Understanding the Cell Membrane - AP Biology

The plasma membrane acts as a selective barrier and defines the boundaries of the cell. It allows for the passage of oxygen, nutrients, and wastes. The nucleus is enclosed by the nuclear envelope, not the plasma membrane.

The plasma membrane, also called cell membrane, is composed of a phospholipid bilayer that separates the outside of the cell from the inside of the cell. It is selectively permeable, and contains many proteins that facilitate the transduction of signals in and out of the cell, and allow for passage of specific substances. The nucleus, endoplasmic reticulum, nucleus, and mitochondria are membrane-bound organelles that exhist in the cytoplasm of eukaryotic cells.

Amphipathic molecules have both hydrophobic and hydrophilic regions (polar and non-polar). Phospholipids are amphipathic because they have a polar head and a non-polar tail. Since integral proteins are embedded within the phospholipid bilayer, the parts that are on either side of the plasma membrane are hydrophilic, and the parts that are in contact with the tails of the phospholipids are hydrophobic.

Tonofibrils are groups of keratin tonofilaments (intermediate filaments) most commonly found in the epithelial tissues, not endocrine tissues, and which play an important structural role in cell makeup.

Hypertonic is the correct answer for the solution with more solute in it. The root "hyper" means more; therefore, a hypertonic solution will have more solute. A hypotonic solution will have less solute in it. Isotonic solutions will have equal concentrations of solutes between the two solutions.

Hypotonic is the correct answer for the solution with less solute in it. The root "hypo" means less; therefore, a hypotonic solution will have less solute. A hypertonic solution will have more solute in it. Isotonic solutions will have equal concentrations of solutes between the two solutions.

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.

Diffusion is defined as the spread of molecules into available space. The random nature of the movement of molecules in an open area means that molecules will seek to spread out—diffuse—evenly within a given area. This means that molecules will evenly distribute themselves when random assortment is permitted within a space.

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.

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.

Osmosis is an example of passive transport. Osmosis does not use energy. Instead, it acts using the concentration gradient. The sodium-potassium pump, proton pump, and electrogenic pump are all examples of active transport. This means that they can pump solutes against their concentration gradients using cellular energy.

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.

Phospholipids are the most abundant because their structure makes it possible for them to form membranes since they have hydrophilic and hydrophobic regions. In the phospholipid bilayer of a cell, the hydrophilic heads are exposed to the intracellular and extracellular compartments, which contain cytosol, and extracellular fluid, both of which are aqueous. The tails of the phospholipids are hydrophobic, and thus "hide" from the aqueous environments on either side of the cell membrane. This structure allows small, uncharged, and fat-soluble molecules to diffuse across the membrane.

Solution A is hypertonic to solution B. Solution A is hypertonic because it has an overall higher number of solutes in it even though B has more types of solutes in it. Diversity of the solutes does not matter when comparing concentrations of solutions, only the overall number of solutes matters. Tonicity can be thought of as relative concentration. Remember that a solution cannot simply be hypertonic or hypotonic; its concentration must be compared to that of another solution.

Solution B is hypotonic to solution A. Solution B is hypotonic because it has an overall lower number of solutes in it even though it has more types of solutes in it. Diversity of the solutes does not not matter when comparing concentrations of solutions, only the overall number of solutes matters. Tonicity can be thought of as relative concentration. Remember that a solution cannot simply be hypertonic or hypotonic; its concentration must be compared to that of another solution.