The microtubule helps to reinforce cell shape, concentrated groups of microtubules near the plasma membrane aid in this process. The microtubule is found in both plant and animal cells.
Microfilaments
The microfilament is made up of two strands of G actin. They are universal within eukaryotic cells. The microfilament functions in the following manners; muscle contraction, cytoplasmic streaming, cell division, as well as the maintenance and changes in cell shape.
Intermediate filaments
Intermediate filaments are hollow tubes about half of the size of a microtubule. They are made up of multiple proteins; the type depends on the cell. Like the microtubule and microfilament the intermediate filaments maintain the shape of the cell.
Centrioles
An animal cell has a pair of centrioles within its microtubule organizing center, located near the nucleus. The centrioles, which have a diameter of about 150 nm and exist at 90 degree angles to each other, are made up of nine sets of three microtubules. The centriole plays an important role in the process of mitosis within an animal cell.
Surface of the Cell
Plasma membrane (Cell membrane)
The plasma membrane is the structure that surrounds a eukaryotic cell; both the plant and animal cell possess a plasma membrane (see figures A and B). The plasma membrane is semi-permeable, like a filter, it allows some products to enter and exit the cell via diffusion, while disallowing other objects.
The plasma membrane is a phospholipid bilayer, this means that the membrane is made up of both phosphates and lipids. The outer wall and inner wall of the cell membrane are phosphates as they are hydrophilic; where as the interior of the cell is made up of lipids that are hydrophobic. This creates a region inside the bilayer cell membrane that does not contain liquids.
Although the plasma membrane is semi-permeable, it does not allow for the transport of some molecules that are too large to fit through the membrane. This is why the membrane also contains proteins, which participate in the active transport of large molecules, also known as endocytosis and exocytosis.
Cell Wall
The only eukarytoic cells in which cell walls are present are the plant cells. The plant cell wall is much thicker than the plasma membrane, it is made up, mainly, of polysaccharide cellulose that is embedded in a matrix of other polysaccharides as well as some small amounts of protein. A plant cell, when young usually possesses a thin flexible cell wall called the primary cell wall, after the cell matures it usually strengthens this main wall by adding pectin and other hardened substances to it, or by building a secondary cell wall.
The cell wall is used by the plant to maintain a more rigid, flat surface than the animal cell, it is needed by the cell as the central vacuole would make it harder for the cell to remain intact without the presence of a cell wall.
Intracellular Junctions
As the name suggests, intracellular junctions, are the connections which hold cells together, cells can communicate through these junctions. There are three types of intracellular junctions that exist between animal cells; they are tight junctions, desmosomes, and gap junctions. The plant cells connect with plasmodesmata.
Plasmodesmata
The plant cells only connect in one way. This is by their connecting cell walls, which are separated by a thin layer, called the middle lamella. The cell walls are perforated with channels called plasmodesmata, these channels connect the cytoplasm of two adjacent cells, hence making the entire plant one working unit.
Tight Junctions
The tight junction between animal cells, most often epithelial cells, holds cells together so tightly that they prohibit the transport of substances through this intracellular space. Specialized proteins in the plasma membrane bond to similar protein in the other plasma membrane allowing for a tight bond.
Desmosomes
A desmosome occurs when intracellular filaments puncture the surface of both plasma membranes. These filaments anchor themselves in both cells and keep them together by forming a junction.
Gap Junctions
The gap junction allows for the transfer of material between one cell and the other, acting like a tunnel joining the cells, just like the plasmodesmata of plant cells. The gap junction occurs when a patch of proteins called a connexon connects with a similar patch of proteins on the membrane of the other animal cell. The intracellular pores created by these gap junctions allows for the transport of inorganic ions, sugars, amino acids, vitamins, and other small molecules between the cytoplasm of both cells.