Although there are differences among eukaryotes, overall, eukaryotic cells share many characteristics, such as the following:
Eukaryotic Cell Envelope & External Structures
- Cell Wall: The cells of plants, algae and fungi have thick, protective cell walls, which provide support, help maintain the shape of the cell, and prevent the cell from taking in too much fresh water and bursting.
- Plasma Membrane: All cells, both prokaryotic and eukaryotic, have a plasma membrane, made mainly of phospholipids and proteins, which functions as a barrier, regulating the movement of materials between the inside and the outside of the cell.
- Cilia & Flagella: These extensions of the cell are covered with plasma membrane and supported internally with a structural system of microtubules—kind of like a bone covered in skin. Flagella, which are longer, and cilia, which are shorter, aid in cell movement. Cilia, which are able to beat together in a coordinated manner, can also help direct materials around the outside of the cell.
Eukaryotic Membrane-bound Organelles
The main distinction between prokaryotic and eukaryotic cells is the presence of membranous organelles, a feature that only eukaryotes have. Organelles separate function within the eukaryotic cell, like a bunch of tiny, specialized factories that work together to help the cell run.
- The Endomembrane System: Organelles that are membranous have an additional handy feature, a built-in internal transportation system. Because membranous organelles are enclosed by the same type of material as the plasma membrane is made of (phospholipids and proteins), cellular supplies can easily be shipped when a piece of one membrane-bound organelle breaks off, forming a vesicle that travels within the cell, and then fuses with a different membrane-bound organelle. Material can also enter (endocytosis) or exit (exocytosis) the cell via this method.
- Nucleus: The nucleus is typically the largest and most visible organelle in a eukaryotic cell. Bound by a double-layer nuclear membrane, the nucleus contains the cell’s genome—the main genetic instructions in the form of DNA (deoxyribonucleic acid).
- Endoplasmic Reticulum: Functioning mainly as a factory for making and shipping proteins and lipids, the ER is network of hollow tubes, extending off of the nuclear membrane. There are two types of ER, rough and smooth. Rough endoplasmic reticulum is covered with ribosomes, non-membrane-bound organelles which are the sites of protein synthesis within the cell. Smooth endoplasmic reticulum is not associated with ribosomes, and specializes in the synthesis and transport of lipids.
- Golgi Body: These pancake-like stacks of vesicles as another type of factory within the cell. The Golgi body modifies cellular molecules and coordinates the packaging and shipment of materials out of the cell. It is the only organelle that can generate lysosomes.
- Lysosomes: This specialist vesicle contains lysozymes, enzymes that can degrade organic materials. They function in cellular digestion and the recycling of materials within the cell.
- Peroxisomes: A type of specialist vesicle required by cells that use aerobic respiration (oxygen to extract energy from food) and made by the endoplasmic reticulum. Peroxisomes are armed with enzymes that break down dangerous oxygen free radicals.
Other Eukaryotic Cell Components and Organelles
- Mitochondria: These tiny powerhouses of the cell, are double-membrane bound organelles which extract energy from food to produce ATP (adesnosine-5’- triphosphate), a multi-purpose that carries energy for use within the cell.
- Cytoplasm: The inside of the cell, between the nucleus and plasma membrane, is filled with a gel-like fluid in which the organelles are suspended. Cytoplasm includes both the liquid (called cytosol) and the suspended organelles.
- Cytoskeleton: Composed of microtubules, intermediate filaments and microfilaments, this network of fibers provides an inner framework for the cell. The cytoskeleton supports the cells structure, anchors and helps transport organelles, and aids in cell division.
- Microtubule Organizing Center (MTOC): This eukaryotic structure is where microtubules are assembled and anchored. In animal cells the MTOC is called the centrosome, which consists of two centrioles. In plant cells, the nuclear envelope appears to function as the main MTOC.