Cell Membrane and Organelles - GRE Subject Test: Biology

As the name implies, gap junctions are literally a gap in the plasma membranes of two adjacent cells that connect their cytoplasms. These junctions allow for electrical synapses and rapid cell signaling. While most notably present in cardiac muscle, gap junctions are also present in some neural cells and receptors.

Adherens junctions anchor cells through interactions with the actin cytoskeleton. Desmosomes use cadherin proteins to anchor cells via interactions with intermediate filaments. Tight junctions are used to create barriers that selectively allow molecules through layers of epithelial cells.

All of the choices describe functions of different types of cell junctions. Anchoring junctions (such as adherens junctions and desmosomes) use the cytoskeletons of each cell, as well as certain transmembrane proteins, to anchor the cells together. Gap junctions create gaps in the plasma membrane between two adjacent cells that connect their cytoplasms. Tight junctions are capable of forming barriers that are nearly impermeable to fluid flow.

All of the choices presented describe functions of cell junctions. Adherens junctions and desmosomes are responsible for anchoring neighboring cells to one another. Hemidesmosomes help anchor the extracellular matrix in place, binding the cell membrane to proteins in the basal lamina or matrix. Gap junctions are an example of junctions that connect the cytoplasm of two neighboring cells and allow for communication via ions and other small molecules.

Cellular junctions allow cells to block materials from moving between cells, or for providing communication between cells. Tight junctions are junctions between cells that form a tight seal. This prevents water and other fluids from moving past the cells. Tight junctions are essential for maintaining concentration gradients, preventing osmosis from equilibrating ion concentrations between two regions.

Desmosomes serve to anchor the cytoskeletons of two adjacent cells, helping with force transduction. Gap junctions allow for cellular communication by creating perforations between cells through which ions and small molecules can flow. Villi are not a type of cell junction, and are structures that serve to increase cell surface area.

In order to have the action potential spread evenly and completely throughout the cardiac muscle cells, the cells need junctions that allow for ions to move between them. This action is accomplished by gap junctions between the cells. The intercalated discs between cardiac muscle cells are composed of gap junctions, and allow for electrical stimulation to travel from one cell to the next. This feature means that cardiac muscle can be depolarized and contract simultaneously.

Tight junctions are used to prevent fluid flow between cells. Desmosomes help with force transduction by linking the cytoskeletons of adjacent cells. Valves are not a type of cellular junction, and are macrostructures that prevent the backflow of fluids in vessels of the body.

The question asks about membranes in general, not just the cell plasma membrane; therefore, all of the answers are true. The plasma membrane's most important functions are protecting the internal environment of the cell and selectively allowing nutrients into the cytoplasm (semi-permeability). The final answer describes a function of the inner-membrane of the mitochondria, which houses the electron transport chain.

Membranes are semi-permeable, meaning that they only allow certain compounds to cross without protein assistance. The two factors that determine permeability across a membrane are size and polarity. Polar and charged compounds very rarely cross the membrane by simple diffusion, and often require a carrier protein or pump in order to cross. Glucose and amino acids are polar, and a sodium ion carries a positive charge. These compounds will be blocked by the hydrophobic interior of the membrane, formed by the fatty acids tails of the phospholipids bilayer.

Nonpolar compounds, on the other hand, can very easily cross the cell membrane. Even larger nonpolar molecules, like steroid hormones, can easily cross the membrane via simple diffusion.

Energy is not necessary when a compound is being moved down its electrochemical gradient. Diffusion, facilitated diffusion, and osmosis all involve a compound moving from a higher to a lower concentration. Since this is the spontaneous direction of flow, no energy input is required.

In order to move a compound against its electrochemical gradient, energy is needed. This type of transport is called active transport.

Because leucine is a hydrophobic amino acid, it would make sense that it would be most stable in a hydrophobic environment. The interior of the phospholipid bilayer is a hydrophobic environment; therefore, leucine and other hydrophobic amino acids are more commonly found in the membrane-spanning portions of transmembrane proteins.

Polar and hydrophilic amino acids are most commonly found in the cytosolic and exoplasmic regions of the membrane, as these regions interact with the aqueous environment outside of the membrane.

Because the molecule is a sugar, it is too large to passively diffuse across the plasma membrane and contains polar regions that would make this impossible.

There are several other mechanisms by which the sugar could enter the cell. One of these is receptor-mediated endocytosis. In this process the sugar would bind to receptors on the plasma membrane, which stimulates a budding event and eventually leads to the formation of a vesicle inside the cell.

Symport is another potential mechanism. In symport, the import of a molecule is coupled with the import of another molecule through the same transmembrane protein. For example, glucose has a symport mechanism with sodium ions.

Of the given choices, only channel and carrier proteins allow substances to cross the membrane. While channel proteins create an open pore through which substances can cross, carrier proteins will change their shape in order to allow substances to cross the membrane.

There are many factors that determine the fluidity of cell membranes. Membranes that are composed of fully saturated, long fatty acid tails are generally less fluid then the opposite conditions. In addition, lower temperatures result in a less fluid membrane.

Membranes that have fatty acid tails with double bonds are more fluid because the double bonds make it difficult for multiple phospholipids to float next to one another. The shape of the double bond adds a another dimension to the lipid, preventing the tails from packing together. Unsaturated fatty acids are thus more fluid than saturated fatty acids.

In higher temperatures, the cell membrane is going to become increasingly fluid due to the increased movement of the phospholipids. The cell membrane can control its fluidity in high temperatures by both increasing the saturated fatty acid tail content of the phospholipids, as well as making the fatty acid tails longer. Cholesterol can also help by acting as a buffer at high temperatures.

Although cell walls often serve very similar functions for different species, the composition of the cell walls can vary significantly. Plant cell walls employ cellulose, while bacteria use peptidoglycan. Fungal cell walls use the polymer chitin.

The correct answer is peptidoglycan. Cellulose composes the cell walls of plants, whereas chitin composes the cell walls of fungi. Starch and glycogen are stored polymers of glucose in plants and animals, respectively.

A Gram-positive cell has the following basic structural characteristics: stains dark purple in the Gram stain, has a thick peptidoglycan layer, and possesses no outer membrane beyond this layer. Thus, there is also no periplasmic space. Acid-fast stains are only used for specific bacteria that have waxy cell walls.

Lysosomes have the function of digesting foreign materials and large structures, such as organelles. They do this by maintaining an acidic pH of approximately 5 and utilizing special proteins called acid hydrolyases, which are specifically designed to function at low pH levels. In order to maintain this low pH, lysosomes must be membrane-bound and have a highly regulated flow of protons.

Prokaryotes do not have membrane-bound organelles, including lysosomes.

Peroxisomes are primarily responsible for the breakdown of very long chain fatty acids and D-amino acids, as well as the synthesis of special types of phospholipids known as plasmalogens. If the cell cannot catabolize very long chain fatty acids, the issue is most likely within the peroxisomes.

The Golgi apparatus is very important for protein packaging. Mitochondria are crucial to generating energy for the cell. The smooth endoplasmic reticulum has various functions, including lipid and carbohydrate metabolism.

The function of breaking down cellular contents is done by the lysosome. Lysosomes have an acidic interior, which is useful for breaking down macromolecules that are no longer being used in the cell.

The smooth endoplasmic reticulum, in contrast, helps to break down foreign material, such as toxins. Mitochondria primarily serve to produce ATP for cellular energy. The Golgi apparatus works in tandem with the rough endoplasmic reticulum and helps to group and package proteins for vesicular transport.

Proton pumps use ATP to continuously pump hydrogen ions into the interior of the lysosome, thus maintaining an acidic environment in which the enzymes are most optimally efficient at breaking down the debris/macromolecules. The membrane of the lysosome is selectively permeable due to these types of transporters, and protects the rest of the cell from the very acidic environment. Recent journal articles suggest that the lysosome's membrane will be deliberately disrupted, releasing the acid and the hydrolases into the cytosol during apoptosis.