Chapter 5

active transport

active transport

method of transporting material that requires energy

amphiphilic

molecule possessing a polar or charged area and a nonpolar or uncharged area capable of interacting with both hydrophilic and hydrophobic environments

antiporter

transporter that carries two ions or small molecules in different directions

aquaporin

channel protein that allows water through the membrane at a very high rate

carrier protein

membrane protein that moves a substance across the plasma membrane by changing its own shape

caveolin

protein that coats the plasma membrane's cytoplasmic side and participates in the liquid uptake process by potocytosis

channel protein

membrane protein that allows a substance to pass through its hollow core across the plasma membrane

clathrin

protein that coats the plasma membrane's inward-facing surface and assists in forming specialized structures, like coated pits, for phagocytosis

concentration gradient

area of high concentration adjacent to an area of low concentration

diffusion

passive transport process of low-molecular weight material according to its concentration gradient

electrochemical gradient

a combined electrical and chemical force that produces a gradient

electrogenic pump

pump that creates a charge imbalance

endocytosis

type of active transport that moves substances, including fluids and particles, into a cell

exocytosis

process of passing bulk material out of a cell

facilitated transport

process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins

fluid mosaic model

describes the plasma membrane's structure as a mosaic of components including phospholipids, cholesterol, proteins, glycoproteins, and glycolipids (sugar chains attached to proteins or lipids, respectively), resulting in a fluid character (fluidity)

glycolipid

combination of carbohydrates and lipids

glycoprotein

combination of carbohydrates and proteins

hydrophilic

molecule with the ability to bond with water; “water-loving”

hydrophobic

molecule that does not have the ability to bond with water; “water-hating”

hypertonic

situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in water moving out of the cell

hypotonic

situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in water moving into the cell

integral protein

protein integrated into the membrane structure that interacts extensively with the membrane lipids' hydrocarbon chains and often spans the membrane

isotonic

situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net water movement into or out of the cell

osmolarity

total amount of solutes dissolved in a specific amount of solution

osmosis

transport of water through a semipermeable membrane according to the water's concentration gradient across the membrane that results from the presence of solute that cannot pass through the membrane

passive transport

method of transporting material through a membrane that does not require energy

peripheral protein

protein at the plasma membrane's surface either on its exterior or interior side

pinocytosis

a variation of endocytosis that imports macromolecules that the cell needs from the extracellular fluid

plasmolysis

detaching the cell membrane from the cell wall and constricting the cell membrane when a plant cell is in a hypertonic solution

potocytosis

variation of pinocytosis that uses a different coating protein (caveolin) on the plasma membrane's cytoplasmic side

primary active transport

active transport that moves ions or small molecules across a membrane and may create a difference in charge across that membrane

pump

active transport mechanism that works against electrochemical gradients

receptor-mediated endocytosis

variation of endocytosis that involves using specific binding proteins in the plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles

secondary active transport

movement of material that results from primary active transport to the electrochemical gradient

selectively permeable

membrane characteristic that allows some substances through (also known as semipermeable)

solute

substance dissolved in a liquid to form a solution

symporter

transporter that carries two different ions or small molecules, both in the same direction

tonicity

amount of solute in a solution

transport protein

membrane protein that facilitates a substance's passage across a membrane by binding it

transporter

specific carrier proteins or pumps that facilitate movement

uniporter

transporter that carries one specific ion or molecule

Which plasma membrane component can be either found on its surface or embedded in the membrane structure?

protein

Which characteristic of a phospholipid contributes to the fluidity of the membrane?

double bonds in the fatty acid tail

What is the primary function of carbohydrates attached to the exterior of cell membranes?

identification of the cell

Plasma membranes range from

5 to 10 nm in thickness

A plasma membrane's principal components are

lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and proteins

A phospholipid is a molecule consisting of

glycerol, two fatty acids, and a phosphate-linked head group

Carbohydrates are present only on the

plasma membrane's exterior surface and are attached to proteins

The membrane's main fabric comprises

amphiphilic, phospholipid molecules

The phospholipids' hydrophilic regions form

hydrogen bonds with water and other polar molecules on both the cell's exterior and interior

the membrane surfaces that face the cell's interior and exterior are

hydrophilic.

the cell membrane's interior is

hydrophobic and will not interact with water

phospholipids form an excellent two-layer cell membrane that

separates fluid within the cell from the fluid outside the cell

A phospholipid molecule consists of a

three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon

vital to the plasma membrane's structure because, in water

phospholipids arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out

lipid bilayer

a double layered phospholipid barrier that separates the water and other materials on one side from the water and other materials on the other side

Single-pass integral membrane proteins usually have a

hydrophobic transmembrane segment that consists of 20–25 amino acids

These carbohydrate chains may consist of

2–60 monosaccharide units and can be either straight or branched

carbohydrates form

specialized sites on the cell surface that allow cells to recognize each other

This recognition function is very important to cells, as it allows the

immune system to differentiate between body cells (“self”) and foreign cells or tissues (“non-self”)

The glycocalyx is highly

hydrophilic and attracts large amounts of water to the cell's surface

The membrane is not like a balloon, however, that can expand and contract; rather, it is

fairly rigid and can burst if penetrated or if a cell takes in too much water

Plasma membranes are asymmetric: the

membrane's interior is not identical to its exterior

Non-polar and lipid-soluble material with a low molecular weight can

easily slip through the membrane's hydrophobic lipid core

Ions such as

sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes

concentration gradients are a form of

potential energy, which dissipates as the gradient is eliminated

Molecules move constantly in a

random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.

A substance moves into any space available to it until it

evenly distributes itself throughout

lack of a concentration gradient in which the

substance has no net movement dynamic equilibrium

A concentration gradient exists that would allow these materials to

diffuse into the cell without expending cellular energy

Facilitated transport proteins shield these materials from the

membrane's repulsive force, allowing them to diffuse into the cell

Passage through the channel allows polar compounds to avoid the

plasma membrane's nonpolar central layer that would otherwise slow or prevent their entry into the cell

Channel proteins are either

open at all times or they are “gated,” which controls the channel's opening

When all of the proteins are bound to their ligands, they are

saturated and the rate of transport is at its maximum

glucose transport proteins, or GLUTs, are involved in

transporting glucose and other hexose sugars through plasma membranes within the body

Channel proteins facilitate diffusion at a rate of

tens of millions of molecules per second; whereas, carrier proteins work at a rate of a thousand to a million molecules per second

Osmosis is the movement of

free water molecules through a semipermeable membrane according to the water's concentration gradient across the membrane

If the solution's volume on both sides of the membrane is the same, but the solute's concentrations are different, then there are

different amounts of water, the solvent, on either side of the membrane

A solution with low osmolarity has a

greater number of water molecules relative to the number of solute particles

A solution with high osmolarity has

fewer water molecules with respect to solute particles

the solute cannot move across the membrane, and thus

the only component in the system that can move the water, moves along its own concentration gradient

Scientists use three terms

hypotonic, isotonic, and hypertonic to relate the cell's osmolarity to the extracellular fluid's osmolarity that contains the cells

In a hypotonic environment

water enters a cell, and the cell swells

In an isotonic condition

the relative solute and solvent concentrations are equal on both membrane sides

In a hypertonic solution

water leaves a cell and the cell shrinks

If either the hypo- or hyper- condition goes to excess, the cell’s functions become

compromised, and the cell may be destroyed

If the cell swells, and the spaces between the lipids and proteins become too large, the cell will

break apart

when excessive water amounts leave a red blood cell, the cell

shrinks, or crenates

Various living things have ways of

controlling the effects of osmosis a mechanism we call osmoregulation

The interior of living cells is electrically negative with respect to the extracellular fluid in which they

are bathed, and at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than the extracellular fluid

To move substances against a concentration or electrochemical gradient, the cell must use

energy.

An important membrane adaptation for active transport is the

presence of specific carrier proteins or pumps to facilitate movement

These three types of carrier proteins are also in

facilitated diffusion, but they do not require ATP to work in that process

Some examples of pumps for active transport are

Na+-K+ ATPase, which carries sodium and potassium ions, and H+-K+ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins

Two other carrier proteins are

Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

One of the most important pumps in animal cells is the

sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells

The process consists of the following six steps

1. With the enzyme oriented towards the cell's interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.

2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.

3. As a result, the carrier changes shape and reorients itself towards the membrane's exterior. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.

4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.

5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the cell's interior.

6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Secondary active transport uses the

kinetic energy of the sodium ions to bring other compounds, against their concentration gradient into the cell

As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an

electrochemical gradient.