Lysosome - Cell Biology - Lecture Slides, Slides of Cell Biology

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Lysosome

Lysosome „ Lysosomes are large, spherical^ organelles

that contain enzymes (acid^ hydrolases

„^ They break up food so it is easier to digest. „^ They are found in animal cells, while in

plant cells^ the same roles are performed by the

vacuole. „^ They digest excess or worn-out

organelles, food particles, and engulfed^ viruses

or^ bacteria. „^ The^ membrane^ around a lysosome allows the digestive^ enzymes^ to work at the 4.5 pH

they require. „^ Lysosomes fuse with^ vacuoles

and dispense their enzymes into the^ vacuoles

, digesting their contents.

Lysosome structure „ The size of lysosomes varies from 0.1–1.2^ μm

. „^ At^ pH^ 4.8, the interior of the lysosomes is acidic compared to theslightly alkaline^ cytosol^ (pH 7.2). „^ The lysosome maintains this pH differential by pumping

• protons (H ions) from the cytosol across the

membrane^ via^ proton pumps^

and chloride^ ion channels. „ The lysosomal membrane protects the cytosol, and therefore therest of the^ cell, from the^ degradative enzymes

within the lysosome. „^ The cell is additionally protected from any lysosomal acid^ hydrolases^ that leak into the cytosol as these enzymes are pH-sensitive and function less well in the alkaline environment of thecytosol.

Primary & secondary lysosomes

Residual bodies „ In lysosomal digestion,^ residual bodies

are vesicles containing indigestible materials. „ Residual bodies are either

secreted^ by the cell via exocytosis^ (this generally only occures in

macrophages), or they become^ lipofuscin granules

that remain in the cytosol indefinitely. „ Longer-living cells like neurons and muscle cells usuallyhave a higher concentration of lipofuscin than other morerapidly-proliferating cells.

The Functions of Lysosomes „ Lysosomes are the cells' garbage disposal system. „ They are used for the digestion of

macromolecules^ from phagocytosis^ (ingestion of other dying cells or largerextracellular material, like foreign invading microbes) „ endocytosis^ (where^ receptor proteins

are recycled from the cell surface) „ And^ autophagy^ (wherein old or unneeded organelles orproteins, or microbes that have invaded the cytoplasmare delivered to the lysosome). „ Autophagy may also lead to

autophagic cell death, a form of^ programmed self-destruction

, or^ autolysis, of the cell, which means that the cell is digesting itself.

Enzymes „ Some important enzymes found within lysosomesinclude: „ Lipase, which digests^ lipids „ Amylase, which digest^ carbohydrates^ (e.g., sugars) „ Proteases, which digest^ proteins „ Nucleases, which digest^ nucleic acids „ phosphoric acid monoesters. „ Lysosomal enzymes are synthesized in the cytosol andthe endoplasmic reticulum, where they receive a mannose-6-phosphate^ tag that targets them for thelysosome

Autophagy „ hydrolytic enzymes degrade the materials „ Intracellular materials, such as old organelles, arebrought into a lysosome by a process calledautophagy. „ For example, when a mitochondrion comes to theend of its ten-day life, it is engulfed by membranederived from the^ endoplasmic reticulum

„^ The newly enclosed mitochondrion then fuses witha lysosome, resulting in its degradation by the acidhydrolases.