Membrane Transport

Cell Biology September 10, 2013

Membrane transport

• Plasma Membrane is selectively permeable

• Membrane Transport refers to the ability to move ions and organic molecules across membranes selectively

• Membrane transport proteins are major players for membrane transport

Ion concentrations inside and outside of a typical mammalian cell

Types of membrane proteins

• Based on the passage of small diffusible molecules through cell membranes, membrane proteins are divided into:

• Transporters - Can shift small molecules from one side of the membrane

to the other by changing its shape

• Channels - Form tiny hydrophilic pores in the membrane through

which solutes can pass by diffusion

Net flux

• Influx- refers to movement of substance into the cell • Efflux- refers to movement of substance out of the cell

• Movement across a membrane either by passive or active transport results from net flux leading to a particular ion or compound enter or exit from the cell

Influencing factors for movement of solutes across membranes

Movement of solute across membranes depends upon: - Concentration gradient; The region along which the density of a chemicalsubstance increases or decreases

- Electrochemical potential; The sum, or combined effect of concentration gradient and the charge gradient across the membrane

Mechanisms of solute movement across membranes

• Simple diffusion (across lipid bilayer/ aqueous protein lined channel)

- Direct movement of solute across the membranes based on the differences of solute concentrations on both sides

• Facilitated diffusion (passive transport)- Movement of solutes across their concentration gradient

(charge, solute concentration, etc. ) with the help of transport proteins (no input of energy)

• Active transport - Movement solutes against their concentration gradient and is

energy dependent

Diffusion

• Diffusion refers to spontaneous (unassisted) movement of substances from a region of higher to a region of lower concentration

• Diffusion is always movement toward equilibrium and is therefore a spontaneous process.

• Because membranes have a hydrophobic interior, simple diffusion is typically a means of transport only for gases, nonpolar molecules, or small polar molecules such as water, glycerol, or ethanol.

Oxygen diffusion across membranes

• Oxygen is a gas that traverses the hydrophobic lipid bilayer readily and therefore moves across membranes by simple diffusion.

• This behavior enables erythrocytes in the circulatory system to take up oxygen in the lungs and release it in body tissues

Diffusion of oxygen in lungs and tissues

• In the capillaries of body tissues, oxygen concentration is low, oxygen is released from hemoglobin and diffuses passively from the cytoplasm of the erythrocyte into the blood plasma and from there into the cells lining the capillaries

• In the lungs, oxygen diffuses from the inhaled air in the lungs, where its concentration is higher, into the cytoplasm of the erythrocytes, where its concentration is lower

Facilitated diffusion • Transmembrane proteins play an essential role in this process.

Consider the uptake of glucose into the erythrocyte.

- The cell needs glucose for its metabolic processes. The concentration of glucose in blood plasma (approximately 5 mM) is much higher than the concentration of glucose within the erythrocyte.

- However, because of its large size, glucose cannot simply diffuse through the lipid bilayer.

- It is moved rapidly across the erythrocyte membrane, down its concentration gradient, by associating with a glucose transporter or permease that changes conformation and allows passage of glucose