Lysosomes, vacuoles and peroxisomes

What are lysosomes

Membrane bound sacs containing a range of hydrolytic enzymes (acid hydrolases) for intracellular digestion

How are lysosomes created

By the addition of hydrolytic enzymes to early endosomes from golgi apparatus

Function of lysosmes- digest

• Excess or worn out organelles (autophagy)

• Food particles

• Engulfed viruses or bacteria (phagocytosis)

Function of lysosomes

Fuse with and dispense enzymes into vacuoles to digest their contents

Why can lysosmes be called suicide bags

Due to their role is autolysis- can lead to cell digestion or cell death

What can lysosomes heal

They heal cell membrane by serving as a membrane patch

What does the membrane surrounding lysosmes allow

Allows for digestive enzymes to work at acidic pH- protects cytosol from degradative enzymes

What happens if enzymes leak into cytosol

Their potential for cell damage is reduced as not at optimum pH

pH of lysosmes interor and cytosol

interior is more acidic (pH 5)

cytosol- pH 7.2

What does the lysosomes single membrane do

Stabilises low pH- pumps in protons (H+) from cytosol via proton pumps and chloride ion channels)

What are lysosomes different enzymes

Group called acid hydrolases

• Nucleases

• Proteases

• Glycosidases

• Lipases

• Phosphatases

• Sulphatases

• Phospholipases

What are the different pathways leading to lysosomes

Endocytic and autophagic

What are peroxisomes

Ubiquitous membrane bound organelles in eukaryotic cells

Function of perioxisomes

Participate in the metabolism of fatty acids and other metabolites

• Beta oxidation

• Fatty acids broken down 2 C at a time, coverted to acetyl-CoA which is then transprted back to cytosol

• In animal cells, beta oxidation can also occur in mitochondria

• In yeast and plant cells, beta oxidation is exclusive for perioxisomes

Peroxisome functions 2:

Contain enzymes that rid cells of toxic peroxides

Part of the secretory pathway

• More dynamic than lysosmes and can replicate by enlarging and then dividing

Peroxisome enzymes

• Different peroxisomes contain oxidases with different substrate specificity but the all contain catalase

• remove H from specifc organic substrates (labelled R) using O2 in an oxidative reaction, producing toxic H2O2

What does catalase use to oxidize other substances

H2O2

Oxidises substartes including:

• Phenols

• Formic acid

• Formaldehyde

• Alcohol

Enzymes in peroxisomes

Enzymes that oxidase

• Fatty acids

• Alcohol

• Toxic compounds

• D-amino acids

  Will generate H2O2 (brok

Importance of catalase

Eliminates poisonous H2O2

Important in liver and kidney cells- peroxisomes detoxify toxc substances entering blood

Converts excess H2O2 to H20

25% of the etahnol we drink is oxidized to acetaldehyde in this way

Comparison of lysosomes and peroxisomes

• Lysosomes contain will contain a full range of hydolytic enzymes- if they differ in appearence its because of the material they are digesting

• Peroxisomes tend to specialise and contain enzymes specific for particular substrates

What is zellwegers syndrome

Children born with this have ‘empty’ peroxisomes- enzmes fail to be imported into the organelle

These die shortly after birth- proves that peroxisomes are essential for functioning of the eukaryotic cell

Other functions of peroxisomes

• Regulate 02 tension in the cell- use more 02 when the pressure rises, mitochondria cannot do this

• Rold 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 glyoysome- carry out the glyoxylate cycle

What are plasmalogens

unique class of membrane glycerophospholipids that contain:

• a fatty alcohol

• fatty acid

• a polar head group attached to a glycerol backbone by an ester linkage

Peroxisomes synthesise them

What does a deficiency of plasmologens cause

Profound abnormalities in the myelination of nerve cells- peroxismol disorders lead to neurological disease

What are vacuoles

Membrane-bound compartments that can serve a variety of secretory, excretory and storage functions

Animal cells have many smaller ones but not all have them

Plant cells have one large one- typically occupy more than 30% of the cell volume (can occupy as much as 90%)

What is a vacuoles membrane called

tonoplast

Functions of vacuoles

• 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 the cell

• Maintain an acidic internal pH

• Export unwanted substances from the cell

• Major role in autophagy-maintianing a balance between biogenesis (production) and degredation (or turnover) of many substance and cell strucutres

• 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’

Types of vacuoles- contractile

Used to pump excess h20 out of cell to reduce osmotic pressure to prevent bursting- cytolysis or osmotic lysis

Vacuoles in plants

• Large amounts of cell sap- water, enzymes, inorganic ions, salts, toxic by produtcs removed from the cytosol to avoid interfercne 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

What is the main role of the central vacuole in plants

To maintain turgor pressure against cell wall

• Proteins in tonoplast control H20 flow into and out of the vacuole through active transport

• k+ pumped into and out of the vacuolar interior

• H20 diffuses into the vacuole by osmosis

• Pressure on cell wall

Turgor pressure

It declines if H20 lost and cell plasmolyses

It is helpful for cellular elongation- cell wall is degraded by the action of auxins and less rigid wall is expanded by the pressure coming from within the vacuole

Vacuoles can help plant cells reach considerable size

What are vacuoles in animal cells 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

Types of endocytosis- pinocytosis

• Substances ingested are in solution i.e. NOT visible under microscope

What is diaminobenzidine

What is used to stain peroxisomes

It is polymerised by catalase

In osmium tetroxide-treated cells, electron dense osmium is then deposited, staining the peroxisomes