Lysosomes are spherical membrane-bound organelles that are generated by the Golgi apparatus. They contain hydrolytic enzymes, so they function as part of the cell’s recycling system. In this article, we will look at the structure, synthesis, and function of lysosomes, and consider their relevance to clinical practice.


Lysosomes are acidic membrane-bound organelles found inside cells, usually about 1 micrometre in length. Lysosomes contain numerous hydrolytic enzymes that catalyze hydrolysis reactions. The membrane surrounding the lysosome is vital in ensuring that these enzymes do not leak into the cytoplasm and damage the cell from within. To maintain the acidic pH of the lysosome, protons are actively transported to the organelle through the lysosomal membrane.


The lysosome and the enzymes it contains are synthesized separately. Lysosomal proteins are formed in the same way as any other protein. The first step is the initiation of the production of mRNA strands from the relevant DNA segments. The mRNA strands proceed to the rough endoplasmic reticulum, where ribosomes build hydrolytic enzymes.

Importantly, these are tagged with mannose-6-phosphate within the Golgi apparatus to target them to the lysosome. As a result, vesicles containing these enzymes bud off from the Golgi apparatus. Two enzymes are responsible for the binding of the mannose-6-phosphate tag: N-acetylglucosamine phosphotransferase and N-acetylglucosamine phosphoglucomutase.

This vesicle, now in the cytoplasm, then joins with a late endosome which is another acidic membrane-bound organelle. The late endosome has proton pumps within its membrane that keep its internal environment acidic. Low pH causes dissociation of the mannose-6-phosphate receptor protein. This receptor can then be recycled back to the Golgi apparatus. The phosphate group is also removed from the mannose-6-phosphate tag, to prevent the entire protein from returning to the Golgi apparatus. The late endosome may eventually mature into a lysosome, having received enzymes from the Golgi apparatus.


Hydrolytic enzymes contained within the lysosome allow foreign particles to be destroyed. Lysosomes play an important role in phagocytosis. When macrophages phagocytize foreign particles, they contain them inside a phagosome. The phagosome will then join with a lysosome to form a phagolysosome. These enzymes are critical in oxygen-independent killing mechanisms. Lysosomes also help defend against pathogen entry through endocytosis by degrading pathogens before they reach the cytoplasm.

Facilitated Diffusion

Facilitated diffusion

What happens if a substance needs help to move across the plasma membrane? How facilitated diffusion works? Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Although facilitated diffusion involves transport proteins, it is still passive transport because the solute moves down the concentration gradient.

Nonpolar small molecules can easily diffuse through the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot. Instead, they diffuse across the membrane via transport proteins. A transport protein completely crosses the membrane and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.

A channel protein, a type of transport protein, acts like a pore in the membrane that quickly lets water molecules or small ions through. Water channel proteins (aquaporins) allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane.

A gated channel protein is a transport protein that opens a “gate,” allowing a molecule to pass through the membrane. Closed channels have a binding site that is specific for a given molecule or ion. A stimulus causes the “gate” to open or close. The stimulus can be chemical or electrical signals, temperature, or mechanical force, depending on the type of channel closed.

For example, sodium-gated channels in a nerve cell are stimulated by a chemical signal that causes them to open and allow sodium ions to enter the cell. Glucose molecules are too large to diffuse easily across the plasma membrane, so they move across the membrane through closed channels. In this way, glucose diffuses very quickly through the cell membrane, which is important because many cells depend on glucose for energy.

A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins “carry” the ion or molecule across the membrane by changing shape after binding of the ion or molecule. Carrier proteins are involved in passive and active transport.

Facilitated diffusion across the cell membrane.

Facilitated diffusion across the cell membrane. Channel proteins and carrier proteins (but not a gated channel protein) are shown. Water molecules and ions move through the channel proteins. Other ions or molecules are also transported across the cell membrane by carrier proteins. The ion or molecule binds to the active site of a carrier protein. The carrier protein changes shape and releases the ion or molecule to the other side of the membrane. The carrier protein then returns to its original shape.

Ion channels

Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) are important for many cellular functions. Because they are charged (polar), these ions do not diffuse across the membrane. Instead, they move through ion channel proteins where they are shielded from the hydrophobic interior of the membrane. Ion channels allow the formation of a concentration gradient between the extracellular fluid and the cytosol. Ion channels are very specific, allowing only certain ions to pass through the cell membrane. Some ion channels are always open, others are “closed” and can open or close. Gated ion channels can open or close in response to different types of stimuli, such as electrical or chemical signals.

Gastric Secretion


The gastric mucosa secretes 1.2 to 1.5 litres of gastric juice per day. Gastric juice renders food particles soluble, initiates digestion (particularly of protein), and converts gastric contents into a semiliquid mass called chyme, thus preparing it for further digestion in the small intestine. Gastric juice is a variable mixture of water, hydrochloric acid, electrolytes (sodium, potassium, calcium, phosphate, sulfate, and bicarbonate), and organic substances (mucus, pepsin, and protein).

This juice is very acidic due to its hydrochloric acid content and is rich in enzymes. As noted above, the walls of the stomach are protected from digestive juices by the membrane on the surface of the epithelial cells that line the lumen of the stomach; this membrane is rich in lipoproteins, which are resistant to attack by acids. The gastric juice of some mammals (eg, calves) contains the enzyme renin, which binds milk proteins and thus removes them from solution and makes them more susceptible to the action of a proteolytic enzyme.

The gastric secretion process can be divided into three phases (cephalic, gastric, and intestinal) that depend on the primary mechanisms that cause the gastric mucosa to secrete gastric juice. The phases of gastric secretion overlap and there is an interplay and some interdependence between the neural and humoral pathways. The cephalic phase of gastric secretion occurs in response to stimuli received by the senses, that is, taste, smell, sight, and hearing.

This phase of gastric secretion has a completely reflex origin and is mediated by the vagus nerve (tenth cranial nerve). Gastric juice is secreted in response to vagal stimulation, either directly by electrical impulses or indirectly by stimuli received through the senses. Ivan Petrovich Pavlov, the Russian physiologist, originally demonstrated this method of gastric secretion in a now-famous experiment with dogs. The gastric secretion phase is mediated by the vagus nerve and by gastrin release. The acidity of the gastric contents after a meal is buffered by protein, so it generally stays around pH3 (acidic) for about 90 minutes.

Acid continues to be secreted during the gastric phase in response to distension and to peptides and amino acids that are released from proteins as digestion proceeds. The chemical action of free amino acids and peptides stimulates the release of gastrin from the antrum into circulation. Therefore, there are mechanical, chemical, and hormonal factors that contribute to the gastric secretory response to eating. This phase continues until the food has left the stomach.

The intestinal phase is not fully understood due to a complex process of stimulation and inhibition. Amino acids and small peptides that promote gastric acid secretion are infused into the circulation; however, at the same time, the chyme inhibits acid secretion. Gastric acid secretion is an important inhibitor of gastrin release. If the pH of the antral contents falls below 2.5, no gastrin is released. Some of the hormones that are released from the small intestine by the products of digestion (especially fats), notably glucagon and secretin, also suppress acid secretion.

Absorption and emptying

Although the stomach absorbs a few of the products of digestion, it can absorb many other substances, including glucose and other simple sugars, amino acids, and some fat-soluble substances. The pH of gastric contents determines whether some substances are absorbed. At low pH, for example, the environment is acidic and aspirin is absorbed from the stomach almost as quickly as water, but as the pH of the stomach increases and the environment becomes more basic, aspirin is absorbed more slowly.

Water moves freely from the gastric contents through the gastric mucosa into the blood. However, the net absorption of water from the stomach is small because water moves just as easily from the blood through the gastric mucosa into the lumen of the stomach. Absorption of water and alcohol may be delayed if the stomach contains food and especially fat, probably because fat slows gastric emptying and most water in any situation is absorbed in the small intestine.

The rate of emptying of the stomach depends on the physical and chemical composition of the food. Liquids empty faster than solids, carbohydrates faster than proteins, and proteins faster than fats. When the food particles are small enough in size and nearly soluble, and when the receptors in the duodenal bulb (the area where the duodenum meets the stomach) have a fluidity and hydrogen ion concentration of a certain level, the duodenal bulb and the second part of the duodenum relax, allowing emptying of the stomach to begin.

During a duodenal contraction, the pressure in the duodenal bulb increases more than in the antrum. The pylorus prevents reflux into the stomach by closing. The vagus nerve plays an important role in voiding control, but there are indications that the sympathetic division of the autonomic nervous system is also involved. Several of the peptide hormones of the digestive tract also have an effect on intragastric pressure and gastric movements, but their role in physiological circumstances is unclear.