TRANSPORT ACROSS CELL MEMBRANES CLASS 3B + 4 LEARNING OBJECTIVES PART ONE To know the principles of membrane transport Phospholipids on their own form liposomes which are proteinfree lipid bilayers. The cell membrane has membrane proteins that many molecules rely on to cross the plasma membrane. Permeable: small nonpolar molecules such as O , CO2, N 2nd2steroid hormones. Semipermeable: small uncharged polar molecules (H O, ethanol and glycerol) and larger uncharged polar 2 molecules (amino acids, glucose and nucleosides) Impermeable: ions such as H+, K+, Ca 2+ To recognise the difference between transporter and channel Transporter: carriers which bind the specific solute to be transported and undergo a series of conformational changes in order to transfer the bound solute across the membrane. Can be passive or active. Channel: hydrophilic pores that extend across the lipid bilayer that permit diffusion. Selectively permeable. To understand the mechanism of osmosis The electrochemical gradient has 2 components: concentration gradient and membrane potential influence. Osmosis: water moves passively across the cell membrane down a concentration gradient. Water molecules diffuse rapidly through aquaporins channels which selectively conduct H O2molecules. H O 2s only semi permeable to the membrane therefore aquaporins are used to transport more. To be familiar with transporters and their different methods of functioning Passive transporters: move solute across its electrochemical gradient. Glucose carrier: dependent only on concentration gradient. Active transporters: move solute against its electrochemical gradient, mediated by carrier proteins and coupled to a source of metabolic energy. Coupled transporters: couple the uphill transport of one solute across the membrane to the downhill transport of another. Symport Antiport Uniport ATPdriven pumps: couple uphill transport to the hydrolysis of ATP. Light driven pumps: use energy from sunlight (bacteria) + + + Na pump: hydrolyses ATP to ADP to transport Na out of the cell, it couples with the inward movement of K . Not only a carrier protein but also an enzyme (ATPase). Uses 30 of the ATP for the cell. Animal cells would burst wit2+ut it due to the high solute 2+ncentration inside the cell. Driven by addition and removal of phosphate group. Ca pump: actively transports Ca out of the cell. Membranebound ATPase in the sarcoplasmic reticulum of muscle cells receives Ca . Ca ATPase is similar to Na ATPase because it is phosphorylated and dephosphorylated during the pumping cycle. PART TWO To understand the structure and function of ion channels Channels are the simplest way for watersoluble molecules to cross the plasma membrane. Ion channels are concerned with the movement of inorganic ions (Na+, K+, Cl) and are very selective. Ion channels can transport ions at a faster rate than carrier proteins. The flow of ions through the ion channel changes the voltage across the membrane (membrane potential) which alters the electrochemical driving forces for transmembrane movements of other ions.