Cytology Flashcards
Organelles and particles (general)
Membranes form barrier between cell and external environment (plasma membrane).
They subdivide the cell into compartments.
These compartments are organelles.
Membranes:
What is there environment/function/
What are they composed of?
What is there size and what do you stain with?
- Membranes allow each type of organelle to maintain novel ionic and enzymatic interior environments.
- Membranes are composed of lipids, proteins, and carbohydrates.
- Membranes are ~7nm and can be seen through an electron microscope when stained with osmium tetroxide.
Membrane Lipids (phospholipids, cholesterol, and glycolipids)
- amphipathic
- uncharged hydrophobic tails
- polar hydrophilic heads.
- hydrophobic center acts as an anchor to membrane proteins that can move freely within the lipid bilayer.
Fluid Mosaic Model
- membrane is highly permeable for small, uncharged molecules that cross the membrane by simple diffusion.
- all other molecules require membrane transport proteins to provide them with passage across the membrane.
Structural membrane proteins: integral and peripheral
- Integral membrane proteins have a hydrophobic region which is embedded into the hydrophobic core of the membrane.
- Transmembrane proteins are integral membrane proteins that extend all the way through the membrane.
- Peripheral membrane proteins are not embedded within the lipid bilayer and attach to integral proteins or to the hydrophilic heads of the membrane lipids.
Transport proteins: types and functions
- Channel proteins: form “pores,” which allow passage of small molecules across the membrane via passive diffusion. Channels are ion-selective.
- Carrier proteins “drag” other molecules through the membrane by hiding them in a cleft in the protein.
- Pumps use ATP energy to pump ions actively through the membrane.
Receptor proteins
Some transmembrane proteins bind to specific molecules in the extracellular matrix. This specific binding can result in a conformational change in the transmembrane protein that can serve as a signal that allows the cell to adapt to its environment.
Structural or anchoring proteins
Attach cell to its surroundings. These transmembrane proteins use their cytoplasm domains to link to the elements of the cytoskeleton, and their extracellular domains to link to the extracellular proteins.
Carbohydrates on membrane
Mostly oligosaccharides are attached to the extracellular domains of membrane proteins and lipids to form glycoproteins and glycolipids.
-Large amount of carbohydrates attached to proteins and lipids on the external surface give cell a “fuzzy” coating called the glycocalyx.
Particles and Inclusions
Particles and inclusions are used for synthesis and storage. The two main types are ribosomes and glycogen particles.
Ribosomes: Function? Size? 3 types in cell? Polysomes?
Ribosomes are RNA/protein particles that catalyze protein synthesis. They bring together mRNA and tRNA to synthesize a polypeptide.
- 15-25nm, small subunit and large subunit, each subunit consists of one or more ribosomal RNA (rRNA) and many specialized proteins
- Free ribosomes, mitochondrial ribosomes, Bound to RER. Most cellular proteins are synthesized on free ribosomes. Golgi, lysosomes, secretory granules, and plasma membrane are synthesized by RER ribosomes. 20% of mitochrondial proteins are synthesized by mitochondrial ribosomes, the rest are from free ribosomes.
- Polysomes are a string of ribosomes connected to mRNA during protein synthesis.
Glycogen particles
Storage form of polysaccharides
Smooth Endoplasmic reticulum
- does not have ribosomes and has more tubular appearance that the rough ER.
- Lipid and steroid metabolism. Phospholipids of the cell’s membrane are produced in SER, so SER is involved in membrane synthesis and recycling. SER is prominent in steroid-secreting cells.
- Detoxification of noxious substances. Well developed in liver cells and contains enzymes that can modify and detoxify such toxins as carcinogens and pesticides.
- Involved in glycogen metabolism
Sarcoplasmic Reticulum
- speacialized form of SER found in muscle tissue.
- involved in storage and transport of Ca++, which regulates the contractions of muscle cells.
Rough Endoplasmic Reticulum
- has ribosomes bound to its surface.
- formed mostly of cisternae.
- prominent in cells specialized for protein secretion, so it is the principal site of synthesis of proteins destined for export out of the cell.
- Proteins from the Golgi, lysosomes, secretory granules, and the plasma membrane are synthesized in the RER.
Golgi Apparatus (series of stacked, flattened, membrane-limited cisternae that is polarized, and tubular extensions)
- receives proteins synthesized in the RER.
- side receiving vesicles from the RER is cis Golgi.
- medial Golgi, forms the middle cisternae
- mature proteins are transported from the trans Golgi
Posttranslational modification of Golgi
- add or remove sugar residues, sulfate or phosphate groups.
- early stages in cis Golgi, intermediate steps in medial Golgi, and final modifications are made in the trans Golgi.
Sorting and packaging of proteins into transport vesicles by Golgi.
What are the three destinations?
- occurs mainly in the trans Golgi.
- Most proteins coming from the Golgi apparatus bear specific signal sequences, which direct them to their destination.
1. Secretory vesicles
2. Lysosomes
3. Constitutive pathway
Secretory vesicles
- maturation process in which secretory proteins are retained within the vesicle. Mature secretory vesicles eventually fuse with the plasma membrane to release the secretory product into the extracellular space in response to a certain signal.
- example: pancreatic acinar cells
Lysosomes
- produced by the Golgi
- involved in digesting the material taken up from outside the cell and degradation of senescent organelles.
- low pH and hydrolytic enzymes.
- -mannose-6-phosphate is the principal sorting signal.
I-cell disease (mucolipidosis)
- type of lysosomal storage diseases
- Mutation of one of the enzymes of mannose-6-phosphate and lysosomal proteins are secreted into the intercellular space.
3 classes of Lysosomes
Primary lysosomes- have not yet received substrates for digestion.
Secondary lysosomes-fusion of primary lysosomes with their target.
Lipofuscin granules (residual bodies)- senescent lysosomes with indigestible material
Constitutive pathway
Proteins in the trans Golgi, which are not destined for either lysosomes or secretory granules are sorted into small vesicles, which are transported directly to the plasma membrane, where the vesicle fuses with the plasma membrane.
-Integral membrane proteins and proteins that are secreted continuously into the extracellular space (collagen) reach the plasma membrane by this route.
Peroxisomes
Size and function?
- Small (0.5 microns), membrane-bound organelles containing oxidative enzymes, particularly catalase.
- oxidize long chain fatty acids and converse ethanol to acetaldehyde.
- oxidative enzymes produce hydrogen peroxide as a product of oxidation reaction and catalase is important in degrading H2O2.