Transport Across Cell Membranes

Examples of Cell Membrane Transport

Pumps and Transporters Present in Different Organisms

Animal cells contain Na+/K+ pumps and Ca2+ pumps, while plant, fungal, and bacterial cells contain H+ pumps; all utilize transporters.

Transport of ions and molecules across cell membranes is important for all cells. Different types of cells have evolved a variety of transmembrane molecules that interact in complex ways. Primary active transport, generating an electrochemical gradient powered by the hydrolysis of ATP, is carried out in animal cells mainly by Na+/K+ pumps and Ca2+ pumps. In plant, fungal, and bacterial cells, a gradient is generating by H+ pumps. All of these pumps require energy input to move ions across the plasma membrane against their electrochemical gradient.

ATP-binding cassette (ABC) transporters are a large family of ATPase pumps that are used to transport small molecules across the plasma membrane. Both bacterial and eukaryotic cells use specialized ABC transporters to move a variety of substrates into and out of cells. For example, some ABC transporters move drugs and other foreign substances from the liver into bile as a form of detoxification. Some drug-resistant cancer cells show an overexpression of ABC transporters. And several genetic illnesses, such as cystic fibrosis, involve faulty ABC transporters.

All cells also utilize transmembrane proteins in both secondary active transport and passive transport. Ion channels specific to potassium, calcium, and anions have been identified in both plants and animals. Sodium channels are present only in animal cells. Similarly, cotransporters in animal cells most often rely on Na+ gradients, while those in plants and bacteria utilize H+ gradients.

In animal cells, secondary active transport systems typically use the energy stored in Na+ gradients, while plant and bacterial cells more often drive secondary active transport with H+ gradients.

Role of K+ Leak Channels

Potassium leak channels are important in maintaining membrane potential.

Differing concentrations of solutes on either side of a membrane result in a chemical gradient of each solute. When the solutes are charged particles such as ions, however, they also contribute to a difference in electrical potential across the plasma membrane. The Na+/K+ pump transports three Na+ ions out of the cell for every two K+ ions it transports in, which results in a membrane potential. However, positive and negative ions move across the cell membrane by other processes as well, and the number of movements required to alter the membrane potential is small.

A potassium (K+) leak channel is an ungated channel specific for K+ ions that aids in maintaining resting membrane potential. This channel allows K+ ions to move in either direction across the cell membrane. K+ ions are driven across the membrane in response to change in electrical potential but also in response to their concentration gradient. The equilibrium maintained by the electrical potential attracting K+ ions into the cell and their leakage out of the cell along their concentration gradient establishes the resting potential of the plasma membrane at a value of about –70 mV. Maintaining the resting potential is critical to the proper functioning of neurons, skeletal muscle cells, and cardiac muscle cells, all of which use voltage-gated ion channels to propagate action potentials.

Generation of Resting Membrane Potential

The balance of actions of the Na+/K+ pump and the potassium leak channel results in a stable resting membrane potential (the electrical difference between intercellular and extracellular ions). The Na+/K+ pump establishes the electrochemical gradient, and the potassium leak channel allows the cell to respond to minute fluctuations in ion concentration.

Voltage-Gated Ion Channels in Neurons

Voltage-gated Na+ and K+ ion channels enable a nerve impulse to travel from one end of a neuron to the other.

A neuron is a cell in the nervous tissue that transmits electrical and chemical signals throughout the body. These cells have a highly elongated shape, consisting of the cell body (which contains the nucleus and most of the volume of the cell), an axon, and dendrites. The axon is an extension from the neuronal cell body that transmits the signal to receiving cells. Each dendrite, which collectively forms a branched array, is an extension from the neuronal cell body that receives input from other cells. The signals that travel along the axons are electrical excitations called action potentials. An action potential is a rapid change in membrane potential because of changes in the flux of potassium and sodium ions inside and outside the cell. This allows a nerve or muscle cell to transmit an electrical signal. Chemical signaling between neurons occurs at the synapses where the axons and dendrites meet, but electrical signals allow the nerve impulses to travel along the axon much more quickly.

The resting membrane potential of a cell is maintained by the Na+/K+ pumps and the K+ leak channels. When an electrical signal is received, it may alter the electrical potential of a region of the cell membrane either by depolarization or hyperpolarization. Depolarization is a change of the voltage across the membrane above resting membrane potential. Hyperpolarization is a change in the voltage across the membrane below resting membrane potential.

Phases of an Action Potential

The resting membrane potential of a cell is maintained by the Na+/K+ pumps and the K+ leak channels. When an electrical signal is received, the electrical potential of a region of the cell membrane is altered, either by depolarization or hyperpolarization.
A depolarization of membrane potential may passively spread across the plasma membrane, but if it is large enough, it will trigger the opening of voltage-gated Na+ channels. Na+ ions flow into the cell, which depolarizes the membrane further and triggers voltage-gated channels in the adjacent areas to open. By this self-propagating process, the action potential travels down the axon, and thus the electrical impulse is spread.

The depolarization of the membrane in a given region continues until the membrane potential is nearly at the Na+ ion equilibrium. At this point, the voltage-gated Na+ channels are inactivated, and voltage-gated K+ channels open. K+ ions flow into the cell and rapidly restore the membrane potential to the K+ ion equilibrium potential. Once the membrane potential is restored, the voltage-gated K+ channels close, and the neuron is ready to fire another action potential.

Through these impulses, messages are sent from sensory neurons to the brain for information processing. Additional messages can be sent from the brain to motor neurons to initiate a movement in response to the information. In the case of simple reflexes, the sensory neuron communicates directly with a motor neuron, bypassing information processing by the brain. For example, when a doctor taps a patient just below the knee, the sensory nerve sends an impulse to the spinal cord, where it is relayed to a motor nerve. This causes the front thigh muscle to contract and the opposing muscle to relax. So, the leg jerks up. This involves only two nerves and one synapse. The brain doesn't even know what's happening until the leg is in motion.

Propagation of Action Potential

A nerve cell transmits an electric signal via an action potential, which is a change in membrane electrical charge resulting from the rapid influx of sodium and potassium ions into the axon.