AP Biology 2.6 Facilitated Diffusion Study Notes - New Syllabus Effective 2025
AP Biology 2.6 Facilitated Diffusion Study Notes- New syllabus
AP Biology 2.6 Facilitated Diffusion Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the structure of a molecule affects its ability to pass through the plasma membrane.
Key Concepts:
- Facilitated Diffusion
2.6.A.1 – Facilitated diffusion
Facilitated diffusion, like passive diffusion, involves the movement of molecules in the direction determined by their relative concentrations inside and outside of the cell. No external source of energy is provided, so molecules travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane. However, facilitated diffusion differs from passive diffusion in that the transported molecules do not dissolve in the phospholipid bilayer. Instead, their passage is mediated by proteins that enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior. Facilitated diffusion therefore allows polar and charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane.
Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins. Carrier proteins bind specific molecules to be transported on one side of the membrane. They then undergo conformational changes that allow the molecule to pass through the membrane and be released on the other side. In contrast, channel proteins form open pores through the membrane, allowing the free diffusion of any molecule of the appropriate size and charge.
Carrier proteins are responsible for the facilitated diffusion of sugars, amino acids, and nucleosides across the plasma membranes of most cells. The uptake of glucose, which serves as a primary source of metabolic energy, is one of the most important transport functions of the plasma membrane, and the glucose transporter provides a well-studied example of a carrier protein. The glucose transporter was initially identified as a 55-kd protein in human red blood cells, in which it represents approximately 5% of total membrane protein. Subsequent isolation and sequence analysis of a cDNA clone revealed that the glucose transporter has 12 α-helical transmembrane segments—a structure typical of many carrier proteins. These transmembrane α helices contain predominantly hydrophobic amino acids, but several also contain polar amino acid residues that are thought to form the glucose-binding site in the interior of the protein.
In living systems, the lipid based membrane creates compartments which allow the transport of a selective concentration of water-soluble substances. The ions, small molecules, proteins, and other solutes have different concentration across the membranes. Hydrophilic, polar or charged molecules cannot cross the membrane.
2.6.A.2 – Movement of large polar molecules through membranes
Not every molecule can cross the cell membranes. The molecules should be small and non-polar to traverse the membrane. For eg., glucose is a large molecule that cannot diffuse across the cell membrane. Ions like sodium, potassium, and calcium are charged and are repelled by the cell membrane. Amino acids and nucleic acids are polar and too large to cross the cell membrane. Also, the water movement across the membrane in bulk is difficult at times.
To facilitate these transfer of substances across the membrane, certain integral membrane proteins or the transmembrane proteins are required. They are channel proteins and carrier proteins.
Transmembrane proteins are the proteins present in the cell membrane that facilitate the movement of certain molecules across the membrane. There are certain channel proteins and carrier proteins that accelerate the transport process. 1. Channel Proteins: These help in the entry and exit of substances in the cell. There are two types of channel proteins, open channel proteins, and gated channel proteins. Open channel proteins create a pore in the cell membrane and allow the charged molecules to pass through. The gated channel proteins are either closed or open and regulate the entry and exit of substances.
2. Carrier Proteins: These are present on the cell membrane. They carry the molecules, change the confirmation of the molecules and release the molecules to the other side. Temperature and saturation affect the carrier proteins.
There are certain transmembrane proteins that help in the movement of solutes and ions.
2.6.A.3 – Aquaporins
Aquaporins are ubiquitous family of membrane proteins that facilitate the rapid transport of water across cell membranes and thus play a critical role in maintaining water homeostasis in all living cells. In cell membranes, aquaporins exist as homo tetramers with each monomer forming an independent water channel. Aquaporin family members mediate the bidirectional water flow driven by an osmotic gradient. They enable highly efficient water permeation with flow rates in order of 109 $s^{−1}$.
In addition to water, some aquaporin family members, called aquaglyceroporins transport certain small, neutral solutes such as glycerol, ammonia, urea, arsenite. Aquaporins have a remarkable property of effective water conductance while blocking the flow of protons and thus maintain the electrochemical gradient across cell membranes. In eukaryotic organisms, aquaporins perform a wide variety of physiological functions, such as concentrating urine in kidneys, maintaining lens transparency in eyes maintaining water homoeostasis in brain, cell migration during tumor growth, facilitating a rapid response to shock in yeast , regulation of cell osmolarity in plants, driving the opening and closing of flower petals. Physiological significance of aquaporins is further highlighted by the fact that a number of diseases including brain edema, tumor, obesity manifest abnormal functioning of these water channels.