Cell Structure and Function PDF

Cell Structure and Function Overview  The cell is the unit of living system.  Structurally; it consists of an aggregate of molecules that enable the survival and growth of the whole organism these molecules are cell organelles.  Most organelles are membrane-enclosed structures. Their...

Cell Structure and Function Overview  The cell is the unit of living system.  Structurally; it consists of an aggregate of molecules that enable the survival and growth of the whole organism these molecules are cell organelles.  Most organelles are membrane-enclosed structures. Their membranes are composed of the same components as plasma membranes that form the outer boundaries of cells.  Organelles do not float freely within the cytosol but reside within the framework established by the cytoskeleton  Each organelle carries out a specific function. Cytosol + Cell organelles (liquid matrix found inside cells) Cytoplasm (Composed of all materials contained within plasma membrane boundaries) Eukaryotic Cell  Usually, multicellular and contain membrane-bound organelles such as the nucleus, mitochondria, and endoplasmic reticulum.  Cell size 10-100 um Cell organelles are:  Nucleus  Mitochondria  Cytoskeleton  Ribosomes  Lysosomes  Centrosome  Endoplasmic reticulum  Peroxisomes  Golgi apparatus  Vesicles 1- Nucleus (plural = nuclei)  All eukaryotic cells except mature erythrocytes (red blood cells) contain a nucleus.  It is surrounded by a nuclear envelope with nuclear pores to permit the transfer of materials between the nucleus and the cytosol.  The interior of the nucleus contains the nucleoplasm; the fluid in which the DNA (which carries our genetic information) is found.  DNA combines with specific proteins to form chromosomes.  Within the nucleus, there is a sub-organelle called the nucleolus. The nucleolus is the site of ribosome production.  Nucleus function  During mitosis, chromosomes undergo replication of their DNA and separation into daughter chromosomes.  DNA directs the protein biosynthesis inside the cell 2- Ribosomes  Each ribosome has a large and a small subunit.  They are composed of proteins and ribosomal RNA (rRNA).  Ribosomes are found within the cytosol either free or bound to the endoplasmic reticulum.  Function:  Ribosomes are the cellular machinery for protein synthesis. 3- Endoplasmic reticulum (ER)  The endoplasmic reticulum appears like a series of interconnected, flattened tubes surrounding the nucleus.  The ER appears to arise from the outer membrane of the nuclear envelope by out- folding, or from the plasma membrane by in-folding.  The endoplasmic reticulum is divided into: A. Rough endoplasmic reticulum (rER). Regions of ER ribosomes are bound to the outer membrane. Bound ribosomes and the associated ER are involved in the production and modification of proteins. B. Smooth endoplasmic reticulum (sER) Smooth endoplasmic reticulum refers to the regions of ER without attached ribosomes, its function is a site for lipid synthesis. 4- Golgi apparatus (Golgi complex)  It appears as flat, stacked, membranous sacs;  Three regions are described within the Golgi complex o Cis face: which is closest to ER o Cisternae (medial) o Trans face: which is near the plasma membrane  Proteins from the rough ER enter at its cis face (entry face), which is usually oriented toward the nucleus.  They are then transported through the Golgi and exit from its trans face (exit face).  As they pass through the Golgi, proteins are modified and sorted for transport to their destinations within the cell. (medial)  Function:  Receiving protein synthesized in ribosomes (pack it and sort it)  Performing modifications to it to make it well well-functioning protein o Phosphorylation (addition of phosphate) o Proteolysis (enzyme-mediated breakdown of protein) o Glycosylation (addition of carbohydrate)  Secretion of cell products (protein) to a cell outside through the formation of vacuoles called secretory vesicles in which secretory products are concentrated and transported to their destinations.  How proteins are transported within the cell. 5- Lysosomes and peroxisomes  Involved in digestion and detoxification  Lysosomes: are vesicles filled with potent enzymes that have an acidic internal pH, known as “acid hydrolases” that break down macromolecules.  Peroxisomes: Contain enzymes as “peroxidase” and “catalase” responsible for detoxification of oxidative metabolites that damage cells.  Lysosomes 1) Hydrolyze or break down macromolecules (proteins, nucleic acids, carbohydrates, and lipids) that have reached the end of their functional life. accumulation of non-function macromolecules is very toxic to cells. 2) Degrade materials that are taken up by the cell through endocytosis or phagocytosis as the accumulation of this material is very toxic to cells.  Peroxisomes 1) Breakdown of fatty acids and purines (AMP and GMP). 2) Detoxification of hydrogen peroxide (a toxic by-product of many metabolic reactions). 3) Detoxification of various toxic substances and metabolites through peroxidative reactions catalyzed by peroxidase. 4) Synthesis of myelin (the substance that forms a protective sheath around many neurons). 5) In liver cells, peroxisomes participate in cholesterol and bile acid synthesis. 6- Mitochondrion (plural = mitochondria)  The mitochondrial membrane is a double phospholipid bilayer membrane that forms the outer boundary of the organelle.  The inner mitochondrial membrane forms folded structures called cristae that protrude into the mitochondrial lumen known as the mitochondrial matrix.  Function:  Mitochondria is the power-house of the cell  Their unique membranes are used to generate ATP through respiration (greatly increasing the energy yield from the breakdown of carbohydrates and lipids).  All enzymes of the electron transport chain, energy liberation, and ATP formation are located in the inner membrane