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# M1 Classification of Living Things The totality of all living things is divided into three groups, the domains. These are further subdivided. Cells without a cell nucleus are referred to as prokaryotes or prozytes. They are subdivided into eubacteria (also called bacteria) and archaea. All living...

# M1 Classification of Living Things The totality of all living things is divided into three groups, the domains. These are further subdivided. Cells without a cell nucleus are referred to as prokaryotes or prozytes. They are subdivided into eubacteria (also called bacteria) and archaea. All living things with a cell nucleus are eukaryotes. Examples are single-celled organisms like amoebae, ciliates, and flagellates. The cells of eukaryotes are called eucytes. # M2 Bacteria Bacteria are found everywhere on Earth – in soil, air, water, and other living things. Their size ranges from 0.3 to 700 µm. Different bacteria have diverse lifestyles: Some, called aerobic bacteria, require oxygen, while others, called anaerobic bacteria, do not. Most bacteria obtain their energy by breaking down complex organic molecules (heterotrophy). Only a few produce their own energy using light or chemical processes (autotrophy). Bacteria are enclosed by a cell membrane, which also holds specialized enzymes; this is similar to the mitochondrion in eukaryotes. An additional rigid cell wall provides structural support, mostly made of peptidoglycans. Some bacteria have a protective outer capsule or a loosely bound slime layer. Bacteria possess a circular chromosome, which comprises roughly one-third of the cell volume. Unlike eukaryotes, bacterial DNA is present as a single chromosome. Many bacteria also have smaller circular DNA molecules called plasmids, providing beneficial properties, such as antibiotic resistance. Bacteria reproduce asexually via cell division, duplicating their DNA first, then dividing. Inside the cytoplasm are ribosomes, where proteins are synthesized. Bacteria also have surface appendages called pili for attachment and flagella, which are helical protein structures for movement, allowing them to propel through their environment. Certain bacteria can form dormant, resistant structures called spores in response to unfavorable conditions (lack of nutrition). Spores can survive harsh conditions like lack water or nutrients for extended time periods. Most types of bacteria are harmless to humans, but some can cause severe diseases like cholera, diphtheria, and typhoid fever. # M3 Archaea Archaea were initially discovered in extreme environments like hot springs and highly saline or acidic waters, but they are also found in less extreme habitats such as soil and water. Most Archaea are anaerobic, meaning they do not require oxygen. Some are autotrophs or heterotrophs. Unlike bacteria and eukaryotes, archaea lack membrane-bound nuclei or organelles and have a cytoskeleton. Their cell walls are composed of different materials compared to that of bacteria (typically not peptidoglycans but other unique macromolecules)