Animal Cell: Structure, function, types, and definition
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.
The animal cell contains a 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 a cell wall, a large vacuole, and plastids. Due to the lack of a cell wall, the shape and size of the animal cells are mostly irregular. Animal cells increase in size by increasing cell numbers. Plant cells mainly increase cell size by becoming larger.
Definition 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: 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 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/
- The animal cell membrane is a thin, quasifluid structure present both extracellularly and intracellularly. Extracellularly, it is present around the protoplast and intracellularly, it is present around most of the cell organelles in a eukaryotic cell. It separates cell organelles from cytosol.
- The thickness of the biomembrane is about 75A°. Under an electron microscope, the cell membrane appears trilaminar (made up of three layers).
- It shows the presence of lipids (mostly phospholipids) arranged in a bilayer. Lipids possess one hydrophilic polar head and two hydrophobic non-polar tails. So, phospholipids are amphipathic.
- Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwiched in between heads.
- Due to this, tails never come in direct contact with aqueous surroundings.
- Animal Cell membrane also shows the presence of proteins and carbohydrates. The ratio of proteins and lipids varies in different cells.
- For example-in human beings, RBCs show approximately 52% protein and 40% lipids.
- Fluid mosaic model: It is a universally accepted model of the cell membrane. It was proposed by Singer and Nicholson in 1972.
- 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 the bilayer.
- The main function of a cell membrane is transport. It controls the entry of selected substances. They are impermeable to other substances.
- The cell membrane also controls cell metabolism through selective permeability of the cell membrane.
- Plasma membrane protects the cell from Mechanical injuries.
- Different enzymes are located in the Biomembrane for performing different metabolic processes like the Synthesis of ATP.
- The cell membrane contains different receptors for hormones, Antibodies, Antigens, Neurotransmitters, etc.
- The cell membrane separates the cell from its external environment and cell organelles from the cytosol.
- Cell membrane used in endocytosis, exocytosis, electron transport, impulse transmission, Cell junction, cell movements, Osmosis, Cell recognition, etc.
2. Cytoplasm :
- The cell contains a ground substance called cytoplasmic matrix or cytosol. This colloidal jelly-like material is not static.
- It shows streaming movements called cyclosis.
- The cytoplasm contains water as a major component along with organic and inorganic molecules like sugars, amino acids, vitamins, enzymes, nucleotides, minerals, and waste products.
- It also contains various cell organelles like endoplasmic reticulum, Golgi complex, mitochondria, plastids, nucleus, microbodies, and cytoskeletal elements like microtubules.
- Cytoplasm acts as a source of raw materials as well as a seat for various metabolic activities taking place in the cell.
- Animal Cytoplasm provides support to the cell and suspends cell organs and cellular molecules within it.
- In the Cytoplasm most of the metabolic reactions of the cells.
- Many metabolic processes occur in the cytoplasm like protein synthesis, the first stage of cellular respiration (known as glycolysis), mitosis, meiosis, etc.
- One of the Key functions of cytoplasm is that it provides proper shape to cells. It fills the cells thus allowing the organelles to remain in place.
- 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.
- The cytoplasm contains many enzymes that are crucial in the breakdown of the waste products and to performing enzymatic reactions
3. Endoplasmic Reticulum (ER):
- This little network within the cytosol is present in all eukaryotic cells except ova and mature red blood corpuscles.
- The membrane of the Endoplasmic reticulum is continuous with the nuclear envelope at one end and extends till the cell membrane.
- It thus acts as an intracellular supporting framework and helps in maintaining the position of various cell organelles in the cytoplasm.
- The outer surface of the endoplasmic reticular membrane may or may not be studded with ribosomes. Accordingly, it is called rough or smooth ER.
- Smooth and rough ER differ in their functions.
- Smooth ER is involved in various processes in different cells. Depending on cell type, it helps in the synthesis of lipids (ex. steroid secreting cells of cortical region of the adrenal gland, testes, and ovaries), detoxification of drugs and poisons (liver cells), and storage of calcium ions (muscle cells).
- 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 a membrane that buds off from the transitional region of ER. Such membrane-bound proteins depart from ER as transport vesicles.
- Mechanical support: The membranous network of the endoplasmic reticulum provides mechanical support to an otherwise colloidal complex of the cytoplasmic matrix.
Localization of enzymes: E.R. membranes are sites for several enzymes and cytochromes to carry out specific reactions.
Large surface area: The large surface area is useful for the rapid synthesis of biomolecules.
Desmotubules: With the help of desmotubules, ER of one cell communicates with ER of adjacent cells.
Conduction of Information: It conducts information from outside to the inside of a cell and between different organelles of the same cell.
Intracellular Transport: ER functions as a circulating system of the cell for quick transport of materials.
Vacuoles: It forms vacuoles.
Nuclear Membranes: During telophase, part of the nuclear envelope is formed by the endoplasmic reticulum.
Membranes to Golgi Apparatus: ER provides membranes to Golgi apparatus for production of vesicles and Golgian vacuoles.
Storage: Glycosomes or glycogen storing particles seem to be formed by ER.
4. Golgi complex :
- Golgi complex or Golgi apparatus or Golgi body; various terms are used to denote this assembly, manufacturing cum packaging, and transport unit of the cell.
- Golgi complex essentially consists of stacks of membranous sacs called cisternae.
- The diameter of cisternae varies from 0.5 to 1 µm. A cell may have few to several cisternae depending on its function.
- 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).
- Transport vesicles that pinch off from transitional ER merge with the cis face of Golgi cisterna and add its contents into the lumen.
- The Golgi body carries out two types of functions, modification of secretions of ER and production of its own secretions.
- Cisternae contain specific enzymes for specific functions. Refining of the product takes place in an orderly manner.
- Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin.
- Cell wall matrix: Carbohydrates of cell wall matrix are synthesized in Golgi Bodies, e.g. pectic compounds, mucopolysaccharides.
- Special carbohydrates: Golgi bodies take part in the synthesis of special carbohydrates like galactose and sialic acid.
- Animal tissue matrix: Chondroitin sulfate and hyaluronic acid are formed by Golgi Apparatus.
- Acrosome: It is a special structure present near the tip of sperm that contains hydrolytic enzymes for digesting the protective coverings of the egg.
- Hormones: Hormone synthesis in endocrine glands is mediated through Golgi bodies.
- Vitellogenesis: In oocytes, the yolk is deposited around the Golgi complex.
5. Lysosomes :
- Lysosomes can be considered as dismantling and restructuring units of a cell.
- These are membrane-bound vesicles containing hydrolytic enzymes. The enzymes in lysosomes are used by most eukaryotic cells to digest (hydrolyze) macromolecules. The lysosomal enzymes show optimal activity in acidic pH.
- Lysosomes are polymorphic in nature.
- We can classify lysosomes as, Primary lysosomes; which are nothing but membrane-bound vesicles in which enzymes are in an inactive state.
- Secondary lysosomes or hybrid lysosomes, which are formed by fusion of lysosome with endocytic vesicle containing materials to be digested, are represented as heterophagic vesicles.
- Lysosomes which bring about digestion of the cell’s own organic material like a damaged cell organelle, are called autophagic vesicles (or suicide bags).
- Animal cell Lysosomes are taken part in intracellular digestion. The ingested food digested by the activity of lysosomes
- The key role of lysosomes in Fertilization. Acrosome present sperm is actually a special lysosome that contains hydrolytic enzymes for clearing the path of sperm into an egg.
- Another important function of Cell lysosomes is it performs Phagocytosis and Autophagocgtosis.
- Animal cell Lysosomes provide Nourishment to the eggs, lysosomes take part in the digestion of stored food.
- During metamorphosis, the larval or embryonic organs are resorbed through the agency of lysosomes.
- The aged and dead cells are removed using lysosomes by the process of Autophagocytosis.
- Leucocytes and macrophages of the body are specialized to ingest bacteria and other microorganisms, foreign proteins, etc. Such harmful foreign matter is destroyed by the lysosome.
- Lysosomal proteases produce Specific hormones like thyroid hormones, thyroxine, and triiodothyronine.
- Some common storage diseases due to lysosomal inefficiency are Hunter’s syndrome, Tay-Sachs disease, Gaucher’s disease.
6. Vacuoles :
- 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.
- vacuole can occupy as much as 90% of the total volume of the cell.
- The vacuoles are bound by a semipermeable membrane, called the tonoplast membrane.
- This membrane helps in maintaining the composition of vacuolar fluid; the cell sap, different from that of the cytosol.
- Intake of food or foreign particle by phagocytosis involves the formation of food vacuole. In freshwater unicellular forms like Paramoecium, excretion, and osmoregulation take place by contractile vacuoles.
- Vacuoles maintain the turgidity of the cell.
- One of the important functions of Vacuoles is storage, it store mineral salt, sugar, insulin, amino acids, esters, and waste products.
- Contractile vacuoles are useful in the osmoregulation and excretion of waste products.
- Animal cell Vacuoles play role in Endosmosis,
- Animal cell Vacuoles functions in cell growth by enlargement of cells during their growth and by providing stored material.
- Food vacuoles are formed by the phagosomes and lysosomes for the digestion of food.
- Air vacuoles are used to store Gases, provide Mechanical protection and protection from harmful radiations.
7. Mitochondria (Singular :
- These are important cell organelles involved in aerobic respiration.
- Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBCs). Their shape may be oval or spherical or spiral strip-like.
- It is a double membrane-bound organelle. The outer membrane is permeable to various metabolites due to the presence of a protein -Porin or Parson’s particles.
- The inner membrane is selectively permeable to a few substances only.
- Both membranes are separated by a space-outer chamber.
- Inner membrane shows several finger-like or plate-like folds- cristae.
- Inner membrane bears numerous particles- exosomes and cytochromes/electron carriers.
- Inner membrane encloses a cavity- inner chamber, containing a fluid- matrix. The matrix contains a few coils of circular DNA, RNA, 70S types of ribosomes, lipids, and various enzymes of the Krebs cycle and other pathways.
- The most important function of mitochondria is to produce the energy currency of the cell, ATP through respiration.
- Mitochondria are generally used in the breakdown of nutrients to get energy through oxidative phosphorylation.
- Mitochondria generate ATP from glycolysis-derived pyruvate, fatty acids, and amino acids through oxidative phosphorylation.
- Another important function of mitochondria is it Maintenance of proper concentration of calcium ions in the cell.
- The mitochondria of Liver cells have certain enzymes that help in the detoxification of Ammonia.
- It helps in apoptosis or programmed cell death.
8. Ribosomes :
- You are aware that ribosomes are protein factories of the cell. They use genetic information to synthesize proteins.
- Ribosomes were first observed as dense particles in the electron micrograph of a cell by scientist Pallade in 1953.
- Ribosomes are made up of Ribosomal RNA and proteins. They do not have any membranous covering around them
- In a eukaryotic cell, ribosomes are present in mitochondria, plastids, and the cytosol.
- Ribosomes in the cytoplasm are either found attached to the outer surface of the Rough Endoplasmic Reticulum and nuclear membrane or freely suspended in the cytoplasm.
- Both are similar in structure and are 80S type. Each ribosome is made up of two subunits; a large and a small subunit.
- 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 the pancreas produce pancreatic digestive enzymes.
- Free ribosomes come together and form chains called polyribosomes for protein synthesis.
- Free ribosomes generally produce enzymatic proteins that are used up in cytoplasm like enzymes required for the breakdown of sugar.
- Both types of ribosomes can interchange position and function. The number of ribosomes is high in cells actively engaged in protein synthesis.
- Fist important function of ribosomes is that they are sites for protein synthesis or polypeptide synthesis.
- Ribosomes are generally two types of free ribosomes are attached ribosomes.
- Free ribosomes are used to synthesize structural and enzymatic proteins which are used inside the cell for different functions. The attached ribosomes are used to synthesize proteins for transport.
- Another key role of Ribosomes is they provide enzymes (e.g. Peptidyl transferase) and factors for the condensation of amino acids for the formation of a polypeptide chain.
- Ribosome carries rRNA to provide attachment points for mRNA and tRNAs.
- Ribosomes have a tunnel for mRNA so that it can be translated properly.
- A newly synthesized polypeptide chain provides protection from cytoplasmic proteolytic enzymes by enclosing it in the groove of a larger subunit of ribosome till it gains Secondary structure.
9. Nucleus :
- The structure of the 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.
- A nuclear envelope is a double-walled delimiting membrane of the nucleus.
- Two membranes are separated from each other by perinuclear space (10 to 50nm). The outer membrane is connected with the endoplasmic reticulum at places.
- The nucleoplasm or karyolymph contains various substances like nucleic acids, protein molecules, minerals, and salts.
- It contains a chromatin network and nucleolus.
- The nucleolus is another component that is not bound by a cell membrane.
- The nucleolus is made up of rRNA and ribosomal proteins and it is best known as the site of ribosome biogenesis.
- They appear as dense spherical bodies present near the chromatin network. They produce rRNA and ribosomal proteins which are then transported to the cytoplasm and are assembled together to form ribosomes.
Chromatin material :
- The nucleus contains genetic information in the form of chromosomes which are nothing but DNA molecules associated with proteins.
- The chromatin material contains DNA, histone and non-histone proteins, and RNA.
- In some regions of chromatin, DNA is more and is genetically active called euchromatin. Some regions that contain more proteins and less DNA and are genetically inert are called heterochromatin
- The nucleus contains entire genetic information, hence play important role in heredity and variation.
- It is the site for the synthesis of DNA, RNA, and ribosomes. It plays important role in protein synthesis.
- Chromosome number being constant for a species, it is important in phylogenetic studies. The nucleus thus is the master .cell organelle.
Read: Nucleus Structure and Functions, definitions
10. Centrioles and centrosomes :
The 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 an 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 using non-tubulin proteins.
At the proximal end of the centriole, there
is a set of tubules called a hub. The peripheral triplets are connected to the hub using radial spokes. Due to this proximal end of the centriole looks like a cartwheel. The centrosomes help in the assembly of the spindle apparatus during cell division. It forms a basal body of cilia and flagella.
- 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.
- 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
- The most striking difference between animal and plant cells is that plant cells have a wall that provides them with greater rigidity.
- The plant cell has plastids the animal cell does not have plastids.
- The animal cell has organelles called lysosomes, the plant does not.
- The animal cell has a very small number of vacuoles, while the plant has a large number of them.
- In the animal cell, the mitochondria are responsible for generating energy, whereas in the plant cell the chloroplasts carry out photosynthesis.
- The nutrition of plant cells is autotrophic, while in animals it is heterotrophic.
- Animal cells have different shapes, while plant cells have only one prismatic shape.
- 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?
Name the organelles which are present in animal cell
Answer: Animal’s cell contains many organelles like
- Endoplasmic reticulum
- Golgi Complex
What is the powerhouse of an animal cell?
Answer: Mitochondria is a powerhouse of cells because it stores and provides energy for the cell in the form of ATP.
Does the size of an Animal cell depend on the size of the animal?
Answer: It doesn’t: a cell’s size is the function of simple physics: the ratio of a cell’s surface area to volume directly impacts the cell’s 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 the lysosome?
Answer: Lysosomes another name is the 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 cholesterol levels.
What are the functions of the rough endoplasmic reticulum in animal cells?
Answer: There are two types of endoplasmic reticulum namely the Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum.
The major function of the smooth endoplasmic reticulum is the synthesis of lipids. The lipids such as phospholipids, cholesterol, and lipoproteins.
The major function of the rough endoplasmic reticulum is the synthesis of proteins. Initial glycosylation and N-linked glycosylation occur in the 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 Animalia 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 cells.
What is cyclosis in animal cells?
Answer: The cytoplasm (cell fluid) remains in a 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 the vacuole in an 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 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 to storage facility during the exocytosis.
Which is the 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.