Plasma Membrane and Transport - MCAT Biological and Biochemical Foundations of Living Systems

Nonpolar molecules and very small polar molecules can freely pass through the lipid bilayer, while large, polar molecules and ions need to be aided by transport proteins. Sodium and potassium are both charged ions that would not be able to cross the membrane. Glucose and citrate are too large, and also contain polar regions.

Carbon dioxide is the only answer choice that is both small and nonpolar enough to simply diffuse across the membrane.

Transport of molecules along their gradients does not require an input of energy, while transporting molecules against their gradients requires cellular energy. Facilitated diffusion refers to the transport of a molecules along its gradient through a protein channel medium. The molecule cannot passively diffuse, usually because of size or polarity, but can still be transported without use of energy.

Active transport of any kind, including the sodium-potassium pump and any ATPases, will require energy to transport a molecule against its natural gradient.

Because there is a smaller concentration of solute inside the cell than in the solution, it is said to be in a hypertonic solution. This will cause water from inside the cell to move outside the plasma membrane to account for this difference; therefore, the cell will crenate (shrink).

Membrane potential is calculated by subtracting the potential outside the cell from the potential inside the cell.

A neuron usually has a negative resting membrane potential because the inside of the cell is more negative than the outside of the cell. This difference in polarity results from the uneven movement of sodium and potassium ions by the sodium-potassium pump. The amount of sodium ions pumped out of the cell (three) is higher than the amount of potassium ions pumped into the cell (two). The net export of positive ions contributes to the negative resting membrane potential.

Remember that depolarization is the first step of an action potential, during which the membrane potential increases rapidly. This rapid increase is attributed to the movement of sodium ions into the cell. Since sodium ions are positively charged, the inside of the cell becomes more positive and the outside of the cell becomes more negative. This change in polarity causes an increase in membrane potential. When a neuron is excited past the threshold stimulus, an action potential ensues. An action potential causes huge alterations in membrane potential due to the movement of ions during depolarization, repolarization, and hyperpolarization. The sodium-potassium pump constantly moves sodium and potassium ions against their concentration gradients, which helps maintain the negative resting membrane potential.

During resting membrane potential, a sodium ion inside the cell will not have a higher electrical potential energy than a sodium ion outside the cell. Recall that the electrical potential energy is higher when the electrical potential is higher. The resting membrane potential is negative; therefore, the inside of the cell has a lower potential than the outside of the cell. Since the potential inside the cell is lower, a sodium ion inside the cell will have a lower potential than a sodium ion outside the cell.

Integral membrane proteins are proteins that span the entire membrane, whereas peripheral membrane proteins are proteins that associate only with only one side of the membrane (the "periphery").

Plasma membrane channels are proteins that facilitate the exchange of ions and other molecules between the extracellular and intracellular sides of a cell. To accomplish this task, a channel must span through the membrane (phospholipid bilayer); therefore, membrane channels are classified as integral membrane proteins. Recall that the inside of a phospholipid bilayer is extremely hydrophobic. Since like dissolves like, a membrane channel must contain hydrophobic regions that can interact with the interior of the phospholipid bilayer.

Amphipathic molecules contain both polar and nonpolar regions. Integral proteins (and most peripheral proteins) are amphipathic molecules. Phospholipids are also amphipathic molecules because they contain a polar head and a nonpolar tail.

Water molecules are small and can travel through the membrane via simple diffusion. The rate of simple diffusion, however, is extremely slow due to the polarity of water molecules and their reluctance to enter the hydrophobic core of the membrane. Faster transportation of water molecules occurs via facilitated diffusion by specialized membrane channels called aquaporins; therefore, water can be transported via simple diffusion and facilitated diffusion.

Hydrophobic regions on membrane channels are essential to span the hydrophobic portions of the phospholipid bilayer. Although they are hydrophobic, lipids are not the main components of the hydrophobic regions. The hydrophobic regions in a membrane channel consist of hydrophobic amino acids that contain nonpolar side chains.

Remember that facilitated diffusion and simple diffusion are both forms of passive transport; therefore, these processes do not require energy and transport molecules from a region of high concentration to low concentration. They don’t move molecules against their electrochemical gradient.

Facilitated diffusion occurs at a much higher rate than simple diffusion. Facilitated diffusion uses membrane channels to transport molecules; therefore, it is much easier for molecules to traverse through a channel (facilitated diffusion) than the phospholipid bilayer (simple diffusion).

To answer this question you need to know the directionality of the sodium-potassium pump and the number of ions pumped each cycle. Remember that each cycle of the sodium-potassium pump moves three sodium ions to the outside of the cell and two potassium ions to the inside of the cell. The amount of sodium ions outside the cell will increase by three and the amount of potassium ions inside the cell will increase by two.

The final result after one cycle of the sodium-potassium pump will be 33 sodium ions outside the cell and 22 potassium ions inside the cell.

Two main factors are required for a membrane potential. First, there must be a concentration or electrochemical gradient of ions. A potential difference occurs between two regions in space that have an uneven distribution of charges; therefore, for a membrane potential there must be a concentration gradient of ions (a difference between the ion concentrations) between the outside and the inside of the cell. Second, there must be a semipermeable membrane that separates the interior of the cell from the exterior. If a semipermeable membrane isn’t present, then the ions can undergo simple diffusion, which will equilibrate the concentration of ions. This would diminish the concentration gradient and the potential difference would become zero.

Neurotransmitters are not required for establishing a membrane potential. They are chemicals that bind to receptors on membranes and initiate a cellular response.

Remember that plasma membrane receptors are found in several types of cells.

G protein coupled receptors are a class of receptors that have a G protein attached to the intracellular side. Upon ligand binding, the G protein dissociates from the receptor and binds to subsequent ion channels or effector proteins to initiate a signal cascade.

Membrane receptors are also found in B-cells, which are part of the immune system. Surfaces of B-cells contain several B-cell receptors with antibodies embedded into each receptor. These antibodies are very specific and bind foreign antigens. Binding of an antigen initiates an immune response that eventually leads to the destruction of the antigen.

Recall that membrane receptors span the membrane; therefore, the membrane-spanning region of a receptor must contain hydrophobic residues. Hydrophobic residues are amino acids with nonpolar side chains.

In signal transduction, a ligand binds to the extracellular side of the plasma membrane receptor. This initiates a cellular response that is facilitated by the intracellular side. The intracellular region can activate a G protein, bind to an effector, or initiate other cellular responses. These responses often result in a signal cascade that affects transcription factors and alters gene expression.

The trick to understanding this question is to remember that the sodium chloride actually consists of equal parts sodium ions and chlorine ions; therefore, the internal concentration of solute inside the red blood cell is actually twice the concentration of solute outside the cell. Since the cell contains a higher solute concentration compared to the extracellular environment, the solution is considered hypotonic. Water will flow toward the more concentrated region via osmosis, entering the cell and causing it to swell. Eventually, the swelling can cause the cell to lyse.

Cell containment, as the name suggests, is the process by which the contents of a cell (organelles, cytoskeleton, etc.) are contained within a region of space inside the cell. This means that there must be a barrier that prevents the contents from leaking out of the cell. This barrier is the phospholipid bilayer, or cell membrane. Phospholipid bilayers can be found in plasma membranes or in the membranes that cover the organelles. Recall that most organelles in an eukaryotic cell contain a phospholipid bilayer that separates the contents of the organelle from the cytosol.

When a membrane channel, such as the sodium-potassium pump, requires energy (ATP) to transport molecules it means that the channel is moving molecules against their concentration gradient. This mode of transport is called active transport.

Recall that the sodium-potassium pump moves three sodium ions out of the cell and two potassium ions into the cell per cycle. Since it uses active transport, the sodium-potassium pump must move both sodium and potassium ions against their respective concentration gradients. This means that the concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell.

Note that symporters exist in which facilitated diffusion of one ion is used to pull a second ion against its concentration gradient without the use of ATP. In this manner, ATP is not always necessary to transport an ion against its concentration gradient. When both ions are moving against their gradients, however, or when only one ion is being transported, ATP will be needed.

Plasma membranes of the cell are permeable to molecules that pass through the phospholipid bilayer easily, namely small nonpolar molecules. Due to this specificity in permeability, membrane proteins are often required to transport molecules across the bilayer. Simple diffusion occurs when a substance passes through a membrane without the aid of an intermediary. All forms of facilitated transport, along with active transport, require the aid of specific membrane proteins. Thus, simple diffusion is the correct answer.

The major components of the plasma membrane of an animal cell are lipids and proteins, with a small amount of carbohydrate components. The major lipid components are glycerophospholipids, sphingolipids, and some cholesterol. The amount of cholesterol varies depending upon certain factors, such as temperature, and helps maintain the fluidity of the membrane. Thus, the correct answer is phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate.

The major purpose of cholestrol in the plasma membrane is to maintain membrane fluidity. Carbohydrates and glycoproteins function in cell-to-cell recognition, and proteins function in the transport of particles through the membrane. Charged particles can't freely pass through the membrane unless it is through a carrier protein.

The core of the lipid bilayer of all eukaryotic cells contains lipid; therefore, transmembrane proteins have a hydrophobic-rich series of residues in the area that spans the membrane.

Students should know the main players in establishing action potentials are K+ and Na+. Further, Na+ inward flow through open channels brings an action potential to a peak depolarization of about +60 mV, which is sodium's equilibrium potential