Lecture 3: 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 protons (H+)
from cytosol via proton
pumps and chloride ion
channels
Peroxisomes
> Ubiquitous membrane-bound organelles in eukaryotic cells
Peroxisome contain a crystal of urate oxidase
Peroxisome function
> Participate in the metabolism of fatty acids and other metabolites
- Beta-oxidation
- Fatty acids broken down 2
C 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 I
> Different peroxisomes contain oxidases with different substrate specificity but they all contain catalase
Remove H from specific organic substances (labelled R) using O2 in a oxidative reaction, producing toxic H2O2
Catalase in turn uses H2O2 to oxidise other substances
Peroxisome enzymes II
Enzymes that oxidise:
> Fatty acids
> alcohol
> Toxic compounds
> D-amino acids
> Generate H2O2 (broken down by catalase)
Catalyse and detoxification
> Uses H2O2 to oxidize 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 oxidized to acetaldehyde in this way
Also, catalase converts excess H2O2 to H2O
Comparison of lysosomes and peroxisomes
> Lysosomes will contain a full range of hydrolytic 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
Separation of lysosomes from peroxisomes
–> Normal rat OR Triton treated rat
–> Equilibrium density centrifugation of liver homogenate
–> Fractions collected and assayed for enzyme activity
Separation of lysosomes from peroxisomes: Triton rat Vs normal rat
Densities from normal rat:
> Mitochondria (cytochrome oxidase)
> lysosomes (Acidic phosphatase)
>Peroxisomes (Urate Oxidase)
Densities from Triton rat:
> Lysosomes (Acidic phosphatase)
> Mitochondria (cytochrome oxidase)
> Peroxisomes (Urate Oxidase)
Lysosomes accumulate triton and become different densities
Zellweger’s Syndrome
> Children born with Zellweger’s 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 the eukaryotic cell
Other functions of peroxisomes
> Regulate O2 tension in the cell
- Use more O2 when the
pressure rises
- Mitochondria cannot 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 the 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 disease
A Plasmalogen
Contains:
> A fatty alcohol
> A fatty acid, and
> A polar head group
> Attached to glycerol backbone
> By an ester linkage
Vacuoles
> Membrane-bound compartments that can serve a variety of secretory, excretory, and storage functions
Membrane called the tonoplast
Animal cells have many smaller ones
- Not all animal cells have
them
Plant cells have one 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
»_space; Cytolysis or osmotic
lysis
In plants
> Large amounts of cell sap
- Water, enzymes, inorganic
ions (K+ and Cl-), salts (e.g.
Ca2+), toxic byproducts
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 I
> Main role of the central vacuole is to maintain turgor pressure against cell wall
-Proteins in tonoplast
control H2O flow into and
out of vacuole through
active transport
- K+ pumped into and out of
the vacuolar interior
- H2O diffuses into the
vacuole by osmosis
- Pressure on the cell wall
Turgor II
> Turgor pressure declines if H2O lost and cell plasmolyses
Turgor pressure helpful for cellular elongation
- Cell wall is particularly
degraded by the action of
auxins
- Less rigid wall is expanded
by the pressure coming off
from within the vacuole
Vacuoles can help plants reach a considerable size
Animal cells
> Vacuoles involved in exocytosis and endocytosis
Vesicles are types of vacuoles
