Definition of Animal Cell:

• Animal cells are the basic structural and functional unit of life of 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 means that they have a nucleus and a specialized structure called organelles that perform various functions.

What is Animal Cell

• 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.
• They are also referred to as Eukaryotic cells which means that they have a nucleus and a specialized structure called organelles that perform various functions.
• 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 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.

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, and 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 makeup 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 a 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.

Animal Cell Structure:

• 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 Cell 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 Structure:

1. Animal Cell membrane/Plasma membrane/biomembrane:

1. The animal cell membrane is a thin, quasi-fluid 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. 
2. The thickness of the biomembrane is about 75A°. Under an electron microscope, the cell membrane appears trilaminar (made up of three layers). 
3. 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. 
4. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwiched in between heads. 
5. Due to this, tails never come in direct contact with aqueous surroundings.
6. The animal Cell membrane also shows the presence of proteins and carbohydrates. The 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 a universally accepted model of the cell membrane. It was proposed by Singer and Nicholson in 1972. 
9. According to this model, it is made up of a 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.

2. Cytoplasm:

1. The cell contains a 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 a 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 a seat for various metabolic activities taking place in the cell.

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. The membrane of the Endoplasmic reticulum is continuous with the nuclear envelope at one end and extends to the cell membrane. 
3. It thus acts as an intracellular supporting framework and helps in maintaining the position of various cell organelles in the cytoplasm. 
4. The outer surface of the 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 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). 
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 a membrane that buds off from the transitional region of ER. Such membrane-bound proteins depart from ER as transport vesicles.

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 the cell.
2. The Golgi complex essentially consists of stacks of membranous sacs called cisternae. 
3. The diameter of cisternae varies from 0.5 to 1 µm. A cell may have a 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 the cis face of Golgi cisterna and add its contents into the lumen. 
6. The Golgi body carries out two types of functions, modification of secretions of ER and production of its secretions.
7. Cisternae contain specific enzymes for specific functions. The refining of the product takes place in an orderly manner. 
8. Golgi bodies also manufacture their products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin. 

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 (hydrolyze) macromolecules. The lysosomal enzymes show optimal activity in acidic pH.
3. Lysosomes are polymorphic. 
4. We can classify lysosomes as, Primary lysosomes; which are nothing but membrane-bound vesicles in which enzymes are in an inactive state.
5. Secondary lysosomes or hybrid lysosomes, which are formed by the fusion of lysosomes with endocytic vesicle-containing materials to be digested, are represented as heterophonic vesicles.
6. Lysosomes which bring about digestion of the cell’s organic material like a damaged cell organelle, are called autophagic vesicles (or suicide bags).

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. Vacuoles can occupy as much as 90% of the total volume of the cell.
3. The vacuoles are bound by a semipermeable membrane, called the 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 the formation of a food vacuole. In freshwater unicellular forms like Paramoecium, excretion, and osmoregulation take place by contractile vacuoles. 
6. Vacuoles maintain the turgidity of the cell.

7. Mitochondria:

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. The outer membrane is permeable to various metabolites due to the presence of a protein -Porin or Parson’s particles. 
4. The inner membrane is selectively permeable to a few substances only. 
5. Both membranes are separated by a space-outer chamber. 
6. The inner membrane shows several finger-like or plate-like folds- cristae. 
7. The inner membrane bears numerous particles- exosomes, and cytochromes/electron carriers. 
8. The inner membrane encloses a cavity- an 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.

8. Ribosomes:

1. You are aware that ribosomes are protein factories of the cell. They use genetic information to synthesize proteins.
2. Ribosomes were first observed as dense particles in the 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 the cytosol. 
5. 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. 
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 the pancreas produce pancreatic digestive enzymes. 
8. Free ribosomes 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 the breakdown of sugar. 
10. Both types of ribosomes can interchange position and function. The number of ribosomes is high in cells actively engaged in protein synthesis.

9. Nucleus:

1.  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 a nuclear envelope, nucleoplasm, nucleolus, and chromatin network.
2. A nuclear envelope is a double-walled delimiting membrane of the nucleus. 
3. Two membranes are separated from each other by perinuclear space (10 to 50nm). The outer membrane is connected with the endoplasmic reticulum at places. 
4. The nucleoplasm or karyolymph contains various substances like nucleic acids, protein molecules, minerals, and salts. 
5. It contains a chromatin network and nucleolus. 
6. The nucleolus is another component that is not bound by a cell membrane.
7. The 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 the chromatin network. They produce rRNA and ribosomal proteins which are then transported to the cytoplasm and are assembled to form ribosomes.

Chromatin material :

1. The 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 proteins and less DNA and are genetically inert are called heterochromatin
4. The nucleus contains entire genetic information and hence plays an important role in heredity and variation. 
5. It is the site for the synthesis of DNA, RNA, and ribosomes. It plays an important role in protein synthesis. 
6. Chromosome number being constant for a species, it is important in phylogenetic studies. The nucleus thus is the master .cell organelle.

10. Centrioles and centrosomes:

1. The centrosome is usually found near the nucleus of an animal cell.
2. It contains a pair of cylindrical structures called centrioles.
3. The cylinders are perpendicular to each other and are surrounded by an amorphous substance called pericentriolar material.
4. Each cylinder of centriole is made up of nine sets of triplet microtubules made up of tubulin. Evenly-spaced triplets are connected using non-tubulin proteins. At the proximal end of the centriole, there is a set of tubules called a hub.
5. The peripheral triplets are connected to the hub using radial spokes. Due to this proximal end of the centriole looks like a cartwheel.
6. The centrosomes help in the assembly of the spindle apparatus during cell division. It forms a basal body of cilia and flagella.
7. Some of these structures are also present in plant cells. Although they are similar, there are some basic differences between these two types of cells.
8. For example, the plant cell has chloroplasts, which are responsible for photosynthesis and the green color of plants.
9. In addition, they work in different ways according to the function of cell organelles.

Functions of the animal cell:

Differences between the Animal Cell and the Plant Cell

Frequently Asked Questions about Animal Cell

1. What is Eukaryotic Animal Cell?
Answer: 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 a cell wall, plastids, and vacuoles, and have plasma membrane containing cytoplasm and cell organelles like mitochondria, Golgi bodies, endoplasmic reticulum, nucleus, ribosomes, lysosomes, which also contains enzymes and proteins. e.g. neurons (nerves cells), epithelial cells, endothelial cells, hepatocytes (liver cells), gastric cells (stomach cells), cardiocytes (heart cells), etc.

2. Name the organelles which are present in animal cell 
Answer: An animal’s cell contains many organelles like

1. Nucleus
2. Mitochondria
3. Value
4. Lysosome
5. Endoplasmic reticulum
6. Golgi Complex
7. Ribosome
8. Centriole
9. Cytoplasm.

3. 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.

4. 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.

5. 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.

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

7. 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.

 8. Why do plant cells have cell walls, but animal cells don’t?
Answer: The basic thing is that plants need rigid structures 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.

9, 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.

10. 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.

11. 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.

12. 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.

13. 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.