Biology: Cells to AP Biology · Module 2 of 15
Cell Structure & Function
Prokaryotic vs. eukaryotic cells, the organelles inside them, and how the membrane controls what crosses it.
Cell Theory & Prokaryotic vs. Eukaryotic Cells
- Cell theory: all living things are made of cells; the cell is the basic unit of life; all cells arise from pre-existing cells.
- Prokaryotic cells (bacteria, archaea) lack a nucleus and membrane-bound organelles; eukaryotic cells (plants, animals, fungi, protists) have both.
- Eukaryotic cells are generally larger and more complex, compartmentalizing functions into organelles.
A cell has no nucleus and no membrane-bound organelles. What type of cell is it?
Solution. A prokaryotic cell, such as a bacterium.
Organelles & Their Functions
- The nucleus stores DNA and controls the cell's activities.
- Mitochondria perform cellular respiration to produce ATP; chloroplasts (plants only) perform photosynthesis.
- The rough ER (with ribosomes) makes and processes proteins; the Golgi apparatus packages and ships them.
- Lysosomes contain digestive enzymes that break down waste and worn-out organelles.
A cell that secretes large amounts of protein (like a pancreas cell) would be expected to have an especially large amount of which organelle?
Solution. Rough endoplasmic reticulum, since its ribosomes synthesize the proteins that will be secreted.
Cell Membrane & Transport
- The cell membrane is a phospholipid bilayer with embedded proteins (the "fluid mosaic model") and is selectively permeable.
- Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their concentration gradient without using cell energy.
- Active transport moves substances against their concentration gradient and requires ATP.
- Osmosis is the diffusion of water across a membrane, from low solute concentration toward high solute concentration.
A red blood cell is placed in saltwater (higher solute concentration outside than inside). What happens to the cell?
Solution. Water leaves the cell by osmosis, moving toward the higher outside solute concentration, causing the cell to shrink.
Surface Area to Volume Ratio & Cell Size
- As a cell gets larger, its volume grows faster than its surface area, reducing the surface-area-to-volume ratio.
- A smaller ratio makes it harder for a cell to exchange nutrients and waste efficiently, limiting how large a cell can effectively grow.
- Cells often solve this by dividing (mitosis) rather than continuing to grow larger.
Why do most cells stay microscopic rather than growing as large as, say, a golf ball?
Solution. As a cell grows, volume increases faster than surface area, so a large cell would not have enough membrane surface to bring in nutrients and remove waste fast enough to support its volume.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the key structural difference between prokaryotic and eukaryotic cells?
2. Which organelle produces ATP through cellular respiration?