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Definition of animal cell
- Animals are a large group of diverse living organisms that make up to three-quarters of all species on earth. With their ability to move, to respond to stimuli, respond to environmental changes and adapt to different modes of feeding defense mechanisms and reproduction, all these mechanisms are enhanced by their constituent elements in the body. However, animals cannot manufacture their own food like plants and hence they depend on plants in one way or another.
- All living things are made up of cells that make up their body structure. Some of these living things are single-celled (unicellular) and other organisms are made up of more than one cell (Multicellular).
- A cell is the smallest (microscopic) structural-functional unit of life of an organism. The cells that constitute an animal are called Animal cells and those that constitute plants are known as plant cells.
- Most cells are covered by a protective membrane known as the cell wall which gives the cells their shape and rigidity.
- An animal cell is a eukaryotic cell that lacks a cell wall, and it is enclosed by the plasma membrane. The cell organelles are enclosed by the plasma membrane including the cell nucleus. Unlike the animal cell lacking the cell wall, plant cells have a cell wall.
- Since animal cells lack a rigid cell wall it allows them to develop a great diversity of cell types, tissues, and organs. The nerves and muscles are made up of specialized cells that plant cells cannot evolve to form, hence giving these nerve and muscle cells have the ability to move.
Animal cell size and shape
- Animal cells come in all kinds of shapes and sizes, with their size ranging from a few millimeters to micrometers. The largest animal cell is the ostrich egg which has a 5-inch diameter, weighing about 1.2-1.4 kg and the smallest animal cells are the neurons of about 100 microns in diameter.
- Animal cells are smaller than the plant cells and they are generally irregular in shape taking various forms of shapes, due to lack of the cell wall. Some cells are round, oval, flattened or rod-shaped, spherical, concave, rectangular. This is due to the lack of a cell wall. Note: most of the cells are microscopic hence they can only be seen under a microscope in order to study their anatomy.
- But animal cells share other cellular organelles with plant cells as both have evolved from eukaryotic cells.
- As noted earlier, animal cells are eukaryotic cells with a membrane-bound nucleus. therefore they have their genetic material in the form of DNA enclosed in the nucleus. They also have several structural organelles within the plasma membrane which perform various specific functions for proper cell function and generally to maintain the body normal mechanisms.
Animal cell structure
Figure: Diagram of Animal Cell
The animal cell is made up of several structural organelles enclosed in the plasma membrane, that enable it to function properly, eliciting mechanisms that benefit the host (animal). The working together of all cells gives an animal its ability to move, to reproduce, to respond to stimuli, to digest and absorb food, etc. Generally, the combined effort by all animal cells is what enables the normal functioning of the body.
Animal cell organelles
The major cell organelles include:
Plasma membrane (Cell membrane) – Definition, Structure, and Functions with Diagram
Definition of Plasma membrane (Cell membrane)
It is a thin semipermeable protein-membrane layer that surrounds an animal cell.
Figure: Diagram of Plasma membrane (Cell membrane)
Structure of Plasma membrane (Cell membrane)
- Thin semi-permeable membrane
- It contains a percentage of lipids making a semi-permeable barrier between the cell and its physical environment.
- It has some protein components a
- It is very consistent around the cell
- All living cells have a plasma membrane.
Functions of Plasma membrane (Cell membrane)
- To enclose and protect the cell content
- To also regulate the molecules that pass into and out of the cell, through the plasma membrane. Therefore it controls homeostasis.
- The proteins are actively involved in transporting materials across the membrane
- The proteins and lipids allow cell communication, and carbohydrates (sugars and sugar chains), which decorate both the proteins and lipids and help cells recognize each other.
Nucleus – Definition, Structure, and Functions with Diagram
Definition of Nucleus
- This is a spherical structured organelle found majorly at the center of a cell surrounded by a double-layered nuclear membrane separating it from the cytoplasm.
- It is held together to the cytoplasm with the help of the filaments and microtubules.
- It holds other cells organelles including the nucleolus, nucleosomes, and chromatins.
- A cell has one nucleus which divides producing multinucleated cells e.g. the skeletal muscle cell fibers.
- Some cells lose their nuclei after maturations e.g. the red blood cells.
Figure: Diagram of Nucleus
Structure of Nucleus
- The double-layered membrane is a continuous channel of membranous from the endoplasmic reticulum network.
- The membrane has pores which allow entry of large molecule
- Nucleoli (Singular; nucleolus) are tiny/small bodies found in the nucleus
- The nucleus and its component organelles are suspended in the nucleoplasm (House of the chromosomal DNA and genetic materials)
Functions of Nucleus
- The primary role of the nucleus is to control and regulate cell activities of growth and maintain cell metabolisms.
- It also carries the genes that have hereditary information of the cell.
- The chromosomal DNA and genetic materials, which are made up of genetic coded ultimately make up their proteins’ amino acid sequences for use by the cell.
- Therefore, the nucleus is the information center.
- It is the site for Transcription (formation of mRNA from DNA) and the mRNA is transported to the nuclear envelope.
Definition of Cytoplasm
- This is a gel-like material that contains all the cell organelles, enclosed within the cell membrane.
- These organelles include; Mitochondria, ribosomes, Endoplasmic reticulum, Golgi apparatus, lysosomes intermediate filaments, microfilaments microtubules, vesicles.
Figure: Diagram of Cytoplasm
Mitochondria – Definition, Structure, and Functions with Diagram
Definition of Mitochondria
- These are membrane-bound organelles located in the cytoplasm of all eukaryotic cells
- The number of mitochondria found in each cell varies widely depending on the function of the cell it performs.
- For example, erythrocytes do not have mitochondria while the liver and muscle cells have thousands of mitochondria.
Figure: Diagram of Mitochondria
Structure of Mitochondria
- They are rod-shaped or oval or spherically shaped, with a size of 0.5 to 10 μm.
- Mitochondria have two special membranes – outer and inner membrane.
- They have a mitochondrial gel-matric in the central mass.
- The membranes bend into folds known as cristae.
Functions of Mitochondria
- Their primary function is to generate energy for the cell i.e they are the power generators, producing energy in form of Adenosine Tri-phosphate (ATP), by converting nutrients and oxygen into energy enabling the cell to perform its function and to also release excess energy from the cell.
- Mitochondria also store calcium which assists in cell signaling activity, generating cellular and mechanical heat and mediating cellular growth and death.
- The outer membrane is permeable, allowing the transport of small molecules and a special channel to transport large molecules.
- The inner mitochondrial membrane is less permeable thus allowing very small molecules into the mitochondrial gel-matrix in the central mass. The gel matrix is composed of the mitochondria DNA and enzymes for the Tricarboxylic Acid (TCA) cycle or the Kreb’s Cycle.
- The TCA cycle uses up the nutrients, converting them into by-products that the mitochondria use for producing energy. These processes take place in the inner membrane because the membrane bends into folds called the cristae, where the protein components used for the main energy production system cells, known as the Electron Transport Chain (ETC). ETC is the main source of ATP production in the body.
- The ETC involves several sequences of oxidation-reduction reactions to transport electrons from one protein component to another, thus producing energy that is used for phosphorylation of ADP (Adenosine diphosphate) to ATP. This process is called the chemiosmotic coupling of oxidative phosphorylation. This mechanism gives energy to most cellular activities including muscle movement and they power up the general brain function.
- Some if not all proteins and molecules that make up the mitochondria come from the cell nucleus. The mitochondrial nucleus genome has 37 genes of which 13 of these genes produce most of the components of the ETC. However, the mitochondrial DNA is very vulnerable to mutations because they don’t possess a large DNA repair mechanism, a common element found in other nuclear DNAs.
- Moreover, Reactive Oxygen Species ((ROS)) also called free radicals are produced in the mitochondrion, because of the preference for abnormal production of free electrons. These electrons are neutralized by antioxidant proteins in the mitochondrion. However, some of the free radicals can damage mitochondrial DNA (mtDNA).
- Equally, consumption of alcohol can cause damage to the mtDNA because excess ethanol in the body causes saturation of the detoxifying enzymes leading to the production and leakage of highly reactive electrons into the cytoplasmic membrane and into the mitochondrial matrix, combining with other cellular molecules forming numerous radicals that significantly cause cell damage.
- Most organisms inherit the mtDNA from their mother. This is because the maternal egg donates most of the cytoplasm to the embryo while the mitochondria inherited from the father’s sperm is destroyed. This causes the origin of inherited and acquired mitochondrial diseases due to mutations transmitted into the embryo from the maternal and paternal DNA or maternal mtDNA. Such diseases include Alzheimer’s disease and Parkinson’s disease. When mutated mtDNA accumulates over time has been linked to aging and the development of certain cancers and diseases.
- Naturally, mitochondria play a major role in programmed cell death (apoptosis) and due to mutations in the mtDNA can inhibit cell death-causing the development of cancer.
Ribosomes – Definition, Structure, and Functions with Diagram
Definition of Ribosomes
- They are small organelles majorly made up of 60% RNA cytoplasmic- granules and 40% proteins.
- All living cells contain ribosomes, which may be freely circulating in the cytoplasm and some are bound to the endoplasmic reticulum.
- It is the site for protein synthesis.
Figure: Diagram of Ribosome
Structure of Ribosomes
- Ribosomes are made up of ribosomal proteins and ribosomal RNA (rRNA). In a eukaryotic cell, ribosomes constitute half ribosomal RNA and half ribosomal proteins.
- Each ribosome is made up of two subunits i. e large subunit and small subunit with their own distinct shapes. These subunits are designated as the 40s and 60s in the animal cell.
Functions of Ribosomes
- Ribosomes that occur as free particles are attached to the endoplasmic reticulum membrane occurring in large numbers accounting for about a quarter of the cell organelles. A single replicated cell has about 10 million ribosomes.
- The ribosomal subunits are the site for genetic coding into proteins. On the ribosomes, the mRNA helps determine the coding for Transfer RNA (tRNA) which also determines the protein amino acid sequences. This leads to the formation of the rRNA which are involved in the catalyzation of peptidyl transferase creating the peptide bond found between the amino acid sequences that develop the proteins. The formed proteins then detach from the ribosomes, migrating to other cell parts for utilization by the cell.
Endoplasmic Reticulum (ER) – Definition, Structure, and Functions with Diagram
Structure of Endoplasmic Reticulum (ER)
- This is a continuous folded membranous organelle found in the cytoplasm made up of a thin network of flattened interconnected compartments (sacs) that connects from the cytoplasm to the cell nucleus.
- Within its membranes, there are membranous spaces called the cristae spaces and the membrane folding are called cristae.
- There are two types of ER based on their structure and the function they perform including Rough Endoplasmic reticulum and the Smooth endoplasmic reticulum.
Figure: Diagram of Endoplasmic Reticulum (ER)
Functions of Endoplasmic Reticulum (ER)
- Manufacturing, processing and transporting proteins for cell utilization both in and out of the cell. This is because it is directly connected to the nuclear membrane providing a passage between the nucleus and the cytoplasm.
- The ER has more than half the membranous cell content, hence it has a large surface area where chemical reactions take place. They also contain the enzymes for almost all the cell lipid synthesis hence they are the site for lipid synthesis.
The variation in physical and functional characteristics differentiate the ER into two types i.e Rough endoplasmic reticulum and Smooth endoplasmic reticulum.
Types of Endoplasmic Reticulum
- Rough Endoplasmic Reticulum (Rough ER) – Rough ER is called “rough” because there surface is covered with ribosomes, giving it a rough appearance. The function of the ribosomes on rough ER is to synthesis proteins and they have a signaling sequence, directing them to the endoplasmic reticulum for processing. Rough ER transports the proteins and lipids through the cell into the cristae. They are then sent into the Golgi bodies or inserted into the cell membrane.
- Smooth Endoplasmic Reticulum (Smooth ER) – Smooth ER is not associated with ribosomes and their unction is different from that of the rough endoplasmic reticulum, despite lying adjacent to the rough endoplasmic reticulum. Its function is to synthesis lipids (cholesterol and phospholipids) that are utilized for producing new cellular membranes. They are also involved in the synthesis of steroid hormones from cholesterol for certain cell types. It also contributes to the detoxification of the liver after the intake of drugs and toxic chemicals.
- There is also a specialized type of smooth ER known as the sarcoplasmic reticulum. Its function is to regulate the concentration of Calcium ions in the muscle cell cytoplasm.
Golgi apparatus (Golgi bodies/Golgi complex) – Definition, Structure, and Functions with Diagram
Structure of Golgi apparatus (Golgi bodies)
- These are membrane-bound cell organelles found in the cytoplasm of a eukaryotic cell, next to the endoplasmic reticulum and near the nucleus.
- Golgi bodies are supported together by cytoplasmic microtubules and held by a protein matrix
- It is made up of flattened stacked pouches known as cisternae.
- These cisternae maybe 4- 10 in number for animal cell Golgi bodies though some organisms like single-celled organisms have about 60 cisternae.
- They have three primary compartments known as cis (Cisternae Nearest the Endoplasmic Reticulum), medial (central layers of cisternae) and the trans (cisternae farthest from the endoplasmic reticulum).
- Animal cells have very few (1-2) Golgi bodies while plants have a few hundred.
Figure: 2D and 3D Diagram of Golgi apparatus (Golgi bodies or Golgi complex)
Functions of Golgi apparatus (Golgi bodies)
- Their primary function is to transport, modify and pack proteins and lipids into the Golgi vesicles to deliver them to their target sites. Animal cells contain one or more Golgi bodies while plants have a few hundred.
- Cis and trans Golgi network make up the outer layer of cisternae at the cis and trans face and they are responsible for sorting proteins and lipids received at the cis face and released by the trans face, by the Golgi bodies.
- The cis face collects the proteins and lipids, of fused vesicles in clusters. The fused vesicles move along the microtubules through a specialized compartment known as the vesicular-tubular cluster. This compartment is found between the endoplasmic reticulum and the Golgi apparatus.
- The vesicle clusters fuse with the cis Golgi network, delivering the proteins and lipids into the cis face cisternae and as they move from the cis face to the trans face, they get modified to functional units. These functional units get delivered to intracellular and extracellular components of the cell.
- Modification mechanisms include:
- Cleaving of oligosaccharides chains
- Attachment of sugar moieties of different side chains
- Adding fatty acids and/or phosphate groups by phosphorylation, and/or removing monosaccharides e.g. the removal of the mannose moieties takes place in the cis and the medial cisternae while adding of galactose takes place in the trans cisternae.
- Sorting of the modified proteins and lipids occurs in the trans-Golgi network and packed into the trans vesicles, which then delivers them to the lysosomes or sometimes to the cell membrane for exocytosis. Assisted by ligands bound to receptors triggering fusion and protein secretion.
Lysosomes – Definition, Structure, and Functions with Diagram
It is also known as cell vesicles; Lysosomes were discovered by Christian Rene de Duve, a Belgian cytologist in the 1950s.
Figure: 2D and 3D Diagram of Lysosomes
Structure of Lysosomes
- They are round subcellular organelle found in almost all eukaryotic cells
- Lysosomes are very acidic organelles containing the digestive enzymes and therefore each of the lysosomes is surrounded by a membrane to protect it from the outer environment.
Functions of Lysosomes
- This is the site for digestion of cell nutrients, excretion, and cell renewal.
- Lysosomes break down macromolecules components from the outside of the cell into simpler elements that are transported into the cytoplasm via a proton pump to build new cell materials.
- These macromolecule components include old cells and parts, cell waste products, microorganisms, and cell debris.
- The digestive enzymes found in the lysosomes are called hydrolytic enzymes or acid hydrolases, breaking down large molecules into smaller molecules that can be utilized by the cell.
- These enzymes also break down large molecules e. g proteins, carbohydrates, lipids, into small molecules e.g. amino acids and simple sugars, fatty acids, respectively.
- Note: The enzymes are active only on the inside of the acidic lysosome and their acidity protects the cell from degrading itself when there is lysosomal leakage because the cell pH is neutral to slightly alkaline.
Cytoskeleton – Definition, Structure, and Functions with Diagram
Structure of Cytoskeleton
- This is a fibrous network that’s formed from and by different proteins of long chains of amino acids.
- These proteins are found in the cell cytoplasm of the eukaryotic cells.
- They are also made up of 3 types of tiny filaments: Actin filaments (Microfilaments), Microtubules, Intermediate filaments.
Figure: Diagram of Cytoskeleton
Functions of Cytoskeleton
- The cytoskeleton functions to create a network organizing the cell components and to also maintain the cell shape.
- It also provided a uniform movement of the cell and its organelles, by the filament system network found in the cell’s cytoplasm.
- It also organizes some of the cell components maintaining the cell shape
- It plays a major role in the movement of the cell and some cell organelles in the cytoplasm.
- The tiny filaments include:
- Actin filaments; also known as microfilaments; it’s a meshwork of fibers running parallel to each other and they play a primary role in giving the cell its shape; they change consistently, helping the cell to move and to also mediate certain cell activities such as adherence ability to substrates and cleavage mechanisms during mitotic cell division
- Microtubules- these are long filaments that assist in mitosis moving daughter chromosomes to new forming daughter cells.
- Intermediate filaments– they are more stable filaments in comparison to the actin and microtubules. They form the true skeleton of the cell, and the hold the nucleus in its rightful position within the cell.
- It also allows the cell’s elasticity factor enabling it to endure physical tension.
- Other proteins that may be added as part of the cytoskeleton of the cell include septin ((assembles the filaments) and spectrin (help maintain the structure of the cell by pulling together the cell membrane with the intracellular surface of the cell).
Microtubules – Definition, Structure, and Functions with Diagram
Structure of Microtubules
- These are long, straight, hollow cylinders filaments that are constructed from 13-15 sub-filaments (protofilament) strand of a special globular protein called tubulin, found only in eukaryotic cells.
- They are found throughout the cytoplasm of the animal cell.
Figure: Diagram of Microtubules
Functions of Microtubules
- Transportation of some organelles like the mitochondria and the vesicles i.e. transporting vesicles from the neuron cell body to the axon tips, and back to the cell body
- Structural support, they give characteristic support to the Golgi bodies, holding them within the gel-matrix of the cytoplasm.
- They provide the rigid and organized component of the cytoskeleton of the cell, enabling a cell to take up a particular shape.
- They are the main elements that make up the locomotive projections of a cell (cilia and flagella)
- They also play a role in forming the spindle fibers of the chromosome of the cell during mitotic cell division.
Centrioles – Definition, Structure, and Functions with Diagram
This is distinctly found in the animal cell, which has the ability to replicate or make copies by itself. It is made up of 9 microtubule bundles and their primary function is to assist in organizing the cell division process.
Figure: Diagram of Centrioles
Structure of Centrioles
- It is a small structure that is made up of 9 sets of microtubules, placed in groups of three hence they are triplet microtubules.
- As triplets, they remain very strong together hence they have been observed to be in structures like cilia and flagella.
- The triplet microtubules are held together by proteins, giving the centriole its shape.
- They are found in the centrosome, creating and holding microtubules within the cell.
- The triplet microtubules are surrounded by a pericentriolar matrix containing molecules that build up the microtubules.
- Each microtubule within the triplet microtubule complex is made up of tubulin subunits that join together forming long hollow tubes that look like straw (microtubules).
Functions of Centrioles
- The centriole microtubules allow the transportation of substances that are linked together with a glycoprotein to any cell location. the glycoprotein linkage acts as a signaling unit to move specific proteins.
- The centrioles anchor the microtubules that extend from it and contain the factors needed to create more tubules.
- Mitosis is achieved by replication of each centriole which makes duplicates of each centriole (4 centrioles). The newly formed centrioles divide into two centrosomes, each centriole at an angle to the second centriole. The microtubules between the centrosomes, push the pairs of centrioles apart, to the opposite ends of the cell. When the centrioles are in place, the microtubules extend to the cell cytoplasm, to seek for the chromosome. The microtubules then bind to the chromosome at the centromere. The microtubules are then unassembled fro the centriole moving the chromosomes apart.
Peroxisomes – Definition, Structure, and Functions with Diagram
These are tiny bodies found in the cytoplasm.
Figure: Diagram of Peroxisome
Structure of Peroxisomes
- They are spherically shaped, bound by a membrane and they are the most common micro-bodies in the cell cytoplasm.
Functions of Peroxisomes
- Peroxisomes functions include:
- Lipid metabolism
- Chemical detoxification by moving hydrogen atoms from various oxygen molecules to produce hydrogen peroxide, hence neutralizing body poison such as alcohol.
- Its mechanism in Reactive Oxygen species is highly essential.
Cilia and Flagella – Definition, Structure, and Functions with Diagram
These are locomotive projections found on the surface of the cell.
Figure: Diagram of Cilia and Flagella
Structure of Cilia and flagella
- They are made of strands of filaments. these filaments have partial and complete microtubules that extend the projections. Partial microtubules don’t extend to the tip of the cilium and the complete microtubules extend to the tip of the cilium.
- The microtubules also have motor proteins known as dynein making a link between the partial microtubules to the complete microtubules.
- The whole collection is combined together as extensions on the plasma membrane of the cell.
Functions of Cilia and flagella
- Sperm cells have flagella allowing it to swim to the ova for fertilization. For single cells, such as sperm, this enables them to swim.
- Cilia in the animal cell helps move fluids away from and past immobile cells.
- Cilia help move surface particles especially on the epithelial lining of the nostrils, and moving mucus over the surface of the cell.
Endosome – Definition, Structure, and Functions with Diagram
These are vesicles bound by membranes and formed by a mechanism of endocytosis. They are found in the cell cytoplasm.
Figure: Diagram of Endosomes
Structure of Endosome
- They are membranous organelles that are bound to the cell membrane.
Functions of Endosome
- Its main function involves folding in of the plasma membrane. The folding allows diffusing in of molecules through the extracellular fluids.
- Their primary role is to remove waste materials from the cell by endocytic processes such as exocytosis and phagocytosis
Vacuoles – Definition, Structure, and Functions with Diagram
These are fluid-filled cell organelles enclosed by a membrane.
Figure: Diagram of Vacuole
Structure of Vacuoles
- They are membrane-bound sacs found within the cell cytoplasm.
- The vacuole sac has a single membrane surrounding it known as a tonoplast and this membrane resembles the plasma membrane.
Functions of Vacuoles
- their primary function is to store food, water, carbohydrates in the form of sugars and waste materials.
- Tonoplast is a regulator controlling the inflow and outflow of small across a protein pump
- acts as the guard for what kinds of matter are allowed passage to and from vacuoles
- They also remove toxic substances and waste materials from the cell as a protection strategy.
- They also remove poorly folded proteins from the cell.
- Vacuoles also can be able to change their functionality to provide necessary roles that suit the cell, by being able to change shape and size.
Microvilli – Definition, Structure, and Functions with Diagram
These are surface protrusions found in the intestinal lining, on egg cell surfaces, and on white blood cells.
Figure: Diagram of Microvilli
Structure of Microvilli
- These are surface protrusions formed from accessory proteins of the actin filaments. The accessory proteins bundle together to form microvilli on the surface of the cell membrane
Functions of Microvilli
- In the small intestines, they increase the surface area for the absorption of digested food and water. Some microvilli may be found in the ear for detection of sound and they transmit the sound waves to the brain through an electric signal.
- They also help to anchor the sperm to the egg for easy fertilization.
- In white blood cells, they also act as anchors allowing the white blood cells freely moving in the circulatory system to attach to possible pathogens.
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