Plant Cell Organelles

Plants have all the typical features of eukaryotic cells: plasma membrane, cytoplasm, nucleus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and cytoskeleton.

Despite all these common components, plant and animal cells have some exclusive organelles that differentiate them:

• Animal cell: Lysosomes (organelles that digest macromolecules), and centrioles (cylinders of microtubules in the centrosome, involved in cellular division).
• Plant cell: Vacuoles (membrane-bounded vesicles with varied functions), plastids (organelles with diverse functions including photosynthesis), and cell wall (protective layer, covering the outside of the plasma membrane).

We will discuss the structure and function of vacuoles, plastids, and the cell wall. Technically, a cell wall is not an organelle, but we include it here as it is an important and distinctive structure in plant cells.

Vacuoles are abundant in plants and fungi, and have diverse functions. They are membranous sacs, similar to vesicles in structure, and sometimes these terms are used interchangeably. In general, vacuoles are larger (they are formed by the fusion of several vesicles) and can persist longer than vesicles. The bilayer membrane that delimits a vacuole is called the tonoplast. Vacuoles are mainly formed by the fusion of vesicles from the trans side of the Golgi apparatus (the one facing the plasma membrane) and are, therefore, part of the endomembrane system.

Depending on the tissue or organ, they will perform different functions and a cell can have several vacuoles with different functions:

• They perform most of the lysosome’s functions in plant and fungi cells. Thus, they contain hydrolytic enzymes.
• In mature plants cells, small vacuoles fuse to form a larger central vacuole. Plant cells grow mainly by adding water to this vacuole (comprising up to 80% of a cell’s volume). When the central vacuole is full, it exerts hydrostatic pressure against the cell wall. This pressure is important in plants, as it gives mechanical support to the cell when they are swollen or turgid. When you forget to water a plant, it becomes flaccid because there is no hydrostatic pressure against the wall. The central vacuole also serves as a reservoir of inorganic ions, maintaining the balance of pH in the cytoplasm.
• Storage of nutritious molecules in seeds and pigments in flowers. They can also store toxic or unpalatable compounds used against herbivores (animals that eat plants).
• Waste products and toxic compounds for the cell (like heavy metals absorbed from the soil) are also stored away by vacuoles.

Some protists form food vacuoles through phagocytosis, and others that live in freshwater have contractile vacuoles to expel the excess of water.

Plastids are a group of organelles that produce and store nutritious molecules and pigments (molecules that absorb visible light at specific waves) in plant and algae cells (Figure 2). They are present in the cytoplasm of different types of cells, surrounded by a double phospholipid bilayer membrane, and have their own DNA. They have specialized tasks depending on the cell function. They are very versatile and can change functions during cell life and some have specialized functions. We focus on three main groups of plastids:

• Chromoplasts produce and store carotenoid pigments (a range of yellow, orange, and red colors) that give flowers and fruits their characteristic color. Coloration in plants serves to attract pollinators.
• Leucoplasts lack pigments, thus, are more common in non-photosynthetic tissues. They store nutrients in cells of seeds, roots, and tubers. Amyloplasts convert glucose to starch for storage (Figure 2B). They are present mainly in specialized tissues of seeds, roots, tubers, and fruits. Proteinoplasts (or aleuroplasts) store proteins in seeds. Elaioplasts synthesize and store lipids.
• Chloroplasts perform photosynthesis, transferring energy from the sunlight into ATP molecules which are used to synthesize glucose. The inner membrane encloses numerous piles of interconnected fluid-filled membranous discs called thylakoids. Thylakoids contain several pigments incorporated into their membrane. Chlorophyll is the more abundant and the main pigment that captures the energy from sunlight (Figure 2A).

Figure 2: A) Photosynthetic cells containing numerous oval-shaped chloroplasts. B) Amyloplasts containing starch granules.

Plant cells, along with fungi and some protists cells, have an external cell wall covering their plasma membrane (Figure 3). This wall protects the cell, gives structural support, and maintains the shape of the cell, thus preventing excess water uptake. In plants, the wall is made up of polysaccharides and glycoproteins. The exact composition of the wall depends on the plant species and the type of cell, but the main component is the polysaccharide cellulose (made up of glucose forming long, straight chains of up to 500 molecules). Other polysaccharides found in cell walls are hemicellulose and pectin.

Structurally, the cell wall is composed of cellulose fibers and hemicellulose molecules embedded in a pectin matrix. The different types of plant cells can be identified by the characteristics of their cell wall.

Cell walls from adjacent cells are glued by another layer of pectin (sticky polysaccharides, like the ones we eat in jelly) called the middle lamella. The components of the wall can be replaced if degraded or during cell growth. In some cells, the wall can become completely rigid when its composition changes and the cell stops growing.

Figure 3. This diagram shows the basic parts of a typical cell wall.

The cell wall is responsible for the rigidity of plants and for keeping them upright. This results from the hydrostatic pressure from the central vacuole against the wall, as mentioned above. This is, in part, what gives them their crunchiness when we eat celery or a carrot, for example.

Plant cells still need to communicate with each other, even with a stiff cell wall. Channels called plasmodesmata allow direct communication between the cytoplasm of neighboring cells (Figure 4). The plasma membrane between neighboring cells is continuous along these channels, thus cells are not completely separated by their plasma membranes.

Figure 4. This diagram shows how a plasmodesma acts as a channel between two adjacent plant cells.