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Unit 1: Characteristics of Life and Biological Molecules

The molecules and properties that make life possible, from water's odd behavior to the four classes of biological macromolecules.

8 topics · 29 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.

What Makes Something Alive

  1. How it worksHow do biologists check a mystery organism against the seven traits that define life?
  2. Going furtherWhat happens to the checklist for life when you try to apply it to a virus?
  3. Common mistakeWhy isn't being made of cells by itself enough to prove something is alive?
Your turn1 of 2
A car burns gasoline, converts it into motion and heat, and eventually breaks down and stops running. A student argues this means the car meets the definition of a living thing because it uses energy. Which reasoning explains why the car still isn't alive?

Levels of Biological Organization

  1. How it worksHow do cells stack up into tissues, organs, and organ systems inside your body?
  2. Going furtherWhat happens to a level of organization when one organ inside it fails?
  3. Ties togetherHow does the organization inside your body mirror the organization of a whole ecosystem?
Your turn1 of 2
A single skin cell on your arm is alive, carries out metabolism, and responds to touch. Why can't that one cell keep your whole body alive on its own the way a full organism can?

Water and the Properties of Life

  1. How it worksHow does a water molecule's bent shape give it a positive end and a negative end?
  2. How it worksWalk me through how hydrogen bonds let many water molecules cling to each other at once.
  3. Common mistakeWhy doesn't ice sink to the bottom of a lake the way a frozen block of most liquids would?
  4. Going furtherWhat would happen to a lake's fish if ice sank instead of floating in winter?
Your turn1 of 2
Ice floats because it is about 9 percent less dense than liquid water. If a fully submerged block of liquid water at 0 degrees C has a volume of 294 cubic centimeters, what volume would the same mass occupy once it freezes into ice?

Carbohydrates

  1. How it worksHow do glucose molecules link together to build starch and glycogen?
  2. How it worksWalk me through how your body breaks a starch chain back down into single sugar molecules.
  3. Going furtherWhat makes cellulose so hard for humans to digest even though it's also just chains of glucose?
  4. Common mistakeWhy isn't sugar automatically bad for you just because it's a carbohydrate?
Your turn1 of 2
A slice of bread and a spoonful of vegetable oil contain roughly the same number of calories, yet the bread gives you a quicker burst of usable energy than the oil does. What actually explains this difference?

Lipids

  1. How it worksHow do three fatty acid tails attach to a glycerol backbone to build a fat molecule?
  2. How it worksHow does one phospholipid end up with a water-loving head and a water-fearing tail?
  3. Common mistakeWhy isn't all dietary fat equally bad for your arteries?
  4. Going furtherWhat happens to a saturated fat's shape that makes it solid at room temperature?
Your turn1 of 2
A triglyceride forms when three fatty acid tails attach to a single glycerol backbone, releasing one water molecule at each attachment point. If a sample contains 142 glycerol molecules, each fully reacting with three fatty acids, how many total water molecules are released?

Proteins and Amino Acids

  1. How it worksHow do amino acids link together through peptide bonds to form a chain?
  2. How it worksWalk me through how a protein chain folds from a flat strand into its final 3D shape.
  3. Common mistakeWhy doesn't heating an egg white just rearrange its amino acid sequence?
  4. Going furtherWhat happens to a protein's function the moment heat or acid makes it lose its shape?
Your turn1 of 2
Amino acids link together end to end through peptide bonds to form a protein chain, with one peptide bond formed between each pair of neighboring amino acids. If a protein chain contains 285 amino acids, how many peptide bonds hold the chain together?

Nucleic Acids at a Glance

  1. How it worksHow do four different bases spell out genetic information along a DNA strand?
  2. Going furtherWhat changes when a cell needs to copy DNA's message into RNA instead?
  3. Common mistakeWhy isn't DNA itself the molecule that directly builds a protein?
Your turn1 of 2
In double-stranded DNA, adenine always pairs with thymine and cytosine always pairs with guanine, so the percentage of A equals the percentage of T. If a DNA sample is 31 percent adenine, what percentage of its bases are guanine?

Enzymes and Activation Energy

  1. How it worksHow does an enzyme's shape let it grab one specific substrate and no other?
  2. How it worksHow does an enzyme lower the activation energy a reaction needs to get started?
  3. Going furtherWhat happens to an enzyme's activity when the temperature climbs past its ideal range?
  4. Common mistakeWhy isn't an enzyme used up or changed after it catalyzes a reaction?
Your turn1 of 2
An uncatalyzed reaction has an activation energy of 117 kJ/mol. An enzyme lowers this activation energy by 52 percent. What is the new, catalyzed activation energy?