Create account

Cytology · Year 1 · Medical University of Sofia

23

Lysosomes and endosomes. Peroxisomes

Free notes for topic 23 of the Cytology syllabus, open without an account. Written by a senior student against the syllabus question and checked line by line by a second student before publishing. How content is made

Updated

Summary

note

Lysosomes are the organelles of intracellular digestion, 0.05 to 0.5 μm, spherical membrane-limited vesicles containing more than 40 different hydrolytic enzymes. Their matrix is at pH 5, which is what those enzymes need. They are synthesised in the rER and modified and packaged in the Golgi.

A primary lysosome holds inactive enzymes; it fuses with a phagosome to become a secondary lysosome, which is larger and includes the substance to be degraded. Autophagy is self-eating: the controlled digestion of damaged organelles.

Endosomes are intracellular sorting organelles important in endocytosis. Absorbed particles are either passed on to lysosomes or returned to the plasma membrane as recycling endosomes.

Peroxisomes are the sites for oxygen utilisation. They carry oxidase, which produces hydrogen peroxide, and catalase, which destroys it.

1. Lysosomes

note

Lysosomes are organelles of intracellular digestion, measuring 0.05 to 0.5 μm.

  • They are spherical membrane-limited vesicles that contain more than 40 different hydrolytic enzymes.
  • They are abundant in cells with great phagocytic activity, for example macrophages.
  • The lysosomal matrix has a pH of 5, which is favourable for enzymatic activity.
  • Lysosomes are synthesised in the rER and get modified and packaged in vesicles in the Golgi apparatus to form lysosomes.

The acid pH is a safety mechanism as well as a working condition: enzymes that need pH 5 do very little damage if they leak into a cytoplasm held near neutral.

Primary and secondary lysosomes

note

Primary lysosomes contain inactive enzymes. They fuse with a phagosome to become a secondary lysosome.

Secondary lysosomes are larger and include the substance to be degraded. That material arrived by endocytosis.

Autophagy

note

Autophagy is self-eating.

  • It is the process of controlled digestion of damaged organelles, in which the cell uses lysosomes to dispose of excess or non-functioning organelles.
  • Autophagosomes are produced, which fuse with lysosomes for digestion of the enclosed material.
  • Amino acids, nucleotides and so on are released for reuse, as sources of energy or in new synthetic reactions.

Yoshinori Ohsumi received the Nobel Prize for discovering autophagy, in relation to intermittent fasting.

2. Endosomes

note

Endosomes are intracellular sorting organelles that are important in endocytosis.

The process:

  • A particle strikes the membrane from the extracellular side.
  • The membrane inverts the particle inside, until it is completely enclosed.
  • This results in a vesicle that fuses with the endosome.
  • Finally the absorbed particles are either transported further to lysosomes, or returned as recycling endosomes back to the plasma membrane.

The word sorting is the important one. An endosome is a decision point: keep and digest, or send back.

3. Peroxisomes

note

Peroxisomes are the sites for oxygen utilisation, measuring 0.5 to 3 μm.

They are spherical organelles enclosed by a single layer membrane.

They contain two enzymes: oxidase and catalase.

The oxidase and catalase pair

note

This is the part worth understanding rather than memorising, because the two enzymes exist to undo each other.

A peroxisome is a compartment for oxidation reactions. Oxidases oxidise substrates by removing hydrogen atoms, in a reaction that produces hydrogen peroxide, H₂O₂, which is toxic for the cell.

That is why peroxisomes contain catalase, which decomposes H₂O₂ to H₂O and O₂, neutralising the toxicity.

So the organelle carries out a reaction whose by-product would poison the cell, and carries the antidote in the same compartment. The membrane keeps both inside.

Other functions

note

  • β-oxidation of long chain fatty acids.
  • Involvement in lipid biosynthesis, the plasmalogens.
  • An important role in cellular respiration.
  • In plants, peroxisomes also carry out the recycling of carbon from phosphoglycolate during photorespiration.

All 95 Cytology topics · exam study support, not clinical guidance. SuperMed is not affiliated with the Medical University of Sofia.

Start free. No card needed.

Every account starts free, with Medical Ethics, Urology and Computer Science open in full and free topics in Cytology, Latin Language, Medical Chemistry, Medical Physics, Medical Biology, Anatomy, Biophysics, Medical Psychology, MCS (Disaster Medicine), Physiology, Biochemistry, Social Medicine, Medical Genetics, General Pathology, Pharmacology, Pathophysiology, Microbiology, Internal Medicine, Nephrology, Cardiology, Pulmonology, Gastroenterology, Internal Medicine (Clinical), Obstetrics and Gynecology, Pediatrics, Forensic Medicine and Deontology, Physiotherapy and Rehabilitation, Anesthesiology and Reanimation, Dental Materials (Dental Medicine), Preclinics of Prosthetic Dental Medicine, Medical Psychology (Dental Medicine), SMME (Social Medicine & Medical Ethics), Preclinics of Pediatric Dental Medicine, Hygiene and Epidemiology, General Surgery, Preclinics of Conservative Dental Medicine, Preclinics of Oral and Maxillofacial Medicine, Dental Public Health, Dental Prophylaxis, Diagnostic Imaging, Dental Allergology, Dental Implantology, Physiotherapy, Cytology & Histology, Medical Chemistry, Medical Physics, Anatomy I, Biophysics, Latin with Medical Terminology, Medical Biology, Biochemistry, Human Physiology, Social Medicine and Medical Ethics, Anatomy II, Microbiology, Medical Psychology, Pathophysiology, Disaster Medicine, Hygiene, Ecology and Occupational Diseases, Propaedeutics of Internal Diseases, General and Operative Surgery, Roentgenology and Radiology, Clinical Laboratory, Medical Physics (Dental Medicine), Cytology, Histology and Embryology, Human Biology (Dental Medicine), Latin and Medical Terminology, Anatomy and Histology (Dental Medicine), Chemistry (Dental Medicine), Biophysics (Dental Medicine), Dental Materials (Dental Medicine), Preclinic of Prosthetic Dentistry, Social Medicine with Medical Ethics, Biochemistry (Dental Medicine), Human Physiology (Dental Medicine), Microbiology (Dental Medicine), Pediatric Dentistry - Part I, Medical Psychology (Dental Medicine), Pathoanatomy (Dental Medicine), Dental Prevention, Pathophysiology (Dental Medicine), Conservative Dentistry - Part I, Special Surgery, including Local Anaesthesia and Analgesics in Dental Medicine, Human Biology, Physics, Chemistry, Latin Language with Medical Terminology, Anatomy and Histology, Cytology, General Histology and Human Embryology, Biophysics, Social Medicine and Biostatistics, Biochemistry, Human Physiology, Disaster Medicine, Pathophysiology, General Pathology, Hygiene and Ecology, Medical Psychology, Chemistry (Dental Medicine), Physics and Biophysics, Cytology, General Histology and Human Embryology (Dental Medicine), Human Biology, Human Genetics and Regenerative Medicine, Latin Language (Dental Medicine), Computer Technique, Dental Materials (Dental Medicine), Dental Morphology, Occlusion and Function of the Jaws, Anatomy and Human Histology, Biochemistry (Dental Medicine), Pre-clinics of Pediatric Dentistry, Human Physiology (Dental Medicine), Social Medicine and Medical Ethics (Dental Medicine), Medical Psychology (Dental Medicine), Disaster Medicine (Dental Medicine), Propaedeutics of Conservative Dentistry, Pharmacology (Dental Medicine), Pathophysiology (Dental Medicine), Pathoanatomy (Dental Medicine), Hygiene, Preventive Medicine and Epidemiology, Anaesthesiology and Emergencies, Otorhinolaryngology (Dental Medicine), Propaedeutics of Oral and Maxillofacial Surgery and Local Anaesthesia, Public Dental Health, Cytology, General Histology and Human Embryology, Physics, Chemistry, Biology, Latin Language with Medical Terminology, Human Anatomy, Biophysics, Physiology, Biochemistry, Medical Psychology, Medical Informatics, Social Medicine, Microbiology, Medical Ethics, Pathophysiology, Medical Statistics, Communication Skills, General and Operative Surgery, Propaedeutics of Internal Diseases, Medical Genetics, Hygiene, Ecology and Occupational Diseases, Radiology, Disaster Medicine, Ophthalmology and Clinical Laboratory. Super opens the rest. Super is €14.99 a month.

Create a free account