Understanding the Cell Membrane - AP Biology

The fluid mosaic model describes the mosaic structure of the cell membrane. According to this model, the cell membrane is composed of a variety of protein molecules randomly bobbing around in a sea of phospholipids. Due to phospholipid polarity, they naturally form a bilayer in accordance with their dual hydrophobic (tails) and hydrophilic (heads) natures.

Integral proteins have both a hydrophobic region and hydrophilic region. The hydrophobic region allows them to sit within the cell membrane, while their hydrophilic regions face the aqueous solutions on either side of the membrane.

An amphipathic molecule is one that has both hydrophilic and hydrophobic regions. In the case of phospholipids, the tail is hydrophobic and the head is hydrophilic.

Endocytosis occurs when the cell engulfs an external substance and takes it into the cell by the infolding of the cell membrane. Exocytosis occurs when the cell secretes a substance outside the cell.

Actin is what makes up microfilaments. Chitin is indeed in cell walls of fungi. Cellulose is the main ingredient to plant cell walls and peptidoglycan are found in the cell walls of cyanobacteria and bacteria. Also, cellulose is made of polysaccharides, therefore, this is also an incorrect answer because it is a component of the cell wall.

Glucose need a channel protein to pass through the membrane because it is too large. Substances that pass through the membrane via simple diffusion need to be relatively small and not charged. Carbon dioxide and oxygen gas are both small nonpolar molecules that can easily diffuse through the plasma membrane.

Only two choices are correct, facilitative diffusion and active transport because they both need carrier proteins for transport across the cell membrane. Simple diffusion does not require carrier proteins.

Isotonic is the correct term to describe solutions with equal solute concentrations. The root word "iso" means same; therefore, isotonic solutions have the same concentration of solute. A hypotonic solution will have less solute in it. A hypertonic solution will have a higher concentration of solute between the two solutions.

Osmosis is the process by which water will diffuse from the hypotonic side of the membrane to the hypertonic side. Water will naturally travel to areas with higher solute concentration in order to lower the concentration and make it equal to the concentration of its surroundings.

Osmosis is defined as a form of passive transport since it does not require an expenditure of energy. Osmosis follows a concentration gradient while active transport acts against the concentration gradient. Also, active transport requires energy and passive transport does not.

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.

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).