The cell is the morphological unit of all modern life forms, of which there are two major classifications: eukaryotes (Greek: eu, good or true, karyon, kernel or nut), which have a nucleus that encloses their DNA, and prokaryotes (Greek: pro, before), which do not have a nucleus. We will discuss the similarities and differences between these two cell types, as well as viruses, which are non-living protein particles. Further, this article will provide all the relevant facts for your medical education turning attention to a key theme in human physiology: the homeostasis.
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influenza viruses

Image : “influenza viruses ” by Medicalgraphics. License: CC BY-SA 3.0


Eukaryotic cells, whether unicellular or multicellular, are vastly more complex than prokaryotic cells. They are typically 10-100mm in diameter, and thus have a thousand to a million times more volume than prokaryotic cells. These cell types are best characterized by their membrane-enclosed organelles. The cytoplasm is a semifluid present inside the eukaryotes surrounded by the plasma membrane.

The nucleus contains chromatin, which is a complex of DNA and histone proteins, and the nucleolus (the site of ribosome synthesis). In addition to the nucleus that encodes genes, eukaryotes possess an endoplasmic reticulum, studded on its outer surface with ribosomes where the synthesis of many of their cellular components occurs. These components are further modified subsequently in the Golgi apparatus. Aerobic respiration takes place in the mitochondria or chloroplasts in photosynthetic cells.

Eukaryotes possess other organelles, such as lysosomes and peroxisomes, which function as the digestive system of the cell. Vacuoles occur more frequently in plant cells than in animal cells, where they serve as storage deposits of water and other materials. The cytosol comprises of the enzymes of various pathways, for example, those of glycolysis. Cytosol also contains the cytoskeleton, which is a microscopic network of protein filaments that serve to give the cell its shape, and the ability to migrate.

Cellular iron homeostasis

Image: “Diagram of cellular iron homeostasis. Generalized to include cell type-specific proteins.” by Neodop. License: CC BY-SA 4.0


Prokaryotes are the most numerous organisms on the planet. They are also the most widespread, due to their highly adaptive metabolisms that allow them to survive in a great array of different habitats.

Prokaryotes are simple single-celled organism that does not have nucleus and membrane-bound organelles, they are classified into bacteria and archaea. They have a relatively simple cell structure in comparison to eukaryotes. The vast majority of bacteria are unicellular, with a notable exception being that some can form colonies of independent cells or filaments. Prokaryotes range in size from 1-10nm, and morphologically, they display one of three basic shapes: spheroidal (cocci), rod-like (bacilli), and coiled (spirilla).


Eukaryotic vs. prokaryotic cells

Prokaryotic cells do not possess a cell membrane, however, they do have a cell wall. Inside of the cell, their metabolic functions occur regionally, without the localization (an important feature of eukaryotic cells). Their replication, transcription, and translation material is housed in the cytosol, and their DNA is circular rather than in the linear form that eukaryotic cells possess.

Plasma membrane, cytoplasm, DNA and ribosomes are key components of prokaryotic cells. The outermost covering that differentiates cell’s interior from the surrounding environment is termed as plasma membrane. The jelly-like cytosol inside the plasma membrane and outside the nucleoid is cytoplasm. DNA is the genetic constituent of the cell, is present in the nucleoid. The cellular structures responsible for the synthesis of proteins are termed as ribosomes.

Average prokaryote cell

Typical prokaryotic cell

The organelles that are present in eukaryotic cells give them a level of complexity that is absent in prokaryotic cells. Bacteria are vastly more efficient than eukaryotes in many aspects.

The bacterial flagellum is smaller and simpler in structure and is made up of a protein called flagellin. It is proton-gradient driven, capable of rotary movement and attachment to host cells and other surfaces. Rod-like structures named as pili are present in some of the bacterias to transfer DNA molecules to other bacterias and for locomotion.

This is different from eukaryotic flagella, which are larger, more complex in structure, and are comprised of proteins called tubulin, in an apparent 9+2 arrangement. The eukaryotic flagella are ATP-driven, which allows it to perform the bending movement. Archaea may also have most of these cell surface features, but their versions of a particular feature are typically different from those of bacteria.


Viruses are simpler than cells and are not considered as ‘living organisms’ because they lack the metabolic repertoire to reproduce without a host cell. They are small protein particles surrounded by a protein coat that instead replicate inside of the cells they infect. They range in size from 20-300 nm.

Viruses are not capable of movement, and they are incapable of replicating outside of a host cell. They also do not possess a metabolic pathway. Outside the host cell, they exist in a form known as a virion, which is comprised of DNA or RNA enclosed in a protein capsid, or in some cases enveloped in a matrix of lipids derived from the host cell’s membrane.


Image: “Basic structure of viruses. A. nonenveloped virus, B. enveloped virus. 1 – Capside, 2- Nucleic acid, 3 – Capsomer, 4 – Nucleocapsid, 5 – Virion, 6 – Envelope, 7 – Spike (envelope glycoproteins). Examples are the viruses showing icosahedral symmetry.” by Y_tambe. License: CC BY-SA 3.0

A virus is classified according to its genetic material into either a DNA or RNA virus, with RNA viruses forming the vast majority of this group. Their genomic material can also be either single-stranded or double-stranded in nature and are either linear or circular in nature. Some viruses have the unique ability to integrate DNA produced by reverse transcription into the host’s genome and are called retroviruses. An important example of retroviruses is the lentivirus, which causes human immunodeficiency virus (HIV).

Homeostasis Definition in Biology

Homeostasis reflects the ability of an organism to maintain a relatively stable metabolism and to function normally despite many constant changes. The changes that are part of normal metabolism may be internal or external, and the organism must respond appropriately.

This is accomplished by the use of control systems comprised of three parts, receptor, control center, and effector.

A receptor is a body structure that detects changes in a variable, which can either be a chemical or a process that is regulated. Receptors typically are comprised of nerve cells (e.g., skin receptors), that relay signals after experiencing a change in the variable, or stimulus (like touch or temperature). Another example of a receptor is the retina of your eye, which detects changes in the level of light (the stimulus) that enters the eye.

The control center is the structure that interprets input from the receptor and initiates changes through the effector. Think of the control center as the ‘go-between’ for the other two components of the homeostatic system. Most often, the control center is a portion of the nervous system or an endocrine gland.

Calcium regulation

Homeostatic mechanism: Calcium regulationImage: “Overview of calcium regulation” by Mikael Häggström. License: Public Domain

A homeostatic mechanism involving the nervous system provides a relatively quick means of responding to change. For example, your blood pressure must be regulated when you rise up from slumber in the morning. Your endocrine system equips you with a more sustained response, generally lasting several hours or even days through the release of hormones. For example, parathyroid hormone regulates your blood calcium levels in a process that is paramount to the function of your muscles and nerve cells.

Sometimes the receptor and control center is located in the same organ. A good example of this is your pancreas, which acts as a receptor when it detects an increase in blood glucose, and also acts as the control center when it releases insulin.

An effector is a structure that brings about changes to alter the stimulus. Most body structures can serve as effectors. The most common effectors are muscles and glands. For example, smooth muscles in the walls of bronchioles regulate airflow into and out of the lungs.

Most of the processes in your body are controlled by negative feedback. In this type of homeostatic control, the resulting action will always be in the opposite direction of the stimulus. In this way, the variable is maintained within a normal level, or what is called it’s set point.

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Homeostatic mechanisms

Homeostatic mechanisms modulate blood pressure, blood glucose levels, body temperature, body fluid composition, and heart rate. They enable the body to constantly be in a steady-state and to respond to ever-changing circumstances. Through homeostasis, for instance, the body returns the breathing pattern to normal after exercise.

Homeostasis Examples in the Human Body

An example of homeostasis in the human body is the release of insulin by the pancreas when the glucose levels get too high. The maintenance of blood pressure is another, already mentioned, one. A third common example is a human body retaining temperature of 98.6°F. The body can control it by making or releasing heat.

Example body temperature

A good way to think about negative feedback is maintaining your body temperature.

On a very cold day, a decrease in body temperature is detected by the sensory receptors of the skin, which send nerve impulses to the hypothalamus (a component of the brain). The hypothalamus makes a comparison between sensory inputs to your body’s temperature setpoint, and then it initiates motor output responses to the blood vessels in your skin to decrease the diameter.

This effectively decreases the amount of blood circulating to the surface of your body, and less heat would be released through the skin. Nerve impulses will also be sent to your skeletal muscles, which will cause shivering, as well as to the smooth muscle associated with hair follicles in the skin, causing ‘goosebumps’.

Temperature Regulation

Homeostasis example in the human body: Temperature regulation

On a hot day or when you exercise, an increase in body temperature is detected by both the skin’s sensory receptor and your hypothalamus. The hypothalamus will transmit motor input to the blood vessels in the skin to dilate, which effectively allows more blood to be brought to the surface of your body and heat will be released. Nerve impulses are also sent from the hypothalamus to your sweat glands to initiate sweating. Both of these responses will help to cool your body by the loss of heat from its surface. In these examples, regulation occurs through the nervous system.

Positive feedback mechanisms often work in loops, and the initial step of the pathway serves as the stimulus. The end product of the pathway stimulates the activity, instead of turning it off like in negative feedback.

An example of positive feedback occurs when a mother breastfeeds her child. The stimulus of her baby suckling promotes the release of oxytocin hormone, which will stimulate the breast glands to secrete milk. Thus, homeostasis of the body depends on the ability of our body to detect and oppose changes due to both external and internal causes.

Another example is that of regulating blood clotting following damage to a blood vessel. Following a break or a tear in a vessel, platelets soon begin to attach to the site of injury and release chemicals that attract even more platelets. Therefore, the recruitment of platelets ultimately causes the recruitment of even more and more platelets. The accumulation of all of these platelets begins the process of forming a clot in order to seal off the injury.

Homeostatic imbalance

The homeostatic imbalance is the inability to maintain homeostasis. It occurs when cells in the body experience a deficiency, e.g. due to exposure to toxins or other internal and external influences. Diseases that may result from this imbalance include heart failure, dehydration, diabetes, hypoglycemia, hyperglycemia.

Frequently ask questions about homeostasis

Do viruses maintain homeostasis?

Viruses do not maintain their own homeostasis, only living things do. They are not able to control their internal environment. Viruses cannot be thought of as living because they lack the metabolic repertoire to reproduce without a host cell.

Are viruses prokaryotic or eukaryotic?

Viruses are considered neither eukaryotes nor prokaryotes. They are simpler than cells and lack the characteristics of living things. They are small protein particles and are only able to replicate inside of the cells they infect.

Review Questions

The answers are below the references.

1. Which of the following structures are possessed by eukaryotic cells, but are absent in prokaryotes?

  1. Cytosol
  2. Cell membrane
  3. Cell wall
  4. Membrane-bound vesicles such as the nucleus, Golgi apparatus, and endoplasmic reticulum
  5. Ribosomes

2. How do viruses reproduce?

  1. They divide by mitosis.
  2. Sexually, by external fertilization
  3. Replication outside the host
  4. Inserting DNA into the host cell
  5. The divide my Meiosis

3. The two-body systems that regulate homeostasis are the:

  1. Cardiovascular skeletal systems
  2. Nervous and urinary systems
  3. Respiratory and endocrine systems
  4. Digestive and cardiovascular systems
  5. Nervous and endocrine systems
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