Transport Movement across the Cell Membrane Lipids of

Transport Movement across the Cell Membrane

Lipids of cell membrane § Membrane consists primarily of phospholipids u phospholipid bilayer inside cell phosphate hydrophilic lipid hydrophobic outside cell S 1

The Fluidity of Membranes § Phospholipids in the membrane are fluid (can move easily) u Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Figure 7. 5 A S 2

Semi-permeable membrane § Will allow passage through the § membrane But need to control what gets in or out u membrane needs to be semi-permeable sugar aa lipid H 2 O salt NH 3 So how does a semipermeable membrane work? S 3

Phospholipid bilayer § What molecules can get through directly? inside cell NH 3 outside cell lipid salt sugar aa Small lipids can slip directly through the phospholipid cell membrane, H 2 O S 4

Getting through cell membrane § Diffusion u Passive transport of small molecules § high low concentration gradient § Facilitated Diffusion u u Passive transport of larger or polar molecules through a protein channel § high low concentration gradient § Active transport u diffusion against the concentration gradient § low high u u uses a protein channel (pump) requires ATP energy S 5

Diffusion (passive transport) § movement from high low concentration S 6

Diffusion of two solutes (passive) § Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances S 7

Osmosis, the diffusion of water § Water goes from HIGH to LOW concentration “passive transport” u no energy needed (does not require ATP) u diffusion osmosis S 8

Simple diffusion across membrane Which way will lipid move? lipid inside cell low lipid lipid high outside cell lipid lipid

Facilitated Diffusion through a Channel § Movement from high to low sugar inside cell sugar low Which way will sugar move? high outside cell sugar sugar

Semi-permeable cell membrane § But the cell still needs control u membrane needs to be semi-permeable § specific channels allow specific material in & out inside cell salt outside NH 3 cell H 2 O aa sugar

Active Transport (needs ATP energy) § Membrane proteins act as a PUMP for specific molecules u u shape change transports a substance from one side of membrane to the other requires energy in the form of ATP shape change transports high low “The Doorman” S 9

Active transport (Low to High) § Cells may need molecules to move against concentration situation protein pump u requires energy § ATP for NRG Na+/K+ pump in nerve cell membranes S 10

Transport summary

How about large molecules? § Moving large molecules into & out of cell through vesicles & vacuoles u Endocytosis (moving into cell) § phagocytosis = “cellular eating” § pinocytosis = “cellular drinking” § receptor-mediated endocytosis u Exocytosis (moving out) exocytosis

Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by chemical signal

More than just a barrier… § Expanding our view of cell membrane beyond just a phospholipid bilayer barrier u phospholipids plus…

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

Membrane Proteins § Proteins determine most of membrane’s specific functions u cell membrane & organelle membranes each have unique collections of proteins § Membrane proteins: u u peripheral proteins = loosely bound to surface of membrane integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane protein

Membrane Carbohydrates § Play a key role in cell-cell recognition ability of a cell to distinguish neighboring cells from another u important in organ & tissue development u basis for rejection of foreign cells by immune system u

Any Questions? Fluid Mosaic Model