension pulls water from soil into roots.

Question 21

Quick Summary

Answer 21

-The strongest force that moves the xylem sap is the transpirational pull.
-There must be an unbroken thread of water all the way from
leaves to roots for the transpirational pull to occur. This is performed by capillary action.

Question 22

Pressure Flow Hypothesis (“source” to “sink”)

Answer 22

1) Sugar is actively transported into the phloem sieve tube.
2) Water entering the sieve tube increases the pressure of the phloem sap.
3) Sugar-rich sap travels by bulk flow through the phloem sieve tubes into regions of lower
pressure.
4) Sugar is transported into
the sink, and water is drawn back into the xylem.

Question 23

Symbiotic Relationships in Roots

Answer 23

• nitrogen fixation: process of converting atmospheric nitrogen to ammonia by nitrogen-fixing bacteria
• Plants cannot use atmospheric nitrogen as is, but ammonia is further converted into ammonium and nitrate ions, which plants can use
• Mycorrhizae (“fungus root”) are relationships between plant roots and fungi. Plants receive large amounts of nitrogen from these relationships.

Question 24

Symbiosis with Bacteria

Answer 24

-Bacteria benefit by finding a secure environment on the plant’s root, and by obtaining carbon from the host plant.
-Plant benefit by obtaining nitrogen for synthesizing proteins and other organic molecules.
Step 1: Roots emit chemical signals called flavonoids. Flavonoids attract rhizobia. Rhizobia are the type of bacteria capable of nitrogen fixation. When rhizobia contact the flavonoids, they respond by producing nod factors, which
bind to proteins on the root hair membranes. This interaction is species specific (each plant species produces specific flavonoid, and each rhizobium responds specifically).
• Step 2: Once rhizobia get inside the root hair, the bacteria emit signals that stimulate root hairs to elongate and to form an infection thread. The infection thread refers to the invagination of the root hair.
• Step 3: The bacteria penetrate the cortex within the infection thread.
• Step 4: Bacteria inside the infection thread divide rapidly. Also the cortex cells divide rapidly, and together
they form a nodule.
• Step 5: The nodule develops its own vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant.

Question 25

Symbiosis with Fungi (mycorrhizae)

Answer 25

-The benefits for plants are:
• They receive large amounts of nitrogen (in northern forests) or
phosphorus (in tropical forests and grasslands) from fungi.
• The area for water uptake is greatly increased.
• Fungi secrete growth factors that stimulate root growth.
• Fungi protect the plant from pathogenic bacteria from the soil.
• The benefit for fungi is:
• They have a steady supply of sugar

Question 26

Carnivorous Plants

Answer 26

• (live in acid bogs on soil poor in nitrogen; require the nutrients from insects; still perform photosynthesis). Examples: venus fly trap,
  pitcher plant, sundews. These plants have different tactics of forming traps.

Question 27

Parasitic Plants

Answer 27

-(grow on host trees; penetrate xylem of the host forming haustorium=bridge; some still perform photosynthesis). Examples: mistletoe, dodder (strangleweed – it sucks both organic and inorganic material from its host) and Indian pipe (aka ghost plant) (a parasite on mycorrhizal fungi).

Question 28

Epiphytes

Answer 28

-(anchor to trees using those for support, but are NOT parasites; they absorb water and minerals from rain through their leaves). Examples: many orchids, staghorn ferns, bromeliads (pineapple) – these are usually found in tropical forests because they require high humidity.