Structure and Function of Life
Core ideas
All living things are made of cells, the smallest units that carry out life's functions. Cell theory states that organisms consist of one or more cells, the cell is the basic unit of life, and new cells come only from existing cells. Cells share a cell membrane that controls what enters and leaves, cytoplasm, ribosomes, and genetic material. Plant and animal cells both contain mitochondria, which release energy from food during cellular respiration. Plant cells also have a rigid cell wall for support and chloroplasts, where photosynthesis captures light energy to make sugar.
Structure matches function. Root hair cells are long and thin to absorb water, muscle cells contain many mitochondria, and nerve cells are elongated to carry signals quickly. Cells are organized into tissues, tissues into organs, organs into organ systems, and systems into a whole organism.
Materials move into and out of cells by diffusion, from high to low concentration, and water crosses membranes by osmosis. Photosynthesis and cellular respiration form a cycle: plants store energy in glucose, and all organisms, plants included, release that energy through respiration.
Traps to avoid
- Claiming plant cells lack mitochondria, or that only animals carry out respiration.
- Confusing the cell wall (rigid support) with the cell membrane (selective barrier).
- Reversing diffusion so it appears to move from low to high concentration.
- Mixing levels of organization, such as calling an organelle an organ.
Rule of thumb and worked example
To compare two cells, first list what both must have (membrane, ribosomes, mitochondria, DNA), then add what only one has. A leaf cell and a muscle cell both release energy in mitochondria, but only the leaf cell has a chloroplast and a cell wall.