Turgor pressure in plants: examples and meaning

Turgor pressure in plants is the force exerted by stored water on a cell wall. It is also known as hydrostatic pressure and is defined as the pressure in a fluid measured at a specific point within it when it is at equilibrium. Turgor pressure is generally caused by the osmotic flow of water and is found in plants, fungi, and bacteria and the phenomenon has also been observed in protists with cell walls.

This system is not found in animal cells because the lack of a cell wall would cause the eukaryote cell to lyse when subjected to excessive pressure. 

Table of Contents

Turgor pressure meaning

Turgor pressure is the actual pressure developed by the fluid in a turgid plant cell as a result of endosmosis. This is the pressure exerted by water on the cell wall of a plant cell.

Turgor pressure example

When considering a balloon being filled with water as an example of turgor pressure, one can see that as more water is drawn into the balloon, the balloon expands. The pressure exerted by the water against the balloon’s walls is similar to the turgor pressure exerted against the cell wall.

What is turgor pressure?

This is the pressure created and exerted by a fluid (water) in a cell that presses the cell membrane against the cell wall. Turgor is the feature that causes living plant tissue to be rigid.

Turgor pressure is critical to the plant’s essential processes as it stiffens and rigidifies the plant cell. The plant cell also becomes flaccid if it lacks turgor pressure. For example, a  wilting plant could result from prolonged flaccidity due to a lack of turgor pressure. Turgor pressure is also important in the formation of stomata as the turgid guard cells open a gap for gas exchange. By so doing, carbon dioxide could enter the system and be used in photosynthesis.

How turgor pressure affects plants.
This is a turgor pressure picture showing its effects.

Mechanism of turgor pressure

All cells are enclosed or surrounded by a lipid bilayer cell membrane that allows water to flow into and out of the cell while restricting solute flow. When the cell is immersed in a hypertonic solution, water flows out, reducing the cell’s volume. While in a hypotonic solution, water flows into the membrane, increasing the volume of the cell, whereas, in an isotonic solution, water flows in and out of the cell at the same rate.

The inflow of water into the cell via osmosis makes the cell to be turgid a phenomenon known as turgidity and it occurs when the cell’s membrane pushes against the cell wall, indicating that turgor pressure is high.

Likewise, when the cell loses water as a result of osmosis, the cell becomes flaccid because of the low turgor pressure. This is visible in plants as wilted anatomical structures and it is more specifically referred to as plasmolysis.

How does turgor pressure affect plants?

Turgor pressure affects plants by causing them to be rigid but a loss of turgor pressure causes wilting of the plants.

How does a plant cell control its internal turgor pressure?

At the cellular level, the plant has features that allow it to regulate its internal turgor pressure. These features include the presence of cell walls that prevent cell lysis (bursting) during high water influx, The presence of cell vacuole that is larger than any of the subcellular components. The vacuole causes water to enter the cell via osmoregulation.

Factors that control a plant cell internal turgor pressure

Cell wall

The plant cell wall is a tough, rigid structure made mostly of cellulose. It can consist of a single or double layer of cellulosic material. As the plant cell matures, the primary cell wall secretes a secondary cell wall beneath and on top of the cell membrane. The lignin deposit in the “secondary cell wall” is distinctively rich, which aids in cell waterproofing.

The cell wall prevents the plant cell from bursting due to water influx and this is because the cell is able to withstand the osmotic pressure exerted by the water molecules and as a result, the cell is kept turgid. Turgor pressure occurs when plant cells become turgid due to the pressure exerted by water molecules against the cell wall.

Plant cells have both a cell membrane and a cell wall, whereas animal cells only have one. The cell wall acts as a barrier between the cell membrane and the outside world. It aids in the resistance to osmotic pressure, which is caused by the osmotic flow of water caused by the different amounts of solutes in extracellular fluid and intracellular fluid.

Osmoregulation by cell vacuole

Osmoregulation is the process of controlling the water potential in order to maintain an optimal osmotic pressure inside the cell. It is the process by which a cell maintains a suitable concentration of solutes and amount of water within the cell in relation to the surrounding fluid.

A vacuole in plants is a large membrane-bound vesicle in the cytoplasm. Water, inorganic molecules, and organic molecules are all found in the vacuole and it regulates the osmotic flow of water to maintain turgor pressure. Because it has the ability to absorb and store ions, sugars, and other solutes and this causes the intracellular fluid to be hypertonic in comparison to the extracellular fluid (which, in this case, is hypotonic relative to the cell).

Hence, water draws into the cell because there are more solutes inside the cell than in the extracellular fluid making the cell become turgid.

Turgor pressure and stomata

Stomata are plant pores that allow for gas exchange. They are typically found on the surface of the leaf’s lower epidermal layer and can also be seen on certain plant stems. Stomates are actually openings created when two guard cells are opened and when the two guard cells are turgid, they create an opening. The osmotic pressure draws water in, causing the guard cells to expand in volume, or essentially swell.

Because the inner wall of the pore is more rigid than the wall on the opposite side of the cell, the swelling causes the guard cells to bow apart from each other. The opening created by the turgid guard cells is critical to stomata function as it allows carbon dioxide to enter through these openings. The CO2 is taken in as one of the reactants in photosynthesis when the stomata are open due to turgor pressure on the guard cells. During the intake of carbon dioxide, oxygen which is a byproduct of photosynthesis, and plants are expelled through the stomata.

Tropism and turgor pressure

This section will explore the relationship between tropisms and turgor movements, which are two types of plant movement that allow the plant to respond to stimuli. Tropism is the external stimuli while turgor pressure is the internal stimuli that affect plant movements.

Tropisms are plant responses to stimuli that cause long-term growth toward or away from the stimulus. This growth is caused by cell elongation that occurs at different rates on different sides of the plant, causing the plant to bend in one direction.

Phototropism, a light-induced reaction, causes the plant to bend toward the light source (see Essential Processes, Auxins). Thigmotropism, a touch reaction, causes plant parts to thicken or coil as they touch or are touched by environmental entities. Gravitropism or geotropism is a reaction to gravity, which causes plant parts to grow upward or downward. When a plant is turned on its side, the shoot begins to grow upward (against gravity), while the roots grow downward, following the pull of gravity.

While turgor movements happen faster than tropisms and are easily reversed. Instead of differential cell growth, they rely on changes in turgor pressure (exerted by water on cell walls) within specific plant cells. Turgor movements are mainly accountable for many plant responses, such as when leaves or flowers droop and fold up at specific times of day or night, or in response to an external touch. For example, a Venus flytrap relies on changes in turgor pressure to close its “jaws” around insects that land on the plant.

Turgor pressure and osmotic pressure

The osmotic pressure is the lowest pressure that must be provided to a solution in order to prevent the inward flow of its pure solvent across a semipermeable membrane, whereas the turgor pressure is the pressure inside the cell that pushes the plasma membrane against the plant cell wall.

Differences between wall pressure and turgor pressure

Turgor Pressure
Wall Pressure
It is the force exerted on the cell wall by the contents of the cell.
It is the pressure exerted on the contents of the cell by the cell wall.
It promotes the expansion and proliferation of plant cells.
It makes it hard to maintain the plant upright.
It’s referred to as hydrostatic pressure.
It acts as a counterweight to the turgor pressure.
A table shows the difference between turgor pressure and wall pressure.

Functions of turgor pressure in plants

  • Rigidity
  • Stomata formation
  • Growth
  • Seed dispersal
  • Nastic movements
  • Flowering and reproductive organs

Rigidity

Turgor pressure is essential for plants, particularly those that live on land. This pressure gives them the necessary turgidity and rigidity to stay upright against gravity while positioning themselves toward the source of light.

Stomata formation

Turgor pressure inside the stomata regulates when the stomata can open and close, which affects the plant’s transpiration rates. This function is also important because it regulates water loss within the plant. Because lower turgor pressure may indicate that the cell has a low water concentration, closing the stomata would aid in water conservation. While high turgor pressure keeps the stomata open for the necessary gas exchanges for photosynthesis.

Growth

The effect of turgor pressure on extensible cell walls is commonly referred to as the cell’s driving force of development. When turgor pressure rises, cells expand and apical cells, pollen tubes, and other plant structures such as root tips extend.  Plants may grow through asphalt and other hard surfaces due to high turgor pressures.

Seed dispersal

Turgor pressure is employed to disperse seeds in Ecballium elaterium (squirting cucumber). The fruit detaches from the stem due to a build-up of pressure inside the fruit. As the fruit falls to the ground, the seeds and water inside are discharged.

Nastic movements

Some plant cells with good turgor pressure use a technique similar to that used by stomates to get into a sleeping position at night. These plants are erect during the day to collect light for photosynthesis. When these plants sleep at night, their leaves and flowers close and droop.

This phenomenon is caused by a type of nastic movement called the nyctinasty which involves the pulvinar cells near the base of a plant leaf (or leaflet) or at the apex of the petiole. This plant drooping behavior is found in the Mimosa pudica plant where the leaves lose turgor pressure while the pulvinar cells increase it in reaction to contact.

Flowering and reproductive organ

The petals of Gentiana kochiana and Kalanchoe blossfeldiana have been seen to bloom due to the volatile turgor pressure of cells on the plant’s adaxial surface. It has been found that drying endothecium cells create an outward bending force during processes like anther dehiscence, resulting in pollen discharge.

This suggests that because these structures are dehydrated, they have reduced turgor pressures. Pollen tubes are cells that lengthen when pollen arrives on the stigma, which is located at the tip of the carpal. Because of the increased turgor pressure, these cells expand their tips swiftly. A plant cell with good turgor pressure always has blooming flowers.

Turgor pressure in other taxa

  • Fungi
  • Protists
  • Animals
  • Diatoms
  • Cyanobacteria

Turgor pressure, as previously indicated, may be present in species other than plants and can play a significant part in their development, mobility, and nature.

Fungi

Turgor pressure has been found to play a significant role in substrate penetration in fungi. High turgor pressures have been seen in the hyphae of species like Saprolegnia ferax, Magnaporthe grisea, and Aspergillus oryzae. They could permeate plant cells and synthetic materials like polyvinyl chloride. Invasive hyphal growth is attributed to turgor pressure, as well as the coenzymes released by the fungi to infiltrate these substrates. In summary, hyphal growth is proportional to turgor pressure, and when turgor pressure lowers, growth slows.

Protists

Because some protists lack cell walls, they are unable to experience full turgor pressure due to the contractile vacuole of these few protists regulating the amount of water in the cell. To avoid lysing in a hypotonic solution, protist cells include a vacuole that pumps water out of the cells to maintain osmotic balance.

Animals

Animal cells do not have a cell wall, hence there is no turgor pressure. Because the cell wall protects cells from being lysed by excessive turgor pressure in organisms with cell walls.

Diatoms

Heterokontophyta diatoms have polyphyletic turgor-resistant cell walls. During the life cycle of these organisms, carefully managed turgor pressure is responsible for cell expansion and sperm release, but not for processes like seta growth.

Cyanobacteria

Water-blooming cyanobacterium is usually gas-vacuolate cyanobacterium. Due to the accumulation of gases within their vacuole, they have the potential to float. The lesser the capacity of the gas-vacuoles in various cyanobacteria, the higher the turgor pressure.

Measurement of turgor pressure

In the measurement of turgor pressure in plants, many aspects must be considered such as if the cell is turgid or flaccid. Other cellular systems to consider include the protoplast, solutes within the protoplast (solute potential), cell transpiration rates, and cell wall tension.

The units used to measure turgor pressure are bars, MPa, and newtons per square meter.

Because of their size or other characteristics, not all procedures or methods of measurement can be utilized for all organisms. A diatom, for example, does not have the same features as a plant, thereby limiting the methods that can be used to calculate turgor pressure.

Thus the different methods used in the calculation of turgor pressure are listed below.

  • Water potential equation
  • Pressure-bomb technique
  • Atomic force microscopes that use a type of scanning probe microscopy (SPM)
  • Pressure probe
  • Micro-manipulation probe

Negative turgor pressure

While most cells have a positive turgor pressure, the xylem of a transpiring plant has a negative turgor pressure. The xylem is a vascular tissue that transports water and nutrients from the roots to the shoots and leaves, so this is not surprising to indicate the fact that this plant tissue has a negative turgor pressure. The xylem experiences high surface tension and negative turgor pressure because of water loss through transpiration.

FAQ on turgor pressure in plants

Why is turgor pressure important to a plant?

1. It maintains the cell’s shape.
2. It gives the rigidity of the cells and keeps their structure.
3. It serves as the foundation for the opening and closing of stomata. When the turgor pressure is high, the stomatal pore opens, and when it is low, the stomatal pore closes.
4. Cells and organelles continue to be stretched.
5. It supports soft tissues such as the parenchyma.
6. It is in charge of keeping leaves oriented correctly and making their surface visible to light.
7. Turgor pressure helps seeds to grow and develop.

What are two different ways a plant could control turgor pressure?

The 2 different ways a plant could control turgor pressure are by pumping ions and opening and closing stomates to control evaporation from leaves. Of course, this necessitates the use of water and in order to absorb water from its surroundings, a cell’s internal fluid or cell sap must have a higher solute concentration or a lower water potential.
Another way is by actively transporting proteins to the inside or outside of cells in order to import ions or other solutes to increase or decrease, respectively, solute concentration inside the cell, or by changing water concentration levels within the plant through leaf evaporation.

What type of plant cells would exhibit the most turgor pressure?

Guard cells are the ones that exhibit the most turgor pressure amongst the various plant cells.

What happens to turgor pressure when the central vacuole fills with water?

The vacuole, when filled with water, pushes the cytoplasm into a thin strip adjacent to the membrane and pushes outwards like a water-filled balloon. This turgor pressure is what keeps the cell firm and gives plant structures like leaves their distinctive shape.

What is turgor pressure in plants?

The force within the cell that pushes the plasma membrane against the cell wall is known as turgor pressure.

How does a plant cell control its internal turgor pressure?

Water moving into and out of plant cells’ vacuoles regulates turgor pressure.

What must the turgor pressure equal if there is no net diffusion between the solution and the cell?

Turgor pressure must equal the solute concentration on the opposite side of the cell, like in a normal isotonic solution.

What is plasmolysis

It is the loss of water from a cell resulting in a drop in turgor pressure in plants whereby the plasma membrane detaches from the cell wall.

What causes turgor pressure?

Turgor is produced by the osmotically driven inflow of water into cells through a selectively permeable membrane.

What large structure in a plant cell helps it maintain turgor pressure?

The central vacuole is a large structure in a plant and the objective is to maintain turgor pressure against the cell wall constant.

How to calculate turgor pressure?

Turgor pressure can be calculated using any of the following methods listed below.
1. Water potential equation
2. Pressure-bomb technique
3. Atomic force microscopes that use a type of scanning probe microscopy (SPM)
4. Pressure probe
5. Micro-manipulation probe

What is the relationship between osmosis and turgor pressure?

Osmosis regulates turgor pressure within cells, which causes the cell wall to expand during growth.

Describe plant wilting in terms of turgor pressure

Wilting is caused by osmosis, which causes water loss from plant cells. This causes the cell’s contents to pull away from the cell wall (plasmolysis), and because the cell contents are no longer pushing against the cell wall, there is no turgor pressure and the cells are flaccid.

How is plasmolysis related to turgor pressure in plant cells?

Plasmolysis is the process by which cells sap exosmosis. It means that the cell sap escapes the protoplast and causes the cell to shrink. This occurs whenever water exits the cell. As a result, the cell’s turgor pressure decreases as it becomes flaccid.

What is high turgor pressure?

High turgor pressure means the cell has a high concentration of water and it keeps the stomata open for the necessary gas exchanges for photosynthesis.

What process contributes directly to the turgor pressure that opens and closes stomata?

Active transport is the process that contributes directly to the turgor pressure resulting in the opening and closing of the stomata.

What happens to a plant with low turgor pressure

Lower turgor pressure may indicate that the cell has a low water concentration, and closing the stomata would aid in water conservation.

Describe how turgor pressure builds up

Turgor pressure is observed when a plant cell’s environment osmotically drives the inflow of water into cells through a selectively permeable membrane.

What two parts of a plant cell work together to make turgor pressure?

The plant cell will store water in the central vacuole, which will cause the vacuole to expand into the cell’s sides and the cell wall then pushes against the walls of other cells, producing turgor pressure.

What is a turgor pressure example?

A balloon that is being filled up with water is an example of turgor pressure.

What is the relationship between turgor pressure and pH

The ability of plant cells and cell walls to elongate or expand quickly under low (acidic) pH is referred to as acid growth. In order to sustain turgor pressure in this low pH, the cell wall must be changed.

A video briefly explaining turgor pressure.

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