Understanding the Cell Membrane - AP Biology

The plasma membrane is able to maintain the appropriate level of fluidity by manipulating a variety of factors. More cholesterol in the membrane reduces its permeability, which is useful in hot conditions. Desert animals would also be expected to have very few unstaurated fatty acids, and the fatty acid tails would be longer. All of these factors would help the plasma membrane not be too fluid in the hot sun.

The natural flow of molecules is from areas of high concentration to areas of low concentration. To act against this gradient, energy must be input. Active transport requires the use of energy to move a molecule up its concentration gradient. Diffusion, osmosis, and facilitated diffusion are forms of passive transport that move a molecule down its concentration gradient.

Endocytosis involves the use of vesicles to transport large molecules into the cell. Facilitated diffusion, osmosis, and active transport use diffusion or protein channels to transport molecules.

Note that exocytosis also uses vesicles, but uses them to export large molecules out of the cell or to incorporate them into the cell membrane.

The plasma membrane functions as a selective barrier to allow the flow of oxygen, nutrients, and wastes to and from the cell. The selectively permeable plasma membrane encloses the cytoplasm. The cytoplasm in turn contains the nucleus, mitochondria, and other organelles.

Cellular membranes are considered semipermeable, and allow certain substances to pass through without assistance from proteins. We typically follow the rule of thumb that substances that are small or nonpolar will be able to pass through the membrane. Water and hydrogen gas are both very small and can pass through the membrane relatively easily. Steroid hormones are large, but nonpolar, so they can pass through. Glucose is both large and polar, so it requires a carrier protein in order to cross.

Exocytosis allows the membrane of secretory vesicles to be incorporated into the cellular membrane. This expands the membrane surface, while including the desired proteins into the membrane. Due to fluidity and the mosaic model of the membrane, these proteins can then distribute to other areas on the cell surface.

Endocytosis does the opposite process, and involves a pinching off of the cell membrane in order to transport incoming materials. Ribosomes translate the proteins, however, processing of membrane proteins occurs in the endoplasmic reticulum and Golgi body, ultimately packaging membrane proteins in secratory vesicles for exocytosis.

The cell membrane is the most permeable to small, nonpolar molecules. Sodium ions (Na+) and water (H2O) are both small, but Na+ is charged and H2O is highly polarized. Both will therefore have a difficult time passing freely through the cell membrane. Water has some ability to cross the membrane, but it generally requires facilitated diffusion. Glucose (C6H12O6) is a large, polar molecule, and requires a protein channel in order to cross the membrane. Insulin is a peptide hormone, meaning that it is composed of amino acids. As such, it is both large and polar. Generally, insulin will bind to a receptor on the cell surface in order to elicit an effect; it will rarely cross the membrane at all.

Oxygen (O2), which is small and nonpolar, can easily diffuse across the membrane. This is essential for loading hemoglobin in the lungs and releasing oxygen in capillaries.

Any amino acid with a charged side chain will be polar. Membranes have hydrophobic tails on the interior, and hydrophilic heads facing the outside and inside of the cell. Since polar molecules are charged, they will interact with the hydrophilic parts of the environment, and therefore they will not be found within the membrane interior.

This question is tricky because we need to remember that if there are 2.0 moles of NaCl in the water, then there are 4.0 moles of solute because it will dissociate to one and one ; therefore, this is an isotonic environment and there will be no net movement of water. When a compound dissociates in solution, it is the ion concentration that will affect the movement of water, rather than the amount of initial solid.

Nonpolar molecules can pass through the plasma membrane with relative ease. Even larger nonpolar molecules, such as steroid hormones, can pass through the plasma membrane easily. Passing through the membrane without the need for assisting proteins is known as passive diffusion.

Facilitated diffusion involves the use of membrane channel proteins to allow molecules to pass (example: potassium leak channels). Active transport requires the metabolism of ATP to pump a molecule against its concentration gradient (example: sodium-potassium pump). Secondary active transport uses ATP to generate an electrochemical gradient, then uses the gradient to transport molecules and perform work (example: ATPase).

If there was no expenditure of energy when determining the voltage across the plasma membrane, there would be equal electrical charge on both sides of the bilayer as the ions travel to reach equilibrium. This means that ATP must be used in order to establish a resting potential, keeping the ions away from electrical equilibrium.

The sodium-potassium pump is an example of how ions can be pumped against their electrochemical gradients in order to establish a negative voltage inside the cell. The cell membrane is not permeable to sodium or potassium.

Passive transport is simple diffusion, without the aid of any protein channels. Since the plasma membrane is composed of a lipid bilayer, the molecules that most readily cross it are small and hydrophobic.

Remember that water is polar; hydrophobic molecules are nonpolar, like lipids and oils. Steroids are derived from cholesterol and are extremely lipid-soluble, so they can cross the membrane unassisted. Large, polar molecules (such as glucose or peptides) and charged ions need channel proteins to facilitate their crossing. Water can diffuse across the membrane, but only when it is moving down its concentration gradient. The answer choice given has water moving from a hypertonic (high-solute, or low-water) environment into a cell, which cannot occur passively.

While the availability of nutrients is a tempting answer, it is important to remember that even in incredibly nutrient-rich environments cells reach a maximum size. The concentration of water in the cytoplasm and the amount of membrane bound organelles will not really have any effect on the size of the cells.

The correct answer is the surface area to volume ratio because it dictates the amount of chemical activity carried out per unit of time. The volume of the cell determines its metabolic needs: larger cells hold more biological material and require more energy and maintenance. The surface area of the cell determines its ability to transport nutrients and absorb nutrients, providing the tools to maintain the cell's volume. A large surface area to volume ratio is essential for the cell.

Phospholipids found in the plasma membrane are comprised of a polar head group, two fatty acids, and a glycerol backbone. The phosphate group carries a negative charge, allowing it to interact with polar aqueous environments. The two fatty acid tails form the hydrophobic region of the bilayer interior. The glycerol backbone forms the structural component for linking the polar and non-polar regions.

Glycogen is a polymer of glucose sugars, and is not found in phospholipids. Triglycerides are composed of three fatty acids and a glycerol backbone.

The plasma membrane is only permeable to small nonpolar molecules. Charged ions, polar solutes, and large solutes can excluded, and require protein mediators to cross the membrane.

Oxygen and carbon dioxide are small molecules with no net dipole; though the bonds themselves may be polar, the symmetry of these molecules causes them to carry no net polarity. Oxygen and carbon dioxide diffuse across the membranes of alveoli in the lungs during reoxygenation. Progesterone is a steroid hormone, indicating that it is a small cholesterol derivative. Steroid hormones are capable of diffusing across the membrane due to their lipophilic nature.

Ions do not undergo simple diffusion. Instead, they are carried across the cell membrane by alternative means, such as active transport. This is essential for maintaining electrochemical gradients, such as the proton gradient in mitochondria and the sodium-potassium gradient in neurons.

Transport proteins span the exterior of the cell and allow substances to pass through the membrane. Some transport proteins are involved in facilitated diffusion, which is a type of passive transport. Other transport proteins are involved in active transport such as the sodium-potassium pump.

Membranes are permeable to small, nonpolar molecules like oxygen. Though oxygen is commonly involved in polar bonds, the diatomic molecule has no net dipole, allowing it to cross the membrane. This function is essential for gas exchange, in which oxygen passes from the alveoli of the lungs into adjacent capillaries.

Cholesterol is too large to freely cross the membrane, and requires the assistance of a facilitating protein channel. Glucose is also too large and carries a polar aldehyde or ketone group, preventing it from freely crossing the membrane. Potassium ions are small, but charged. Though they could pass between the lipids of the membrane, they would be repelled by the hydrophobic region in the lipid bilayer interior.

Cells obtain all of their nutrients across their plasma membranes. To ensure cellular function it is essential to maximize surface area, which corresponds to the amount of plasma membrane. This creates a large area available for diffusion of the necessary nutrients. While it may be more efficient for an organism to produce a few large cells instead of many small cells, the advantage of a large area for diffusion outweighs the cost of energy. Essentially, the surface area is the region of nutrient source and the volume (cytoplasm) is the region of nutrient consumption. For the cell to survive, it must maximize the amount of source nutrients and minimize the amount of nutrient consumption.

The statements that smaller cells are difficult to be detected by predators and require fewer nutrients for survival are both true, but are not the major reasons why cells are microscopic in size.

The cell membrane is primarily made of a phospholipid bilayer. The phospholipids consist of a polar phosphate head and two nonpolar lipid tails, oriented so that the tails of each side of the bilayer face one another. Numerous proteins are found in the membrane as well, contributing to the fluid-mosaic model.

Cholesterol molecules are found in the lipid interior of the membrane, between the two layers. Its primary function is to prevent adhesion of the nonpolar tails. By stopping the tails from sticking to one another, the cholesterol helps maintain the fluidity of the membrane.

Glucose, RNA, and ribosomes are not found in the cell membrane.