Cell Structure

Background

Cells represent the most basic biological units of all organisms, whether it be simple, single-celled organisms like bacteria, or large, multicellular organisms like elephants and giant redwood trees. In the mid 19^th century, the Cell Theory was proposed to define a cell, which states:

• Every living organism is made up of one or more cells.
• The cells are the functional units of all organisms.
• All cells arise from preexisting cells.

All cells share common features such as having a plasma membrane, a cytoplasm, DNA, and ribosomes. A plasma membrane is a phospholipid bilayer that surrounds the cell. This thin and fluid layer around the cells serves to isolate the cell’s contents from its environment and regulates the material exchange with its environment, while also facilitating interactions with other cells. Inside the plasma membrane, the cell is filled with a gel-like fluid called cytoplasm that contains organic molecules, salts, and other materials that are vital for the cell’s functions. Therefore, biochemical reactions that support life, which are known as metabolic processes, take place inside the cytoplasm. The types of metabolic processes that a cell can execute depend on its genetic information. All cells use DNA as the genetic material, which is the hereditary blueprint to construct cellular structures and products. Finally, all cells use ribosomes to synthesize their protein products.

There are two types of cells based on their genetic material location: prokaryotic, which means “before nucleus”, and eukaryotic, which means “true nucleus”. Therefore, while both types of organisms have DNA, prokaryotes like bacteria have nucleoids, or “nucleus-like” components instead of a nucleus, whereas eukaryotes possess true, membrane-bound nuclei to contain their DNA. Moreover, prokaryotes are relatively small, around 0.1–5.0 micrometers (µm), in comparison to eukaryotes that can typically range from 10-100 µm in size. The small size of prokaryotes allows quick and effortless distribution of materials within the cell and execution of metabolic processes, as well as swift removal of waste or other products from the cell. Hence, eukaryotic cells possess specialized structures known as organelles, such as mitochondria or Golgi apparatus, to enable the execution of vital functions.

The Eukaryotic Cell

The eukaryotic cell is a shared derived trait of all eukaryotes, meaning that it had a single origin that has since been inherited by all eukaryotes. The earliest eukaryotic cells are seen in fossils about 2.4 billion years ago, and are recognizable because they are larger than prokaryotic cells¹. The origin of this cell type resulted from an endosymbiotic event in which one amoeba-like cell engulfed a micrococcal bacteria and formed a stable coexistence². The engulfed bacteria evolved into the first energy-producing organelles, mitochondria, which are the aerobic metabolism organelles in the cell. Mitochondria have their own separate genome and are similar in size to prokaryotes. They contain two layers of membranes that enclose two distinct compartments. Some of the reactions that break down high-energy biomolecules occur in the inner compartment, whereas the outer compartment houses the reactions that capture the energy released from these compounds into adenosine triphosphate (ATP) molecules to be used as the energy currency of the cell.

Nuclei and mitochondria are not the only shared structures of eukaryotic cells. Other ubiquitous eukaryotic organelles are the smooth and rough endoplasmic reticulum (ER), Golgi apparatus, lysosomes and vacuoles. Endoplasmic reticulum simply means “network inside the plasma” and as its name indicates it is a large network of membranes inside the cell, especially around the nucleus. Parts of the rough ER extend from the nuclear membrane and are distinguished from the smooth ER by their rough appearance due to numerous ribosomes on their surface. Rough ER is the site for protein synthesis, such as the proteins that are embedded in the plasma membrane or the proteins that are secreted from the cell. In contrast, smooth ER produces lipid-based products but also contains enzymes to detoxify harmful chemicals. Hence liver cells contain abundant smooth ER. Also, muscle cells contain significant amounts of smooth ER because of the calcium-storing function of this organelle, which is essential for muscle contraction. The Golgi apparatus sorts, modifies, and packages cellular products inside vesicles, which fuse with the plasma membrane to release the products. Some of the proteins that are produced in the rough ER are intracellular digestive enzymes. These enzymes are packed in the Golgi apparatus into special vesicles called lysosomes. The major function of the lysosomes is to digest the food particles engulfed by the cell as well as old cell parts. Vacuoles are sacs of cell membrane that serve as storage units within cells. They may serve to store water to regulate the cell’s water content as well as store metabolic products or even poisonous molecules, depending on the cell type and the organism.

Kingdom-specific Organelles

Eukaryotic cells also developed distinct organelles, specific for each kingdom. For instance, kingdom Plantae and Animalia are both eukaryotic, however the organelles of plant and animal cells differ in key ways that allow them to carry out their lives as producers and consumers, respectively. Terrestrial plants need to grow tall and have rigid stems to hold leaves, which they use for photosynthesis. They also need to be able to retain water taken up by roots. Their cells reflect these specific needs. Unlike animal cells, plant cells have chloroplasts, which are used for photosynthesis and often contain the green pigment chlorophyll. Additionally, they are surrounded by cell walls, which are rigid outer layers made of cellulose to support growth and water retention. Because they need to store large amounts of water to maintain the water pressure in the cell, they have larger vacuoles than animal cells. In addition, plant cells also have another type of specialized storage organelles called plastids, which contain pigments as well as photosynthetic products such as starch. These differences are noticeable and distinguish plant cells from animal cells: plant cells generally have a regular, rectangular shape due to their rigid cell walls, while animal cells are rounded and more irregular.

Microscopy

Some cells, such as frog oocytes, are large enough to be seen with the naked eye, yet most cells cannot be seen without any visual aid. Therefore, scientists use microscopy techniques to study cell structures and distinguish cell types from one another. While microscopes are able to magnify objects that are difficult or impossible to be see with the human eye, most tissues naturally lack pigmentation. Therefore, solutions have been created that can selectively stain cells based on their molecular composition. This allows researchers to distinguish between organelles in a cell, tissue types in a plant stem, and fat layers in animals, just to name a few examples. Methylene blue dye stains nucleic acids of dead cells, binding to negatively charged DNA. Safranin solution is another biological dye that stains cell nuclei red. Cells only need to be in the staining solutions for a short period of time, and can be mounted immediately following the staining step. The commonly used mounting techniques are wet mount and oil immersion. A wet mount is created by collecting a sample and putting it on a glass slide with liquid between the slide and the coverslip. Cell samples are suspended in liquids like water or glycerol. Glycerol is better to use with live cultures, because it prevents the proliferation of bacteria³. Immersion oil can be added on top of the coverslip to improve viewing of the sample at high magnification. This is accomplished because the oil has the same refractive index as glass, which means that it allows light to pass through it as well as glass would. The glass-air interface scatters light more than oil or glass, so the clarity of the image is affected when samples are mounted “dry”, or without oil. Once the cells are stained and mounted, they are ready to be studied under the microscope.

There are various microscopy techniques from electron scanning technology that has allowed researchers to view objects on an atomic level to fluorescent live cell imaging that allows real-time monitoring of the movement of molecules within individual cells⁴. Bright-field microscopy is the simplest microscopy technique, requiring only a halogen source light, a condenser lens to focus the light, an ocular lens to view the image, and an objective lens to magnify the image. With any microscopy technique, it is important to understand the parts of a microscope before using one. In general, compound microscopes used for bright-field imaging have an eyepiece at the top of the scope, which is attached to the head and objectives. The eyepiece has a magnification of 10X, and objective lenses are set to a particular magnification in a range of 4X-100X. There are between three and five objectives on a standard microscope. The objectives point down to the stage, which is where a specimen is placed for viewing. The stage often has mechanical parts and stage clips to hold a slide and move it around while viewing. An aperture is a hole in the stage for light to pass through. This light is controlled by an adjustable condenser lens above an illuminator, or light source. To control the zoom of the stage for the object being viewed, microscopes feature coarse and fine focus adjustment knobs. The coarse focus knob moves on a larger scale than the fine focus, but they are on the same axis. The fine focus is useful when the object on the stage has been brought close to the objectives. It is important not to let the objective lens touch the object on the stage, as it can scratch the lens. Objects should always be first viewed on the lowest magnification objective and clearly focused before switching to higher magnification objectives.

Microscopy is an important tool for many aspects of the medical field including research, diagnosis, and treatment. This has the application of using nanotechnology in medicine, as a new treatment method in place of a more invasive surgery⁵. Surgeons also make use of microscopes, some of which have been modified to be mounted on a surgeon’s head and are operated with foot pedals. These are of much lower magnification than even the light microscopes used today, but they facilitate the safe execution of delicate procedures, such as optical and neurosurgery.

References

1. Bengtson S, Rasmussen B, Ivarsson M, Muhling J, Broman C, Marone F, Stampanoni M, Bekker A. Fungus-like mycelial fossils in 2.4-billion-year-old vesicular basalt. Nature Ecology & Evolution. 2017, Vol. 1, Article number: 0141.
2. Vellai T, Vida G. The origin of eukaryotes: the difference between prokaryotic and eukaryotic cells. Proc. R. Soc. Lond. B. 1999, Vol. 266, 1571-1577.
3. Gouet V, Roger G, Fonty C, Andre P. Effects of glycerol on the growth, adhesion, and cellulolytic activity of rumen cellulolytic bacteria and anaerobic fungi. Current Microbiology. 24, 1992, Vol. 4, 197-201.
4. Cognet L, Leduc C, Lounis B. Advances in live-cell single-particle tracking and dynamic super-resolution imaging. Curr Opin Chem Biol. 2014, Jun; 20:78-85.
5. Asiyanbola B, Soboyejo W. For the surgeon: an introduction to nanotechnology. J Surg Educ. 2008, Vol. 65, 2 (155-61).