MCAT Biochemistry Review

Chapter 8: Biological Membranes

Conclusion

Understanding biological membranes becomes increasingly important as you progress in your medical career. At this point, you should have a strong foundation of knowledge about the fluid mosaic model and how membranes exist dynamically. We've also covered the components of cell membranes, with a special emphasis on lipids and the phospholipid bilayer. We reviewed some basic physical properties of the cell, including cell–cell junctions. We also examined membrane transport, such as passive transport (simple diffusion, facilitated diffusion, and osmosis) and active transport, before briefly touching upon endocytosis and exocytosis. Finally, we reviewed specialized membranes within cells. Up to now, you have been exposed to each of the classes of molecules and some of their applications both experimentally and within the cell. This comprehensive review should provide you with a better understanding of what will be expected of you on Test Day and briefly introduce you to topics that you will learn more about in medical school.

The first seven chapters of MCAT Biochemistry Review focused on various types of biomolecules, their structures, and their functions. In this chapter, we applied this knowledge of biomolecules to make sense of biological membranes. In the remaining four chapters, we'll turn our attention to the metabolic pathways by which the body builds, stores, and burns these biomolecules.

Concept Summary

Fluid Mosaic Model

·        The fluid mosaic model accounts for the presence of lipids, proteins, and carbohydrates in a dynamic, semisolid plasma membrane that surrounds cells.

·        The plasma membrane contains proteins embedded within the phospholipid bilayer.

·        The membrane is not static.

o   Lipids move freely in the plane of the membrane and can assemble into lipid rafts.

o   Flippases are specific membrane proteins that maintain the bidirectional transport of lipids between the layers of the phospholipid bilayer in cells.

o   Proteins and carbohydrates may also move within the membrane, but are slowed by their relatively large size.

Membrane Components

·        Lipids are the primary membrane component, both by mass and mole fraction.

o   Triacylglycerols and free fatty acids act as phospholipid precursors and are found in low levels in the membrane.

o   Glycerophospholipids replace one fatty acid with a phosphate molecule, which is often linked to other hydrophilic groups.

o   Cholesterol is present in large amounts and contributes to membrane fluidity and stability.

o   Waxes are present in very small amounts, if at all; they are most prevalent in plants and function in waterproofing and defense.

·        Proteins located within the cell membrane act as transporters, cell adhesion molecules, and enzymes.

o   Transmembrane proteins can have one or more hydrophobic domains and are most likely to function as receptors or channels.

o   Embedded proteins are most likely part of a catalytic complex or involved in cellular communication.

o   Membrane-associated proteins may act as recognition molecules or enzymes.

·        Carbohydrates can form a protective glycoprotein coat and also function in cell recognition.

·        Extracellular ligands can bind to membrane receptors, which function as channels or enzymes in second messenger pathways.

·        Cell–cell junctions regulate transport intracellularly and intercellularly.

o   Gap junctions allow for the rapid exchange of ions and other small molecules between adjacent cells.

o   Tight junctions prevent paracellular transport, but do not provide intercellular transport.

o   Desmosomes and hemidesmosomes anchor layers of epithelial tissue.

Membrane Transport

·        Concentration gradients help to determine appropriate membrane transport mechanisms in cells.

·        Osmotic pressure, a colligative property, is the pressure applied to a pure solvent to prevent osmosis and is used to express the concentration of the solution.

o   It is often better conceptualized as a “sucking” pressure in which a solution is drawing water in, proportional to its concentration.

·        Passive transport does not require energy because the molecule is moving down its concentration gradient or from an area with higher concentration to an area with lower concentration.

o   Simple diffusion does not require a transporter. Small, nonpolar molecules passively move from an area of high concentration to an area of low concentration until equilibrium is achieved.

o   Osmosis describes the diffusion of water across a selectively permeable membrane.

o   Facilitated diffusion uses transport proteins to move impermeable solutes across the cell membrane.

·        Active transport requires energy in the form of ATP or an existing favorable ion gradient.

o   Active transport may be primary or secondary depending on the energy source. Secondary active transport can be further classified as symport or antiport.

·        Endocytosis and exocytosis are methods of engulfing material into cells or releasing material to the exterior of cells, both via the cell membrane. Pinocytosis is the ingestion of liquid into the cell from vesicles formed from the cell membrane and phagocytosis is the ingestion of bacteria by phagocytes.

Specialized Membranes

·        The composition of cell membranes is fairly consistent; however, there are some cells that contain specialized membranes.

·        Membrane potential is maintained by the sodium–potassium pump and leak channels.

o   The electrical potential created by one ion can be calculated using the Nernst equation.

o   The resting potential of a membrane at physiological temperature can be calculated using the Goldman–Hodgkin–Katz voltage equation, which is derived from the Nernst equation.

·        The mitochondrial membrane differs from the cell membrane:

o   The outer mitochondrial membrane is highly permeable to metabolic molecules and small proteins.

o   The inner mitochondrial membrane surrounds the mitochondrial matrix, where the citric acid cycle produces electrons used in the electron transport chain and where many other enzymes important in cellular respiration are located. The inner mitochondrial membrane also does not contain cholesterol.

Answers to Concept Checks

·        8.1

1.    Flippases are responsible for the movement of phospholipids between the layers of the plasma membrane because it is otherwise energetically unfavorable. Lipid rafts are aggregates of specific lipids in the membrane that function as attachment points for other biomolecules and play roles in signaling.

2.    Lipids, including phospholipids, cholesterol, and others, are most plentiful; proteins, including transmembrane proteins (channels and receptors), membrane-associated proteins, and embedded proteins, are next most plentiful; carbohydrates, including the glycoprotein coat and signaling molecules, are next; nucleic acids are essentially absent.

·        8.2

1.    The hydrophilic region is at the top of this diagram. While you need not be able to recognize it, the head group is phosphatidylcholine in this example. The hydrophobic region is at the bottom and is composed of two fatty acid tails. The tail on the left is saturated; the tail on the right is unsaturated, as evidenced by the kink in its chain.

2.    Cholesterol provides membrane fluidity by interfering with the crystal structure of the cell membrane and occupying space between phospholipid molecules. Cholesterol also provides stability by cross-linking adjacent phospholipids through interactions at the polar head group and hydrophobic interactions at the nearby fatty acid tail.

3.    Transmembrane proteins are most likely to serve as channels or receptors. Embedded membrane proteins are most likely to have catalytic activity linked to nearby enzymes. Membrane-associated (peripheral) proteins are most likely to be involved in signaling or are recognition molecules on the extracellular surface.

4.    Gap junctions allow for the intercellular transport of materials and do not prevent paracellular transport of materials. Tight junctions are not used for intercellular transport but do prevent paracellular transport. Gap junctions are in discontinuous bunches around the cell, while tight junctions form bands around the cell.

·        8.3

1.    The primary thermodynamic factor responsible for passive transport is entropy.

2.    As osmotic pressure increases, more water will tend to flow into the compartment to decrease solute concentration. Osmotic pressure is often considered a “sucking” pressure because water will move toward the compartment with the highest osmotic pressure.

3.    Primary active transport uses ATP as an energy source for the movement of molecules against their concentration gradient, while secondary active transport uses an electrochemical gradient to power the transport. Symport moves both particles in secondary active transport across the membrane in the same direction, while antiport moves particles across the cell membrane in opposite directions.

·        8.4

1.    The membrane potential, which results from a difference in the number of positive and negative charges on either side of the membrane, is maintained primarily by the sodium–potassium pump, which moves three sodium ions out of the cell for every two potassium ions pumped in, and to a minor extent by leak channels that allow the passive transport of ions.

2.   

The exact value is – 64.0 mV

3.    The inner mitochondrial membrane lacks cholesterol, which differentiates it from most other biological membranes. There is no pH gradient between the cytoplasm and the intermembrane space because the outer mitochondrial membrane has such high permeability to biomolecules (the proton-motive force of the mitochondria is across the inner mitochondrial membrane, not the outer mitochondrial membrane).

Equations to Remember

(8.1) Osmotic pressure: Π = iMRT

(8.2) Nernst equation:

(8.3) Goldman–Hodgkin–Katz voltage equation:

Shared Concepts

·        Biochemistry Chapter 3

o   Nonenzymatic Protein Function and Protein Analysis

·        Biochemistry Chapter 5

o   Lipid Structure and Function

·        Biology Chapter 1

o   The Cell

·        Biology Chapter 10

o   Homeostasis

·        General Chemistry Chapter 9

o   Solutions

·        Physics and Math Chapter 5

o   Electrostatics