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Chapter 7 Introduction to Cell Pathology
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Introduction to Cell Pathology
Chapter 7
Clark
Key Terms
Key Terms
Cell membrane: lipoprotein structure that separates the interior of a cell from the external environment; regulates what can enter and leave a cell.
Cytoplasm: lies within the cell membrane; contains organelles for producing proteins, energy, and so on.
Diffusion: movement of solutes from an area of high concentration to an area of low concentration across a concentration gradient.
Endocytosis: the process of engulfing substances and moving them into a cell by extending the cell membrane around the substance; pinocytosis and phagocytosis are two kinds of endocytosis.
Endoplasmic reticulum: fine network of interconnected channels known as cisternae found in the cytoplasm; site of chemical reactions within the cell.
Exocytosis: removal of substances from a cell by pushing them through the cell membrane.
Genes: sequences of DNA that control basic cell functions and allow for cell division
Golgi apparatus: a series of flattened sacs in the cytoplasm that prepare hormones or other substances for secretion and may produce lysosomes and store other synthesized proteins.
lipoprotein: structure composed of proteins and lipids; the bipolar arrangement of the lipids monitors substances passing in and out of the cell.
lysosomes: encapsulated digestive enzymes found within a cell; they digest old or damaged areas of the cell and are responsible for destroying the cell when the membrane ruptures and the cell dies.
Mitochondria: rod-shaped organelles that produce energy within the cell in the form of adenosine triphosphate (ATP).
Mitosis: cell division resulting in two identical daughter cells
Nucleus: the part of a cell that contains the DNA and genetic material; regulates cellular protein production and cellular properties
organelles: distinct structures found within the cell cytoplasm.
Osmosis: movement of water from an area of low solute concentration to an area of high solute concentration in an attempt to equalize the concentrations
Ribosomes: membranous structures that are the sites of protein production within a cell
The Cell
The
Cell
The cell is the basic structural unit of the body.
- The cells that make up living organisms, which are arranged into tissues and organs, all have the same basic structure.
- Each cell has a nucleus, a cell membrane, and cytoplasm, which contains a variety of organelles..
Cell Nucleus
Cell
Nucleeus
Each cell is “programmed” by the genes, or sequences of DNA, that allow for cell division, produce specific proteins that allow the cell to carry out its functions, and maintain cell homeostasis or stability.
- The nucleus is the part of a cell that contains all genetic material necessary for cell reproduction and for the regulation of cellular production of proteins.
- The nucleus is encapsulated in its own membrane and remains distinct from the rest of the cytoplasm.
- A small spherical mass, called the nucleolus, is located within the nucleus.
- Genes are responsible for the formation of messenger RNA and transcription RNA, which are involved in production of the proteins unique to the cell.
- The DNA necessary for cell division is found on long strains called chromatin.
- These structures line up and enlarge during the process of cell division (mitosis).
Cell Membrane
Cell
Membrane
The cell is surrounded by a thin barrier called the cell membrane, which separates intracellular fluid from extracellular fluid.
- The membrane is essential for cellular integrity and is equipped with many mechanisms for maintaining cell homeostasis.
Lipoproteins
Lipoproteins
The cell membrane is a lipoprotein structure, meaning that it is mainly composed of proteins and lipids—phospholipids, glycolipids, and cholesterol; the bipolar arrangement of the lipids monitors substances passing in and out of the cell.
- The phospholipids, which are bipolar in nature, line up with their polar regions pointing toward the interior or exterior of the cell and their nonpolar region lying within the cell membrane.
- These properties allow the membrane to act as a barrier to regulate what can enter the cell.
- The freely moving nature of the membrane allows it to adjust to the changing shape of the cell so that areas of the membrane can move together to repair the membrane should it become torn or injured.
- Some of the outward-facing phospholipids have a sugar group attached to them; these are called glycolipids.
- Cholesterol is found in large quantities in the membrane, and it works to keep the phospholipids in place and the cell membrane stable.
Receptor Sites
Receptor
Sites
One type of protein located on the cell membrane is known as a receptor site.
- This protein reacts with specific chemicals outside the cell to stimulate a reaction within the cell.
- For example, the receptor site for insulin reacts with the hormone insulin to cause activation of ATP (adenosine triphosphate protein) within the cell.
- This reaction alters the cell’s permeability to glucose.
- Receptor sites are very important in the functioning of neurons, muscle cells, endocrine glands, and other cell types, and they play a very important role in clinical pharmacology.
Channels
Channels or pores within the cell membrane are made by proteins in the cell wall that allow the passage of small substances in or out of the cell.
- Specific channels have been identified for sodium, potassium, calcium, chloride, bicarbonate, and water; other channels may also exist.
- Some drugs are designed to affect certain channels specifically.
- For example, calcium-channel blockers prevent the movement of calcium into a cell through calcium channels.
Cytoplasm
Cytoplasm
The cell cytoplasm lies within the cell membrane and outside the nucleus and is the site of activities of cellular metabolism and special cellular functions.
- The cytoplasm contains many organelles, which are structures with specific functions such as producing proteins and energy.
- The organelles within the cytoplasm include the mitochondria, the endoplasmic reticulum, free ribosomes, the Golgi apparatus, and the lysosomes.
Mitochondria
Mitochondria are rod-shaped “power plants” within each cell that produce energy in the form of ATP, which allows the cell to function.
- Mitochondria are plentiful in very active cells such as muscle cells and are relatively scarce in inactive cells such as bone cells.
- Mitochondria, which can reproduce when a cell is very active, are always very abundant in cells that consume energy.
- For example, cardiac muscle cells, which must work continually to keep the heart contracting, contain a great number of mitochondria.
- The mitochondria can take carbohydrates, fats, and proteins from the cytoplasm and make ATP via the Krebs cycle, which depends on oxygen.
- Cells use the ATP to maintain homeostasis, produce proteins, and carry out specific functions.
Endoplasmic Reticulum
Endoplasmic Reticulum
Much of the cytoplasm of a cell is made up of a fine network of interconnected channels known as cisternae, which form the endoplasmic reticulum.
- The undulating surface of the endoplasmic reticulum provides a large surface for chemical reactions within the cell.
- Many granules that contain enzymes and ribosomes, which produce protein, are scattered over the surface of the rough endoplasmic reticulum.
- Production of proteins, phospholipids, and cholesterol takes place in the rough endoplasmic reticulum.
- The smooth endoplasmic reticulum is the site of further lipid and cholesterol production and the production of cell products, such as hormones.
- The breakdown of many toxic substances may also occur here in particular cells.
Free Ribosomes
Free
Ribosomes
Ribosomes that are not bound (free) to the surface of the endoplasmic reticulum exist throughout the cytoplasm.
- These free-floating ribosomes produce proteins that are important to the structure of the cell and some of the enzymes that are necessary for cellular activity.
Golgi Apparatus
The Golgi apparatus is a series of flattened sacs that may be part of the endoplasmic reticulum.
- These structures prepare hormones or other substances for secretion by processing them and packaging them in vesicles to be moved to the cell membrane for excretion from the cell.
- In addition, the Golgi apparatus may produce lysosomes and store other synthesized proteins and enzymes until they are needed.
Lysosomes
Lysosomes are membrane-covered organelles that contain specific digestive enzymes that can break down proteins, nucleic acids, carbohydrates, and lipids and are responsible for digesting worn or damaged sections of a cell when the membrane ruptures and the cell dies.
- Lysosomes form a membrane around any substance that needs to be digested and secrete the digestive enzymes directly into the isolated area, protecting the rest of the cytoplasm from injury.
- Lysosomes become very important clinically when cell death (from disease or a drug effect) leads to the death of neighboring cells when lysosomes are released from the dead cell and lyse or digest the proteins and membrane of neighboring cells, causing those cells to die and release their lysozymes.
- A decubitus ulcer is a good example of cell death leading to the death of neighboring cells and becoming a potentially out-of-control reaction.
Cell Properties
Cell
Properties
Cells have certain properties that allow them to survive.
- Endocytosis involves incorporation of material into the cell by extending the cell membrane around the substance.
- This process allows cells to absorb nutrients, enzymes, and other materials.
- Phagocytosis is a similar process; it allows the cell, usually a neutrophil or macrophage, to engulf a bacterium or a foreign protein and destroy it within the cell by secreting digestive enzymes into the area.
- Exocytosis is the opposite of endocytosis and involves removing substances from a cell by pushing them through the cell membrane.
- Hormones, neurotransmitters, enzymes, and other substances produced within a cell are excreted into the body by this process
Homeostasis
The main goal of a cell is to maintain homeostasis, which means keeping the cytoplasm stable within the cell membrane.
- Each cell uses a series of active and passive transport systems to achieve homeostasis.
- For a cell to produce the energy needed to carry out cellular metabolism and other processes, the cell must have a means to obtain necessary elements from the outside environment.
- In addition, it must have a way to dispose of waste products that could be toxic to its cytoplasm.
- To accomplish this, the cell moves substances across the cell membrane, either by passive transport or by active (energy-requiring) transport.
Passive Transport
Passive
Transposrt
Passive transport happens without the expenditure of energy and can occur across any semipermeable membrane.
There are essentially three types of passive transport:
- Diffusion
- Diffusion is the movement of a substance from a region of higher concentration to a region of lower concentration.
- Osmosis
- Osmosis, a special form of diffusion, is the movement of water across a semipermeable membrane from an area that is low in dissolved solutes to one that is high in dissolved solutes.
- Facilitated diffusion
- Sometimes a substance cannot move freely on its own in or out of a cell. Such a substance may attach to another molecule, called a carrier, to be diffused.
Active Transport
Sometimes a cell requires a substance in greater concentration than is found in the environment around it or needs to maintain its cytoplasm in a situation that would normally allow chemicals to leave the cell.
- When this happens, the cell must move substances against the concentration gradient using active transport, which requires energy.
- When a cell is deprived of oxygen because of a blood supply problem or insufficient oxygenation of the blood, systems of active transport begin to malfunction, placing the cell’s integrity in jeopardy.
Cell Cycle
Cell
Cycle
Most cells have the ability to reproduce themselves through the process of mitosis and the genetic makeup of a particular cell determines the rate at which that cell can multiply.
- Some cells reproduce very quickly (e.g., the cells lining the gastrointestinal tract have a generation time of 72 hours), and some reproduce very slowly (e.g., the cells found in breast tissue have a generation time of a few months).
- In some cases, certain factors influence cell reproduction.
- Erythropoietin, a hormone produced by the kidney, can stimulate the production of new red blood cells.
- Active leukocytes release chemicals that stimulate the production of white blood cells when the body needs new ones.
- Regardless of the rate of reproduction, each cell has approximately the same life cycle.
- The life cycle of a cell, called the cell cycle, consists of four active phases and a resting phase
Phases of a Cell Cycle
Phases of a
Cell Cycle
G0 Phase:
During the G0 phase, or resting phase, the cell is stable.
- It is not making any proteins associated with cell division and is basically dormant as far as reproduction goes.
- Cells in the G0 phase cause a problem in the treatment of some cancers.
- Cancer chemotherapy usually works on active, dividing cells, leaving resting cells fairly untouched.
G1 Phase:
When a cell is stimulated to emerge from its resting phase, it enters what is called the G1 phase or gathering phase, which lasts from the time of stimulation from the resting phase until the formation of DNA.
- During this period, the cell synthesizes substances needed for DNA formation.
- The cell is actively collecting materials to make these substances and producing the building blocks for DNA.
S Phase:
The next phase, called the S phase or synthesis phase, involves the actual synthesis of DNA, which is an energy-consuming activity.
- The cell remains in this phase until the amount of cellular DNA has doubled.
G2 Phase
After the cellular DNA has doubled in preparation for replication, the G2 phase begins, another gathering phase.
- During this phase, the cell produces all of the substances required for the manufacture of the mitotic spindles.
M Phase
After the cell has produced all of the substances necessary for formation of a new cell, or daughter cell, it undergoes cell division, the cell splits to form two identical daughter cells, a process called mitosis.