Animal Cell Structure
Cells have many organelles that perform various functions important to the cell's survival.
- The cell membrane is located around the outside of the cell. It is a protein lipid bilayer. The hydrophilic heads of the lipids point outwards while the hydrophobic tails occupy the space between the two lipid layers. Several types of proteins are imbedded in the membrane: channel, transport, recognition, receptor, and electron transfer. Channel proteins provide passageways through the membrane for small substances to diffuse through. Transport proteins are involved in the active transport of substances across the membrane. Recognition proteins recognize other cells. Receptor proteins are receptor sites for hormones and other chemicals. Electron transfer proteins are involved in the transfer of electrons in processes like photosynthesis and cellular respiration. Because the proteins constantly shift throughout the cell membrane, it is referred to as a fluid mosaic model. The functions of the cell membrane include: holding cellular material, regulating the movement of materials across the membrane, providing a surface for many chemical reactions, and identifying the cell to the body's immune system.
- Cell junctions connect one cell to another. Gap junctions are found in animals and are very, very small channels that allow various ions and other small substances to pass from one cell to another. Tight junctions are seals around cells to prevent leakage. They are important for containing liquids like stomach acids. Desmosomes are spot welds that hold cells together.
- The nucleus controls the cell's activities and contains all the genetic material (46 chromosomes in humans).
- The nucleolus is involved in the synthesis of ribosomal RNA. It is a dark body inside the nucleus.
- The nuclear membrane keeps DNA inside the nucleus but allows mRNA and proteins through. It is a double membrane with large pores.
- Ribosomes assemble proteins from RNA codes. They are found free-floating in the cytoplasm throughout the cell or attached to the endoplasmic reticulum.
- The smooth endoplasmic reticulum is a series of long canals running throughout the cell. It detoxifies the cell and converts foodstuffs.
- The rough endoplasmic reticulum is a series of long canals running throughout the cell with ribosomes attached. It transports proteins to the golgi bodies for packaging.
- Golgi bodies (also apparatus or complex) store and package cellular secretions for export out of the cell (usually through the use of vacuoles). Salivary, oil, and digestive glands have very active golgi bodies.
- Lysosomes digest and remove worn out cell organelles. In essence, they are vacuoles filled with digestive enzymes.
- Mitochondria produce most of the cell's energy. They are composed of two membranes (an outer and a folded inner membrane) and are common in muscle cells.
- Centrioles anchor spindle fibers during cell division. They are composed of microtubules and are only found in animal cells.
- The cell's cytoskeleton provides the cell with shape and support. It is involved in cell movement (cytoplasmic streaming, muscle contraction, ameboid movement, and cell division). The cytoskeleton is composed of actin filaments, intermediate filaments, and microtubules.
- Vacuoles are "bubbles" of material in the cell. Usually vacuoles hold water. They can, however, hold solutions and solid material as well.
- Some cells have microvilli to aid in movement or absorption.