BIOL-208 Lecture Notes - Lecture 6: Vapor Pressure, Tonicity, Osmotic Concentration
17 Dec 2016
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Chapter 6: Water Relations
- Water Availability
- Water moves, creating concentration gradients. The magnitude of these
gradients means organisms lose or gain water from environments
- Consider basic physical behaviour of water in terrestrial and aquatic
environments, including microclimate the organism uses
- Content in air
- Terrestrial organisms and water availability
- How much water is in the surrounding air?
- Water vapour from land as evaporation
- Evaporation: water loss by land organisms
- Relative humidity: Water vapour density divided by saturation of water
vapour density multiplied by a hundred
- Water vapour density: water vapour per unit volume of air
- Saturation water vapour density: quantity of water vapour air can
potentially hold
- Changes with temperature: as temperature rises, so does the water
content in the air
- Total atmospheric pressure is the pressure exerted by all gases in the air,
such as carbon dioxide, nitrogen gas, etc
- Water Vapour Pressure (WVP): partial pressure due to water vapour
- Saturation Water Vapour Pressure (SWVP): pressure exerted by water
vapour in air saturated with water
- Vapour pressure deficit: difference between WVP and SWVP at a
particular temperature
- Warm and dry? Relative humidity is lower and VPD is higher. Water is
drawn out involuntarily, is pulled out by concentration gradient
- Warm and moist? Air is near saturation and VPD is low
- Higher VPD indicates a steeper concentration gradient and that the water
vapour content is below saturation
- Where the VPD is high, the rate of evaporative water loss by organisms is
higher
- A low VPD indicates that the water vapour content is near saturation
- Where the VPD is low, the rate of evaporative water loss by organisms is
lower
- Aquatic environments
- Water: concentration gradient
- Pure water > freshwater > ocean > saline
- Aquatic organism
- Aqueous solution
- Bounded by a permeable membrane
- Floating in another aqueous solution
- Water movement in aquatic environments
- Diffusion: movements of solutions
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- Osmosis: diffusion of only water through semipermeable
membrane
- Isosmotic: internal and external at same concentration
- Hyperosmotic: internal water at lower concentration than external,
so it is salty inside
- Hypoosmotic: internal water at higher concentration than external,
so it is watery inside
- Isosmotic environment
- Internal concentrations of water and salt equal their
concentrations in the environment
- Salts and water diffuse at approximately equal rates into
and out of an isosmotic organism
- Hyperosmotic environment
- Compared to the environment, a hyperosmotic aquatic
organism has a lower internal concentration of water and a
higher internal concentration of salts
- Salts diffuse out of a hyperosmotic organism at a higher
rate, while water diffuses in at a higher rate
- Hypoosmotic environment
- Compared to the environment, a hypoosmotic aquatic
organism has a higher internal concentration of water and
a lower internal concentration of salts
- Salts diffuse into a hypoosmotic organism at a higher rate,
while water diffuses out at a higher rate
- Soils and plants
- Flows down a water potential gradient
- Water potential (Ψ): Capacity to perform work
- Dependent on free energy content
- Pure water: Ψ = 0
- Ψ in nature is generally negative
- Ψsolute measures the reduction in Ψ due to dissolved substances
- So it is harder for water to move up plant from roots to stem to
leaves to fruit if it has any
- Dry air has the lowest water potential
- Water potential at the top of the plant is lower
- Water potential of the soil is higher
- Water potential of solution Ψsolutes: reduction in water potential due to
dissolved substances
- Matric forces: water’s tendency to adhere to walls; reduces plant water
potential
- Negative pressure: created by water evaporating from leaves also
reduces plant water potential
- Plant Ψ: Ψplant = Ψsolutes + Ψmatric + Ψpressure
- Plants: matric forces are very small (so they are ignored)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
The magnitude of these gradients means organisms lose or gain water from environments. Consider basic physical behaviour of water in terrestrial and aquatic environments, including microclimate the organism uses. Relative humidity: water vapour density divided by saturation of water vapour density multiplied by a hundred. Water vapour density: water vapour per unit volume of air. Saturation water vapour density: quantity of water vapour air can potentially hold. Changes with temperature: as temperature rises, so does the water content in the air. Total atmospheric pressure is the pressure exerted by all gases in the air, such as carbon dioxide, nitrogen gas, etc. Water vapour pressure (wvp): partial pressure due to water vapour. Saturation water vapour pressure (swvp): pressure exerted by water vapour in air saturated with water. Vapour pressure deficit: difference between wvp and swvp at a particular temperature. Relative humidity is lower and vpd is higher. Water is drawn out involuntarily, is pulled out by concentration gradient.
