Name : Poulani Roy. College Rollno. : 19MB0001 . Registration no. :0291905030913.
Carrier , channels and proteins
Integral proteins that control membrane permeability fall into three broad classes—carriers,
channels and pumps—each with distinct properties .. These proteins allow cells to control solute
traffic across membranes, an essential feature of many physiological processes.
Carriers : They are enzyme-like proteins that provide passive pathways for solutes to move
across membranes down their concentration gradients from a region of higher concentration to one
of lower concentration. Each conformational change in a carrier protein translocates a limited
number of solutes across the membrane. Carriers use ion gradients as a source of energy to
perform a remarkable variety of work. Some carriers use translocation of an ion down its
concentration gradient to drive another ion or solute up a concentration gradient.
❖ Do not have pores that extend completely across the membrane .
❖ Binding to carrier protein is like enzyme binding site action.
❖ Can be either active or passive.
❖ Passive action is sometimes called facilitated diffusion.
❖ Moderate speed.
➢ Slower than in a channel.
❖ 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.
Channels : They are ion-specific pores that typically open and close transiently in a regulated
manner. When a channel is open, a flood of ions passes quickly across the membrane through the
channel, driven by electrical and concentra-tion gradients. The movement of ions through open
channels controls the electrical potential across membranes, so that changes in channel activity
produce rapid electrical signals in excitable membranes of nerves, muscles, and other cells.
❖ Transmembrane proteins that work as selective pores.
❖ Transport through these is Passive .
❖ The size of the pore determines its specificity.
❖ Movement down the gradient in electrochemical potential.
❖ Very fast transport.
❖ Limited to ions and water.
❖ It involves transient binding of the solute to the channel protein.
❖ Channel proteins have structures called gates(Open & close pore in response to signals
-Light,Hormone binding )
❖ Only Potassium can diffuse either inward or outward -All others must be expelled by active
❖ Aquaporins: Integral membrane proteins that form water selective channels
➢ allows water to diffuse faster.
➢ Facilitates water movement in plants.
Pumps:They are enzymes that utilize energy from adenosine triphosphate (ATP), light, or
(rarely) other sources to move ions (generally, cations) and other solutes across membranes at
relatively modest rates. They establish concentration gradients between membrane-bound
❖ To carry out active transport: The membrane transporter(pump) must couple the uphill
transport of a molecule with an energy releasing event This is called Primary active
❖ Energy source can be
➢ The electron transport chain of mitochondria .
➢ The electron transport chain of chloroplasts .
❖ Movement against the electrochemical gradient.
❖ Unidirectional .
❖ Very slow.
❖ Significant interaction with solute .
❖ Direct energy expenditure.
Fig 1 . Properties of the three types of proteins that transport ions and other solutes across
membranes,The triangle represents the concentration gradients of Na+ (blue) and glucose (green)
across the membrane.
Fig 2 : Membrane transport across the plasma membrane-Channels ( passive transport ), Carriers
(passive transport) & Pumps (active transport).