Unit 1: How Do Living Things Survive and Reproduce?
Cells as the functional unit of life
Unit 1 builds the foundation for everything else: cells are the smallest unit that can carry out all the processes of life. You must be able to compare prokaryotic and eukaryotic cells, describe the roles of organelles (nucleus, mitochondrion, chloroplast, ribosome, endoplasmic reticulum, Golgi body, lysosome, vacuole, cell membrane, cell wall), and explain the fluid mosaic model of the membrane, including diffusion, facilitated diffusion, osmosis and active transport. Know that surface-area-to-volume ratio limits cell size, and that exchange surfaces (villi, alveoli, root hairs) increase it.
Systems, inputs and outputs
Living things survive by coordinating systems that obtain and use matter and energy. Understand photosynthesis and cellular respiration as complementary reactions, the role of enzymes (specificity, active site, effect of temperature and pH, denaturation), and how organisms regulate water and solute balance. At the organism level, relate structure to function across plant and animal systems.
Traps examiners set
- Confusing osmosis (water only, down its own gradient, through a semipermeable membrane) with diffusion of solutes.
- Claiming active transport moves substances "down" a gradient; it moves them against it and needs energy.
- Saying an enzyme is "killed" or "used up" by heat or by reaction; it is denatured or unchanged.
- Treating SA:V as increasing with cell size; it decreases.
- Describing a hypothesis as a prediction, or failing to identify variables and controls.
Worked example and rule of thumb
A potato cylinder loses mass in a concentrated salt solution. Reasoning: the external solution has lower water potential than the cell contents, so water leaves by osmosis through the semipermeable membranes, and mass falls. Rule of thumb: follow the water, not the salt - water moves from higher water potential (dilute) to lower (concentrated).
Investigations
Describe controlled experiments: independent, dependent and controlled variables, replicates, and sources of error. Distinguish accuracy from precision and reliability from validity.