תא Flashcards
(143 cards)
1
Q
eukaryote
A
חד תאים אצות פיטריות צומח בעלי חיים
2
Q
prokaryote
A
חיידקים - תאים פשוטים יותר מבנה: 1 .דופן תא 2 .קרום תא DNA .3 4 .ציטופלסמה
3
Q
minerals
A
מולקולות המתפרקות במים ליון חיובי ושלילי
4
Q
acids
A
מולקולה המשחררת במים יוני מימן
5
Q
base
A
מולקולה הקושרת אליה במים יוני מימן חופשיים
6
Q
pH
A
רמת יוני המימן החופשיים שיש בתמיסה
7
Q
proteins
A
פולימר. אבן הבניין: חומצה אמינית:
בטבע קיימות 20 חומצות אמיניות שונות
כשאבני הבניין מתחברות זו לזו נוצרת שרשרת
("פפטיד"
כשמספר "פפטידים" מתחברים - נוצר חלבון
תלת ממדי
רצפים שונים של חומצות אמיניות אחראיים למגוון
אינסופי של חלבונים
8
Q
protein structure
A
מבנה ראשוני
מבנה שניוני
מבנה שלישוני
מבנה רביעוני
9
Q
Primary structure of protein
A
רצף חומצות אמיניות
10
Q
Secondary structure of protein
A
התקפלות ספונטנית
“סלילי אלפא” או “משטח בטא”
11
Q
Tertiary structure
A
קיפול של השרשרת
הפפטידית למבנה תלת מימדי
12
Q
Quaternary structure
A
חיבור של מספר
שרשראות פפפטידיות (תת יחידות)
13
Q
protein functions
A
מבנה וצורה נשאים הגנה חיסונית בקרה אנזימים
14
Q
lipids
A
מולקולות שאינן נמסות (מתפרקות) במים (הידרופוביות)
ליפידים
לדוגמה: גליצרידים (“שומנים”), פוספוליפידים, כולסטרול, גליצרול
15
Q
lipid functions
A
מתווכי דלקת וקרישת דם מאגר אנרגיה לטווח ארוך "שומן") (גליצרידים סטרואידים (כולסטרול) תמיכה מבנית קרומי התא מעטפת מיילין
16
Q
sugar types
A
חד סוכרים (monosaccharides : (מולקולות אורגניות טבעתיות
דו סוכרים (disaccharides
רב סוכרים
:(polysaccharides)
17
Q
glycogen
A
רב-סוכר המיוצר בכבד
ובשרירים (מאגר גלוקוז
קצר טווח)
18
Q
sugar functions
A
1. נוקלאוטיד (RNA ,DNA) 2. גלוקוז - מקור מידי לאנרגיה 3. גליקופרוטאינים: חלבון עם קבוצות סוכר פרוטאוגליקאנים: רב-סוכר עם קבוצות חלבון 4. קצר טווח לאנרגיה גליקוגן – מאגר
19
Q
signal transduction
A
מעבר של אות מחוץ התא אל תוך התא.
20
Q
Cytoskeleton types
A
- microfilaments
- intermediate filaments
- microtubules
21
Q
Cytoskeleton functions
A
צורה לתא
- עגינה לאברונים
- תנועת של בועיות בתא
- תנועת הכרומוזומים
- תנועת התא כולו
22
Q
microfilaments
A
Microfilaments (mIˉ-kroˉ-FIL-a-ments) are
the thinnest elements of the cytoskeleton. They are composed of the proteins actin and myosin and are most prevalent at the edge of a cell (Figure 3.15a). Microfilaments have two general functions: They help generate movement and provide mechanical support. With respect to movement, microfilaments are involved in muscle contraction, cell division, and cell locomotion, such as occurs during the migration of embryonic cells during development, the invasion of tissues by white blood cells to fight infection, or the migration of skin cells during wound healing.
They anchor the cytoskeleton to integral proteins in the plasma membrane. Microfilaments also provide mechanical support for cell extensions called microvilli
23
Q
intermediate filaments
A
As their name suggests, intermediate filaments are thicker than microfilaments but thinner than
microtubules . Several different proteins can compose intermediate filaments, which are exceptionally strong. They are found in parts of cells subject to mechanical stress; they help stabilize the position of organelles such as the nucleus and help attach cells to one another.
24
Q
microtubules
A
Microtubules (mIˉ-kroˉ-TOO-buˉls), the largest of the cytoskeletal components, are long, unbranched hollow
tubes composed mainly of the protein tubulin. The assembly of microtubules begins in an organelle called the centrosome (discussed shortly). The microtubules grow outward from the centrosome toward the periphery of the cell . Microtubules help determine cell shape. They also function in the movement of organelles such as secretory vesicles, of chromosomes during cell division, and of specialized cell projections, such as cilia and flagella.
25
centriole
are cylindrical structures, each composed of nine clusters of three microtubules (triplets) arranged in a circular pattern
26
cilia
(SIL-e¯-a eyelashes; singular is cilium) are numerous, short, hairlike projections that extend from the surface of the cell. Each cilium contains a core of 20 microtubules surrounded by plasma membrane (Figure 3.17a). The microtubules are arranged such that one pair in the center is surrounded by nine clusters of two fused microtubules (doublets). Each cilium is anchored to a basal body just below the surface of the plasma membrane. A basal body is similar in structure to a centriole and functions in initiating the assembly of cilia and flagella.
27
flagellum
are similar in structure to cilia but are typically much longer. Flagella usually move an entire cell. A flagellum generates forward motion along its axis by rapidly wiggling in a wavelike pattern
28
Endoplasmic reticulum
The ER extends from the nuclear envelope (membrane around the nucleus), to which it is connected and projects throughout the cytoplasm. The ER is so extensive that it constitutes more than half of the membranous surfaces within the cytoplasm of most cells. Cells contain two distinct forms of ER, which differ in structure and function.
29
Ribosomes
consists of two subunits, one about half the size
of the other . The large and small subunits are made separately in the nucleolus. Once produced, the large and small subunits exit the nucleus separately, then come together in the cytoplasm.
Some ribosomes are attached to the outer surface of the nuclear membrane and to an extensively folded membrane called the endoplasmic reticulum. These ribosomes synthesize proteins destined for specific organelles, for insertion in the plasma membrane,
or for export from the cell. Other ribosomes are “free” or unattached to other cytoplasmic structures. Free ribosomes synthesize proteins used in the cytosol. Ribosomes are also located within mitochondria, where they synthesize mitochondrial proteins.
30
Golgi Complex
The first step in the transport pathway is through an organelle called the Golgi complex (GOL-je¯). It consists of 3 to 20 cisternae (sisTER-ne¯ cavities; singular is cisterna), small, flattened membranous sacs with bulging edges that resemble a stack of pita
bread (Figure 3.20). The cisternae are often curved, giving the Golgi complex a cuplike shape. Most cells have several Golgi complexes, and Golgi complexes are more extensive in cells that secrete proteins, a clue to the organelle’s role in the cell.
Different enzymes in the entry, medial, and exit cisternae of the Golgi complex permit each of these areas to modify, sort, and package proteins into vesicles for transport to different destinations.
The entry face receives and modifies proteins produced by the rough ER. The medial cisternae add carbohydrates to proteins to form glycoproteins and lipids to proteins to form lipoproteins. The exit face modifies the molecules further and then sorts and
packages them for transport to their destinations.
31
secretion vesicles Goldgi
These vesicles deliver the proteins to the plasma membrane, where they are discharged by exocytosis into the extracellular fluid. For example, certain pancreatic cells release the hormone insulin in
this way
32
transport vesicles
vesicle that moves protein from rER to Golgi
33
lysosome
Lysosomes (LIˉ-so¯-so¯ms; lyso- dissolving; -somes bodies) are membrane-enclosed vesicles that form from the Golgi complex (Figure 3.22). They can contain as many as 60 kinds of powerful digestive and hydrolytic enzymes that can break down a wide variety of molecules once lysosomes fuse with vesicles formed during endocytosis. Because lysosomal enzymes work best at an acidic pH, the lysosomal membrane includes active transport pumps that import hydrogen ions (H). Thus, the lysosomal interior has a pH of 5
34
מחלת גושה (disease s’Gaucher
מחלה תורשתית רצסיבית אוטוזומלית (שלוש צורות).
• נובעת מחסרונו של אחד מאנזימי הליזוזום (גלוקו-צרברו-זידאז) שנועד לסלק
• נזק בעיקר לתאי הכבד, הכליות מרכיבים שומנים מקרום התא.
הריאות, ומח העצם ( בצורות
מסוימות גם במוח).
35
Tay-Sachs disease
inherited condition characterized by the absence
of a single lysosomal enzyme called Hex A. This enzyme normally breaks down a membrane glycolipid called ganglioside GM2 that is especially prevalent in nerve cells. As the excess ganglioside GM2 accumulates, the nerve cells function less efficiently. Children with Tay-Sachs disease typically experience seizures and muscle rigidity.
They gradually become blind, demented, and uncoordinated and usually die before the age of 5. Tests can now reveal whether an adult is a carrier of the defective gene.
36
peroxisome
similar in structure to lysosomes, but
smaller, are the peroxisomes (pe-ROKS-i-so¯ms; peroxi- peroxide; -somes bodies; see Figure 3.1). Peroxisomes, also called microbodies, contain several oxidases, enzymes that can oxidize (remove hydrogen atoms from) various organic substances. For instance, amino acids and fatty acids are oxidized in peroxisomes
as part of normal metabolism. In addition, enzymes in peroxisomes oxidize toxic substances, such as alcohol.
Thus, peroxisomes are very abundant in the liver, where detoxification of alcohol and other damaging substances occurs.
A by-product of the oxidation reactions is hydrogen peroxide (H2O2), a potentially toxic compound, and associated free radicals such as superoxide.
However, peroxisomes also contain the enzyme catalase, which decomposes H2O2. Because production and degradation of H2O2
occur within the same organelle, peroxisomes protect other parts of the cell from the toxic effects of H2O2. Peroxisomes also contain enzymes that destroy superoxide. Without peroxisomes, byproducts of metabolism could accumulate inside a cell and result
in cellular death. Peroxisomes can self-replicate. New peroxisomes may form from preexisting ones by enlarging and dividing.
They may also form by a process in which components accumulate at a given site in the cell and then assemble into a peroxisome.
37
Proteasomes
Continuous destruction of unneeded, damaged, or faulty proteins is the function of tiny barrel-shaped structures consisting of four stacked rings of proteins around a central core called proteasomes (PRO¯-te¯-a-so¯ms protein bodies). For example, proteins that are part of metabolic pathways need to be degraded after they have accomplished their function. Such protein
destruction plays a part in negative feedback by halting a pathway once the appropriate response has been achieved. A typical body cell contains many thousands of proteasomes, in both the cytosol and the
nucleus. Discovered only recently because they are far too small to discern under the light microscope and do not show up well in electron micrographs, proteasomes were so named because they contain
myriad proteases, enzymes that cut proteins into small peptides.
Once the enzymes of a proteasome have chopped up a protein into
smaller chunks, other enzymes then break down the peptides into amino acids, which can be recycled into new proteins.
38
Mitochondria
Because they generate most of the ATP through aerobic (oxygenrequiring) respiration, mitochondria
are referred to as the “powerhouses” of the cell. A cell may have as few as a hundred or as many as several thousand mitochondria, depending on its activity. Active cells that use ATP at a high rate—such as those
found in the muscles, liver, and kidneys—have a large number of mitochondria. For example, regular exercise can lead to an increase in the number of mitochondria in muscle cells, which allows muscle cells to function more efficiently. Mitochondria are usually located
within the cell where oxygen enters the cell or where the ATP is used, for example, among the contractile proteins in muscle cells.
A mitochondrion consists of an outer mitochondrial membrane and an inner mitochondrial membrane with a small fluid-filled space between them (Figure 3.23). Both membranes are similar in structure to the plasma membrane. The inner mitochondrial membrane contains a series of folds called mitochondrial cristae (KRISte¯ ridges). The central fluid-filled cavity of a mitochondrion, enclosed by the inner mitochondrial membrane, is the mitochondrial matrix. The elaborate folds of the cristae provide an enormous surface area for the chemical reactions that are part of the aerobic phase of cellular respiration, the reactions that produce most of a cell’s ATP . The enzymes that catalyze these reactions are located on the cristae and in the matrix of the mitochondria. Mitochondria even have their own
DNA, in the form of multiple copies of a circular DNA molecule that contains 37 genes. These mitochondrial genes control the synthesis of 2 ribosomal RNAs, 22 transfer RNAs, and 13 proteins that build mitochondrial components.
39
apoptosis
Mitochondria also play an important and early role in apoptosis (ap-o¯p-TO¯-sis or ap-o¯-TO¯-sis a falling off), the orderly, geneti cally programmed death of a cell. In response to stimuli such as large numbers of destructive free radicals, DNA damage, growth factor
deprivation, or lack of oxygen and nutrients, certain chemicals are released from mitochondria following the formation of a pore in the outer mitochondrial membrane. One of the chemicals released into
the cytosol of the cell is cytochrome c, which while inside the mitochondria is involved in aerobic cellular respiration. In the cytosol,
however, cytochrome c and other substances initiate a cascade of activation of protein-digesting enzymes that bring about apoptosis.
40
nucleus
The nucleus is a spherical or oval-shaped structure that usually is the most prominent feature of a cell . Most cells have a single nucleus, although some, such as mature red blood cells, have none. In contrast, skeletal muscle cells and a few other types of cells have multiple nuclei.
41
nucleoplasm
חומר בגרעין
42
Chromatin
Human somatic (body) cells have 46 chromosomes, 23 inherited from each parent. Each chromosome is a long molecule of DNA that is coiled together with several proteins (Figure 3.25). This complex of DNA, proteins, and some RNA is called chromatin (KRO¯-matin).
43
PASSIVE PROCESSES types
1 Simple diffusion
2 .Facilitated diffusion
3 .Osmosis
44
ACTIVE PROCESSES
1 . Primary active transport/ Secondary active transport
2 .Endocytosis/ Exocytosis
3 .Phagocytosis/ Pinocytosis
45
diffusion
Movement of molecules or ions down a concentration gradient due to their kinetic energy until they reach equilibrium.
46
simple diffusion
Simple diffusion is a passive process in which substances move freely through the lipid bilayer of the plasma membranes of cells without the help of membrane transport proteins .
Nonpolar, hydrophobic molecules move across the lipid bilayer through the process of simple diffusion. Such molecules include oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids;
and fat-soluble vitamins (A, D, E, and K). Small, uncharged polar molecules such as water, urea, and small alcohols also pass through the lipid bilayer by simple diffusion.
.
47
facilitated diffusion
Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can cross the plasma membrane by a passive process called facilitated diffusion. In this process, an integral membrane protein assists a specific substance across
the membrane. The integral membrane protein can be either a membrane channel or a carrier.
Diffusion of ions through channels is generally slower than free diffusion through the lipid bilayer because channels occupy a smaller fraction of the membrane’s total surface area than lipids. Still, facilitated diffusion through channels is a very fast process: More than a million potassium ions can flow through a K channel in one second!
Substances that move across the plasma membrane by carriermediated facilitated diffusion include glucose, fructose, galactose, and some vitamins. Glucose, the body’s preferred energy source for making ATP, enters many body cells by carrier-mediated
facilitated diffusion as follows
48
osmosis
Osmosis (oz-MO¯-sis) is a type of diffusion in which there is net movement of a solvent through a selectively permeable membrane. Like the other types of diffusion, osmosis is a passive process. In living systems, the solvent is water, which moves by
osmosis across plasma membranes from an area of higher water concentration to an area of lower water concentration. Another way to understand this idea is to consider the solute concentration: In osmosis, water moves through a selectively permeable membrane from an area of lower solute concentration to an area
of higher solute concentration. During osmosis, water molecules pass through a plasma membrane in two ways: (1) by moving between neighboring phospholipid molecules in the lipid bilayer via simple diffusion, as previously described, and (2) by moving through aquaporins (ak-wa-POR-ins; aqua- water), integral membrane proteins that function as water channels.
49
Active Transport
Active process in which a cell expends energy to move a substance across the membrane against its concentration gradient by transmembrane proteins that function as carriers. Polar or charged solutes.
50
Ion channel (integral)
Forms a pore through which a specific ion can flow to get across membrane. Most plasma membranes include specific channels for several common ions.
51
Carrier (integral)
Transports a specific substance across membrane by undergoing a change in shape. For example, amino acids, needed to synthesize new proteins, enter body cells via carriers. Carrier proteins are also known
as transporters.
52
Receptor (integral)
Recognizes specific ligand and alters cell's function in some way. For example, antidiuretic hormone binds to receptors in the kidneys and changes the water permeability of certain plasma membranes.
53
Enzyme (integral and peripheral)
Catalyzes reaction inside or outside cell (depending on
which direction the active site faces). For example,
lactase protruding from epithelial cells lining your
small intestine splits the disaccharide lactose in the
milk you drink.
54
Cell identity marker (glycoprotein)
Distinguishes your cells from anyone else's (unless
you are an identical twin). An important class of such
markers are the major histocompatibility (MHC) proteins.
55
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.
56
symporter
A symporter is an integral membrane protein that is involved in the transport of many differing types of molecules across the cell membrane. The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter.
both directions - into the cell
57
antiporter
```
An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions, one into the cell and one out of the cell.
one in one out
```
58
uniporter
A uniporter is a membrane transport protein that transports a single species of substrate (charged or uncharged) across a cell membrane. It may use either facilitated diffusion and transport along a diffusion gradient or transport against one with an active transport process.
one in
59
Endocytosis
During endocytosis (en-doˉ-sIˉ-TO¯ -sis; endo- within), materials move into a cell in a vesicle formed from the plasma membrane.
60
Endocytosis types
Here we consider three types of endocytosis:
| receptor-mediated endocytosis, phagocytosis, and bulk-phase endocytosis.
61
pinocytosis
In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell through an invagination of the cell membrane, resulting in a suspension of the particles within a small vesicle inside the cell. These pinocytotic vesicles then typically fuse with early endosomes to hydrolyze (break down) the particles.
62
Exocytosis
Exocytosis (/ˌɛksoʊsaɪˈtoʊsɪs/) is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell (exo- + cytosis) by secreting them through an energy-dependent process.
63
DNA
פולימר כפול של הנוקלאוטידים: A ,C ,T ,G
כל שני נוקלאוטידים מחוברים זה לזה דרך הבסיסים החנקניים,
תמיד אותם זוגות: T-A C-G) קומפלמנטריות)
64
RNA
U ,C ,G ,A :הנוקלאוטידים של פולימר
65
genes
Within the nucleus are most of the cell’s hereditary units, called genes, which control cellular structure and direct cellular activities. Genes are arranged along chromosomes.
66
Protein Synthesis
1 .transcription
| 2 .translation
67
intron
Not all parts of a gene actually code for parts of a protein. Regions within a gene called introns do not code for parts of proteins.
68
exon
They are located between regions called exons that do code for segments of a protein.
69
rna polymerase
The enzyme RNA polymerase (po-LIM-er-a¯s) catalyzes transcription of DNA. However, the enzyme must be instructed where to start the transcription process and where to end it. Only one of the two DNA strands serves as a template for RNA synthesis.
70
pre mrna
Immediately after transcription, the transcript
includes information from both introns and exons and is called pre-mRNA. The introns are removed from pre-mRNA by small nuclear ribonucleoproteins (snRNPs, pronounced “snurps”
71
poly a
Polyadenylation is the addition of a poly tail to a RNA transcript, typically a messenger RNA. The poly tail consists of multiple adenosine monophosphates; in other words, it is a stretch of RNA that has only adenine bases.
72
mrna
The resulting product is a functional mRNA molecule that passes through a pore in the nuclear envelope to reach the cytoplasm, where translation takes place.
73
alternative splicing
Although the human genome contains around 30,000 genes, there are probably 500,000 to 1 million human proteins. How can so many proteins be coded for by so few genes? Part of the answer lies in alternative splicing of mRNA, a process in which the premRNA transcribed from a gene is spliced in different ways to produce several different mRNAs. The different mRNAs are then translated into different proteins. In this way, one gene may code for 10 or more different proteins.
74
trna
Transfer ribonucleic acid (tRNA) is a type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein. tRNAs function at specific sites in the ribosome during translation, which is a process that synthesizes a protein from an mRNA molecule.
75
anticodon
An anticodon is a trinucleotide sequence complementary to that of a corresponding codon in a messenger RNA (mRNA) sequence. An anticodon is found at one end of a transfer RNA (tRNA) molecule.
76
genetic code
Genetic code is the term we use for the way that the four bases of DNA--the A, C, G, and Ts--are strung together in a way that the cellular machinery, the ribosome, can read them and turn them into a protein. In the genetic code, each three nucleotides in a row count as a triplet and code for a single amino acid.
77
centromere
The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids (a dyad). During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first thought to be genetic loci that direct the behavior of chromosomes.
78
chromatid
A chromatid is one of two identical halves of a replicated chromosome. During cell division, the chromosomes first replicate so that each daughter cell receives a complete set of chromosomes.
79
cell hypertrophy
Hypertrophy (/haɪˈpɜːrtrəfi/, from Greek ὑπέρ "excess" + τροφή "nourishment") is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It is distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number.
80
cell hyperplasia
Hyperplasia, or hypergenesis, is an increase in the amount of organic tissue that results from cell proliferation. It may lead to the gross enlargement of an organ, and the term is sometimes confused with benign neoplasia or benign tumor. Hyperplasia is a common preneoplastic response to stimulus
81
cell atrophy
Atrophy is defined as a decrease in the size of a tissue or organ due to cellular shrinkage; the decrease in cell size is caused by the loss of organelles, cytoplasm and proteins.
82
גידול (neoplasm
היפרפלסיה פתולוגית
83
גידול שפיר
התאים נשארים במקום הגידול
84
גידול סרטני / ממאיר (Tumor
התאים מתנתקים מהגידול המקורי ומתיישבים
| במקומות שונים בגוף ("גרורות", metastasis
85
אנפלסיה (anaplasia
חוסר יכולת של התא להתמיין
86
דיספלסיה (dysplasia
חוסר יכולת של התא לשגשג
87
נמק (necrosis
מוות של תאים מטראומה או פתולוגיה. התא נהרס ומפזר
| את האברונים, תהליך דלקתי.
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ligand
Each type of receptor recognizes and binds a specific
type of molecule. For instance, insulin receptors bind the hormone insulin. A specific molecule that binds to a receptor is
called a ligand (LI¯-gand; liga tied) of that receptor.
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linkers
Integral proteins may also serve as linkers that anchor proteins
in the plasma membranes of neighboring cells to one another
or to protein filaments inside and outside the cell. Peripheral
proteins also serve as enzymes and linkers.
90
cellidentity markers
Membrane glycoproteins and glycolipids often serve as cellidentity markers. They may enable a cell to (1) recognize other
cells of the same kind during tissue formation or (2) recognize and
respond to potentially dangerous foreign cells.
91
electrochemical gradient
The combined influence of the
concentration gradient and the electrical gradient on movement of a
particular ion is referred to as its electrochemical gradient.
92
aquaporins
integral membrane proteins that function as water channels.
93
isotonic solution
Any solution in which a cell—for example, a red blood cell (RBC)—maintains its normal shape and volume is an isotonic solution (Iˉ-soˉ-TON-ik; iso- same)
94
hypotonic solution
a solution that has a lower concentration of solutes than the cytosol inside the RBCs. In this case, water molecules enter the cells faster than they leave, causing the RBCs to swell and eventually to
burst. The rupture of RBCs in this manner is called hemolysis (he¯-MOL-i-sis; hemo- blood; -lysis to loosen or split apart); the rupture of other types of cells due to placement in a hypotonic solution is referred to simply as lysis. Pure water is very hypotonic and causes rapid hemolysis.
95
hypertonic solution
A hypertonic solution (hIˉ-per-TON-ik; hyper- greater
than) has a higher concentration of solutes than does the cytosol inside RBCs (Figure 3.9). One example of a hypertonic solution is a 2% NaCl solution. In such a solution, water molecules move out of the cells faster than they enter, causing the cells to shrink. Such shrinkage of cells is called crenation (kre-NAˉ-shun).
96
pumps
In primary active transport, energy derived from hydrolysis of ATP changes the shape of a carrier protein, which “pumps” a substance across a plasma membrane against its concentration gradient. Indeed, carrier proteins that mediate primary active transport are often called pumps. A typical body cell expends about 40% of the ATP it generates on primary active transport. Chemicals that turn off ATP production—for example, the poison cyanide—are lethal because they shut down active transport in cells throughout the body
97
primary active transport
In primary active transport, energy derived from hydrolysis of ATP changes the shape of a carrier protein, which “pumps” a substance across a plasma membrane against its concentration gradient.
98
secondary active transport
In secondary active transport, the energy stored in a Na or H concentration gradient is used to drive other substances across the membrane against their own concentration gradients. Because a Na or H gradient is established by primary active transport, secondary active transport indirectly uses energy obtained from the hydrolysis of ATP.
99
Receptor-mediated endocytosis
Receptor-mediated endocytosis is a highly selective
type of endocytosis by which cells take up specific ligands.
(Recall that ligands are molecules that bind to specific receptors.)
A vesicle forms after a receptor protein in the plasma membrane recognizes and binds to a particular particle in the extracellular fluid. For instance, cells take up cholesterol-containing low-density lipoproteins (LDLs), transferrin (an iron-transporting protein in
the blood), some vitamins, antibodies, and certain hormones by receptor-mediated endocytosis. Receptor-mediated endocytosis of LDLs (and other ligands) occurs as follows:
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clathrin-coated vesicle
endocytosis .Vesicle formation. The invaginated edges of the membrane around the clathrin-coated pit fuse, and a small piece of the membrane pinches off. The resulting vesicle, known as a clathrin-coated vesicle, contains the receptor–LDL complexes.
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Phagocytosis
is a form of endocytosis in which the cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses (Figure 3.13). Only a few body cells, termed phagocytes (FAG-oˉ-sIˉts), are able to carry
out phagocytosis. Two main types of phagocytes are macrophages, located in many body tissues, and neutrophils, a type of white blood cell.
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bulk-phase endocytosis, or
| pinocytosis
Most body cells carry out bulk-phase endocytosis, also called
pinocytosis (pi-noˉ-sIˉ-TO¯-sis; pino- to drink), a form of endocytosis in which tiny droplets of extracellular fluid are taken up
(Figure 3.14). No receptor proteins are involved; all solutes dissolved in the extracellular fluid are brought into the cell. During
bulk-phase endocytosis, the plasma membrane folds inward and
forms a vesicle containing a droplet of extracellular fluid. The
vesicle detaches or “pinches off” from the plasma membrane and
enters the cytosol. Within the cell, the vesicle fuses with a lysosome, where enzymes degrade the engulfed solutes. The resulting
smaller molecules, such as amino acids and fatty acids, leave the
lysosome to be used elsewhere in the cell. Bulk-phase endocytosis occurs in most cells, especially absorptive cells in the intestines and kidneys.
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Transcytosis
Transport in vesicles may also be used to successively move a substance into, across, and out of a cell. In this
active process, called transcytosis (tranz-sIˉ-TO¯-sis), vesicles undergo endocytosis on one side of a cell, move across the cell, and then undergo exocytosis on the opposite side. As the vesicles fuse with the plasma membrane, the vesicular contents are released
into the extracellular fluid. Transcytosis occurs most often across the endothelial cells that line blood vessels and is a means for materials to move between blood plasma and interstitial fluid. For instance, when a woman is pregnant, some of her antibodies cross
the placenta into the fetal circulation via transcytosis
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pericentriolar material
Surrounding the centrioles is pericentriolar
material (per-e¯-sen-tre¯-O¯ -lar), which contains hundreds of ring-shaped complexes composed of the protein tubulin. These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a critical role in cell division, and for
microtubule formation in nondividing cells. During cell division, centrosomes replicate so that succeeding generations of cells have the capacity for cell division.
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Rough ER
Rough ER is continuous with the nuclear membrane
and usually is folded into a series of flattened sacs. The outer surface of rough ER is studded with ribosomes, the sites of protein synthesis. Proteins synthesized by ribosomes attached to rough ER enter spaces within the ER for processing and sorting. In some
cases, enzymes attach the proteins to carbohydrates to form glycoproteins. In other cases, enzymes attach the proteins to phospholipids, also synthesized by rough ER. These molecules (glycoproteins and phospholipids) may be incorporated into the membranes of organelles, inserted into the plasma membrane, or
secreted via exocytosis. Thus rough ER produces secretory proteins, membrane proteins, and many organellar proteins
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Smooth ER
extends from the rough ER to form a network of
membrane tubules (Figure 3.19). Unlike rough ER, smooth ER does not have ribosomes on the outer surfaces of its membrane.
However, smooth ER contains unique enzymes that make it functionally more diverse than rough ER. Because it lacks ribosomes, smooth ER does not synthesize proteins, but it does synthesize
fatty acids and steroids, such as estrogens and testosterone. In liver cells, enzymes of the smooth ER help release glucose into the bloodstream and inactivate or detoxify lipid-soluble drugs or
potentially harmful substances, such as alcohol, pesticides, and carcinogens (cancer-causing agents)
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Transfer vesicles
Transfer vesicles that bud from the edges of the cisternae move specific enzymes back toward the entry face and move some partially modified proteins toward the exit face.
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membrane vesicles
Other processed proteins leave the exit face in membrane vesicles that deliver their contents to the plasma membrane for incorporation into the membrane. In doing so, the Golgi complex adds new segments of plasma membrane as existing
segments are lost and modifies the number and distribution of membrane molecules.
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autophagy
Lysosomal enzymes also help recycle worn-out cell structures. A lysosome can engulf another organelle, digest it, and return the digested components to the cytosol for reuse. In this way, old organelles are continually replaced. The process by which entire worn-out organelles are digested is called autophagy (aw-TOF-a-je¯; auto- self; -phagy eating)
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nuclear envelope
A double membrane called the nuclear envelope separates the nucleus from the cytoplasm. Both layers of the nuclear envelope are lipid bilayers similar to the plasma membrane. The outer membrane of the nuclear envelope is continuous with rough ER and resembles it in structure.
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nuclear pores
Many openings called nuclear pores extend
through the nuclear envelope. Each nuclear pore consists of a circular arrangement of proteins surrounding a large central opening that is about 10 times wider than the pore of a channel protein in the plasma membrane.
Nuclear pores control the movement of substances between the nucleus and the cytoplasm. Small molecules and ions move through the pores passively by diffusion. Most large molecules, such as RNAs and proteins, cannot pass through the nuclear pores by diffusion. Instead, their passage involves an active
transport process in which the molecules are recognized and selectively transported through the nuclear pore into or out of the nucleus. For example, proteins needed for nuclear functions move from the cytosol into the nucleus; newly formed RNA molecules move from the nucleus into the cytosol in this manner.
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nucleoli
Inside the nucleus are one or more spherical bodies called nucleoli (noo-KLE¯-o¯-li; singular is nucleolus) that function in producing ribosomes. Each nucleolus is simply a cluster of protein, DNA, and RNA; it is not enclosed by a membrane. Nucleoli are the sites of synthesis of rRNA and assembly of rRNA and proteins into ribosomal subunits. Nucleoli are quite prominent in
cells that synthesize large amounts of protein, such as muscle and liver cells. Nucleoli disperse and disappear during cell division and reorganize once new cells are formed.
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genome
The total genetic information carried in a cell or an organism is its genome (JE¯-no¯m).
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nucleosome
In cells that are not dividing, the chromatin appears as a diffuse, granular mass. Electron micrographs reveal that chromatin has a beads-on-a-string structure. Each bead is a nucleosome (NOO-kle¯-o¯-so¯m) that consists of double-stranded DNA wrapped twice around a core of eight proteins called histones, which help organize the coiling and folding of DNA. The string between the
beads is called linker DNA, which holds adjacent nucleosomes together. In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger-diameter chromatin fiber, which then folds into large loops. Just before cell division takes place, however, the DNA replicates (duplicates) and the loops
condense even more, forming a pair of chromatids (KRO¯-matids).
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proteome
Just as genome means all of the genes in an organism, proteome (PRO¯-te¯-o¯m) refers to all of an organism’s proteins
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gene expression
In the process called gene expression, a gene’s DNA is used as a template for synthesis of a specific protein. First, in a process aptly named transcription, the information encoded in a specific region of DNA is transcribed (copied) to produce a specific molecule of RNA (ribonucleic acid). In a second process, referred to
as translation, the RNA attaches to a ribosome, where the information contained in RNA is translated into a corresponding sequence of amino acids to form a new protein molecule
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base triplet
DNA and RNA store genetic information as sets of three nucleotides. A sequence of three such nucleotides in DNA is called a base triplet.
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codon
Each DNA base triplet is transcribed as a complementary sequence of three nucleotides, called a codon. A given codon specifies a particular amino acid.
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genetic code
The genetic code is the set of rules that relate the base triplet sequence of DNA to the corresponding codons of RNA and the amino acids they specify.
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promoter
The segment of DNA where transcription begins, a special nucleotide sequence called a promoter, is located near the beginning of a gene
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terminator
Transcription of the DNA strand ends at another special nucleotide sequence called a terminator, which specifies the end of the gene (Figure 3.27a). When RNA polymerase reaches the terminator, the enzyme detaches from the transcribed RNA molecule and the DNA strand.
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guanin cap
the beginning of mRNA
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mitosis
A somatic cell (so¯-MAT-ik; soma body) is any cell of the body other than a germ cell. A germ cell is a gamete (sperm or oocyte) or any precursor cell destined to become a gamete. In somatic cell division, a cell undergoes a nuclear division called mitosis (mı¯-TO¯ -sis; mitos thread)
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meiosis
Reproductive cell division is the mechanism that produces gametes, the cells needed to form the next generation of sexually reproducing organisms. This process consists of a special twostep division called meiosis, in which the number of chromosomes in the nucleus is reduced by half. stages: meiosis I and meiosis II
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Somatic Cell Division
When a cell reproduces, it must replicate (duplicate) all its chromosomes to pass its genes to the next generation of cells.
The cell cycle consists of two major periods: interphase, when a cell is not dividing, and the mitotic (M) phase, when a cell is dividing
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cytokinesis
a cytoplasmic division called cytokinesis (sı¯-to¯-ki-NE¯ -sis; cyto- cell; -kinesis movement)
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G0
Exit from cell cycle (nondividing cell)
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G1 phase
Cell metabolically active; duplicates organelles and
| cytosolic components; centrosome replication begins.
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S phase
DNA replicated (8 hours)
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G2 phase
Cell growth continues; enzymes and other proteins are synthesized; centrosome replication completed.
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Mitotic Phase
```
The mitotic (M) phase of the cell cycle, which results in the formation of two identical cells, consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) to form two identical cells. The events that occur during mitosis and cytokinesis are plainly visible under a microscope because chromatin condenses
into discrete chromosomes.
```
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Nuclear Division: Mitosis
biologists divide the process into four stages: prophase, metaphase, anaphase, and telophase. However, mitosis is a continuous process; one stage merges seamlessly into the next.
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centromere
A constricted region called a centromere (SENtro¯-me¯r) holds the chromatid pair together.
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kinetochore
At the outside of each centromere is a protein complex known as the kinetochore (ki-NET-o¯-kor)
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mitotic spindle
Later in prophase, tubulins in the pericentriolar material of the centrosomes start to form the mitotic
spindle, a football-shaped assembly of microtubules that attach to the kinetochore (Figure 3.32b). As the microtubules lengthen, they push the centrosomes to the poles (ends) of the cell so that the spindle extends from pole to pole. The mitotic spindle is responsible for the separation of chromatids to opposite poles of the cell. Then, the nucleolus disappears and the nuclear envelope breaks down
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Metaphase
Metaphase (MET-a-faˉz). During metaphase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle (Figure 3.32c).
This midpoint region is called the metaphase plate.
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Anaphase
Anaphase (AN-a-faˉz). During anaphase, the centromeres split, separating the two members of each chromatid pair, which move toward opposite poles of the cell . Once separated, the chromatids are termed chromosomes. As the chromosomes are pulled by the microtubules of the mitotic spindle during anaphase, they appear V-shaped because the centromeres lead the way, dragging the trailing arms of the chromosomes toward the pole.
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Telophase
Telophase (TEL-o¯-faˉz). The final stage of mitosis, telophase, begins after chromosomal movement stops.
The identical sets of chromosomes, now at opposite poles of the cell, uncoil and revert to the threadlike chromatin form. A nuclear envelope forms around each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks up.
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Prophase I
Prophase I is an extended phase in which the chromosomes shorten and thicken, the
nuclear envelope and nucleoli disappear, and the mitotic spindle forms. Two events that are not seen in mitotic prophase occur during prophase I of meiosis (Figure 3.33b). First, the two sister chromatids of each pair of homologous chromosomes pair off, an event
called synapsis (sin-AP-sis). The resulting four chromatids form a structure called a tetrad (TE-trad; tetra four). Second, parts of the chromatids of two homologous chromosomes may be exchanged with one another. Such an exchange between parts of
nonsister (genetically different) chromatids is called crossing-over. This process, among others, permits an exchange of genes between chromatids of homologous chromosomes. Due to crossing-over,
the resulting cells are genetically unlike each other and genetically unlike the starting cell that produced them. Crossing-over results in genetic recombination—that is, the formation of new combinations of genes—and accounts for part of the great genetic variation
among humans and other organisms that form gametes via meiosis
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metaphase I
In metaphase I, the tetrads formed by the homologous pairs of chromosomes line up along the metaphase plate of the cell, with homologous chromosomes side by side
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anaphase I
During anaphase I, the members of each homologous pair of chromosomes separate as they are pulled to opposite poles of the cell by the microtubules attached to the centromeres. The paired chromatids, held by a centromere, remain together. (Recall that during
mitotic anaphase, the centromeres split and the sister chromatids separate.)
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Telophase I
Telophase I and cytokinesis of meiosis are similar to
telophase and cytokinesis of mitosis. The net effect of meiosis I is that each resulting cell contains the haploid number of chromosomes because it contains only one member of each pair of the homologous chromosomes present in the starting cell.
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Meiosis II
The second stage of meiosis, meiosis II, also consists of four phases: prophase II, metaphase II, anaphase II, and telophase II (Figure 3.33a). These phases are similar to those that occur during mitosis; the centromeres split, and the sister chromatids separate and move toward opposite poles of the cell.
