General characteristics of the plant cell
From atoms to molecules and then to cells, living organisms have different characteristics.
The cell theory formed in 1838-1839 by Schleiden and Schwann establishes that all eukaryotic organisms are composed of cells, considered the smallest independent units of life.
The plant cell (Fig. 1) is eukaryotic, but how does it differ from the animal cell?
The difference is given by the cellular compartmentalization and the development of membranes and organelles, in plant cells, of meristematic origin. It is difficult to provide a single generalized description since each cell goes against differentiation and then specializes in organs and tissues. Furthermore, organisms such as algae and fungi have a different composition than other plant cells.
Let’s see in detail the common cellular components that characterize them.
The cell wall of the plant cell
It is divided into primary wall, secondary wall and middle lamella, that are the outer layer of the plasma membrane present in all plant cells.
The cell wall has different tasks based on cell differentiation. It is a structure with such rigidity as to regulate turgor pressures, regulates osmosis and cell growth, provides protection and resistance. It’s considered a metabolically active compartment, essential in the transport of water and solutes and cellular communications. It consists mainly of cellulose, hemicellulose, pectins and proteins.
Middle lamella is the outermost portion, composed of pectic substances.
The primary wall contains a living protoplast, it contains waterproofing and hydrophobic molecules.
Various additional layers form the secondary wall, often specialized post-mortem, lignified. Processes present in tracheas and vessels.
On the wall, we find the plasmodesmata, points of contact that join adjacent cells, therefore responsible for the movement of water and solutes.
Endomembrane systems in the plant cell
Plant cells contain several types of membranes; the main one is the plasma membrane or plasmalemma, which surrounds the cell.
The plasma membrane
Plasma membrane (Fig. 3) surrounds the cell and is composed of lipids, proteins, and carbohydrates; they are permeable to water and diffuse through the lipid component.
It is responsible for all exchanges between the cell and the external environment. It contains the cytoplasm, which has all the vital chemicals, including salts, ions, sugars, a large number of enzymes and proteins, and most of the RNA.
The nucleus (Fig. 4) contains almost all of the cell DNA. Its purpose is to contain nucleic acids, provide for DNA duplication, transcription, and RNA maturation.
The continuity between nucleus and cytoplasm is guaranteed by nuclear pores whose function is the active transport of molecules to and from the cytoplasm.
In the core can be distinguished: a double membrane, which contains a filamentous material, chromatin. Chromatin consists of proteins and nucleic acids and nucleoli, immersed in the nuclear substance.
Christian de Duve in 1965 classified peroxisomes (Fig. 5). They are structurally simple organelles with a granular matrix with the metabolic task of oxidation of substrates such as reactive oxygen species (ROS).
Ribosomes (Fig. 6) are particles immersed in the cytoplasm and are responsible for protein synthesis. They consist of 3 molecules of rRNA and proteins that associate to form two subunits. A plant cell has three types of ribosomes: cytoplasmic, mitochondrial, and plastidial. They are classified according to their sedimentation rate S.
They are organelles typical of eukaryotic cells. Metabolic activity is observed inside them, and they obtain energy through the process known as cellular respiration. The structure of mitochondria is the same in both plant and animal cells: they show a “bean” shape with a thickness of 1 µm and a variable length of 1-3 µm.
Mitochondria (Fig. 7) have two membranes, one internal and one external. The respiratory chain takes place in the inner membrane, consisting of a matrix containing enzymes and transport systems that conduct the flow of ions.
The mitochondrial crests have the role of amplifying the inner surface by allowing a greater number of enzymes and ions. Mitochondria also have the privilege (which they share with chloroplasts, present only in plant cells) of possessing a DNA molecule independent from the main one of the cell, present in the nucleus (mtDNA). The mtDNA molecule is found in the internal matrix.
The endoplasmic reticulum
The endoplasmic reticulum (ER, Fig. 8) is the largest and most adaptable compartment of eukaryotic cells. The ER is home to the synthesis and accumulation of some reserve proteins, lipids, and triglycerides. Formed by continuously moving tubules and cisterns, it forms a three-dimensional structure in the cytoplasm.
It is divided into rough RE (with ribosomes) and smooth RE. It plays a role in the biogenesis of various cellular components such as the vacuole and redox processes.
We can define the Golgi apparatus (Fig. 9) as a set of numerous stacks, consisting of a dozen superimposed cisterns (variable number based on cell differentiation), distributed in the cytoplasm. Through a tubular system, they sort the newly synthesized protein content at the level of the RE.
The vacuoles in the plant cell
Vacuoles (Fig. 10) are organelles surrounded by a membrane called tonoplast, consisting mainly of phospholipids and rich in proteins. The vacuole content is the “vacuolar juice” formed by various substances, solutes, and crystallines, with a pH of around 5.0.
They perform various functions such as cellular homeostasis, turgor, detoxifying processes, and reserve functions. It is also involved in both biotic and abiotic plant stress response.
They are characteristic organelles responsible for photosynthetic processes and metabolic activities. There are different types and forms:
Chloroplasts are organelles present in all eukaryotic autotrophic organisms, in which photosynthesis takes place.
Two phospholipid double layers form these organelles. The fluid inside the chloroplast is called the stroma; it contains many enzymes involved in the metabolism of the organelle, starch granules, circular DNA, and ribosomes.
The chloroplast contains grains, stacks of discoid membranous systems called thylakoids. With other defined intergranal or stomatal thylakoids, they create a closed system of membranes with a central area called the lumen. The space between the thylakoids is called partition, and the part in contact with the stroma is called the margin. The photochemical apparatus responsible for photosynthesis is present in the thickness of the thylakoid membrane.
We refer as leucoplasts (Fig. 12) to any colorless plastid, assigned to the task of storing nutrients. We can distinguish: elaioplasts (storing lipids), proteinoplasts (proteins), and amyloplasts (starch and carbohydrates).
Original article: Caratteristiche generali della cellula vegetale by Umberto Lazzaro
- Botanica generale e diversità vegetale G.Pasqua, G.Abbate, C.Forni et al.
- Botanica: fondamenti di biologia delle piante
Libro di James D. Mauseth