the volume of a eukaryotic cell.6 The nucleus is separated from other parts of the cell by a nuclear envelope, or more specifically, a double-layered membrane. This space between the double layers is defined as the perinuclear space and connects with the endoplasmic reticulum. The inner surface area of the nuclear envelope is lined with protein. This lining is called the nuclear lamina. In turn, the lamina binds to chromatin and other components found inside the nucleus.
Very small holes occupy the surface of the envelope. These holes are called nuclear pores and allow for the passage of molecules between the cytoplasm and the nucleus. The nuclear pores are responsible for overseeing passage of molecules between the cytoplasm in the nucleus. Some of these molecules are allowed to pass through the membrane.
Eukaryotic and prokaryotic cells both contain common features such as DNA, cytoplasm, a plasma membrane, a nuclear weight region, a nucleus, and ribosomes.
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are allowed to pass into the nucleus through these nuclear pores, and so too are molecules that will provide the energy for genetic material construction.
During cellular division (mitosis) the nuclear envelope disintegrates, but will later form again as the two cells formed from mitosis complete formation and as the chromatin unravels and also eventually evaporates.
Chromatin or chromatin fibers are defined as a complex arrangement of DNA and proteins found inside the cell nucleus. The nucleus contains molecules that when laid out would encompass nearly 2 meters (6 feet) of DNA.7 They’re literally packed inside every nucleus of every human cell. The chromatin fibers are an extraordinary feat of “packaging” wonder that is precisely structured and organized into very dense fibers or streams called chromatin.
This packaging is facilitated by special proteins that bind and fold DNA into an incredibly complex series of loops and coils.
Nuclear “sap” is considered similar to the cytoplasm of a cell, but it’s not exactly the same. Nuclear sap is a substance found inside a new nucleus, also known as nucleoplasm. Its main function is to act as a suspension substance for organelles that also helps the nucleus maintain its structure and shape. It’s also a major component of transportation for cellular metabolism. Enzymes and nucleotides are dissolved in the nucleoplasm.
This fluid is typically found in the nuclei of eukaryotic cells. This sap-like structure is likened to protoplasm and is constructed of various molecules, dissolved ions, and water. Nuclear sap is completely encompassed within the nuclear membrane, also known as the nuclear envelope. This fluid, also gel-like in nature as cytoplasm or cytosol, contains chromatin materials.
The nucleolus is an organelle found within the nucleus. The nucleolus is responsible for the manufacture of ribosomes. Ribosomes are structures that produce protein. The nucleus of a cell may contain as many as four nucleoli, but every species has a predetermined set or fixed number of nucleoli.
During cellular division, this nucleolus literally disappears.
Endosomes are small areas found inside eukaryotic cells. They’re located in the cytoplasm. Endosomes are described as part of the pathway utilized in the rejuvenation or recycling of surface receptors. Endosomes can be characterized as a collection of organelles that serve in the function of identifying, sorting, and as a delivery vessel of materials from the cellular surface, and even as a transport vehicle from the Golgi apparatus to the lysosome.
Endosomes play a role in the “recycling” of molecules from the plasma membrane through the endoplasmic pathway. This pathway is composed of unique compartments; each involved in the replenishment of degraded molecules. Three types of endosomes are typically defined as:
•early endosome
•recycling or sorting endosome
•late endosome
Molecules are transported along this pathway for degradation in the lysosome (think of a lysosome as a trash truck pulling up to the curb for garbage disposal). They are then recycled back into the plasma membrane.
The process is known as endocytosis and follows the endocytic pathway. Endocytosis is a necessary component of cellular structures because the majority of substances that are vital for optimal function are large, polarized molecules that are unable to passively make their way through the plasma membrane.
■Characteristics of all Cells
Cells share a number of common characteristics depending on various responses to their environment, among which is their ability to move and engage in metabolic processes. They have the ability to grow (anabolism)as well as literally self-destruct (catabolism). One of the most fascinating aspects of cellular characteristics is their ability to reproduce (mitosis.)
Every cell has the potential to respond to their environment. They can literally be irritated into responses or stimulated by a number of factors. They have the potential to sense changes in their immediate environment and respond to those changes. This response to environment is achieved through their nuclear receptors, which can trigger a number of controlled responses.
This type of “communication” between cells as well as their environment is known as homeostasis.
Cells are capable of receiving and processing simultaneous signals. Cells don’t only receive signals, but can transmit them. For example, signaling molecules known as neurotransmitters can travel short distances between adjacent neurons or between a neuron and a muscle cell. Others can send signals much further. A prime example is FSH or follicle-stimulating hormone, which is sent by the hypothalamus to the female ovary, signaling (via FSH) the ovary to release an egg.
In the skin, sensory cells respond to external cues such as touch. Cells inside the ear respond to sound waves. Cells found in blood vessels identify and respond to changes in blood pressure.
This is done through the presence of protein receptors. These receptors bind to “signaling” molecules to trigger physiological responses.8 Some receptors are found deep inside a cell while others are found on its surface, while still others are located in the nucleus.
Motility
How do cells move? They migrate. Cells utilize two basic methods of transportation: contractibility or self-propulsion. Contractibility is contraction. For example, think of contraction of a muscle cell or fibers in the act of bending the elbow and bringing the hand to the face. This contracting ability helps divide daughter cells during mitosis. Contraction occurs when molecular “motors” act on cytoskeletal filaments or microtubules, compelling them to draw toward one another.
Cells can move in specific directions. This directional mobility is called chemotaxis and describes how cells move after triggering by an external signal or stimulus. In many cases, this external signal is caused by the influence of a short peptide or molecule (known as a chemoattractant). The cells automatically move toward the direction of the increased signal concentration. This type of movement is typical in wound-healing scenarios. A damaged or injured cell releases a chemoattractant. This signals the attraction of microphages and fibroblasts of the immune system.
For example, a chemical “scent” or trail is left by the movement of a damaged cell. Like bloodhounds following a scent, leukocytes, vital for defense, respond to the area ready to do battle. This is known as positive chemotaxis.9
Another type of cellular movement in
