Molecules and Cells
Core concepts
Cells are the basic units of structure and function. Prokaryotic cells lack a membrane-bound nucleus and organelles; eukaryotic cells have both. The plasma membrane is a phospholipid bilayer with embedded proteins, controlling what enters and leaves. Diffusion moves substances down a concentration gradient without energy; osmosis is diffusion of water across a semipermeable membrane; facilitated diffusion uses channel or carrier proteins but still needs no ATP; active transport (for example, the sodium-potassium pump) moves substances against a gradient and requires energy. Mitochondria carry out aerobic respiration, converting glucose and oxygen into ATP, carbon dioxide, and water. Chloroplasts perform photosynthesis, capturing light energy to build glucose and release oxygen. Ribosomes assemble proteins; the nucleus stores DNA and directs activity. Enzymes are protein catalysts that lower activation energy; they are specific to substrates and can be denatured by heat or extreme pH.
Common traps
Questions often confuse osmosis with diffusion of solutes, or claim that facilitated diffusion uses ATP. Remember that a cell placed in a hypertonic solution loses water and shrinks; a hypotonic solution causes water uptake. Do not assume all cells have mitochondria; bacteria do not. Distinguish the products of respiration from those of photosynthesis.
Worked example
A plant cell is placed in a solution with a higher solute concentration than its cytoplasm. Water leaves the cell by osmosis, and the cell becomes flaccid or plasmolyzed. Rule of thumb: water always moves toward the side with more solute (lower water concentration).