AP Biology Science Practice 1: Models and Representations

Main ideas, concepts, and lessons

• Science practices are core skills for doing science effectively

  • There are seven overarching science practices that students should learn and apply.
  • In AP Biology, teachers build these skills through the year so students can apply knowledge on the AP Biology test.

• Models and visual representations help turn “mental ideas” into shared, testable understanding

  • A model is not just a private mental picture; it’s a visual/conceptual representation that others can use.
  • Models make complex biological processes easier to understand and apply on exams.

• Example: DNA structure requires models to interpret microscopic evidence

  • An electron microscope image may not immediately show the “double helix” as it’s often imagined.
  • The visible structure at that scale may reflect DNA wrapped around histone proteins, forming a fiber.
  • Watson and Crick created a model to explain how DNA works, enabling understanding beyond what is directly seen.

• The role of science practices across AP Bio “big ideas”

  • AP Biology is organized around four big ideas:
    • Evolution
    • Free energy
    • Information
    • Systems
  • The speaker provides example models for each:
    • Evolution → natural selection model (bacteria with differential survival/resistance)
    • Free energy → photosynthesis model (light reactions + Calvin cycle, including NADPH and ATP carriers)
    • Information → operon model (RNA polymerase and a repressor controlling gene expression)
    • Systems → energy pyramid model (plants → herbivores → carnivores)

AP Biology science practice skill: 5 things you can do with models/representations

The video highlights five model-related tasks typical of AP Biology exam questions:

1) Create models and representations

• Example (directional selection / beetle color):
  • Start with a hypothetical beetle population with wide variation in color matching tree trunk coloration.
  • Build a graph showing how the population changes when the environment changes so tree trunks darken.
  • Conceptual steps shown:
    • Represent beetle color frequencies as a normal distribution (light ↔ dark on the x-axis).
    • After trunks darken:
      • Light-colored beetles are more visible to predators and die.
      • The surviving distribution shifts toward darker coloration.
    • Label:
      • Pre-evolution (before environmental change)
      • Post-evolution (after the shift), illustrating directional selection

2) Describe models and representations

• Example (osmosis with a U-tube):
  • A U-tube contains:
    • Water (blue side)
    • Dissolved salts on the other side
    • A semipermeable membrane that allows water only
  • Description of what happens over time:
    • Salt cannot cross the membrane; water moves via diffusion/osmosis.
    • Water moves from lower salt concentration (higher water concentration) to higher salt concentration.
    • Water levels change:
      • Increase on the high-salt side
      • Decrease on the low-salt side
    • Motion continues until salt-to-water concentration is balanced (equilibrium):
      • Water still fluctuates, but there is no net change.

3) Refine models and representations

• Example (translation and changing mRNA):
  • Given a translation model:
    • mRNA moves through a ribosome
    • tRNA enters the A site and contributes an amino acid
  • If the mRNA sequence changes:
    • The amino acid sequence changes.
  • If amino acids change:
    • Each amino acid differs by its R group
    • Different R groups cause different chemical interactions
    • The protein folds differently:
      • Secondary and tertiary structure changes
    • Therefore, the protein’s properties change.

4) Use models and representations

• Example (bacterial transduction → genetic variation):
  • A model of transduction in bacteria using a bacteriophage is shown.
  • Key reasoning steps:
    • A bacteriophage infects bacteria and injects DNA.
    • Instead of viral DNA being packaged, it may be bacterial DNA.
    • When that phage infects another bacterium, it injects bacterial DNA into a new bacterium.
  • Result:
    • Transferred DNA produces genetic variation in the receiving bacteria.

5) Re-express models and representations

• Example (signal transduction via insulin):
  • Model includes:
    • Insulin receptor
    • GLUT glucose transporters
  • Question type: how changing elements affects cellular response.
  • The model’s function:
    • Insulin docks with the insulin receptor
    • This triggers signal transduction
    • Signal transduction opens GLUT transporters so glucose enters the cell
  • Consequences based on changes:
    • No insulin → no signaling → GLUT doesn’t open → glucose can’t enter
    • Type 2 diabetes scenario (receptor docking problem):
      • Insulin is present, but receptors don’t function properly
      • Still no effective signaling → GLUT not opened → glucose uptake fails

Additional historical/scientific context

• The video concludes by emphasizing:
  • Models make biological processes easier to understand and are used by scientists.
  • A “most famous model” mentioned is the Watson and Crick DNA model.
  • Their success came from knowing DNA’s chemical components and base ratios, and then building a structure that could be visualized.

Speakers / sources featured

• Mr. Anderson (speaker)
• James Watson (credited with DNA model)
• Francis Crick (credited with DNA model)
• “McCarty” (mentioned as contributing to the discovery that DNA transforms bacteria; first name not provided in subtitles)
• Bacteriophage / Watson-Crick context (scientific entities/models discussed; not additional human sources)