ACT® · Science: Background Knowledge - Life Science
Cell Structure and Processes
A cell's surface-area-to-volume ratio falls as it grows, its organelles build and ship proteins in a fixed order, and water crosses its membrane by osmosis until both sides reach equal concentration.
A cell feeds itself and dumps waste through its surface, but it grows by volume. Surface area scales with the square of the radius, volume with the cube, so for a sphere the ratio reduces to 3/r. Double the radius and the ratio drops by half. That's why cells stay small or fold their membranes into villi and cristae: more surface packed into the same space keeps exchange fast enough to support the volume inside.
Making and shipping a protein is an assembly line. Ribosomes on the rough endoplasmic reticulum build the protein and feed it into the ER's interior, where it gets folded and sometimes carbohydrate tags attached. Vesicles bud off and carry it to the Golgi apparatus, which sorts, modifies, and packages it into new vesicles addressed for their destination. Those vesicles travel to the plasma membrane and fuse, releasing the protein outside. A pulse-labeling experiment that tracks a tagged protein over minutes shows exactly this order: rough ER first, Golgi next, vesicles and the outside last.
Osmosis is water diffusing across a membrane from the side with more free water (less dissolved solute) to the side with less free water (more solute), until concentrations equalize on both sides. The membrane itself is not a static wall with holes; it's a fluid double layer of lipids studded with proteins that act as channels, pumps, and receptors, constantly shifting and selectively controlling what crosses. That selective, dynamic structure is what makes both the shipping pathway and osmotic balance possible in the first place.
Key terms
- Surface-area-to-volume ratio
- The amount of membrane surface a cell has per unit of internal volume, which falls as a cell gets larger.
- Rough endoplasmic reticulum
- A ribosome-studded organelle membrane where proteins destined for secretion or membranes are built and folded.
- Golgi apparatus
- An organelle that sorts, modifies, and packages proteins arriving from the ER into vesicles for their final destination.
- Osmosis
- The diffusion of water across a selectively permeable membrane from a region of higher water concentration to lower.
- Fluid mosaic model
- The description of the cell membrane as a flexible double layer of lipids with proteins moving freely within it.
Watch it explained
- How it worksHow do the organelles of a cell divide the labor of making, packing and shipping one protein?
- How it worksWalk me through osmosis moving water across a membrane until the two sides come into balance.
- Worked exampleFind which way water flows when a cell sitting at 0.9 percent salt is dropped into distilled water.
- Common mistakeWhy isn't a cell membrane a solid wall with a few holes punched through it?
Check yourself
End of topic.