1.4

The cell membrane acts as a barrier, controlling what substances can enter and leave the cell. Movement through the cell can occur passively (without using energy), or actively (using energy). The cell membrane also has the capacity to engulf a particle, so it can be moved into the cell by endocytosis or out of the cell by exocytosis.

As previously stated, passive movement of particles or ions across the cell membrane does not require an expenditure of energy. A difference in the number of particles on either side of the membrane creates a chemical gradient, while a difference in charged particles or ions creates an electrical gradient. As particles can influence both, they are collectively referred to as an electrochemical gradient. The three processes of passive movement include (1) diffusion, (2) osmosis, and (3) facilitated diffusion and are depicted in Figure 1.8.

Figure 1.8 Mechanism of membrane transport: In diffusion, particles move freely to become equally distributed across the membrane.

Diffusion is the process by which molecules move from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side. This flow of molecules continues until they are equally distributed (their concentrations are equal) on both sides of the membrane. Diffusion through the membrane often occurs easily with lipid-soluble molecules like oxygen, alcohol, carbon dioxide, and fatty acids. In other instances, substances can diffuse through small openings (pores) in the membrane. The rate (speed) of diffusion is also impacted by several factors. The number of openings in the cell membrane can determine how fast or slow transport occurs. Temperature also affects this, as diffusion rates increase in direct proportion to increased temperatures.

Figure 1.9 Mechanism of membrane transport: In osmosis, osmotically active particles regulate the flow of water.

Osmosis is the movement of water from an area of low solute (few substances dissolved in water) to an area of high solute (many particles in the water) concentration until an equilibrium is achieved. Water moves through channels called aquaporins. The pressure that water generates as it moves through the membrane is called osmotic pressure.

Figure 1.10 Mechanism of membrane transport: Facilitated diffusion uses a carrier system.

Facilitated diffusion uses a transport protein to help lipid insoluble or large molecules pass through the membrane, that otherwise would not be able to pass through. The transmembrane movement of glucose is an example of this process. Glucose associates with unique transport proteins on the outer surface of the membrane, are moved through the membrane, and then are released into the intracellular space. The speed at which this occurs depends on the number of available transport proteins and the rate at which they can bind to and release the substance they are transporting.

Because small ions like sodium and potassium have an electrical charge, it is difficult to pass through the lipid layer of the cell membrane. In many instances, cells require a rapid movement of ions either in or out of the cell in order to function. In order to hasten their movement, their way across is via facilitated diffusion through selective ion channels. Ion channels are made of integral proteins that span the width of the cell membrane. Specific stimuli cause these proteins to open a channel (or gate) through which the ions can easily move across. Such channels are very selective, only allowing passage of ions for which they are matched.

Figure 1.11 Mechanism of membrane transport: In active transport, selected molecules are transported across the membrane using the energy-driven (Na/K-ATPase) pump.

Active transport is used when cells use energy to move ions against an electrical or chemical gradient. The most studied active transport system is the sodium-potassium (Na⁺/K⁺)-ATPase pump. This pump moves sodium from inside the cell to the extracellular region and returns potassium to the inside of the cell. Energy used for this process occurs by splitting the high-energy phosphate bond in ATP by the enzyme ATPase. If this process did not occur, sodium would remain in the cell and water would follow, causing the cell to swell.

There are two types of active transport systems, primary and secondary.

In primary active transport, ATP is used directly to transport the substance.

In secondary active transport, energy is derived from the primary active transport of one substance, usually sodium, for the cotransport of a second substance as depicted in Figure 1.12, top panel.

When sodium leaves the cell by primary active transport, a concentration gradient is developed. This results in storage of energy because sodium is wanting to diffuse back into the cell. Two groups of secondary active transport are available.

Cotransport (symport) systems consist of sodium ions and the solute being transported in the same direction.

Counter-transport (antiport) systems consist of sodium ions and the solute being transported in the opposite direction. An example of cotransport occurs in the intestine when glucose and amino acid absorption is paired with sodium transport.

Figure 1.12 Secondary active transport systems. Top: Symport or cotransport carries the transported solute (S) in the same direction as the sodium ion. Bottom: Antiport or counter-transport carries the solute and sodium in the opposite direction.

Biomacromolecules such as proteins, polysaccharides, and nucleic acids, because of their extremely large size, cannot be transported across cell membranes by carrier proteins and require special transport methods.

Endocytosis occurs when a molecule outside of the cell is enclosed in an invagination of the cell membrane forming a vesicle inside the cell. This process includes pinocytosis and phagocytosis.     

Figure 1.13 Mechanism of membrane transport: In pinocytosis, the membrane forms a vesicle that engulfs the particle and transports it across the membrane, where it is released.

Pinocytosis, or “cell drinking,” engulfs small solid or fluid particles such as proteins and electrolytes.

Phagocytosis means “cell eating” as the membrane engulfs and then kills microorganisms or other particulate matter. Once the particle is enclosed, a phagosome is formed. These move into the cytoplasm, where it joins with a lysosome to be destroyed. Macrophages and leukocytes   (particularly neutrophils), types of white blood cells, are programmed for this.

Exocytosis is the opposite of endocytosis in that it secretes intracellular substances into the extracellular space. It is important for removing cellular debris and releasing hormones made in the cell.

Membrane Potentials

A membrane potential is the difference in voltage between the inside and outside of a cell. Membrane potentials in nerve and muscle cells are needed to generate nerve impulses, or action potentials and muscular contractions. In other cells, changes in the membrane potential can cause hormone secretion.

Electrical potential is defined as the ability of separated electrical charges of opposite polarity (+ and -) to do work and is measured in volts (V).

Potential difference is the difference between the separated charges.

As the potential difference from one side of the membrane to the other is relatively small, it is measured in millivolts (mV). Extra and intracellular fluids are electrolyte solutions containing 150-160 mmol/L of positively charged ions and an equal concentration of negatively charged ions. These ions generate and conduct membrane potentials. The accumulation of the ions on the surface of the membrane contributes to the resting membrane potential (RMP).

A diffusion potential describes the voltage generated by ions that diffuse across the cell membrane. In order for ions to diffuse, the membrane must be permeable to that ion, and a concentration gradient must exist. An equilibrium potential occurs when the diffusion and electrical forces are balanced on both sides of the membrane.