Question

The semi-permeable cell membrane allows something to pass through and prevents others from passing. Describe what types of things would be allowed to pass through a cell membrane. Briefly Explain.

Answer 1

The selectivity of semi-permeable cell membrane ( lipid bilayer) enables cell to discriminate between different molecules to allow or prevent them from passing in or out. Lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like oxygen (O₂) and carbon dioxide (CO₂)- easily diffuse through the hydrophobic lipid bilayer. However, it is impermeable to ions, small hydrophilic molecules (glucose) and larger biomolecules RNA and nuclei acids- proteins). Two major types of trnasport systems exist for passage of molecules in cells, namely, passive and active transport.

Passive transport: It inconstitutes the transport of molecules without the expense of energy on the basis of movement of molecules from higher concentration to lower concentration. It does not work against concentration gradient. Includes: simple diffusion. osmosis, filtration, facilitated diffusion.

Simple diffusion: Greater the concentration gradient and permeability of the cell membrane, greater will be the rate of passive diffusion for molecules like gases and water. It occurs down the concentration gradient and the rate depends upon concentration gradient, hydrophobicity, size, charge, if the molecule has a net charge.

Osmosis: movement of solvent (water) across membrane in response to the concentration gradient of solute. Isotonic ( no net movement across, equal movement of water in both directions), Hypertonic (water moves out, cell shrinks) and Hypotonic ( water moves in and cell swells.)

Filtration: Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by for instance, the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through the membrane.

Facilitated passive diffusion: Involves movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane . Also, carrier-mediated diffusion, large molecules, such as glucose, insoluble in lipids are too large to fit into the porins (proteins) , therefore, bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane.Transporter proteins- Permease or Porter or carrier protein are specific but some can transport multiple compounds. Binding of the solute with the transporter protein changes the 3D structure of the transporter protein and this change in shape allows the solute to carried across the membrane.

Active transport: Involves transport of molecules agsinst concentration gradients with the expense of cellular energy. Proteins such as permease or transporter protein carry the molecules across the membrane and the energy required to transport is obtained by ATP or Ion gradient. The substances transported by active transport are glucose, aminoacids, organic acids and inorganic ions (SO4–, PO4–, K+ etc). Active transport can be primary or secondary. For example: Lac operon (transport of lactose by permease), Na-K pump, etc.

Primary active transport: is also called direct actice transport and utilizes hydrolysis of energy rich molecules such as ATP provide energy required for transport of molecules form lower concentration to higher concentration across membrane. eg. Sodium potassium pump for maitaining membrane potential.

Secondary active transport: exhibits no direct coupling of ATP. Instead, the electrochemical potential difference created by pumping ions out of the cell is instrumental in this. Also, one type of molecule migrates from higher concentration to lower concentration, releasing energy. which in turn is used to transport other molecule from its lower concentration to higher concentration across cell membrane. The two main forms of active transport are antiport and symport. It is also known as co-transport.

Antiport: two species of ion or solutes are pumped in opposite directions across a membrane. eg- the sodium-calcium antiporter (exchange)

Symport: uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient), i.e the movement of two solutes in the same direction. eg: glucose symporter SGLT1 co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell.

Other larger nolecules abd entities are transported via : taken up ( endocytosis) and sent out (exocytosis) , cell drinking ( pinocytosis), ingested ( phagocytosis-cell eating).