Unit 1: Chemistry of Life
Ice floats on liquid water. Explain why, and why this matters for life in lakes during winter.
Hydrogen bonds hold water molecules in an open lattice in ice, so ice is less dense than liquid water. The floating ice insulates the water below, so aquatic organisms can survive.
Why can't a single cell grow larger and larger without limit?
As a cell grows, its volume increases faster than its surface area, so the surface area-to-volume ratio drops. The membrane can no longer exchange nutrients and wastes fast enough.
Explain how an enzyme speeds up a reaction. Does it change the free energy change (delta G) of the reaction?
It lowers the activation energy by stabilizing the transition state. It does not change delta G, and the enzyme is not used up.
A human cell has 46 chromosomes in G1. How many chromosomes and how many chromatids does it have in prophase? How many chromosomes does each daughter cell have at the end?
In prophase it has 46 chromosomes made of 92 chromatids. Each daughter cell has 46 chromosomes.
Compare the organelles you would expect to find in large numbers in a pancreatic enzyme-secreting cell and in a muscle cell. Explain why.
Pancreatic cell: lots of rough ER and Golgi, because it makes and secretes proteins. Muscle cell: lots of mitochondria, because it needs a lot of ATP for contraction.
Solution A has a pH of 3 and solution B has a pH of 6. How many times higher is the H+ concentration in A than in B?
1000 times higher (the pH scale is logarithmic, and 3 pH units = 10 x 10 x 10).
Calculate the surface area-to-volume ratio of a cube-shaped cell with sides of 3 cm.Formulas (SA=6a^2), (V=a^3)
Surface area = 6 x 3 x 3 = 54 cm2. Volume = 27 cm3. Ratio = 2:1.
In an experiment, the rate of an enzyme reaction rises as substrate concentration increases, then levels off. Explain why it levels off.
All the enzyme active sites are occupied (the enzyme is saturated), so adding more substrate cannot increase the rate.
In an onion root tip, 200 cells are counted: 150 in interphase, 20 in prophase, 10 in metaphase, 10 in anaphase, and 10 in telophase. The whole cell cycle takes 20 hours. How long does a cell spend in interphase, and how long in prophase?
Interphase = 150/200 x 20 = 15 hours. Prophase = 20/200 x 20 = 2 hours.
Compare the surface area-to-volume ratio of spherical cells with radii of 5 micrometers and 10 micrometers. (SA = 4 pi r2, V = 4/3 pi r3.) Which cell exchanges materials more efficiently?
The ratio simplifies to 3/r. The 5 micrometer cell has a ratio of 0.6 and the 10 micrometer cell has 0.3. The smaller cell exchanges materials more efficiently.
A mutation changes one amino acid far away from an enzyme's active site, yet the enzyme stops working. Explain how this is possible.
The new amino acid changes R-group interactions, which changes the tertiary structure (3D shape). This can distort the active site so the substrate no longer fits.
A plant cell has a solute potential of -0.5 MPa and a pressure potential of +0.2 MPa. It is placed in a solution with a water potential of -0.1 MPa. Calculate the cell's water potential and say which way water moves.
Cell water potential = -0.5 + 0.2 = -0.3 MPa. Water moves from higher to lower water potential, so water moves into the cell.
Both chloroplasts and mitochondria make ATP by chemiosmosis. For each, say where the H+ ions accumulate, where the electrons come from, and what the final electron acceptor is.
Chloroplast: H+ accumulate in the thylakoid space, electrons come from water, final acceptor is NADP+. Mitochondrion: H+ accumulate in the intermembrane space, electrons come from NADH and FADH2, final acceptor is oxygen.
One molecule of epinephrine binds a receptor on a liver cell and leads to the release of thousands of glucose molecules. Explain how a tiny signal produces such a large response.
Signal amplification. Each step in the signal transduction cascade (receptor, G protein, enzyme, second messenger, further enzymes) activates many molecules in the next step.
Design an experiment to test whether temperature affects the activity of catalase. Name the independent variable, the dependent variable, and two controls.
Independent variable: temperature. Dependent variable: rate of oxygen or foam production (or H2O2 breakdown). Controls: same enzyme and substrate concentration, same pH, same volume, and a control with no enzyme or boiled enzyme.
A protein is made of two polypeptide chains. One has 150 amino acids and the other has 100. How many peptide bonds are there in total, and how many water molecules were released when they were made?
149 + 99 = 248 peptide bonds, and 248 water molecules released (dehydration synthesis).
A potato core has a mass of 5.0 g. After sitting in a sucrose solution it has a mass of 4.2 g. Calculate the percent change in mass and say whether the solution was hypotonic, hypertonic, or isotonic to the potato cells.
(4.2 - 5.0) / 5.0 x 100 = -16%. The potato lost water, so the solution was hypertonic to the potato cells.
A poison blocks the electron transport chain. Predict what happens to the proton gradient and to ATP production. Why does NADH build up?
The proton gradient cannot form, so ATP synthase makes much less ATP. NADH cannot give up its electrons, so it is not converted back to NAD+, and it builds up.
A cell has DNA damage and a nonfunctional p53 protein. What happens at the G1 checkpoint, and what is the possible consequence?
The checkpoint fails to stop the cycle. The cell does not pause for DNA repair or trigger apoptosis, so the damaged DNA is copied and passed on. This can lead to tumor formation.
In some metabolic pathways, the final product inhibits the first enzyme in the pathway. What is this called, and why is it efficient for the cell?
Feedback inhibition (negative feedback, often allosteric regulation). It stops the cell from wasting energy and materials making more product than it needs.
A DNA molecule has 200 base pairs, and 20% of its bases are adenine. How many guanine bases does it contain?
120. There are 400 bases in total. A = 80, so T = 80. The remaining 240 bases are G and C, so G = 120.
A pancreatic cell secretes a digestive enzyme. Trace the path of this protein from synthesis to leaving the cell.
Ribosome on the rough ER, then the ER lumen, then a transport vesicle, then the Golgi apparatus (modified and packaged), then a secretory vesicle, then exocytosis at the plasma membrane.
During a sprint, muscle cells run low on oxygen. Which pathway keeps them making ATP, why is NAD+ regeneration important, and how much ATP is made per glucose?
Lactic acid fermentation (after glycolysis). Regenerating NAD+ lets glycolysis keep running. The net yield is 2 ATP per glucose.
A cancer drug prevents the spindle from forming. At which stage will the treated cells be stuck, and why?
Without spindle fibers, chromosomes cannot attach to the spindle and line up at the equator, so the spindle assembly checkpoint blocks entry into anaphase.
Explain how a single DNA base mutation could lead to uncontrolled cell division.
The mutation can change the amino acid sequence, which changes the protein's shape and function. If the protein is a growth signal receptor or a tumor suppressor like p53, the pathway may stay constantly on or checkpoints may fail, so the cell cycle is no longer controlled.