Objectives 9.2.10 & 9.2.11

Asia BowmanShantel Butler

William MiddletonDestinee MiguestShanese Stuckey

IB Biochemical Biology P.2

9.2.10 Outline four adaptations of xerophytes that

help to reduce transpiration.

What are Xerophytes?! Plants that are adapted to arid climates. Xerophytes have evolutionary adaptations

that reduce transpiration These plants have various leaf modifications

that reduce the rate of transpiration

Adaptations:

1. Xerophytes have small, thick leaves This limits water loss by reducing surface area

relative to leaf volume.

2. Stomata are recessed in “crypts” protects the stomata from hot, dry wind

3. Trichomes Breaks up the flow of air, allowing the chamber

of the crypt to have a higher humidity than the surrounding atmosphere.

Adaptations:4. One of the most elegant adaptations to arid

habitats is found in ice plants and other succulent plants of the family Crassulaceae and in representatives of many other plant families.

CO2 is assimilated by an alternative photosynthetic pathway known as CAM, for crassulacean acid metabolism.

Because the leaf takes in its CO2 at night, the stomata can close during the day, when transpiration is most severe.

9.2.11 Outline the roles of phloem in active

translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

Translocation of Phloem Sap Xylem sap flows in a direction that generally does not

allow it to function in exporting sugar from leaves to other parts of the plant… on the other hand

The prevalent solute in phloem sap is sugar, primarily the disaccharide sucrose in most plant species.

The phloem transports the organic products of photosynthesis throughout the plant.

Phloem sap may also contain minerals, amino acids, and hormones in transit from one part of the plant to another.

Phloem

Translocation: the transport via phloem of food in a plant.

sugar source: a plant organ in which sugar is being produced by either photosynthesis or the break down of starch.

sugar sink: an organ that is a net consumer or storer of sugar.

Examples: Mature leaves = primary sugar sources Sugar sink supplied by phloem = growing roots, shoot tips,

stems, and fruit

Phloem Storage organ depends on the season. Example:

Sugar sink during the summer In the spring the storage organ becomes a source as its

starch is broken down to sugar which is carried away in the phloem to the growing buds of the shoot system.

Other solutes may be transported to sinks along with sugar.

Example: Minerals that reach leaves in xylem may later be transferred in the phloem to developing fruit.

Loading of sucrose into phloem

Loading Sugar from the mesophyll cells of a leaf and other sources must

be loaded into sieve-tube members before it can be exported to sugar sinks. From mesophyll cells to sieve-tube members through symplast,

passing from cell to cell through plasmodesmata Sucrose reaches sieve-tube members through a mixture of

symplastic and apoplastic paths Sample path:

Sucrose diffuses through cytoplasm (symplast) from mesophyll cells into small veins

Much of the sugar moves from the cells into the walls (apoplast) of the companion cells near sieve-tube members Sieve-tube member- a chain of living cells that form sieve-tubes in

phloem

Companion cells pass the sugar they accumulate into the sieve-tube member through plasmodesmata linking the cells Plasmodesmata- an open channel in the cell wall of plants

through which cytosol connect adjacent cells Transfer cells- companion cells that have been modified

with numerous ingrowths on their walls to increase the cells’ surface area and enhance the transfer of solutes between apoplast and symplast

Sieve-tube members accumulate sucrose to concentrations two to three times higher than concentrations in mesophyll Phloem loading requires active transport

Unloading At the sink end of a sieve tube, phloem

unloads the sucrose The concentration of free sugar in the sink is

lower than that in the sieve tube because the unloaded sugar is consumed during growth of the sink cells or is converted to starch

TRIVIA!!!1. Imagine cutting a live twig from a tree and

examining the cut surface of the twig with magnifying glass. You locate the vascular tissue and observe a growing droplet of fluid exuding from the cut surface. The fluid is most likely:

a. Phloem sapb. Xylem sapc. Guttation fluidd. Fluid of the transportation stream

Answer: A . Phloem sap

TRIVIA!!!2. The movement of sap from a sugar source

to a sugar sinka. occurs through the apoplast of sieve-tube

members.b. May translocate sugars from the breakdown of

stored starch in a root up to developing shootsc. Is similar to the flow of xylem sap in depending on

tension, or negative pressure.d. Depends on the active pumping of water into

sieve tubes at the source end

Answer B: May translocate sugars from the breakdown of stored starch in a root up to

developing shoots

TRIVIA!!!

3. What cells mediate the photosynthesis-transpiration compromise? Answer: guard cells

TRIVIA!!!

4. ______________ have evolutionary adaptations that reduce transpiration.

Answer: xerophytes

TRIVIA!!!

5. The prevalent solute in phloem sap is _______, primarily the disaccharide sucrose in most plant species.

Answer: sugar