Lysosomes, Peroxisomes and Vacuoles

What are lysosomes?

Membrane-bound sacs containing a range of hydrolytic enzymes for intracellular digestion

What is the membrane of lysosomes called and why is it useful

lumen - vital to ensure these enzymes do not leak out into the cytoplasm and damage the cell from within. Allows digestive enzymes to work at acidic pHs

How is the acidic pH of lysosomes maintained?

protons are actively transported into the organelle across the lysosomal membrane.

How are lysosomes created?

Addition of hydrolytic enzymes to early endosomes from Golgi apparatus

What are the functions of lysosomes?

1. Digestion: excess or worn out organelles (autophagy), food particles, engulfed viruses or bacteria (phagocytosis)

2. Fuse with dispense enzymes into vacuoles to digest their contents

3. Heal cell membrane by serving as a membrane patch

What are the pathways leading to lysosomes?

1. Bacterium engulfed via phagocytosis to produce a phagosome

2. Food molecules entering organelle engulfed via endocytosis to produce early endoscopes and late endoscopes which then form into lysosomes

3. Endoplasmic reticulum engulf organelle to produce an autophagosome via autophagy

What are peroxisomes?

Single membrane–bound vesicles in eukaryotic cells. They contain digestive enzymes for breaking down toxic materials in the cell and oxidative enzymes for metabolic activity.

How do you distinguish between lysosomes and peroxisomes?

Their interiors must be analysed

What are the functions of peroxisomes?

Rid cells of toxic peroxides.

Participate in the oxidation of fatty acids and other metabolites (beta-oxidation) a major source of metabolic energy.

Part of secretory pathway

Regulate O2 tension in the cell

Role in bile acids and bile protein production

Contain antioxidative enzymes in higher plants

What type of peroxisome is found in germinating seeds?

Glyoxysomes - carries out glyoxylate cycle

Which oxidase is found in all peroxisomes?

Catalase

What are the functions of catalase?

Uses H2O2 to oxidise substrates such as phenols, formic acid, formaldehyde and alcohol

Eliminates poisonous H2O2

Converts H2O2 to H2O

How do you separate lysosomes from peroxisomes?

1. Centrifuge sample homogenate

--fractions collected and assayed for enzyme activity

2. Centrifuge homogenate from original sample that has been treated with triton

--lysosomes accumulate triton so become a different density

What do peroxisomes synthesis & what are they?

Plasmalogens - lipids constituting 80-90% of the lipid in myelin sheath

What do plasmalogens contain?

A fatty acid

A fatty alcohol

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

What are vacuoles?

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

What is the vacuolar membrane called?

Tonoplast

What are the functions of the vacuole?

Remove unwanted structural debris

Isolate material that might be harmful

Contain waste products

Maintain internal HP or turgor within cell

Maintain acidic internal pH

Major role in autophagy (maintaining balance between biogenesis and degradation)

What are the 2 types of vacuole?

Food vacuoles (used by some protists and macrophages as a stage in phagocytosis)

Contractile vacuoles

(used to pump excess H2O out of cell to reduce osmotic pressure)

How is turgor pressure maintained in a cell?

1. Proteins in tonoplast control H2O flow into and out of the vacuole through active transport

2. K+ pumped into and out of the vacuolar interior

3. H2O diffusses into the vacuole by osmosis

--puts pressure on cell wall

What are the 2 types of endocytosis?

Phagocytosis - material contacts membrane which infolds; infolds are pinched off leaving engulfed material in membrane-enclosed vacuole and cell membrane intact

Pinocytosis - substances ingested are in solution (not visible under microscope)