When sensitive plant leaves contact with raindrops, they rapidly close. Their leaves reopened after a while. How do they respond to external environment like animals, even if they have no nervous system? It turns out that the plant central large vacuole is orchestrating all the things. Along with chloroplasts and cell wall, it is considered one of the three typical characteristics in plants.
However, not all terrestrial plant cells possess these features, especially in some meristematic tissues or young plants. Many tiny vacuoles that is difficult to observe under an optical microscope are dispersed within cytoplasm. They originate from ER where vesicles carrying membrane proteins or hydrolases bud from the smooth ER and move to Golgi apparatus. Proteins are further processed and folded, then encapsulated in vesicles that separate from the trans face. These vesicles fuse together to enlarge themselves, and eventually become mature central vacuoles. A selectively permeable phospholipid bilayer separates cell sap from cytoplasm. Similar to plasma membrane, its surface is also dotted with carrier proteins and ion channels that regulate molecular entry and exit. However, the vacuole membrane contains more phospholipids and fewer steroids, as its volume is more frequently altered to adapt to physiological needs and environmental changes. The internal fluid is called cell sap. This is a highly heterogeneous organelle. Its size, shape, number, and even the color vary greatly each plant cell.
Central Large Vacuole: Water and Nutrient Pool of Terrestrial Plants
It usually occupies 30% of cell volume but fills 90% when saturated with water. It acts like a reservoir filled with water and nutrients. Cytoplasm and other organelles are compressed into a thin layer close to the cell wall that ensures it doesn’t absorb excessive water and burst. Turgor pressure makes stems and leaves appear upright. Water is released to sustain life activities in drought. Especially in desert cacti, their succulent stems also have a crucial task in storing carbon dioxide during CAM metabolism. The carbon dioxide absorbed at night is converted into malic acid and stored in the central vacuole. Although stomata are closed during the day, the malic acid is transported to chloroplasts to release carbon dioxide for photosynthesis.
They are also short-term reserves of energy and nutrients. Most amino acids are found here, so plant cells can continue to propagate for several generations even on nitrogen-deficient media. Long-term energy are stored in amyloplasts, proteinoplasts, and oleoplasts. Mature cells are rich in sucrose and fructose, and give them a strong sweet flavor, as seen in sugarcane stems and beet roots. Unripe fruits contain many organic acids that taste sour. Additionally, pigments give flowers and fruits vibrant colors to attract animals for pollination or seed dispersal. Some pigments change color based on pH, so they are often used as natural acid-base indicators.
Many plants have roots, stems, leaves, and fruits that taste unpleasantly bitter due to alkaloids which interfere with animal nervous systems to result in muscle paralysis or even death. This is one of the strategies plants use to defend against herbivores. Some less toxic alkaloids are used as commodities or medicines. Tobacco, coffee, cocoa, and poppies are all common commodities that are pleasurable and addictive. One of the extractions from cinchona tree bark is quinine, a valuable remedy for malaria in ancient Europe.
Large Central Vacuoles: Regulators of Osmotic Pressure
Simple diffusion and facilitated transport through water channels seem to be the way water molecules enter plant roots. This is indeed the case in moist ecosystems, but water spontaneously flows out in arid and saline soils. Active transport is the correct method in hypertonic environments. Cells don’t grab water molecules and pull them inward directly. Instead, they consume ATP to move potassium and sodium minerals from soil into their central vacuoles to create a high concentration. The hypertonic internal environment causes water molecules to be "passively" absorbed; in reality, this process is cotransport or secondary active transport. In addition to minerals, plants also inject oligosaccharides, amino acids, and organic acids to further increase the concentration. Sometimes salts in here become so concentrated that you may see crystallization whose most common component is calcium oxalate.
Transpiration and gas exchange are regulated by ion concentrations and moisture. When a plant needs to open its stomata, guard cells absorb potassium and chloride ions to condense its cell sap. They absorb water and swell to open stomata. Closing stomata is the reverse process. They shrink or even split into many small vacuoles.
Large Central Vacuoles as Digestive Organs in Plant Cells
In plants, they almost replace all the functions of lysosomes. Vacuoles contain various acid hydrolases to break down proteins, nucleic acids, lipids, and polysaccharides. Chitinases are absent in most animal lysosomes. When fungi invade, plants synthesize and accumulate chitinases to combat these intruders. Chitinases break down chitin in the fungal cell walls, and destroy the fungi directly. The vacuole membrane can also invaginate to engulf cytoplasmic material. For larger substances, a biological membrane originating from the ER wraps aging or damaged organelles to form a double-membrane vesicle that fuses with central vacuole. Autophagy is very active in plants preparing for winter. Ice crystals will easily pierce cells, so plants engulf some cytoplasm, and break it down into monosaccharides and amino acids for condensed cell sap and the lowered freezing point. When an entire cell ages or is too damaged to repair, autolysis is triggered. The membrane disintegrates to release internal hydrolases to dissolve the whole cell.