What is a Cell?
The smallest functional unit of a living organism is known as the cell. It is composed of cytoplasm enclosed by an outer layer known as the cell membrane. A cell is classified as a prokaryotic cell and a eukaryotic cell. A prokaryotic cell does not contain a nucleus, and the genetic material is freely suspended in the cell cytoplasm; for example, bacteria are composed of a prokaryotic cell. Whereas a eukaryotic cell contains a nucleus and membrane-bound cell organelles with a specific function, plants and animal cells are examples of eukaryotic cells.
Plant Cells and Animal Cells
Plant cells are eukaryotic cells having a true nucleus and specialized structures known as organelles. Each organelle carries out specific functions. Plant cells vary in many factors from other eukaryotic cells. The most distinguishing feature of plant cells is the presence of a cell wall and chloroplast. In addition, plant cells are generally larger than animal cells.
The size of animal cells varies from microscopic to millimeters. Ostrich egg is the largest known animal cell. Animal cell shapes vary from flat, circular, cuboidal and columnar. A membrane-bound nucleus is present in animal cells. The organelle present in animal cells is almost similar to plant cells except for a few such as chloroplasts. Animal cells lack the cell wall.
The Various Organelles Present in the Plant Cell which Perform Specific Functions
The Various Organelles are as follows.
The cell membrane is semi-permeable in nature. It comprises protein and fat. It allows the movement of ions and solutes for the proper functioning of the cell.
The nucleus is present only in the eukaryotic cell. It is a membrane-bound structure. The nucleus regulates the activities of the cell. It contains the genetic material which encodes for proteins (both structural and regulatory proteins).
The chloroplast in the plant cell is disc-shaped and is enclosed by a phospholipid membrane. The chloroplast stroma is filled and contains circular DNA (Deoxyribonucleic acid). In chloroplast, chlorophyll is present, which absorbs photons from the sunlight and performs photosynthesis. Photosynthesis is a process where plants convert water and carbon dioxide into glucose and oxygen in the presence of sunlight.
The vacuole occupies a plant cell, 30% of cell volume. The vacuole is surrounded by a tonoplast and maintains turgor pressure in the cell. In addition, the cell sap is present in the vacuole. Vacuole is different from contractile vacuole. The function of the contractile vacuole is in osmoregulation; excess water is pumped out of the cell by osmoregulation. The contractile vacuole is found in protists and unicellular algae.
The cytoplasm is surrounded by the cell membrane and consists of water, protein, and salts. In the cytoplasm, all other organelles are present. The cytoplasm is the ground material of the cell. Various metabolic activities which help in cell survival occur in the cell cytoplasm.
In-plant cell endoplasmic reticulum (ER) is a single-membrane structure. The endoplasmic reticulum membrane encloses its lumen. The endoplasmic reticulum is of two types, smooth ER and rough ER. Smooth ER is responsible for the synthesis of carbohydrates and lipids. Rough Er has ribosomes present on its membrane and thus is involved in protein synthesis destined to different cell organelles, plasma membrane or outside the cell.
C.Benda coined the term mitochondria. Mitochondria are also known as the powerhouse of the cell. Aerobic respiration occurs in mitochondria. It is a double-membrane structure consisting of the outer membrane and inner membrane. The intermembrane space is the space between the outer and inner membrane. Mitochondria have their own DNA and ribosome. It is capable of synthesizing its proteins and is therefore known as a semiautonomous organelle. It helps synthesize adenosine triphosphate (ATP), which is known as the energy currency and is very important for cell survival and sustenance.
The outermost layer which surrounds the cell membrane is called the cell wall. It is composed of cellulose. The main function of the cell wall is to provide shape, support, and protection to the plant cell.
Structural Components of Plant Cell Wall
Different types of carbohydrates are present in the cell wall of the plant cells and are mainly classified into two major categories: microfibrillar polysaccharide and lignin.
- Microfibrillar Polysaccharides in plant cells are composed of cellulose. Cellulose is the polymer of glucose molecules. Cellulose microfibrils are formed by the parallel association of many cellulose chains. High tensile strength is provided by extensive non-covalent bonding in between the adjacent chains. The characteristic nature of cellulose is insolubility, chemical stability, and resistance towards chemical and enzymatic attack. An enzyme cellulase degrades cellulose. The synthesis of cellulose microfibrils is performed by a plasma membrane-bound enzyme complex known as cellulose synthase.
In a matrix consisting of protein and polysaccharides, cellulose microfibril is embedded in plant cells. Then, by plasma membrane-bound enzymes present in the Golgi complex, the synthesis of major polysaccharides occurs. Finally, the formed polysaccharide is delivered to the cell wall by the process known as exocytosis of small vesicles. Hemicellulose and pectin are two types of matrix polysaccharides. Hemicellulose is a highly branched and heterogeneous polysaccharide. It is attached with cellulose microfibrils with the help of a hydrogen bond. Pectin is a heterogeneous polysaccharide made up of acidic sugar such as galacturonic acid. Pectin forms a connection between plant cells and helps in adjusting pH and maintaining ion balance.
- Lignin is a highly branched phenolic polymer. It comprises three simple phenolic alcohols such as coniferyl alcohol, coumaryl alcohol, sinapyl alcohol. Lignin precursor is synthesized by phenylalanine and is secreted to the cell wall. Lignin protects the plant cell from pathogenic attack and also provides mechanical strength to the plant cell.
In the plant cell wall, structural proteins are also present along with the polysaccharides. These structural proteins are classified based on the composition of amino acids. These amino acids include hydroxyproline-rich glycoprotein, glycine-rich protein, arabinogalactan protein, and proline-rich protein.
Primary and Secondary Cell Walls
The plant cell wall is divided into two types: the primary cell wall and the secondary cell wall. At first, a primary cell wall is formed, which is thin and flexible. It is composed of 25% cellulose, 25% hemicellulose, 35% pectin. After the maturation of the cell, it stops growing. And strengthening of the wall occurs. The secretion of hardening substances performs the strengthening into the primary wall. Between the plasma membrane and primary wall, secondary cell wall formation occurs. As compared to the primary cell wall, the secondary cell wall is more complex in structure and composition. It is thick and layered. There are three layers of the secondary cell wall present in the wood. It is composed of 45% cellulose, 20-30% hemicellulose, and is impregnated with lignin.
The middle lamella is located at the junction where the primary cell walls of neighboring cells are in contact. Thus, it acts as cementing material. It is made up of pectin: mainly calcium and magnesium pectate.
Context and Applications
This topic is significant in the professional exams for both undergraduate and graduate courses, especially for
- Bachelors in Biology
- Bachelors in Zoology
- Bachelors in Botany
- Masters in Botany
- Masters in Life science
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