Lecture 3 0 Membrane and Membrane Transport Genalin

Lecture 3. 0 – Membrane and Membrane Transport Genalin Lagman Taguiam Spring 2012 -2013

Course Outcomes • On completion of the week’s activity, the students should be able to • describe the basic transport mechanisms of cell membrane (selective permeability, diffusion, osmosis, active transport, passive transport, exocytosis, endocytosis, phagocytosis); • define the terms hypertonic, hypotonic, isotonic.

Plasma Membrane • encloses cell contents • mediates exchanges with the extracellular environment • plays a role in cellular communication • acts as a selectively permeable barrier

Plasma Membrane – substances move across the plasma membrane by passive processes, which depend on the kinetic energy of molecules or on pressure gradients, and by active processes, which depend on the use of cellular energy adenosine triphosphate (ATP)

Membrane Transport • diffusion – movement of molecules (driven by kinetic energy) down a concentration gradient – Movement of electrolytes and other substances from an area of higher concentration to an area of lower concentration – fat-soluble solutes can diffuse directly through the membrane by dissolving in the lipid

Diffusion

Membrane Transport • facilitated diffusion – passive movement of certain solutes across the membrane either by their binding with a membrane carrier protein or by their moving through a membrane channel – as with other diffusion processes, it is driven by kinetic energy, but the carriers and channels are selective

Diffusion

Membrane Transport • osmosis – diffusion of a solvent, such as water, through a selectively permeable membrane – water diffuses through membrane pores (aquaporins) or directly through the lipid portion of the membrane from a solution of lesser osmolarity (measure of solute concentration) to a solution of greater osmolarity

net osmosis ceases when the solute concentration on both sides of the plasma membrane reaches equilibrium

Osmosis – the presence of solutes unable to permeate the plasma membrane leads to changes in cell tone that may cause the cell to swell or shrink

Osmosis • hypotonic – solutions that cause a net water gain of the cells

Osmosis • hypotonic – contain lesser amount of solutes than the serum of the blood

Osmosis • isotonic – solutions that cause neither gain nor loss of water from cells

Osmosis • isotonic – contain the same amount of solutes or electrolytes as the serum of the blood

Osmosis • hypertonic – solutions that cause a net loss of water from cells

Osmosis • hypertonic – contain more amount of solutes than the serum of the blood

Membrane Transport • filtration – occurs when a filtrate is forced across a membrane by hydrostatic pressure – non selective and limited only by pore size – the pressure gradient is the driving force

Membrane Transport • active transport – “solute pumping” – depends on a carrier protein and ATP – substances transported move against concentration or electrical gradients

Membrane Transport • vesicular transport also requires ATP – ¹exocytosis • anchors the vesicles to the plasma membrane, ejects substances (hormones, wastes, secretions) from the cell – ²endocytosis • brings substances into the cell in protein-coated vesicles – if the substance is particulate, the process is called phagocytosis – if the substance is dissolved molecules, the process is pinocytosis

References • Marieb, E. N. (2010). Human anatomy and physiology (8 th Ed. ). San Francisco, Pearson. • Marieb, E. N. (2006). Human anatomy and physiology (7 th Ed. ). San Francisco, Pearson. • Tortora, G. J. (2006). Principles of anatomy and physiology. (11 th. Ed. ). New Jersey, John Wiley and Sons, Inc.