Plant Cells.. Membrane.. Nutrients traffic.. Regulation.. Solute transport Plant Cells.. Membrane.. Nutrients traffic.. Regulation.. Dr. Abdul Latif Khan

Ion transport in roots As all plant cells are surrounded by a cell wall, Ions can be carried through the cell wall space with out entering an actual cell The apoplast Just as the cell walls form a continuous space, so do the cytoplasms of neighboring cells The symplast

Ion transport in roots All plant cells are connected by plasmodesmata. In tissues where large amounts of intercellular transport occurs neighboring cells have large numbers of these. As in cells of the root tip Ion absorption in the root is more pronounced in the root hair zone than other parts of the root. An Ion can either enter the root apoplast or symplast but is finally forced into the symplast by the casparian strip.

Ion transport in roots Once the Ion is in the symplast of the root it must exit the symplast and enter the xylem Called Xylem Loading. Ions are taken up into the root by an active transport process Ions are transported into the xylem by passive diffusion

Membrane transport Facilitate the passage of ions and other polar molecules Arabidopsis thaliana contains 849 membrane proteins (4.8% of genome) Three types of membrane transporters enhance the movement of solutes across plant cell membranes Channels – passive transport Carriers – passive/active transport Pumps- active transport Artificial vs Natural membranes. Different proteins for different molecules

Passive transport Diffusion: Diffusion is the net movement of material from an area of high concentration to an area with lower concentration Facilitated diffusion: Facilitated diffusion, also called carrier-mediated diffusion, is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Osmosis: Osmosis is the diffusion of water molecules across a selectively permeable membrane. 

Diffusion Facilitated Diffusion

Simple diffusion Movement down the gradient in electrochemical potential Movement between phospholipid bilayer components Bidirectional if gradient changes Slow process

Channels Transmembrane proteins that work as selective pores Transport through these passive The size of the pore determines its transport specifity Movement down the gradient in electrochemical potential Unidirectional Very fast transport Limited to ions and water

Channels Sometimes channel transport involves transient binding of the solute to the channel protein Channel proteins have structures called gates. Open and close pore in response to signals Light Hormone binding Only potassium can diffuse either inward or outward All others must be expelled by active transport. K+ form the environment, opening of stomata Potassium channels: Inward or outward, open depending upon Release of K+ into xylem Closing of stomata

Remember the aquaporin channel protein? There is some diffusion of water directly across the bi-lipid membrane. Aquaporins: Integral membrane proteins that form water selective channels – allows water to diffuse faster Facilitates water movement in plants Alters the rate of water flow across the plant cell membrane – NOT direction

Carriers Do not have pores that extend completely across membrane Substance being transported is initially bound to a specific site on the carrier protein Carriers are specialized to carry a specific organic compound Binding of a molecule causes the carrier protein to change shape This exposes the molecule to the solution on the other side of the membrane Transport complete after dissociation of molecule and carrier protein

(A) In the initial conformation, the binding sites on the protein are exposed to the outside environment and can bind a proton. (B) This binding results in a conformational change that permits a molecule of S to be bound. \figures\ch06\pp06091.jpg

binding sites and their substrates to the inside of the cell. (C) The binding of S causes another conformational change that exposes the binding sites and their substrates to the inside of the cell. (D) Release of a proton and a molecule of S to the cell’s interior restores the original conformation of the carrier and allows a new pumping cycle to begin. \figures\ch06\pp06092.jpg

Active transport Movement of molecules (ions, glucose and amino acids) across a cell membrane in the direction against their concentration gradient, i.e. moving from an area of lower concentration to an area of higher concentration It uses chemical energy : ATP - ADP Two types: Primary and Secondary Primary active transport: directly uses metabolic energy to transport molecules across a membrane transmembrane ATPases: sodium potassium pump, calcium pump, proton pump vacuolar ATPase ABC (ATP binding cassette) transporter

Active transport Secondary active transport: coupled transport or co-transport, energy is used to transport molecules across a membrane Two types: Antiport and Symport Antiport: two species of ion or other solutes are pumped in opposite directions across a membrane e.g.  sodium-calcium antiporter: three sodium ions into the cell to transport one calcium out Symport: downhill movement of one solute from high to low concentration

(A) In a symport, the energy dissipated by a proton moving back into \figures\ch06\pp06100.jpg (A) In a symport, the energy dissipated by a proton moving back into the cell is coupled to the uptake of one molecule of a substrate (e.g., a sugar) into the cell. (B) In an antiport, the energy dissipated by a proton moving back into the cell is coupled to the active transport of a substrate (for example, a sodium ion) out of the cell.

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