Bio 10: The Fundamentals of
Biology
Fall 2005 - 1082
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Chapter 5: Cell Division
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Cell Increase and Decrease
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Cell division increases the number of
somatic (body) cells, and consists of:
Mitosis (division of nucleus)
Cytokinesis (division of cytoplasm)
Apoptosis (cell death) decreases the
number of cells.
Both cell increase and apoptosis occur
during normal development and growth.
The Cell Cycle
• The cell cycle is an orderly sequence of
events that occurs from the time when a
cell is first formed until it divides into two
new cells.
• Most of the cell cycle is spent in
interphase.
• Following interphase, the mitotic stage of
cell division occurs.
The stages of interphase
• G1 stage – cell growth, cell doubles its
organelles, accumulates materials for DNA
synthesis
• S stage – DNA synthesis occurs, and DNA
replication results in duplicated
chromosomes
• G2 stage – cell synthesizes proteins
needed for cell division
The cell cycle
The Mitotic Stage
• Following interphase is the M stage,
including mitosis and cytokinesis.
• During mitosis, sister chromatids of each
chromosome separate, and become the
nuclei of the two daughter cells.
• The cell cycle ends when cytokinesis, the
cleaving of the cytoplasm, is complete.
Control of the cell cycle
•
The cell cycle is controlled at three
checkpoints:
1. During G1 prior to the S stage
2. During G2 prior to the M stage
3. During the M stage prior to the end of
mitosis
• DNA damage can also stop the cell cycle
at the G1 checkpoint.
Apoptosis
• Apoptosis is programmed cell death.
• Apoptosis occurs because of two sets of
enzymes called capsases.
• The first set, the “initiators” receive a
signal to activate the second set, the
“executioners”.
• The second set of capsases activate
enzymes that tear apart the cell and its
DNA.
Maintaining the Chromosome
Number
• When a eukaryotic cell is not dividing, the
DNA and associated proteins is a tangled
mass of thin threads called chromatin.
• At the time of cell division, the chromatin
condenses to form highly compacted
structures called chromosomes.
• Each species has a characteristic number
of chromosomes.
Overview of Mitosis
• The diploid number of chromosomes is
found in the somatic (non-sex) cells.
• The diploid (2n) number of chromosomes
contains two chromosomes of each kind.
• The haploid (n) number of chromosomes
contains one chromosome of each kind.
• In the life cycle of many animals, only
sperm and eggs have the haploid number
of chromosomes.
• The nuclei of somatic cells undergo
mitosis, a nuclear division in which the
number of chromosomes stays constant.
• Before nuclear division occurs, DNA
replication takes place, duplicating the
chromosomes.
• A duplicated chromosome is made of two
sister chromatids held together in a region
called the centromere.
• Sister chromatids are genetically identical.
• At the end of mitosis, each chromosome
consists of a single chromatid.
• During mitosis, the centromeres divide and
then the sister chromatids separate,
becoming daughter chromosomes.
Mitosis overview
• Following mitosis, a 2n parental cell gives
rise to two 2n daughter cells, or 2n → 2n.
• The cells of some organisms (algae, fungi)
are haploid as adults; n → n.
• Mitosis occurs when tissues grow or when
repair occurs.
• Following fertilization, the zygote divides
mitotically, and mitosis continues
throughout the lifespan of the organism.
Mitosis in Detail
• During mitosis, the spindle distributes the
chromosomes to each daughter cell.
• The spindle contains fibers made of
microtubules that disassemble and assemble.
• Centrosomes, that divide during interphase,
organize the spindle.
• Centrosomes contain centrioles and asters.
• Mitosis has four phases: prophase, metaphase,
anaphase, and telophase.
Late Interphase
Early Prophase
Late Prophase
Metaphase
Anaphase
Telophase
How Plant Cells Divide
• Plant cells lack centrioles and asters, but
have a centrosome and spindle and the
same four stages of mitosis.
• Meristematic tissue, in shoot and root tips,
retains the ability to divide throughout life.
• Lateral meristems accounts for the ability
of trees to grow in girth.
Cytokinesis in Plant and Animal
Cells
• Cytokinesis, or cytoplasmic cleavage,
accompanies mitosis.
• Cleavage of the cytoplasm begins in
anaphase, but is not completed until just
before the next interphase.
• Newly-formed cells receive a share of
cytoplasmic organelles duplicated during
the previous interphase.
Cytokinesis in Plant Cells
• The rigid cell wall surrounding plant cells
does not permit cytokinesis by furrowing.
• The Golgi apparatus releases vesicles that
microtubles move to the cell plate forming
between the two new cells.
• New plant cell walls form and are later
strengthened by cellulose fibers.
Cytokinesis in plant cells
Cytokinesis in Animal Cells
• In animal cells, a cleavage furrow begins
at the end of anaphase.
• A band of actin and myosin filaments,
called the contractile ring, slowly forms a
constriction between the two daughter
cells.
• A narrow bridge between the two cells is
apparent during telophase, then the
contractile ring completes the division.
Cytokinesis in animal cells
Cell Division in Prokaryotes
• The process of asexual reproduction in
prokaryotes is called binary fission.
• The two daughter cells are identical to the
original parent cell, each with a single
chromosome.
• Following DNA replication, the two
resulting chromosomes separate as the
cell elongates.
Reducing the Chromosome
Number
•
Meiosis reduces the chromosome
number such that each daughter cell has
only one of each kind of chromosome.
• The process of meiosis ensures that the
next generation will have:
1) the diploid number of chromosomes
2) a combination of traits that differs from
that of either parent.
Overview of meiosis
Overview of Meiosis
• Meiosis requires two nuclear divisions and
four haploid nuclei result.
• Humans have 23 pairs of homologous
chromosomes, or 46 chromosomes total.
• Prior to meiosis I, DNA replication occurs.
• During meiosis I, synapsis occurs.
• Meiosis I separates homologous pairs
of chromosomes.
• Daughter cells are haploid, but
chromosomes are still in duplicated
condition.
• No replication of DNA occurs between
the two divisions.
• Meiosis II separates sister
chromatids.
• In many life cycles, haploid daughter
cells mature into gametes.
• Fertilization restores the diploid
number of chromosomes during
sexual reproduction.
Independent assortment
Meiosis in Detail
• The same four phases seen in mitosis –
prophase, metaphase, anaphase, and
telophase – occur during both meiosis I
and meiosis II.
• The period of time between meiosis I and
meiosis II is called interkinesis.
• No replication of DNA occurs during
interkinesis because the DNA is already
duplicated.
Meiosis I in an animal cell
Meiosis II
Sources of Genetic Variation
•
As a result of meiosis followed by
fertilization, there are three sources of
genetic recombination:
1) Independent alignment of paired
chromosomes along the metaphase I
plate
2) Crossing-over during prophase I
3) Combining of chromosomes of
genetically different gametes
Comparison of Meiosis with Mitosis
• In both mitosis and meiosis, DNA
replication occurs only once during
interphase.
• Mitosis requires one division while meiosis
requires two divisions.
• Two diploid daughter cells result from
mitosis; four haploid daughter cells result
from meiosis.
• Daughter cells from mitosis are genetically
identical to parental cells; daughter cells
from meiosis are not genetically identical
to parental cells.
• Mitosis occurs in all somatic cells for
growth and repair; meiosis occurs only in
the reproductive organs for the production
of gametes.
Comparison of Meiosis I to Mitosis
• Meiosis I:
• Prophase I - pairing of
homologous
chromosomes
• Metaphase I –
homologous pairs line up
at metaphase plate
• Anaphase I –
homologous
chromosomes separate
• Telophase I – daughter
cells are haploid
• Mitosis:
• Prophase has no such
pairing
• Metaphase –
chromosomes align at
metaphase plate
• Anaphase – sister
chromatids separate
• Telophase – diploid cells
Comparison of Meiosis II to Mitosis
• The events of meiosis II are like those of
mitosis except in meiosis II, the nuclei
contain the haploid number of
chromosomes.
• At the end of telophase II of meiosis II,
there are four haploid daughter cells that
are not genetically identical.
• At the end of mitosis, there are two diploid
daughter cells that are identical.
Meiosis compared to mitosis
Chapter 6 Metabolism: Energy and
Enzymes
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Cells and the Flow of Energy
• Energy is the ability to do work.
• Living things need to acquire energy; this
is a characteristic of life.
• Cells use acquired energy to:
• Maintain their organization (a lack of, is
referred to as “entropy” which is chaos)
• Carry out reactions that allow cells to
develop, grow, and reproduce
Forms of Energy
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There are two basic forms of energy.
Kinetic energy is the energy of motion.
Potential energy is stored energy.
Food eaten has potential energy because
it can be converted into kinetic energy.
• Potential energy in foods is chemical
energy.
• Organisms can convert chemical energy
into a form of kinetic energy called
mechanical energy for motion.
Two Laws of Thermodynamics
• The flow of energy in ecosystems occurs
in one direction; energy does not cycle.
• The two laws of thermodynamics explain
this phenomenon.
• First Law: Energy cannot be created or
destroyed, but it can be changed from one
form to another.
• Second Law: Energy cannot be changed
from one form to another without loss of
usable energy.
• The ultimate source of energy for
ecosystems is the sun, and this energy is
passed from plants to animals.
Metabolic Reactions and Energy
Transformations
• Metabolism is the sum of all the chemical
reactions that occur in a cell (a combo of
anabolism and catabolsim).
• Reactants are substances that participate
in a reaction; products are substances that
form as a result of a reaction.
ATP: Energy for Cells
• ATP (adenosine triphosphate) is the energy
currency of cells.
• ATP is constantly regenerated from ADP
(adenosine diphosphate) after energy is
expended by the cell.
• Use of ATP by the cell has advantages:
• 1) It can be used in many types of reactions.
• 2) When ATP → ADP + P, energy released
is sufficient for cellular needs and little
energy is wasted.
The ATP cycle
Metabolic Pathways and Enzymes
• Cellular reactions are usually part of a
metabolic pathway, a series of linked
reactions, illustrated as follows:
•
E1
E2
E3
E4
E5 E6
A → B → C → D → E →F → G
• Here, the letters A-F are reactants or
substrates, B-G are the products in the
various reactions, and E1-E6 are enzymes.
• An enzyme is a protein molecule that
functions as an organic catalyst to speed a
chemical reaction.
• An enzyme brings together particular
molecules and causes them to react.
Energy of activation (Ea)
Enzyme-Substrate Complexes
• Every reaction in a cell requires a specific enzyme.
• Enzymes are named for their substrates (and usually end
with the letters “-ase” ):
•
•
•
•
•
Substrate
Lipid
Urea
Maltose
Ribonucleic acid
Enzyme
Lipase
Urease
Maltase
Ribonuclease
Enzymatic reactions:
can be to either build OR break down things!
Enzyme activity and temperature and pH:
it’s that darn homeostasis again!!
• As the temperature rises, enzyme activity
increases because more collisions occur
between enzyme and substrate.
• If the temperature is too high, enzyme activity
levels out and then declines rapidly because
the enzyme is denatured.
• Each enzyme has an optimal pH at which the
rate of reaction is highest.
Rate of an enzymatic reaction as a
function of temperature and pH
Enzyme Inhibition
• Enzyme inhibition occurs when an active
enzyme is prevented from combining with
its substrate.
• When the product of a metabolic pathway
is in abundance, it binds competitively with
the enzyme’s active site, a simple form of
feedback inhibition.
• Other metabolic pathways are regulated
by the end product binding to an allosteric
site on the enzyme.
Feedback inhibition
Enzyme Cofactors
• Presence of enzyme cofactors may be
necessary for some enzymes to carry out their
functions.
• Inorganic metal ions, such as copper, zinc, or
iron function as cofactors for certain enzymes.
- which is why it is critical that you consume
certain metals in your diet…..mmmm, metal!
Enzyme Cofactors and Coenzymes
• Organic molecules, termed coenzymes, must be present for
other enzymes to function.
• Some coenzymes are vitamins – now you know why you need
them in your diet….and why a lack of them can result in
disease – without them your enzymes cannot perform their
functions in catabolism or anabolism of certain substances…..
• EX: see page 276, Rickets (Vita D), Dermatitis (niacin), and
Scurvy (Vita C – ya limey!)…..
Photosynthesis
• The overall reaction for photosynthesis
can be written:
• 6CO2 + 6H2O + energy → C6H12O6 + 6O2
• During photosynthesis, hydrogen atoms
are transferred from water to carbon
dioxide, and glucose is formed.
• The Energy to form glucose comes from
the sun! Solar-powered…plants!
Cellular Respiration
• The overall equation for cellular respiration
is opposite that of photosynthesis:
• C6H12O6 + 6O2 → 6CO2 + 6H2O +
Energy
Organelles and the Flow of Energy
• During photosynthesis, chloroplasts capture solar energy and
use it to convert water and carbon dioxide into carbohydrates
that provide food for other living things.
• Cellular respiration, the breakdown of glucose into carbon
dioxide and water, occurs in mitochondria.
• It is the cycling of molecules between chloroplasts and
mitochondria that allows a flow of energy from the sun through
all living things.
Relationship of chloroplasts to
mitochondria
• Each reaction requires a specific enzyme.
• Substrate concentration, temperature, pH,
and enzyme concentration affect the rates
of reactions.
• Most metabolic pathways are regulated by
feedback inhibition.
• Photosynthesis and cellular respiration
involve oxidation-reduction reactions and
account for the flow of energy through all
living things.
Chapter 7:
Cellular Respiration
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Cellular respiration
Cellular Respiration takes place in 4 phases
Lets follow ONE molecule of glucose through its complete metabolism
Cellular Respiration takes place in 4 phases
1. Glycolysis is the breakdown of glucose into
pyruvate, 2 ATP molecules are made
Step 1:
2
Cellular Respiration takes place in 4 phases
2. In the transition reaction, pyruvate is broken down
into acetyl CoA, no ATP is made
Step 2:
2
Cellular Respiration takes place in 4 phases
3. The Citric Acid Cycle also called the “Krebs” cycle, and
breaksdown Acetyl-CoA into CO2….2 ATP are made
Step 3:
2
2
MITOCHONDRIA
Cellular Respiration takes place in 4 phases
4. The Electron Transport System uses the ELECTRONS
removed from glucose molecules to provide
engergy to make TONS of ATP…..about 32-34 ATPs!!!
Step 4:
2
2
MITOCHONDRIA
32
Cellular Respiration takes place in 4 phases
Therefore, ONE molecule of glucose generates
2+2+32/34 ATP molecules…….a total of 36-38!
Step 2:
Step 1:
2
Step 3:
Step 4:
2
32
Where is all this occuring?!?!
Cell
Outside of the
Mitochondria!
ATP is not only produced….but also NADH and FADH!
ATP is the currency to run machinery within the cell…..
But NADH and FADH2 run the electron transport system
*which then makes the ATP!*
NAD+ and FAD
• Each step of cellular respiration requires a
separate enzyme.
• Some enzymes use the oxidationreduction coenzyme NAD+ (nicotinamide
adenine dinucleotide).
• FAD (flavin adenine dinucleotide) is
sometimes used instead of NAD+.
The function of NADH and FADH2 is to carry and then
donate electrons to the electron transport system…..
The function of NADH and FADH2 is to carry and then
donate electrons to the electron transport system…..
*remember electron transport is occuring across the mitochondria’s cristae!*
What happens when breakdown of glucose is incomplete?
FERMENTATION!!!!
• When oxygen is available, pyruvate enters the
mitochondria, where it undergoes further breakdown,
through the citric acid and electron transport cycles.
• If oxygen is not available, fermentation occurs and
pyruvate undergoes reduction.
• Fermentation is an anaeorbic process and does not
require oxygen. (“an-aeorobic" means, without oxygen)
• In humans, pyruvate is reduced to lactic acid during
fermentation.
In humans…..
In bacteria or yeast…..
The Fermentation Process…… remember, it’s ANAEOROBIC
this situation only occurs when oxygen levels are LOW!
In humans…..
In bacteria or yeast…..
Notice, that in low oxygen you only make 2 ATP, compared to 36!
this is why, when you have an oxygen debt, you get a
lactic acid buildup in your muscles! AND, you pant to try to
Bring more oxygen into your body to complete cellular respiration
Efficiency of Fermentation
• Two ATP produced during fermentation are
equivalent to 14.6 kcal; complete oxidation
of glucose to CO2 and H2O represents a
yield of 686 kcal per molecule of glucose.
• Thus, fermentation is only 2.1% efficient
compared to cellular respiration.
• (14.6/686) x 100 = 2.1%
Advantages and Disadvantages of
Fermentation
• Fermentation can provide a rapid burst of
ATP in muscle cells, even when oxygen is
in limited supply.
• Lactate, however, is toxic to cells.
• Initially, blood carries away lactate as it
forms; eventually lactate builds up,
lowering cell pH, and causing muscles to
fatigue.
• Oxygen debt occurs, and the liver must
reconvert lactate to pyruvate.
But we don’t just eat carbohydrates…..so
what happens with proteins and lipids?
?
?
?? ?
?
? ? ?
?
?
The metabolic pool concept
Catabolism
catabolic reactions, break it down
• Molecules aside from glucose can enter
the catabolic reactions of cellular
respiration.
• When a fat is used for energy, it breaks
down into glycerol and three fatty acids;
glycerol is converted to PGAL, and the
fatty acids are converted to acetyl-CoA,
thus both types of molecules can enter the
citric acid cycle.
Fat breaks down into glycerol and
three fatty acids; glycerol is converted
to PGAL, and the fatty acids are
converted to acetyl-CoA, and both
molecules can then enter the
citric acid cycle.
Catabolism
catabolic reactions, break it down
• The carbon backbones of amino acids can
also enter the reactions of cellular
respiration to provide energy.
• The amino acid first undergoes
deamination, or the removal of the amino
group in the liver; the amino group
becomes ammonia (NH3) and is excreted
as urea.
• Where the carbon portion of the amino
acid enters the reactions of respiration
depends on its number of carbons.
Anabolism
Anabolic reactions build things up
• The substrates of the pathways of cellular
respiration can also be used as starting
materials for synthetic reactions.
• This is the cell’s metabolic pool, in which one
type of molecule can be converted into another.
• In this way, dietary carbohydrates can be
converted to stored fat, and come substrates of
the citric acid cycle can be transaminated into
amino acids.
Phases of Complete Glucose
Breakdown
• The oxidation of glucose by removal of
hydrogen atoms involves four phases:
• Glycolysis – the breakdown of glucose to
two molecules of pyruvate in the
cytoplasm with no oxygen needed; yields
2 ATP
• Transition reaction – pyruvate is oxidized
to a 2-carbon acetyl group carried by CoA,
and CO2 is removed; occurs twice per
glucose molecule
• Citric acid cycle – a cyclical series of oxidation
reactions that give off CO2 and produce one ATP
per cycle; occurs twice per glucose molecule
• Electron transport system – a series of carriers
that accept electrons removed from glucose and
pass them from one carrier to the next until the
final receptor, O2 is reached; water is produced;
energy is released and used to synthesize 32 to
34 ATP
• If oxygen is not available, fermentation occurs in
the cytoplasm instead of proceeding to cellular
respiration.
Citric acid cycle
Overview of the electron transport
system