Lysosomes: acid hydrolases and intracellular digestion
A lysosome is an organelle enclosed by a membrane and filled with acid hydrolases, enzymes that split large molecules by adding water, to digest material inside the cell.
Remember
A lysosome is where the cell digests and recycles: its acid hydrolases, working in an acidic setting, break down large molecules, engulfed bacteria and viruses, and worn-out organelles.
Memory aid
the lysosome is the cell's garbage disposal and recycling centre
The lysosome works like the cell's garbage disposal and recycling centre, where material is broken down and its parts are reused.
A lysosome holds acid hydrolases: digestive enzymes that split proteins, nucleic acids, carbohydrates and lipids by adding water. The membrane keeps these powerful enzymes away from the rest of the cell, so the cell is not digested from within.
They are called acid hydrolases because they work best in an acidic setting. Inside a lysosome the pH is about 5, far more acidic than the rest of the cell. A pump in the lysosomal membrane, the V-ATPase, uses ATP to move protons (hydrogen ions, H⁺) into the lumen, the fluid-filled space inside. If the pump is blocked, the lumen becomes less acidic and the enzymes work poorly, so digestion slows.
The lysosome deals with three kinds of material. It breaks down macromolecules, which are large molecules such as proteins, nucleic acids, lipids and polysaccharides, taken in from outside the cell. It destroys bacteria and viruses that the cell has engulfed. It also disposes of worn-out organelles by autophagy, which means self-eating: the cell digests its own damaged parts. The small building blocks released are reused, so the lysosome is often called the cell's recycling centre.
The hydrolases are made in the rough endoplasmic reticulum (the ribosome-studded membrane network), and the Golgi apparatus packages them into vesicles that become lysosomes or fuse with existing ones.
A peroxisome is also a small organelle enclosed by a membrane, but it deals with toxins and hydrogen peroxide, a by-product that its own enzymes then neutralise. Mitochondria make ATP, and the Golgi apparatus modifies and sorts proteins.
Key facts
- Contents
- acid hydrolases, digestive enzymes that split proteins, nucleic acids, carbohydrates and lipids by adding water.
- Acidity
- the lumen sits at about pH 5, much lower than the cytoplasm.
- Acid supply
- the V-ATPase pump in the lysosomal membrane uses ATP to move protons into the lumen.
- Origin of the enzymes
- they are made in the rough endoplasmic reticulum and packaged by the Golgi apparatus.
- Jobs
- digesting macromolecules and engulfed bacteria and viruses, and recycling worn-out organelles by autophagy.
- Look-alikes
- peroxisomes handle toxins and hydrogen peroxide, mitochondria make ATP, and the Golgi apparatus modifies and sorts proteins.
Exam traps
- Blocking the lysosomal proton pump does not raise enzyme production, because the pump affects how well the hydrolases work, not how many are made, and they are made on ribosomes of the rough endoplasmic reticulum, upstream of the pump.
- A less acidic lumen does not speed digestion up, because lysosomal enzymes are active at a much lower pH than the cytoplasm, so less acid slows the breakdown of macromolecules.
- The Golgi apparatus is not the site of digestion, because it modifies, sorts and packages proteins, including the lysosomal enzymes, and sends them on.
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