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.
- Cristae: Inwardly-projecting, shelf-like membranes of the mitochondria where electrons flow along the cytochrome enzyme system.
- Chloroplast: Membrane-bound organelle and the site of photosynthesis and ATP production in autotrophic plant cells. Like mitochondria, chloroplasts contain their own circular DNA molecules. In fact, chloroplast DNA, including the protein-coding RBCL gene, is often used at the family level to show the relationships between genera and species within plant families. Intron regions from chloroplast DNA are also used to construct family trees. Introns are sections of messenger RNA that are removed prior to translation at the ribosome. Comparative DNA between different genera and species of a plant family can be shown with computer generated evoltionary trees called cladograms.
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.
- Grana: Region of chloroplast composed of stacks of thylakoid membranes. This is the site of the "light reactions" where ATP and NADPH2 are generated. These two products are utilized in the "dark reactions" where carbon dioxide is converted ("reduced") to glucose.
- Stroma: Region of the chloroplast where the "dark reactions" occur. Carbon dioxide (CO2) is gradually converted into glucose through a series of reactions called the Calvin cycle.
- Endoplasmic Reticulum: A complex system of membrane-bound channels extending throughout the cytoplasm of cells. Like the emergency room of a hospital, the endoplasmic reticulum is often abbreviated as ER.
- Smooth Endoplasmic Reticulum: Does not contain attached ribosomes.
- Rough Endoplasmic Reticulum: Studded (dotted) with attached ribosomes on the side of the membrane that faces the cytoplasm.
- Ribosome: Organelle site of protein synthesis. The ribosome is composed of large and small subunits separated by a central groove. A strand of messenger RNA (m-RNA) fits into the groove and the ribosome moves along the m-RNA in a 5' to 3' direction. Molecules of cloverleaf-shaped transfer-RNA (t-RNA), each with a unique amino acid, temporarily attach to the m-RNA at the ribosome in a process called translation. Transfer-RNA anticodons hook up with m-RNA codons and the amino acids bond together by dehydration synthesis. As the ribosome moves toward the 3' end of the m-RNA strand, the amino acid chain (polypeptide) grows longer and longer. Finally the completed polypeptide leaves the ribosome site and moves away to become a protein utilized within the cell or secreted from the cell. The simplified animated gif images below illustrate this remarkable process. A series of several ribosomes moving along the same m-RNA strand is called a polyribosome. Ribosomes are composed of ribosomal RNA and they are not membrane-bound. They occur in prokaryotic as well as eukaryotic cells. In eukaryotic cells, ribosomal RNA is synthesized in the nucleolus. The large and small subunits of ribosomes are synthesized by specific genes. One gene in the nucleolus codes for the smaller subunit of the ribosome. The gene is called SSU rDNA or small subunit ribosomal DNA. Base sequences from this gene are sometimes used to compare taxa at the species level. The results from comparative DNA studies using mitochondrial and chloroplast DNA are illustrated in computer generated evoltionary trees called cladograms.
Ricin from the castor bean (Ricinus communis) is a potent cytotoxic protein that is lethal to eukaryotic cells by inactivating the organelle sites of protein synthesis called ribosomes. Just one single ricin molecule that enters the cytosol of a cell (the semifluid medium between the nucleus and plasma membrane) can inactivate over 1,500 ribosomes per minute and kill the cell. One of the two protein subunits of ricin (RTA) is a deadly enzyme that removes purines (such as adenine) from ribosomal RNA, thus altering its molecular structure and function.
- Nucleolus: Dark-staining body within the nucleus where ribosomal RNA is synthesized. Plant nuclei in onion root tip cells may have several nucleoli.
- Nucleus: Membrane-bound organelle containing chromatin, a term applied to all the chromosomes collectively when they are in a tenuous (threadlike) stage. During the prophase of mitosis, the chromosomes become shorter and thicker, and appear as distinct doubled bodies called "chromosome doublets."
- Cell (Plasma) Membrane: The living membrane that surrounds the cytoplasm of all cells. It is composed of a phospholipid bilayer with embedded glycoproteins. In the "sandwich model" the two phospholipid layers are sandwiched between two layers of protein. The membranes of organelles are also composed of a phospholipid bilayer, including vacuoles, nuclei, mitochondria and chloroplasts. [Riubosomes are not membrane-bound.] Embedded glycoproteins in plasma membranes include membrane transport "carrier molecules" and cell recognition antigens. The plasma membrane is permeable to water molecules by osmosis, but not to other molecules and ions by simple diffusion. Ions pass through the plasma membrane via carrier molecules by active transport and facilitated diffusion. Active transport requires ATP.
- Cell Wall: A cellulose layer that surrounds the plasma membrane of plant cells. Because it is very porous, the cell wall is permeable to molecules and ions that cannot pass through the plasma membrane by simple diffusion. During plasmolysis, the cell membrane loses water and its contents shrink up into a ball, while the outer cell wall remains intact. Shrubs and trees have a thickened secondary cell wall containing lignin, a brown phenolic polymer that imparts great strength and hardness to wood. Ironwoods such as lignum vitae sink in water because of the density of their heavy, thick-walled, lignified cells.
- Vacuole: A membrane-bound, fluid-filled sac inside plant and animal cells. Contractile vacuoles of protists, such as the Paramecium, are specialized organelles for expelling excess water. Food vacuoles of the Amoeba digest smaller cells captured by phagocytosis. Plant cells have large central vacuoles that occupy much of the cell volume.
- Large Central Vacuole: A membrane-bound, fluid-filled sac that occupies much of the volume of a plant cell. For this reason, the chloroplasts, nucleus and other organelles are displaced to the periphery of the cytoplasm (around the central vacuole). In addition to water, this large vacuole stores salts, water soluble pigments (anthocyanins), and potentially toxic molecules in the form of crystals. In the crystalline state, oxalates are relatively innocuous to the plant cell. Crystals of calcium oxalate may be needle-like (raphide crystals) or many faceted like a glistening diamond (druse crystals). Plant cells with high levels of calcium oxalate can be toxic to humans. The primary reason that wolffia (world's smallest flowering plant) is more palatable to humans as a high protein food source is that its vacuoles lack raphide crystals that are abundant in other duckweeds (Lemna & Spirodela). Comparative chloroplast DNA studies have shown that the duckweed family (Lemnaceae) is closely related to the arum family (Araceae). In fact, members of both families have cells containing abundant raphide crystals of calcium oxalate. Chewing on leaves of the cultivated arum called "dumb cane" (Dieffenbachia) can cause difficulty in talking and swallowing. Symptoms of ingestion include burning pain, inflammation and swelling of the tongue, throat and larynx tissues. A proteolytic enzyme in the leaves called dumbcaine is injected into the cells via microscopic punctures by thousands of needlelike raphide crystals. Mast cells (basophils), special white blood cells in connective tissue, may also be injured. In allergic reactions, sensitized mast cells release stinging histamines into the afflicted tissues.
- Amyloplast (Starch Grain): A membrane-bound organelle containing concentric layers of starch (amylopectin). This organelle is commonly found in subterranean storage organs, such as tubers (potatoes), corms (taro & dasheen), and storage roots (sweet potatoes). Amyloplasts are also found in bananas and other fruits.
- Centrioles Nonmembrane-bound organelles that occur in pairs just outside the nucleus of animal cells. Each centriole is composed of a cylinder or ring of 9 sets of microtubule triplets with none in the middle (9 + 0 pattern). During cell division a pair of centrioles moves to each end of the cell, forming the poles of the mitotic spindle. Centrioles also give rise to basal bodies that control the origin of cilia and flagella in motile cells of protists. In cross section, flagella and cilia have 9 sets of microtubule doublets surrounding a pair of single microtubules in the center (9 + 2 pattern). This characteristic pattern also occurs in motile cells of higher organisms, such as human sperm.
- Centrosome: The microtubule organizing center that forms the mitotic spindle in dividing cells. In animal cells the centrosome includes a pair of centrioles surrounded by radiating strands of microtubules called the aster.
- Microtubules: Protein filaments composed of a polymer called tubulin. The centrosome of animal cells (including a pair of centrioles and radiating aster) are composed of microtubules. Microtubules are involved in cell movement, cell shape and the formation of mitotic spindles during cell division (mitosis). Some cancer chemotherapy drugs cause the dissolution (depolymerization) of tubulin in microtubules, thus destroying mitotic spindles and effectively stopping cell division in tumor cells.
- Cytoplasm: All the contents of a cell within the plasma membrane. The nucleus and its contents (nucleoplasm) are generally excluded from the cytoplasm. The semifluid medium between the nucleus and the plasma membrane is called cytosol.
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