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Organ Systems

A multicellular organism may be divided into many levels of organisation. The smallest unit is the organelle, a specialised structure which performs a specific task within the cell, which is the next level of organisation. Cells then group together based on function into structures known as tissues, which are then grouped together into an organ, which performs a task. Organs often work together to provide a specific function and are grouped together into organ systems. Humans, animals and plants are all made of organ systems!

What is an organelle?

As described above, an organelle is a small structure within a cell which is designed to perform a specific function. They may be contained within a membrane, or simply be free-floating functional units within the cytoplasm. Some key examples of organelles are the nucleus, mitochondria and ribosomes present in our cells!

Check out the Animal and Plant Cells article to learn more about sub-cellular structures or organelles!

It is generally believed that some organelles, specifically the mitochondria and chloroplast, may have once been free-living organisms which were engulfed by an early cell, but instead of dying, they developed a symbiotic relationship with the cell. Over time they lost components not necessary in their new living arrangement, eventually becoming the organelles we know today. This theory is known as the endosymbiotic theory.

What is a cell?

The cell is the next largest unit of organization. Cells are small, membrane-enclosed spaces which contain organelles, which form the basic units from which larger structures are formed. They may either be the entire organism, as is the case with bacteria or amoebas (unicellular organisms), or they may be constituents of a larger multicellular organism, like humans.

In multicellular organisms, cells may be specialised in function. Some examples of this are muscle cells or nerve cells, each of which is highly specialised in terms of structure for their specific function. The conversion of non-specialised cells to specialised is referred to as differentiation. Cells of a similar type and function tend to group together, forming larger structures which are known as tissues.

Undifferentiated cells are known as stem cells. There are three main sub-types of stem cells: totipotent, pluripotent and multipotent, each being more limited in the type of cell it can become. Totipotent cells may become any type of cell within the body, including extra-embryonic tissue (placental cells). Pluripotent cells may become any type of cell within the body, excluding placental cells and multipotent stem cells may become several cell types, but not all.

What is a tissue?

The complex nature of eukaryotic organisms makes it difficult for a single cell alone to perform a function. Therefore, two or more cells with similar structures group together to perform a specific function are named a tissue. There are four main types of tissue:

  • Epithelial tissue: Epithelial tissues are formed of thin continuous layers of cells and line various internal and external surfaces within the body. The most visible example of epithelial tissue is the skin.

  • Connective tissue: As the name suggests connective tissue is any tissue that connects and supports other tissues. An example of connective tissue that might not be very obvious is blood, and a more common example is tendons.

  • Muscular tissue: Muscular tissue makes up the muscles that move our body and our heart! This includes the skeletal muscle, cardiac muscle and smooth muscle.

  • Nervous tissue: The nervous tissue transmits signals throughout the body and is comprised of neurons, the actual cells which transmit signals and neuroglia, cells which support the nervous system.

Eukaryotes or eukaryotic organisms are organisms with eukaryotic cells, meaning cells with membrane-bound organelles like a nucleus. Read more about this in our Eukaryotes and Prokaryotes article!

What is an Organ and an Organ System?

An organ refers to a group of tissues that come together to perform a specific function.

This allows for the formation of things like the pumps that make up our heart, or a tube that is capable of moving food like the small intestine. An organ system is a group of organs also working together to perform a specific function. The organ systems come together to form an organism. There are many organ systems in the human body.

What are the main organ systems in the human body and their functions?

The main organ systems in the human body are the nervous system, respiratory system, endocrine system, circulatory system, digestive system, muscular system, skeletal system, urinary system, lymphatic system, excretory system, integumentary system and reproductive systems.

  • Nervous system: The brain, spinal cord and nerves make up the nervous system. It controls all activities of the other systems.

  • Respiratory system: Starting from the nostrils to the lungs, the respiratory system controls our breathing.

  • Endocrine system: The endocrine system secretes hormones, that regulate activities in our bodies. It is made up of the glands like the ovary, testis, thymus and pancreas.

  • Circulatory system: The circulatory system is responsible for the transport of blood all around the body. It is made up of the heart and blood vessels.

  • Digestive system: The digestive system is responsible for the digestion of food substances.

  • Muscular system: The muscular system is responsible for the movement of the body using muscles.

  • Skeletal system: The skeletal system provides the body structure and support. It is made up of bones.

  • Urinary system: The urinary system is responsible for excreting metabolic waste and other substances out of the body in the form of urine. It is made up of the kidneys, ureter, bladder and urethra.

  • Lymphatic system: Made up of the red bone marrow, thymus, lymphatic vessels, thoracic duct, spleen and lymph nodes, the lymphatic system is responsible for protecting the body against infection as well as draining excess fluids from cells and tissues.

  • Integumentary system: The integumentary system is responsible for protecting the body from the external environment. It is made up of the skin, nails and hair.

  • Reproductive system: The reproductive system enables us to produce offspring. It is made up of the penis, testis, prostate gland and scrotum in males and the ovary, uterus, vagina, and fallopian tube in females.

Diagram of Human Organ Systems

Here is a diagram showing an overview of many of the main organ systems of the body discussed above.

Examples of Organ Systems

Two main systems of relevance, the digestive system and the circulatory system, are explored below, along with non-communicable diseases which often affect human organ systems.

Overview Of The Digestive System

The digestive system, like all organ systems, is formed of a variety of organs working together to achieve a certain function. In the case of the digestive system, it is to process and extract nutrients and water from the food and liquids we consume. It does this by breaking down large molecules into smaller molecules and then absorbing these small molecules into the body via diffusion, osmosis and active transport.

The organs that make up the digestive system are the organs of the digestive tract, a series of hollow organs, whose lumen is technically outside of the body! The digestive tract is comprised of the mouth, oesophagus, stomach, small intestine, large intestine and anus. These are supported by the liver, pancreas and gallbladder, which produce and store substances that support digestion. The various organs of the digestive system all coordinate their actions to work together and efficiently extract nutrients and water from the food and fluids consumed.

The mouth begins chemical digestion by secreting enzymes, as well as physically mashing up the food by chewing. The partially digested food then flows down the oesophagus into the stomach, where acid and enzymes continue to break it down. It then flows into the small intestine, where additional enzymes and substances are added by the pancreas and gall bladder to absorb nutrients. Finally, it travels through the large intestine where bacteria digest the last remnants and water is absorbed before the waste is released in faeces.

Read our article Human Digestive System to learn more about how all these organs contribute to digestion!

Overview Of The Circulatory System

The circulatory system is responsible for, as the name suggests, circulating blood around the body. It is comprised of the heart and blood vessels, along with the blood itself. It is responsible for feeding cells with nutrients and oxygen, as well as removing waste products. It also carries components of the immune system, regulates water in the body and, by way of the endocrine system, acts as a communication system within the body.

The heart, as you know, pumps blood around the body, through the blood vessels. These blood vessels consist of the arteries, veins and capillaries. Arteries carry high pressure, oxygenated blood away from the heart around the body. Veins carry deoxygenated, relatively low-pressure blood back to the heart. Capillaries bridge between smaller versions of the previous two types, known as arterioles and venules, and penetrate into tissues and organs. Capillaries are very small and have thin walls, making them the site of the majority of entry to and exit from the blood.

Read our article Circulatory System to learn more about how blood travels around the body!

Non-Communicable Diseases in Organ Systems

While the body's organ systems are affected by many infectious diseases, meaning diseases caused by microorganisms like bacteria or viruses, they can also suffer from diseases which are not caused by infectious pathogens. These are termed non-communicable diseases. Two of the main non-communicable diseases affecting humans are coronary heart disease and cancer, each of which has its own set of risk factors.

Coronary heart disease is a disease resulting from a build-up of fatty acids in the arteries supplying the heart with blood. It causes limited or no blood supply to areas of the heart, causing symptoms ranging from mild chest pain to death.

Cancer is a disease characterised by the uncontrolled division of cells within the body, sometimes forming a tumour, usually stemming from damage or mutation to the genes that control these processes within cells. A key characteristic of cancer is that the cells can spread around the body, whereas a benign tumour stems from the same division of cells but doesn't spread to new areas. The symptoms of cancer vary significantly and depend on the cells and tissues affected.

Risk Factors are anything which increases the likelihood of a disease occurring. Some examples are exposure to radiation or carcinogenic chemicals increasing the chance of cancer, or consumption of lots of fatty foods increases the risk of coronary heart disease.

Check out the articles Non-Communicable Diseases and Communicable Diseases to learn the differences between them!

Plant Organs

Just like humans, plants also have organ systems. They function in the same way as in any other organism, however, tend to be quite simpler. Plants have two organ systems, the root and shoot systems. The root system acts somewhat like a digestive system in humans, except instead of absorbing resources from consumed foods, it absorbs resources from the environment. The shoot system consists of stems and leaves, along with the reproductive organs of the plant.

Check out our article Plant Organs to learn more about these systems!

Organ Systems - Key takeaways

  • Organisms can be broken down into several organisation levels (organelles, cells, tissues, organs, organ systems)
  • Organ systems consist of several organs all working together to achieve a common purpose, such as the digestion and absorbance of substances from food and liquids consumed in the digestive system.
  • The key organ systems of the body are the: nervous system, respiratory system, endocrine system, circulatory system, digestive system, muscular system, skeletal system, Urinary system, lymphatic system, excretory system, integumentary system and reproductive system.
  • Organ systems may be impacted by communicable and non-communicable diseases.

Frequently Asked Questions about Organ Systems

An organ system is a group or organs working together to provide a certain function within the body. 

The digestive system contains the mouth, oesophagus, stomach, small intestine, large intestine and anus. it also contains the liver, pancreas and gallbladder.

The circulatory system is comprised of the heart, veins, arteries and blood. 

Five of the main organ systems within the body are the nervous, respiratory, endocrine, circulatory and digestive systems. 

Organ systems work together by each performing a key role to allow the organism as a whole, and by extension the entire organism, to survive. An example of this is the circulatory system providing nutrients to, and removing waste from, the other organ systems in the body.  

Final Organ Systems Quiz

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What are enzymes?

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Enzymes are biological catalysts. This means that they speed up chemical reactions in the body.

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What type of molecule are enzymes?

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Enzymes are proteins. This is important because being proteins allows them to have the specific 3D shape required for their active site.

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What is the lock and key theory?


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The lock and key theory suggest that enzymes and substrates are exactly specific to each other. It suggests that one type of substrate fits exactly into the active site of one particular enzyme.

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 What factors affect enzyme activity?


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Temperature, pH, enzyme concentration and substrate concentration all affect enzyme activity. Any of these factors could be a limiting factor.

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What is a limiting factor?


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A limiting factor is something that causes enzyme activity to plateau. Temperature, pH, enzyme concentration and substrate concentration can all act as limiting factors.

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What is pepsin?

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Pepsin is an enzyme that is found in the stomach. It has an optimum pH of around 2, meaning it can function in the stomach acid. Pepsin breaks down proteins into polypeptide chains.

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What is an active site?


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An active site is the part of the enzyme that the substrate binds to. This is a specific 3D shape, meaning that only one type of substrate can bind to a specific active site/enzyme.

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 What is an enzyme?


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Biological catalyst

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Which enzyme is found in saliva?


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Pepsin

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Which factor involves kinetic energy?


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pH

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What is the optimum pH for pepsin?


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2

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Where is amylase found?


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Stomach

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If I have 100 enzymes and 80 substrates, what is likely to be the limiting factor?

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Temperature

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What type of molecules are enzymes?


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Carbohydrates

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What happens when we go too far above the optimum pH of an enzyme?


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Enzyme activity decreases

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What is digestion?

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Digestion is the process of breaking down larger insoluble substances into smaller soluble substances

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Where does digestion start?

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Digestion starts in the mouth both mechanically and chemically. Mechanical digestion refers to the teeth and chemical digestion refers to the actions of the enzymes maltase and amylase.

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Where is amylase produced?


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Amylase is produced in both the salivary glands and the pancreas

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What are enzymes?


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Enzymes are biological catalysts. This means that they speed up chemical reactions in the body.

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What does amylase break starch into?

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Amylase breaks down starch into glucose.

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What are the two roles of bile?


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Bile emulsifies fats and neutralises the contents of the small intestine

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What are villi?


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Villi are small finger-like structures in the small intestine. Their role is to help increase the surface area of the small intestine for the absorption of nutrients

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What is absorption?


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Absorption is the movement of nutrients from the small intestine into the blood

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What is assimilation?


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Assimilation is the movement of nutrients from the blood into body cells that require them

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Where does most absorption take place?


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Most absorption takes place in the small intestine, more specifically in the ileum

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Where does digestion start?


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Mouth

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Where is amylase produced?


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Pancreas and stomach

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What are lipids broken down into?


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Fatty acids and amino acids

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Where does most absorption take place?


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Ileum

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What is ingestion?

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When food leaves the alimentary canal via the rectum

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What are carbohydrates broken down into?

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Carbohydrates are broken down into glucose and other small sugars

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What are proteins broken down into?

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Proteins are broken down into amino acids

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What are lipids broken down into?


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Lipids are broken down into glycerol and fatty acids

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What does qualitative mean? (In reference to a biomolecular test)


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It refers to tests in which results are decided on based on whether something is present or absent. Results are not given by counting or calculations.

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What is the test for proteins?


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The test for proteins is the Biuret solution. A positive test goes from blue to purple

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What is the test for carbohydrates?


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The test for carbohydrates is Benedict’s solution. A positive test goes from blue to brick red, with colours like green and orange also being observed.

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What two tests can we use for lipids?


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We can use the Sudan III test or the emulsion test to check for the presence of lipids in foods

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What test do we use for starch?


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To test for the presence of starch in food, we use the Iodine test. A positive test changes from yellow/brown to brown/black.

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What would a positive biuret test show?


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Blue

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What would a positive Benedict’s test show?


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White

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 What would a positive emulsion test show?


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Red

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What are proteins broken down into?


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Amino acids

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What are lipids broken down into?


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Amino acids


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Which of the following refers to the quality of something?

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Qualitative

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What would a negative iodine test show?

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Yellow/Brown

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_________ carry oxygenated blood away from the heart. The blood they carry also contains _________ and other important solutes that respiring cells require.

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Arteries, glucose

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_________ carry deoxygenated blood towards the heart. The blood they carry also contains _________ and other waste products and metabolites.

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Veins, urea

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What is the role of a capillary?


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Capillaries connect veins and arteries. They are the site of gas exchange and so are extremely thin and permeable?

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Which blood vessel contains valves?


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Veins contain valves. This is because veins carry blood at low pressure. The valves prevent backflow and ensure unidirectional blood flow.

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Which blood vessel has the most narrow lumen?


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Arteries

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