Sentence 1: You have earlier observed cells in an onion peel and/or human cheek cells under the microscope. Let us recollect their structure.
Recall that fascinating glimpse into the cellular realm you might have had through a microscope. By revisiting the structure of onion (plant) and cheek (animal) cells, we can establish a foundation for understanding the diverse components that make up a cell. Imagine a tiny factory – the cell wall in a plant cell acts like a sturdy outer shell, providing support and protection, similar to the walls of a factory building [1]. Animal cells, on the other hand, have a more flexible plasma membrane as their outermost boundary, allowing for processes like nutrient intake and waste removal.
Sentence 2: The onion cell which is a typical plant cell, has a distinct cell wall as its outer boundary and just within it is the cell membrane. The cells of the human cheek have an outer membrane as the delimiting structure of the cell.
Here, we delve deeper into the distinction between plant and animal cells based on their outer boundaries. Plant cells have a rigid cell wall, typically composed of cellulose, a complex sugar that provides structural support – like the scaffolding that holds up a building [1]. Animal cells, lacking a cell wall, rely on the plasma membrane, a thin, flexible phospholipid bilayer that controls what enters and leaves the cell, acting as a selective gatekeeper.
Sentence 3: Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA. Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic.
Now, we venture into the cell’s interior, where we encounter the nucleus, the control center. This membrane-bound structure safeguards the genetic material, deoxyribonucleic acid (DNA), which holds the blueprints for an organism’s traits [2]. Think of the nucleus as the CEO’s office within the cellular factory, housing the crucial instructions for every cellular operation. Based on the presence or absence of a well-defined nucleus, cells are classified as:
- Eukaryotic: These cells have a distinct nucleus, like the cells in your cheek. Examples include animal cells, plant cells, and protists (single-celled organisms like amoeba).
- Prokaryotic: These cells lack a membrane-bound nucleus, and their genetic material floats freely within the cytoplasm. Bacteria are a prime example of prokaryotic cells.
Sentence 4: In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’.
Imagine the cytoplasm as the bustling factory floor. It’s a jelly-like substance filling the cell’s interior and serves as the platform for numerous cellular activities. Here, essential processes like protein synthesis (building blocks for cellular structures) and energy production occur, keeping the cell alive and functioning – like the various departments within a factory working together to create a product.
Sentence 5: Besides the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, microbodies and vacuoles. The prokaryotic cells lack such membrane bound organelles.
Eukaryotic cells possess specialized compartments called organelles, each with a specific function. These membrane-bound structures act like mini-factories within the larger cellular factory, enhancing efficiency. Here’s a breakdown of some key organelles:
- Endoplasmic reticulum (ER): A network of membranes involved in protein and lipid (fat) production, resembling the assembly lines within a factory.
- Golgi complex: Responsible for processing, modifying, and packaging molecules for transport within or outside the cell, functioning like a quality control and shipping department.
- Lysosomes: Break down waste materials and foreign invaders within the cell, acting as the cellular cleanup crew.
- Mitochondria: The “powerhouse of the cell,” responsible for cellular respiration, the process of generating energy for cellular functions, similar to the power plant of a factory.
- Microbodies: Perform specialized metabolic tasks, such as breaking down toxins, acting like specialized workshops within the factory.
- Vacuoles: Storage sacs for water, nutrients, or waste products (more prominent in plant cells), functioning like warehouses within the factory.
Sentence 6: Ribosomes are non-membrane bound organelles found in all cells – both eukaryotic as well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER.
This introduces ribosomes, the protein-making factories of the cell. Unlike most organelles, ribosomes are not membrane-bound. These tiny structures are essential for all living things, as they translate the genetic instructions from DNA into proteins, the workhorses of the cell. Ribosomes can be found free-floating in the cytoplasm or attached to the rough endoplasmic reticulum (RER) in eukaryotic cells. Additionally, plant cells have chloroplasts, specialized organelles containing chlorophyll, a pigment that captures sunlight for photosynthesis (the process of converting light energy into food).
Sentence 7: Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.
This mentions centrosomes, structures unique to animal cells. These play a crucial role in organizing cell division, ensuring the accurate separation of chromosomes during cell reproduction. Imagine centrosomes as the coordinators during a factory expansion, ensuring an orderly division of equipment and resources between the new and existing facilities.
Sentence 8: Cells differ greatly in size, shape and activities (Figure 8.1). For example, Mycoplasmas, the smallest cells, are only 0.3 µm in length while bacteria could be 3 to 5 µm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter.
Cells exhibit a remarkable diversity in size, shape, and function. Mycoplasmas, a type of bacteria, are some of the smallest cells, measuring a mere 0.3 micrometers (µm) in length – that’s thousands of times smaller than the width of a human hair! In contrast, the egg of an ostrich holds the record for the largest isolated single cell, reaching up to 17 centimeters (cm) in diameter. Even within multicellular organisms, cells can vary significantly. For instance, human red blood cells are tiny, disc-shaped cells about 7.0 µm in diameter, specialized for oxygen transport, while nerve cells can be incredibly long and thin, extending over a meter in length to transmit signals throughout the body. The shape of a cell often reflects its specific function.
Sentence 9: The four basic shapes of bacteria are bacillus (rod-like), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).
This delves deeper into the world of prokaryotes, specifically bacteria. Bacterial cells come in various shapes, with four main categories:
- Bacillus: Rod-shaped bacteria, common among many familiar bacteria like E. coli.
- Coccus: Spherical bacteria, such as Staphylococcus bacteria, some of which can cause infections.
- Vibrio: Comma-shaped bacteria, exemplified by Vibrio cholerae, the bacterium responsible for cholera.
- Spirillum: Spiral-shaped bacteria, including the corkscrew-shaped Spirochetes, some of which can cause syphilis.
Sentence 10: The organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions. All prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. In higher classes you will learn that this plasmid DNA is used to monitor bacterial transformation with foreign DNA.
This summarizes the key features of prokaryotic cells. Despite their diverse shapes and functions, they share a fundamental organization. Most prokaryotes have a cell wall for protection and a cell membrane for regulating entry and exit of materials. Unlike eukaryotes, they lack a defined nucleus, and their genetic material (DNA) resides freely within the cytoplasm. Additionally, some bacteria possess plasmids, small circular DNA molecules separate from the main chromosome. These plasmids can encode genes for beneficial traits, such as antibiotic resistance, giving certain bacteria an advantage in specific environments.