Animal Cell : Structure, function, types and defination

Animal Cell : Structure, function, types and defination

The cell is the basic unit of life. All organisms are made up of cells (or, in some cases, a single cell). Most cells are very small, being invisible without using a microscope. They are covered by a cell membrane and can have different shapes.

An animal cell is one of the most complicated things that we know of probably in the entire Universe. It has thousands of proteins and enzymes and somehow has a complete copy of our own, unique DNA inside of the nucleus of each cell. Remember, a single copy of the Human Genome is 3 billion, 200 million base pairs long.

Animal cell contains membrane bound nucleus, it also contains other membrane bound cellular organelles. These cellular organelles carry out specific functions that are necessary for the normal functioning of the cell. Animal cells lack cell wall, a large vacuole and plastids. Due to the lack of the cell wall, the shape and size of the animal cells are mostly irregular. Animal cells increase in size by increasing in cell numbers. Plant cells mainly increase cell size by becoming larger.

Defination of Animal Cell:

Animal cells are the basic Structural and functional unit of life of the Animals. Animal cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cells, DNA in animal cells is housed within the nucleus. They are also referred to as Eukaryotic cells which clearly means that they have a nucleus and a specialized structure called organelles that perform various functions.

Animal cell Shape : Animal cells come in various sizes and tend to have round or irregular shapes. Plant cells are more similar in size and are typically rectangular or cube shaped.

Animal Shape Size : Animal cells are generally smaller than plant cells. Animal cells range from 10 to 30 micrometers in length, while plant cells range from 10 and 100 micrometers in length.

Read : Somatic Cell Structure and Functions 

Types of Animal Cells

● Epithelial cells: They are those that form the superficial and support structures of the tissues. In other words, they are the ones that form the skin, tissues, glands and are responsible for covering the animal's organs.

● Conjunctive cells: Their main objective is to form a structure that can interconnect all the internal parts and keep them in their correct place. An example of these cells can be the bone cells that make up bones.

● Blood cells: They are those that allow each nutrient, molecule and vitamin that is necessary to maintain the life and health of the organism to travel through the entire circulatory system. In addition, they prevent some external agents that can be harmful from spreading throughout the body. Its main activity is linked to everything that is movement within the body.

● Nerve cells: Animals are living beings that are managed by different environments that make them always be on the alert, that is why nerve cells become one of the most important in any animal organism. These are responsible for processing information and making it travel throughout the body in the form of a nerve impulse.

● Muscle cells: These are the ones that form the fibers that are responsible for carrying out actions of contracting and relaxing, as needed. These cells work closely with those of the nervous system since it is from there that the order comes to either relax or contract.

Read : What is Cell 

Structure of  Animal Cells:

Animal organisms can be made up of trillions of cells. These cells come in all shapes and sizes and their structure adapts to their function. For example, the body's nerve cells, neurons, have a very different shape and function than red blood cells. Nerve cells conduct electrical impulses throughout the nervous system. They are elongated and thin, with projections that extend outward to communicate with other nerve cells in order to conduct and transmit nerve impulses. The main role of red blood cells is to transport oxygen to the cells of the body. Their small, flexible disc shape allows them to maneuver through small blood vessels to supply oxygen to organs and tissues. Organelles are smaller cell structures in the cytoplasm. Each has its specific function for the proper functioning of the cell and all act together. They are like the gears of a machine. See which are the organelles of the animal cell :

Components of Animal Cell 

1. Animal Cell membrane / Plasma membrane/ 
 biomembrane :
  1. Animal cell membrane is thin, quasifluid structure present both extracellularly and intracellularly. Extracellularly, it is present around protoplast and intracellularly, it is present around most of the cell organelles in eukaryotic cell. It separates cell organelles from cytosol. 
  2. Thickness of biomembrane is about 75A° . Under electron microscope, cell membrane appears trilaminar (made up of three layers). 
  3. It shows presence of lipids (mostly phospholipids) arranged in bilayer. Lipids posses one hydrophilic polar head and two hydrophobic non-polar tails. So, phospholipids are amphipathic. 
  4. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwitched in between heads. 
  5. Due to this, tails never come in direct contact with aqueous surrounding.
  6. Animal Cell membrane also shows presence of proteins and carbohydrates. Ratio of proteins and lipids varies in different cells. 
  7. For example-in human beings, RBCs show approximately 52% protein and 40% lipids. 
  8. Fluid mosaic model : It is universally accepted model of cell membrane. It was proposed by Singer and Nicholson in 1972. 
  9. According to this model, it is made up of phospholipid bilayer and proteins. Proteins are like icebergs in the sea of lipids. Proteins can change their position. Some proteins are intrinsic i.e. occur at different depths of bilayer.

Functions of Cell Membrane:
  1. Main function of cell membrane is transport. It controls the entry of selected substances. They are impermeable to other substances.
  2. Cell membrane also controls cell metabolism through selective permeability of cell membrane.
  3. Plasma membrane protects the cell from Machanical injuries.
  4. Different enzymes are located in the Biomembrane for performing different metabolic processes like Synthesis of ATP.
  5. Cell membrane contains different receptors for harmones, Antibodies, Antigens, Neurotransmitters etc.
  6. Cell membrane separates the cell from its external environment and cell organelles from cytosol.
  7. Cell membrane used in endocytosis, exocytosis, electron transport, impulse transmission, Cell junction, cell movements, Osmosis, Cell recognition etc.

2. Cytoplasm :
  1. The cell contains ground substance called cytoplasmic matrix or cytosol. This colloidal jelly like material is not static. 
  2. It shows streaming movements called cyclosis. 
  3. The cytoplasm contains water as major component along with organic and inorganic molecules like sugars, amino acids, vitamins, enzymes, nucleotides, minerals and waste products. 
  4. It also contains various cell organelles like endoplasmic reticulum, Golgi complex, mitochondria, plastids, nucleus, microbodies and cytoskeletal elements like microtubules. 
  5. Cytoplasm acts as a source of raw materials as well as seat for various metabolic activities taking place in the cell.

Functions of Cytoplasm
  1. Animal Cytoplasm provides support to the cell and suspend cell organs and cellular molecules within it.
  2. In Cytoplasm most of the metabolic reactions of the cells.
  3. Many metabolic processes occur in the cytoplasm like protein synthesis, the first stage of cellular respiration (known as glycolysis), mitosis, and meiosis etc.
  4. One of the Key functions of cytoplasm is that it provides proper shape to cell. It fills the cells thus allowing the organelles to remain in place.
  5. The Animal cell cytoplasm acts as a regulator and protects the cell's genetic material as well as cellular organisms from damage by motion and collision with other cells.
  6. Cytoplasm contains many enzymes that are crucial in the break down of the waste products and to perform enzymatic reactions

3. Endoplasmic Reticulum (ER):
  1. This little network within the cytosol is present in all eukaryotic cells except ova and mature red blood corpuscles.
  2. Membrane of Endoplasmic reticulum is continuous with nuclear envelope at one end and extends till cell membrane. 
  3. It thus acts as intracellular supporting framework and helps in maintaining position of various cell organelles in the cytoplasm. 
  4. The outer surface of endoplasmic reticular membrane may or may not be studded with ribosomes. Accordingly, it is called rough or smooth ER. 
  5. Smooth and rough ER differ in their functions. 
  6. Smooth ER is involved in various processes in different cells. Depending on cell type, it helps in synthesis of lipids (ex. steroid secreting cells of cortical region of adrenal gland, testes and ovaries), detoxification of drugs and poisons (liver cells) and storage of calcium ions (muscle cells). 
  7. Rough ER is primarily involved in protein synthesis. (e.g. pancreatic cells that secrete insulin). These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins. These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles.

Common functions of SER and RER : 
  1. Mechanical support : The memoranous network of endoplasmic reticulum provides mechanical support to otherwise colloidal complex of cytoplasmic matrix.
  2. Localisation of enzymes: E.R. membranes are sites for a number of enzymes and cytochromes to carry out specific reactions.
  3. Large surface area: The large surface area is useful for rapid synthesis of biomolecules.
  4. Desmotubules: With the help of desmotubules, ER of one cell communicates with ER of adjacent cells.
  5. Conduction of Information: It conducts information from outside to inside of cell and between different organelles of the same cell.
  6. Intracellular Transport: ER functions as circulating system of the cell for quick transport of materials.
  7. Vacuoles: It forms vacuoles.
  8. Nuclear Membranes: During telophase, part of the nuclear envelope is formed by endoplasmic reticulum.
  9. Membranes to Golgi Apparatus: ER provides membranes to Golgi apparatus for production of vesicles and Golgian vacuoles.
  10. Storage: Glycosomes or glycogen storing particles seem to be formed by ER.

4. Golgi complex :
  1. Golgi complex or Golgi apparatus or Golgi body; various terms are used to denote this assembly, manufacturing cum packaging and transport unit of cell.
  2. Golgi complex essentially consists of stacks of membranous sacs called cisternae. 
  3. Diameter of cisternae varies from 0.5 to 1 ┬Ám. A cell may have few to several cisternae depending on its function. 
  4. The Golgi sacs show specific orientation in the cell. Each cisterna has a forming or ‘cis’ face (cis: on the same side) and maturing or ‘ trans’ face (trans: the opposite side).
  5. Transport vesicles that pinch off from transitional ER merge with cis face of Golgi cisterna and add its contents into the lumen. 
  6. Golgi body carries out two types of functions, modification of secretions of ER and production of its own secretions.
  7. Cisternae contain specific enzymes for specific functions. Refining of product takes place in an orderly manner. 
  8. Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin. 

Functions of Golgi Bodies  :
  1. Cell wall matrix: Carbohydrates of cell wall matrix are synthesized in Golgi Bodies, e.g. pectic compounds, mucopolysaccharides.
  2. Special carbohydrates: Golgi bodies takes part in synthesis of special carbohydrates like galactose and sialic acid.
  3. Animal tissue matrix: Chondroitin sulphate and hyaluronic acid are formed by Golgi Apparatus.
  4. Acrosome: It is a special structure present near the tip of sperm that contains hydrolytic enzymes for digesting the protective coverings of the egg.
  5. Hormones: Hormone synthesis in endocrine glands is mediated through Golgi bodies.
  6. Vitellogenesis: In oocytes, yolk is deposited around Golgi complex.

5. Lysosomes :
  1. Lysosomes can be considered as dismantling and restructuring units of a cell. 
  2. These are membrane bound vesicles containing hydrolytic enzymes. The enzymes in lysosomes are used by most eukaryotic cells to digest (hydrolyse) macromolecules. The lysosomal enzymes show optimal activity in acidic pH.
  3. Lysosomes are polymorphic in nature. 
  4. We can classify lysosomes as, Primary lysosomes; which are nothing but membrane bound vesicles in which enzymes are in inactive state.
  5. Secondary lysosomes or hybrid lysosomes, which are formed by fusion of lysosome with endocytic vesicle containing materials to be digested, represented as heterophagic vesicle.
  6. Lysosomes which bring about digestion of cells own organic material like a damaged cell organelle, are called autophagic vesicles (or suicide bags).

Functions of Lysosomes
  1. Animal cell Lysosones are takes part in lntracellular digestion. The ingested food digested by the activity of lysosomes
  2. The key role of lysosomes in Fertilization. Acrosome present sperm is actually a special lysosome which contains hydrolytic enzymes for clearing the path of sperm into egg.
  3. Another important function of Cell lysosomes is it performs Phagocytosis and Autophagocgtosis. 
  4. Animal cell Lysosomes provides Nourishment to the eggs, lysosomes take part in digestion of stored food.
  5. During metamorphosis, the larval or embryonic organs are resorbed through the agency of lysosomes.
  6. The aged and dead cells are removed by means of lysosomes by the process of Autophagocytosis .
  7. Leucocytes and macrophages of the body are specialized to ingest bacteria and other microorganisms, foreign proteins, etc. Such harmful foreign matter is destroyed by lysosome.
  8. Lysosomal proteases produce Specific harmones like thyroid hormones, thyroxin and triiodothyronine.
  9. Some common storage diseases due to lysosomal inetficiency are Hunter's syndrome, Tay-Sach's disease, Gaucher's disease.

6. Vacuoles :
  1. Vacuoles are membrane bound sacs prominently found in plant cells. In animal cells, whenever present they are few in number and smaller in size. Generally, there are two or three permanent vacuoles in a plant cell.
  2. vacuole can occupy as much as 90% of the total volume of the cell.
  3. The vacuoles are bound by semipermeable membrane, called tonoplast membrane. 
  4. This membrane helps in maintaining the composition of vacuolar fluid; the cell sap, different from that of the cytosol.
  5. Intake of food or foreign particle by phagocytosis involves formation of food vacuole. In fresh water unicellular forms like Paramoecium, excretion and osmoregulation takes place by contractile vacuoles. 
  6. Vacuoles maintain turgidity of the cell.

Functions of  vacuoles:
  1. One of the important function of Vacuoles is storage, it store mineral salt, sugar,  insulin, amino acids,  esters and waste products.
  2. Contractile vacuoles is useful in osmoregulation and excretion of waste products.
  3. Animal cell Vacuoles play role in Endosmosis, 
  4. Animal cell Vacuoles functions in cell growth by enlargement of cell during there growth and by providing stored material.
  5. Food vacuoles formed by the phagosomes and lysosomes for digestion of food.
  6. Air vacuoles use to store Gases, provide Machanical protection and protection from harmful radiations.

7. Mitochondria (Singular : 
 Mitochondrion) :

  1. These are important cell organelles involved in aerobic respiration. 
  2. Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBCs). Their shape may be oval or spherical or spiral strip like. 
  3. It is a double membrane bound organelle. Outer membrane is permeable to various metabolites due to presence of a protein -Porin or Parson’s particles. 
  4. Inner membrane is selectively permeable to few substances only. 
  5. Both membranes are separated by a space-outer chamber. 
  6. Inner membrane shows several finger like or plate like folds- cristae. 
  7. Inner membrane bears numerous particles- oxysomes and cytochromes / electron carriers. 
  8. Inner membrane encloses a cavity- inner chamber, containing a fluid- matrix. Matrix contains few coils of circular DNA, RNA, 70S types of ribosomes, lipids and various enzymes of Krebs cycle and other pathways.

Function of mitochondria
  1. The most important function of mitochondria to produce the energy currency of the cell, ATP through respiration.
  2. Mitochondria generally used in breakdown of nutrients to get energy through oxidative phosphorylation.
  3. Mitochondria generate ATP from glycolysis-derived pyruvate, fatty acids, and amino acids through oxidative phosphorylation.
  4. Another important function of mitochondria is it Maintenance of proper concentration of calcium ions in the cell.
  5. The mitochondria of Liver cells have certain enzymes that help in the detoxification of Ammonia.
  6. It helps in apoptosis or programmed cell death.

8. Ribosomes :
  1. You are aware that ribosomes are protein factories of the cell. They use the genetic information to synthesise proteins.
  2. Ribosomes were first observed as dense particles in electron micrograph of a cell by scientist Pallade in 1953. 
  3. Ribosomes are made up of Ribosomal RNA and proteins. They do not have any membranous covering around them
  4. In a eukaryotic cell, ribosomes are present in mitochondria, plastids and in cytosol. 
  5. Ribosomes in cytoplasm are either found attached to outer surface of Rough Endoplasmic Reticulum and nuclear membrane or freely suspended in cytoplasm. 
  6. Both are similar in structure and are 80S type. Each ribosome is made up of two subunits; a large and a small subunit.
  7. Bound ribosomes generally produce proteins that are transported outside the cell after processing in ER and Golgi body. e.g. Bound ribosomes of acinar cells of pancreas produce pancreatic digestive enzymes. 
  8. Free ribosome come together and form chains called polyribosomes for protein synthesis.
  9. Free ribosomes generally produce enzymatic proteins that are used up in cytoplasm like enzymes required for breakdown of sugar. 
  10. Both types of ribosomes can interchange position and function. Number of ribosomes is high in cells actively engaged in protein synthesis.

Functions of ribosomes
  1. Fist important function of ribosomes is that they are sites for protein synthesis or polypeptide synthesis.
  2. Ribosomes are  generally two types free ribosomes are attached ribosomes. 
  3. Free ribosomes are used to synthesize structural and enzymatic proteins which is  use inside the cell for different functions. The attached ribosomes are used to synthesise proteins for transport.
  4. Another key role of Ribosomes is it provide enzymes (e.g. Peptidyl transferase) and factors for condensation of amino acids for formation of polypeptide chain.
  5. Ribosome carries rRNA to providing attachment points for mRNA and tRNAs.
  6. Ribosomes has a tunnel for mRNA so that it can be translated properiy.
  7. Newly synthesised polypeptide chain provides protection from cytoplasmic proteolytic enzymes by enclosing it in tne groove of larger subunit of ribosome till it gains Secondary structure.

9. Nucleus :

  1.  Structure of nucleus of a plant cell becomes distinct in a non-dividing cell or during interphase. Such an interphase nucleus is made up of nuclear envelope, nucleoplasm, nucleolus and chromatin network.
  2. Nuclear envelope is a double walled delimiting membrane of nucleus. 
  3. Two membranes are separated from each other by perinuclear space (10 to 50nm). Outer membrane is connected with endoplasmic reticulum at places. 
  4. The nucleoplasm or karyolymph contains various substances like nucleic  acids, protein molecules, minerals and salts. 
  5. It contains chromatin network and nucleolus. 
  6. Nucleolus is another component which is not bound by cell membrane.
  7. Nucleolus is made up of rRNA and ribosomal proteins and it is best known as the site of ribosome biogenesis. 
  8. They appear as dense spherical bodies present near chromatin network. They produce rRNA and ribosomal proteins which are then transported to cytoplasm and are assembled together to form ribosomes.
Read : Nucleotide

Chromatin material :
  1. Nucleus contains genetic information in the form of chromosomes which are nothing but DNA molecules associated with proteins.
  2. The chromatin material contains DNA, histone and non-histone proteins and RNA
  3. In some regions of chromatin, DNA is more and is genetically active called euchromatin. Some regions that contain more of proteins and less DNA and are genetically inert, are called heterochromatin
  4. The nucleus contains entire genetic information, hence play important role in heredity and variation. 
  5. It is the site for synthesis of DNA, RNA and ribosomes. It plays important role in protein synthesis. 
  6. Chromosome number being constant for a species, it is important in phylogenetic studies. Nucleus thus is the master .cell organelle.
Read : Nucleus Structure and Functions,  definitions 

10. Centrioles and centrosomes :
Centrosome is usually found near the
nucleus of an animal cell. It contains a pair
of cylindrical structures called centrioles.
The cylinders are perpendicular to each other and are surrounded by amorphous substance called pericentriolar material. Each cylinder of centriole is made up of nine sets of triplet microtubules made up of tubulin. Evenly spaced triplets are connected to each other by means of non-tubulin proteins.

At the proximal end of centriole, there
is a set of tubules called hub. The peripheral triplets are connected to hub by means of radial spokes. Due to this proximal end of centriole looks like a cartwheel. The centrosomes help in assembly of spindle apparatus during cell division. It forms basal body of cilia and flagella.

Some of these structures are also present in plant cells. Although they are similar, there are some basic differences between these two types of cell. For example, the plant cell has chloroplasts, which are responsible for photosynthesis and the green color of plants. In addition, they work in different ways according to the function of cell organelles.

Functions of the animal cell
  1. There are many functions that this cell can perform, however one of the most basic refers to the nutrition of organisms since these cells are responsible for absorbing nutrients and converting them into substances that can be used by each cell and of which they can obtain energy while favoring the expulsion of waste.
  2. In cells their shape and size can vary so their functions can also change but they are always destined to fulfill a specific task such as, for example, building supportive tissue or detecting or transmitting certain sensations and cellular respiration, among other

Differences between the Animal Cell and the Plant Cell

  1. The most striking difference between animal and plant cells is that plant cells have a wall that provides them with greater rigidity.
  2. The plant cell has plastids the animal cell does not have plastids.
  3. The animal cell has the organelles called lysosomes, the plant does not.
  4. The animal cell has a very small number of vacuoles, while the plant has a large number of them.
  5. In the animal cell the mitochondria is responsible for generating energy, whereas in the plant cell the chloroplasts carry out photosynthesis.
  6. The nutrition of plant cells is autotrophic, while in animals it is heterotrophic.
  7. Animal cells have different shapes, while plant cells have only one prismatic shape.
  8. Eukaryotic cells have a defined nucleus in their nuclear envelope and contain DNA, these characteristics are found in the animal or plant cell .

Frequently Asked Questions about Animal Cell

What is Eukaryotic Animal Cell ?
The eukaryotic animal cell is a eukaryotic cell characterized by the presence of a nucleus, Cytoplasm and Animal cell membrane. It differs from the plant cell by the absence of a cell wall and chloroplasts. In addition, smaller and more abundant vacuoles can be found compared to those of a plant cell. Animal cell lacks cell wall, plastids and vacuoles and having plasma membrane contains cytoplasm and cell organelles like mitochondria, golgi bodies, endoplasmic reticulum, nucleus, ribosomes, lysosomes, also contains enzymes and proteins. e.g. neuron (nerves cells), epithelial cells, endothelial cells, hepatocytes (liver cells), gastric cells (stomach cells), cardiocytes (heart cells) etc.

Name the orgnells which are present in animal cell 
Answer : Animals cell contains many organelles like
  1. Nucleus
  2. Mitochondria
  3. Vacule
  4. Lysosome
  5. Endoplasmic reticulum
  6. Golgi Complex
  7. Ribosome
  8. Centriule
  9. Cytoplasam.

What is the powerhouse of an animal cell?
Answer : Mitochondria is a power house of cell because it stores and provides energy for the cell in the form of ATP.

Does the size of a Animal cell depend on the size of the animal?
Answer : It doesn't: a cell's size is function of simple physics: the ratio of a cell's surface area to volume directly impacts the cells ability to absorb and transport nutrients within its volume, placing an upper limit on the size to which it can grow and remain viable.

What is the other name of lysosome?
Answer : Lysosomes another name is suicidal bag. Because they burst themselves to digest the other dead organelles in the cell.

Read : Prokaryotic cell structure and function 

How is fluidity reduced in animal cells?
Answer : The fluidity in animal cells is reduced by the cholesterol levels.

What are the functions of the rough endoplasmic reticulum in animal cells?
Answer : There are two types to endoplasmic recticulum namely Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum.

The major function of smooth endoplasmic reticulum is the synthesis of lipids. The lipids such as the phospholipids, cholesterol and the lipoproteins.

The major function of rough endoplasmic reticulum is the synthesis of proteins. Initial glycosylation and N-linked glyosylation occurs in rough endoplasmic reticulum.

Why do plant cells have cell walls, but animal cells don't?
Answer : The basic thing is that plants need rigid structure to support them as well as to defend themselves whereas, animals do not possess any sort of cell wall since animals always move, they are not static in one particular place, therefore they need to be flexible and as a result of that cell walls are absent.

Which is the largest cell in the animalia kingdom?
Answer : The largest cell in the animelia kingdom  at present is Ostrich’s egg. The ostrich egg is equal to the size of 24 chicken eggs. But the biggest egg ever laid belongs to the elephant bird. Elephant birds have been extinct since the 1600s, and their eggs were 15 times bigger than an ostrich egg – equal to 200 chicken eggs! That’s a lot of cell.

What is cyclosis in animal cells?
Answer : The cytoplasm (cell fluid) remains in constant circular motion. This motion is known as cyclosis. Cyclosis is present only in only eukaryotic cells.

What are the main differences between plant and animal cells?
Answer : The main difference is that plants in general carry out photosynthesis using an organelle called a chloroplast. They convert (sun)light into chemical energy, glucose. Plants use this chemical energy to grow and reproduce and to feed their neighbor animal cells. Animals can not carry out photosynthesis. They depend on plants for food somewhere in their food chain.

What is the function of vacuole in animal cell 
Answer : Vacuoles are storage bubbles found in cells. They are found in both animal and plant cells but are much larger in plant cells. In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis. In most of the cases, the vacuoles present in the animal cell are smaller in size but at the same time, they are large in numbers The functions of the animal vacuoles are that they cater storage facility during the exocytosis.

Which is longest animal cell
Answer : The nerve cells/neurons are the longest animal cells they help in controlling everything going on in the body by conveying messages and commands by superior organs.