Cell biology Flashcards
1
Q
The cell is made up of…
A
Phospholipid molecules makeup the basic component of cell membranes.
Hydrophilic Head
Phosphate & Glycerol
Negatively charged
Hydrophobic Tail
Fatty acids
Nonpolar
2
Q
Describe the phospholipid is bilayyer and what it enables
A
Phosphate groups are hydrophili and arrange adjacent to Intracellular (ICF) or Extracellular (ECF) fluid.
Fatty acid tails makeup the inner membrane creating a hydrophobic or fluid free environment.
These physiochemical properties enable a fluid membrane structure
3
Q
Because the phosphate groups are polar and hydrophilic, they are….
A
attracted to water in either the intracellular fluid. Intracellular fluid (ICF) or Extracellular fluid (ECF).
4
Q
Because the lipid tails are hydrophobic, they meet in
A
the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space.
5
Q
Integral Proteins
A
Run through membrane bilayer.
Channel proteans
receptor proteins
glycoproteins
6
Q
Channel proteins
A
Proteins that recognise external signals, such as receptor proteins, and in turn induce signalling changes within the cell
7
Q
glycoproteins with associated bound carbohydrates….
A
enable cell recognition.
8
Q
Peripheral Proteins
A
These are attached to the internal or external layer of the cell membrane, and are usually associated with receptor proteins to influence cell signalling.
9
Q
concentration gradient
A
is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient.)
10
Q
Diffusion
A
is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept..
11
Q
Molecules diffuse across the cell membrane at a rate dependant on
A
Concentration
Size
Charge
12
Q
What molecules can cross the phospholipid bilayer
A
. Very small polar molecules, such as water, can cross via simple diffusion due to their small size.
Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer
13
Q
Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are
A
repelled by the hydrophobic tails in the interior of the phospholipid bilayer.
14
Q
Molecules needed by the cell that are charged or too large cross via
A
Channel Proteins
Carrier Proteins
15
Q
Facilitated diffusion
A
is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and/or polarity.
A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.
16
Q
active transport
A
Where a molecule needs to move against it’s concentration gradient it requires a form of active transport, where energy is used to overcome opposite electrochemical gradients.
17
Q
sodium-potassium pump
A
transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used.
18
Q
Name some examples of Vesicular driven active absorption of large extracellular molecules or particles:
A
Phagocytosis
Pinocytosis
Receptor-mediated Endocytosis
19
Q
Endocytosis
A
the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane. Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle.
20
Q
Phagocytosis
A
(“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens, such as invading bacterial cells, phagocytize them, and digest them.
21
Q
pinocytosis
A
(“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles.
22
Q
Receptor-mediated endocytosis
A
is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes.
23
Q
What is an example of receptor-mediated endocytosis
A
Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.
24
Q
exocytosis
A
(taking “out of the cell”) is the process of a cell exporting material using vesicular transport. Many cells manufacture substances that must be secreted. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane.
Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis. Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.
25
The Endoplasmic Reticulum is responsible for:
Protein synthesis
Lysosome formation
Vesicle formation
Lipid synthesis
26
The endomembrane system is responsible for
protein and lipid synthesis, and cellular packaging.
27
The endomembrane system is responsible for protein and lipid synthesis, and cellular packaging.
It comprises of the:
Endoplasmic reticulum
Golgi apparatus
Vesicles
28
There are three major components of the endoplasmic reticulum:
Rough ER & Ribosomes
Smooth ER
29
endoplasmic reticulum (ER)
is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions.
30
The Golgi apparatus Packages proteins from the rough ER and directs transport either:
Intracellular
Targeted through cytoskeleton network.
Extracellular
Excreted from the cell via exocytosis
31
Golgi apparatus
responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post-office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released from the opposite side after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted.
32
mitochondrion (plural = mitochondria)
is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane.
The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. It is along this inner membrane that a series of proteins, enzymes, and other molecules perform the biochemical reactions of cellular respiration.
These reactions convert energy stored in nutrient molecules (such as glucose) into adenosine triphosphate (ATP), which provides usable cellular energy to the cell.
33
cytoskeleton
is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and transportation of substances within the cell.
34
microtubules
A very important function of microtubules is to set the paths (somewhat like railroad tracks) along which the genetic material can be pulled (a process requiring ATP) during cell division, so that each new daughter cell receives the appropriate set of chromosomes. Two short, identical microtubule structures called centrioles are found near the nucleus of cells. A centriole can serve as the cellular origin point for microtubules extending outward as cilia or flagella or can assist with the separation of DNA during cell division. Microtubules grow out from the centrioles by adding more tubulin subunits, like adding additional links to a chain.
35
microfilament
is a thinner type of cytoskeletal filament. Actin, a protein that forms chains, is the primary component of these microfilaments. Actin fibers, twisted chains of actin filaments, constitute a large component of muscle tissue and, along with the protein myosin, are responsible for muscle contraction. Like microtubules, actin filaments are long chains of single subunits (called actin subunits). In muscle cells, these long actin strands, called thin filaments, are “pulled” by thick filaments of the myosin protein to contract the cell.
Actin also has an important role during cell division. When a cell is about to split in half during cell division, actin filaments work with myosin to create a cleavage furrow that eventually splits the cell down the middle, forming two new cells from the original cell.
36
intermediate filament
is a filament intermediate in thickness between the microtubules and microfilaments. Intermediate filaments are made up of long fibrous subunits of a protein called keratin that are wound together like the threads that compose a rope. Intermediate filaments, in concert with the microtubules, are important for maintaining cell shape and structure. Unlike the microtubules, which resist compression, intermediate filaments resist tension—the forces that pull apart cells. There are many cases in which cells are prone to tension, such as when epithelial cells of the skin are compressed, tugging them in different directions. Intermediate filaments help anchor organelles together within a cell and also link cells to other cells by forming special cell-to-cell junctions.
37
nuclear envelope.
This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores.
38
nuclear pore
a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.
39
nucleolus
. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.
40
Principal components of the nucleus
Nuclear Envelope
Nucleus membrane
Nuclear Pore
Transfer channel
Nucleolus
Synthesis of RNA
41
DNA
A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups. The two sides of the ladder are not identical but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine.
Within the nucleus are threads of chromatin composed of DNA and associated proteins. Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace,
42
Stages of DNA replication
1. Initiation
Separation of DNA
2. Elongation
Synthesis of complementary strands
3. Termination
Completion of two complementary strands
43
Describe stage 1 of dna replication
Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA.
44
Describe stage 2=of dna replication
Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.
45
Describe stage 3 of dna replication
Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.
46
semiconservative
Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is “semiconservative,” because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is replicated.
47
proteome
Just as the cell’s genome describes its full complement of DNA, a cell’s proteome is its full complement of proteins. Protein synthesis begins with genes.
48
Gene
A gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein. Gene expression, which transforms the information coded in a gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are Therefore, a gene, which is composed of multiple triplets in a unique sequence, provides the code to build an entire protein, with multiple amino acids in the proper sequence.
49
triplet
a section of three DNA bases in a row that codes for a specific amino acid. Similar to the way in which the three-letter code d-o-g signals the image of a dog, the three-letter DNA base code signals the use of a particular amino acid. For example, the DNA triplet CAC (cytosine, adenine, and cytosine) specifies the amino acid valine.
50
Transcription
Transcription is the process by which messenger RNA copies the code for a specific gene. This occurs in the nucleus. Like all molecular mechanisms this takes place over three stages/steps:
51
Stage 1 of dna transcription
Stage 1: Initiation. A region at the beginning of the gene called a promoter—a particular sequence of nucleotides—triggers the start of transcription
52
Stage 2 of dna transcription
Stage 2: Elongation. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A, C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA. This process builds a strand of mRNA.
53
Stage 3 of dna transcription
Stage 3: Termination. When the polymerase has reached the end of the gene, one of three specific triplets (UAA, UAG, or UGA) codes a “stop” signal, which triggers the enzymes to terminate transcription and release the mRNA transcript.
54
Describe dna translation
Stages:
1. Initiation
mRNA binds to ribosome in the Rough ER.
2. Elongation
tRNA with anticodon to the mRNA sequence adds its respective amino acid to the polypeptide chain
3. Termination
Stop region terminates translation
55
Describe the elongation stage of translation
The elongation stage involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and codon sequences are bound (remember, they are complementary base pairs), the tRNA presents its amino acid cargo and the growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various enzymes and requires energy. The tRNA molecule then releases the mRNA strand, the mRNA strand shifts one codon over in the ribosome, and the next appropriate tRNA arrives with its matching anticodon.
56
Describe the termination stage of translation
This process continues until the final codon on the mRNA is reached which provides a “stop” message that signals termination of translation and triggers the release of the complete, newly synthesized protein. Thus, a gene within the DNA molecule is transcribed into mRNA, which is then translated into a protein product.
57
Interphase
```
Made up of the following sub-phases:
Gap 1 (G1 Phase)
Synthesis (S Phase)
Gap 2 (G2 Phase)
Resting (G0 Phase)
```
Interphase is the period of the cell cycle during which the cell is not dividing. The majority of cells are in interphase most of the time
58
Cell cycle
One “turn” or cycle of the cell cycle consists of two general phases: interphase, followed by mitosis and cytokinesis.
59
G1 phase
G1 phase (gap 1 phase) is the first gap, or growth phase in the cell cycle
cells will vary the most in their duration of the G1 phase. It is here that a cell might spend a couple of hours, or many days.
60
S phase
The S phase (synthesis phase) is period during which a cell replicates its DNA.
The S phase typically lasts between 8-10 hours and the G2 phase approximately 5 hours.
61
G2 phase
second gap phase, during which the cell continues to grow and makes the necessary preparations for mitosis.
62
G0
a resting phase of the cell cycle. Cells that have temporarily stopped dividing and are resting (a common condition) and cells that have permanently ceased dividing (like nerve cells) are said to be in G0.
63
Each copy of the chromosome is referred to as a
sister chromatid and is physically bound to the other copy.
64
centromere
the structure that attaches one sister chromatid to another. Because a human cell has 46 chromosomes, during this phase, there are 92 chromatids (46 × 2) in the cell.
65
Meiosis
nuclear division that forms haploid cells from diploid cells
In meiosis, the starting nucleus is always diploid and the daughter nuclei that result are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome replication followed by two rounds of nuclear division. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II, prometaphase II, and so on.
66
Prophase 1
Chromosomes condense. Nuclear envelope fragments
Homoglous chromosomes bind firmly together along their length.
Chiasmata form between non sister chromatids
Crossing over occurs at chiasmata
Spindle fibres emerge from centromeres
67
Metaphase 1
the homologous chromosomes are arranged at the metaphase plate—roughly in the midline of the cell, with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator.
The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the crossing over events between nonsister chromatids, are responsible for much of the genetic variation in offspring
68
How does meiosis create genetically diverse gametes
Meiosis I creates genetically diverse gametes in two ways. First, during prophase I, crossover events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make their way into the gametes.
69
Anaphase 1
Anaphase I
In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.
70
Telophase and cytokines
In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes “decondense” and nuclear envelopes form around the separated sets of chromatids produced during telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow
Two haploid cells are the result of the first meiotic division of a diploid cell. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present.
71
Meiosis 2
In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II are similar to mitosis, except that each dividing cell has only one set of homologous chromosomes, each with two chromatids. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis. In terms of chromosomal content, cells at the start of meiosis II are similar to haploid cells in G2, preparing to undergo mitosis
72
Prophase 2
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The MTOCs that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed
73
Pro metaphase 2
The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
74
Metaphase2
The sister chromatids are maximally condensed and aligned at the equator of the cell.
75
Anaphase 2
The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Nonkinetochore microtubules elongate the cell.
76
Telophase II and Cytokinesis
The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. If the parent cell was diploid, as is most commonly the case, then cytokinesis now separates the two cells into four unique haploid cells. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.
77
In meiosis, Dna synthesis occurs in
S phase of interphase
78
In meiosis, Synapsis of homologous chromosomes occurs in
Prophase 1
79
In meiosis, Crossover occurs in
Phrophase 1
80
In meiosis,homologous chromosomes line up at metaphase plate occurs in
Metaphase 1
81
In meiosis, Sister chromosomes line up at metaphase plate occurs in
Metaphase 2
82
In mitosis, Dna synthesis occurs in
S phase
83
In mitosis, Sister chromosomes line up at metaphase plate occurs in
During metaphase
84
What is the number and genetic composition of daughter cells in meiosis
Four haploid cells at the end of meiosis 2
85
What is the number and genetic composition of daughter cells in mitosis
Two diploid cells at the end of mitosis
86
What is the cell control cycle
A very elaborate and precise system of regulation controls direct the way cells proceed from one phase to the next in the cell cycle and begin mitosis. The control system involves molecules within the cell as well as external triggers. These internal and external control triggers provide “stop” and “advance” signals for the cell. Precise regulation of the cell cycle is critical for maintaining the health of an organism, and loss of cell cycle control can lead to cancer.
87
checkpoint
is a point in the cell cycle at which the cycle can be signalled to move forward or stopped. At each of these checkpoints, different varieties of molecules provide the stop or go signals, depending on certain conditions within the cell. A cyclin is one of the primary classes of cell cycle control molecules.
88
cyclin-dependent kinase (CDK)
is one of a group of molecules that work together with cyclins to determine progression past cell checkpoints. By interacting with many additional molecules, these triggers push the cell cycle forward unless prevented from doing so by “stop” signals, if for some reason the cell is not ready
89
G1 checkpoint
the cell must be ready for DNA synthesis to occur.
90
G2 checkpoint
the cell must be fully prepared for mitosis.
91
The metaphase checkpoint
ensures that all sister chromatids are properly attached to their respective microtubules and lined up at the metaphase plate before the signal is given to separate them during anaphase.
92
Describe cancer in the context of cell cycle control
Most people understand that cancer or tumors are caused by abnormal cells that multiply continuously. If the abnormal cells continue to divide unstopped, they can damage the tissues around them, spread to other parts of the body, and eventually result in death. In healthy cells, the tight regulation mechanisms of the cell cycle prevent this from happening, while failures of cell cycle control can cause unwanted and excessive cell division. Failures of control may be caused by inherited genetic abnormalities that compromise the function of certain “stop” and “go” signals. Environmental insult that damages DNA can also cause dysfunction in those signals. Often, a combination of both genetic predisposition and environmental factors lead to cancer.
93
The process of a cell escaping its normal control system and becoming cancerous may actually happen throughout the body quite frequently. Fortunately.....
certain cells of the immune system are capable of recognising cells that have become cancerous and destroying them.
94
Stem Cells
an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to their potential to differentiate
95
totipotent
The first embryonic cells that arise from the division of the zygote are the ultimate stem cells
they have the potential to differentiate into any of the cells needed to enable an organism to grow and develop.
96
pluripotent
stem cell is one that has the potential to differentiate into any type of human tissue but cannot support the full development of an organism
97
multipotent
has the potential to differentiate into different types of cells within a given cell lineage or small number of lineages, such as a red blood cell or white blood cell.
98
oligopotent
limited to becoming one of a few different cell types.
99
unipotent
cell is fully specialized and can only reproduce to generate more of its own specific cell type.
100
There are different stem cells present at different stages of a human’s life. They include the
the embryonic stem cells of the embryo, foetal stem cells of the foetus, and adult stem cells in the adult.
101
epithelial stem cell
which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin.
102
Adult bone marrow has three distinct types of stem cells:
hematopoietic stem cells, which give rise to red blood cells, white blood cells, and platelets; endothelial stem cells, which give rise to the endothelial cell types that line blood and lymph vessels; and mesenchymal stem cells, which give rise to the different types of muscle cells.
103
The primary mechanism by which genes are turned “on” or “off” is through
transcription factors
104
transcription factors
is one of a class of proteins that bind to specific genes on the DNA molecule and either promote or inhibit their transcription.
105
Epithelial tissue
, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands.
106
Connective tissue
, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body.
107
Muscle tissue
I s excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart
108
Nervous tissue
is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.
109
tissue membrane
is a thin layer or sheet of cells that covers the outside of the body (for example, skin), the organs (for example, pericardium), internal passageways that lead to the exterior of the body (for example, abdominal mesenteries), and the lining of the moveable joint cavities. There are two basic types of tissue membranes: connective tissue and epithelial membranes
110
Connective Tissue Membranes
The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints
111
synovial membrane
is a type of connective tissue membrane that lines the cavity of a freely movable joint. For example, synovial membranes surround the joints of the shoulder, elbow, and knee. Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity. The hyaluronan effectively traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body fluids.
112
Epithelial Membranes
composed of epithelium attached to a layer of connective tissue, for example, your skin
113
mucous membrane
composite of connective and epithelial tissues. Sometimes called mucosae, these epithelial membranes line the body cavities and hollow passageways that open to the external environment, and include the digestive, respiratory, excretory, and reproductive tracts. Mucous, produced by the epithelial exocrine glands, covers the epithelial layer.
114
The underlying connective tissue,
called the lamina propria (literally “own layer”), help support the fragile epithelial layer.
115
serous membrane
an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that is supported by connective tissue. These membranes line the coelomic cavities of the body, that is, those cavities that do not open to the outside, and they cover the organs located within those cavities. They are essentially membranous bags, with mesothelium lining the inside and connective tissue on the outside. Serous fluid secreted by the cells of the thin squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Serous membranes are identified according locations. Three serous membranes line the thoracic cavity; the two pleura that cover the lungs and the pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs.
116
cutaneous membrane
The skin
a stratified squamous epithelial membrane resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens.
117
squamous
(flattened and thin)
118
cuboidal
(boxy, as wide as it is tall)
119
columnar
(rectangular, taller than it is wide)
120
simple epithelium
where every cell rests on the basal lamina
121
stratified epithelium
only the basal layer of cells rests on the basal lamina
A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space.
122
Pseudostratified
(pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer.
123
Transitional describes
a form of specialized stratified epithelium in which the shape of the cells can vary.
124
endothelium
is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue.
125
mesothelium
is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.
126
simple cuboidal epithelium
, the nucleus of the box-like cells appears round and is generally located near the centre of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.
127
simple columnar epithelium,
the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.
128
Pseudostratified columnar epithelium
is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighbouring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.
129
Simple Epithelium
where every cell rests on the basal lamina
The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell
130
Stratified squamous epithelium
is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium.
131
Stratified cuboidal epithelium and stratified columnar epithelium
can also be found in certain glands and ducts, but are uncommon in the human body.
132
transitional epithelium
, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended..
133
Merocrine secretion
is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example.
134
Apocrine secretion
accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break down; this causes body odor. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and golgi regions remain intact after secretion.
135
holocrine secretion
involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells.
136
Connective tissue proper
includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibres suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibres that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibres are loosely organized, leaving large spaces in between.
Fibroblasts are present in all connective tissue proper.
Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.
137
Supportive connective tissue
—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibres in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts.
138
fluid connective tissue
, in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.
139
Loose connective tissue
is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.
Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver. Reticular cells produce the reticular fibres that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.”
140
Adipose tissue
consists mostly of fat storage cells, with little extracellular matrix. A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its colour to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.
141
Areolar tissue
shows little specialization. It contains all the cell types and fibres previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibres, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.
142
Dense connective tissue
Dense connective tissue contains more collagen fibres than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibres are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fibre orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibres are parallel. Dense regular elastic tissue contains elastin fibres in addition to collagen fibres, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic.
143
dense irregular connective tissue
, the direction of fibres is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibres crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibres run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibres. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching
144
cartilage
The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.
145
Hyaline cartilage,
the most common type of cartilage in the body, consists of short and dispersed collagen fibres and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood.
146
Fibrocartilage
is tough because it has thick bundles of collagen fibres dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage.
147
Elastic cartilage
contains elastic fibres as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.
148
Blood and lymph are
fluid connective tissues.
149
Blood
Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow. Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body.
150
Lymph
contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.
151
Skeletal muscle
is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a by-product of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fibre.
152
Cardiac muscle
forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibres, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibres that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle.
153
Smooth muscle tissue contraction is responsible for
involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations.
154
Neurons
display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as neurotransmitters. Neurons categorized as multipolar neurons have several dendrites and a single prominent axon.
155
Bipolar neurons
possess a single dendrite and axon with the cell body, while unipolar neurons have only a single process extending out from the cell body, which divides into a functional dendrite and into a functional axon. When a neuron is sufficiently stimulated, it generates an action potential that propagates down the axon towards the synapse. If enough neurotransmitters are released at the synapse to stimulate the next neuron or target, a response is generated.
156
Astrocyte
cells, named for their distinctive star shape, are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier, the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection but are not nervous tissue because they are related to macrophages.
157
Oligodendrocyte
cells produce myelin in the central nervous system (brain and spinal cord) while the Schwann cell produces myelin in the peripheral nervous system.
