Integral Proteins Some transmembrane transport proteins are porins that form aqueous channels through membranes Some integral proteins anchored to the membrane provide for cell recognition Some integral proteins anchored to the membrane participate in cell signaling

Bacterial outer membranes

Examples of proteins anchored in the plasma membrane by lipids and glycolipids

Membrane Mobility Proteins and phosphlipids cannot move quickly between faces of the membrane Lateral movement through the membrane is allowed Membrane proteins associated with the cytoskeleton or other proteins are restricted from moving Plasma membranes of some cells are divided into functional domains

Mobility of membrane proteins

Membrane Mobility Functional domains of membranes restrict protein movement to the area with which they are associated Lipid rafts inhibit free diffusion of some membrane proteins Interaction of lipid rafts with the cytoskeleton stabilizes them

A polarized intestinal epithelial cell

Structure of lipid rafts

Glycocalyx Carbohydrate groups of glycoproteins and glycolipids that are exposed on the membrane surface make up the glycocalyx The glycocalyx provides protection for the cell surface Carbohydrate groups in the glycocalyx act as cell surface markers

The Glycocalyx

Binding of selectins to oligosaccharides

Small Molecule Transport Passive diffusion is the simplest method for molecules to cross the plasma membrane – Molecules dissolve in the bilayer to diffuse across then dissolve in the aqueous solution on the other side – No membrane proteins are needed – Molecule transport is down a concentration gradient – Passive diffusion is a nonselective process

Molecule Transport Facilitated diffusion occurs along a concentration gradient or as a result of electric potential It requires proteins to mediate transport of the molecules across the bilayer Carrier proteins and channel proteins aid in facilitated diffusion across membranes

Active Transport Active transport uses energy provided by another reaction to drive uphill transport of molecules in energetically unfavorable directions Ion pumps are used to maintain ion gradients across membranes through active transport coupled to hydrolysis of ATP ABC transporters convey nutrients in prokaryotes and remove toxins in eukaryotes

Model for operation of the Na + -K + pump

Ion gradients across the plasma membrane of a typical mammalian cell

Structure of an ABC transporter

Ion-Gradient Driven Active Transport Active transport driven by ion gradients couples molecule transport in an unfavorable direction with transport of another molecule in an energetically favorable direction Symport vs. Antiport

Active transport of glucose

Glucose transport by intestinal epithelial cells (Part 1)

Glucose transport by intestinal epithelial cells (Part 2)

Glucose transport by intestinal epithelial cells (Part 3)

Examples of antiport

Endocytosis Eukaryotes surround material for internalization in an area of plasma membrane that buds off internally in the process of endocytosis Phagocytosis: cell eating – Occurs in specialized cells – Produce phagosomes and phagolysosomes Pinocytosis: cell drinking – Receptor-mediated endocytosis is the best known form

Phagocytosis

Examples of phagocytic cells

Receptor-Mediated Endocytosis Allows for selective uptake of specific macromolecules Molecules bind with receptors in clathrin- coated pits Pits bud to form clathrin-coated vesicles with the assistance of dynamin Vesicles fuse with early endosomes where contents are sorted to their destination

Clathrin-coated vesicle formation (Part 1)

Clathrin-coated vesicle formation (Part 2)

Formation of clathrin-coated pits (Part 1)

Formation of clathrin-coated pits (Part 2)

Receptor-Mediated Endocytosis Cholesterol uptake in mammals has provided understanding of receptor mediated endocytosis on the molecular level Fluid phase endocytosis also functions via receptors, allowing nonspecific uptake of extracellular fluids Endocytosis may also occur without the use of clathrin

Structure of LDL

The LDL receptor

Clathrin-Independent Endocytosis Cells may use caveolae for uptake of molecules, independent of clathrin Caveolin organizes creation of caveolae Caveolae carry out receptor-mediated endocytosis via transmembrane receptors, but cavoelin and caveolae lipids also function as receptors for molecule uptake Macropinocytosis may also be used to uptake fluids in a clathrin-independent manner

Caveolae

Sorting in early endosomes

Recycling of synaptic vesicles

Protein sorting by transcytosis

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