Diversity and Unity of Cells

Section 2: Diversity and Unity of Cells

Through recall and discussion, you have initially understood the diversity and unity of cells. In middle school, cells are usually observed using the low-power lens of an optical microscope, and the materials for observation are limited. Now, let's try to use a high-power lens to observe more types of cells.

Prokaryotic and Eukaryotic Cells

Through microscopy, we understand the diversity of cells and also observe that cells have similar basic structures, such as cell membranes, cytoplasm, and nuclei, reflecting the unity of cells.

There is a class of cells without a well-defined nucleus, such as Escherichia coli and other bacterial cells. Scientists classify cells into two main categories based on whether they have a nucleus with a nuclear membrane: eukaryotic cells and prokaryotic cells. Organisms composed of eukaryotic cells are called eukaryotes, such as plants, animals, and fungi. Organisms composed of prokaryotic cells are called prokaryotes.

Prokaryotes are mainly various bacteria that are widely distributed. One type of bacteria is called cyanobacteria (formerly known as blue-green algae, see Figure 1-4). Have you seen them? Cyanobacteria cells are larger than those of other bacteria; most bacteria have diameters of 0.5 to 5.0 μm, while cyanobacteria cells have diameters around 10 μm, and some can even reach 70 μm, such as Oscillatoria. Generally, we cannot distinguish cyanobacteria with the naked eye, but you may have seen them when they exist in colonies. Nutrient-rich freshwater bodies contaminated by eutrophication lead to the proliferation of cyanobacteria and green algae, forming unpleasant blooms (see Figure 1-5), affecting water quality and aquatic life.

Cyanobacteria cells contain phycocyanin and chlorophyll and are autotrophic organisms capable of photosynthesis. Most types of bacteria are heterotrophic organisms that live saprophytically or parasitically. Bacterial cells all have cell walls, cell membranes, and cytoplasm, lack a nucleus enclosed by a nuclear membrane, and do not have chromosomes, but they have circular DNA molecules located in specific regions within the cell, called nucleoids (see Figure 1-6).

Thus, we have gained further understanding of the diversity of cells: diverse eukaryotic cells, diverse prokaryotic cells, and the differences between eukaryotic and prokaryotic cells.

Prokaryotic and eukaryotic cells share similar cell membranes and cytoplasm, both using DNA as genetic material, which again demonstrates the unity of prokaryotic and eukaryotic cells.