Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online




Last Update:  04/08/16

Learning Goals/Objectives for Chapter 6A:  After class and this reading, students will be able to

  • define flux (J) of solute (A) across a membrane;
  • write mathematical relationship that show how flux J depends on the concentration gradient of solute across the membrane (dA/dx) and also on the difference of solute concentration across the membrane (ΔA) for passive diffusion;
  • differentiate between passive diffusion, facilitated diffusion mediated by a receptor transporter, and active transport
  • write chemical equations which show the physical steps in the process of passive and facilitated diffusion
  • derive a mathematical equation and graphs which shows the dependencies of flux J as a function of Aout and AR for facilitated diffusion assuming rapid equilibrium binding of ;
  • differentiate between carrier proteins, permeases or transport proteins on one hand and channels on the other;

A4.  Membrane Pores

The Nuclear Pore Complex

Channels have pores which can be gated open and allow selective flow of ions.  Porins have larger entrances which allow larger molecules to pass the bilayer.  The biggest pore structure known is the nuclear pore complex, which has a combined molecular mass around 125,000,000!  It's job is to shuttle small molecules through passive diffusion down a concentration gradient through the pore.  In addition it moves large molecules and molecular structures (proteins, RNA, and perhaps ribosomes) across the nuclear membrane in a process which requires energy.  The proteins that comprise this complex are called nucleoporins,  of which there appears to be around 30 in yeast.    Large proteins that pass through the pore must first be bound to a cargo receptor, which can move the "cargo" across the pore with concomitant GTP hydrolysis.

Figure:  Movement of Molecules and Particles Through Membranes

Diffusion Transpoprt Endocytosis

Porins: TBA


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Archived version of full Chapter 6A:  Passive and Facilitated Diffusion

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