Kingdoms and domains of life

Notes of revisiting the topic:

1. Linnaeus originally placed all living things into either plant or animal kingdom, and nowaday we place living things (e.g. not virus) into six (Animalia 动物界, Plantae 植物界, Fungi 真菌界, Protista 原生生物界, Archaea/Archaeabacteria 古细菌界, and Bacteria/Eubacteria 细菌界in US's textbook) or five (Animalia, Plantae, Fungi, Protista and Monera in other countries) kingdoms.
2. Protista is any eukaryotic organism that is not an animal, plant, or fungi. 原生生物是除动物植物真菌之外的一切真核生物，包括海带、紫菜、藻类、变形虫等。所以海带不是植物，长知识了！
3. Archaea/Archaeabacteria 古细菌界 vs. Bacteria/Eubacteria 细菌界: Archaea (/ar-kee-ə/， ending sounds like IKEA) and Bacteria are both prokaryotes. Archaea is similar to Bacteria in structure (e.g. No membrane-bound organelles or nucleus, circular chromosome), but is closer to eukaryote in biochemistry (e.g. similar transcription and translation to Eukarya, e.g. TFs bound to the promoter, RNA pol II). Archaea was originally found in the extreme environment, e.g. extreme hot spring in Yellowstone, but now it's found everywhere in the world, e.g. in the gut of a human. 
4. Not all extremophiles are Archaeons. Thermus aquaticus, originally found in Yellowstone National Park, is not an Archaea, but a thermophilic Bacteria. The famous Taq enzyme for PCR is from the bacteria. T. aquaticus 最初是由Thomas Brock在1969年发现的，1976年DNA聚合酶polymerase 首次从T. aquaticus中分离出来，1988年Kary Mullis等发现并提纯了这个酶，指出其可用于PCR，5年后（1993）因此获得诺贝尔化学奖。
5. Six kingdoms are grouped into three domains (Bacteria, Archaea, and Eukarya), although both Bacteria and Archaea are Prokarya (原核，无核膜）. The main characteristics that scientists focus on when determining which kingdom an organism belongs to are summarized below:
      • Cell Type: Organisms are either prokaryotic or eukaryotic （有无核膜）.
      • Cell Walls: Some organisms have cell walls and others do not （有无细胞壁）.
      • Body Type: Organisms can be unicellular or multicellular （单细胞 or 多细胞）.
      • Nutrition: Many organisms are autotrophs and others are heterotrophs, respectively depending on whether they make their own food or receive nutrients by consuming other organisms （自养 or 它养.