Plant Cell Definition
Plant cells are the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic cells, which have a true nucleus along with specialized structures called organelles that carry out different functions. Animals, fungi, and protists also have eukaryotic cells, while bacteria and archaea have simpler prokaryotic cells. Plant cells are differentiated from the cells of other organisms by their cell walls, chloroplasts, and central vacuole.
Type Of Plant Cell
- Prokaryotic Cell
- Eukaryotic Cell
Prokaryotic Cell Definition
The single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro = before; karyon– = nucleus).
Prokaryotic cells are cells that do not have a true nucleus or most other cell organelles. Organisms that have prokaryotic cells are unicellular and called prokaryotes. Bacteria and archaea are prokaryotes. Prokaryotic cells can be contrasted with eukaryotic cells, which are more complex. Eukaryotic cells have a nucleus surrounded by a nuclear membrane and also have other organelles that perform specific functions in the cell.
Cells fall into one of two broad categories: prokaryotic and eukaryotic. The single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro = before; karyon– = nucleus).
Components of Prokaryotic Cells
All cells share four common components:
(1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment;
(2) cytoplasm, consisting of a jelly-like region within the cell in which other cellular components are found;
(3) DNA, the genetic material of the cell; and
(4) ribosomes, particles that synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.
A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid.
Unlike Archaea and eukaryotes, bacteria have a cell wall made of peptidoglycan, comprised of sugars and amino acids, and many have a polysaccharide capsule (Figure 1). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion, while most pili are used to exchange genetic material during a type of reproduction called conjugation.
Examples of Prokaryotic Cells
Bacteria are single-celled microorganisms that are found nearly everywhere on Earth, and they are very diverse in their shapes and structures. There are about 5×1030 bacteria living on Earth, including in our own bodies; in the human gut, bacteria outnumber human cells 10:1.
The cell walls of bacteria contain peptidoglycan, a molecule made of sugars and amino acids that gives the cell wall its structure and is thicker in some bacteria than others. Bacteria contain certain structures unique to them as previously mentioned, such as the capsule, flagella, and pili. Most bacteria have just one chromosome that is circular, which can range from about 160,000 base pairs (bp) to 12,200,000 bp. They also contain plasmids, which are small circular pieces of DNA that replicate independently of the chromosome. Some bacteria can form endospores. These are tough, dormant structures that the bacteria can reduce themselves to under starvation conditions when not enough nutrients are available. They do not need nutrients and are resistant to extreme temperatures, UV rays, and chemicals. When environmental conditions become favorable again, the endospore can reactivate.(Plant Cell)
Archaea are similar in size and shape to bacteria, and they are also unicellular. Since bacteria and archaea are the two types of prokaryotes, this means that all prokaryotes are unicellular. Some archaea are found in extreme environments, such as hot springs, but they can be found in a variety of locations, such as soils, oceans, marshlands, and inside other organisms, including humans.
In nature, the relationship between form and function is apparent at all levels, including the level of the cell, and this will become clear as we explore eukaryotic cells. The principle “form follows function” is found in many contexts. It means that, in general, one can deduce the function of a structure by looking at its form, because the two are matched. For example, birds and fish have streamlined bodies that allow them to move quickly through the medium in which they live, be it air or water.(Plant Cell)
A eukaryotic cell is a cell that has a membrane-bound nucleus and other membrane-bound compartments or sacs, called organelles, which have specialized functions. The word eukaryotic means “true kernel” or “true nucleus,” alluding to the presence of the membrane-bound nucleus in these cells. The word “organelle” means “little organ,” and, as we learned earlier, organelles have specialized cellular functions, just as the organs of your body have specialized functions.
Eukaryotic Cell Cycle
The cell cycle is the life cycle of a cell. During this cycle, it grows and divides. Checkpoints exist between all stages so that proteins can determine whether the cell is ready to begin the next phase of the cycle.(Plant Cell)
Quiescence, also known as senescence or resting, is a phase in which the cell is not actively dividing. It is also known as Gap 0, or G0. This stage is considered the start of the cell cycle, although it is one that cells can reach and then stop dividing indefinitely, which ends the cell cycle. Liver, stomach, kidney cells, and neuron are all examples of cells that can reach this stage and remain in it for long periods of time. It can also occur when a cell’s DNA is damaged. However, most cells do not go into the G0 stage at all, and can divide indefinitely throughout the life of an organism.
In interphase, the cell grows and takes in nutrients in preparation for division. Interphase takes up about 90 percent of the cell cycle. It consists of three parts: Gap 1, Synthesis, and Gap 2.
- Gap 1 (G1) is also known as a growth phase. The cell gets larger and increases its stock of proteins, along with organelles such as the energy-producing mitochondria.
- Synthesis (S) is the phase in which DNA replicates. During synthesis, the chromosomes replicate so that each chromosome is made up of two sister chromatids. At the end of this phase, there is double the amount of DNA in the cell.
- Gap 2 (G2) is another growth phase. The cell becomes even larger in order to prepare for mitotic division.
Mitosis, or M phase, is when the cell begins to organize its duplicated DNA for separation into two daughter cells. The chromosomes separate so that one of each chromosome goes into each daughter cell. This results in the daughter cells having identical chromosomes to the parent cell. Mitosis itself is divided into prophase, metaphase, anaphase, and telophase, which mark various points in the DNA separation process. Mitosis is then followed by a process called cytokinesis, during which the cell separates its nuclei and other organelles in preparation for division and then physically divides into two cells.
Structure Of Eukaryotic Cell
Eukaryotic plant cell are developed and advanced form or cell which is similar to animal cell in several ways. However, these cells are bigger than the animal cells and have some added cell organelles.(Plant Cell)
Cell wall: It is the outermost layer which is present only in plant cell. It is slightly rigid and provides specific shape to the cell.
Plasma membrane: It lies below the cell wall and surrounds the entire cell. The thin layer acts as a protection layer.
Cytoplasm: It is the semi-fluid or jelly like structure which hold all cell organelles in place. It also prevents cells from being flaccid.
Ribosome: It is a complex molecule of RNA-protein which synthesizes protein in the cell.
Rough endoplasmic reticulum (ER): Endoplasmic reticulum which has ribosomes attached to its surface is known as rough ER. Its major function is to carry out the protein synthesizing process called translation.
Smooth endoplasmic reticulum (ER): Endoplasmic reticulum which lacks ribosome is called smooth endoplasmic reticulum. Basically, smooth ER’s function is to produce molecules according to the need of specific cell. In plant cell, smooth ER manufactures lipids or fatty acids.
Centrosome: It is round organelle present near to the nucleus. During mitosis, when the nuclear membrane breaks down, microtubules associated with centrosome interact with the chromosome and builds mitotic spindle.
Nuclear membrane: Nuclear membrane which is also called nucleolemma, nuclear envelope or karyotheca, is the covering membrane of nucleus. It is a bilayer of lipid which encases the genetic material.
Nucleus: Regarded as the brain of the cell, nucleus is the controlling unit. It controls every activity of the cell. Particularly, in plant cell, it is not located in the center; also it’s not towards the edge of the cell.
Nucleolus: It is the spherical body inside the nucleus. Its major function is to produce ribosomal subunits and to distribute them throughout the cell. These units combine together to form ribosomes which are responsible for producing protein.
Vacuole: It is an empty space or bubble found in the cytoplasm. It is much larger and is located more to the center in a eukaryotic plant cell. It helps in storing foods and other nutritional substances which support cell survival. It also stores waste products in order to prevent contamination of the entire cell. Usually, a single plant cell has single vacuole.
Mitochondria: Often referred to as the powerhouse of the cell, mitochondria are the cell organelles where biochemical respiration occurs. They have double layer of which the inner layer is folded inwards creating layers called cristae.
Golgi bodies: Golgi bodies are the composite of vestibules and folded membranes. It carries out various functions such storing and absorbing lipids, protein, and certain enzymes. Specifically in plant cells, Golgi bodies play vital role in synthesizing polysaccharides such as pectin, microfibrils of a-cellulose, hemicellulose, along with various mucilaginous products essential to form cell wall.
Chloroplast: It is colored pigment found in all plant cells and in algae too, which photosynthesis (the process of food for plant in presence of water, carbon dioxide gas and sunlight).
Amyloplast: It is non-pigmented organelles found only in certain plant cells. It synthesizes starch granules from polymerization of glucose and stores them. It can also revert the process to form glucose from starch granules in cases when plant is in need of energy.
Examples of Eukaryotic Cells
Plant cells are unique among eukaryotic cells for several reasons. They have reinforced, relatively thick cell walls that are made mostly of cellulose and help maintain structural support in the plant. Each plant cell has a large vacuole in the center that allows it to maintain turgor pressure, which is pressure from having a lot of water in the cell and helps keep the plant upright. Plant cells also contain organelles called chloroplasts which contain the molecule chlorophyll. This important molecule is used in the process of photosynthesis, which is when a plant makes its own energy from sunlight, carbon dioxide, and water.(Plant Cell)
Like plant cells, fungal cells also have a cell wall, but their cell wall is made of chitin (the same substance found in insect exoskeletons). Some fungi have septa, which are holes that allow organelles and cytoplasm to pass between them. This makes the boundaries between different cells less clear.
Animal cells do not have cell walls. Instead, they have only a plasma membrane. The lack of a cell wall allows animal cells to form many different shapes, and allows for the processes of phagocytosis “cell eating” and pinocytosis “cell drinking” to occur. Animal cells differ from plant cells in that they do not have chloroplasts and have smaller vacuoles instead of a large central vacuole.
Protozoa are eukaryotic organisms that consist of a single cell. They can move around and eat, and they digest food in vacuoles. Some protozoa have many cilia, which are small “arms” that allow them to move around. Some also have a thin layer called a pellicle, which provides support to the cell membrane.(Plant Cell)