Movement of Molecules across Membranes

Membranes are selectively permeable. This means that not every molecule may freely pass from one side of the lipid bilayer to the other. The movement of molecules is accomplished through three modes of transport:

1. Diffusion directly through the Lipid Bilayer

2. Facilitated Transport through the Bilayer by Proteins

3. Bulk Flow via the processes of Endocytosis and Exocytosis

Chapter 2.1

1. Diffusion through the Lipid Bilayer

In this process, lipid soluble molecules enter on one side of the membrane and emerge on the other side. Molecules that diffuse will exhibit net movement (over-all movement) from a region of high concentration to a region of low concentration. This will proceed until both regions have an equal concentration of the molecule (unless pressure builds up). When molecules move from high concentration to low concentration, they are said to be move down the concentration gradient.

Types of Molecules that will Diffuse through a Lipid Bilayer

1. Small Non-polar Molecules

As size increases, the rate of diffusion decreases. Non-polar molecules will not interact with the polar lipid heads which are

found on the outer surfaces of the membrane. As well, they will not interact with the polar water molecules located in the environment and in the cytoplasm. As a result, non-polar molecules pass through the non-polar interior of the membrane.

Examples: O2 (Oxygen Gas), N2 (Nitrogen Gas), CO2 (Carbon Dioxide Gas)

2. Small Slightly Polar Molecules

As the degree of molecular polarity increases, the rate of diffusion decreases. As size increases, the rate of diffusion decreases. The polar regions of molecules will interact with the polar lipid heads and

water molecules. As a result, they do not readily enter the interior of the membrane. If they do enter the hydrophobic interior of the membrane, they simply pass through since they do not interact with the non-polar lipid tails located in this region.

Examples: Ethanol, Glycerol, Glucose, Urea

Diffusion through the Lipid Bilayer

Type of Molecule Rate of Diffusion ExamplesSmall Non-polar Rapid Diffusion – Rate

Decreases with Increasing Size

O2, N2, CO2

Small Slightly Polar Slow Rate of Diffusion – Rate Decreases with Increasing Net Polarity and Size

Ethanol > Glycerol > Glucose(in regard to rate of diffusion)

Small Charged Don’t Diffuse – Membrane is Highly Impermeable to Charged Particles

Ions – Na+, Cl-, K-, NO3 -

2. Facilitated Transport through the Membrane by Proteins

Facilitated transport (translocation) involves proteins that move molecules and/or that are charged, too polar, or too large to diffuse through the lipid bilayer. Protein facilitated transport includes both passive transport (not requiring ATP energy) and active transport (requiring ATP energy).

Passive Transport via Proteins is called Facilitated Diffusion

When uncharged molecules move from high concentration to low concentration they are said to move down the concentration gradient. However, charged ions and molecules are said to move down the electrochemical gradient. Molecules and ions are either move through channels within a protein or are carried across when protein changes its conformation (shape).

1. Protein Channels

Some integrin proteins in the bilayer have a channel (passageway) that from one side of the membrane to the other located within their structure. The passageway within a channel protein will allow only one specific molecule or ion to pass through it. For example, some channel proteins only allow K+ ions to pass through the membrane and are called potassium leak channels.

Some protein channels have the ability to open in response to a messenger molecule (ligand) that binds to the channel protein or in response to a change in membrane potential (charge difference between the two sides of the membrane). Channels that open and close are called gated channels.

Water is a highly polar molecule and cannot diffuse through the lipid bilayer. However, water appears to rapidly diffuse across membranes. This “diffusion” of water across the membrane has traditionally been called osmosis. The reason water can move from high to low concentration across membranes is due to special proteins called aquaporins. These proteins have channels that allow water molecules to pass freely through the membrane. In actuality, water molecules move via facilitated diffusion using aquaporin protein channels.

2. Carrier Proteins

Some integrin proteins may attach to molecules that normally cannot diffuse through the lipid bilayer or to molecules that diffuse slowly through the bilayer. These molecules are called transport proteins or carrier proteins, since they are directly bond to the specific molecules and/or ions they transport across the membrane. When the molecules and/or ions bind the transport protein, a conformational change in the transport protein’s structure occurs. The change in the transport protein’s shape then facilitates the movement of the molecules and/or ions through the membrane. Some carrier proteins transport molecules in both directions across the membrane, while others transport molecules in just one direction.

In passive transport, no ATP energy is used and molecules always move down the concentration gradient from high to low concentration.

Since proteins are large, the time required for flip-flop is significant. It is now thought that flip-flop is not the main mode for moving molecules into cells.

Comparison of Passive Transport Rate by Protein Channels and Carrier Proteins

The rate of movement by carrier proteins is limited by the number of proteins present in the membrane and by the rate at which protein movement occurs. Protein channels are open spaces through which molecules freely migrate, and the movement of molecules or ions through protein channels is not limited. As a result, protein channels allow for faster rates of movement at high concentrations.

Active Transport via Proteins

Carrier Proteins

Some integrin proteins attach to molecules and/or ions that normally cannot diffuse through the lipid bilayer or molecules that diffuse slowly through the bilayer. With the expenditure of energy (ATP or membrane potential), the protein will deposit the specific molecules and/or ions that it attaches to on the other side of the membrane. Some carrier proteins transport molecules and/or ions in both directions across the membrane, while others transport in just one direction.

Velocityof MovementacrossMembrane

Concentration Difference

Carrier Protein

Channels Protein

Channel Protein – Rate of movement increases as the concentration gradient increases.

Carrier Protein – Rate of transport is limited by the number of carrier proteins present, since each protein has a maximum rate at which it can transport molecules and/or ions.

Molecules move molecules and/or ions up the concentration gradient. In this process, ATP energy or potential energy in the form of an electrochemical gradient (membrane potential) is used.

Since proteins are large, the time required for flip-flop is significant. It is now thought that flip-flop is not the main mode for moving molecules into cells.

3. Bulk Flow - Endocytosis and Exocytosis

Endocytosis is the inward movement of substances into a cell. This involves changing the position of the cell membrane to surround a large molecule or food particle. Microfilaments attached to the cell membrane are responsible for membrane movement and fluidity of the membrane makes it possible to pinch the membrane off to form a vesicle (food vacuole) inside the cytoplasm of the cell. Three types of endocytosis exist: phagocytosis, pinocytosis, and receptor mediated endocytosis.

Phagocytosis – pseudopods (extensions of the membrane) extend around a particle and fuse together to enclose it in a membrane bound phagosome (food vacuole).

Pinocytosis - the cell membrane invaginates to form a long, narrow channel. Fluid and particles around the cell enter into the channel and the membrane pinches-off to form a vesicle.

Receptor Mediated Endocytosis –specific molecules bind to receptors embedded in the plasma membrane. Then, the plasma membrane moves inward enclosing the particle and receptors in a vesicle. After the particles in the vesicle are used-up, the receptors may be recycled to the plasma membrane if the vesicle fuses with it.

Exocytosis is the outward movement of large molecules or substances that are enclosed in a vesicle or vacuole from the cell. This occurs when vesicles in the cytoplasm fuse with the cell membrane and deposit (secrete) the contents of the vacuole or vesicle in the environment outside the cell.

After a lysosome and food vacuole fuse, digestion occurs. The valuable biomolecules produced by digestion move into the cytoplasm and the wastes that remain are carried to the membrane for exocytosis.

Vesicles that pinch off from the trans-cisternae of the Golgi apparatus move to the membrane and processed proteins (hormones and enzymes) are secreted.