Intracellular transport
Intracellular transport

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Intracellular transport

6.5 Lysosomes

Lysosomes are membrane-bound organelles responsible for intracellular digestion of substances derived from both inside and outside the cell. Within their membrane an acidic interior (pH ~ 5) is maintained by the action of proton pumps in the membrane. The lysosomes contain a class of enzymes, hydrolases, that catalyse hydrolysis of covalent bonds in proteins, lipids, carbohydrates and nucleic acids.

The lysosomal hydrolases break down complex molecules into simpler, low molecular mass compounds that can be reused. The enzyme content of lysosomes varies in different tissues according to the needs of the tissue.

Lysosomes are not just involved in breaking down material arriving via endocytosis, but also degrade intracellular debris, such as defective organelles and macromolecules. Molecules destined to be degraded are tagged (most frequently by ubiquitination) and then taken up by lysosomes, or are first encapsulated within endosomes that fuse with lysosomes. This process is called autophagy. Products of lysosomal degradation are released from lysosomes into the cytosol and are re-used by the cell. Material that cannot be digested accumulates in vesicles called residual bodies, whose contents are removed from the cell by exocytosis. Some residual bodies contain high concentrations of heterogenous pigmented substances, including polyunsaturated fatty acids and proteins, called lipofucsin or ‘age pigment’. Under particular conditions some lysosomal enzymes are secreted from the cell for the digestion of extracellular material in connective tissue.

  • What do you suppose is the consequence of a genetic deficiency in individual lysosomal enzymes?

  • People who are genetically deficient in lysosomal enzymes are unable to break down particular macromolecules that have been directed to the lysosome for recycling. The macromolecules accumulate in the cell, or they may be released to accumulate in the extracellular space.

The deficiencies result in so-called ‘lysosomal storage diseases’. What accumulates depends on which type of enzyme is deficient. For example, the lack of some endoglycosidases leads to the accumulation of heparan sulfate-containing glycoproteins. As a consequence the undegraded molecules accumulate in tissues and cause damage, particularly in the brain where they produce loss of neurons and progressive dementia. Most of these conditions are rare but extremely serious, and affected individuals often die in childhood (Figure 39).

Figure 39 A section of brain tissue, stained with haematoxylin and eosin, from a patient who died of type-A Niemann–Pick disease, an autosomal recessive condition that causes progressive neural damage in children. In this disease, the enzyme responsible for one of the steps in the breakdown of sphingomyelin is absent, causing accumulation of sphingolipids. Note the numerous abnormal distended neurons (arrows).

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371