Membrane Transport

Nestor T. Hilvano, M.D., M.P.H.(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain,

Textbook, 2012.)

Learning Objectives1. Describe plasma membrane and give its

functions.

2. Differentiate the types of transport of molecules across the membrane.

3. Compare and contrast diffusion and osmosis.

4. Discuss briefly what happens when a red blood cells is placed in hypotonic solution? In hypertonic solution?

Plasma MembraneFunctions: • Regulates passage of ions and macromolecules

in and out of the cell• Allows some substances to cross more easily

than others• Allows information to be transferred across the

membrane• Possesses devices for cell attachment• Receptors for hormones • Ion pumps for regulating the internal

environment

Components of Plasma Membrane • The fluid mosaic model states that a

membrane is a fluid structure with a “mosaic” of various proteins embedded in it

• Selectively permeable• Composed mainly of Bilayer of phospholipids

(selective barriers), and some proteins, cholesterol, carbohydrates

Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Transport of Molecules Across the Plasma Membrane

a) Passive transport- move from [high] to [low]; requires no ATP

b) Active transport- move from [low] to [high]; needs ATP

c) Facilitated transport- via carrier protein

d) Exocytosis and endocytosis

Passive Transport

• Cell does not expend energy

• Diffusion – particles spread out from area of high concentration to an area of low concentration gradient.

i.e. O2 and CO2 are small and nonpolar molecules that easily cross the membrane.

• Osmosis – is the net diffusion of water across a selectively permeable membrane

Figure 5.3A

Molecules of dye Membrane

Pores

Net diffusion Net diffusion Equilibrium

Figure 5.4

Osmosis

Solute moleculewith cluster ofwater molecules

Watermolecule

Selectivelypermeablemembrane

Solutemolecule

H2O

Lowerconcentration

of solute

Higherconcentration

of solute

Equalconcentrations

of solute

p. 76

Tonicity • Tonicity is important in maintaining cell

volume (ability to cause a cell to loss or gain water)

• Isotonic solution – solute concentration is the same both intracellular and extracellular

An animal cell (RBC) placed in:

• Hypotonic – will gain water, swell and lyse

• Hypertonic – will lose water and crenate

Facilitated Transport • Uses specific transport proteins in the membrane• Does not requires energy• Substances move down its concentration gradient• Channel proteins- ions channels (gated channel) that

open or close in response to stimulus • Carrier proteins- binds to protein and changes it’s

shape

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

Active Transport

• Requires expenditure of energy (ATP)

• Molecules move against the concentration gradient

• i.e. Na+ - K+ pump in nerve impulse transmission

Faciliatated diffusion and active transport: http://www.youtube.com/watch?v=1ZFqOvxXg9M&feature=related

Endocytosis and Exocytosis • Transport of large molecules

• Exocytosis – export bulky materials

i.e. tearing, insulin release into the blood

stream

• Endocytosis – cells take in macromolecules by forming vesicles/vacuoles from its plasma membrane.

a) phagocytosis, b) pinocytosis

Homework 1. Define – diffusion, osmosis, endocytosis,

exocytosis, channel proteins, carrier proteins, isotonic solution, hypotonic solution, and hypertonic solution.

2. Describe the structure of plasma membrane and give its functions. What is it’s primary component?

3. Compare and contrast passive transport, facilitated transport, and active transport across a membrane as to concentration gradients, energy expenditure, use of carrier/transport proteins, and cite an example of substances.

4. Discuss the effect of hypotonic and hypertonic solution to red blood cells.