BIOL101- MasteringBiology- CH. 6b

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In cells, membrane proteins are responsible for the passage of substances that can’t cross the membrane on their own. <b>Channel and carrier proteins are involved in facilitated diffusion. Pumps are involved in active transport.</b> In terms of their properties or functions, which statement most accurately describes how these three types of membrane proteins compare?

A- Channel and carrier proteins are integral membrane proteins; pumps are peripheral membrane proteins.
B- Among the three types, only pumps are thought to undergo a shape change as part of their function.
C- Among the three types, only pumps can concentrate substances on one side of the membrane.
D- Pumps are selective; channel and carrier proteins are not selective.

C- Among the 3 types, only pumps can concentrate substances on one side of the membrane. Only pumps use energy (from ATP) to move substances against electrochemical gradients.

One type of active transport is called cotransport. In cotransport, the energy available from an ion moving through a transport protein down its electrochemical gradient is coupled with the movement of another solute (small polar molecule or ion) through the same protein but against its concentration or electrochemical gradient. The solute that is being transported against its gradient may move into the cell or out of the cell (but not both), depending on the type of transport protein that catalyzes this process.
Before beginning this tutorial, watch the clip of the Membrane Transport animation showing cotransport of sodium and glucose.

Part A – The glucose-sodium cotransporter See figure. In cotransport, the energy required to move one solute against its concentration or electrochemical gradient is provided by an ion moving into the cell down its electrochemical gradient. The ion that moves into the cell down its gradient is usually the same ion that is pumped out of the cell by an active transport pump: for example, Na+ in animal cells using the sodium-potassium pump, or H+ in plants and prokaryotes using the proton pump. In the case of the glucose-sodium cotransporter in animals, Na+ moves back into the cell down its electrochemical gradient, providing the energy for glucose to move into the cell against its concentration gradient. The energy for glucose transport into the cell is supplied indirectly by the sodium-potassium pump’s hydrolysis of ATP, and directly by the Na+ electrochemical gradient created by the pump.

Which statement accurately summarizes a feature of passive transport?

a- It makes the cell interior and exterior more different from one another.
b- It is a spontaneous process.
c- It involves no change in entropy.
d- No membrane proteins are involved.

b. it is a spontaneous process. Passive transport is powered by diffusion along an electrochemical gradient and does not require an input of energy.

In terms of structure, how do channel proteins differ from carrier proteins?
Drag the appropriate items to their respective bins.

Channel: -form pores in membrane -open or close in response to a signal -have different functional groups in the interior of pore that function as a molecular filter Carrier: -change shape to shuttle molecules or ions across membrane -undergo conformational changes caused by binding of specific molecules

One of the roles of plasma membrane proteins is to____.

a. allow passage of select molecules that could not otherwise cross the lipid bilayer
b. provide a structure on which the membrane lipids can assemble a bilayer
c. form one side of the membrane bilayer
d. dissolve the lipids in the membrane

a. allow passage of select molecules that could not otherwise cross the lipid bilayer Some membrane proteins function as transporters to move charged or large, polar molecules across the membrane.

Which of the following membrane activities requires energy from ATP?

a. movement of Na+ ions from a lower concentration in a mammalian cell to a higher concentration in the extracellular fluid
b. movement of glucose molecules into a bacterial cell from a medium containing a higher concentration of glucose than inside the cell
c. movement of carbon dioxide out of a paramecium
d. movement of water into a cell
e. facilitated diffusion of chloride ions across the membrane through a chloride channel

a. movement of Na+ ions from a lower concentration in a mammalian cell to a higher concentration in the extracellular fluid This is an example of active transport, which requires energy for particles to from area of low concentration to high concentration, or against their concentration gradient.

The physical property characteristic that distinguishes lipids from most other cellular macromolecules is that _____.

a- lipids readily form hydrogen bonds with water and other polar solvents
b- lipids contain carbon
c- lipids are very large molecules
d-lipids are largely hydrophobic

d-lipids are largely hydrophobic

Under what circumstances does membrane transport require energy?

a-whenever an ion moves through a phospholipid bilayer membrane
b-whenever a solute is moved <b>against</b> its electrochemical gradient
c-wherever large molecules are moved within a cell
d-whenever oxygen moves through a phospholipid bilayer membrane

b-whenever a solute is moved against its electrochemical gradient this is the definition of active transport

Part A – Diffusion
All molecules have energy that causes thermal motion. One result of thermal motion is diffusion: the tendency of substances to spread out evenly in the available space. Although the motion of each individual molecule is random, there can be directional motion of an entire population of molecules.
Consider a chamber containing two different types of dye molecules, purple and orange. The chamber is divided into two compartments (A and B) by a membrane that is permeable to both types of dye. Initially (left image), the concentration of the orange dye is greater on side A, and the concentration of the purple dye is greater on side B. With time, the dye molecules diffuse to a final, equilibrium state (right image) where they are evenly distributed throughout the chamber.

Each dye molecule and the water molecules that surround it are in constant motion due to their thermal energy. Any individual molecule’s motion is random because of the frequent collisions among all of the molecules. If a concentration gradient exists for a population of molecules, the motion of the individual molecules in that population will result in a net (directional) movement from higher to lower concentration. For example, in the initial condition, there is a concentration gradient for the orange dye. As a result, the orange dye molecules diffuse down the concentration gradient, with net movement from side A to side B. Once diffusion has eliminated the concentration gradient and equilibrium is reached, net movement stops, but the random motion of each molecule continues.

Part B – Permeability of the lipid bilayer
Some solutes are able to pass directly through the lipid bilayer of a plasma membrane, whereas other solutes require a transport protein or other mechanism to cross between the inside and the outside of a cell. The fact that the plasma membrane is permeable to some solutes but not others is what is referred to as selective permeability.
Which of the following molecules can cross the lipid bilayer of a membrane directly, without a transport protein or other mechanism? Select all that apply.

carbon dioxide

carbon dioxide, water, lipids, oxygen -Small nonpolar (hydrophobic) molecules, such as dissolved gases (O2, CO2, N2) and small lipids, can pass directly through the membrane. They do so by interacting directly with the hydrophobic interior of the lipid bilayer. -Very small polar molecules such as water and glycerol can pass directly through the membrane, but much more slowly than small nonpolar molecules. The mechanism that permits small polar molecules to cross the hydrophobic interior of the lipid bilayer is not completely understood, but it must involve the molecules squeezing between the hydrophobic tails of the lipids that make up the bilayer. -Polar molecules such as glucose and sucrose have very limited permeability. -Large molecules such as proteins cannot pass through the lipid bilayer. -Ions and charged molecules of any size are essentially impermeable to the lipid bilayer because they are much more soluble in water than in the interior of the membrane.

Part C – Facilitated diffusion via channels and carrier proteins
The majority of solutes that diffuse across the plasma membrane cannot move directly through the lipid bilayer. The passive movement of such solutes (down their concentration gradients without the input of cellular energy) requires the presence of specific transport proteins, either channels or carrier proteins. Diffusion through a transport protein in the plasma membrane is called facilitated diffusion.

only channels: -provide a continuous path across membrane – allow water molecules and small ions to flow quickly across membrane only carries: -undergo change in shape to transport solutes across membrane -transport primarily small polar organic molecules both channel & carrier: -transport solutes down a concentration or electrochemical gradient -are integral membrane proteins -provide hydrophilic path across membrane

What is the function of Structure E?

stabilization of the phospholipids
cell-cell communication
transport across the plasma membrane
structural support of the cell
detection of environmental change

stabilization of the phospholipids cholesterol helps stabilize structure of the plasma membrane.

Identify Structure D.

extracellular matrix
phospholipid bilayer of membrane

Phospholipid bilayer of membrane phospholipids can be recognized by the presence of a head and two tails

Identify Structure A.

extracellular matrix

glycoprotein Structure A is composed of both a carb and protein.

Which of the following allows water to move much faster across cell membranes?

the hydrophobic interior of a cell membrane
the sodium-potassium pump
peripheral proteins

aquaporins they are especially for water transport across membranes

Decreasing the saturation of the fatty acid chains on a particular type of phospholipid would result in the formation of _____.

<b>more fluid bilayers</b>
less fluid bilayers

more fluid bilayers decrease saturation>more kinks>more fluid

Where would you most likely find an integral membrane protein?

<b>a- spanning the cell membrane, with parts of the protein visible from both the inside and the outside of the cell</b>
b-on the inside surface of the cell membrane
c-floating freely in the cytoplasm
d-on the outside surface of the cell membrane

a- spanning the cell membrane, with parts of the protein visible from both the inside and the outside of the cell because it is the longest duhh jk.

The movement of molecules through a plasma membrane channel protein is always _____.

<b>down the molecule’s electrochemical gradient</b>
toward the inside of the cell
against the molecule’s electrochemical gradient
"paid for" by an expenditure of cellular energy

down the molecule’s electrochemical gradient channels are responsible for facilitated diffusion.

Which of the following processes includes all others?

diffusion of a solute across a membrane
facilitated diffusion
<b>passive transport</b>
transport of an ion down its electrochemical gradient

passive transport they’re all down the concentration gradient and do not require input of energy>>definition of passive transport

A membrane protein that spans the phospholipid bilayer one or more times is _____.

an integrin
a transmembrane protein
a peripheral protein
a glycoprotein
an associated protein

a transmembrane protein

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