🧬 Biology Study Guide

πŸ“š High School / AP Biology 🎯 Key Concepts: Cell Biology, Photosynthesis, Genetics, Evolution, Ecosystems
Before You Begin
πŸ“Œ After this unit, you'll be able to
🌱 Why study biology?
Think of it intuitively

All life processes are energy conversion chains. ATP (adenosine triphosphate) is the cell's energy currency. Food (glucose) gets converted to ATP, and when ATP breaks down, that energy powers muscle contraction, protein synthesis, and nerve signals.

Biology is the science of life and living systems. Why do we need sleep? How do cancer cells multiply uncontrollably? Why don't antibiotics work against viruses? All have biological answers. Understanding photosynthesis connects to food security and carbon neutrality; genetics connects to gene therapy, GMOs, and CRISPR. Every major challenge of the 21st century β€” from pandemic preparedness to climate adaptation β€” requires biological literacy.

⚑ 30-second summary

1. Cell Biology

Cell Theory

  1. All living things are made of one or more cells.
  2. The cell is the basic unit of life.
  3. All cells come from pre-existing cells (cell division).

Prokaryotic vs. Eukaryotic Cells

FeatureProkaryoteEukaryote
NucleusNo membrane-bound nucleusTrue nucleus with membrane
SizeSmaller (1–10 Β΅m)Larger (10–100 Β΅m)
OrganellesFew; no membrane-boundMany membrane-bound organelles
ExamplesBacteria, ArchaeaAnimal, plant, fungal, protist cells

Key Organelles (Eukaryotic)

2. Photosynthesis and Cellular Respiration

Photosynthesis

Plants, algae, and cyanobacteria convert light energy into chemical energy (glucose).

6COβ‚‚ + 6Hβ‚‚O + light energy β†’ C₆H₁₂O₆ + 6Oβ‚‚

Cellular Respiration

Cells break down glucose to release ATP energy (aerobic).

C₆H₁₂O₆ + 6Oβ‚‚ β†’ 6COβ‚‚ + 6Hβ‚‚O + ATP (β‰ˆ36–38 ATP)

3. DNA, Genes, and Protein Synthesis

4. Genetics β€” Mendelian Inheritance

Gregor Mendel's experiments with pea plants established the laws of inheritance.

Key Terms
❌ Classic Mistake β€” "Dominant" Doesn't Mean Better
Dominant alleles are healthier, more common, or genetically superior to recessive ones "Dominant" only means the allele is expressed in the heterozygous (Bb) state β€” it says nothing about health or frequency
Huntington's disease is caused by a dominant allele. Sickle cell anemia by a recessive one. Dominance is about expression, not advantage.

Punnett Squares AP Exam

Cross: Bb Γ— Bb (heterozygous Γ— heterozygous)
    B    b
B | BB | Bb |
b | Bb | bb |
Genotype ratio: 1 BB : 2 Bb : 1 bb
Phenotype ratio: 3 dominant : 1 recessive (75% : 25%)
πŸ› Origin of the Concept

Darwin's 1831–1836 Beagle voyage to the GalΓ‘pagos Islands β€” where he observed different beak shapes among finches on different islands β€” planted the seed of natural selection. After 20 years of meticulous research, he published On the Origin of Species in 1859, only accelerating after Alfred Russel Wallace independently arrived at the same idea. Darwin's theory merged with Mendel's genetics in the 1940s into the Modern Evolutionary Synthesis β€” the cornerstone of modern biology.

5. Natural Selection and Evolution

Darwin's Four Postulates
  1. Individuals in a population show variation in traits.
  2. Some variation is heritable (passed to offspring).
  3. More offspring are produced than can survive β€” competition for resources.
  4. Individuals with favorable traits have greater reproductive success β†’ those traits increase in the population.
Evidence for Evolution

6. Ecosystems

AP Biology Big Ideas: (1) Evolution explains biodiversity. (2) Cells are the basic units of life. (3) Living systems use energy and molecular building blocks. (4) Living systems interact with their environments. Keep these themes in mind when studying any biology topic.

7. Biogeochemical Cycles

Matter is recycled through ecosystems via biogeochemical cycles. Unlike energy (which flows in one direction and is lost as heat), nutrients cycle repeatedly between living organisms and the environment.

Carbon Cycle
Nitrogen Cycle
Why is the 10% Energy Rule important?

If a field of grass contains 10,000 kcal of energy, primary consumers (rabbits) can obtain at most ~1,000 kcal, secondary consumers (foxes) ~100 kcal, and tertiary consumers ~10 kcal. This is why food chains rarely exceed 4–5 trophic levels β€” there is simply not enough energy left to sustain a higher level.

πŸ“ Practice Problems

Exercise 1 β€” Cell Organelles

Match each function to the correct organelle.

  1. Produces ATP through aerobic respiration β†’ ( )
  2. Converts light energy into glucose β†’ ( )
  3. Synthesizes proteins from mRNA β†’ ( )
  4. Modifies and packages proteins for secretion β†’ ( )
  5. Stores DNA and directs cell activities β†’ ( )
β–Ά Show Answers

1. Mitochondria   2. Chloroplast   3. Ribosome   4. Golgi Apparatus   5. Nucleus

Exercise 2 β€” Photosynthesis and Respiration

  1. Write the balanced equation for photosynthesis.
  2. Where in the chloroplast does the Calvin Cycle occur?
  3. How many ATP molecules are produced per glucose molecule in aerobic respiration?
  4. What is the final electron acceptor in the electron transport chain?
β–Ά Show Answers

1. 6COβ‚‚ + 6Hβ‚‚O + light energy β†’ C₆H₁₂O₆ + 6Oβ‚‚
2. The stroma
3. Approximately 36–38 ATP
4. Oxygen (Oβ‚‚), which becomes water (Hβ‚‚O)

Exercise 3 β€” Genetics AP Exam

A pea plant that is heterozygous for seed color (Yy, where Y = yellow is dominant) is crossed with a homozygous recessive plant (yy).

  1. Draw the Punnett square for this cross.
  2. What are the genotype ratios of the offspring?
  3. What percentage of the offspring will have yellow seeds?
β–Ά Show Answers
   Y    y
y | Yy | yy |
y | Yy | yy |

Genotypes: 1 Yy : 1 yy (50% : 50%)
Phenotypes: 50% yellow seeds (Yy) : 50% green seeds (yy)

Exercise 4 β€” Evolution

  1. What is the difference between homologous and analogous structures? Give an example of each.
  2. A population of bacteria is exposed to an antibiotic. Most die, but a few with a random mutation survive and reproduce. What evolutionary process does this illustrate?
  3. What type of evidence would show that two species share a common ancestor approximately 5 million years ago?
β–Ά Show Answers

1. Homologous: same underlying structure, different function β€” shared common ancestor (e.g., human arm and whale flipper). Analogous: different structure, same function β€” convergent evolution (e.g., bird wing and insect wing).
2. Natural selection β€” bacteria with the favorable mutation (antibiotic resistance) survive and pass on that trait.
3. Molecular evidence (DNA/protein sequence comparison), fossil record, or radiometric dating of fossils.

πŸ”— Bridge to Next Concept

Why do species on opposite sides of the world sometimes look remarkably similar β€” or completely different β€” from close relatives?

When plate tectonics split ancient landmasses apart, isolated populations evolved independently. Earth science explains the geographic stage on which evolution performed its experiments.

Earth Science
πŸ”“ Master This to Unlock
Earth Science β€” species distribution & continental drift Ecology β€” populations, communities, matter cycles

Biology is the intersection of chemistry, earth science, and medicine. Now that you understand cells and genetics, the energy flows of ecosystems and the evolutionary history shaped by geology are ready to explore.

Pre-Test Checklist
Practice

Basic Q. What happens to ATP when energy is released in a cell?

Show answer
ATP (adenosine triphosphate) loses one phosphate group: ATP β†’ ADP + Pi (inorganic phosphate) + energy. The reverse reaction (ADP + Pi β†’ ATP) stores energy β€” this is what happens during cellular respiration and photosynthesis.

Intermediate Q. Write the overall equation for photosynthesis and label the reactants and products.

Show answer
6COβ‚‚ + 6Hβ‚‚O + light energy β†’ C₆H₁₂O₆ + 6Oβ‚‚. Reactants: carbon dioxide and water. Products: glucose (stores chemical energy) and oxygen (released as a byproduct).

Advanced Q. Explain what "semi-conservative replication" means and why it matters for genetic inheritance.

Show answer
Each of the two DNA strands serves as a template for a new complementary strand. Result: each daughter cell gets one original strand + one new strand. This guarantees that exact genetic information is passed on during every cell division.
🧠
Spaced Repetition β€” Ebbinghaus Curve

Review this material at increasing intervals to commit it to long-term memory.

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Up Next
Earth Science

From life to our planet β€” explore plate tectonics, weather systems, and geological history.

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βœ“ NGSS Standards aligned βœ“ Reviewed Apr 2026 πŸ” Accuracy verified Found an error? Let us know