Fitness (College Board AP® Biology): Study Guide

Variation in molecules within cells

• Variation in the structure of molecules within cells allows organisms to respond to different stimuli, increasing biological fitness

  • Suitable responses to stimuli provide a survival advantage to the organism, increasing the chances of reproduction in a given environment

• In order to respond to stimuli cells need to have the right types and number of molecules to function effectively

Examples of molecular variation

Phospholipids

• The phospholipids within a cell membrane affect membrane function

• Phospholipids can differ in ways that influence membrane fluidity

  • E.g. fatty acid chain saturation

    • Phospholipids with saturated fatty acid chains pack tightly together, resulting in a more rigid membrane

    • Phospholipids with unsaturated fatty acids pack less tightly together, resulting in a more fluid membrane

• Altering the proportion of phospholipids within membranes allows organisms to maintain optimal membrane fluidity, ensuring proper membrane function in a range of environments

  • Fluid membranes aid survival in colder climates where membranes tend to become more rigid

  • Stable membranes aid survival in warmer environments where membranes tend to become more fluid

Hemoglobin

• In humans hemoglobin is responsible for binding oxygen in the lungs and transporting it to the tissues

• Two types of hemoglobin in humans are:

  • adult hemoglobin

  • fetal hemoglobin

• Adult and fetal hemoglobin differ in their oxygen affinity

  • Fetal hemoglobin must bind oxygen from the mother's blood across the placenta; oxygen concentration here is low and so fetal hemoglobin needs a higher affinity for oxygen

  • Adult hemoglobin must bind oxygen directly from the air that enters the alveoli; oxygen concentrations are high in the lungs so the affinity for oxygen is lower

Chlorophylls

• Chlorophylls are photosynthetic pigments which absorb light energy

• Chlorophyll exists in two forms:

  • chlorophyll a

  • chlorophyll b

• Having a range of different chlorophyll types gives plants the ability to absorb a range of light wavelengths, maximising energy available for photosynthesis