Lysosomes

A simple description of lysosomes is that they are tiny sacs filled with fluid containing enzymes (i.e. proteins that act as biological catalysts) which enable the cell to process its nutrients and are also responsible for destroying the cell after it has died.

• Lysosomes are the main sites of digestion, that is the break-down of structures, within cells.
  There are, however, some circumstances (diseases / conditions) in which lysosomes begin to 'break-down' living cells - not just useless parts of cells or potentially harmful structures.
• A defining characteristic of lysosomes is that each one is bounded by only a single membrane.

What is a Lysosome ?

^§ i.e. 50x10^-9m to 1x10^-6m, which is the same as 0.00005 to 0.001 mm (millimetres).
See scientific numbers for more about how these very small numbers are expressed.

Structure of Lysosomes

• The outer surface is formed by a single membrane, a phospholipid bilayer that can fuse with some other membrane-bound organelles.
• Approx. spherical shape of diameter ranging up to one micrometre (1 μm).
• A single lysosome contains many enzyme molecules.
• The enzymes contained within lysosomes are known collectively as acid hydrolases and work best at in acidic environments, i.e. at low pH. The interior of lysosomes is acidic (about pH 4.8 to 5) compared with the slightly basic (about pH 7.2) intracellular fluid, which is also called cytosol, that surrounds organelles such as lysosomes within cells.

Formation of Lysosomes

Above: A very simple representation of a primary lysosome being formed from the Golgi body within the cell, then going on to form a secondary lysosome by
merging with a vesicle.

The Golgi apparatus (also known as the Golgi complex, the Golgi body, or simply the Golgi), which is present in the vast majority of eukaryotic cells, forms tiny vesicles that separate, some descriptions say "bud", from the ends of the Golgi cisternae. Vesicles formed in this way that contain enzymes such as proteases and lipases, are primary lysosomes.

Secondary lysosomes are formed when primary lysosomes fuse with other membrane-bound vesicles.

See the diagram on the right.

Importance of Lysosomes

If they were not enclosed, the enzymes contained within lysosomes could cause damage to other structures within the cell. The rest of the cell is therefore protected by these enzymes being isolated within a membrane - each such membrane and its contents forming an organelle known as a lysosome. In addition to holding potentially harmful enzymes apart from other structures within the cell, lysosomes perform many functions concerned with removing unwanted materials from cells (see below).

Functions of Lysosomes

The functions of lysosomes concern the different ways in which the enzymes contained within the membrane (that defines and encloses the lysosome) affect other materials, which can originate from either outside or inside the cell.

Note that the functions of lysosomes are listed differently in different textbooks - both in terms of the number of functions listed and the words used to describe the various processes. Technical (biology) terms are indicated in bold green below, with explanations of these words being given in standard font.

• Release enzymes outside of the cell (exocytosis)
  which may serve the purpose of destroying materials around the cell.
• Break-down 'digestion' of materials from inside the cell (autophagy)
  i.e. by fusing with vacuoles from inside the cell.
  This could include digesting worn-out organelles so that useful chemicals locked-up in their structures can be re-used by the cell.
• Break-down 'digestion' of materials from outside the cell (heterophagy)
  i.e. by fusing with vacuoles from outside the cell.
  This could include breaking-down material taken-in by phagocytes, which include many types of white blood cells - also known as leucocytes. Specific mechanisms of heterophagy can be:
  • phagocytic - by which cells engulf extracellular debris, bacteria or other particles - only occurs in certain specialized cells
  • pinocytic - by which cells engulf extracellular fluid
  • endocytic - by which cells take-up particles such as molecules that have become attached to the outer-surface of the cell membrane.
• Recycle the products of biochemical reactions that have taken place following materials being brought into the cell by endocytosis (general term for this 'recycling' function: biosynthesis)
  Different materials (chemicals) are processed in different ways, e.g. some structures may be processed/degraded within lysosomes and others are taken to the surface of the cell.
• Completely break-down cells that have died (autolysis)

In general, the functions of lysosomes involve breaking-down i.e. processing to 'make safe' or make use of, or removing from the cell e.g. by exocytosis, useless and potentially harmful materials such as old worn-out parts of the cell or potential threats such bacteria. Lysosomes can therefore be thought of as the rubbish disposal units within cells.

More about Lysosomes

How many lysosomes are present in a typical cell ?
A 'typical cell' or even a 'typical animal cell' is a very vague concept, for example there are about 200 different types of cells in the human body alone. However, as a general guide, many human cells contain hundreds of lysosomes while phagocytic cells often contain thousands of lysosomes. Erythrocytes (red blood cells) do not contain any lysosomes.

Lysosomes are particularly abundant in secretory cells e.g. epithelial cells, and in phagocytic cells.
For example, there are many lysosomes in liver cells - two of the main functions of the liver being (1.) secretion of bile and bile salts, and (2.) phagocytosis of bacteria and dead or foreign materials. There are also many lysosomes in cells in the kidneys where the third process by which the kidneys clean blood (regulating its composition and volume) is tubular secretion, which involves substances being added to the tubular fluid.

Some human diseases called lysosomal storage diseases are due to lysosome enzyme disorders.