← MCAT®

Bio/Biochem 2A: Assemblies of Molecules, Cells, and Groups of Cells

The membrane as a gate, the organelles as a factory floor, the cytoskeleton as both scaffold and engine, and the tissues they build.

23 topics · 89 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

2A. The Fluid Mosaic Model

  1. How it worksHow do phospholipids arrange themselves into a bilayer the moment they hit water?
  2. How it worksWalk me through what a membrane protein does as it drifts sideways through the lipid sea.
  3. Common mistakeWhy isn't the plasma membrane a rigid sheet with proteins nailed into fixed spots?
  4. Going furtherWhat happens to membrane fluidity as the temperature drops toward freezing?
Your turneasy1 of 3
A membrane protein is tagged with a fluorescent dye and a small patch of one cell's surface is bleached with a laser. Over the next few minutes the patch recovers to 81 percent of its original brightness and then stops improving. What percentage of that protein population is immobile?

2A. Membrane Lipids: Phospholipids, Steroids, and Waxes

  1. How it worksHow does cholesterol wedge between phospholipid tails and buffer a membrane at both temperature extremes?
  2. How it worksShow me how a phospholipid, a steroid, and a wax differ in shape and in what they do to a membrane.
  3. Common mistakeWhy doesn't adding cholesterol simply make a membrane stiffer at every temperature?
Your turneasy1 of 3
A purified plasma membrane is found to contain 52 cholesterol molecules for every 118 phospholipid molecules. Treating these two as the entire lipid content, what percentage of the lipid molecules in this membrane is cholesterol?

2A. Membrane Proteins and Membrane Dynamics

  1. How it worksHow do integral and peripheral membrane proteins sit differently in the bilayer?
  2. How it worksWalk me through a transmembrane helix threading its greasy stretch through the lipid core.
  3. Going furtherWhat changes about a membrane protein's job once sugar chains are hung off its outer face?
  4. Common mistakeWhy isn't every protein in a membrane free to wander anywhere across the cell surface?
Your turneasy1 of 3
An alpha helix rises 0.15 nm along its axis per amino acid residue. To cross the hydrophobic core of a bilayer that is 3.85 nm thick, how many residues does one membrane-spanning helix need?

2A. Passive Diffusion Across the Membrane

  1. How it worksHow does a concentration gradient turn random molecular jiggling into net one-way movement?
  2. How it worksShow me which molecules slip straight through the lipid core and which get stopped cold.
  3. Worked exampleFind which of CO2, glucose, and a sodium ion crosses a bare bilayer fastest, and explain the order.
  4. Common mistakeWhy doesn't diffusion stop the instant the concentrations match on both sides?
Your turneasy1 of 3
Oxygen sits at 78 micromolar outside a cell and 21 micromolar inside. Across this membrane a difference of 1 micromolar drives a net flux of 6 nanomoles per second per square centimetre. What is the net inward flux of oxygen?

2A. Osmosis and Osmotic Pressure

  1. How it worksHow does water move toward the side with more solute when the solute itself cannot cross?
  2. Worked exampleFind the osmotic pressure of 0.15 M NaCl at 310 K, and explain why that 0.15 gets doubled.
  3. Common mistakeWhy isn't osmotic pressure a push from the solute rather than a pull on the water?
  4. Going furtherWhat happens to a plant cell and an animal cell dropped into the same hypertonic bath?
Your turnhard1 of 3
A red blood cell of volume 90 femtolitres has an interior osmolarity of 335 milliosmolar and is dropped into a bath held at 327 milliosmolar. Water crosses freely, the solutes do not, and the bath is effectively infinite. What volume does the cell settle at?

2A. Facilitated Diffusion and Membrane Channels

  1. How it worksHow does a carrier protein change shape to hand glucose across without spending any ATP?
  2. How it worksWalk me through the difference between a channel that stays open and a carrier that cycles.
  3. Going furtherWhat happens to the transport rate once every carrier in the membrane is already occupied?
  4. Common mistakeWhy isn't facilitated diffusion a kind of active transport just because a protein is involved?
Your turnmedium1 of 3
A glucose transporter behaves like an enzyme, with a maximum transport rate of 18 millimoles per minute and a half-maximal concentration of 7 millimolar. What is the transport rate when glucose outside the cell sits at 28 millimolar?

2A. Active Transport and the Sodium-Potassium Pump

  1. How it worksWalk me through one full cycle of the sodium-potassium pump, three sodium out and two potassium in.
  2. Worked exampleFind the net charge a cell exports after 1000 turns of the sodium-potassium pump.
  3. Common mistakeWhy isn't the sodium-potassium pump just a channel that happens to be picky about ions?
  4. Going furtherWhat happens to cell volume and voltage when ouabain locks that pump shut?
Your turneasy1 of 3
A neuron carries 657 thousand sodium-potassium pumps in its membrane and each one completes 44 full cycles per second. How many thousands of potassium ions are carried into the cell every second?

2A. Secondary Active Transport

  1. How it worksHow does a sodium gradient built by one pump pay for a different molecule's uphill trip?
  2. How it worksShow me a symporter and an antiporter riding the same gradient in opposite directions.
  3. Worked exampleWalk me through how one intestinal cell pulls glucose out of a nearly empty gut lumen.
  4. Common mistakeWhy doesn't blocking the sodium-potassium pump leave secondary transport running normally?
Your turnmedium1 of 3
An intestinal cell absorbs 93 micromoles of glucose through SGLT1, which carries two sodium ions inward with each glucose. The sodium-potassium pump must then export all of that sodium at three sodium ions per ATP. How many micromoles of ATP does this cost?

2A. Membrane Potential

  1. How it worksHow do unequal ion concentrations plus a picky membrane add up to a voltage?
  2. Worked exampleFind the potassium equilibrium potential for 140 mM inside and 5 mM outside at 37 degrees C.
  3. Common mistakeWhy isn't the resting potential caused by extra negative ions piling up in the cytoplasm?
  4. Going furtherWhat happens to the voltage the instant the membrane becomes permeable to sodium?
Your turnmedium1 of 3
A cell holds potassium at 142 millimolar inside and 9 millimolar outside. At body temperature the equilibrium potential for a singly charged positive ion is 61 times the base-ten logarithm of the outside concentration over the inside one, in millivolts. Find the potassium equilibrium potential.

2A. Membrane Receptors

  1. How it worksHow does a ligand binding on the outside twist the part of the receptor sitting inside?
  2. Going furtherWhat happens to a cell's sensitivity when it pulls receptors off its surface after hours of signal?
  3. Common mistakeWhy isn't receptor specificity about the signal being small enough to squeeze through?
Your turneasy1 of 3
One epinephrine molecule binds one receptor, which activates 29 G proteins before the ligand leaves. Each activated G protein switches on an adenylate cyclase that produces 619 molecules of cyclic AMP. How many cyclic AMP molecules result from that single binding event?

2A. Endocytosis and Exocytosis

  1. How it worksWalk me through a clathrin coat bending a flat patch of membrane into a sealed vesicle.
  2. How it worksHow does a secretory vesicle fuse with the surface and dump its cargo outside the cell?
  3. Going furtherWhat happens to total surface area during a burst of exocytosis with no endocytosis to match it?
  4. Common mistakeWhy isn't phagocytosis the same process as pinocytosis just scaled up?
Your turnhard1 of 3
A stimulated secretory cell releases 7 thousand vesicles, each a sphere of radius 48 nanometres, and none of that membrane is retrieved. Taking the area of a sphere as 4 π r squared, how many square micrometres are added to the cell surface?

2A. Intercellular Junctions: Gap, Tight, and Desmosomes

  1. How it worksHow does a tight junction seal the gap between two cells so nothing leaks past?
  2. How it worksShow me ions passing straight from one cytoplasm into the next through a gap junction pore.
  3. Common mistakeWhy isn't a desmosome able to stop fluid leaking between the two cells it holds together?
  4. Ties togetherHow do gap junctions in cardiac muscle let the whole heart contract as one unit?
Your turneasy1 of 3
A gap junction channel is built from two connexons facing each other across the intercellular gap, and each connexon is a ring of six connexin subunits. A junctional plaque between two cardiac cells contains 433 channels. How many connexin subunits does the plaque contain in total?

2A. Defining Characteristics of Eukaryotic Cells

  1. How it worksHow does splitting a cell into compartments let incompatible chemistry run at the same time?
  2. How it worksWalk me through the three features that define a eukaryote: a nuclear envelope, organelles, and mitosis.
  3. Going furtherWhat happens to the surface-area-to-volume ratio as a cell grows bigger?
Your turneasy1 of 3
Treat a cell as a sphere of radius 22 micrometres. Using surface area 4 π r squared and volume four thirds π r cubed, find its surface-area-to-volume ratio in reciprocal micrometres.

2A. The Nucleus and Nucleolus

  1. How it worksHow does a nuclear pore let a ribosomal subunit out while keeping chromatin in?
  2. How it worksWalk me through the nucleolus from rRNA transcription to a finished subunit leaving the nucleus.
  3. Common mistakeWhy isn't the nucleolus a membrane-bound organelle like the ones surrounding it?
  4. Going furtherWhat happens to the nuclear envelope the moment a cell enters mitosis?
Your turneasy1 of 3
A nucleus carries 41 hundred nuclear pore complexes, and each exports on average 7 ribosomal subunits per minute. How many ribosomal subunits leave this nucleus every second?

2A. Mitochondria

  1. How it worksHow does folding the inner membrane into cristae multiply the ATP a mitochondrion can make?
  2. How it worksShow me why the intermembrane space and the matrix have to stay chemically separate.
  3. Common mistakeWhy isn't mitochondrial self-replication the same thing as the cell splitting its mitochondria in two?
  4. Ties togetherHow does an ancient bacterium living inside another cell explain everything odd about mitochondria?
Your turneasy1 of 3
Oxidative phosphorylation delivers about 2.5 ATP per NADH and about 1.5 ATP per FADH2. A round of metabolism sends 11 NADH and 4 FADH2 to the electron transport chain. How much ATP does that yield?

2A. The Endoplasmic Reticulum, Rough and Smooth

  1. How it worksWalk me through a ribosome docking on the rough ER and threading a new protein into the lumen.
  2. How it worksHow does the smooth ER build membrane lipids and detoxify drugs in the very same tubes?
  3. Going furtherWhat happens inside a liver cell's smooth ER after weeks of heavy drug exposure?
  4. Common mistakeWhy isn't every protein a cell makes threaded through the endoplasmic reticulum?
Your turneasy1 of 3
A hepatocyte's smooth endoplasmic reticulum occupies 5 percent of the cell volume. After weeks of a drug that its detoxifying enzymes handle, that figure rises to 22 percent. By what factor has the smooth ER expanded?

2A. The Golgi Apparatus and Protein Trafficking

  1. How it worksWalk me through a secreted protein's whole route from rough ER through the Golgi to the cell surface.
  2. How it worksHow do sugar tags added inside the Golgi act as shipping labels for different destinations?
  3. Worked exampleTrace where a lysosomal enzyme ends up when its mannose-6-phosphate tag never gets added.
  4. Common mistakeWhy isn't the Golgi just a warehouse holding proteins until the cell needs them?
  5. Going furtherWhat changes about the route when a protein is bound for the membrane instead of the outside?
Your turneasy1 of 3
A newly made secretory protein spends 34 minutes folding in the ER lumen, then crosses a Golgi stack of 6 cisternae at 9 minutes per cisterna, and is released as soon as it reaches the trans face. How long is the whole journey from synthesis to the cell surface?

2A. Lysosomes and Peroxisomes

  1. How it worksHow does the lysosome's acid interior both power its enzymes and protect the cytoplasm?
  2. How it worksShow me a peroxisome chopping a long fatty acid down and neutralizing the peroxide it makes.
  3. Common mistakeWhy isn't a peroxisome just a lysosome that specializes in lipids?
  4. Going furtherWhat happens to a cell over years when one single lysosomal enzyme is missing from birth?
Your turnmedium1 of 3
A lysosome holds its lumen at pH 5.07 while the surrounding cytosol stays at pH 7.2. How many times more concentrated are hydrogen ions inside the lysosome than outside it?

2A. Microfilaments and Intermediate Filaments

  1. How it worksHow does a ring of actin tighten like a drawstring and cleave one cell into two?
  2. How it worksWalk me through why intermediate filaments bear tension while microfilaments generate movement.
  3. Common mistakeWhy isn't the cytoskeleton a fixed scaffold that holds its shape the way a skeleton does?
Your turneasy1 of 3
Actin monomers add to a filament so that each one advances its length by 2.7 nanometres. How many monomers are in a single microfilament 7 micrometres long?

2A. Microtubules, Centrioles, and Motor Proteins

  1. How it worksHow do a microtubule's plus and minus ends give kinesin and dynein opposite directions to walk?
  2. How it worksWalk me through a centrosome nucleating microtubules and organizing them into a spindle.
  3. Worked exampleTrace where an axon's vesicles pile up when dynein is disabled and kinesin still works.
  4. Going furtherWhat happens to a cell's microtubules when colchicine stops tubulin from polymerizing?
Your turnhard1 of 3
A vesicle is carried by kinesin at 2 micrometres per second along an axon that is 79 centimetres long, travelling without pause from the cell body to the terminal. How many days does the trip take?

2A. Cilia and Flagella

  1. How it worksWalk me through the nine outer doublets and two central tubules that make an axoneme bend.
  2. How it worksHow does dynein sliding one doublet past its neighbor turn into a whip-like beat?
  3. Common mistakeWhy isn't a bacterial flagellum built the same way as a eukaryotic flagellum?
  4. Going furtherWhat happens to airway clearance and fertility together when the dynein arms are missing?
Your turneasy1 of 3
Each motile axoneme has the 9 plus 2 arrangement: nine outer doublets, each built from two microtubules, surrounding two single central microtubules. An airway cell carries 224 cilia. How many individual microtubules does that add up to?

2A. Epithelial Tissue

  1. How it worksHow do simple, stratified, squamous, cuboidal, and columnar combine into every epithelium's name?
  2. How it worksWalk me through an epithelial cell carrying different transporters on its apical and basal faces.
  3. Going furtherWhat happens to a barrier's job when one layer of cells becomes twenty stacked layers?
  4. Common mistakeWhy isn't epithelium free-floating tissue with nothing anchoring it underneath?
Your turneasy1 of 3
An intestinal absorptive cell has an apical face that would measure 72 square micrometres if it were flat, but its microvilli multiply that area by a factor of 25. What is the actual apical surface area of one cell?

2A. Connective Tissue

  1. How it worksHow does the matrix between the cells, not the cells themselves, set each connective tissue's properties?
  2. How it worksShow me collagen, elastin, and ground substance doing three different mechanical jobs.
  3. Common mistakeWhy isn't blood disqualified as connective tissue just because it flows?
  4. Ties togetherHow do the junctions holding an epithelium together anchor it to the connective tissue below?
Your turneasy1 of 3
A tendon is 85 percent collagen by dry mass. A dried tendon sample weighs 7 grams. How many grams of collagen does it contain?

AP®, SAT® and PSAT/NMSQT® are trademarks registered by the College Board (PSAT/NMSQT® jointly with the National Merit Scholarship Corporation). ACT® is a registered trademark of ACT Education Corp. GRE® is a registered trademark of ETS. LSAT® is a registered trademark of the Law School Admission Council. MCAT® is a registered trademark of the Association of American Medical Colleges. ISEE® and SSAT® are registered trademarks of their owners. None of these organizations is affiliated with, or endorses, Jentoo.