3.1 The Cell Membrane

What materials can easily diffuse through the lipid bilayer, and why?

Only materials that are relatively small and nonpolar can easily diffuse through the lipid bilayer. Large particles cannot fit in between the individual phospholipids that are packed together, and polar molecules are repelled by the hydrophobic/nonpolar lipids that line the inside of the bilayer.

Why is receptor-mediated endocytosis said to be more selective than phagocytosis or pinocytosis?

Receptor-mediated endocytosis is more selective because the substances that are brought into the cell are the specific ligands that could bind to the receptors being endocytosed. Phagocytosis or pinocytosis, on the other hand, have no such receptor-ligand specificity, and bring in whatever materials happen to be close to the membrane when it is enveloped.

What do osmosis, diffusion, filtration, and the movement of ions away from like charge all have in common? In what way do they differ?

These four phenomena are similar in the sense that they describe the movement of substances down a particular type of gradient. Osmosis and diffusion involve the movement of water and other substances down their concentration gradients, respectively. Filtration describes the movement of particles down a pressure gradient, and the movement of ions away from like charge describes their movement down their electrical gradient.

sodium-potassium pump

also, Na+/K+ ATP-ase) membrane-embedded protein pump that uses ATP to move Na+ out of a cell and K+ into the cell

Because they are embedded within the membrane, ion channels are examples of ________. a. receptor proteins b. integral proteins c. peripheral proteins d. glycoproteins

b. integral proteins

Choose the answer that best completes the following analogy: Diffusion is to ________ as endocytosis is to ________. a. filtration; phagocytosis b. osmosis; pinocytosis c. solutes; fluid d. gradient; chemical energy

b. osmosis; pinocytosis

Ion pumps and phagocytosis are both examples of ________. a. endocytosis b. passive transport c. active transport d. facilitated diffusion

c. active transport

glycocalyx

coating of sugar molecules that surrounds the cell membrane

The diffusion of substances within a solution tends to move those substances ________ their ________ gradient. a. up; electrical b. up; electrochemical c. down; pressure d. down; concentration

d. down; concentration

amphipathic

describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in water solubility

hypertonic

describes a solution concentration that is higher than a reference concentration

hypotonic

describes a solution concentration that is lower than a reference concentration

isotonic

describes a solution concentration that is the same as a reference concentration

hydrophilic

describes a substance or structure attracted to water

hydrophobic

describes a substance or structure repelled by water

concentration gradient

difference in the concentration of a substance between two regions

electrical gradient

difference in the electrical charge (potential) between two regions

facilitated diffusion

diffusion of a substance with the aid of a membrane protein

osmosis

diffusion of water molecules down their concentration gradient across a selectively permeable membrane

pinocytosis

endocytosis of fluid

phagocytosis

endocytosis of large particles

receptor-mediated endocytosis

endocytosis of ligands attached to membrane-bound receptors

exocytosis

export of a substance out of a cell by formation of a membrane-bound vesicle

selective permeability

feature of any barrier that allows certain substances to cross but excludes others

extracellular fluid (ECF)

fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other reservoirs in the body

intracellular fluid (ICF)

fluid in the cytosol of cells

interstitial fluid (IF)

fluid in the small spaces between cells not contained within blood vessels

passive transport

form of transport across the cell membrane that does not require input of cellular energy

active transport

form of transport across the cell membrane that requires input of cellular energy

single phospholipid molecule

has a phosphate group on one end, called the "head," and two side-by-side chains of fatty acids that make up the lipid tails

endocytosis

import of material into the cell by formation of a membrane-bound vesicle

cell membrane

membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various molecules; also known as plasma membrane

peripheral protein

membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane

integral protein

membrane-associated protein that spans the entire width of the lipid bilayer

vesicle

membrane-bound structure that contains materials within or outside of the cell

channel protein

membrane-spanning protein that has an inner pore which allows the passage of one or more substances

ligand

molecule that binds with specificity to a specific receptor molecule

diffusion

movement of a substance from an area of higher concentration to one of lower concentration

receptor

protein molecule that contains a binding site for another specific molecule (called a ligand)

glycoprotein

protein that has one or more carbohydrates attached

cell recognition proteins

serve to mark a cell's identity so that it can be recognized by other cells