What is the cell organelle defination?
Have you ever thought about what eukaryotic cells are made of? Although cells are the basic units of life, they are not simple protoplasm. The subcellular structures of eukaryotes known as organelles within cells complete specific functions, and these subunits are usually separately enclosed within their own lipid bilayer. Organelles are to cells what organs are to the body, and the name "organelle" means the little organ. They are well-defined and enclosed in membranes in eukaryote, while in prokaryote, organelles are not enclosed by membranes and are far simpler.
Why do eukaryotes have membrane bounded cell organelles?
Eukaryotes are usually ten times the diameter of prokaryotes, and a thousand times in volume. This vast size means a smaller surface-to-volume ratio. To minimize interference between biochemical reactions and to increase the membrane area, eukaryotic cells are divided into many small compartments to perform specific tasks. These organelles occupy about half the space of eukaryotic cell. They work as efficiently and orderly as a modern super factory’s workshops responsible for specific processes, whereas prokaryotes are more like modestly equipped workrooms.
Eukaryotic Cell Organelles Structure & Functions
Nucleus: DNA, chromosomes, control center
The oval or spherical nucleus is the control center of the cell. It is surrounded by a double-layered membrane known as the nuclear envelope, with pores that allow molecules to enter and exit. The nucleus also contains a small spherical structure called the nucleolus where ribosomes are made. Genetic material (DNA) controls the synthesis of proteins to regulate the cell's life activities. DNA is filamentous most of the time, while it folds into short rods that are visible under a microscope as chromosomes during cell division.
What organelles do photosynthesis and aerobic respiration occur in?
Mitochondria: Aerobic respiration, Cell Powerhouse
Mitochondria are organelles enclosed by a double membrane. The smooth and highly permeable outermost membrane makes ions and small organic molecules pass easily. The inner membrane contains more proteins and has structures that fold inward, called cristae, which increase the surface area for chemical reactions. Ions and organic substances enter and exit the inner membrane with the help of carrier proteins, making it less permeable.
Mitochondria are the sites responsible for cellular aerobic respiration. Over 100 enzymes are located in their membranes and matrix, and most of them are related to aerobic respiration. The ATP produced by mitochondria is transported across the membrane to other parts of the cell to participate in life activities. Cells with vigorous metabolism and high energy consumption have more mitochondria with dense cristae; conversely, cells with weaker metabolism have fewer mitochondria with sparser cristae.
Chloroplasts: Photosynthesis, Food Producer
Chloroplasts are found in plant cells, algae and some protozoa. They convert light energy into chemical energy through a process called photosynthesis. Chloroplasts are surrounded by a double membrane. The outer membrane envelops the entire organelle and the porins allow molecules under 10,000 molecular weight to pass through. The low-permeability inner membrane acts as a barrier between the cytoplasm and the chloroplast matrix, allowing only some small molecules to pass. Larger molecules require the help of transport proteins on the inner membrane to enter and exit the chloroplast.
The space enclosed by inner membrane is called matrix where located many flat sacs called thylakoids. They are interconnected each other to form a continuous entity. The chlorophyll for sunlight absorption is discovered in thylakoids membranes where ATP and NADPH are produced. The fixation of carbon dioxide to sugars takes place in the matrix. Chloroplasts and mitochondria also have their own DNA and ribosomes. It is suggested they may have evolved from ancient bacteria engulfed by a larger cell.
Ribosomes – Machines for Protein synthesis
Ribosomes are organelles responsible for synthesizing proteins. They consist of a large subunit and a small subunit. Each subunit contains ribosomal RNA (rRNA) and proteins. Ribosomes exist freely in the cytoplasm or be attached to the endoplasmic reticulum (ER). Proteins manufactured by free ribosomes stay in the cytoplasm or enter other organelles, while those made by ribosomes attached to the ER are exported outside the cell or inserted into the cell membrane.
Endoplasmic Reticulum Processes Proteins and Lipids
The single-layer membrane and its enclosed cavity make up the continuous structure of endoplasmic reticulum. It extends from the nucleus to the plasma membrane. It occupies about 10% of the cell's volume, and half of the membrane system coming from it. The flat areas with attached ribosomes are called rough ER. The peptides synthesized here become the ER transmembrane proteins or enter its interior for further processing. Another part of the ER, without ribosomes, is called the smooth ER. These tubular structures are involved in lipid synthesis, detoxification and calcium storage. Organic substances produced in the ER are usually transported to the Golgi apparatus for further processing.
Golgi body: Protein Modification and Transport
The Golgi apparatus, also known as the Golgi complex, is an organelle made up of multiple stacked flat membrane sacs. These sacs are surrounded by vesicles of varying sizes. Precursors from ER enter the Golgi apparatus from cis region and then move through the sacs with the help of vesicles. During this process, the precursors are modified with sugars, phosphates or other groups. Finally, vesicles carrying the finished products bud off from the trans region and leave. These vesicles are delivered to other organelles such as lysosomes and peroxisomes, or secreted outside the cell.
Lysosomes: Digestive Enzymes, the Principal Sites of Intracellular Digestion
The single membrane lysosomes are involved in intracellular digestion of extracellular materials and worn-out organelles. Usually, hundreds of lysosomes are found in animal cells. There are about 40 types hydrolytic enzymes in their aquatic internal environment, such as hydrolases for proteins, nucleic acids, saccharides and lipids. All of these enzymes are optimally active in about 5 PH acidic conditions. The membrane proteins in lysosome are highly glycosylated to protect them from digestion. Lysosomes can not only digest large molecules ingested by the cell, but also degrade worn-out organelles and useless biomolecules. The final products, such as amino acids, sugars, and nucleotides, are transported into the cytosol by carrier proteins on the membrane for recycling.
Central Vacuole of plant cell: Digestive Enzymes, Pigments
In plants, there are cell organelles similar to lysosomes called central vacuoles, enclosed by a single membrane. They contain various enzymes for breaking down macromolecules such as proteins, carbohydrates, lipids, and nucleic acids. The cell sap in the plant central vacuole contains ions, sugars, amino acids, proteins, alkaloid and so on. Some plant central vacuoles also contain pigments that display a colorful appearance. The high osmotic pressure of cell sap allows plant cells to absorb water from outside. This pressure supports soft plant tissues and keeps them in a swollen state. Immature plant cells contain many small vacuoles budded from the endoplasmic reticulum. These small vacuoles fuse into one large central vacuole during cell growth. Over 90% volume is occupied by vacuole in mature plant cells.
Peroxisomes
Peroxisomes are small single-membrane cellular organelles. They contain oxidases and catalases. The number of these organelles and the types of enzymes they contain depend on the cell's environmental conditions. People who drink alcohol frequently tend to have a higher number of peroxisomes. The hydrogen peroxide produces by oxidases will quickly decomposes into water by catalase. A portion of alcohol and fatty acids is metabolized in peroxisomes.
Non-organelle Subcellular Structures
Besides cell organelles, the cell membrane, cell wall and cytoskeleton are also vital subcellular structures. They play important roles in maintaining the stability of the intracellular environment and the cell shape.
Cell Membrane, Plasma Membrane
The main component of cell membrane is phospholipid molecules that are arranged in two layers. The hydrophilic phosphate heads face the intracellular and extracellular aqueous environments. The hydrocarbon tails are hydrophobic and point toward the interior of the bilayer. Various proteins embedded in the cell membrane are served as receptors, transporters, enzymes, or anchors for other molecules. The cell membrane's selective permeability isolates the cell from its surrounding environment. It controls the entry and exit of molecules to keep homeostasis within the cell.
Cell Wall
The cell wall is an additional protective and supportive coating outside the cell membrane of plants, fungi and bacteria. It helps maintain the cell's shape and size, and prevents the cell from bursting due to osmotic pressure. The cell wall is made from different materials depending on the type of cell. For example, plant cell walls are primarily composed of cellulose.
The cable like structure from the interwound cellulose gives plants toughness and strength. Lignin in some plant cells is inserted into the gaps of cellulose to make the plant more rigid and less prone to bending. Bacterial cell walls are made of mesh-like structure called peptidoglycan which contains sugars and short peptide chains. The cell walls of fungi are made of chitin. It is a chain structure composed of thousands of N-acetyl glucosamine units.
Cytoskeleton
Biologists discovered a network structure that runs through the entire cytoplasm by the electron microscope in 1960. Then, it was named as cytoskeleton. Before this discovery, biologists believed that organelles were suspended in the cytosol and moved by free diffusion. It is generally thought the cytoskeleton exists in eukaryotic cells, but prokaryotes also have a cytoskeleton. However, it is not as complex as eukaryotes to finely control the intracellular life activities.
The components of the cytoskeleton in eukaryotes include microtubules, microfilaments, and intermediate filaments. Organelles and enzymes are attached to the cytoskeleton and move along it with the help of motor proteins and ATP. The relatively stable intermediate filaments give the cell and some organelles a stable structure. For example, the genetic material inside the nucleus is located in a cage made of intermediate filaments. Meanwhile, microtubules and microfilaments are in a dynamic state of assembly and disassembly, which is crucial for rapid changes in cell shape and precise control of life activities. For example, this is particularly important for the amoeba or immune cells engulfing prey and the equitable distribution of chromosomes during mitosis.
Since animal cells lack the rigid cell walls like plants, they rely more on the cytoskeleton system to control cell shape and movement.