Cells At Work

We have all heard of cells. The building block of life that everyone is made out of. It does everything from transporting oxygen, to letting us see, to making our nails. But do we know how they actually work?

Cell Basics (Definition, Types)

There is a basic cell structure which most cells follow. Different groups also have different typical cells, such as animal cells, plant cells, bacterial cells.

• Prokaryotic cells are much smaller

• Plant cells have chloroplast and cell wall, while animal plants do not

• Humans have animal cells

All of us are made of carbohydrates, proteins, and lipids (fats), amongst other substances. Cells, too, mostly consist of these biomolecules. They also produce the various substances essential to our bodies: 

• (proteins) enzymes for digestion, collagen 

• (carbohydrate) glycogen as an energy reserve 

• (lipid hormone) testosterone for development of male characteristics 

Cells in Organisation of Living Organisations

Cell make up the whole organism in this manner:

• Cell: a cell

• Tissue: a group of specialised cells carrying out the same function

• Organ: a group of tissues carrying out the same function

• Organ system: a group of… I think you get it by now

• Organism: an individual

To illustrate:

Organism → organ system (circulatory system) → organ (heart) → tissue (cardiac muscle) → cell (muscle cell)

This organisation initially appears to only apply to animals. However, the roots, shoots, and leaves of a plant are all considered organs. Yet, sponges, which are animals, have no tissues since their specialised cells are not organised into groups.

Cell Structure

Now, let’s travel inside a cell! :D 

Cell membrane and cell wall (the doors and walls of a cell)

The cell membrane is made of lipids. We all know how oil and water don’t mix. Membranes utilise this to form a layer that water and other substances cannot pass through. It has proteins embedded in it, so that some substances such as ions can pass though. Larger molecules enter via endocytosis.

(The yellow part is the “oil,” and the red part is attached to the yellow part, preventing the “oil” from moving away.)

Cell walls form another layer on top of the cell membrane. 

• Animal cell: not present

• Plant cell: cellulose cell wall, gives plant the box like shape, lignin in some cell act as a “skeleton” to support the plant

• Fungal cell (mushrooms, yeast): chitin cell wall protects the fungi

Cytosol

The gel-like substance within the cell. It surrounds the organelles, which are the organs of the cells.

Organelles

Only eukaryotic cells have organelles. This includes plants, animals, but excludes prokaryotes like bacteria. They include the mitochondria, rough endoplasmic reticulum, etc.

Nucleus

The big round thing inside a cell. Sometimes called the control centre of the cell, it contains DNA, which folds to form one or many chromosomes. Our genes are small areas along the chromosomes. Transcription occurs to form mRNA, the transportable form of the genes. We don’t want the actual genes to go out as that is dangerous.

Ribosome 

mRNA is translated into protein by the ribosome. Free ribosomes produce proteins that stay in the cell, while bound ribosomes produce proteins that enter the endomembrane pathway and either stay on the cell membrane or are secreted out.

Endomembrane system

Bound ribosomes on the rough endoplasmic reticulum (RER) synthesise protein, and the smooth endoplasmic reticulum (SER) synthesise lipids. Both proteins and lipids are transported to the Golgi apparatus where they are further modified, then to the cell membrane, where they then leave the cell or are embedded into the cell membrane.

Mitochondria

The powerhouse of the cell. Most stages of respiration, which produces energy in the form of ATP, occur in the mitochondria. Glycolysis however, does not, so cells without mitochondria can still produce energy by glycolysis. This includes bacteria, and even our muscle cells when they run out of oxygen, although we cannot sustain this for very long.

Chloroplast

Photosynthesis occurs by using light energy to create food from carbon dioxide. RuBisCO, an essential enzyme in this process, sometimes binds to oxygen instead of carbon dioxide, making the “wrong” product. Most plants are C3 plants, which face this problem of inefficiency. However, some plants are C4 and CAM plants, which modify their photosynthesis to purposely concentrate carbon dioxide around RuBisCO, decreasing the likelihood of RuBisCO binding to oxygen.

Cells Differences

Different cells do different things, so it makes sense that they need different structures.

Name: alpha and beta cells

Location: Islets of Langerhans, Liver

They produce insulin and glucagon - protein hormones needed to regulate blood sugar - and thus have a more rough endoplasmic reticulum to secrete more protein out the cell

Name: red blood cells

Location: blood vessels

These cells are carriers of oxygen. They have haemoglobin that binds to oxygen reversibly, releasing it to other cells as they travel through the circulatory system. Adaptations include a biconcave shape for a high surface area to release more oxygen, and no organelles* to contain more haemoglobin.

*There exist nucleated red blood cells with organelles. They can fight against disease, and are common in non-mammals, and found in human umbilical cord blood.

A Cell as an Organism

Single-cells organisms exist. They are your prokaryotes: bacteria and archaea, but also your eukaryotes: various protists including some fungi, amoebas, dinoflagellates (some of which cause algae blooms), and many more.

Eukaryotes have a more complex cell structure than prokaryotes, because their organelles allow for more functions to be carried out. This is even more apparent in protists, who need more organelles as functions cannot be divided between cells. Well. Sometimes there are colonial or occasionally multicellular protists, whereby function is divided, and it allows us to consider how the transition from single celled organisms to complex multicellularity occurred (it occurred about 5 times), but I digress.

Contractile vacuole

Contractile vacuoles expand and contract to regulate the water and salt balance in the cell (osmoregulation). When too much water enters the cell, the cell can burst. The vacuole takes in water from the cytosol and expels it into the surroundings.

Not a mitochondria

The mitochondria is the powerhouse of the cell. Once, it was thought that all eukaryotes had mitochondria, later on it was thought that some don’t. Then mitochondria-related organelles were found in all of these cells that lacked mitochondria, and they carry out other functions, so it was believed that all eukaryotic cells had a mitochondria or MROs. In 2016, scientists discovered that the protist Monocercomonoides sp. had no MROs.

Conclusion

Cells are very diverse, carrying out many functions - and their structure corresponds to said function. 

References

Karnkowska, A. et al. (2016) ‘A Eukaryote without a MItochondrial Organelle’, Current Biology, 26(10). doi: https://doi.org/10.1016/j.cub.2016.03.053 

Katrina B. V, et al. (2023) ‘A conserved pressure-driven mechanism for regulating cytosolic osmolarity’ Current Biology, 33(16). doi: https://doi.org/10.1016/j.cub.2023.06.061

Chico V, Nombela I, Puente-Marín S, et al. (2019) ‘Nucleated Red Blood Cells Contribute to the Host Immune Response Against Pathogens’, Immune Response Activation and Immunomodulation, IntechOpen. doi: http://dx.doi.org/10.5772/intechopen.80545 

Kang, N. Y., Zhang, Y. (2021) ‘Revisiting the question of nucleated versus enucleated erythrocytes in birds and mammals’, American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 321(4) doi: https://doi.org/10.1152/ajpregu.00276.2020