The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

Active transport is the movement of molecules against their concentration gradient (from an area of low concentration to an area of high concentration, requiring energy, usually in the form of ATP. Passive transport is the movement of molecules with their concentration gradient (from an area of high concentration to an area of low concentration), and does not require energy input. Facilitate diffusion is the movement of molecules with their concentration gradient across the cell membrane using transmembrane proteins (carrier proteins or channels), and does not require energy. Osmosis is the movement of a solvent (usually water), from an area with a lower concentration of solute to an area of higher concentration of solute; this process does not require energy.

This is the definition of passive transport. Active transport requires energy for molecules to move. Also, the Na+/K+ pump requires energy, and thus is a form of active transport. Vesicle transport, including endocytosis and exocytosis, also requires energy.

Only small, nonpolar molecules and small, uncharged, polar molecules can freely diffuse across the lipid bilayer of the cell. Glucose is a large molecule that cannot freely diffuse across the lipid bilayer, even if this would be favored by the concentration gradient. Facilitated diffusion is the movement of molecules across the lipid bilayer using carrier proteins or channels, which does not require energy input.

Glycolysis (the process of breaking down glucose) takes place in the cytoplasm, or cytosol—the aqueous portion of the cytoplasm. It is in the cytoplasm where the enzymes required for glycolysis are found.

The citric acid cycle takes place in the mitochondrial matrix, and the electron transport chain takes place along the inner mitochondrial membrane in order to pump protons into the intermembrane space.

A hypotonic solution is a solution in which there is a lower concentration of solutes in the solution than in the cell. Thus, water (the solvent) will enter the cell, causing the cell to swell.

Ubiquitin ligases add ubiquitin to their ligands. The addition of ubiquitin acts as a signal that a protein has become ineffective and is ready for degradation. When multiple ubiquitin residues have been added to a protein molecule, it is transported to the lysosome in the cell to be digested.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

Several different types of proteins can change the structure of a ligand, such as isomerases.

As the size of a cell increases, the surface area to volume ratio decreases, as surface area is a squared function, while volume is a cubic function. Due to the decreasing surface area to volume ratio, there is less area for the diffusing molecules to actually enter the cell, thus decreasing the rate at which diffusion can occur.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

A phosphatase removes a phosphate group from its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

The cell membrane is a phospholipid bilayer, where the hydrophobic heads prevent hydrophilic molecules (such as charged ions) from crossing. Small, uncharged molecules are able to passively diffuse across the cell membrane when favored by the concentration gradient.