Chapter 7 Mastering Biology

nonpolar molecules that repel water molecules. Salad oil is predominantly made up of carbon and hydrogen atoms, which share electrons almost equally, forming nonpolar covalent bonds. Substances that are nonpolar due to their large number of nonpolar bonds do not have an affinity for water and are termed hydrophobic ("water-fearing"). Substances that contain polar bonds are hydrophilic ("water-loving") because they contain atoms with partial charges due to those polar bonds. For example, a hydrogen atom with a partial positive charge can form a hydrogen bond with a partial negative charge on the oxygen atom of a water molecule.

The polar heads interact with water; the nonpolar tails do not. A phospholipid is similar to a fat molecule but has only two fatty acids attached to glycerol rather than three. The third hydroxyl group of glycerol is joined to a phosphate group, which has a negative electrical charge in the cell. The fatty acids, referred to as the "tails" of the phospholipid, are hydrocarbons that are hydrophobic and therefore do not interact with water. The phosphate group and its attachments form a hydrophilic "head" that has an affinity for water.

Most prokaryotic cells have no internal membranes; eukaryotic cells do. All cells are enclosed by a plasma membrane, a selective barrier that forms a boundary between the cell and the external environment. The existence of cells depends on the properties of phospholipids that make up the plasma membrane. Eukaryotic cells have extensive, elaborately arranged internal membranes as well that divide the cell into compartments called organelles, but most prokaryotic cells do not have internal membranes. All cells have DNA that stores the genetic information of the cell.

Potential energy is the energy possessed by matter due to its location or structure. Energy is defined as the capacity to cause change—for instance, by doing work. Potential energy is the energy that matter possesses because of its location or structure. Matter has a natural tendency to move from higher states of potential energy toward the lowest possible state of potential energy, such as water running downhill from a dam. As water runs downhill, the energy released can be used to do work.

Phospholipids form a selectively permeable structure. Their structure allows some substances to penetrate easily and blocks others.

The fluid aspect of the membrane is due to the lateral and rotational movement of phospholipids, and embedded proteins account for the mosaic aspect. This is what the term "fluid mosaic" refers to.

in the interior of the membrane The steroid cholesterol, wedged between phospholipid molecules in the plasma membranes of animals, helps stabilize the membrane.

energy, carbon, and nitrogen storage right answer feedback: Correct. Proteins are not present in biological membranes to act as stores of energy, carbon, and nitrogen.

Membrane proteins with short sugar chains form identification tags that are recognized by other cells. Cell-cell recognition is an important function of membrane proteins, and this cell-cell recognition is important in tissue formation during embryogenesis.

Membrane carbohydrates function primarily in cell-cell recognition. Variations in carbohydrate structure distinguish one species from another, one individual from another, and even one cell type from another.

on the outside (external) surface of the membrane Membrane carbohydrates are covalently bonded to lipids or proteins and extend out from the external side of the plasma membrane as a means of cell identification.

All of the listed choices are aspects of the sidedness of the plasma membrane.

carbon dioxide Hydrophobic molecules, such as hydrocarbons, carbon dioxide, and oxygen, can dissolve in the membrane and cross it with ease.

a large, polar molecule The combination of being polar and large means that this molecule will be the slowest one from the choices to move across the membrane.

Passive transport permits the solute to move in either direction, but the net movement of solute molecules occurs down the concentration gradient of the molecule. Passive transport can occur in either direction, but the direction of net diffusion is down the concentration gradient of the solute.

the diffusion gradient in cell B is steeper As long as a metabolically active cell converts oxygen to water during cellular respiration shortly after it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.

It is a passive process. Diffusion is the tendency of molecules to spread out in the available space. A substance will diffuse from an area of higher concentration to an area of lower concentration without energy input.

1.0M This solution is hypertonic to the plant cell. Water will leave the cell, and eventually the plasma membrane will pull away from the cell wall, resulting in plasmolysis.

Water would leave the cell by osmosis, causing the volume of the cytoplasm to decrease. The added salt makes the solution hypertonic compared to the cell. Water will leave the cell by osmosis.

Both cells would lose water; the red blood cell would shrivel, and the plant plasma membrane would pull away from the cell wall. Seawater will cause both cells to lose water.

Facilitated diffusion of solutes may occur through channel or transport proteins in the membrane. The passageways for facilitated diffusion may be either protein pores or carrier proteins.

Facilitated diffusion requires the hydrolysis of ATP. Facilitated diffusion, like simple diffusion, needs only a concentration gradient—no energy input is required.

Nothing will happen, because the two solutions are isotonic to one another. Osmotic pressure is produced by the concentration of dissolved substances and is not influenced by the relative sizes of the solutes.

a hypertonic sucrose solution When a cell is placed in a hypertonic environment, water will leave the cell, causing it to shrink.

A 30% salt solution is hypertonic to the bacteria, so they lose too much water and undergo plasmolysis. If a cell is placed in a hypertonic solution, it will lose water to its environment, shrivel, and probably die.

The sodium-potassium pump hydrolyzes ATP and results in a net positive change outside the cell membrane. This is how the sodium-potassium pump generates voltage across the cell membrane.

To pump glucose up its concentration gradient, sodium moves down its concentration gradient, and the distribution of sodium ions across the membrane forms an electrochemical gradient that drives this mechanism. The movement of sodium down its gradient drives glucose up its gradient, and because sodium is at different concentrations on either side of the membrane and as sodium has a +1 charge, an electrochemical gradient also exists.

Active transport requires energy from ATP, and facilitated diffusion does not. Active transport can move substances against the concentration gradient, but it requires energy in the form of ATP.

facilitated diffusion of Ca2+ into the cell down its electrochemical gradient Both the electrical and chemical (concentration) gradients contribute the energy to move Ca2+ into the cells by facilitated diffusion as long as there is a channel or carrier that is specific for Ca2+.

Electrogenic pumps create a voltage difference across the membrane. An electrogenic pump creates a net charge difference across a membrane (a membrane potential).

Cotransport proteins allow a single ATP-powered pump to drive the active transport of many different solutes. The electrochemical gradient created by a single ATP-dependent pump can drive the transport of many different solutes using cotransport proteins.

The movement of protons through the cotransport protein cannot occur unless sucrose moves at the same time. The obligate coupling of proton movement to sucrose movement prevents the energy of the proton gradient from being lost if sucrose is not present.

receptor-mediated endocytosis In receptor-mediated endocytosis, only a specific molecule, called a ligand, can bind to the receptor. Without receptor binding occurring first, endocytosis does not proceed.

exocytosis and smooth ER and rough ER In exocytosis, vesicles derived from the endomembrane system fuse with the plasma membrane, thus increasing the number of phospholipids in the plasma membrane and increasing its surface area. The smooth ER is largely responsible for production of lipids destined for the membrane, and the rough ER produces proteins destined for the plasma membrane.

Endocytosis is the procedure that cells use to import large molecules across their plasma membrane.

pinocytosis: the uptake of water and small solutes into the cell by formation of vesicles at the plasma membrane Pinocytosis is the uptake of liquid and the solutes dissolved in the liquid.