Campbell Biology Chapter 7 Review: Membrane Structure and Function

7.1 The fluid mosaic: lipids and proteins

pp. 125–131

On the AP exam? Yes

This is Topic 2.3: know what each part of the membrane does and how the fluid mosaic model describes it. You won't be asked about the history of membrane models or the lab methods used to study them.

In the course: Topic 2.3 Plasma Membrane, Topic 2.1 Cell Structure and Function, Topic 4.2 Introduction to Signal Transduction (notes, videos and more questions)

Key points

Membranes are mostly phospholipids and proteins, with short sugar chains attached on the outside. A phospholipid is amphipathic: its phosphate head is attracted to water and its two fatty acid tails avoid it. In water, phospholipids line up in two layers, heads facing the watery fluid on each side and tails meeting in the middle.
The fluid mosaic model pictures a membrane as a flexible, oily sheet with many different proteins set into it or stuck to its surface. Earlier models put the proteins in flat layers over the surfaces, but evidence showed they sit within the bilayer.
Lipids and many proteins drift sideways within their own layer, and lipids do it very fast. Switching from one layer to the other is rare, because a polar head would have to pass through the oily core. Some proteins are held still by the cytoskeleton inside or by fibers outside the cell.
Fatty acid tails with double bonds have kinks, so they can't pack tightly, and they keep a membrane fluid in the cold. In animal cells, cholesterol evens out fluidity: it limits lipid movement when it's warm and keeps the lipids from packing solid when it's cold. Over generations, natural selection has matched each species' membrane lipids to the temperatures it lives in.
Integral proteins reach into the hydrophobic core, and most of them, the transmembrane proteins, cross it completely. The parts in the core are made of nonpolar amino acids, while the parts facing water are polar or charged. Peripheral proteins sit loosely on one surface.
Membrane proteins do most of a membrane's jobs: moving substances across, acting as enzymes, receiving signals, letting cells recognize each other, linking neighboring cells, and anchoring to the cytoskeleton and the matrix outside the cell.
Sugar chains on glycoproteins and glycolipids face the outside, where they act as markers that other cells can read. A membrane's two faces differ because the ER and Golgi build membrane with a set orientation, and vesicles carry it to the cell surface.

Key terms (12)

plasma membrane: The thin boundary around every cell that controls what goes in and out. It's built mainly from phospholipids and proteins.
phospholipid bilayer: Two layers of phospholipids, tails pointing inward and heads facing the water on each side. It's the basic fabric of every cell membrane.
amphipathic: Having one part that's attracted to water and another part that avoids it, like a phospholipid with its polar head and nonpolar tails.
fluid mosaic model: The current picture of a membrane: a fluid sheet of lipids with a patchwork of proteins and other molecules that can move around in it.
integral protein: A membrane protein that reaches into the bilayer's oily core. You need a detergent to pull it out.
transmembrane protein: An integral protein that spans the whole bilayer, with parts sticking out on both sides. Most channels, carriers and receptors are this kind.
peripheral protein: A protein loosely attached to one surface of a membrane, often to the head groups or to an integral protein, without entering the core.
cholesterol: A steroid tucked between phospholipids in animal cell membranes. It keeps fluidity from swinging too far as temperature changes.
unsaturated fatty acid: A fatty acid tail with one or more C=C double bonds. The kinks they cause keep tails from packing tightly, so membranes stay fluid.
glycoprotein: A protein with sugar chains attached. In the plasma membrane, the sugars stick out on the outside of the cell.
glycolipid: A lipid with a sugar chain attached. Like glycoproteins, glycolipids put their sugars on the cell's outer face.
cell-cell recognition: A cell's ability to tell other cells apart by the molecules on their surfaces. It's key to building tissues and to immune defense.

Check yourself: 7.1 The fluid mosaic: lipids and proteins

4 questions on 7.1 The fluid mosaic: lipids and proteins. Pick an answer to see if you got it, and why.

Question 1 of 4

A transmembrane protein crosses the plasma membrane once. The stretch of about 20 amino acids that sits inside the bilayer's core would most likely be rich in amino acids with which kind of R group?

Question 2 of 4

Researchers made artificial membranes from the same phospholipids, with or without cholesterol, and measured how fluid each was at two temperatures (invented data; higher numbers mean more fluid). Membrane | Fluidity at 10 °C | Fluidity at 37 °C No cholesterol | 0.8 | 4.6 With cholesterol | 1.9 | 3.2 Which claim is best supported by these data?

Question 3 of 4

A researcher attaches a glowing tag to two different plasma membrane proteins, X and Y, in separate cells. She then uses a laser to bleach the glow from a small spot of each membrane. Within seconds the bleached spot in the X cells glows again, but the bleached spot in the Y cells stays dark for many minutes. Which explanation is best supported?

Question 4 of 4

An enzyme that cuts sugar chains off proteins is too large and too polar to cross membranes. When it's added to intact cells, it removes the sugar chains from most plasma membrane glycoproteins. When it's added to sealed vesicles made from the same plasma membranes but turned inside out, it removes almost none. What do these results show?

0 of 4 answered

7.2 What can cross a membrane, and why

pp. 131–132

On the AP exam? Yes

Topic 2.4 tests this directly: be ready to predict whether a molecule crosses the bilayer on its own from its size, polarity and charge.

In the course: Topic 2.4 Membrane Permeability, Topic 2.6 Facilitated Diffusion (notes, videos and more questions)

Key points

Cells constantly trade materials with their surroundings: nutrients and O₂ in, wastes and CO₂ out, and ions in both directions. A membrane is selectively permeable, which means some substances cross easily, some slowly, and some hardly at all.
The hydrophobic core of the bilayer does most of the sorting. Small nonpolar molecules, like O₂, CO₂ and N₂, dissolve in it and pass straight through.
Ions and polar molecules are held back by the core. Ions carry a shell of water and almost never cross on their own. Large polar molecules like glucose cross only very slowly, and small uncharged polar ones like water get through in small amounts.
Transport proteins let hydrophilic substances skip the oily core. Channel proteins form water-filled tunnels, and carrier proteins bind a substance and change shape to move it across.
Each transport protein is choosy, moving one substance or a few closely related ones. Aquaporins are channels that let large amounts of water cross quickly.
So a membrane's permeability depends on its lipids and on its set of transport proteins, which is why different cell types let different things through.

Key terms (10)

selective permeability: A membrane's habit of letting some substances through easily and others slowly or not at all.
hydrophobic: Avoiding water. Nonpolar molecules and the fatty acid tails in the middle of the bilayer are hydrophobic.
hydrophilic: Attracted to water. Ions and polar molecules are hydrophilic, which is why the oily membrane core blocks them.
nonpolar molecule: A molecule with no separated charges, like O₂, CO₂ or N₂. Small ones pass straight through the lipid bilayer.
polar molecule: A molecule with partly positive and partly negative ends, like water or glucose. Large ones need proteins to cross membranes quickly.
ion: An atom or molecule with an electric charge, such as Na⁺, K⁺ or Cl⁻. Ions need channel or carrier proteins to cross a membrane.
transport protein: A membrane protein that moves a specific substance across the membrane. Channels and carriers are the two main kinds.
channel protein: A transport protein that forms a tunnel through the membrane so certain ions or molecules can pass through very quickly.
carrier protein: A transport protein that binds a specific substance and changes shape to move it to the other side.
aquaporin: A channel protein just for water. It lets cells such as plant root cells move lots of water very quickly.

Check yourself: 7.2 What can cross a membrane, and why

4 questions on 7.2 What can cross a membrane, and why. Pick an answer to see if you got it, and why.

Question 1 of 4

An artificial membrane is made of phospholipids only, with no proteins. Which list ranks these substances from fastest to slowest at crossing it?

Question 2 of 4

Testosterone, a steroid hormone, binds a receptor protein inside its target cells. Glucagon, a hormone made of 29 amino acids, binds a receptor on the outside of the plasma membrane. Which explanation best accounts for the difference?

Question 3 of 4

Researchers made sealed lipid vesicles with or without a purified membrane protein, Q, and measured how fast each substance entered (invented data; arbitrary units). Substance | Vesicles without Q | Vesicles with Q NH₃ (ammonia) | 50 | 51 K⁺ | 0.01 | 30 Na⁺ | 0.01 | 0.02 Which conclusion is best supported?

Question 4 of 4

Frog egg cells normally swell only slowly when placed in dilute water. Researchers injected some eggs with mRNA coding for a membrane protein. When placed in dilute water, these eggs swelled quickly and burst within minutes, while uninjected eggs did not. What does this result best show?

0 of 4 answered

7.3 Diffusion, osmosis and facilitated diffusion

pp. 132–135

On the AP exam? Yes

Topics 2.5–2.7 cover all of this. The book explains osmosis with solute concentration, but the course also uses water potential (Ψ = Ψp + Ψs and Ψs = −iCRT), so practice both.

In the course: Topic 2.5 Membrane Transport, Topic 2.6 Facilitated Diffusion, Topic 2.7 Tonicity and Osmoregulation, Topic 2.4 Membrane Permeability (notes, videos and more questions)

Key points

Molecules are always moving at random. Diffusion is the net spread of a substance from where it's more concentrated to where it's less, which we call moving down its concentration gradient. It needs no energy from the cell, because the gradient itself stores potential energy.
Every substance follows its own gradient, whatever other substances are doing. At equilibrium molecules still cross, but equally in both directions, so nothing changes overall.
Passive transport is diffusion through a membrane that costs the cell no energy. If a cell keeps using up a substance, its gradient stays and the substance keeps moving in.
Osmosis is the diffusion of water through a membrane that lets water pass but holds back some solutes. Water moves from higher water potential to lower. With no pressure difference, that means toward the side with more dissolved particles. Water potential is Ψ = Ψp + Ψs, and solute potential is Ψs = −iCRT, so adding solute makes Ψ more negative.
Tonicity compares solutes that can't cross the membrane. In a hypertonic solution a cell loses water, in a hypotonic one it gains water, and in an isotonic one there's no net change. Cells without walls burst in hypotonic surroundings and shrivel in hypertonic ones, so they need osmoregulation, like the contractile vacuoles of freshwater protists.
Cells with walls (plants, fungi, bacteria) swell until the wall pushes back. Firm, turgid cells are healthy for most plants. Isotonic cells are limp (flaccid), and in hypertonic surroundings the membrane shrinks away from the wall (plasmolysis) and the plant wilts.
Facilitated diffusion is passive transport through proteins: channels, including aquaporins and gated ion channels that open in response to a voltage change or a chemical signal, and carriers. The protein speeds things up but never changes the direction, which is always down the gradient.

Key terms (15)

diffusion: The overall spread of a substance from where it's more concentrated to where it's less, caused by random motion. It needs no energy input.
concentration gradient: A difference in concentration from one place to another. Substances diffuse down it, from high to low.
passive transport: Movement across a membrane down a gradient with no energy spent by the cell. Simple diffusion, osmosis and facilitated diffusion all count.
osmosis: Diffusion of water through a membrane that blocks some solutes, from higher water potential to lower, usually toward the side with more solute.
water potential (Ψ): A measure of how likely water is to move, in bars or megapascals. Water flows from higher Ψ to lower Ψ, and Ψ = Ψp + Ψs.
solute potential (Ψs): The part of water potential set by dissolved solutes, found with Ψs = −iCRT. More solute makes it more negative.
tonicity: How a surrounding solution will make a cell gain or lose water, based on solutes that can't cross the membrane.
hypertonic: Describes a solution with more non-crossing solute than the cell. The cell loses water to it.
hypotonic: Describes a solution with less non-crossing solute than the cell. The cell gains water from it.
isotonic: Describes a solution with the same concentration of non-crossing solutes as the cell, so there's no net water movement.
osmoregulation: How an organism controls its water and solute balance, for example by pumping water out with a contractile vacuole.
turgid: Firm from water pressing the membrane against the cell wall. Turgid cells hold up soft plant parts like leaves.
plasmolysis: When a walled cell loses so much water that its membrane separates from the wall. It makes plants wilt and can kill them.
facilitated diffusion: Passive transport through a channel or carrier protein, down the gradient, with no energy input.
gated channel: An ion channel that opens or closes in response to a signal, such as a voltage change or a chemical binding to it.

Check yourself: 7.3 Diffusion, osmosis and facilitated diffusion

4 questions on 7.3 Diffusion, osmosis and facilitated diffusion. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Find the solute potential (Ψs) of a 0.20 M NaCl solution at 25 °C in an open beaker. Assume NaCl splits completely into Na⁺ and Cl⁻ (i = 2). Use Ψs = −iCRT, with R = 0.0831 L·bar/(mol·K).

Question 2 of 4Calculator allowed

Cores cut from a turnip were weighed, soaked in sucrose solutions for 24 hours, and weighed again (invented data). Sucrose (M) | Change in mass (%) 0.0 | +18 0.2 | +7 0.4 | −3 0.6 | −11 0.8 | −17 The sucrose concentration at which the cores would neither gain nor lose mass is the best estimate of how concentrated the turnip cells are. Where does that point fall?

Question 3 of 4

A single-celled protist lives in a freshwater pond and pumps out extra water with a contractile vacuole. Which outcome is most likely if a drug stops the cell from making ATP?

Question 4 of 4

In the liver, glucose enters cells through a carrier protein after a meal. Hours later, when blood sugar has dropped, glucose leaves liver cells through the same carrier. No ATP is used either way. What explains the change in direction?

0 of 4 answered

7.4 Active transport, pumps and cotransport

pp. 135–138

On the AP exam? Yes

Topic 2.8 covers ATP-powered pumps, the sodium-potassium pump and the membrane potential. Cotransport isn't named in the course, but exam questions may describe one and ask you to reason about it.

In the course: Topic 2.8 Mechanisms of Transport, Topic 2.5 Membrane Transport, Topic 2.6 Facilitated Diffusion, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Active transport moves a substance against its gradient, from low concentration to high. That takes energy, usually from ATP, and it always needs a protein; a channel can only let things flow down a gradient.
The sodium-potassium pump is the key example in animal cells. Each cycle uses one ATP to move 3 Na⁺ out and 2 K⁺ in. ATP adds a phosphate to the pump, which changes its shape and how tightly it holds each ion.
Cells have a voltage across their plasma membrane, the membrane potential, usually between about −50 and −200 mV, with the inside negative. A negative inside attracts positive ions and repels negative ones.
For an ion, both concentration and charge matter. Together they make its electrochemical gradient, and an ion moving passively follows that combined gradient, not just its concentration.
An electrogenic pump moves net charge and so builds voltage. The sodium-potassium pump (3 positive charges out, 2 in) is the main one in animals, while plants, fungi and bacteria rely mainly on proton pumps that push H⁺ out. In a resting cell, much of the voltage also comes from K⁺ leaking out through channels, down the gradient the pump built.
In cotransport, a protein lets one ion, often H⁺ or Na⁺, flow back down its gradient and uses that energy to drag a second substance up its own gradient. The cotransporter uses no ATP itself, but ATP was spent to build the first gradient, so it's still active transport.
Proton gradients also drive ATP synthase in mitochondria and chloroplasts, which you'll meet in Topics 3.4 and 3.5.

Key terms (9)

active transport: Moving a substance from lower to higher concentration through a membrane, which takes energy (usually ATP) and a transport protein.
sodium-potassium pump: An animal-cell pump that uses one ATP per cycle to move 3 Na⁺ out and 2 K⁺ in, keeping both gradients steep.
phosphorylation: Adding a phosphate group to a molecule. When ATP phosphorylates a pump, the pump changes shape.
membrane potential: The voltage across a membrane, caused by uneven charges on the two sides. In most cells the inside is negative.
electrochemical gradient: The combined push on an ion from its concentration difference and from the membrane's voltage. Ions diffuse down it.
electrogenic pump: A pump that moves net charge across a membrane and so creates voltage, like the sodium-potassium pump or a proton pump.
proton pump: A pump that moves H⁺ across a membrane, building an H⁺ gradient and a voltage that can power other work.
cotransport: Transport where one substance flowing down its gradient powers the movement of another substance against its gradient through the same protein.
ATP: The cell's main short-term energy carrier. Breaking off its last phosphate releases energy that powers most pumps.

Check yourself: 7.4 Active transport, pumps and cotransport

4 questions on 7.4 Active transport, pumps and cotransport. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A single sodium-potassium pump completes 120 cycles in one second. In that second, how many ions does it move, and how many ATP molecules does it use?

Question 2 of 4

During a nerve impulse, Na⁺ channels open briefly and some Na⁺ rushes into the cell. Which process returns that Na⁺ to the outside over time?

Question 3 of 4

Ca²⁺ is about 10,000 times more concentrated outside a typical animal cell than in its cytoplasm, and the cell's interior carries a negative charge relative to the fluid outside. If calcium channels open, what happens?

Question 4 of 4

Cultured gut-lining cells were placed in solutions with the amino acid alanine. After 30 minutes, researchers compared the alanine concentration inside the cells to the concentration outside (invented data). Condition | Alanine inside ÷ alanine outside Normal solution (with Na⁺) | 9.0 Na⁺ replaced by another cation that cells can't use | 1.0 Normal solution, Na⁺/K⁺ pumps blocked 2 hours before | 1.3 ATP levels were normal in all three cases. Which conclusion is best supported?

0 of 4 answered

7.5 Vesicle traffic: exocytosis and endocytosis

pp. 138–139

On the AP exam? Yes

Topic 2.5 covers endocytosis and exocytosis and the fact that they need energy. The course doesn't name the three kinds of endocytosis, but knowing them helps you explain scenarios.

In the course: Topic 2.5 Membrane Transport, Topic 2.1 Cell Structure and Function (notes, videos and more questions)

Key points

Large molecules like proteins and polysaccharides, and large amounts of material, cross the plasma membrane inside vesicles instead of through transport proteins. This bulk transport needs energy.
In exocytosis, a vesicle, often from the Golgi, rides cytoskeleton tracks to the cell surface, fuses with the plasma membrane, and releases its contents outside. Its membrane merges into the surface membrane. Cells use it to release hormones and signal molecules and, in plants, to deliver material for building walls.
In endocytosis, a patch of plasma membrane sinks inward and pinches off as a vesicle carrying material into the cell. It looks like exocytosis in reverse, but different proteins run it.
In phagocytosis, the cell extends its membrane (pseudopodia) around a large particle such as a bacterium and seals it in a sac that later fuses with a lysosome for digestion. In pinocytosis, the cell takes in tiny droplets of fluid, along with whatever is dissolved in them.
In receptor-mediated endocytosis, specific receptors bind their target molecules (ligands) and gather in coated pits that pinch inward. This lets a cell collect lots of one substance even when it's scarce outside, and the receptors are sent back to the surface to be reused.
Exocytosis adds membrane to the cell surface and endocytosis removes it. In a cell that isn't growing, the two roughly balance.

Key terms (10)

bulk transport: Moving large molecules or large amounts of material across the plasma membrane inside vesicles. It takes energy.
vesicle: A small sac of membrane that carries materials within a cell or to and from its surface.
exocytosis: Releasing material from a cell when a vesicle fuses with the plasma membrane and empties outside. It's how cells secrete proteins like hormones.
endocytosis: Taking material into a cell by folding the plasma membrane inward and pinching off a vesicle.
phagocytosis: Endocytosis of a large particle, like a bacterium, which the cell surrounds with its membrane and later digests. Its name means "cell eating."
pseudopodium: A temporary bulge of a cell's membrane and cytoplasm, used to crawl or to wrap around something to engulf it (plural: pseudopodia).
pinocytosis: Endocytosis of tiny droplets of fluid along with whatever is dissolved in them. It doesn't pick out particular molecules.
receptor-mediated endocytosis: Endocytosis triggered when specific receptors bind their target molecules, letting a cell gather lots of one substance.
ligand: A molecule that fits and binds a particular receptor protein, the way a hormone binds its receptor.
coated pit: A dent in the plasma membrane lined with coat proteins on the inside, where receptors gather before pinching off as a vesicle.

Check yourself: 7.5 Vesicle traffic: exocytosis and endocytosis

4 questions on 7.5 Vesicle traffic: exocytosis and endocytosis. Pick an answer to see if you got it, and why.

Question 1 of 4

A plasma cell, a type of immune cell, releases thousands of antibody proteins every second. Which process does it use to get these large proteins out?

Question 2 of 4

Macrophages are white blood cells that engulf bacteria. In macrophages treated with drug X, bacteria are engulfed normally but then survive for days inside membrane sacs in the cytoplasm. Which step does drug X most likely block?

Question 3 of 4

Cells were given two fluorescent substances at the same time: ligand L, which binds a specific surface receptor, and a dye that binds nothing and simply dissolves in the fluid. Uptake was measured under three conditions (invented data; % of uptake at 37 °C). Condition | Ligand L | Dye 37 °C | 100 | 100 37 °C + drug that drains ATP | 6 | 9 37 °C + large excess of unlabeled ligand L | 12 | 98 Which conclusion is best supported?

Question 4 of 4

In a fruit fly mutant, endocytosis stops working at warm temperatures. When the flies are warmed, their nerve endings keep releasing signal molecules by exocytosis for a short time, then stop, while the plasma membrane at each nerve ending gets larger. What best explains this?

0 of 4 answered