Water in Plants
Molecular movement
• Molecules and ions are constantly in random motion
• If a drop of ink is diluted with water and observed through a microscope,
the ink particles appear to be in constant motion
• Brownian movement – result of the bombardment of visible ink particles
by invisible water molecules, which are in constant motion
Diffusion:
• If a perfume bottle cap is removed, perfume molecules will
eventually become dispersed throughout the room
• The movement of molecules or ions from a region of higher
concentration to a region of lower concentration – diffusion
• Movement occurs along a diffusion gradient – from higher
concentration to lower concentration
• When molecules have become evenly distributed in the available space,
they are in a state of equilibrium
• Rate of diffusion depends on several factors, including pressure,
temperature, and density of the medium
• Gas molecules occupy a space that becomes available to them relatively
rapidly, while liquids do so more slowly, and solids are even slower
• Large molecules move much slower than small ones
• In nature wind and water currents distribute molecules much faster than
they ever could be distributed by diffusion alone
Osmosis
• Membranes through which different substances diffuse
at different rates are described as semipermeable
• Plant cell membranes are semipermeable
, • In plant cells, osmosis is the diffusion of water through a semipermeable
membrane from a region where water is more concentrated to a region
where it is less concentrated
• Osmosis ceases if the concentration of water on both sides of the
membrane is equal
• In plant physiology – we prefer to define osmosis in terms of potentials
• Osmotic potential (ψs ) of a solution – a measure of the potential of
water to move from one cell to another as influenced by solute
concentration
• Water enters a cell by osmosis until osmotic potential is balanced by the
resistance to the expansion of the cell wall
• Vacuoles and cells become turgid - turgor pressure that develops against
cell walls - called pressure potential (ψp ).
o (Turgor pressure released when biting crisp carrot/celery stick)
• water potential (ψw ) of a plant cell – osmotic and pressure potentials
combined
o ψw = ψs + ψp
• If ψw of 2 adjacent cells differ, water will move from cell with higher ψw
to cell with lower ψw
• Osmosis - primary means by which water enters plants from their
surrounding environment
o Water from soil enters cell walls and intercellular spaces of
epidermis and root hairs
o Travels along cell walls until it reaches the endodermis, crosses
differentially permeable membranes and cytoplasm of
endodermal cells - into the xylem
o Flows from xylem to leaves, transpires (diffuses) into atmosphere
through the stomata
• Water movement – due to water potential gradients: from relatively
high soil water potential to successively lower water potentials in roots,
stems, leaves, and atmosphere
Molecular movement
• Molecules and ions are constantly in random motion
• If a drop of ink is diluted with water and observed through a microscope,
the ink particles appear to be in constant motion
• Brownian movement – result of the bombardment of visible ink particles
by invisible water molecules, which are in constant motion
Diffusion:
• If a perfume bottle cap is removed, perfume molecules will
eventually become dispersed throughout the room
• The movement of molecules or ions from a region of higher
concentration to a region of lower concentration – diffusion
• Movement occurs along a diffusion gradient – from higher
concentration to lower concentration
• When molecules have become evenly distributed in the available space,
they are in a state of equilibrium
• Rate of diffusion depends on several factors, including pressure,
temperature, and density of the medium
• Gas molecules occupy a space that becomes available to them relatively
rapidly, while liquids do so more slowly, and solids are even slower
• Large molecules move much slower than small ones
• In nature wind and water currents distribute molecules much faster than
they ever could be distributed by diffusion alone
Osmosis
• Membranes through which different substances diffuse
at different rates are described as semipermeable
• Plant cell membranes are semipermeable
, • In plant cells, osmosis is the diffusion of water through a semipermeable
membrane from a region where water is more concentrated to a region
where it is less concentrated
• Osmosis ceases if the concentration of water on both sides of the
membrane is equal
• In plant physiology – we prefer to define osmosis in terms of potentials
• Osmotic potential (ψs ) of a solution – a measure of the potential of
water to move from one cell to another as influenced by solute
concentration
• Water enters a cell by osmosis until osmotic potential is balanced by the
resistance to the expansion of the cell wall
• Vacuoles and cells become turgid - turgor pressure that develops against
cell walls - called pressure potential (ψp ).
o (Turgor pressure released when biting crisp carrot/celery stick)
• water potential (ψw ) of a plant cell – osmotic and pressure potentials
combined
o ψw = ψs + ψp
• If ψw of 2 adjacent cells differ, water will move from cell with higher ψw
to cell with lower ψw
• Osmosis - primary means by which water enters plants from their
surrounding environment
o Water from soil enters cell walls and intercellular spaces of
epidermis and root hairs
o Travels along cell walls until it reaches the endodermis, crosses
differentially permeable membranes and cytoplasm of
endodermal cells - into the xylem
o Flows from xylem to leaves, transpires (diffuses) into atmosphere
through the stomata
• Water movement – due to water potential gradients: from relatively
high soil water potential to successively lower water potentials in roots,
stems, leaves, and atmosphere
