Acid growth refers to the ability of plant cell

Plant cell

Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...

s and plant cell walls to elongate or expand quickly at low (acidic) pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. This form of growth does not involve an increase in cell number. During acid growth, plant cells enlarge rapidly because the cell walls are made more extensible by expansin

Expansin

Expansin refers to a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem function, and other developmental processes...

, a pH-dependent wall-loosening protein. Expansin loosens the network-like connections between cellulose microfibril

Microfibril

The microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, examples are hair and sperm tail. Its most frequently observed structural pattern is 9+2...

s within the cell wall, which allows the cell volume to increase by turgor

Turgor pressure

Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...

 and osmosis

Osmosis

Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

. A typical sequence leading up to this would involve the introduction of a plant hormone

Plant hormone

Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...

 (auxin

Auxin

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

, for example) that causes proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s (H⁺ ions) to be pumped out

Proton pump

A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.-Function:...

 of the cell into the cell wall. As a result the cell wall solution becomes more acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic. This activates expansin activity, causing the wall to become more extensible and to undergo wall stress relaxation

Wall stress relaxation

Wall stress relaxation refers to the reduction in tensile stress in cell walls of plants, fungi and bacteria, as a result of movement or rearrangement of the polymeric network that gives the wall its tensile strength. Wall stress usually comes from cell hydrostatic pressure or turgor pressure,...

, which enables the cell to take up water and to expand.