All animal organs and systems comprises of structural building blocks called cells. Higher organisms like plants and animals have developed cellular structures. They have membrane-bound cell organelles like the nucleus (plural nuclei), mitochondria, vacuoles, etc. Animal and plant cells are termed eukaryotic cells.
Although both animal and plant cells are eukaryotic, a plant cell has a cell wall, and an animal cell lacks a cell wall. Different animal cells are based on the tissues or organs they form. The animal cells are skin, muscle, blood, fat, nerve, stem, and sex cells. Although cells’ types, functions, and locations vary, the essential structural components of all animal cells are the same.
The essential structural components in the animal cell include the nucleus, lysosome, mitochondria, vacuoles, Golgi bodies, ribosomes, endoplasmic reticulum, etc. Cell membrane surrounds these cell organelles and the organelles work together to keep the cell active by performing their functions.
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Structural Composition of Animal Cell
Animal cells are eukaryotic cells bound by cell membranes instead of rigid cell walls and have membrane-bound cell organelles. Most animal cells are visible only under the microscope due to their size (ranging from 1 µm to a few mm). The largest known animal cell is the ostrich egg (130-170 mm in diameter).
The shape of animal cells is diverse. An animal cell can be oval, round, rod-shaped, concave, curved, or rectangular. The membrane-bound cell organelle- nucleus encloses the genetic material (DNA).
The following are the cellular structure of animal cells with their functions:
Cell membrane
The outermost layer of animal cells is the cell membrane. The cell membrane function increases in animal cells due to the lack of cell walls. The cell membrane helps transport the material in and out of the cell and provides a protective covering for the cell organelles.
It is also called the plasma membrane and comprises fatty acid-based lipids and proteins. The cell membrane is composed of two types of lipids; sterols and phospholipids. These lipids are amphiphilic (attract to water and organic solvents), making them a suitable cell membrane building block. The proteins in the cell membrane are also of two types; extrinsic and intrinsic proteins.
The extrinsic proteins loosely bind to the electrically charged phosphoryl group and intrinsic proteins. The intrinsic proteins embed inside the phospholipid bilayer.
Cytoplasm
The gel-like substance inside the cell membrane is called cytoplasm. All the cell organelles lie inside the cytoplasm. The cytoplasm is composed of water, salts, and organic salts. Inside the cytoplasm, cellular growth, expansion, and activity by cell organelles occur.
It has three parts; cytosol, cell organelles, and cell inclusions. The cytosol is the liquid part of the cytoplasm. The cell organelles are membrane-bound, like the nucleus, mitochondria, or non-membrane-bound ribosomes or centrioles. The cell inclusions are the nutrients or pigments dissolved in the cytoplasm.
Nucleus
The nucleus is the membrane-bound cellular organelle that separates the nucleus from other organelles. The nuclear membrane has double layer with pores that help interact with the cytoplasm.
The membrane also binds the nucleolus and DNA, the cell’s genetic material. The nucleolus is the spherical structure inside the nucleus that helps to produce and assemble ribosomes. Here the ribosomal RNA also gets produced, which is transported to the cell’s cytoplasm for protein synthesis.
DNA (deoxyribonucleic acid) is the hereditary material of an animal cell. Its structure is a double helix, and the DNA forms by deoxyribose sugar and nucleotides. DNA tightly bind to histone protein and forms a complex known as chromatin. The chromatin condenses to form chromosomes.
Mitochondria
Mitochondria are the cellular organelles bound by membranes. They are also known as the cell’s powerhouse because mitochondria produce the chemical energy required for the proper functioning of the cell. Different metabolic processes like the Krebs cycle, urea cycle, heme biosynthesis, cardiolipin synthesis, quinone, and steroid biosynthesis occur inside the mitochondria.
Oxidative phosphorylation coupled with the Kreb cycle helps in the production of the energy currency of the cell ATP. ATP is the small molecule that stores the chemical energy required for proper cell functioning.
Ribosomes
The ribosome is the non-membrane-bound cell organelle responsible for protein synthesis. It comprises of proteins and RNA molecules. DNA codes for a specific protein, and the mRNA carries the code to ribosome. The ribosome reads the code carried by mRNA and produces amino acids. The linking of amino acids forms the polypeptide chains. The polypeptide chains fold to produce specific proteins.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a group of flattened sacs that forms a network. The endoplasmic reticulum is composed of a network of membranes called cisternae. It processes and transports proteins. It also stores calcium and region for lipid metabolism.
ER is of two types; smooth and rough. Rough ER has ribosomes around it, and smooth ER has no ribosomes. The smooth ER synthesizes lipids, removes toxic substances, and acts as storage. The rough ER plays a vital role in protein synthesis.
Golgi Bodies
Golgi bodies, apparatus, or complexes consist of membrane-bound sacs named after the discoverer Camillo Golgi. It receives proteins from ER and packages proteins into vesicles. Unlike ER, the Golgi complex does not interconnect. It also helps in the packaging and processing of lipid molecules.
Centrioles
Centrioles are cylindrical structures usually occurring as pairs. These are located near the nuclear envelope. These organize the assembly of microtubules, a cytoskeleton, during the division of cells. They also decide the location of the nucleus and other cellular organelles in the cell.
Microtubules
These are hollow rods that constitute a significant component of the cytoskeleton of eukaryotic cells. Microtubules are made up of alpha- and beta-tubulin. As the animal cell lacks a cell wall, the microtubules maintain the rigidity and shape of the function. Besides rigidity and maintaining the cell’s shape, microtubules help in cell motility and intracellular transport and are also involved in mitosis.
Microfilaments
Microfilaments or actin filaments are proteinous structures inside the cell. These are structural components of the cytoskeleton and provide shape and movement to the cells. Microfilaments consists of a group of proteins called actin. When associated with myosin filaments, microfilaments help in cell contraction and movement.
Peroxisomes
Peroxisomes are membrane-bound organelles. These structures contain enzymes that help detoxify alcohol, break down fats, and form bile acid. These enzymes also contribute to the oxidation of biomolecules.
Lysosomes
Lysosomes are a group of vesicles bound by a membrane that contains digestive enzymes. These enzymes break down large molecules like organelles, carbohydrates, lipids, and proteins into small parts so cells can reuse them. These enzymes also help to protect cells against invading bacteria and viruses.
Vacuoles
Vacuoles are another type of vesicle that are membrane-bound. Unlike plant vacuoles, animal vacuoles are not well-developed and numerous. But their function is the same as plant vacuoles, which remove waste from the cell.
Types of Animal Cell
Animal cells form different tissues and organs of the human body. Based on the organs/tissue formed by the cells, they are of the following types:
- Skin cells: Skin cells are the cells that form the epidermal tissue, which protects the body from the external environment. The skin cells are of four different types; keratinocytes, melanocytes, Langerhans, and Merkel cells.
- Nerve cells: These are the cells that form the communication channel of the nervous system. The nerve cells have a cell body inside: the nucleus, an axon (major branching fiber), and multiple dendrites (minor branching fiber).
- Gamete cells: The gamete cell is the reproductive cells in the body. These cells are usually haploid. There are two types of gametes; female or ova or egg cells and male or sperm cells. Combining the two gametocytes gives rise to a diploid embryo.
- Muscle cells: Muscle cells or myocytes form the tissues of skeletal muscles and the heart. Muscle cells are of three types in the human body: cardiac muscle cells- found in the heart; smooth muscle cells- which form the muscular layer of vessels; and skeletal muscle cells- found in muscle cells connecting the skeleton.
- Blood cells: Blood consists of different cells, namely white blood cells, red blood cells (erythrocytes), and platelets (thrombocytes). White blood cells are of different types: lymphocytes, monocytes, eosinophils, basophils, macrophages, and neutrophils.
- Fat cells: Fat cells or adipocytes are special cells that can store large globules of fats. These are found underneath the skin, around the internal organs, and between the muscles.
- Stem cells: These are particular types of cells that can develop into any kind of cells in the body, from the brain to muscle cells. These cells help in repairing damaged cells or tissues. These cells are used as therapy for severe diseases like Alzheimer’s.
References
- Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011
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- Eukaryotic Cells | Learn Science at Scitable. Retrieved June 15, 2020, from https://www.nature.com/scitable/topicpage/eukaryotic-cells-14023963/
- Alberts B., Johnson A., Lewis J., et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Compartmentalization of Cells. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26907/
- Lehninger, A., Nelson, D., Cox, M., & Osgood, M. (2005). Lehninger principles of biochemistry. W.H. Freeman.
