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All living things are made of cells. Cells are microscopic; first discovered in 1665. Cells are surrounded by a membrane.
1) Prokaryotic cells - in bacteria. They have a simple structure (no nucleus or organelles). They are small (1 - 5 micrometers across). Usually have an external cell wall.
2) Eukaryotic cells - in plants, fungi, animals. The DNA is organized into chromosomes in the nucleus. They have specialized organelles and are larger (10 - 50 micrometers across). Prokaryotes and Eukaryotes
Microscopes
1) Light microscopes - focus light on the specimen. Advantages: living cells, color, portable, cheap. Disadvantage : magnification only up to 1000.
2) Electron microscopes - focus a beam of electrons on the specimen. Advantage : high magnification (up to 1 million ) Disadvantages : dead cells, black/white, not portable, expensive.
The Cell
Why are cells so small? They need to keep a large surface area compared to volume. Cells consist of : membrane, nucleus and cytoplasm.
Nucleus: roughly 5 micrometers across. It contains genes (DNA) arranged in chromosomes, and is surrounded by nuclear membrane which has pores. The nucleolus (inside the nucleus) produces ribosomes.
Cytoplasm : contains many organelles
Organelles
1). Ribosomes : are made of RNA and protein. Ribosomes use RNA to produce proteins.
2). ER (Endoplasmic reticulum)