Cytosol

Inside a cell, there is a special liquid.

This liquid is called the cytosol. It is mostly made of water. Many tiny things float in the water. These things include salts and proteins.

The liquid is very crowded. It is like a busy soup. This soup helps the cell stay alive. It also helps the cell do its work. The liquid stays inside the cell walls.

Inside a cell, there is a busy liquid called the cytosol.

It is the liquid part of the cytoplasm. Most of the cytosol is water. In a typical cell, water makes up about 70% of the volume.

Many things float in this water. These include ions, which are tiny charged particles. It also holds large molecules like proteins. Proteins take up about 20% to 30% of the space. Because there are so many molecules, the liquid is very crowded. This is called macromolecular crowding. It means the molecules have less room to move. This crowding helps molecules find each other to work together. The cytosol also has different levels of organization. It is not just a random mix. For example, it can have concentration gradients. This means some parts have more of a substance than others. Small sparks of calcium can move through the liquid. These sparks act as signals for the cell. In bacteria, the cytosol holds the genome. The genome is the set of instructions for the cell.

Some bacteria even use special parts called carboxysomes to hold things in the cytosol.

The cytosol is a busy liquid found inside cells.

It is often called the cytoplasmic matrix or groundplasm. This liquid makes up part of the cytoplasm. In many cells, the cytosol is the main liquid area. It is surrounded by a cell membrane. Other parts of the cell, called organelles, sit inside this liquid. The cytosol is like a soup that fills the space between these parts. It is very important for keeping the cell working well.

This liquid works by holding many different things at once. Most of the cytosol is made of water. Water makes up about 70% of a typical cell's volume.

Many small things called ions float in this water. These include potassium and sodium ions. The cell keeps different amounts of these ions inside than outside. This helps the cell manage water through a process called osmoregulation. It also helps nerve and muscle cells send signals.

Large molecules like proteins also float in the liquid.

Scientists have studied the cytosol for a long time. The term "cytosol" was first used in 1965 by H. A. Lardy. Before this, people used names like hyaloplasm. In the past, the nature of cell fluid was not well understood. Early researchers sometimes looked at extracts made by breaking cells apart. They used a machine called an ultracentrifuge to separate parts. Today, we use the word cytosol to describe the liquid in a living, intact cell. This helps us avoid confusion with older scientific methods.

There are many interesting facts about what is inside the cytosol. Proteins can take up 20% to 30% of the volume. This creates a state called macromolecular crowding. This means there is very little empty space for molecules to move. This crowding helps proteins find each other to work together. In bacteria, the cytosol holds the genome in a mass called a nucleoid. In eukaryotes, the genome stays in the nucleus. The nucleus is separated from the cytosol by tiny holes called nuclear pores.

Even though it is a liquid, the cytosol is very organized. It is not just a random mix of things. Small molecules can form concentration gradients. This means one area might have more of a substance than another. For example, calcium can create tiny "calcium sparks." These sparks can join to form larger calcium waves.

Some bacteria even use carboxysomes to enclose parts of the cytosol. These structures help keep specific parts of the cell separate. This organization helps the cell perform its many jobs.

The cytosol is the complex liquid phase found within a living cell. It is also known as the cytoplasmic matrix or groundplasm. While it is part of the cytoplasm, it specifically refers to the liquid that surrounds the organelles. It does not include the fluids found inside organelles, such as the mitochondrial matrix. In eukaryotic cells, the cytosol is bounded by the cell membrane. In prokaryotes, it is even more vital because most metabolic chemical reactions happen directly in the cytosol.

The cytosol functions as a highly organized aqueous environment. It is a mixture of water, dissolved ions, and various macromolecules. Water makes up about 70% of a typical cell's total volume. Although it is mostly water, the cytosol is not a simple, thin solution. The presence of many large molecules makes it very crowded. This state is called macromolecular crowding. This crowding increases the effective concentration of molecules. It can change how chemical reactions happen by helping proteins find and bind to one another.

Many different types of substances occupy this space. Small molecules, called metabolites, are incredibly diverse. For example, plants may produce up to 200,000 different small molecules. In single cells like baker's yeast or E. coli, estimates suggest there are under 1,000 metabolites. Large macromolecules, specifically proteins, are also very abundant. Proteins can occupy 20% to 30% of the cytosolic volume. These proteins often work together in large enzyme complexes to carry out metabolic pathways. Some bacteria even use specialized structures called carboxysomes to enclose and separate parts of the cytosol.

Ionic concentrations in the cytosol are strictly controlled and differ from the outside environment. For instance, the cytosol has a high concentration of potassium ions and a low concentration of sodium ions. In mammalian cells, potassium levels are between 139 and 150 millimolar, while sodium is only about 12 millimolar. This difference is essential for osmoregulation, which is how cells manage water balance. If these levels were equal, water would rush into the cell via osmosis. To prevent this, the cell uses a protein called Na+/K+-ATPase to pump sodium out and bring potassium in. This process creates a negative membrane potential, which is vital for nerve and muscle cells.

Calcium ions play another specialized role through a process called calcium signaling. The concentration of calcium in the cytosol is kept extremely low, at less than 0.0002 millimolar. When a cell receives a signal, such as a hormone, calcium channels open. This allows calcium to flood into the cytosol. This sudden increase acts as a "second messenger" to activate other molecules like calmodulin. Additionally, cells can use molecules called osmoprotectants to survive extreme drying. In a state called cryptobiosis, the cytosol and these molecules turn into a glass-like solid. This helps stabilize the cell's internal structures during desiccation.

Our understanding of the cytosol has changed as technology has improved. The term was first introduced in 1965 by H. A. Lardy. Originally, he used it to describe liquid produced by breaking cells apart using ultracentrifugation. This extract was actually a cytoplasmic fraction, not the liquid of a living cell. Today, we use "cytosol" specifically for the liquid in an intact, living cell. Some scientists also use the term "aqueous cytoplasm" to avoid confusion. Before this, researchers used names like hyaloplasm. As microscopes and chemical tools improved, we realized the cytosol was much more organized than previously thought.

Even without internal membranes, the cytosol shows high levels of organization. It is not just a random mixture of parts. It can form concentration gradients, where certain areas have more of a substance than others. For example, "calcium sparks" are tiny, short-lived increases in calcium concentration. These sparks are about 2 micrometres in diameter and last only a few milliseconds. These small sparks can merge into larger "calcium waves" within the cell. This organization allows the cell to direct chemical activity to specific locations. This complexity ensures that the many different biological processes can happen efficiently in one shared space.