Cell Structure and Function
All living things are made of cells, the smallest units of life. Cells fall into two broad categories.
Prokaryotic vs. eukaryotic
- •Prokaryotic cells (bacteria and archaea) have no membrane-bound nucleus; their DNA floats in the cytoplasm. They are small and lack most organelles.
- •Eukaryotic cells (plants, animals, fungi, protists) enclose their DNA in a nucleus and contain membrane-bound organelles.
Key organelles
- •Nucleus stores DNA and directs protein synthesis.
- •Mitochondria carry out cellular respiration, producing ATP. They have their own DNA and a double membrane.
- •Ribosomes build proteins; they are not membrane-bound.
- •Endoplasmic reticulum (ER): rough ER (studded with ribosomes) makes proteins; smooth ER makes lipids.
- •Golgi apparatus modifies, sorts, and ships proteins.
- •Chloroplasts (plants only) carry out photosynthesis.
Worked example
Why does a muscle cell, which needs lots of energy, contain many mitochondria? Because mitochondria generate ATP through respiration, cells with high energy demand (muscle, liver) have more of them. Structure matches function, a recurring theme in biology.
Key takeaways
- •Prokaryotes lack a nucleus; eukaryotes have one plus organelles.
- •Mitochondria make ATP; chloroplasts make sugar.
- •Organelle abundance reflects a cell's job.
