Eukaryotic Cell Definitions: = Typically Found Only In Plant Cells = Typically Found In Animal Cells

• Golgi Apparatus: A series (stack) of flattened, membrane-bound sacs (saccules) involved in the storage, modification and secretion of proteins (glycoproteins) and lipids destined to leave the cell (extracellular) and for use within the cell (intracellular). The Golgi apparatus is abundant in secretory cells, such as cells of the pancreas.

• Golgi Vesicle: A membrane-bound body that forms by "budding" from the Golgi apparatus. It contains proteins (glycoproteins), such as digestive enzymes, and migrates to the cell (plasma) membrane. Golgi vesicles fuse with the cell membrane and discharge their contents into the exterior of the cell through a process called exocytosis. Some Golgi vesicles become lysosomes which are involved in intracellular digestion.

• Pinocytotic Vesicle: A membrane-bound vacuole formed by a specific type of endocytosis called pinocytosis. The plasma membrane invaginates (pinches inwardly) to form a vesicle that detaches and moves into the cytoplasm. Macromolecular droplets and particles up to 2 micrometers in diameter enter the cell within these pinocytotic vesicles. Larger particles (including bacteria) enter special white blood cells (phagocytes) through a form of endocytosis called phagocytosis. The Amoeba is a unicellular protist that ingests food (including algal cells) by phagocytosis.

• Lysosome: A membrane-bound organelle containing hydrolytic (digestive) enzymes. Lysosomes originate as membrane-bound vesicles (called Golgi vesicles) that bud from the Golgi apparatus. They are primarily involved with intracellular digestion. Lysosomes fuse with vesicles (small vacuoles) formed by endocytosis. The contents of these vesicles are digested by lysosomal enzymes. Autodigestion by lysosomes also occurs during embryonic development. The fingers of a human embryo are webbed initially, but are separated from each other by lysosomal enzymes. Cells in the tail of a tadpole are digested by lysosomal enzymes during the gradual transition into a frog.
• Peroxisome: A membrane-bound organelle that contains specific enzymes imported from the cytoplasm (cytosol). For example, certain peroxisomes contain the enzyme catalase which rapidly breaks down toxic hydrogen peroxide into water and oxygen. This reaction can be easily demonstrated by pouring some hydrogen peroxide on raw meat or an open wound.
• Glycolysis: An anaerobic oxidation pathway outside of the mitochondria in which glucose is oxidized to pyruvate with a net gain of 2 ATP molecules. Pyruvate is converted into a 2-carbon acetyl group which enters the Krebs cycle within the mitochondria.

• Mitochondrion: Membrane-bound organelle and the site of aerobic respiration and ATP production. Energy from the step-by-step oxidation of glucose (called the Krebs or citric acid cycle) is used to produce molecules of adenosine triphosphate (ATP). The Krebs cycle starts when a 2-carbon acetyl group combines with a 4-carbon group to form a 6-carbon citrate. Including glycolysis (which occurs outside the mitochondria), a total of 38 ATP molecules are generated from one molecule of glucose.

In eukaryotic cells, including the cells of your body, ATP is produced within special membrane-bound organelles called mitochondria. During this process, electrons are shuttled through an iron-containing cytochrome enzyme system along membranes of the cristae which result in the phosphorylation of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). ATP is the vital energy molecule of all living systems which is absolutely necessary for key biochemical reactions within the cells. The actual synthesis of ATP from the coupling of ADP (adenosine diphosphate) with phosphate (PO₄) is very complicated and involves a mechanism called chemiosmosis. The electron flow generates a higher concentration (charge) of positively-charged hydrogen (H+) ions (or protons) on one side of the membrane. When one side of the membrane is sufficiently "charged," these protons recross the membrane through special channels (pores) containing the enzyme ATP synthetase, as molecules of ATP are produced. In the membranes of prokaryotic bacterial cells, ATP is produced by a similar process. In fact, some biologists believe that mitochondria and chloroplasts within eukaryotic animal and plant cells may have originated from ancient symbiotic bacteria that were once captured by other cells in the distant geologic past. This fascinating idea is called the "Endosymbiont Theory" (or "Endosymbiont Hypothesis" for those who are more skeptical). Chloroplasts and mitochondria have outer phospholipid bilayer membranes and circular DNA molecules like those of prokaryotic bacterial cells. In addition, the layers of thylakoid membranes in the grana of chloroplasts are remarkably similar to photosynthetic cells of cyanobacteria. Acquiring cells and genomes from other organisms is known as symbiogenesis. According to L. Margulis and D. Sagan (Acquiring Genomes: A Theory of the Origins of Species 2002), symbiogenesis is a major factor in the evolution of life of earth. In fact, the author's state that long-term genomic mergers result in much greater evolutionary change than DNA mutations and natural selection.