Transport across boundaries

Learning Outcomes

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

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

CELL DIVISION, CELL DIVERSITY AND CELLULAR ORGANISATION

OCR AS BiologyUnit F211: Cells, exchange and transport

Module 1: Cells