The cell as a living system
Cells are the basic units of life. Some organisms consist of one cell; others contain many specialized cells working together. Every cell has a membrane that separates its interior from its surroundings, genetic material that stores instructions, and ribosomes that build proteins. Eukaryotic cells, including plant and animal cells, keep most of their DNA inside a nucleus. Prokaryotic cells, such as bacteria, do not have a membrane-bound nucleus. Small does not mean simple in function: even a single cell must exchange materials, use energy, and maintain suitable internal conditions.
Structures have specific jobs
The cell membrane regulates movement into and out of a cell. Cytoplasm is the region inside the membrane where many reactions take place. Ribosomes assemble proteins using genetic instructions. In most eukaryotic cells, mitochondria carry out important stages of aerobic respiration. Many plant cells also contain chloroplasts, where photosynthesis occurs, a large central vacuole for storage and support, and a cellulose cell wall outside the membrane. Plant cells still have cell membranes and mitochondria. A cell wall provides support, but it does not replace the selectively permeable membrane.
Moving materials across a membrane
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, due to random molecular motion. Oxygen can diffuse into cells when its concentration is lower inside. Osmosis is the net movement of water across a selectively permeable membrane toward the side with a higher effective concentration of dissolved solutes, in a simple system. Active transport can move substances against a concentration gradient and requires energy. Remember the contrast: diffusion follows the gradient; active transport can work against it. Cells use several transport processes together.
Photosynthesis and respiration are connected
Photosynthesis captures light energy to build sugars from carbon dioxide and water, releasing oxygen. Aerobic respiration transfers energy from organic molecules such as glucose into ATP, using oxygen and releasing carbon dioxide and water. The simplified overall equations look complementary, but the processes involve different reactions and structures. Plants perform respiration as well as photosynthesis: they need usable energy even when light is unavailable. Matter cycles through these processes, while energy enters ecosystems largely as sunlight and eventually disperses as heat. Energy is transferred and transformed, not created by cells.
Further reading
These resources offer a fuller treatment of the underlying concepts. The lesson, quiz wording, and flashcards here were created for this project.
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