Objectives: • Identify cell structures common to all eukaryotes. • Deduce cell features speci c to animals, plants and fungi. • Identify essential features necessary for the cell’s survival. • Apply concept relevant to pedagogical level. 1. CELL WALL • Made up of thick, rigid polysaccharide cellulose • Lignin is made up of a network of phenolic compounds that forms hard and waterproof llers in between components of the cell wall. They have openings known as plasmodesmata that act as bridges of cytoplasmic • material establishing continuity of adjacent cells. • Chitin, a modi ed form of cellulose is another cell wall component that provides mechanical support and protects the cell from swelling and bursting under hypotonic conditions. 2. CELL COAT • Also called the glycocalyx, this is found at the periphery of some cells and is composed of mucopolysaccharides, glycolipids, and glycoproteins. • It is important in cell adhesion and cell-to-cell recognition, since the highly speci c structures of the oligosaccharide chains allow one cell to recognize another. It is therefore responsible for tissue organization but prevents unwanted cell-to-cell interactions. • It also protects the cells from mechanical and chemical damage. 3. PLASMA MEMBRANE • This is the semi-permeable boundary surrounding the protoplasm to distinguish the cytoplasm from the extracellular environment. • It is a lipid bilayer interspersed with proteins—completely embedded in the bilayer or associated with a monolayer—in a mosaic pattern. The other component in some membranes is carbohydrate, linked covalently either to lipids (glycolipids) or to proteins (glycoproteins). • These various components move about making the membrane uid. The membrane is dynamically involved in the transport of small molecules either by passive or active means and large molecules through membrane ow. • It can also detect and transduce signals in the environment by means of membrane receptors. • In multicellular organisms, speci c proteins in the plasma membrane attach the cells to the extracellular matrix or mediate cell-cell adhesion in order to build tissues. fi fl fl fi fi 4. NUCLEUS • Contains the eukaryotic DNA which is enclosed by a nuclear envelope which is made up of two membranes—the inner and outer nuclear membranes—that are separated by a 10-15 nm-wide perinuclear space. • The nuclear envelope is essential in the regulation and transport of proteins of proteins in the cytosol. • The nucleolus, a sub organelle of the nucleus, can be seen as a prominently staining region of the nucleoplasm and it is NOT bound by a membrane. It is the site of ribosomal RNA transcription and the assembly of ribosomal subunits. fi fi EUKARYOTIC CELL STRUCTURE 5. RIBOSOMES • Made up of one small and one large subunit. The small subunit is where the codons of the mRNA are matched to the anticodons of the tRNA while the large subunit contains the peptidase enzyme that catalyzes peptide bond formation to link the amino acids during protein synthesis. • It has 40S and 60S subunits that join together in the cytoplasm to form 80S complexes. These appear as dense granules that may be free in the cytoplasm or bound to the endoplasmic reticulum or nuclear membranes. During protein synthesis, a ribosome initiates translation at the 5’ end of the mRNA and as soon as this ribosome moves out of the 5’ end, a new ribosome can associate itself to the same mRNA. Thus, several ribosomes may assemble in the same mRNA molecule forming large cytoplasmic clusters known as polyribosomes or polysomes. These enable the synthesis of more proteins at a given time. Ribosomes have also been located in the mitochondria and chloroplasts which synthesize some of the proteins needed by these organelles. Chloroplast ribosomes are 70S while mitochondrial ribosomes are a combination of 70S and 80S complexes. 6. ENDOPLASMIC RETICULUM • Forms a network of branching tubules and attened sacs forming the most extensive membrane system in the cell. This system encloses the ER lumen or the ER cisternal space and is continuous with the outer nuclear membrane. The stacks of attened cisternae have ribosomes on its membrane surface in which case it is called the rough ER (rER) but the network of tubules lacks ribosomes and so it is called smooth ER (sER). The rER functions in the synthesis, modi cation, and transport of secretory proteins, transmembrane proteins of the various organelles, and proteins of the extracellular matrix. The sER, on the other hand, is linked with the synthesis of lipids and hormones, storage of Ca2+ for cell signaling responses, and detoxi cation of drugs and certain harmful compounds like pesticides and carcinogens. Muscle cells have modi ed sER known as sarcoplasmic reticulum that releases and takes up Ca2+ which trigger myo bril contraction and relaxation, respectively. 7. GOLGI APPARATUS • The Golgi apparatus or complex is a pile of four to six platelike membrane-bound structures called cisternae and is surrounded by vesicles. • The major function of this organelle is the synthesis of carbohydrates many of which are attached as side chains to the proteins and lipids that came from the ER. • Some of these proteins are delivered to lysosomes while others are targeted into transport vesicles to be subsequently delivered to the plasma membrane, the cell surface or other organelles. fl fi fi fl fi fi 8. LYSOSOMES, VACUOLES, MICROBODIES • Lysosomes are single-membraned organelles containing about 40 types of hydrolytic enzymes that digest macromolecules (eg. proteases, nucleases, glycosidases, lipases, etc.) converting them into their corresponding monomeric subunits. They exhibit heterogeneous morphology which corresponds to their various functions such as the breakdown of intra- and extracellular debris, digestion of materials taken in by the cell via phagocytosis and synthesis of nutrients needed by the cell. • Vacuoles are similar to lysosomes and these are found in plant and fungal cells. These are usually very large occupying 30-90% of the cell volume. Plant vacuoles vary in terms of their size depending on the cell type and growth conditions, and in terms of content and function. Most cells of the vegetative tissues of the plant have a large central vacuole. Vacuoles also perform other functions such as to store water, nutrients, other water-soluble materials or waste products, to increase cell size or to regulate turgor pressure and pH. • Microbodies are a heterogeneous group of small, vesicle-like, single-membraned organelles containing enzymes for oxidation. They are involved in the breakdown of fatty acids, degradation of toxic substances, and photorespiration in plant leaves. 9. MITOCHONDRIA • Though classically depicted as elongated cylinders or ovoid bodies, mitochondria can change shape as they divide and fuse. These are associated with microtubules and so they can move about the cell. The mitochondrial membrane interacts with other intracellular membranes such as that of the ER. Two unit membranes are present, a smooth outer membrane which is freely permeable and an inner one which is a di usion barrier and has infoldings known as cristae. Between these two membranes is the intermembrane space. This organelle is the main site of ATP production via aerobic metabolism of glucose or fatty acids. The inner membrane contains respiratory chain complexes as well as the ATP synthase complex. The inner membrane encloses the matrix which contains DNA, RNA, ribosomes and enzymes of the citric acid cycle except for succinate dehydrogenase. MtDNA is utilized to analyze forensic samples such as old bones, teeth, and hair. It is useful for human identi cation because it does not undergo recombination, it has a high copy number, and it is maternally inherited. COX1, a mitochondrial gene, is also useful in molecular taxonomy and identi cation of plants and animals. 10. PLASTIDS • These are membrane-bound organelles unique to plant cells and green algae. The most important plastid is the chloroplast where photosynthesis occurs. It has the same structural organization as the mitochondria—highly permeable outer membrane, a signi cantly less permeable inner membrane, an intermembrane space and a uid matrix known as the stroma. Similar to the mitochondrial matrix, the stroma also contains DNA, RNA, ribosomes and soluble enzymes for a metabolic pathway called the Calvin cycle. Unlike the mitochondrion, the chloroplast has another membrane-bound compartment in the stroma called thylakoid which are attened disks, either piled together to form grana (granum, sing.), also called grana lamellae, or spanning two or more grana to form stroma lamellae. It is in the thylakoid membrane, not in the inner membrane, where the electrontransport chain and the ATP synthase complex are found along with the light absorbing pigments and other proteins making up the photosystems. The internal aqueous compartment enclosed by the thylakoid membrane is called the lumen. The chloroplast DNA (cpDNA) has protein-coding genes that synthesizes proteins for the translation machinery of the chloroplast as well as for photosynthesis. Similar to mtDNA, cpDNA is also employed in phylogenetic studies. Speci cally, the rbcL gene sequence is widely used to infer phylogenies of angiosperms at di erent taxonomic levels. • Plastids other than chloroplasts are also double-membraned and possess their own genome. These generally function for storage of metabolically important substances like starch grains (amyloplasts), oils (elaioplasts), proteins (proteinolasts), and colored pigments (chromoplasts). ff fl fi fi fi ff fl fi fi 11. CYTOSOL • The cytosol generally refers to the aqueous part outside the membrane-bound organelles and enclosed by the plasma membrane. It comprises about 55% of the cell volume. Almost all proteins in the cell are initially synthesized in the cytosol and then transported to speci c organelles. However, some proteins remain in the cytosol serving as enzymes in certain metabolic pathways. The cytosol is also the main site of protein degradation. 13. CENTRIOLES • These can be found only in animals, protozoans, and some fungi. Centrioles consist of two hollow cylinders lying at right angles to one another. The walls of the cylinder have nine sets of short, modi ed microtubules, each set being a triplet of bers. • The centrioles are embedded in the centrosome from which microtubules are nucleated. The centrioles are duplicated prior to cell division, facilitating the duplication of the centrosome, too. The two centrosomes move to opposite sides of the nucleus to form the two poles of the spindle ber. fi fl fi fl ff fi fi fi fi fi fl fl fi 14. CILIA AND FLAGELLA • Cilia and agella are found in certain eukaryotic cells. Both are composed of microtubules with a diameter of 0.5 μm, but while the cilia are 2-10 μm long, agella are about 100-200 μm in length. In contrast to the 9+0 arrangement of microtubules in the centrioles, both cilia and agella are characterized by 9+2 motif, i.e., nine sets of microtubules with each set made up of two bers and two additional tubules at the center. Some protozoans such as Paramecium and the cells lining the human respiratory tract or the oviduct depend on the whiplike beating of cilia to e ect movement. On the other hand, other protozoans and the sperm move in an undulating manner due to their agellum. fi fi 12. CYTOSKELETON • The spatial and mechanical properties of cells depend on a network of protein laments known as the cytoskeleton. These generally support the cell membranes and organize the contents of the cells. Three distinct classes of proteins make up the cytoskeleton: micro laments, intermediate laments and microtubules. • Micro laments are composed of the protein actin and so it is also called the actin lament. Depending on the cell type, these contribute to the strength and shape of the plasma membrane, cell locomotion, change in cell shape, muscle contraction, attachment of cells to a substrate, sensory response or increase in surface area. • Intermediate laments provide mechanical strength to either a cell part, the cell or even a tissue layer and also functions for cell-cell or cell-matrix adhesion to aid tissue formation. • Microtubules appear as hollow tubules which are made up of tubulin subunits. These are important for intracellular transport, chromosome segregation during cell division, cell locomotion, cell polarity and cellular organization.
