Cell Membrane Transport Mechanisms

Cell Membrane Structure and Function

• The cell membrane is a lipid bilayer with cell membrane transport proteins.
• Penetrating proteins interrupt lipid bilayer function as transport proteins.
• Proteins that allow free movement of water are called channel proteins.
• Carrier proteins bind with molecules or ions and then move them through interstices.

Diffusion vs Active Transport

• Diffusion is the random molecular movement of substances molecule by molecule through intermolecular space or with a carrier protein.
• Active transport is the movement of ions or other substances across the membrane with a carrier protein, from low concentration to high concentration.

Diffusion

• Molecules and ions are constantly moving due to heat, which is a form of kinetic energy.
• Greater motion is associated with higher temperatures.
• Motion ceases at absolute 0 temperature.
• In diffusion, a moving molecule hits a stationary molecule, transferring some kinetic energy and continuing movement.

Diffusion Through the Cell Membrane

• There are two types of diffusion through the cell membrane: simple diffusion and facilitated diffusion.
• Simple diffusion is the kinetic movement of molecules or ions through a membrane without carrier proteins.
• The rate of simple diffusion depends on the amount, velocity, and number of molecules.
• Simple diffusion occurs in two ways: through interstices of the lipid bilayer and through water channels penetrating the membrane.
• Facilitated diffusion is the interaction with a carrier protein.

Diffusion Through the Lipid Bilayer

• Lipid solubility determines the rate of diffusion through the lipid bilayer
• Oxygen (O2), nitrogen (N2), and carbon dioxide (CO2) are examples of lipid-soluble substances that can diffuse rapidly through the lipid bilayer

Diffusion Through Protein Channels

• Cell membranes contain protein "pores" called aquaporins that permit the passage of water and other lipid-insoluble molecules
• Aquaporins act as channels for water molecules to pass through the membrane
• Urea, a molecule 20% larger than water, has a 1000 times slower penetration rate due to its size

Vesicle Transport

• Vesicle transport requires the use of energy
• It involves the transport of larger substances by membranous sacs

Exocytosis

• Exocytosis is the process of secreting materials from the cell to the interstitial fluid outside the cell
• Vesicles fuse with the cell membrane, releasing their contents outside the cell

Endocytosis

• Endocytosis is the process of taking in contents from outside the cell
• The plasma membrane traps substances by folding inward, forming vesicles
• There are several types of endocytosis:

Types of Endocytosis

• Phagocytosis: vesicles fuse with lysosomes, which break down particles into component molecules, also known as "cell eating"
• Pinocytosis: the plasma membrane folds inward and engulfs interstitial fluids containing solutes, also known as "cell drinking"
• Receptor-mediated endocytosis: receptors bind to molecules in interstitial fluids, folding and enclosing to form new vesicles

Cellular Fluids

• Interstitial fluid is the fluid outside of the cell.
• Cytosol is the inner part of the cell.
• The plasma membrane (or cell membrane) separates interstitial fluid and cytosol.

Membrane Transport

• Passive transport does not require energy.
• Active transport requires energy.

Types of Passive Transport

• Simple diffusion: small non-polar solutes move between lipid bilayer.
• Facilitated diffusion: small charged or polar solutes pass through plasma membrane.

Channel-Mediated Transport

• Ions move through a water-filled channel.
• There are three types of channels: leak channels, gated channels, and stimulus channels.

Carrier-Mediated Transport

• Small polar molecules move across the membrane with carrier proteins.
• Glucose binds to a carrier protein, causing a shape change that moves the glucose molecule to the other side.

Osmosis

• Osmosis is the passive movement of water through a selectively permeable membrane.
• It occurs when there is a difference in water concentration.
• Water can move through the membrane in two ways:
  • Between phospholipid molecules that make up the plasma membrane.
  • Through integral protein water channels called aquaporins.
• Molecules move from high concentration to low concentration to achieve equilibrium.

Diffusion

• Movement of substances from high concentration area to low concentration area

Active Transport

• Movement of solute against concentration gradient (from low to high)
• Two types: primary transport and secondary active transport

Primary Transport

• Cellular protein pump called ion pump moves ions across the membrane against their concentration gradient
• ATP binds to sodium ions, breaks down to ADP and Pi, releasing energy
• Energy is used to change the shape of the pump, moving ions to the other side
• Potassium ions move back, reverting the shape of the pump

Secondary Active Transport

• Substance is moved against its concentration gradient by the movement of a second substance down its concentration gradient
• Energy is provided by the movement of the second substance from high to low concentration

Types of Secondary Active Transport

• Symport: two substances moved in the same direction
• Antiport: two substances moved in opposite directions

Vesicular Transport

• Transport by vesicle, a membrane-bound sac filled with material