This lecture, Part 3 of 4, will cover the structure of function of cells. The topics of cell organelles and the cell nucleus will be discussed. This diagram, of a model cell, represents the many components found in cells and describes each of their functions.
It is important to note that not all cells contain everyone of the components shown here. This cell serves as a model for educational purposes. The cell membrane, or plasma membrane, is a phospholipid bilayer structure that separates the cell from its external environment. The external environment of the cell is known as the extracellular or interstitial fluid. The cytoplasm found inside of the plasma membrane consists of the cytosol, also known as the intracellular fluid and organelles. The cytosol or intracellular fluid has high concentrations of proteins potassium ions and low concentrations of sodium compared to extracelluar fluid. Inclusions, such as lipid droplets and glycogen, may also be present in the cytosol.
The organelles are broken down into two main groups: the nonmembranous organelles and membranous organelles. The nonmembranous organelles include the centrioles, the cytoskeleton, the microvilli, cilia, proteasomes, and ribosomes. The membranous organelles include peroxisomes, lysosomes, the golgi apparatus, mitochondria, and the endoplasmic reticulum. Let's take a look at some of these organelles! The centrioles, seen here... ...play a role in the movement of chromosomes during cellular division. The cilia are long, finger-like projections that extend..
...from the outer surface of the cell and help move material over the surface of the cell. Cells with cilia are found in the reproductive & respiratory tracks. Proteasomes, seen here, are scattered throughout the cytoplasm. They contain proteolytic enzymes, which are enzymes that can help break down proteins. Ribosomes contain RNA... and can be found either free or fixed: free throughout the cytoplasm or fixed along the endoplasmic reticulum and we'll discuss that in greater detail coming up. Ribosomes are the sites of protein synthesis.
The remaining organelles on this page will be discussed in more detail on the following slides. The cytoskeleton provides the framework of the cell. It is composed of microfilaments, intermediate filaments, and microtubules. Microvilli are finger-like projections of the plasma membrane that help increase the surface area of the cells they're found in. Microvilli can be found in cells such as cells in the digestive track and cells in the kidney. The endoplasmic reticulum, a membranous organelle, consists of a network of intracellular membranes connected to the nuclear envelope that surrounds the nucleus of the cell. The endroplasmic reticulum's functions include the synthesis of proteins, lipids, and carbohydrates; the storage of substances; the transport of material throughout the endoplasmic reticulum; and the detoxifying of drugs and other toxins through the use of enzymes found in the structure. The endoplasmic reticulum has two different regions: the smooth endoplasmic reticulum and the rough endoplasmic reticulum.
The smooth version does not have ribosomes, while the rough version does. The smooth endoplasmic reticulum is responsible for... the synthesis of phospholipids and cholesterols in all cells, the synthesis of steroid hormones and the reproductive organ cells, ...the synthesis of glycerides in the liver and fat cells, and the synthesis and storage of glycogen in the liver and skeletal muscle cells. The ribosomes and of the rough endoplasmic reticulum synthesis proteins.
Some of these proteins stay in the endoplasmic reticulum and others are transported to the golgi apparatus in vesicles. The Golgi Apparatus is another membranous organelle is composed on flatten membranous discs called cisternae. The function of the Golgi Apparatus is to modify and package secretions, to renew the plasma membrane, and to package enzymes. The three types of vesicles, the Golgi Apparatus, prepares are lysososmes, secretory vesicles, and membrane renewal vesicles. The lysosomes contain digestive enzymes and play a role in the clean up and recycling of cellular materials. Lysosomes also support the autolysis of cells.
If a cell is dead or damaged, the membrane of lysosome will degrade, releasing the digestive enyzmes inside, into the cell. The result is the destruction of the cell's components: autolysis. The secretory vesicles contain secretions that will leave the cell and membrane renewal vesicles fuse with the plasma membrane, bringing new lipids and proteins to the region.
Peroxisomes are designed to break down fatty acids and neutralize free radicals, such as hydrogen peroxide, that are formed through this process. The mitochondria are considered the powerhouse of the cell. They are the major site of ATP production. The number of mitochondria in a cell varies based of the cell's energy needs. For example, red blood cells have no mitochondria, while cardiac muscle cells have many. A mitochondrion is formed of a double membrane, the outer membranes surround the organelle and the inner membrane is folded into cristae, that are surrounded by the matrix.
Here, the outer and inner membrane with its folds, the cristae can be seen. The mitochondria utilizes aerobic metabolism or cellular respiration to produce ATP. The nucleus is the largest, inner cellular structure and can be thought of as the control center of the cell. Most cells have a nucleus, but there are exceptions. Red blood cells have none and skeletal muscles have many.
The nucleus contains DNA, the genetic material that codes for protein synthesis and the traits of the organism. The DNA is found in the nucleoplasm of the nucleus. The double membrane that surrounds the nucleus is called the nuclear envelope and it provides a barrier between the cytosol of the cell and and the nucleoplasm of the nucleus. The communication between these two regions takes place through the nuclear pores that are scattered throughout the envelope. The nucleoli of the nucleus are organelles involved in the synthesis of ribosomal RNA and the construction of ribosomal subunits. Chromatin refers to the loose and uncoiled structure of the nucleosomes, which are composed of DNA and histones. When the cell is preparing for division, the DNA is tightly wound around the histones and the chromosomes are visible. In a nondividing cell, chromatin, the loose, uncoiled form is present.
Both are represented in this diagram here. Let's review with a checkpoint! Can you identify the membranous organelles and list their functions? Feel free to pause this lecture while you try to formulate your response. The membranous organelles includes the endoplasmic reticulum, which produces secretory products and plays a role in intercellular storage and transport. We have the rough endroplasmic reticulum, involved in protein synthesis, and the smooth endroplasmic reticulum involved in lipid and carbohydrate synthesis. The Golgi Apparatus stores and packages secretory products and digestive enzymes found in lysosomes and the lysosomes are responsible for breaking down damaged organelles and pathogens in the cell.
The mitochondria make energy, the ATP for the cell, and peroxisomes that break down fatty acids and neutralize toxic substances that result.
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