Molecules and Cells
What to know
The cell is the basic unit of life, and its chemistry explains its behavior. Candidates should be fluent in the four macromolecules - carbohydrates (energy and structure, sugars and polysaccharides), lipids (nonpolar, membranes and long-term energy storage), proteins (amino acid chains folded into functional shapes, enzymes, transport, signaling), and nucleic acids (DNA and RNA, information storage and transfer). Understand the central dogma: DNA is transcribed to mRNA, which is translated to protein at the ribosome.
Know the membrane as a phospholipid bilayer with embedded proteins, selectively permeable and described by the fluid mosaic model. Compare prokaryotic and eukaryotic cells (prokaryotes lack membrane-bound organelles and a nucleus). Link organelles to functions: mitochondria (aerobic respiration, ATP), chloroplasts (photosynthesis), ribosomes (protein synthesis), rough and smooth ER, Golgi (packaging and shipping), lysosomes, and the nucleus. Trace energy flow: photosynthesis captures light energy into glucose; cellular respiration converts glucose to ATP, beginning with glycolysis in the cytoplasm and continuing in the mitochondria.
Traps
- Passive vs. active transport. Diffusion and osmosis need no energy and move down a gradient; pumps and vesicle transport consume ATP.
- Osmosis direction. Water moves toward the side with more solute (lower water concentration).
- Enzyme claims. Enzymes lower activation energy; they are not consumed and do not change the reaction's energy yield. Denaturation destroys shape and function.
- Respiration location. Glycolysis is cytoplasmic even in eukaryotes.
Rule of thumb
If a question asks where a molecule moves, ask which side has more solute, then move water the other way. If it asks where ATP is made, look to mitochondria (and cytoplasm for glycolysis).
Worked example
A plant cell is placed in a hypertonic solution. Solute outside is higher, so water exits, the cell loses turgor, and the membrane pulls from the wall (plasmolysis).