Cell Ultrastructure Flashcards
What is the difference between Heterochromatin and Euchromatin?
Both are a type of chromatin which differ in a number of features, including its microscopic appearance. Heterochromatin is more condensed as it exists in the ‘30nm fibre’ conformation, which is a transcriptionally inactive state. It therefore appears darker on microscopy. Cells which are quite inactive and produce few proteins have abundant Heterochromatin.
Euchromatin is the DNA and protein aggregate which exists is the less condensed ‘beads on a string’ conformation. Because of this looser structure it is transcriptionally active as transcription factors can access it. Highly active cells, such as fibroblasts, have abundant euchromatin.
The two forms have a role in regulating gene expression.
What is chromatin?
Chromatin is the association of DNA with protein, namely his tone proteins, to form the condensed and folding conformation which allows the genome to be packages into the nucleus.
Describe the differences between eukaryotes and prokaryotes.
Eukaryotes are organisms like mammalian cells, fungi and Protozoa. They have a membrane-bound genome and membranous organelles such as endoplasmic reticulum, peroxisomes, Golgi and mitochondria. Their ribosomes consist of 40S and 60S subunits which translate mRNA which has undergone various post-transcriptional modifications.
Prokaryotes are organisms such as bacteria and have a genome which is free in the cytoplasm of the cell, consisting or a circular piece of DNA and satellite fragments called plasmids. They have no membrane-bound organelles. They have 30S and 50S ribosomal subunits which translate mRNA which has not undergone post-transcriptional processing. Such ribosomes are free in the cytoplasm.
The differences between the cell types make each a unique target for drugs such as antibiotics.
Describe the structure and properties of phospholipids and their role in cell structure.
Phospholipids are one form of lipid which make up about 40% of cell membranes, with the remaining 60% being protein. They consist of a glycerol molecule bound to two fatty acids and a phosphate group, itself bound to a small polar head group such as an amino acid or sugar. This conformation makes it amphipathic and so able to form a phospholipid bilayer. They function to provide a barrier for the regulation of movement into and out of a cell, compartmentalising the cell itself.
What is the structure and function of the glycocalyx?
The glycocalyx is a collection of oligosaccherides and polysaccharides bound to proteins on the surface of a cell membrane, for instance on the inner surface of lysosomes. Its presence allows protection of the membrane from the acidic pH within the lysosome. It also has a role in cell signalling???
How does the structure and function of the rough ER differ from that of the smooth ER?
Imaging using electron microscopy allows the ER to be observed in two different states. The rough ER appears as such due to the studding of ribosomes on its external surface, which are therefore involved in the synthesis of proteins which are either destined for the plasma membrane or for secretion. Within the RER the proteins can be processed and modified in a number of ways before passing onto the Golgi body. The smooth ER however is not associated with ribosomes in such a way and so appears smoother. It is involved in the synthesis of lipids and steroids. the ER is continuous with the nuclear membrane.
What is the structure and function of the Golgi body?
The Golgi body is a membrane-bound organelle present in eukaryotes which structurally consists of a cis-end, where vesicles are received from the ER, and a trans-end, from which secretory vesicles emerge. It functions to modify, concentrate and package proteins prior to secretion from the cell. Lysosomes are also formed from the Golgi.
What is the structure and function of the lysosome? How are they formed?
The lysosome is a membrane-bound organelle present in eukaryotes which are formed from the Golgi body. It contains a variety of enzymes, including proteases, nucelases, phosolipases and lipases. They are involved in the breakdown of a variety of substances, notably pathogens which appear in the organelle following phagocytosis and formation of the phagolysosome. They also degrade old organelles. The pH within the lysosome is acidic due to the function of the proton pump. The glycocalyx protects to membrane from the acidic pH within the lysosome.
Describe the structure and function of the peroxisome, including its formation as it differs from the lysosome.
The peroxisome is a membrane-bound organelle present in eukaryotes which is formed from the endoplasmic reticulum, unlike the lysosome which is formed in the Golgi body. Enzymes of the peroxisome facilitate its functions including reduction of reactive oxygen species, particularly hydrogen peroxide (catalase and superoxide dismutase) and beta-oxidation of long-chained fatty acids to medium-sized fatty acids which can be shuttled into the mitochondria for further oxidation.
Describe the structure of the mitochondria, including the location of important enzymes involved in the metabolic functions.
The mitochondria is an organelle present in eukaryotes which consists of a double membrane and an inter-membrane space. The mitochondria carries its own DNA within the matrix, since it is thought to be descended from prokaryotic cells. The inner membrane is highly folded into cristae in order to increase the surface area upon which metabolic reactions can occur. The matrix contains the genome, which follows the maternal lineage, and enzymes involved in the TCA cycle, production of acetyl choline and beta-oxidation of fatty acids. The inner membrane contains the complexes involved in the ETC, including the ATPase.
Describe the nuclear membrane and the regulation of movement of substances in and out of the nucleus.
The nucleus is a membrane-bound organelle present in eukaryotes which contains the genome of the cell. The nuclear membrane itself is a double-layered structure which is pierces by nuclear pore complexes, through which substances of specific sizes can move in and out as long as the exhibit the nuclear localisation sequences (NLS). The perinuclear space is the space between the inner and outer lipid bilayers and is continuous with the the lumen of the endoplasmic reticulum. The outer membrane therefore forms the membrane of the ER.
What is the nucleolus?
The nucleolus is a structure located within the nucleus of eukaryotes which functions to synthesise ribosomal subunits.
Describe the structure and function of ribosomes in eukaryotes and prokaryotes.
Ribosomes are present in both cell types, however in eukaryotes a consist of the 40S and 60S subunits and in prokaryotes are made up on 30S and 50S subunits. They maybe present in the cytoplasm or bound to ribosomes. They function in the translation of mRNA into protein.
Describe the structure and function of microfilaments
Microfilaments are part of the cytoskeleton of a cell and are made up of helices of actin monomers, which can associate with myosin proteins to allow for movement In an energy dependant way. This is particular relevant for actions such as muscle contraction and cytokinesis.
Describe the structure and function of intermediate filaments
IFs are components of the cellular cytoskeleton which are mostly cytoplasmic. They are involved in maintaining cellular structure and shape. Example include nuclear lamanin and keratin.
