Turgor pressure is the internal pressure within plant cells that pushes the cell membrane against the rigid cell wall. This pressure, maintained by the water content inside the cell, is important for plant physiology, helping plants maintain structural integrity and various biological functions.
The Mechanism of Turgor Pressure
Turgor pressure is generated through the process of osmosis, which involves the movement of water molecules across a semipermeable membrane. In plant cells, water moves from an area of higher water concentration outside the cell into the cell’s interior, into a large organelle called the central vacuole. This influx of water causes the central vacuole to swell and expand, pressing the cell membrane outwards against the cell wall.
The plant cell wall, a strong, inflexible outer layer made of cellulose, resists this outward expansion. As the cell membrane pushes against the cell wall, the wall pushes back, creating turgor pressure. This counter-pressure prevents the cell from bursting, allowing it to become firm or “turgid.” Without a cell wall, animal cells would burst under similar internal pressure.
The Role of Turgor Pressure in Plants
The presence of adequate turgor pressure is important for the structural rigidity and overall health of plants. It provides internal support, allowing non-woody parts of plants, such as stems and leaves, to remain upright and firm. When plant cells are turgid, they contribute to the plant’s firm texture and erect posture.
A reduction in turgor pressure leads to a loss of rigidity, causing plants to wilt. This occurs when the plant loses water faster than it can absorb it, reducing water volume in the central vacuoles and pressure on cell walls. Wilting is often a reversible process; if water becomes available, plant cells can regain turgor and the plant can recover.
Turgor pressure also regulates the opening and closing of stomata, tiny pores on leaf surfaces. These stomata control the exchange of gases like carbon dioxide, oxygen, and water vapor. When the specialized guard cells surrounding the stomata are turgid, the stomata open, facilitating gas exchange. Conversely, when guard cells lose water and turgor pressure decreases, the stomata close, helping the plant conserve water.
Turgor Pressure in Daily Life
Turgor pressure is observable in many everyday situations involving plants and plant products. The crispness of fresh vegetables like lettuce, celery, or carrots is a common example. Their firm texture results from high turgor pressure within their cells, as water-filled cells push against their cell walls.
When these vegetables are left out or stored improperly, they lose water and turgor pressure, becoming limp and soft. Rehydrating them, such as by placing them in cool water, can sometimes restore crispness as water re-enters the cells, increasing turgor. Similarly, cut flowers that wilt after being out of water often regain firmness when placed in a vase. This revival occurs as the flower’s cells absorb water, restoring the turgor pressure that provides structural support to petals and stems.