Studies into the function and appearance of cells reveal the presence of great diversity, which is particularly striking among eukaryotic cells. Even very different types of eukaryotic cells, however, exhibit many common structures and functions. This mix of uniformity and diversity is also reflected in the organisation within cells.
This course looks in more detail at the interior of cells – their subcellular structure, including their ‘ultrastructural’ features (sometimes called ‘fine structure’), which are visible using electron microscopy. Knowledge of the subcellular components of cells and how these components are arranged is fundamental to your understanding of how cells perform their functions: that is, how cells ‘work’.
Schematic diagrams of typical animal and plant cells are shown in Figure 1a and b. You do not need to study this figure in detail now; it will be referred to again during the course of this course.
The diagrams are three-dimensional representations of the cell’s internal structure. Near the centre in both cells is the nucleus, which contains a small nucleolus. The nucleus is surrounded by a double membrane, or envelope, with pores in its surface. Outside of and continuous with the nuclear envelope are the membranous sacs that make up the endoplasmic reticulum. The endoplasmic reticulum nearest the nucleus is studded with ribosomes and is described as rough; while further out from the nucleus is the smooth endoplasmic reticulum, which lacks associated ribosomes. Other membranous sacs present in both cells make up the Golgi apparatus. Also present in the cytosol are numerous free ribosomes, small spherical structures called peroxisomes and several mitochondria; the mitochondria have an outer membrane and also a highly convoluted inner membrane. Both the animal cell and the plant cell also contain cytoskeletal components – microfilaments, intermediate filaments and microtubules; and both cells are bounded by the cell membrane. Present in the animal cell’ but not shown in the plant cell, are lysosomes and also a pair of small cylindrical organelles collectively called the centrosome. The plant cell differs from the animal cell in having a cell wall outside the cell membrane, which is punctuated by connections called plasmodesmata – these are channels, lined by cell membrane, that pass through the cell walls of adjacent cells. Finally, the plant cell contains several chloroplasts, oval organelles containing stacks of green, chlorophyll-containing flattened sacs called grana; the grana are linked by membranes called grana lamellae.
Before beginning your study of cell components, some of the main techniques used to study the interior organisation of cells are briefly outlined.