lysosomes, vacuoles and peroxisomes Flashcards
lysosomes:
membrane bound sacs containing a range of hydrolytic enzymes (acid hydrolases) for intracellular digestion.
created by the addition of hydrolytic enzymes to early endosomes from golgi apparatus.
lysosome function:
digest: excess or worn out organelles (autophagy), food particles, engulfed viruses or bacteria (phagocytosis).
fuse with and dispense enzymes into vacuoles to digest their contents.
cell death and healing:
nicknamed suicide bags or suicide sacs due to role in autolysis. can lead to cell digestion or cell death.
heal cell membrane by serving as a membrane patch.
lysosomes and pH
membrane surrounding a lysosome allows digestive enzymes to work at acidic pH. protects cytosol from degradative enzymes.
if enzymes leak into cytosol, their potential for cell damage is reduced as not at optimum pH.
interior of lysosome is more acidic (pH 5) than the cytosol (pH 7.2).
single membrane stabilises low pH. pumps in proton (H+) from cytosol via proton pumps and chloride ion channels.
peroxisomes:
ubiquitous membrane bound organelles in eukaryotic cells.
peroxisome function:
participate in the metabolism of fatty acids and other metabolites. beta oxidation. fatty acids broken down at 2c at a time, converted to acetyl CoA, which is then transported back to cytosol. in animal cells, beta oxidation can also occur in mitochondria. in yeast and plant cells, beta oxidation is exclusive for peroxisomes.
contain enzymes that rid cells of toxic peroxides.
part of secretory pathway. more dynamic than lysosomes and can replicate by enlarging and then dividing.
peroxisome enzymes:
different peroxisomes contain oxidases with different substrate specificity but they all contain catalase.
remove H from specific organic substrates (labelled R) using O2 in an oxidative reaction, producing toxic h2o2.
catalase in turn uses h2o2 to oxidise other substances.
catalase and detoxification:
uses h2o2 to oxidise substrates including phenols, formic acid, formaldehyde, alcohol. eliminates poisonous h2o2.
important in liver and kidney cells. peroxisomes detoxify toxic substances entering blood.
about 25% of the ethanol we drink is oxidised to acetaldehyde in this way.
also, catalase converts excess h2o2 to h2o.
comparison of lysosomes and peroxisomes:
lysosomes will contain a full range of hydolytic enzymes. if they differ in appearance it is because of the material they are digesting.
peroxisomes tend to specialise and contain enzymes specific for particular substrates.
diaminobenzidine:
diaminobenzidine (DAB) is polymerised by catalase. in osmium tetroxide treated cells, electron dense osmium is then deposited, staining the peroxisomes.
zellwegers syndrome:
children born with zellwegers syndrome have empty peroxisomes. enzymes fail to be imported into the organelle.
these children die shortly after birth. proves that peroxisomes are essential for functioning of eukaryotic cells.
other functions of peroxisomes:
regulate o2 tension in the cell. uses more o2 when pressure rises. mitochondria cant do this.
role in bile acids and bile protein production.
contain antioxidative enzymes in higher plants. superoxide dismutase. components of the ascorbate glutathione cycle. NADP dehydrogenases of pentose phosphate pathway.
germinating seeds contain specialised form of peroxisome called a glyoxysome. carry out glyoxylate cycle.
peroxisomes and plasmalogens:
peroxisomes synthesise plasmalogens. lipids constituting 80-90% of the lipid in myelin (lipid rich membrane) sheath that insulates axons of nerve cells.
deficiency of plasmalogens causes profound abnormalities in the myelination of nerve cells. peroxisomal disorders lead to neurological diseases.
vacuoles:
membrane bound compartments that can serve a variety, excretory and storage functions.
membrane called the tonoplast.
animal cells have many smaller ones. not all animal cells have them.
plant cells have 1 large vacuole. typically occupy more than 30% of the cell volume (can occupy as much as 90%).
vacuole functions:
remove unwanted structural debris.
isolate materials that might be harmful or a threat to the cell.
contain waste products.
maintain internal hydrostatic pressure or turgor within cell.
maintain an acidic internal pH.
contain small molecules.
export unwanted substances from the cell.
major role in autophagy. maintaining a balance between biogenesis (production) and degradation (or turnover), of many substances and cell structures.
store food and other materials needed by a cell.
aid destruction of invading bacteria or misfolded proteins.
types of vacuoles:
food vacuoles- used by some protists and macrophages as a stage in phagocytosis. also called storage sacs
contractile vacuoles- used to pump excess h2o out of cell to reduce osmotic pressure to prevent bursting. cytolysis or osmotic lysis.
in plants:
large amounts of cell sap. water, enzymes, inorganic ions (K+ and Cl-), salts (Ca2+), toxic byproducts and removed from the cytosol to avoid interference with metabolism.
toxins may protect from predators.
transport of H+ from cytosol to vacuole keeps cytoplasmic pH stable, and vacuole acidic so degradative enzymes can act.
pushes cell contents against membrane to keep chloroplasts closer to light.
stores pigments in flowers and fruits.
turgor:
main role of the central vacuole is to maintain turgor pressure against cell wall. proteins in tonoplast control h2o flow into and out of the vacuole through active transport. K+ pumped into and out of the vacuolar interior. h2o diffuses into the vacuole by osmosis. pressure on cell wall.
turgor pressure declines if h2o lost and plasmolyses.
turgor pressure helpful for cellular elongation. cell wall is partially degraded by te action of auxins. less rigid wall is expanded by the pressure coming from within the vacuole.
vacuoles can help plant cells reach considerable sizes.
animals cells:
vacuoles involved in exocytosis and endocytosis.
vesicles are types of vacuoles.
types of endocytosis:
phagocytosis- material contacts membrane, which invaginates. invagination is pinched off, leaving engulfed material in membrane enclosed vacuole and cell membrane intact.
pinocytosis- substances ingested are in solution ie not visible under microscope
