Plant organs are composed of 3 tissue systems: dermal, vascular and ground

3 Basic Plant Tissues

1.      Dermal Tissue or epidermis

a.       Serves a protective layer

b.      Outer covering is called the epidermis

c.       Usually one cell thick, covered by cuticle

2.      Ground Tissue

 

a.       In dicots, divided into 2 parts

1.      Pith- internal to the vascular tissue

2.      Cortex-external to the vascular tissue

b.      Performs photosynthesis

c.       Stores water and carbohydrates

d.      support

3.      Vascular tissue system

a.       Conducts water, mineral nutrients and carbohydrates produced during photosynthesis

Xylem and phloem

·        True vascular tissues

§         Transport and storage of water and nutrients

§         Xylem

·         tissues that conduct water and mineral upward from plant root…

·        Large, hollow, hard walled tubes, can withstand considerable water pressure (Turgid)           

·         Two types (dead at functional maturity)

·        when the living interior of a tracheid or vessel elements disintegrates, the cell’s cell wall remains, forming a nonliving condiut

§         Tracheids- long and thin

1.      contain lignin (strong), assits in cell support

§         Vessel elements-short and thick

1.      both form in parts of the plant that are no longer elongating

1.      both have pits, or thinner regions that allow for the movement of material

 

Transport of xylem sap

• flows upward to veins that branch throughout each leaf
• plants lose an astonishing amount of water by transpiration (maple tree 200L/hr)
• Root Pressure- expenditure of energy to pump mineral ions into the xylem.
• Accumulation of minerals in the stele lowers water potential
• Water moves in forcing water/minerals up against gravity called root pressure
• Guttation- the exudation of water droplets (small herbaceous plants)
• Also due to the fact that during the night transpiration rates are low
• Root pressure can only force water up a few meters….there is more to what is moving water against gravity

 

• Pulling xylem sap: The transpiration-cohesion-tension mechanism
• Transpiration provides the pull
• Cohesion of water due to hydrogen bonding transmit the upward pull along the entire length of the xylem to the roots
• Transpirational Pull:
• Most days the air around the leaf has a lower water potential. Water vapor in the air spaces of the leaves will move out via the stomata…this movement of water vapor is called transpiration
• Creates a NEGATIVE pressure or TENSION…this pull accounts for water being pulled up the xylem
• Two factors that are assisting transpiration
• Cohesion of water molecules…evaporating water pulls on adjacent water
• Adhesions; attraction of water to the hydrophilic walls of the xylem
• Negative pressure will cause a tube to collapse…keep in the mind that rings of xylem prevent this from happening
• Positive pressure will cause a tube to expand..cell walls prevent over expansion
•  
• Bulk Flow- the movement of a fluid driven by pressure….useful over long distance movement of a fluid
• Remember: gradients of water potential drive the osmotic movement of water from to cell to cell within root and leaf tissue
• In contrast, bulk flow, the mechanism for long-distance transport up xylem vessels depends ONLY on pressure
• Plant expends none of its own metabolic energy to lift xylem sap up to the leaves by bulk flow

 

Control of Transpiration

 

• Stomata….guard cells
• In the leaf, the irregular shape of the spongy parenchyma increases the surface area that is exposed to more carbon dioxide BUT it also increases the surface area for evaporation
• Transpiration-to-photosynthesis ratio….amount of water used for each molecule carbon dioxide assimilated
• 600:1 in C3 plants  600 water to 1 carbon
• C4 300:1 or less
•  
• Advantages to transpiration
• Supplies air space with water
• Brings minerals to the leaves
• Also results in evaporative cooling, which can lower the temperature of leaf
• Prevents leaves from reaching a temperature that will denature the enzymes
•  
• Guard cells control the diameter of the stoma by changing shape, widening or narrowing the gap between the 2 cells
• When guard cells take in water by osmosis, they become more turgid and swell.
• When the cells lose water and become flaccid…close the space between
• The changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of K⁺ ion
• Stomata open when guard cells actively accumulate K …       
• This uptake of solute causes the water potential to become more negative within the guard cells, and cells become more turgid as water enters by osmosis
• Stomata closing results from K leaving the guard cells
• In general stomata are open during the day and closed at night
• A second stimulus causing the stomata to open is depletion of CO₂ within air spaces of the leaf
• A third is CIRCADIAN RHYTMS…biological clocks
• Environmental stresses that can cause stomata to close during the daytime
• Water deficiency
• Hormone: abscisic acid, when released causes closing of stomata
• High temperatures
•  
• g

§         Phloem

·        Soft, thinner walled cells that transport carbohydrates (glucose), organic compounds away from areas where made to the rest of the plant.(mature leaves to roots and nonphotosynthetic parts of the shoot system

·        Composed of

o       Sieve tube elements(or members) (cells that actually carry the nutrients.

§         alive at functional maturity

§         contain no nucleus, ribosomes and vascuoles

§         Sieve plates are located at the end of the tubes..contain pores

Companion cells-

lend support to sieve tube cells.

nucleus and ribosomes seem to be helping the sieve tubes members (that lack nuclei

Seems to assit loading the sieve tubes

nonconducting cells

·        Veins: bundles containing strands of both xylem and phloem

·        Mesophyll: the ground tissue that makes up a leaf

§         Meso=middle and phyll, leaf

§         consists mainly of parenchyma cells equipped with

 

Translocation of Phloem Sap

 

Ground Tissue of a Leaf p. 734

2 Kinds of Mesophyll found in plants

·        Epidermis is protective covering. one or more layers

·        Just beneath the upper epidermis of many kinds of leaves is the palisade layer (aka Palisade Mesophyll), which consists of one more rows of closely packed, columnar cells (parenchyma cells)…contains chloroplasts

·        The lower portion of the mesophyll consists of loosely packed, spherical cells called the spongy layer…contains chloroplasts. Air spaces are scattered throughout: allows movement of gases (CO₂ in and O₂ out) and water vapor

·        Stomata: the tiny holes on bottom (mostly) of leaf surface: connect the air spaces of the mesophyll to the outside air. Guard cells surround the stomata

·        Vascular Bundles consists xylem and phloem. Vascular bundles are covered with protective coating of cells, Bundle Sheath, which prevents air bubbles from entering vessels. might impede the movement of water.

 

 

Transport at the cellular level depends in the selective permeability of membranes p. 749

·        This brief section is a review of Chapter 8..worth reading

 

 

 

Proton Pumps play a central role in transport across plant membranes p. 749

·        Proton pump (description and purpose)

·        Membrane potential

·        Cotransport

·        Chemiosmosis

·        Figure 36.2 p. 750 solute transport in plant cells

Differences in water potential drive water transport in plant cells p. 750

·        Osmosis

·        Impact cell wall has on osmosis

·        2 factors that impact osmosis in plant cells

1.      Solute concentration

2.      pressure; due to the presence of a cell wall

·        Water potential- measurement of combined effects of these two factors

1.      Megapascals- MPa

2.      water moves from an area of higher water potential to an area of lower water potential

3.      cell immersed in a solution having a higher water potential than the cell, osmotic uptake of the water will cause the cell to swell

1.      by moving the water has the capacity to do work, Potential

2.      tendency of systems to change spontaneously to a state of lowest free energy.

 

·        Water potential (Osmotic Potential) of water is set at 0

1.      the addition of solutes lowers the water potential…remember “free water”, hydration shell around a solute

2.      the presence of solutes lowers a the potential of a solution

3.      water potential is directly proportional to pressure…increasing pressure raises Y…pushing down with a syringe p. 751

4.      can also create a negative pressure, or TENSION…pulling up on the syringe …opposite …p. 751

·        Y= Y_P + Y_S

·        Pure water Y=0

·        Y=-0.23 (0.1 molar solution for ANY solute)

·        water uptake and loss of water by plants

1.      flaccid cell (draw Figure 36.4 p. 752)

1.      place it in a solution with a higher solute concentration

2.      cell will plasmolyze

3.      dynamic equilibrium

2.      Turgor

1.      dynamic equilibrium

·         

Aquaporins affect the rate of water transport across membranes p. 752

·        What are aquaporins? Role?

·        Do not affect water potential gradient or the direction of water flow, but rather the rate at which water diffuses down it water potential gradient

 

 

 

 

 

 

 

 

Water and minerals that pass from the soil into the root cortex cannot be transported to the rest of the plant until they enter the xylem of the stele

Endodermis, the innermost layer of cells in the root cortex, surrounds the stele and functions as a lacxkt checkpoint for the selective passage of minerals fdorm the

Rate of Transpiration

(Water vapor loss via leaves)

·        90% of water taken in by roots released as water vapor

·        Water passes from soil to the leaves via tracheids

·        Rate of transpiration:  regulated by opening and closing of stomata.

§         Guard cells take in water (via osmosis)…guard cells swell and open stomata

§         Loss of water causes guard cells to shrink….stomata close.

Purpose:

·        Directs the upward movement of water to the leaves

·        Keeps the air spaces of leaves moist

·        Contributes to the cooling of the plant through exudation (passing of a liquid via pores)

·        Increases water pressure in the roots, allowing the roots to uptake more water…if water moves out of the roots it makes room for more water….

·        Limitations: photosynthesis slows down when stomata are closed…prevents CO₂ from entering the leaf. Also causes oxygen levels to increase in the leaf.

·        Other factors that impact water movement in a plant

§         Attraction of water molecules to each other (adhesion)

§         Capillary action (cohesion of water molecules to the walls of the vascular tubes)

§         Osmosis