Starter
Fill in the blanks:
• Pure water has the highest possible water potential, which is
0 kPa.
• Solutions always have a lower water potential than pure
water, so they have negative water potentials.
• water always moves from a region of higher water potential
to a region of lower water potential. (more negative)
• If one cell has a water potential of -6kPa and the next one has
a water potential of -4kPa, water will move from the cell at 4kPa to the cell at -6kPa. i.e. from higher water potential to
lower water potential.
Transport Across a Cell
Membrane
1.1.2,i,j,m
Learning Objectives
• explain the mechanisms of diffusion, facilitated diffusion
(to include channel and carrier proteins). (i)
• describe the mechanism of active transport, with reference
to the structure of the red blood cell membrane; (j)
• explain, in terms of water potential, why the concentration
of plasma proteins, glucose and electrolytes will affect the
water potential of blood; (m)
Facilitated Diffusion
Molecules that are soluble in water or charged
particles such as ions, can not diffuse through
the phospholipid bilayer. So they diffuse
through the plasma membrane with the help
of proteins. This type of diffusion is called
facilitated difusion
Facilitated Diffusion
Some of these proteins are
protein channels that are
permanently open. They are lined
with hydrophilic amino acids and
water
Molecules can also diffuse through
the membrane by binding to carrier
proteins. When the molecule binds
to the carrier protein, it causes the
protein to change shape and
release the molecule on the other
side of the membrane.
Give an example of facilitated diffusion.
Keeping the Osmotic Balance
What happens to the water potential in blood
plasma if glucose concentration increases?
What is going to happen to the red blood cells?
Electrolytes
The concentration of electrolytes in plasma and
in cells that is responsible for maintaining a
water potential balance.
What are electrolytes?
Electrolytes are ions with positive or negative
charge.
Why is it important to keep normal
osmotic balance
• Potassium plasma levels should be between
3.5-5.0 mmol dm⁻³.
• If levels fall below 3.0 mmol dm⁻³, this can
lead to tachycardia and cardiac arrest.
• If levels above 6.0 mmol dm⁻³, this is
associated with bradycardia and heart failure.
Na⁺ is the major extracellular ion.
While
K⁺ is a major intracellular ion.
Active Transport
Define active transport
Comparing methods of transport
Task
In group of three, produce an animation to
show active transport, facilitated transport,
diffusion, osmosis