4. Ion Transport

• enclosed by a porous cell wall
• plasma membrane at inner boundary of the cell wall, encloses the cytoplasm
• within the cytoplasm there is a large vacuole surrounded by tonoplast membrane
• membrane is a phospholipid bilayer - ions and solutes are insoluble and less able to pass through

• membranes are poorly permeable to ions because ions are relatively insoluble in lipids
• Kj = cj^membrane/cj^{aqueous phase in equilibrium}
• Jj = Pj (cj^o - cjⁱ). Pj is called the permeability coefficient

the extent to which a membrane permits or restricts the movement of the substance

• decreases as molecules increase in :
• 1. size
• 2. polarity

oxygen and carbon dioxide are both non-polar molecules and pass through membranes rapidly, water is polar but passes through rapidly because it is small

ions are charged molecules, sugars are large - would move slowly across membranes, except membrane transport proteins facilitate their movement

• the spontaneous downhill movement of molecules down a concentration gradient
• can also be driven by the electric potential difference across the membrane

• 1. concentration difference
• 2. electric potential difference across plasma membrane

• the ability to drive ions into the cell against the electrochemical gradient
• must expend metabolic energy in the form of ATP
• transport across relatively impermeable lipid membranes requires special transport proteins

• two main kinds:
• carriers - can also carry out passive transport
• channels

pumps- really carriers that are involved in active transport

transmembrane proteins that can bind specific ions and other solutes and transport them across membranes as a result of a conformational change in the protein

transport is slow because conformational change in the protein is required, much slower than channel

can transport a much wider range of substrates than channels, can be either passive or active (pumps)

• membrane proteins that carry out primary active transport
• most transport ions, those belonging to ATP-binding cassette family can carry large organic molecules

• electrogenic - ion transport involving the net movement of charge across membrane, often hydrogen ion is principal ion
• electroneutral - ion transport where there is no net movement of charge

• active transport proteins - considered to be pumps, energized by ATP
• transport large organic molecules
• act independently of any proton gradient

transmembrane proteins that function as selective pores in the membrane

• transport specificity depends on:
• size of the pore
• density of electrical charges on the surface of the interior lining

• can involve transient binding of the solutes to the channel proteins
• always passive
• diffuse through rapidly
• proteins have structures called gates that open and close in response to external signals - ex voltage changes

• channels that transport water
• water-selective proteins
• expression and activity appear to be regulated, possibly by protein phosphorylation, in response to water availability

• role: the silencing of plant aquaporins has been linked to poor plant growth and even death of the plant
• seems to serve to facilitate the transcellular symplastic pathway for water transport
• aquaporins have gates that can stop the flow of water through the pore of the protein - can be for when the plant contains low amounts of water due to drought
• gating carried out by interaction which causes a 3D change in the protein so that it blocks the pore and stops the flow of water through the pore

• aquaporin gating:
• dephosphorylation of certain serine residues - drought
• protonation of specific histidine residues - flooding