Chapter 3 Flashcards
1
Q
According to morphology, the two broad groupings of life are
A
- Prokaryotes, which lack a membrane-bound nucleus
- Eukaryotes, which have such a nucleus
2
Q
According to phylogeny, or evolutionary history,
there are three domains:
A
- Bacteria (prokaryotic)
- Archaea (prokaryotic)
- Eukarya – eukaryotic
3
Q
Prokaryotes:
A
– Contain a single, circular
chromosome, tightly coiled
to fit inside the cell
Located in a region called
the nucleoid
– They also may contain
small, circular DNA
molecules called plasmids
4
Q
Although there is wide variation between bacterial and archaeal cells, all prokaryotes contain
A
Plasma membrane, chromosome, and protein-synthesizing ribosomes
5
Q
Prokaryotes have ribosomes, which are
A
– Macromolecular machines
– Consist of RNA molecules and protein
– Used for protein synthesis
6
Q
Many prokaryotes have internal photosynthetic membranes, they are
A
- Develop from folds of the plasma membrane
- The green stripes in this photosynthetic bacterium are membranes that contain the pigments and enzymes required for photosynthesis. This photo has been colourized to enhance the membranes
7
Q
Some bacteria have membrane-bound
compartments called
A
Organnelles
8
Q
Organnelles perform specialized tasks including
A
– Store calcium ions
– Hold magnetite crystals to serve as a compass
– Organize enzymes for building organic compounds
9
Q
Bacteria and archaea contain protein fibres that
A
perform a variety of roles including forming the basis of the cytoskeleton
10
Q
The cytoskeleton
A
– Assists in cell division
– Maintains cell shape
- E.g. FtsZ and MreB – participates in Z-ring formation
11
Q
The cell wall forms a protective
A
exoskeleton
12
Q
Most prokaryotes have a cell wall which
A
– Composed of a tough, fibrous layer
– Surrounds the plasma membrane
13
Q
Many species have an additional layer outside the cell wall composed of
A
glycolipids
14
Q
Many prokaryotes, such as E.Coli have
structures that grow from
their
A
plasma membrane, such as flagella and fimbriae
15
Q
What are flagella
A
long filaments that rotate to propel the cell
16
Q
The needlelike projections that promote attachment to other cells or
surfaces
A
Fimbriae
17
Q
The close up view of a prokaryotic cell
is created by
A
This painting is David Goodsell’s representation of a cross-section through part of a bacterial cell
It is based on electron micrographs of bacterial cells and is drawn to scale.
David Goodsell is a professor at the Scripps, Research Institute in Canada, but a google images search will also show that he is an amazing artist
18
Q
Eukaryotes range in size from
A
microscopic algae
to 100-metre-tall redwood trees
– Protists, fungi, plants, and animals are eukaryotes
– May be multicellular or unicellular
19
Q
Most eukaryotic cells are larger than most
A
prokaryotic cells
20
Q
Eukaryotic cells have a large
A
surface-to-volume ratio
– Difficult for molecules to diffuse across the entire cell
– The fluid portion, the cytosol, has a small volume
20
Q
Eukaryotic cells have a large
A
surface-to-volume ratio
– Difficult for molecules to diffuse across the entire cell
– The fluid portion, the cytosol, has a small volume
21
Q
Organelles break up the large cell volume into smaller membrane-bound organelles
This compartmentalization offers two advantages:
A
- Separation of incompatible chemical reactions
- Increasing the efficiency of chemical reactions
22
Q
Three key differences between eukaryotic and prokaryotic cells
A
- Eukaryotic cells are generally much larger
- Prokaryotic chromosomes are in a nucleoid region;
eukaryotic chromosomes are enclosed in a nucleus - Eukaryotic cytoplasm is compartmentalized into a
larger number of distinct organelles
23
Q
The nucleus stores and transmits
A
Information. The genetic, or hereditary information is encoded in DNA, which is a component of the chromosomes inside the nucleus
24
The nucleus
- The nucleus is large and highly organized . It is surrounded by a double-membrane nuclear envelope.
- Studded with pore-like openings
- The inside surface is linked to the nuclear lamina (Lattice-like sheet of fibrous proteins)
25
The nucleus has a distinct region called the nucleolus, where
Ribosomal RNA is synthesized
Ribosome subunits are assembled
26
Ribosomes are the site of
Protein synthesis. Eukaryotic ribosomes are larger than bacterial and archaeal ribosomes but similar in overall structure and function
27
Ribosomes are complex molecular machines that
Manufacture proteins
28
Ribosomes are not considered organelles because they
Lack a membrane
29
Some ribosomes are free in the cytosol, which
Manufacture proteins that remain in the cytosol or are imported to other organelles (e.g., nucleus)
30
Some ribosomes are attached to the endoplasmic reticulum, which
Manufacture proteins with other fates
31
The Endoplasmic Reticulum Is a
Site of
Synthesis, processing, and storage. The ER is continuous with the nuclear envelope and possesses two distinct regions: on the left, the rough ER is a system of membrane-bound sacs and tubules with ribosomes attached; on the right, the smooth ER is a system of membrane-bound sacs and tubules that lacks ribosomes
32
The endoplasmic reticulum is an organelle that is an extension of the
nuclear envelope
33
Two types of ER are
rough and smooth
34
Rough endoplasmic reticulum (rough ER, RER):
- Is studded with ribosomes
- Synthesizes proteins that will be:
shipped to another organelle, inserted into the plasma membrane, secreted to the cell exterior
35
As proteins are manufactured on the RER, they move to the
lumen (inside of any sac-like structure)
In the RER lumen, proteins are folded and processed
36
Proteins made on the RER may
- Carry messages to other cells
- Act as membrane transporters or pumps
- Catalyze reactions
37
Smooth endoplasmic reticulum (smooth ER,
SER) lacks
Ribosomes
38
Smooth endoplasmic reticulum Contains enzymes that catalyze reactions involving
lipids, that may
Synthesize lipids needed by the organism
Break down lipids and other molecules that are
poisonous
39
Which type of ER is a reservoir for Ca2+ ions
Smooth ER
40
The Golgi Apparatus Is a Site of
Protein Processing, Sorting, and shipping
41
The Golgi apparatus is formed by
a series of stacked, flat, membranous sacs called cisternae
42
The golgi apparatus has a distict
polarity, or sidedness
The cis (“on this side”) surface
is closest to the nucleus
The trans (“across”) surface is
oriented toward the plasma
membrane
43
The functions of the golgi apparatus are
– Processes, sorts, and ships proteins synthesized in the rough ER
– cis side of a Golgi apparatus receives products from the rough ER
– trans side ships them out to other organelles or the cell surface
– Membranous vesicles carry materials to and from the organelle
44
Lysosomes are
Recycling Centres found only in
animal cells
45
The function of lysosomes is
Digest macromolecules and export monomers to the cytosol
46
Enzymes are
acid hydrolases
– Work best at pH 5.0
– Proton pumps in
membrane maintain low
internal pH
47
Collectively, lysosomes, Golgi apparatus, and ER make up the
endomembrane system
48
The function of the endomembrane system is
- Produces, processes, and transports proteins and lipids
– For example, acid hydrolases are
Synthesized in the ER
Processed in the Golgi apparatus
Shipped to lysosomes
49
Vacuoles Are Generally
Storage Centres in Plant and Fungal Cells. Vary in size and function. Some contain digestive enzymes and serve as recycling centers; most are large storage containers
50
Vacuoles are large membrane structures that
- Some specialized for digestion
– Most used to store water and ions to
help the cell maintain its normal
volume
– In seeds, they are filled with proteins
– In flower petals or fruits, they contain
pigments
– May contain noxious compounds to
protect leaves and stems from being
eaten
51
Peroxisomes buds from
ER
52
Peroxisomes are
– Centre of redox reactions
– Liver cell peroxisomes contain
enzymes that oxidize ethanol
53
Specialized plant peroxisomes oxidize
fats to form an energy-storage
compound, they are
glyoxysomes
54
Oxidation often produces
hydrogen peroxide
– In peroxisomes, the enzyme
catalase “detoxifies
55
Mitochondria supply
ATP to cells
56
Mitochondria have two membranes
The inner one is folded into a
series of sac-like cristae
The solution inside the inner
membrane is the mitochondrial matrix
57
Mitochondria have their own
mitochondrial
DNA (mtDNA)
58
Mitochondria manufacture their own
ribosomes
59
Most plant and algal cells have centres where photosynthesis
takes place called
chloroplasts
60
Chloroplasts Are
Sugar- Manufacturing Centres in Plants and Algae
61
chloroplasts have how many membranes
Have three membranes
Innermost membrane contains
flattened sacs called
thylakoids, arranged in stacks
called grana
62
Surrounding the thylakoids is the
stroma
63
Chloroplasts contain their own
DNA and manufacture their own
ribosomes
64
Chloroplasts and mitochondria may once have been
free-living bacteria
65
Endosymbiosis theory states
Bacteria were engulfed
and a mutually beneficial relationship evolved
66
Evidence for endosymbiosis is
– Mitochondria and chloroplasts contain their own DNA
– Synthesize their own small ribosomes
– Grow and divide independently of cell division
67
The cytoskeleton is composed of protein fibres and is responsible for
– Gives cells shape and structural stability
– Aids cell movement
– Transports materials within the cell
– Organizes the organelles and other cellular structures
into a cohesive whole
68
Fungi, algae, and plants have a stiff outer
Cell wall which gives structural support to the cell. Rods or fibres of a carbohydrate run through a stiff
matrix made of other polysaccharides and proteins
69
Structure of each cell component
correlates with its
function
– Size and number of different types of
organelles
– Correlate with cell’s specialized function
70
Cells are dynamic living things which
– Have interacting parts
– Contain constantly moving molecules
71
Your body’s cells use, and synthesize,
approximately how many ATP molecules per second
10 million
72
Cellular enzymes can catalyze more than how many reactions per second
25,000
73
Each membrane phospholipid can travel the breadth of its organelle or cell in under a
minute
74
The hundreds of trillions of mitochondria inside you
are replaced about every
10 days, for as long as
you live.
75
The nuclear envelope is perforated with openings called
nuclear pore complexes
76
the function os nuclear envelope is
– Connects inside of nucleus with the cytosol
– Consists of about 30 different proteins
77
RNA and ribosomes are synthesized in the
nucleus and exported to the cytoplasm
78
binds proteins to form ribosomes
Ribosomal RNA
79
carries information to synthesize
proteins
Messenger RNA (mRNA)
80
Proteins needed in the nucleus are made in the
cytoplasm and imported into the nucleus (in eukaryotes)
– Proteins for copying DNA or synthesizing RNA
– Proteins for ribosomes
81
Nucleotides for building DNA and RNA also enter
Nuclear envelope
82
Most of the proteins found inside organelles
– Are actively imported from the cytosol
– Contain special signal sequences that target them to
the appropriate organelles
83
The endomembrane system functions by
– Proteins made in the RER
– Move to the Golgi apparatus for processing
– Travel to the correct destination
84
The secretory pathway hypothesis
This hypothesis proposes that proteins intended for secretion from the cell are synthesized and processed in a highly prescribed series of steps
Note that proteins are packaged into vesicles when they move from the rough ER to the Golgi and from the Golgi to the cell surface
85
A pulse-chase experiment is used to
track protein movement within a cell
86
describe the model of the secret pathway hypothesis
1. Protein enters the ER while being synthesized by the ribosome and is processed; one or more carbohydrate groups are often added (red dots)
2. Protein exits ER inside a vesicle and travels to the cis face of Golgi apparatus
3. Protein enters the Golgi apparatus and is processed
4. Protein exits the Golgi apparatus in a vesicle and moves to the plasma membrane
5. Protein is secreted from cells
87
The signal hypothesis
- explains how proteins destined for secretion enter the endomembrane system
- All proteins start to be synthesized on free ribosomes
- Proteins bound for the endomembrane system have a zip code
- It directs the growing polypeptide to the RER
- This zip code is a 20-amino-acid-long ER signal sequence
88
According to the signal hypothesis, proteins destined for secretion contain
a short stretch of amino acids that interact with a signal recognition particle (SRP) in the cytosol. This interaction directs the synthesis of the remaining proteins into the rough ER lumen
89
describe the process of signal hypothesis
1. ER signal sequence is synthesized by ribosome
2. ER signal sequence binds to signal recognition particle (SRP) and halts synthesis
3. SRP binds to receptors in the ER membrane
4. SRP is released. Protein synthesis continues. Protein enters ER through the trans colon
5. ER signal sequence is removed. Protein synthesis then proceeds to the completion
The ER signal sequence binds to a signal recognition particle (SRP)
They move to the RER membrane and bind to a receptor there
The SRP is released, and protein synthesis continues through a channel
The growing proteins is fed into the RER lumen
The ER signal sequence is removed
90
In the RER lumen, proteins are
folded
91
Enzymes add carbohydrate side chains in a process called
glycosylation
– Carbohydrates serve as indicator for shipment
– Resulting glycoprotein is ready to be shipped to next
destination
92
Proteins are transported in vesicles that
– Bud off from the ER
– Move away
– Fuse with the membrane on the cis face of the Golgi
apparatus
– Dump their cargo inside
93
How do proteins reach their destinations?
- Each protein that comes out of the Golgi apparatus has a molecular tag
that places it in a particular type of transport vesicle
- Each type of transport vesicle also has a tag that allows it to be transported to the correct destination
94
Proteins are secreted out of a cell by
exocytosis
95
Describe the process of protein sorting and vehicle transport in the golgi apparatus
1. In the endomembrane system, proteins bound for different destinations carry distinct tags that serve as molecular postal codes
2. Proteins are sorted in the Golgi apparatus when they bind to different receptors
3. Transport vesicles bud off the trans face of the Golgi apparatus and travel to their destinations
4. Cytosolic and membrane proteins cause transport vesicles to attach and fuse at destinations
5. Vesicles deliver content
96
Large molecules must be digested by lysosomes before
their monomers can be used by the cell
97
Materials are brought into the cell by
pinching off the plasma membrane in a process called endocytosis
98
The two types of endocytosis are:
1. Receptor-mediated endocytosis
2. Phagocytosis
99
How many pathways exist to recycle material in the lysosome?
Three: receptor-mediated endocytosis, phagocytosis, and autophagy
100
Receptor-mediated endocytosis and phagocytosis involve
bringing in material from the outside and surrounding it with a lipid bilayer from the plasma membrane
101
Endosomes mature into lysosomes or, like phagosomes, will
fuse with existing lysosomes
102
In autophagy, material within the cytoplasm is
encapsulated with an internal membrane before fusing with the lysosome
103
what is receptor-mediated endocytosis
Receptor-mediated endocytosis uses receptors to bind to macromolecules outside the cell. The plasma membrane pinches in to form a vesicle that delivers cargo to early endosome
The early endosome is acidified and matures into the late endosome, and eventually, a lysosome
104
What is phagocytosis?
Phagocytosis brings smaller cells or food particles inside the cell, forming a phagosome
A phagosome is delivered to the lysosome, which fuses with the phagosome and digests its contents
105
What is autophagy
Autophagy encloses a damaged organelle within a membrane, forming an autophagosome that is delivered to a lysosome and digested
Lysosome releases small molecules from digested materials in the cytosol
106
The cytoskeleton is
– Is a dense and complex network of fibres
– Helps maintain cell shape by providing structural
support
– Is not a static structure like scaffolding used at
construction sites
107
the fibrous proteins of the cytoskeleton move and change to
– Alter the cell’s shape
– Shift its contents
– Even move the cell itself
108
The three types of cytoskeletal elements are:
1. Actin filaments (microfilaments)
2. Intermediate filaments
3. Microtubules
109
the cytoskeletal filaments are distinguished by
their size, structure, and type of protein subunit
110
describe the structure of actin filaments
- Actin filaments, or microfilaments, are the smallest cytoskeletal elements
- Formed by polymerization of individual actin molecules into long strands
- Two strands coil around each other
- Grouped together into long bindles or dense networks
- Usually found just inside the plasma membrane
111
describe the function of actin filaments
- Maintain cell shape by resisting tension (pull)
- Move cells via muscle contraction or cell crawling
- Divide animal cells in two
- Move organelles and cytoplasm in plants, fungi, and animals
Help define the cell’s shape
- Cell crawling is caused by actin filaments growing in one direction, moving the cell
112
Actin and motor protein myosin work together to perform which functions
- Uses ATP to change shape and do work: muscle contraction, cytoknesis and cytoplasmic streaming
113
What is cytoknesis
dividing cytoplasm during cell division
114
What is cytoplasmic streaming
flow of cytoplasm
115
describe how actin and myosin collaborate
- Actin and myosin interact to cause movement
- When a myosin “head” attaches to actin and moves, the actin filament slides
- Actin-myosin interactions draw the membrane in, divide a cell in two
- Cytoplasmic streaming in plants- actin-myosin interactions move cytoplasm around a cell
- ATP hydrolysis in the head region of myosin causes the protein to attach to actin and change shape. The movement slides the myosin towards the plus head of actin
- Actin-myosin interactions can divide cells and move organelles and cytoplasm
116
Describe the structure of intermediate filaments
- Are defined by size rather than composition
- Many types exist, each consisting of a different protein
- About 20 types of keratin- found in nails and hair
117
Describe the function of intermediate filaments
- Provide structural support for the cell
- Are not involved in the movement
- Maintain cell shape by resisting tension (pull)
- Acnchor nucleus and some other organelles
118
Describe the structure of microtubules
- Microtubules are the largest cytoskeletal
elements
– Large, hollow tubes made of tubulin dimers
– Have two polypeptides, called α-tubulin and β-tubulin
– Have polarity
– Are dynamic
– Usually grow at their plus ends
119
describe the function of microtubules
- Maintain cell shape by resisting compression (push)
- Move cells via flagella or cilia
Move chromosomes during cell division
- Assist formation of cell plate during plant cell division
- Provide tracks for intracellular transport
120
microtubules originate from
The microtubule organizing center
Plus ends grow outwards
Radiating throughout the cell
121
In animal cells, the microtubule organizing centre is called
The centrosome
It contains two bundles of microtubules called centrioles
122
What is the structure and function of centrosome in animal cells
- Centrosomes are a type of microtubule-organizing centre
- Animal cells typically have a single centrosome at their centres
- The minus ends of the microtubules are attached to the surface of the centrosome while the plus ends extend outwards and can reach the inner surface of the plasma membrane
123
Vesicle transport requires a motor protein called
kinesin
124
What is the function of kinesin
– Uses ATP to do mechanical work
– The head region binds to a microtubule
– The tail region binds to a transport vesicle
125
how does kinesin interact with the microtubule
- Kinesin “walks” along a microtubule
– Through a series of conformational changes
– As it hydrolyzes ATP
- The two head segments act like feet that alternately attach, pivot, and release in response to the gain or loss of a phosphate group from ATP
126
How many distinct regions does kinesin have
3
127
Which elements of the cell work to move the entire cell
Cilia and Flagella
128
long, hair-like projections from the
cell surface that move cells
Flagella
129
Prokaryotic flagella
– Are helical rods made of a protein called flagellin
– Move the cell by rotating the rod like a ship’s propeller
– Are not surrounded by the plasma membrane
130
Eukaryotic flagella
– Consist of several microtubules
– Move the cell by undulating—they whip back and forth
– Are surrounded by the plasma membrane
– Probably evolved independently from prokaryotic
flagella
131
closely related to eukaryotic flagella
Cilia
– Short, hair-like projections
– Cells generally have just one or two flagella
– Some cells have many cilia
132
Cilia and Flagella Differ in
Length and Number
Cilia range in length from 1 to 10 um, while flagella are typically longer and can exceed 1mm.
Cells with flagella typically possess only 1 to 4 flagella
Cilia tend to occur in larger numbers, and certain ciliated cells have up to 14,000 cilia
133
The amount of chemical energy in a covalent bond is based on
– Position of shared electrons
– Distance from positive charges in nuclei
134
The potential energy of a molecule depends on the configuration and position of
its shared
electrons
135
Weaker bonds with equally shared electrons have
high
potential energy
136
Stronger bonds with unequally shared electrons have
low potential energy
137
Equal sharing (nonpolar) have longest, weakest bonds and
high potential energy
138
Unequal sharing (polar) have shortest, strongest bonds and
lowest potential energy
139
Gibbs free energy (G) determines
whether a reaction is spontaneous or requires added energy to proceed
140
Standard free-energy change equation is used to
Calculate the change in G (ΔG) during the reaction:
ΔG = ΔH –TΔS
– ΔH = change in enthalpy
– ΔS = change in entropy
– T = temperature in degrees Kelvin
141
Enthalpy (H) includes
– The potential energy of the molecule (heat content)
– Effect of the molecule on surrounding pressure and
volume
142
Changes in enthalpy (ΔH) are primarily based on
the difference in potential energy
143
Exothermic reactions
– Release heat energy
– ΔH < 0
– Products have less potential energy than reactants
144
Endothermic reactions
– Heat energy is taken up
– ΔH > 0
– Products have higher potential energy than reactants
145
Entropy (S) is
the amount of disorder
146
When the products of a chemical reaction become
less ordered than the reactant molecules
– Entropy increases
– ΔS > 0
– Spontaneous reactions
147
Second law of thermodynamics states that
total entropy always increases in a system
148
ΔG < 0 =
a spontaneous reaction
– These reactions are exergonic
149
ΔG > 0 =
a nonspontaneous reaction that requires
energy input to occur
– These reactions are endergonic
150
ΔG = 0
a reaction that is at equilibrium
151
For most reactions to proceed
– One or more chemical bonds have to break
– Others have to form
152
Substances must collide in a specific orientation that brings the electrons involved
near each other
153
Higher concentrations and higher temperature increase the number of
Collisions
154
Higher concentrations and higher temperature therefore increase
reaction rate
155
Energetic Coupling Allows Endergonic Reactions to Proceed Using the
Free Energy Released from Exergonic
Reactions
156
Reduction–oxidation reactions (redox
reactions) are
-Are chemical reactions that involve electron transfer
- Always occur together
- Represent energetic coupling of two half-reactions
157
Oxidation is
loss of an electron(s)
158
reduction is
gain of an electron(s)
159
What type of reaction is oxidation
exergonic decreased potential energy
160
What type of reaction is reduction
endergonic increased potential energy of the reduced molecule
161
Redox Reactions Transfer Energy via
Electrons
162
During a redox reaction, electrons may be gained or lost in two different ways
– Change in the number of electrons in the valence shell of an atom
– electrons are transferred as new covalent bonds that are formed with other atoms
163
Electrons can be transferred from an
electron donor to an electron acceptor
164
Most electron acceptors gain
potential energy as
they are reduced
165
Reduction often
“adds Hs”
166
Oxidation often
“removes Hs”
167
Electrons are usually accompanied by a
proton (H+)
168
Flavin adenine dinucleotide
(FAD) forms form FADH2 by
Accepts two electrons plus two protons. Will later donate these electrons to other electron acceptors. Acts as an electron shuttle
169
Nicotinamide adenine
dinucleotide (NAD+) forms NADH by
Accepts two electrons plus one proton. Will later donate these electrons
to other electron acceptors. Acts
as an electron shuttle
170
Adenosine triphosphate (ATP) is
is the energy currency for cells
– It provides the fuel for most cellular activities
171
ATP forms bonds between
- three negatively charged phosphate
groups
– Negative charges repel each other
– High-energy bonds store a large amount of potential energy
- stores a large amount of potential energy
172
ATP Hydrolysis Releases
Free Energy
- Hydrolysis of the bond between the two outermost phosphate groups results in formation of ADP and Pi (inorganic phosphate,
H2PO4−)
– In a highly exergonic reaction
Releases 7.3 kilocalories of energy per mole of ATP
173
Energy released during ATP hydrolysis is transferred to a
substrate by phosphorylation
174
Phosphorylation is
- adding a phosphate group
– Usually causes a change in the protein’s shape
175
Exergonic phosphorylation reactions are coupled to
endergonic reactions
In cells, many reactions only occur if one reactant is activated by phosphorylation
The phosphorylated reactant molecule has high enough free energy that the subsequent reaction is exergonic
In this graph, the free energy being tracked on the y-axis represents A,B, and the 35.0kJ/mol that is released when ATP is hydrolyzed
For simplicity, the free energy in ADP and Pi is not shown G represents the change in free energy between the reactants and products for each indicated step
176
Most biological chemical reactions occur fast enough only in the presence of an
enzyme
177
Enzymes are protein
catalysts
178
Enzymes work by
– Bring reactants together in precise orientations
– Make reactions more likely
– Are specific for a single type of reaction
179
Before a reaction can take place, reactants need to
1. Collide in a precise orientation
2. Have enough kinetic energy to overcome repulsion between electrons that come into contact as a bond forms
180
Enzymes bring
substrates together
181
Substrates bind to the enzyme’s
- active site
– Enzymes help them collide in a precise orientation
– Bonds break and form to generate products
182
Many enzymes undergo a
conformational change
– When the substrates are bound to the active site
– This change is called an induced fit
183
Substrates bind via
hydrogen bonding or other interactions with amino acid residues in the active
site
184
An unstable intermediate condition called the
transition state is formed
185
Activation energy (Ea)
is required to strain substrates’ bonds so they can reach the transition
state
186
Interactions between the enzyme and the substrate
– Interactions between substrates and amino acid
residues of the enzyme
– Stabilize the transition state
– Lower the activation energy required for the reaction to
proceed
187
are enzymes consumed during the reactions?
No, enzyme is unchanged
188
Enzyme catalysis has three steps
1. Initiation—substrates are precisely oriented as they bind to the active site
2. Transition state facilitation interactions
between the substrate and active site R-groups
lower the activation energy
3. Termination—reaction products are released from the enzyme
189
What Limits the Rate of Catalysis?
Saturation of
enzyme active
sites
190
The speed of an enzyme-catalyzed reaction
1. Increases linearly at low substrate concentrations
2. Slows as substrate concentration increases
3. Reaches maximum speed at high substrate concentrations
191
Temperature affects
– Folding
– Movement of the enzyme
– Kinetic energy of substrates
192
pH affects
– Enzyme’s structure and function by affecting:
– The charge on carboxyl and amino groups in residue side chains
– The active site’s ability to participate in reactions that involve the transfer of protons or electrons
193
Enzymes Have an Optima
Temperature and pH
194
An Enzyme’s Activity Is Precisely
regulated
* Regulation via noncovalent interactions
– Does not permanently affect the enzyme structure
– Is referred to as “reversible”
195
Competitive inhibition occurs when
a molecule competes with
the substrate for the active site
When the enzyme is in absence of regulation, the substrates cannot bind when a regulatory molecule binds to the enzyme’s active site
196
Allosteric regulation
occurs
when a molecule binds at a location other than the active site
– Causes a change in enzyme shape
– Can activate or deactivate the enzyme
197
Regulation may involve
covalent modifications
– Changes the enzyme’s primary structure
– Can be reversible or irreversible
198
Irreversible changes often result from
cleavage of
peptide bonds
199
Most common reversible modification of enzymes is
the addition of phosphate groups
(phosphorylation)
– Causes a change in shape
– May activate or inactivate the enzyme
200
Enzymes Can Work Together in
Metabolic Pathways
– A series of reactions
– Each catalyzed by a different enzyme
– To build biological molecules
– Much like an assembly line in a factory
201
An initial substrate A is sequentially modified by enzymes 1–3 to produce product D. The B and C molecules are referred to as
intermediates in the pathway
– they serve as both a product and a reactant.
– molecule B is the product of reaction 1 and the reactant
for reaction 2.
202
Distant cells can communicate through
signalling
molecules
203
Neurotransmitters may
open or close channels in
distant cells
204
Hormones are
information-carrying molecules
Secreted from a cell
Circulates in the body
Acts on target cells far from the signalling cell
205
Hormones are usually
– Small molecules—peptides, steroids, or even gases
– Typically present in minute concentrations
– Have a large impact on the activity of target cells
206
Hormones function by
binding to signal receptor molecules
– Change shape and activity after binding to a hormone
– Only cells with appropriate signal receptors will
respond to a particular signalling molecule
207
Hormones may be
soluble or insoluble in lipids
– Lipid-soluble hormones diffuse across the plasma membrane
Receptors are in the target cells’ cytoplasm
208
Lipid-insoluble hormones and large hormones do not cross the
plasma membrane
Receptors are on the cell’s plasma membrane
209
Lipid-soluble steroid hormones
– Bind to receptors inside the cell
– Trigger a change in the cell’s activity directly
210
The hormone–receptor complex
– Is transported to the nucleus
– Where it alters gene expression
211
Some Cell–Cell Signalling Molecules Enter the Cell and Bind to
Receptors in the Cytosol
Because they are lipids, steroid hormones can diffuse across cell membranes and bind to signal receptors located on the cytosol
The hormone-receptor complex may then be transported to the nucleus, where it changes the activity of genes
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describe the processing of lipid-soluble signals
1. The arrival of signal: protein carries steroid hormone to the cell surface
2. Signal entry: hormone diffuses across the plasma membrane into cytosol
3. Signal reception: hormone binds to the receptor, inducing conformational change
4. Direct-signal response: hormone-receptor complex binds to DNA, inducing a change in gene expression
213
Hormones that cannot diffuse across the plasma membrane bind to
membrane receptors
214
When a signal binds at the cell surface, it
– Triggers a complex series of events
– Collectively called signal transduction
– Converts the extracellular hormone signal to an intracellular signal
215
Signal Transduction
converts an extracellular signal to an intracellular signal
A lipid-insoluble signaling molecule will not pass through the membrane to direct a cellular response. Instead, the molecule activates a surface receptor that directs a multistep process to generate intracellular signals. One or more of these intracellular signalling molecules may then be transported to the nucleus to change the activity of genes
1. Signal reception
2. Signal transduction from extracellular signal to intracellular signal. Signal is amplified (in most cases)
3. Signal response: for example, specific proteins are activated, inducing a change in gene expression
216
During signal transduction two things occur
1. The signal may be amplified. By having many steps between the receptor and the response, there is an opportunity for a signalling molecule
to activate several downstream molecules.
2. The signal may be diversified. A cell that receives a signal may undergo several related changes as a result.
217
G proteins are
intracellular peripheral membrane
proteins
– Closely associated with transmembrane signal
receptors
218
When G proteins are activated by a signal receptor
– They trigger production of a second messenger
– Small, nonprotein signalling molecule or ion
– Links the receipt of an extracellular signal to the
production of an intracellular signal
219
G proteins are regulated by guanine nucleotides
– When a G protein binds to guanosine triphosphate
(GTP), its shape is altered and it is activated
– When a phosphate group is removed from the bound GTP to form GDP, the G protein is inactivated
220
G-protein-coupled signalling involves three steps
1. A signalling molecule binds to its membrane receptor
2. The associated G protein exchanges GDP for GTP
Splits into two parts
3. The active G protein subunit activates a nearby membrane enzyme
Catalyzes the production of a second messenger
