单元结构

Prokaryotic cells, found in bacteria and archaea, lack a membrane-bound nucleus and have densely packed DNA in a nucleoid region. Eukaryotic cells, found in animals, plants, and fungi, contain a true membrane-bound nucleus and compartmentalize internal structures into organelles. Eukaryotic cells are typically larger, ranging from 10-100 micrometers, compared to prokaryotes at 0.1-5.0 micrometers.

Eukaryotic cells first appeared approximately 2.4 billion years ago when an amoeba-like cell engulfed a bacterial cell, forming a stable coexistence. The engulfed bacteria evolved into mitochondria, the energy-producing organelles. Over time, this cell-within-cell arrangement led to the development of other specialized structures like chloroplasts, enabling the diversity of eukaryotic life.

Organelles are membrane-bound structures within eukaryotic cells that carry out specific functions. They allow eukaryotic cells to compartmentalize and specialize their internal processes. Key organelles include the nucleus, which contains DNA; mitochondria, which produce energy; the endoplasmic reticulum, which synthesizes proteins and lipids; and lysosomes, which digest cellular materials.

Plant cells contain chloroplasts for photosynthesis and possess a rigid cell wall made of cellulose for structural support and water retention. They also have larger vacuoles to store water and maintain cell pressure. Animal cells lack these structures, resulting in rounded, irregular shapes, while plant cells typically appear rectangular due to their rigid cell walls.

Cell structures are naturally transparent, making them invisible under a light microscope without staining. Biological dyes like safranin and methylene blue selectively bind to cellular components such as DNA and nuclei based on their molecular composition. Staining allows researchers to distinguish between different organelles and cell types, enabling detailed observation of cell structure.

Bright-field microscopy uses a halogen light source directed through a condenser lens to focus light on a specimen. The objective lens magnifies the image, and the ocular lens allows viewing. The stage holds the specimen, and coarse and fine focus knobs adjust clarity. This simple technique is ideal for viewing stained cells and is commonly used in undergraduate laboratory exercises.

Immersion oil has the same refractive index as glass, allowing light to pass through it as effectively as glass would. When placed between the coverslip and objective lens, it eliminates the glass-air interface that normally scatters light and reduces image clarity. This technique enables better visualization of specimens at high magnification by improving light transmission and reducing optical distortion.