Transcript Lecture 7

Chapters 5 & 35
Lecture 7
Movement across membranes
Dr. Angelika Stollewerk
Movement across membranes
Aims:
• To understand the process of diffusion
• To understand the process of osmosis
Movement across membranes
Aims:
• To understand the process of diffusion
• To understand the process of osmosis
These lecture aims form part of the knowledge
required for learning outcome 1
Movement across membranes
Essential reading
• pages 105-107
• pages 764-768
• 5.3 What Are the Passive Processes of Membrane
Transport?
• 35.1 How Do Plant Cells Take Up Water and
Solutes?
5.3 What Are the Passive Processes of Membrane Transport?
Membranes have selective
permeability—some substances can
pass through, but not others
Passive transport—no outside energy
required—diffusion
Active transport—energy required
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion: the process of random
movement toward equilibrium
Equilibrium—particles continue to move,
but there is no net change in distribution
Figure 5.8 Diffusion Leads to Uniform Distribution of Solutes
5.3 What Are the Passive Processes of Membrane Transport?
Net movement is directional until
equilibrium is reached.
Diffusion is net movement from regions of
greater concentration to regions of
lesser concentration.
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion rate depends on:
• Diameter of the molecules or ions
• Temperature of the solution
• Electric charges
• Concentration gradient
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion works very well over short
distances.
Membrane properties affect the diffusion
of solutes.
The membrane is permeable to solutes
that move easily across it; impermeable
to those that can’t.
5.3 What Are the Passive Processes of Membrane Transport?
Simple diffusion: small molecules pass
through the lipid bilayer.
Lipid soluble molecules can diffuse
across the membrane, as can water.
Electrically charged and polar molecules
can not pass through easily.
5.3 What Are the Passive Processes of Membrane Transport?
Osmosis: the diffusion of water
Osmosis depends on the number of
solute particles present, not the type of
particles.
Figure 5.9 Osmosis Can Modify the Shapes of Cells
5.3 What Are the Passive Processes of Membrane Transport?
If two solutions are separated by a
membrane that allows water, but not
solutes to pass through, water will
diffuse from the region of higher water
concentration (lower solute
concentration) to the region of lower
water concentration (higher solute
concentration).
5.3 What Are the Passive Processes of Membrane Transport?
Isotonic solution: equal solute
concentration (and equal water
concentration)
Hypertonic solution: higher solute
concentration
Hypotonic solution: lower solute
concentration
5.3 What Are the Passive Processes of Membrane Transport?
Water will diffuse (net movement) from a
hypotonic solution across a membrane
to a hypertonic solution.
Animal cells may burst when placed in a
hypotonic solution.
Plant cells with rigid cell walls build up
internal pressure that keeps more water
from entering—turgor pressure.
35.1 How Do Plant Cells Take Up Water and Solutes?
Terrestrial plants obtain water and
mineral nutrients from the soil.
Water is needed for photosynthesis; it is
essential for transporting solutes
upward and downward, for cooling the
plant, and for internal pressure that
helps support the plant.
Plants lose large quantities of water to
evaporation, which must be replaced.
Figure 35.1 The Pathways of Water and Solutes in the Plant
35.1 How Do Plant Cells Take Up Water and Solutes?
Osmosis: movement of water through a membrane in
accordance with the laws of diffusion.
Osmosis is passive: no input of energy is required.
Solute potential (osmotic potential): The greater the
solute concentration of a solution, the more
negative the solute potential, and the greater the
tendency for water to move into it from another
solution of lower solute concentration.
35.1 How Do Plant Cells Take Up Water and Solutes?
For osmosis to occur, two solutions must be
separated by a selectively permeable
membrane; permeable to water, but not to the
solute.
Plants have rigid cell walls. As water enters a
cell due to its negative solute potential, entry
of more water is resisted by an opposing
pressure potential (turgor pressure).
35.1 How Do Plant Cells Take Up Water and Solutes?
Water enters plant cells until the pressure
potential exactly balances the solute
potential.
At this point the cell is turgid: it has significant
positive pressure potential.
35.1 How Do Plant Cells Take Up Water and Solutes?
The overall tendency of a solution to take up
water from pure water, across a membrane, is
called water potential (ψ).
Water potential is the sum of its negative solute
potential and positive pressure potential.
ψ = ψs + ψ p
By definition the water potential of pure water is zero.
Figure 35.2 Water Potential, Solute Potential, and Pressure Potential
35.1 How Do Plant Cells Take Up Water and Solutes?
Water always moves across a selectively
permeable membrane toward a region of
lower (more negative) water potential.
Solute potential, pressure potential, and water
potential can be measured in megapascals
(MPa).
35.1 How Do Plant Cells Take Up Water and Solutes?
Osmosis is extremely important to plants.
Physical structure is maintained by the positive
pressure potential. If this is lost, the plant
wilts.
Over long distances in xylem and phloem, flow
of water and dissolved solutes is driven by a
gradient of pressure potential.
35.1 How Do Plant Cells Take Up Water and Solutes?
Bulk flow: movement of a solution due to
difference in pressure potential.
Bulk flow in xylem is between regions of
different negative pressure potential
(tension).
Bulk flow in phloem is between regions of
different positive pressure potential (turgidity).
35.1 How Do Plant Cells Take Up Water and Solutes?
Aquaporins - membrane channel proteins that
water can pass through rapidly.
Abundance in plasma membrane and tonoplast
(vacuole membrane) depends on cell’s need
to obtain or retain water.
Rate of water movement can be regulated but
direction of movement can not.
35.1 How Do Plant Cells Take Up Water and Solutes?
Mineral ions cannot pass membranes without
transport proteins.
Molecules and ions move with their
concentration gradients as permitted by
membrane characteristics.
Concentration of most ions in the soil solution
is lower than in the plant; uptake must be
active transport, requiring energy.
35.1 How Do Plant Cells Take Up Water and Solutes?
Electric charge differences are also important.
Combination of electrical and concentration
gradients is called an electrochemical
gradient. Uptake against this gradient
requires ATP.
35.1 How Do Plant Cells Take Up Water and Solutes?
Plants have a proton pump that moves
protons out of a cell against a gradient.
Accumulation of H+ outside the cell results in
an electric gradient and a concentration
gradient of protons.
Inside of cell is now more negative than
outside, cations such as K+ can move in by
facilitated diffusion.
35.1 How Do Plant Cells Take Up Water and Solutes?
The proton gradient can be harnessed to drive
active transport of anions into the cell against
a gradient; a symport couples movement of
H+ and Cl–.
The proton pump and other transport activities
results in the interior of the cell being very
negative; they build up a membrane potential
of about –120 mV.
Figure 35.3 The Proton Pump in Active Transport of K+ and Cl–
35.1 How Do Plant Cells Take Up Water and Solutes?
Where water is moving by bulk flow, dissolved
minerals are carried along.
Water and minerals also move by diffusion,
and minerals move by active transport (e.g.,
at root hairs).
Ions must cross other membranes to reach the
vessels and tracheids.
35.1 How Do Plant Cells Take Up Water and Solutes?
Movement of ions across membranes can
result in movement of water.
Water moves into a root because the root has
a more negative water potential than the soil.
Water moves from the cortex into the stele
because the stele has a more negative water
potential than the cortex.
35.1 How Do Plant Cells Take Up Water and Solutes?
Water and minerals can move into the stele by two
pathways:
• The apoplast: cell walls and intercellular spaces form
a continuous meshwork that water can move through,
without crossing any membranes.
Water movement is unregulated until it reaches the
Casparian strips of the endodermis.
35.1 How Do Plant Cells Take Up Water and Solutes?
• Symplast: water passes through cells via the
plasmodesmata.
Selectively permeable membranes of root hair
cells control access to the symplast.
Movement across membranes
Check out
5.3 Recap, page 110, first 2 questions only
35.1 Recap, page 752, first 2 questions only
5.3 Chapter summary, page 116, see WEB/CD Activity 5.1
35.1 Chapter summary, page 778
Self Quiz
Page 117: Chapter 5, question 10
Page 778: Chapter 35, questions 1-5
For Discussion
Page 779: Chapter 35,
Movement across membranes
Key terms:
active transport, apoplast, aquaporin, bulk flow,
Casparian strip, diffusion, endodermis, equilibrium,
facilitated transport, gated transport, hydrophilic,
hydrophobic, hypertonic, hypotonic, isotonic,
Megapascals (MPa), osmosis, pericycle, pressure
potential (ψp), semipermeable membrane, solute
potential (ψs), stele, symplast, turgid, water
potential (ψ)