Integral proteins can be located on either surface of the membrane, (inside or outside), or may span the entire membrane. They are “locked” in place, and have many different functions, such as identifying, receiving, and communicating with other molecules and cells.
Peripheral proteins are bound to the plasma membrane or to integral proteins by chemical or covalent bonds, and also aid in cell signaling and receptor modulation. Ultimately, the cell membrane’s unique structure provides a cell with protection, stability, and communication.
Cytoplasm
The cytoplasm can be described as a gel-like fluid matrix that resides inside a cell between the cell membrane and the nucleus, the cell’s spherical epicenter and command center.
The cytoplasm serves as the interior environment that accommodates each cell’s essential structural inclusions, called organelles. Appropriately termed, organelles function like “microscopic organs” within the cell, responsible for the cell’s vitality – similar to how the internal organs are responsible for the human body’s vitality as a whole.
The following organelles, which are found in the cytoplasm, will be discussed further: ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, peroxisomes, centrosomes, cilia, flagella, and microvilli.
The cytoplasm is one of numerous important components of cellular structures. Not only does it help to give the cell its shape, but cytoplasm contains three very important components:
•Cytosol (also known as “cell sap”)
•Organelles
•Inclusions
In medical terminology, an inclusion is defined as something that encloses something. Medical dictionaries define a cellular inclusion as a non-living material found in cellular protoplasm. These substances are unable to carry out metabolic activities, nor are they bound by membranes. A few examples of cellular inclusions4:
•Nutritional substances
•Granules of pigment
•Droplets of fat
•Granules of glycogen
Cytosol (aka “cell sap”) Cytosol, often called the cytoplasmic matrix, is the liquid found inside the cell, and makes up a majority of intracellular fluid. Cytosol is often likened to a jelly or gel like substance.
Figure 2-3 Cellular structure.
Organelles literally means “little organs.” Each organelle in the cytoplasm is responsible for performing specific functions. Organelles are defined as a number of different and unique structures found within a living cell.
Organelles found in cytoplasm have specific responsibilities and functions just like the body’s larger organs. Each of these will be described in further detail.
The nucleus of the cell is an organelle. So too is the cellular wall. Other organelles found within a cell include:
•Centrioles – typically found in animal cells.
•Centrosomes – play a role in mitosis and serve as the processing or organizing center of a microtubule. Centrosomes are necessary in the construction of the mitotic spindle. We explore this topic in more depth later in the chapter.
•Chloroplasts – also part of numerous cellular structures including those of plants. They behave in a similar way to an animal cell’s mitochondria. The outer membrane is permeable although the inner membrane typically allows transportation through membrane transporters. A third membrane is found in the chloroplast and is known as an alkaloid membrane, required for energy generation and involves adenosine triphosphate as part of the energy production chain.
•Endoplasmic reticulum (smooth or rough) – smooth endoplasmic reticulum (ER) plays a role in lipid synthesis, while rough ER is vital in the synthesis of proteins.
•Golgi complex – responsible for “receiving” macromolecules from the endoplasmic reticulum and for sorting and processing so that these macromolecules reach their appropriate destinations inside the cell.
•Lysosomes – mainly defined as the primary catabolic organelle.
•Mitochondria – plays a vital role in the production of energy in eukaryotic cells.
•Peroxisomes – contain enzymes involved in a number of biochemical reactions in the body.
•Ribosomes – contain liposomal RNA proteins and molecules that float freely within the cytoplasm or they can be attached or embedded on the outer membrane surfaces of rough ER.
•Vacuoles – fluid-filled structures most commonly found in fungal and plant cells. They’re involved in waste management, detoxification, storage, and molecular catabolism or degradation. They’re responsible for the maintenance of structure and support of a cell, often known as turgor pressure maintenance.
Ribosomes
Ribosomes are organelles constructed primarily of two microscopic subunits. The subunits; one large and one small, are made up of ribosomal RNA (rRNA).
Ribosomes provide the site for synthesis of proteins via a process called translation, where the genetic code, or DNA, is read and translated from an mRNA molecule into a specific amino acid sequence producing the specified protein.
Ribosomes are sometimes found attached to another intracellular organelle called the endoplasmic reticulum or are simply free-floating in the cell’s cytoplasm. At both locations, their function remains the same – the biosynthesis of proteins. Hence, they have been nicknamed the “protein factories” of a cell.
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a system of membranes arranged into a network of tubules, canals, and connecting sacs that form a convoluted channel in the cytoplasm that travels from the nucleus of a cell and stretches almost to the outer plasma membrane.
As briefly mentioned, there are two types of ER: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
The RER is named due to the ribosomes that are attached to its outer surface, giving it a studded or “rough” appearance. It should be mentioned here that ribosomes are not permanently docked on the RER; they dock only in preparation of synthesizing a protein designed for the secretory pathway. This pathway describes a process of steps that transport proteins out of a cell.
Once synthesized, the proteins are dropped off into the interior of the RER and packaged into vesicles bound for the Golgi apparatus. Two types of proteins are made in the RER:
•secretory proteins
•integral membrane proteins
Figure 2-4 Endoplasmic reticulum structure.
The SER, true to its namesake and discernibly lacking the rough exterior, serves to synthesize
