Lysosomes, vacuoles, peroxisomes, and the plasma membrane

The ER and Golgi make and ship the cell’s products. The remaining members of the endomembrane system handle digestion, storage, and cleanup, and the plasma membrane manages everything that passes between the cell and the outside world.

Lysosomes: digestion with acid

Lysosomes are small organelles in animal cells that break down molecules and macromolecules. They’re packed with acid hydrolases, enzymes that break a covalent bond (a strong bond in which atoms share electrons) by adding a water molecule across it. That kind of reaction is hydrolysis, from hydro (water) and lysis (splitting). With many different hydrolases, a lysosome can digest carbohydrates, proteins, lipids, and nucleic acids.

As their name says, acid hydrolases work best in acid. (The pH scale measures acidity: 7 is neutral, and lower numbers are more acidic.) The inside of a lysosome is kept at about pH 4.8, while the cytosol sits near a neutral pH 7.2 and is buffered (it resists changes in pH). That difference is a built-in safety feature.

Lysosomes get material to digest from two sources:

• Endocytosis: the cell takes in material from outside by wrapping it in a piece of plasma membrane, and the lysosome digests it.
• Autophagy (“self-eating”): the cell breaks down its own worn-out molecules and parts and recycles the building blocks into new ones.

Because each hydrolase digests particular molecules, a mutation that disables one of them leaves the lysosome unable to break down those molecules, even though the rest of the organelle is normal.

Vacuoles: storage, water control, and digestion

Vacuoles are membrane sacs that stand out in plant, fungal, and some protist cells. Despite a name meaning “empty space,” they hold fluid and sometimes solids, and most are built by many small vesicles fusing together. Animal cells have smaller ones, sometimes called storage vesicles, that hold or move materials for a while.

The three main types of vacuoles do very different jobs:

The central vacuole’s push against the wall is turgor pressure. It keeps plant cells firm, holds the whole plant upright, and drives the cell wall to expand as the cell grows. When a plant dries out, turgor pressure drops and the plant wilts.

Food vacuoles form by phagocytosis (“cell eating”): a cell engulfs a large particle, such as a bacterium or a bit of food, in a pocket of its membrane. Macrophages do this using pseudopodia.

Peroxisomes: handling dangerous by-products

Peroxisomes are small organelles with a single membrane, found in all eukaryotic cells, usually by the hundreds. Liver cells have especially many, because peroxisomes break down toxic molecules. A common by-product of that work is hydrogen peroxide:

RH₂ (toxin) + O₂ → R + H₂O₂

Hydrogen peroxide is risky to leave around. Iron ions (Fe²⁺), found naturally in cells, convert it into a free radical (·OH, called the hydroxide free radical), a highly reactive molecule that damages proteins, lipids, and DNA. Peroxisomes avoid that with catalase, an enzyme that breaks hydrogen peroxide into harmless water and oxygen:

2 H₂O₂ → 2 H₂O + O₂

Peroxisomes also help metabolize fats and amino acids. Plant seeds contain glyoxysomes, organelles similar to peroxisomes that convert stored fat into sugar when the seed sprouts. Fat packs more energy per gram than carbohydrate, so fat-storing seeds can be smaller and lighter.

The plasma membrane: the cell’s interface

The plasma membrane, the last member of the endomembrane system, is the boundary between the cell and its surroundings. Its proteins carry out three major functions:

• Membrane transport. Transport proteins move specific ions and molecules in or out, so the membrane is selectively permeable: it lets only certain substances cross. This is the function that most directly supplies metabolism, since it decides which molecules get in and which products get out.
• Cell signaling. Receptors in the membrane recognize signals, such as molecules released by other cells. When a signal binds, it sets off a signal transduction pathway, a chain of steps that makes the cell respond. After a meal, for example, the hormone insulin binds receptors on your cells, and they respond by taking up more glucose.
• Cell adhesion. Proteins in the membranes of neighboring animal cells bind one another, holding cells together into a multicellular body.

Check yourselfA lysosome bursts inside a cell. Why doesn’t the cell digest itself?

The lysosome’s acid hydrolases work best at about pH 4.8. The cytosol is buffered near pH 7.2, where these enzymes are barely active, so the spill does little damage.