Download Ch 36 Notes File

AP Biology
Mrs. Farish
Name ___________________________
Chapter 36: Transport in Plants
I.
Overview of Transport
A. Cellular level
1. Passive diffusion – solutes diffuse down concentration gradients
2. Facilitated diffusion
a. solutes can diffuse through transport proteins that change shape
b. selective channels – allow solutes to cross (exp: some allow K+
but not Na+)
3. Active transport – The Proton Pump
a. hydrolyzes ATP & uses released energy to pump H+ out of cell
1. causes a proton gradient (drives ATP synthesis –
chemiosmosis)
a. [H+] is higher outside cell than inside
b. H+ ions diffuse downhill back into cell
2. generates a membrane potential (negative inside of cell and
positive outside because H is moving outside
a. membrane potential is a voltage – separation of
opposite charges (stored energy) across a membrane
b. energy is used to do cellular work
b. K+ is driven into cell by root hairs due to proton gradient (passive
transport)
c. Co-transport – NO3- ions and sucrose enter with H+ (passive for
H+)
B. Water Potential (Ψ) – psi – measured in MPa megapascals: 1MPa = 10
atmospheres)
1. Ψ is the relative tendency for water to leave one location in favor of
another
2. due to 2 factors
a. movement of water by osmosis (solute concentration)
b. physical pressure due to the cell wall (makes it mores difficult to
predict water movement than in animal cells)
3. water will move across a membrane in the direction of lower water
potential (Ψ = Ψp+ Ψs )
a. solution with high Ψ will move to a solution with low Ψ
b. lots of solutes = low Ψ
c. increase of pressure = increase in Ψ
4. Examples:
a. Flaccid cell (Ψp = 0, Ψs = -0.7) is placed in a solution of higher
solute concentration. Which way will the water move?
Water moves out of the cell since Ψs is negative and the cell will
plasmolyze (shrink)
b. Flaccid cell (Ψp = 0, Ψs = -0.7) is placed in pure water. Which
way will the water move?
Water moves into the cell since Ψ is negative and the cell will
become turgid (turgor pressure causes cell to swell and stay firm)
C. Aquaporins
1. water specific transport proteins that create channels that effect the
RATE in which water diffuses down its water potential gradient
2. may form gated channels that open & close in response to turgor
pressure so cell can regulate rate of water uptake or loss when Ψ is
different than surroundings
D. Transport through cell compartments
1. Tonoplasts: membrane that separates the vacuole from the cytosol
a. regulates the molecular traffic between them
2. Plasmodesmata: connections from one plant cell to another
a. movement of materials through these channels is called Symplastic
3. Apoplastic: movement of materials through cell wall to cell wall
4. Transmembrane: movement of materials from one cell to another with
the materials crossing cells through the cytosol
5. Bulk Flow: for long distance transport
a. movement of a fluid driven by pressure
b. occurs through xylem and phloem quicker than by diffusion
II.
Absorption of water and minerals by roots
A. Root Hairs
1. extensions of epidermal cells that increase the surface area of the root
2. water and solutes switch pathways between apoplast and symplast which
causes selective transport of essential nutrients (exp: K+ instead of
Na+)
B. Mycorrhizae
1. symbiotic structures of fungi with the plant roots
2. supplies an abundance of water and minerals
C. Endodermis
1. innermost layer of cells in the root cortex
2. functions as the last checkpoint for the selective passage of minerals into
the vascular tissue
3. Casparian strip
a. Belt made of suberin (waxy material impervious to water)
b. Found in every endodermal cell from cell wall to cell wall
c. Only way for water and minerals to get to the stele (vascular
cylinder) is to go through the symplast
III.
Transport of Xylem Sap
A. Root pressure (upward push of xylem sap from roots)
1. accumulation of minerals in the stele lowers water potential
2. water flows in from the root cortex generating a positive pressure
forcing water up the xylem
3. can force water up a few meters
B. Transpirational pull (upward by leaves)
1. water vapor exits the stomata due to the air being drier than the air
spaces in a leaf (transpiration)
2. adhesion of the water molecules to the walls of the cells and the
cohesive force of water (surface tension) “pulls” the water up the xylem
3. thin diameter of the tracheids and vessel elements in the xylem
contribute to the adhesive force
4. water goes from roots to leaves
5. Controlling transpirational pull
a. Guard cells
1. must open to obtain CO2 and release O2 for photosynthesis
2. regulate water vapor from leaving the leaf
3. control the diameter of stomata by changing shape
4. stomata open when guard cells actively accumulate K+
causing water to enter the cells
5. in general, stomata are opened in the day and closed at
night
b. transpiration-to-photosynthesis ratio: tells how efficiently a plant
uses water
1. amount of water (g) lost per gram of CO2 assimilated by
photosynthesis
2. C3 plants (most) = 600:1; C4 plants (corn) = 300:1
3. C4 plants assimilate CO2 at a greater rate
IV.
Translocation of Phloem Sap
A. Translocation is the transport of food in a plant
B. Phloem sap is different in consistency than xylem sap (water)
1. mostly made of sucrose
2. may also contain minerals, amino acids, and hormones
C. Sieve tubes carry food from a sugar source to a sugar sink
1. sugar source is an organ that produces sugar (either by photosynthesis or
hydrolysis of starch (mature leaves are sugar sources)
2. sugar sink is an organ that consumes or stores sugar
a. exp: root tips, stems, fruit,
D. Sugar is loaded into the sieve tubes via both symplastic and apoplastic pathways
in many plant species
E. Due to accumulating sugar in higher and higher concentrations, active transport is
necessary to load phloem
1. proton pumps do the work that enables the cells to accumulate sucrose
F. Bulk flow moves sugar due to changes in water potential
1. at the source end of the sieve tube are higher concentrations, which
lowers the water potential and causes water to flow into the tube
2. at the sink end, the pressure is relieved by the loss of water, owing to
water potential being lowered outside the sieve tube by the lowering of
sucrose