An animal cell is a microscopic entity, typically ranging from a few micrometers to about 100 micrometers in diameter. It exhibits an intricate and highly organized internal structure. Observing an animal cell under a microscope reveals components, each with a distinct appearance and role. Specialized staining techniques are often employed to enhance the visibility of these otherwise colorless and translucent structures.
Overall Appearance and Basic Organization
When viewed under a microscope, an animal cell generally presents as a flexible, often irregular or spherical shape. This contrasts with plant cells, which possess a rigid cell wall, giving them a more fixed, rectangular appearance. The outermost boundary is the cell membrane, appearing as a thin, flexible layer that encloses the cell’s contents.
The interior of the cell is filled with cytoplasm, a jelly-like substance that holds all internal structures in place. Under a light microscope, it appears as a transparent, semi-liquid, or gel-like material. It can sometimes show a granular texture due to numerous small inclusions and organelles suspended within it.
Major Internal Structures
The most prominent feature is typically the nucleus, a large, often spherical or oval-shaped organelle usually located centrally. It appears as a distinct, darker, round structure within the cytoplasm. Inside the nucleus, a denser, darker spot called the nucleolus may be visible.
Scattered throughout the cytoplasm are mitochondria, generally seen as small, oval, or sausage-shaped structures. While visible under a light microscope, their internal details, like the characteristic folds of their inner membrane (cristae), become clear only with higher magnification electron microscopes.
The endoplasmic reticulum (ER) forms a network of interconnected membranes, sacs, and tubules throughout the cytoplasm. It exists in two forms: rough ER and smooth ER. Rough ER appears bumpy or granular due to tiny dots, called ribosomes, attached to its outer surface. Smooth ER lacks ribosomes, giving it a smoother, more tubular appearance.
The Golgi apparatus, also known as the Golgi body or Golgi complex, typically appears as a stack of flattened, curved, membrane-bound sacs called cisternae, resembling a stack of deflated balloons or pancakes. These stacks are often located near the nucleus. Lysosomes are generally observed as small, spherical, single-membrane-bound sacs.
Ribosomes are extremely small, appearing as tiny dots or granules. Due to their minute size, they are not typically visible with a standard light microscope, requiring an electron microscope. They can be found free in the cytoplasm or attached to the rough ER. Animal cells may also contain small, temporary vacuoles, which are membrane-bound compartments appearing as clear, empty-looking sacs. These are much smaller and less prominent than the large central vacuole found in plant cells.
A Living, Dynamic View
Observing an animal cell is witnessing a highly dynamic environment. The internal structures are constantly interacting and moving, contributing to its overall appearance and function. Cytoplasmic streaming, the active flow of cytoplasm, can sometimes be observed, where organelles and granules are transported throughout the cell.
Mitochondria are not fixed in position but can divide, fuse, and change shape, and are actively transported within the cell. Lysosomes also exhibit dynamic behavior, moving rapidly around the cell and sometimes deforming into tear-drop shapes or tubular forms. This continuous movement and interaction highlights that the cell’s “look” is defined not just by its fixed structures, but also by constant activity.