Chapter 36 – Plants & Transpiration

• The success of plants depends on their ability to gather and conserve resources from their environment

• The transport of materials is central to the integrated functioning of the whole plant

• Plants also depend on nutrient cycles to obtain materials they need for life processes

Evolution Connection :Underground Plants• Stone plants (Lithops) are adapted to life in the

desert– Two succulent leaf tips are exposed above

ground; the rest of the plant lives below ground

Adaptations for acquiring resources were key steps in the evolution of vascular plants

• The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water

• Early nonvascular land plants lived in shallow water and had aerial shoots

• Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport

Figure 36.2-1

H2Oand minerals

H2O

Figure 36.2-2

H2Oand minerals

O2

CO2

CO2O2

H2O

Figure 36.2-3

H2Oand minerals

O2

CO2

CO2O2

H2O

Light

Sugar

• The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis

• Xylem transports water and minerals from roots to shoots

• Phloem transports photosynthetic products from sources to sinks

Plant Adaptations Cont’

• Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss

Shoots and Light Capture

• Stems serve as pathways for water and nutrients and as supporting structures for leaves

• There is generally a positive correlation between water availability and leaf size

Obtaining of Water and Minerals

• Soil is a resource used by the root system• Root growth can adjust to local conditions

– For example, roots branch more in a pocket of high nitrate than low nitrate

• Roots are less competitive with other roots from the same plant than with roots from different plants

• Roots and soil fungi form mutualistic associations called mycorrhizae

• Mutualisms with fungi helped plants colonize land• Mycorrhizal fungi increase the surface area for

absorbing water and minerals• Nitrogen fixation by bacteria

Transport of Water• Plants must balance water uptake and loss• Osmosis determines the net uptake or water loss

by a cell and is affected by solute concentration and pressure

• Water potential is a measurement that combines the effects of solute concentration and pressure

• Water potential determines the direction of movement of water

• Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa)

• Ψ = 0 MPa for pure water at sea level and at room temperature

Transport of Water

What Affects Water Potential?

• Both pressure and solute concentration affect water potential

• This is expressed by the water potential equation: Ψ ΨS ΨP

• The solute potential (ΨS) of a solution is directly proportional to its molarity

• Solute potential is also called osmotic potential

• Pressure potential (ΨP) is the physical pressure on a solution

• Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast

• Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water

• Water moves in the direction from higher water potential to lower water potential

Water Potential

Figure 36.8a

Solutes have a negative effect on by binding water molecules.

Pure water at equilibrium

H2O

Adding solutes to the right arm makes lower there, resulting in net movement of water to the right arm:

H2O

Pure water

Membrane Solutes

Figure 36.8b

Positive pressure has a positive effect on by pushing water.

Pure water at equilibrium

H2OH2O

Positivepressure

Applying positive pressure to the right arm makes higher there, resulting in net movement of water to the left arm:

Solutes and positive pressure have opposing effects on watermovement.

Pure water at equilibrium

H2OH2O

Positivepressure

Solutes

In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:

Figure 36.8c

• Water potential affects uptake and loss of water by plant cells

• If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis

Water Movement Across Plant Cell Membranes

Figure 36.9a

Plasmolyzedcell at osmoticequilibrium withits surroundings

0.4 M sucrose solution:

Initial flaccid cell:

(a) Initial conditions: cellular environmental

0.9 MPaS

0.9P

0

P

0S

0.9

0.9 MPa

P

0 S

0.7

0.7 MPa

Figure 36.9b

Pure water:

Turgid cellat osmoticequilibrium withits surroundings

(b) Initial conditions: cellular environmental

Initial flaccid cell:

0.7 MPa

P 0 S

0.7

P0

0 MPaS

0

P 0.7S 0.7 0 MPa

• If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid

• Turgor loss in plants causes wilting, which can be reversed when the plant is watered

• Transpiration drives the transport of water and minerals from roots to shoots

The rate of transpiration is regulated by stomata• Leaves generally have broad surface areas and high surface-to-

volume ratios• About 95% of the water a plant loses escapes through stomata• Each stoma is flanked by a pair of guard cells, which control the

diameter of the stoma by changing shape• Stomatal density is under genetic and environmental control

Figure 36.15a

Radially orientedcellulose microfibrils

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal openingand closing (surface view)

• This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells

Figure 36.15b

(b) Role of potassium in stomatal opening and closing

K

H2O

H2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Stimuli for Stomatal Opening and Closing

• Generally, stomata open during the day and close at night to minimize water loss

• Stomatal opening at dawn is triggered by– Light

– CO2 depletion

– An internal “clock” in guard cells

• All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

Nitrogen cycle and plants• Plants need Nitrogen for

– Making proteins– Cell replication: Making copies of the DNA

• Nitrogen is abundant in the air but is not available to plants

• Nitrogen Fixation: conversion of N2 in the atmosphere to ammonia (NH3) and nitrate (NO3

-)– Abiotic Fixation – lightning & radiation– Biotic fixation – Soil bacteria & symbiotic relationships

with plants

Nitrogen cycle and plants• Nitrification: Ammonia is oxidized into nitrite (NO2)

and nitrate (NO3-)

• Assimilation: Nitrates are most easily absorbed by plants and therefore added to the food chain

• Ammonification: Decomposers break down nitrogenous waste and convert it to NH3 for absorbtion

• Denitrification – nitrates converted back to N2 and lost to atmopshere

• Humans affect this cycle by adding excess fertilizer & also by soil depletion

• Excess NH4+ can acidify soils and aquatic ecosystems