Transport across membranes

Learning Outcomes

• explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis;

Exchange across the plasma membrane

• The membrane provides an effective barrier against the movement of substances, however some exchange between the cell and the environment is essential.

Transport across membranes

• Materials can move across cell membranes:– Passively• Diffusion (simple or facilitated)• Osmosis

– Actively• Active transport• Bulk transport

Diffusion

• Net movement of molecules or ions from a region of high concentration to a region of low concentration

• Occurs along a concentration gradient • Result = equilibrium (molecules or ions evenly

spread out within a given space or volume)

Factors affecting the rate of diffusion

• Concentration gradient– Greater the difference in concentration the greater the

rate of diffusion• Temperature– At higher temperature kinetic energy particles increases – Diffusion is faster

• Surface area– Greater the surface area, more particles can cross– Increases rate of diffusion

Factors affecting the rate of diffusion

• Nature of molecules or ions– Large molecules diffuse slower– Non-polar molecules diffuse more easily– The respiratory gases (CO2 and O2) are small

enough to diffuse quickly through the membrane. – Large, polar molecules (glucose and amino acids)

and ions (Na+ and Cl-) cannot diffuse through the phospholipid bilayer

Facilitated Diffusion

• Protein molecules exist in membranes to facilitate diffusion.

• 2 type of protein molecule– Channel protein • transmembrane protein that forms a tunnel through

the bilayer.– Carrier proteins • change shape to help molecules move into and out of

cells.

Facilitated Diffusion

Active Transport

• Energy consuming transport of molecules or ions across a membrane against a concentration gradient, made possible by transferring energy from respiration.

• Energy makes the carrier proteins change shape, transferring ions across the membrane.

Examples of active transport

• Reabsorption in kidneys• Digestion in gut– Helps absorb glucose from our intestines

• Load sugars into phloem• Inorganic ion uptake in root hairs– Magnesium ions are in short supply in the soil but

are needed for photosynthesis

Bulk transport

• This is the method of transporting large quantities of materials into cells (endocytosis) or out of cells (exocytosis)– Endocytosis - Engulfing of material by cell membrane to

form a endocytic vacuole.• 2 forms

– Phagocytosis the uptake of solid material– Pinocytosis the uptake of liquid

– Exocytosis - Process by which materials are removed from cells

Examples of bulk transport

• Hormones released into bloodstream from endocrine glands

• White blood cells engulf invading microorganisms by phagocytosis

• In plant cells materials to build the cell wall are carried outside in vesicles.

OSMOSIS

• Special type of diffusion involving water molecules

• Example:– Two solutions are separated by a partially

permeable membrane. Solute molecules are too large to pass through pores in the membrane, but water molecules are small enough.

What would happen if the membrane were not present?

• Net movement of solute molecules from B to A by diffusion

• Net movement of water molecules from A to B by diffusion

• Equilibrium – concentrations of water molecules and solute molecules in A would equal that in B.

What will happen if the membrane is present?

What will happen if the membrane is present?

• Solute molecules too large to pass through membrane

• Water molecules pass easily from A to B• Net movement of water from A to B until equilibrium

is reached, i.e. solution A has the same concentration of water molecules as solution B.

• The level of liquid A will fall and the level of liquid B will rise

• Equilibrium is brought about by the movement of water molecules alone.

Definition of osmosis

• Water potential Ψ– Tendency of water molecules to diffuse from one place to

another. – Measured in kPa– Pure water has a water potential of 0kPa

• Osmosis– Is the net movement of water molecules from a region of

high water potential to a region of low water potential (down a water potential gradient) across a partially permeable membrane.

Water potential

Highest water potential

0kPaPure water No solute

Lower water potential-50kPa

Dilute solutionSmall amount

of solute dissolved

Very low water potential -500kPa

Concentrated solution

Large amount of solute dissolved

Decre

asin

g w

ate

r pote

ntia

l

Some Important Terms

• Hypotonic– a region of

• higher water potential.• Lower solute concentration

• Hypertonic– a region of

• lower water potential• Higher solute concentration

• Isotonic – a region where there are equal water potentials on either

side of a membrane.

Determining Water Potential in Potato tubers

Salt Soluntion(mol-1)

Starting Mass (g) Finishing mass (g) Change in mass (g)%age change in

mass

0.1

0.2

0.3

0.4

0.5

Osmosis in Red Blood Cells

Osmosis in Plant Cells

Important Terms

• Turgid – the term used to describe a plant cell where the

protoplast exerts a pressure on the cell wall.• Plasmolysed – the term used to describe a plant cell where the

protoplast has shrunk away from the cell wall due to loss of water by osmosis.

Osmosis in red onion cells