The smallest unit of life — and the busiest factory your student will ever tour.
Suggested pace: 4 weeks. Mastery looks like:
Three sentences organize all of biology: every living thing is made of one or more cells; the cell is the basic unit of life; and every cell comes from a preexisting cell. That last sentence, hard-won in the 1800s, ended centuries of belief in spontaneous generation and gave biology its unbroken chain — every cell in your student's body descends, division by division, from earlier cells. Teach the theory first, because everything in this unit is an elaboration of it.
All cells fall into two great categories. Prokaryotic cells — bacteria — are small and structurally simple: no membrane-bound nucleus, their DNA loose in the cytoplasm. Eukaryotic cells — everything from yeast to oak trees to your student — are larger and organized into compartments called organelles, each membrane-wrapped and specialized. The distinction matters constantly: antibiotics, for example, work by attacking features prokaryotes have and our cells lack.
Walk the factory floor with your student. The nucleus is the office, holding the DNA blueprints. Ribosomes are the assembly machines that build proteins. The endoplasmic reticulum is the production line — rough ER studded with ribosomes for proteins, smooth ER for lipids. The Golgi apparatus packages and ships products. Mitochondria are the power plants, running cellular respiration to make ATP. Lysosomes are the recycling crew, digesting waste. Plant cells add three features animal cells lack: a rigid cell wall for support, chloroplasts for photosynthesis, and a large central vacuole that maintains pressure. The memorable rule: every organelle's shape serves its job.
Every cell is wrapped in a plasma membrane built as a phospholipid bilayer — two layers of lipid molecules with water-loving heads facing out and water-fearing tails facing in, studded with protein gates and pumps. The membrane is selectively permeable: it controls what enters and leaves. Small molecules drift through by diffusion, moving from high concentration to low without any energy spent. Osmosis is simply the diffusion of water across the membrane, and it explains why cells swell in pure water and shrivel in salt water — a demonstration your kitchen can run with a potato slice tonight.
When the cell must move materials against the concentration gradient — from low to high — it pays for the privilege: active transport uses protein pumps powered by ATP. Finally, cells reproduce on a schedule called the cell cycle: a long interphase of growth and preparation (including the S phase, when the cell copies its entire DNA), followed by mitosis, the division that produces two identical daughter cells. Checkpoints police the cycle; cancer is what happens when those checkpoints fail. Your student now holds the floor plan that the genetics unit will set in motion.
Every correct answer climbs one rung. Climb forever. Badges at 10, 25, 50, and 100 lifetime rungs.
GENO — a robot you can actually TALK to — has studied this entire unit and is available day or night, in 32 languages, at no cost. Ask him to re-explain any idea on this page, quiz you out loud, or go deeper than the lesson goes.