There are two major types of cells, the basic units of organisms: prokaryotic cells (Bacteria・Archaea) and eukaryotic cells (animals・plants・fungi・and protists). Let us accurately understand their structural differences.
Basic Knowledge
Prokaryotic cells: Cells that do not have a nuclear membrane. The region where DNA exists loosely and freely in the cytoplasm is called the nucleoid. They do not have membrane-bound organelles such as mitochondria and chloroplasts. They are small, about 1〜10 μm. Examples: Escherichia coli・lactic acid bacteria・cyanobacteria・and Archaea.
Eukaryotic cells: Cells with a nucleus surrounded by a nuclear membrane. They contain various membrane-bound organelles. They are about 10〜100 μm. Examples: animals・plants・fungi・and protists.
Main organelles in eukaryotic cells:
・Nucleus (contains DNA and stores genetic information)
・Mitochondria (produce ATP through respiration)
・Chloroplasts (plants only. Photosynthesis)
・Endoplasmic reticulum (synthesis and transport of proteins・lipids)
・Golgi apparatus (processing and secretion of substances)
・Lysosomes (contain digestive enzymes)
・Vacuoles (highly developed in plants. Store substances)
・Cell membrane (phospholipid bilayer+proteins)
・Cell wall (plants = cellulose, fungi = chitin, bacteria = peptidoglycan)
Prokaryotic cell (No nuclear membrane・few organelles. Bacteria・Archaea)
Eukaryotic cell (Has a nuclear membrane・various organelles. Animals・plants・fungi)
Nucleus (Stores DNA. Includes the nuclear membrane and nucleolus)
Mitochondrion (Site of respiration・ATP production・has its own DNA)
Chloroplast (Site of photosynthesis・contains chlorophyll・has its own DNA)
Cell membrane (Phospholipid bilayer+membrane proteins. Fluid mosaic model)
Cell wall (Plants = cellulose, fungi = chitin, bacteria = peptidoglycan)
In Depth
The mitochondria and chloroplasts of eukaryotic cells have their own circular DNA and their own ribosomes, and are surrounded by double membranes. These are also characteristics of prokaryotic cells and support the endosymbiotic theory (Lynn Margulis), which proposes that about 2 billion years ago, prokaryotic cells were engulfed by other cells and formed symbiotic relationships. The ancestor of mitochondria is thought to have been an aerobic bacterium (an α-proteobacterium), while the ancestor of chloroplasts is thought to have been a cyanobacterium.
The phospholipid bilayer of the cell membrane has hydrophilic heads (phosphate groups) arranged outward and hydrophobic tails (hydrocarbon chains) arranged inward. This is an ultimately effective interface separating water from water-insoluble lipids, and is a highly rational chemical arrangement. Membrane proteins float in the membrane and carry out selective transport of substances and information transmission (the fluid mosaic model).
The main component of the cell wall differs among taxonomic groups: plants = cellulose (β-1,4-glucose polymer), fungi = chitin (N-acetylglucosamine polymer), and bacteria = peptidoglycan (sugar+peptide). These have also been extensively studied chemically as polysaccharide structures.
- Prokaryotic cells = no nuclear membrane, eukaryotic cells = nuclear membrane present
- Prokaryotes = Bacteria・Archaea
- Eukaryotic cells = nucleus・mitochondria・endoplasmic reticulum・Golgi apparatus, and so on
- Plant cells = chloroplasts+vacuoles+cell wall (cellulose)
- Animal cells = no chloroplasts・large vacuoles・or cell wall
- Endosymbiotic theory = origin of mitochondria and chloroplasts
- Cell membrane = phospholipid bilayer+proteins
Points to Note
① Prokaryotic cells do not lack a nucleus; they lack a nuclear membrane. They do contain DNA. ② Mitochondria occur in both animals and plants, but chloroplasts occur only in plants. Do not draw chloroplasts in diagrams of animal cells. ③ Vacuoles are highly developed in plants, whereas in animals they are small or nearly absent. ④ The composition of the cell wall differs among taxonomic groups (plants = cellulose, fungi = chitin, bacteria = peptidoglycan). Animal cells have no cell wall.
Practice
- What are cells without a nuclear membrane called?
- What theory explains the origins of mitochondria and chloroplasts?
- What is the main component of the plant cell wall?