4-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 4: Movement across membranes

4-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Functions of membrane

• Plasma membranes control the passage of substances into and out of a cell

– maintain stable conditions inside cell homeostasis

– membranes also control movement in and out of organelles

• Permeability of molecules depends on– size– electrical charge– lipid solubility

4-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Diffusion of solutes in water

• Diffusion is the passive movement of molecules along a concentration gradient

– high concentration → low concentration

• Diffusion of certain substances occurs across membranes

– O2, CO2, alcohol

• Rate of diffusion depends on – permeability– magnitude of concentration gradient

4-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Diffusion of ions

• Diffusion of ions occurs along an electrochemical gradient

• Electrochemical gradient is combination of– chemical gradient

high to low concentration

– electrical gradient difference in charge across membrane (opposites attract,

like repels)

• Direction of net passive movement depends on relative strength of these two gradients

4-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.2: Concentration and electrical gradients

4-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Membrane transporters

• Membrane transporters accelerate the movement of less permeable molecules across membranes

• Transport proteins are specific to one or a small number of solutes

• Rate of transport across membrane depends on number of transport proteins

– rate levels off if all transport proteins are occupied

(cont.)

4-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Membrane transporters (cont.)

• Transport proteins assist movement of molecules down concentration gradient through facilitated diffusion

– requires no energy input

• Channels– conduits allow direct passage from one side of the

membrane to the other

• Carriers– binding of solute on one side of membrane produces

conformational change in protein moving solute through

4-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Channels

• Channels allow molecules to move in or out of cells rapidly

– faster than carriers, which bind and release solutes

• Most channels transport ions– high specificity

calcium channels potassium channels chloride channels

4-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.5: Channels and carriers

4-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Gating mechanisms

• Channels have open and closed states• Channels are opened and closed by signals

– voltage-gated channels respond to changes in voltage across membranes

– ligand-gated channels activated by binding of specific molecules (ligands)

– mechanically-gated channels respond to physical disturbance

4-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Facilitated transport• Properties of facilitated transport that distinguish it

from simple diffusion– transport is faster– transport proteins become saturated as substrate

concentration increases– transport proteins are specific for particular substrates or

types of substrate– transport is inhibited by similar substrates that compete

for the binding site

4-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Aquaporins

• Aquaporins are membrane-spanning proteins that allow water and urea to diffuse across membranes

• Tissues with high water permeability have high concentrations of aquaporins in their cell plasma membranes

• Concentration of some aquaporins in plasma membranes can be controlled by hormones

– ACTH increases deposition of aquaporins in cell membranes of kidney collecting tubules

4-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Active transport

• Active transport uses energy to move solute against electrochemical gradient

• Transport ATPases– direct pumping coupled to ATP hydrolysis

• Co-transport– diffusion of one molecule down its electrochemical

gradient is used to pump a second molecule against its electrochemical gradient

(cont.)

4-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Active transport (cont.)

• Plant cells– H+ pumped from cell by H+ ATPase creates

electrochemical gradient for inward movement of H+

– inward movement of H+ drives the pumping of other solutes against their electrochemical gradients

• Animal cells– gradients of Na+ established by Na+–K+ ATPase are used

for active transport of solutes

4-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Voltage differences

• Difference in voltage across membrane is membrane potential

• Membrane potential due to– negative charge of proteins and other polymers inside

cell– transport of ions across plasma membrane

• Removal of positive ions from cell increases negative charge inside cell

– animal cells c. –50 to –80 mV– plant cells –200 mV

(cont.)

4-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.8: Na+–K+ ATPase

4-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Voltage differences (cont.)

• Functions of membrane potentials– provide favourable electrochemical gradient for passive

uptake of cations– inward electrochemical gradient can be used in active

transport of other ions– changes can be used as signals, either locally or for

transmission between cells

4-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Osmosis

• Water travels passively across membranes by osmosis

– higher free energy → lower free energy

• Free energy levels determined by– dissolved solutes– physical pressure (tension)

• Free energy of water decreased by– solutes– reduced pressure

(cont.)

4-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.9: Diffusion

4-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Osmosis (cont.)

• Water flows across a membrane towards a region of highest solute concentration (= lowest water free energy)

– in plants, pressure is also important as cell walls are rigid

• Overall free energy of water is water potential (Ψ)– sum of

osmotic potential (Ψπ)

pressure potential (ΨP)

Ψ = Ψπ + ΨP

(cont.)

4-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Osmosis (cont.)

• Osmosis is the net passive movement of water from a region of higher water potential to one of lower water potential through a selectively permeable membrane

• Iso-osmotic solutions – have same solute concentration (same osmotic potential)

• Hyperosmotic solutions– more concentrated

• Hypo-osmotic solutions – less concentrated

(cont.)

4-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.10a: Animal cells

4-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 4.10b: Plant cells

4-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Osmosis (cont.)

• Differences in water potential across the plasma membrane of cells without rigid walls are due only to differences in osmotic potential (concentration of solutes)

• In cells with a rigid wall, osmotic potential and pressure potential contribute to water potential

– water enters when cells are placed in solution with less negative water potential

– can only expand by c. 10 per cent before pressure (turgor) causes intracellular water potential to equal that of external solution

4-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Turgor

• Shape of plants maintained by cell turgor• Water continually moves from cells to atmosphere,

which has very low water potential• If lost water is not replaced, pressure potential

(and turgor) is reduced

4-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Vesicle-mediated transport• Large molecules transported in membrane-bound

vesicles• Endocytosis

– plasma membrane encloses substances outside cell– pinches off to form vesicle

phagocytosis (solids) pinocytosis (fluids)

– invagination may be receptor-mediated

• Exocytosis– intracellular vesicles fuse to plasma membrane and

release contents to outside