Eukaryotic cells are more developed cells than prokaryotic cells. These are more developed and usually enveloped. Eukaryotic cells include fungi, animal, and plant cells.
Plant cells are the basic units of life for plants. These cells are responsible for plant growth, reproduction, and overall life. Like all eukaryotic cells, plant cells also consist of various parts like cell membranes, cell walls, plastids, nuclei, and vacuoles.
Various plant cell types are based on their role, including parenchymal, sclerenchymal, collenchymal, and reproductive plant cells.
Table of Contents
Parts of Plant Cells
Like animal cells, plant cell consist of cell membrane, cytoplasm, nucleus, vacuoles, cell organelles like endoplasmic reticulum, ribosomes, and mitochondria. Besides these parts, plant cells have cell organelles like plastids and cell walls.
Cell Wall
A plant cell consists of a rigid part called a cell wall. The properties of the cell wall are as follows:
- The cell wall is most plant cells’ outermost, dead, and rigid part.
- The structure mainly comprises carbohydrates like pectin, hemicellulose, cellulose, lignin and fatty substances like waxes.
- The ultrastructure of the cell wall has a microfibrillar network that lies in a gel-like matrix. Cellulose mainly forms these microfibrils.
- Adjacent cells connect to each other with a pectin-rich cementing substance called middle lamella.
- The cell wall formed right after cell division constitutes the primary cell wall. However, the primary cell wall only covers many kinds of plant cells. The primary cell wall comprises pectin hemicellulose and a loose network of cellulose microfibrils.
- Some plant cells, like phloem and xylem, have an additional layer in the inner surface of the primary cell wall at a later stage. This layer is called the secondary cell wall and comprises mainly cellulose, lignin, and hemicellulose.
- Many plant cells have tunnels passing through the cell wall called plasmodesmata that help communicate with other cells in a tissue.
- The cell wall has an exoskeleton that provides protection and mechanical support to the plant cell. It also determines the shape of plant cells and prevents the desiccation of cell walls.
Cell Membrane
In plant cells, a cell membrane or plasma membrane is a membrane that occurs just inside the cell wall and binds with the cytoplasm. The properties of the cell membrane are as follows:
- This membrane has a tri-laminar (three-layered) structure with a translucent layer sandwiched between two dark layers.
- At the molecular level, this membrane has a continuous bilayer of lipid molecules (phospholipids and cholesterol) with protein embedded in the membrane or attached to both surfaces.
- Some carbohydrate molecules can also adhere to the external surface of the plasma membrane to form either glycoproteins or glycolipids.
- This membrane is selectively permeable, which helps control materials’ selective entrance and exit. This allows for maintaining a constant internal environment (homeostasis).
- Osmosis, diffusion, and active transport help transport small molecules like water, oxygen, carbon dioxide, ions, glucose, etc. Through endocytosis and exocytosis transportation of large-sized molecules occurs.
Cytoplasm
The cytoplasm follows the cell membrane, which has cytosol and cytoplasmic structures.
Cytosol
It is a colloidal organic fluid and a part of the cytoplasm. The cytosol is also called a matrix and forms the aqueous portion of the cytoplasm and the nucleoplasm. It fills the cell’s spaces and constitutes its authentic internal milieu.
Various small molecules required for cellular metabolisms, like glucose, amino acids, vitamins, nucleotides, vitamins, minerals, oxygen, and ions, suspend in the cytosol. It also consists of soluble proteins and enzymes, which form 20-25% of the total protein content of the cell. The soluble enzymes in the cytosol is essential in glycolysis and activation of amino acids for protein synthesis.
Cytosol divides into (i) Ectoplasm and (ii) Endoplasm. The ectoplasm, or cell cortex, is the peripheral layer of cytosol, which is relatively non-granular, dense, clear, and rigid. Endoplasm is the inner portion of the granular and less dense cytosol.
The cytosol also consists of fibers, collectively called cytoskeleton, that help maintain cell shape and mobility, providing anchoring points for the other cellular structures. Three classes of the cytoskeleton are present in the cytosol: microtubules, microfilaments, and intermediate filaments.
The cytoplasm fills with a three-dimensional network of interlinked filaments of cytoskeletal fibers called micro trabecular lattices. This lattice forms an anchor of various cell organelles. This lattice is also flexible and responsible for shape changes during cell movements.
Cytoplasmic structures
The cytoplasmic matrix suspends non-living and living structures. The non-living systems are called cytoplasmic inclusions and include oil drops, triacylglycerols, and starch grains. The residing facilities present in the cytoplasm are the cytoplasmic organelles like Golgi bodies, mitochondria, endoplasmic reticulum, and lysosomes.
Nucleus
The nucleus is the centrally located and spherical cellular component that controls all the activities of the cytoplasm. It also carries the hereditary material, DNA. The nucleus has three structures: Chromatin, nuclear envelope, and nucleolus.
Chromatin
Chromatin is the heart of every eukaryotic cell and contains genes, the hereditary units of the cell. Genes on the chromosomes exist as chromatin networks in the non-dividing cell. Chromatin comes in two forms: euchromatin and heterochromatin.
Euchromatin is the well-dispersed form of chromatin that takes a lighter DNA stain and is genetically active. Heterochromatin is the highly condensed form of chromatin that takes a dark DNA stain and is genetically inert.
Chemically, chromatin consists of a single DNA molecule, an equal amount of five basic types of histone proteins, some RNA molecules, and a variable amount of different types of acidic proteins. The chromatin has its unit structures in the form of nucleosomes. The chromatin binds to the inner part of the nuclear lamina, a 50-80 nm thick fibrous lamina lining the inner side of the nuclear envelope.
Nuclear lamina is formed of three types of proteins, namely lamin A, B, and C. Lamin proteins are homologous in structure to intermediate filament proteins. The lamins anchor parts of interphase chromatin to the nuclear membrane and interfere with chromatin condensation during the interphase of the cell cycle. These also help in the assembly of interphase nuclei after each mitosis.
Nuclear envelope and nucleoplasm
The nuclear envelope consists of two nuclear membranes: an inner nuclear membrane lined by nuclear lamina and an outer nuclear membrane continuous with rough ER. Some areas of the nuclear envelope are interrupted by structures called pares or nucleopores.
The nuclear envelope binds the nucleoplasm, which is rich in the molecules needed for DNA replication, transcription, regulation of gene actions, and processing of various types of newly transcribed RNA molecules.
Nucleolus
This nucleus structure is a darkly stained circular suborganelle called the nucleolus. This structure lacks any limiting membrane and is formed during interphase by the nucleolar organizer’s ribosomal DNA (rDNA).
Ribosomes are manufactured in nucleolus. Here, the ribosomal DNA transcribes most of the rRNA molecules, which undergo processing before their step-wise addition to 70 types of ribosomal proteins to form the ribosomal sub-units.
Vacuole
The cytoplasm of plant cells consists of numerous small or large-sized, hollow, liquid-filled structures called the vacuoles. The vacuoles are supposed to be significantly expanded endoplasmic reticulum or Golgi apparatus.
The plant cell vacuoles are bounded by a single, semipermeable membrane known as a tonoplast. The vacuoles contain water, phenol, flavonols, anthocyanins (blue and red pigment), alkaloids, and storage products like sugars and proteins.
Cell organelles
There are different cell organelles residing in the cytoplasm of plant cells. In this section, we will understand the cell organelles present in the plant cell and their brief description.
Endoplasmic Reticulum (ER)
With the cytoplasm of most animal cells is an extensive network of membrane-limited channels called endoplasmic reticulum (ER). Some ER membranes remain continuous with the plasma membrane and the nuclear envelope.
The outer surface of rough ER has attached ribosomes, whereas smooth ER does not have attached ribosomes. The function of smooth ER is lipid metabolism, glycogenolysis, and drug detoxification.
The membranes of rough ER consist of specific ribosome-specific, transmembrane glycoproteins, called ribophorins I and II, which are attached to the ribosomes while engaged in polypeptide synthesis. As a growing secretory polypeptide emerges from the ribosome, it passes through the RER membrane and accumulates in the RER lumen.
RER pinches off specific tiny protein-filled vesicles, ultimately fusing to cis Golgi. RER also synthesizes membrane proteins and glycoproteins co-translationally inserted into the RER membranes. So, ER is the site for the biogenesis of cellular membranes.
Ribosomes
Ribosomes are tiny spheroidal dense particles that contain approximately equal amounts of RNA and proteins. These are found in all cells and serve as scaffolds for the ordered interaction of the numerous molecules involved in protein synthesis.
Ribosome granules may exist in either a free state in the cytosol or attached to RER. Ribosomes have a sedimentation coefficient of about 80S, composed of two subunits, namely 40S and 60S.
The smaller 40S ribosomal subunit is ellipsoid in shape and has one molecule of 18S ribosomal RNA and 30 proteins (S1, S2, S3, and so on). The larger 60S ribosomal subunit is round and contains a channel through which the growing polypeptide chain exits. It has three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA, and 5S rRNA, and 40 proteins (L1, L2, L3, and so on).
Golgi bodies
These are cup-shaped organelles located near the nucleus in many types of cells. Golgi bodies, apparatus, or complex consists of smooth cisternae. The cisternae are closed, fluid-filled, flattened membranous sacs or cysts. The Golgi bodies are stacked together in parallel rows.
These bodies are surrounded by spherical membrane-bound vesicles which appear to transport proteins to and from them. Golgi bodies have at least three distinct classes of cisternae: cis-Golgi, median Golgi, and trans-Golgi.
Each cisterna has different types of enzymatic activities. The synthesized proteins appear to move in the following direction: RER → cis Golgi → median Golgi → trans-Golgi → secretory vesicles/lysosomes or peroxisomes.
The size and number of Golgi bodies in a cell show the active metabolic, mainly synthetic, state of that cell. The plant cell contains many freely disturbed sub-units of Golgi apparatus called dictyosomes, secreting cellulose and pectin for cell wall formation during cell division.
Golgi bodies perform the following essential functions:
- Packaging of secretory materials that are to be discharged from the cell.
- Processing of proteins, i.e., glycosylation, phosphorylation, sulphation, and selective proteolysis.
- Synthesis of specific polysaccharides and glycolipids,
- Sorting of proteins destined for various locations in the cell.
- Proliferation of membranous element for the plasma membrane.
- Formation of the acrosome of the spermatozoa.
Mitochondria
These are oxygen-consuming ribbon-shaped cellular organelles of immense importance. Two unit membranes bound each mitochondrion. The outer mitochondrial membrane resembles the plasma membrane in structure and chemical composition.
It consists of porins, proteins that render the membrane permeable to molecules having molecular weight as high as 10,000. The inner mitochondrial membrane has many coenzymes, enzymes, and other electron transport chain components.
The inner membrane also consists of proton pumps and many permease proteins to transport various molecules such as citrates, ADP, phosphate, and ATP. It gives finger-like outgrowths (cristae) facing towards the lumen of the mitochondrion and contains tennis racket-shaped F1 particles, which consist of ATPase enzymes for ATP synthesis.
The mitochondrial matrix is the liquid area encircled by the inner membrane. It contains the soluble enzymes of Kreb’s cycle, which completely oxidize the acetyl-CoA to produce CO2, H2O, and hydrogen ions.
Hydrogen ions condense the molecules of NAD and FAD, and both pass on hydrogen ions to the oxidative phosphorylation, generating energy-rich ATP molecules.
Since mitochondria act as the powerhouses of cells, they are abundantly present on those sites where energy is earnestly required. Mitochondria also consists of single or double circular and double-stranded DNA molecules called mtDNA and the 55S ribosomes called mitoribosomes. Since mitochondria can synthesize 10% of their proteins in their protein-synthetic machinery, they are considered semi-autonomous organelles.
Plastids
Palstids are present only in plant cells and contain pigments. Plastids may synthesize and accumulate various substances.
Plastids are of 3 types depending on the primary pigment it contains.
- Chloroplasts: Photosynthesis occurs in this plastid. It is oval and is surrounded by a double membrane. Within the membrane, a sac-like structure called thylakoids contains green-colored pigment chlorophyll. Other secondary pigments, carotenoids, and xanthophylls are also in the thylakoid. The stroma of chloroplasts consists of DNA, RNA, oil droplets, ribosomes, and other materials like scratch grains.
- Leucoplast: Non-pigmented plastids containing storage products like protein bodies, starch grains, and oils. A leucoplast that contains starch grains is called amyloplast.
- Chromoplast: This colorful plastid has red, yellow, and orange pigments. It is usually present in the fruits and flowers of the plant.
References
- Glimn-Lacy, J., & Kaufman, P. (2006). Botany illustrated (2nd ed., pp. 1-3). Springer.
- ESSENTIAL PLANT BIOLOGY. Myerscough.ac.uk. (2013). Retrieved 9 May 2022, from https://www.myerscough.ac.uk/media/3751/plant-cells-and-anatomy-session-booklet.pdf
