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Biology
State Assessment
Review
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3.1.1
Question: Cells are composed of a variety of specialized structures that carry out specific functions.
Eukaryotic cells (cells with a nucleus) are considered much more “complex” than prokaryotic cells (cells
without a nucleus) because they have organized, internal membrane-bound structures. Give specific examples
of eukaryotic cell structures and their basic functions.
Chloroplast: organelle found in some plant cells and certain unicellular organisms where photosynthesis takes
place
Cytoskeleton: internal framework of the cell providing structure, organization and movement
Endoplasmic reticulum: network of membranes within a cell's cytoplasm that produces a variety of molecules
Golgi apparatus: cellular organelle that modifies, stores, and routes cell products
Lysosome: membrane-bound sac containing digestive enzymes that can break down proteins, nucleic acids, and
polysaccharides
Mitochondria: cellular organelles where cellular respiration occurs; “Powerhouse of the cell”
Nucleus: in a cell, the part that houses the cell's genetic material in the form of DNA
Plasma membrane (Cell membrane): thin outer boundary of a cell that regulates the traffic of chemicals
between the cell and its surroundings
Ribosome: cluster of proteins and nucleic acids that constructs proteins in a cell
Vacuole: membrane-bound sac that buds from the endoplasmic reticulum or the Golgi apparatus
This diagram provides an overview of a generalized animal cell.
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3.1.3
Question: Cells function and replicate as a result of information stored in DNA and RNA molecules. Briefly
describe how the information stored in these molecules is eventually expressed as biological traits. Include in
your description the specific cell structures involved in the process.
Information flows from gene to polypeptide.
First, a sequence of nucleotides in DNA (a
gene) is transcribed into RNA in the cell's
nucleus. Then the RNA travels to the
cytoplasm where it is translated into the
specific amino acid sequence of a polypeptide.
The language of genes is written as a sequence of bases along the length of a DNA chain. If the bases are the
language's letters, each gene is like a sentence. Specific strings of bases make up each gene "sentence" on one
DNA strand.
What is the connection between these genes and the polypeptides in a cell? The answer involves RNA, another
kind of nucleic acid with a structure similar to that of DNA. RNA (ribonucleic acid) is any nucleic acid whose
sugar is ribose rather than the deoxyribose of DNA. Another difference between RNA and DNA is that RNA
contains a nitrogenous base called uracil (U) instead of the thymine of DNA. Uracil is very similar to thymine,
and pairs with adenine. The other components of RNA are the same as those for DNA. RNA typically forms a
single, sometimes twisted strand, not a double helix like DNA.
Several RNA molecules play a part in the intermediate steps from gene to protein. In the first step, DNA's
nucleotide sequence is converted to the form of a single-stranded RNA molecule in a process called
transcription. This "transcribed" message leaves the nucleus and directs the making of proteins in the cytoplasm,
while the DNA remains in the nucleus.
When a reporter transcribes a speech, the language remains the same. However, the form of the message
changes from spoken language to written language. Similarly, when DNA is transcribed, the result is RNA—a
different form of the same message. The next step, however, does require changing languages. Much as English
can be translated into Russian, genetic translation converts nucleic acid language into amino acid language. The
flow of information from gene to protein is based on codons. A codon is a three-base "word" that codes for one
amino acid. Several codons form a "sentence" that translates into a polypeptide.
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3.2.1
Question: Heredity information is contained in genes, located in the chromosomes of each cell. Each gene
carries a single unit of information. An inherited trait of an individual can be determined by one or by many
genes, and a single gene can influence more than one trait. Identify examples of the following inheritance
patterns: complete dominance, incomplete dominance (intermediate inheritance), polygenic inheritance, and
multiple alleles.
Complete Dominance: inheritance pattern in which an allele in a heterozygous individual that appears to be the
only one affecting a trait
Example: Mendel found that flower color in pea plants was a trait that exhibits complete dominance. Purple
flowers are dominant over white flowers as shown below.
Incomplete Dominance (Intermediate Inheritance): inheritance pattern in which heterozygotes have a
phenotype intermediate between the phenotypes of the two homozygotes
Example: In the diagram below, black chickens and white chickens have 100% blue offspring.
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Polygenic Inheritance: inheritance pattern in which the combined effect of two or more genes influences a
single trait.
Example: Eye color in humans is a trait that is determined by the interaction of many different genes.
Multiple Alleles: inheritance pattern in which more than two alleles for a gene are found within a population.
Example: Multiple alleles control blood type in humans as shown in the chart below.
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3.2.2
Question: Experiments have shown that all known living organisms contain DNA or RNA as their genetic
material. What are some experiments that led to our current understanding of the function and structure of the
DNA molecule?
Griffith's "Transforming Factor" Is the Genetic Material
In 1928, Frederick Griffith showed that although a deadly strain of bacteria could be made harmless by heating
it, some factor in that strain is still able to change other harmless bacteria into deadly ones. He called this the
"transforming factor."
Avery Shows DNA Is the Transforming Factor
In 1944, Oswald Avery and his colleagues took Griffith's experiments one step further. To test whether protein
was the transforming factor, they treated Griffith's mixture of heat-treated deadly strain and live harmless strain
with protein-destroying enzymes. The bacterial colonies grown from the mixture were still transformed. Avery
and his colleagues concluded that protein could not be the transforming factor. Next, they treated the mixture
with DNA-destroying enzymes. This time the colonies failed to transform. Avery concluded that DNA is the
genetic material of the cell.
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Virus Experiments Provide More Evidence
In 1952, Alfred Hershey and Martha Chase offered further evidence that DNA, not proteins, is the genetic
material. Only the DNA of the old generation of viruses is incorporated into the new generation.
DNA's Structure
In the early 1950s, scientists Rosalind Franklin and Maurice Wilkins produced some intriguing photographs of
DNA using a method called X-ray crystallography. This technique provides clues to the shapes and dimensions
of complex molecules. The photographs showed the basic shape of DNA to be a helix, and revealed the basic
dimensions of the helix.
The Double Helix
In 1953, scientists James Watson and Francis Crick modeled DNA's structure with tin and wire. Their early
models failed to explain DNA's chemical properties. Then one day, Watson saw one of Franklin's X-ray
crystallography photos of DNA. Using the clues provided by Franklin's work, Watson and Crick created a new
model in which two strands of nucleotides wound about each other. This formed a twisting shape called a
double helix.
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3.2.3
Question: DNA (or RNA) specifies the characteristics of most organisms. Describe the overall structure of the
DNA molecule and briefly explain how the molecule is capable of storing information.
The bases pair up between the two intertwined sugar-phosphate
backbones, forming the double helix. A pairs with T, and G pairs
with C. A is also said to be "complementary" to T, and G is
complementary to C.
So, while the sequence of nucleotides along the length of one of the
two DNA strands can vary in countless ways, the bases on the
second strand of the double helix are determined by the sequence of
the bases on the first strand. Each base must pair up with its
complementary base. Base-pairing rules set the stage for
understanding how the information in DNA is passed through
generations.
During DNA copying, the two strands of the
double helix separate. Each single strand acts as
a "negative" for producing a new,
complementary strand. Nucleotides line up one
at a time across from the existing strand as
predicted by the base-pairing rules. Enzymes
link the nucleotides together and form the two
new DNA strands. This process of copying the
DNA molecule is called DNA replication.
During DNA replication, the two strands of the
original parent DNA molecule, shown in blue,
each serve as a template for making a new
strand, shown in yellow.
Replication results in two daughter DNA
molecules, each consisting of one original strand
and one new strand.
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3.3.2
Question: Biologists use evolution theory to explain the earth’s present day biodiversity-the number, variety,
and variability of organisms. Describe the evidence of evolution, including the fossil record, homologous and
vestigial structures, geographic distribution, similarities during development, and genetic similarities. Discuss
Darwin’s ideas of natural selection and descent with modification.
Fossil Record
The fossil record is this chronological collection of life's remains in the rock layers, recorded during the passage
of time. The fossil record provides evidence of Earth's changing life. The oldest fossil evidence of life consists
of chemical traces in rocks from Greenland that are 3.8 billion years old. Fossils of prokaryotes (Bacteria and
Archaea) have been found in rocks of about 3.5 billion years in age. These data fit with the molecular and
cellular evidence that prokaryotes are the oldest form of life. Fossils in younger layers of rock record the
evolution of various groups of eukaryotic organisms.
Fossils of species that became extinct—species that no longer exist—help scientists reconstruct the past. Fossil
evidence suggests that ancient whales evolved from ancestors with hind limbs. This illustration shows an
artist's rendition of what an early whale species, Basilosaurus, may have looked like based on fossil evidence.
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Homologous Structures
Homologous structures are similar structures found in more than one species that share a common ancestor.
Homologous structures support other evidence that evolution is a remodeling process. Structures that originally
functioned one way in ancestral species become modified as they take on new functions.
For example, the forelimbs of all mammals consist of the same skeletal parts. The hypothesis that all mammals
descended from a common ancestor predicts that their forelimbs would be variations of the structural form in
that ancestor.
Vestigial Structures
Vestigial structures are remnants of a structure that may have had an important function in a species' ancestors,
but has no clear function in the modern species. Often, vestigial organs are reduced in size. For example, the
whales of today lack hind limbs, but some have small vestigial hipbones probably derived from their fourfooted ancestors described earlier. Natural selection provides a different explanation for vestigial structures that
is consistent with known processes of inheritance. Natural selection would favor the survival and reproduction
of individuals with genes for reduced versions of those structures.
“Goosebumps” are homologous to the tiny muscles that raise the fur of hairier mammals. Since goosebumps
have no known significant function, they can be considered vestigial structures.
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Geographic Distribution
Many patterns in the geographic distribution of life forms make sense in an evolutionary context. Just as the
fossil record documents the history of species of the past, the geographic distribution of organisms serves as a
clue to how modern species may have evolved. As an example, consider two islands with similar environments
in different parts of the world. The species on each island have more similarities to species on the nearest
mainland than they do to species on the other island. If species evolved from ancestors that lived in one
geographic region, the presence of related species in that region today makes sense.
Similarities in Development
Other clues to evolutionary history come from comparing the development of different organisms. Embryos of
closely related organisms often have similar stages in development.
fish
chicken
pig
human
Comparing the development of organisms supports other evidence of homologous structures. The forelimbs of
the mammals are an example. As the skeletons of the forelimbs take form in the embryos of different
mammals, there is a common pattern in the development of many bones. Structural differences become clear
later in development. The specific type of limb is shaped by differences in the rates at which different bones of
the skeleton grow. This evidence further supports the hypothesis that all mammals are related and descended,
with modification, from a common ancestor.
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Genetic Similarities
In recent decades, biologists have been reading a molecular history of evolution in the DNA sequences of
organisms. The sequences of bases in DNA molecules are passed from parents to offspring. These DNA
sequences determine the amino acid sequences of proteins. These information-rich molecules are the records of
an organism's ancestry (hereditary background). Among siblings, the DNA and protein sequences are very
similar. However, the sequences of unrelated individuals of the same species show more differences.
This idea of molecular comparison extends to studying relationships between species. If two species have
genes and proteins with sequences that match closely, biologists conclude that the sequences must have been
inherited from a relatively recent common ancestor. In contrast, the greater the number of differences in DNA
and protein sequences between species, the less likely they share as close a common ancestry.
This table shows the results of a comparison of the amino acid sequences of
hemoglobin in humans and other vertebrates. The data reveal the same pattern of
evolutionary relationships that researchers find when they compare species using
nonmolecular methods.
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Natural Selection and Descent with Modification
Darwin made two main points in his book. First, he argued from evidence that the species of organisms living
on Earth today descended from ancestral species. In other words, life has a history of change. Darwin proposed
that the descendants of the earliest organisms spread into various habitats over millions of years. In these
habitats, they accumulated different modifications, or adaptations, to diverse ways of life. Darwin called this
process descent with modification. He saw descent with modification as a way to account for the diversity of
life. For example, the jackrabbit and the snowshoe hare are two species of hares that have adapted to living in
different environments. The jackrabbit benefits from fur that blends well in the desert and ears, rich with blood
vessels, that help cool its body. White fur provides protective camouflage in the snowy northern regions of the
snowshoe hare's range.
Darwin's second main point was his argument for natural selection as the mechanism for evolution. Natural
selection is the process by which individuals with inherited characteristics well-suited to the environment leave
more offspring on average than do other individuals. Figure 14-9 models how certain inherited traits can give
individuals some advantage over other individuals of the same species in the same environment. This process,
which you will read more about later in this chapter, can cause a population to change over time. When
biologists speak of "Darwin's theory of evolution," they are referring to natural selection as a cause of evolution.
The result of natural selection is adaptation. This process of natural selection is another way of defining
evolution. But the term evolution can also be used on a much broader scale to mean the history of life, from the
earliest microbes to the enormous diversity of modern organisms.
In this hypothetical population of snails, inherited shell variations make some
snails less likely than others to be attacked by predators. Wide, blunt shells
increase the chances for snails to survive and pass their traits to the next
generation by reproducing.
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3.3.4
Question: Biologists recognize that the primary mechanisms of evolution are natural selection and genetic drift
that lead to variation within and among species. Discuss the overall value of variation within a species in terms
of species survival. Describe the ultimate source(s) of new heritable traits.
Variation refers to differences among members of the same species. You need only look around your classroom
to see how hair color, skin tone, and facial features, for example, vary among just a small group of people. Just
as no two people in a human population are alike, individual variation is widespread in all species. Much of this
variation is heritable and passes from generation to generation. This explains why siblings usually share more
traits with one another and with their parents than they do with unrelated members of the same population.
Ultimate sources of heritable traits: Random assortment of chromosomes during meiosis that produces
variations in eggs and sperm. Crossing-over during meiosis that gives the egg or sperm different combinations
of chromosomes. Fertilization, which determines which egg is fertilized with which sperm. Mutations that can
occur spontaneously or through environmental conditions.
Random Assortments of Chromosomes
The figure below illustrates one way in which meiosis contributes to genetic variety. How the chromosomes in
each chromosome pair line up and separate at metaphase I is a matter of chance, like the flip of a coin. So, the
assortment of chromosomes that end up in the resulting egg or sperm occurs randomly with 223 (8,388,608)
possible combinations!
In a diploid cell with four chromosomes (two homologous pairs), there are two
equally possible ways for the chromosomes inherited from the two parents to
be arranged during metaphase I. This variation in the orientation of
chromosomes leads to gametes with four equally possible combinations of
chromosomes.
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Crossing Over
A second factor that contributes to genetic variation is crossing over—the exchange of genetic material between
paired chromosomes. This exchange occurs during prophase I of meiosis. The figure below shows the results of
crossing over in one tetrad. When crossing over begins, paired chromosomes are closely paired all along their
lengths. There is a precise gene-by-gene alignment Segments of the two chromosomes can be exchanged at
one or more sites.
This diagram illustrates an example of crossing
over in one pair of homologous chromosomes,
shown here side-by-side for ease of viewing. (The
process can occur in all pairs.) Early in prophase
I, a chromatid from one chromosome exchanges a
segment with the corresponding segment from the
other chromosome. These altered chromosomes
give rise to what are known as "recombinant
chromosomes" in the gametes.
So, on top of all the possible chromosome combinations (over 8 million), crossing over adds another source of
variation. Crossing over can produce a single chromosome that contains a new combination of genetic
information from different parents, a result called genetic recombination. Because chromosomes may contain
hundreds of genes, a single crossover event can affect many genes. Since more than one crossover can occur in
each group of chromosomes, it is no wonder that gametes, and the offspring that result from them, can be so
varied.
Fertilization
Fertilization occurs when the egg and sperm cells fuse. Which sperm enters the egg is all by chance, and thus,
the combination of genes for the new offspring contain extreme variation.
Mutation
A mutation is any change in the DNA. Mutations can involve large regions of a chromosome or just a single
nucleotide pair. A mutation changes the code in the DNA molecule, which then can be inherited.
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Variation
So what is the importance of variation? Variation allows a species to survive. Each species has a gene pool, a
reservoir of genes from which the next generation draws its genes. Hence, the species gene pool is where
genetic variation is stored. As the environment changes around a species, different genes may become
beneficial while other genes become lethal. If the entire species has exactly the same genes, the whole species
could be wiped out by on single disease or natural disaster. Variation in the genes, provides a buffer and allows
some organisms of that species to survive and carry on the species. Every time we lose a species, we are losing
a unique gene pool that has undetermined beneficial results to nature and mankind.
The impact of the loss of variation can happen quickly. Since the 1950’s, manmade varieties of corn, wheat and
rice have squeezed out native species of rice. In countries like Indonesia, more than 80% of all rice farmers
plant man-made genetic varieties. Indonesia’s 1500 local rice species have become extinct in 15 years! In
1846, Ireland lost most of its genetically identical potato crop to the potato blight. In 1991, Brazil lost nearly all
of its genetically identical orange trees to citrus canker disease. In 1970, U.S. Farmers lost 1 million dollars due
to a disease that affected genetically identical corn. In 1972, the Soviet Union lost most of its genetically
identical wheat crop to disease. In Bangladesh, 62% of their rice crop is identical, 72% of the rice crop is
identical in Sri Lanka and 74% of the rice crop is identical in Indonesia.
Genetic variation is also what allows for natural selection of organisms. Organisms genetically best suited for
their environment will survive and pass on their genes to succeeding generations, while those with less adaptive
genes will die out before creating offspring.
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3.4.2
Question: Energy flows through ecosystems. Give an example of a food chain within an ecosystem and
describe the flow of energy beginning with sunlight and ending with decomposers. Include the following terms
in your answer: producer (autotroph), consumer (heterotroph), decomposer.
Food Chains
In the desert, a grasshopper munches on a brittlebrush's bright yellow flowers. Suddenly, a mouse seizes the
grasshopper for its own meal. These feeding relationships, from flower to grasshopper to mouse, relate to how
energy and chemicals move through the desert ecosystem. Each of these organisms represents a feeding level,
or trophic level, in the ecosystem. The pathway of food transfer from one trophic level to another is called a
food chain. See another food chain below:
Producers
In all food chains, the producers make up the trophic level that supports all other trophic levels. In terrestrial
ecosystems, plants are the main producers. In aquatic ecosystems, phytoplankton—photosynthetic protists and
bacteria—multicellular algae, and aquatic plants are the main producers.
Consumers
Organisms in the trophic levels above the producers are consumers. They may be categorized according to what
they eat. A consumer (such as a horse) that eats only producers is an herbivore. A consumer (such as a lion) that
eats only other consumers is a carnivore. And a consumer (such as a bear) that eats both producers and
consumers is an omnivore. Consumers eating plants directly are called primary consumers. Consumers that eat
the primary consumers are called secondary consumers. Finally, consumers that eat secondary consumers are
called tertiary consumers.
Decomposers
At each trophic level, organisms produce waste and eventually die. These wastes and remains of dead
organisms are called detritus. Decomposers are consumers that obtain energy by feeding on and breaking down
detritus. Animals that eat detritus, often called scavengers, include earthworms, some rodents and insects,
crayfish, catfish, and vultures. But an ecosystem's main decomposers are bacteria and fungi. These organisms,
found in enormous numbers in the soil and in the sediments at the bottom of lakes and oceans, recycle
chemicals within the ecosystem. Diagrams of food chains generally do not depict the decomposers that break
down the remains of the organisms at each trophic level in the food chain. But all ecosystems do include
decomposers—their role is vital to the ongoing recycling of chemicals in the ecosystems.
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Food Webs
The feeding relationships in an ecosystem are usually more complicated than the simple food chains you have
just read about. Since ecosystems contain many different species of animals, plants, and other organisms,
consumers have a variety of food sources. The pattern of feeding represented by these interconnected and
branching food chains is called a food web.
The figure shows how food chains within a food web are interconnected. For example, the rattlesnake eats
several animal species that may also be eaten by other consumers, such as the hawk. In addition, some
consumers can feed at several different trophic levels. The woodpecker, for instance, is a primary consumer
when it eats cactus seeds, and a secondary consumer when it eats ants or grasshoppers. The hawk can be a
secondary, tertiary, or even quaternary consumer depending on its prey. Note that like food chains, food web
diagrams typically do not show decomposers. In the next section you'll read more about how trophic levels in
food webs relate to energy flow in an ecosystem. See the diagram below.
3.4.3
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Question: Populations do not live in isolation, organisms cooperate and compete in ecosystems. Give
examples of how organisms interact to generate stable ecosystems.
Competition Between Species
An organism, such as an elephant, cannot survive without other organisms. The elephant is part of a herd of
elephants that form a population. The herd is part of a larger community of organisms. A community is a group
of species living in the same geographic area. The elephants' community includes gazelles, giraffes, and birds;
ants, beetles, fungi, and bacteria in the soil; and grasses and trees.
Members of a population may compete for limited resources in the environment. This competition within a
single species limits the growth of the population. Within a community, interspecific competition (competition
between species) takes place when two or more species rely on the same limited resource. For example, in the
African savanna community, many species feed on grasses. In times of drought, the grasses may be in short
supply, and competition may become especially intense.
Predation
Within the same savanna community where grazing animals may compete for grass, other interactions between
species are taking place. For instance, a lion chases down an injured zebra, while nearby an egret targets a fish
for its meal. These two interactions are examples of predation, an interaction in which one organism eats
another. The lion and the egret are examples of predators, the organisms doing the eating. The food species
being eaten are the prey. Because eating and avoiding being eaten are so important to survival, it is not
surprising that many effective adaptations have evolved in both predators and prey.
Symbiotic Relationships
A symbiotic relationship is a close interaction between species in which one of the species lives in or on the
other. There are three main types of symbiotic relationships: parasitism, mutualism, and commensalism.
Parasitism is a relationship in which one organism, the parasite, obtains its food at the expense of another
organism, the host. Usually the parasite is smaller than the host. Both blood-sucking mosquitoes and tapeworms
that live and feed in the intestines of larger animals are examples of parasites. The process of natural selection
works on both the parasite and the host. Parasites that can locate and feed on their hosts efficiently are most
successful. For example, some aquatic leeches locate their hosts first by detecting movement in the water. Then
they confirm their selection by using temperature and chemical cues on the host's skin. Usually the effect of the
parasite does not kill the host quickly, which would result in the death of the parasite as well. Natural selection
has also produced defensive adaptations that help hosts resist parasites. The immune system of humans and
other vertebrates is an example.
Mutualism is a relationship in which both organisms benefit. One example of mutualism occurs inside your
own body. Your large intestine is inhabited by millions of bacteria. The bacteria benefit by having a warm,
moist home with a constant stream of nourishment, your food. In turn, some intestinal bacteria produce vitamin
K. Vitamin K is essential for blood clotting. Both you and the bacteria benefit from this relationship.
Commensalism is a relationship in which one organism benefits, while the other organism is neither harmed
nor helped significantly. For example, a spider crab may place seaweed on its back. The crab benefits by being
camouflaged from its predators. The seaweed does not seem to be significantly affected. True commensalism in
nature is rare, since most interactions harm one species (parasitism) or help both species (mutualism) to some
degree.
3.5.2
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Question: The sun is the primary source of energy for life through the process of photosynthesis. Explain how
the energy from the sun is captured by plants and then transferred to molecules that serve as sources of energy
for life processes.
Through photosynthesis, plants convert the energy of sunlight to chemical energy stored in organic molecules.
Those organic molecules provide fuel for cellular respiration for the plants as well as for other organisms that
eat the plants.
The light reactions and the Calvin cycle together convert light energy to the stored chemical energy of
sugar. The plant can use the sugar to build other organic molecules.
The process of photosynthesis consists of two stages: The light reactions and The Calvin cycle. The light
reactions convert the energy in sunlight to chemical energy. The products of the light reactions are oxygen (a
"waste product"), electrons, and hydrogen ions that the chloroplasts use to make an energy-rich molecule called
NADPH. The overall result of the light reactions is the conversion of light energy to chemical energy stored in
two compounds: NADPH and ATP.
The Calvin cycle makes sugar (carbohydrate) from the atoms in carbon dioxide plus the hydrogen ions and the
high-energy electrons carried by NADPH. The ATP made by the light reactions provides the energy to make
sugar. The Calvin cycle is sometimes referred to as the "light-independent reactions" because, unlike the light
reactions, it does not directly require light to begin. However, this doesn't mean that the Calvin cycle can
continue running in a plant kept in the dark. The Calvin cycle requires two inputs supplied by the light
reactions, ATP and NADPH.
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3.5.3
Question: Food molecules contain energy that is made available to organisms by the process of cellular
respiration. Identify how the energy is stored in food molecules, how it is released from food molecules, and
what molecule it is stored in for later use by the cell.
Many organisms, including both producers and consumers harvest the energy stored in foods through cellular
respiration. Cellular respiration is a chemical process that uses oxygen to convert the chemical energy stored in
organic molecules into another form of chemical energy—a molecule called adenosine triphosphate (ATP).
Cells in plants and animals then use ATP as their main energy supply.
Just like the molecules in gasoline and other fuels, these organic compounds have a form of potential energy
called chemical energy. In the case of chemical energy, the potential to perform work is due to the arrangement
of the atoms within the molecules. Put another way, chemical energy depends on the structure of molecules.
Organic molecules such as the carbohydrates, fats, and proteins have structures that make them especially rich
in chemical energy. The rearrangement of atoms during chemical reactions releases the potential energy. This
energy is then available for work such as contracting a muscle.
Energy is released from food molecules through a process called cellular respiration. The main function of
cellular respiration is to generate ATP for cellular work. In fact, the process can produce up to 38 ATP
molecules for each glucose molecule consumed. Cellular respiration also transfers hydrogen and carbon atoms
from glucose to oxygen atoms, thus forming water and carbon dioxide.
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Stage 1: Glycolysis
The first stage in breaking down a glucose molecule, called glycolysis, takes place outside the mitochondria in
the cytoplasm of the cell. Using two ATP molecules as an initial "investment," the cell splits a six-carbon
glucose molecule in half. The result is two three-carbon molecules, each with one phosphate group. Each threecarbon molecule then transfers electrons and hydrogen ions to a carrier molecule called NAD . Accepting two
electrons and one hydrogen ion converts the NAD to a compound called NADH. The next step is the "payback"
on the ATP investment—four new ATP molecules are produced, a net gain of two ATP molecules. In
summary, the original glucose molecule has been converted to two molecules of a substance called pyruvic
acid. Two ATP molecules have been spent, and four ATP molecules have been produced. The pyruvic acid
molecules still hold most of the energy of the original glucose molecule.
+
+
Stage 2: The Krebs Cycle
The Krebs cycle finishes the breakdown of pyruvic acid molecules to carbon dioxide, releasing more energy in
the process. The enzymes for the Krebs cycle are dissolved in the fluid matrix within a mitchondrion's inner
membrane. Glycolysis produces two pyruvic acid molecules. These pyruvic acid molecules do not themselves
take part in the Krebs cycle. Instead, after diffusing into the mitochondrion, each three-carbon pyruvic acid
molecule loses a molecule of carbon dioxide. The resulting molecule is then converted to a two-carbon
compound called acetyl coenzyme A, or acetyl CoA. This acetyl CoA molecule then enters the Krebs cycle. In
the Krebs cycle, each acetyl CoA molecule joins a four-carbon acceptor molecule. The reactions in the Krebs
cycle produce two more carbon dioxide molecules and one ATP molecule per acetyl CoA molecule. However,
NADH and another electron carrier called FADH trap most of the energy. At the end of the Krebs cycle, the
four-carbon acceptor molecule has been regenerated and the cycle can continue. Glycolysis produces two
pyruvic acid molecules from one glucose molecule. Each pyruvic acid molecule is converted to one acetyl CoA
molecule. Since each turn of the Krebs cycle breaks down one acetyl CoA molecule, the cycle actually turns
twice for each glucose molecule, producing a total of four carbon dioxide molecules and two ATP molecules.
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Stage 3: Electron Transport Chain and ATP Synthase Action
The final stage of cellular respiration occurs in the inner membranes of mitochondria. This stage has two parts:
an electron transport chain and ATP production by ATP synthase. First, the carrier molecule NADH transfers
electrons from the original glucose molecule to an electron transport chain. Electrons move to carriers that
attract them more strongly. In this way the electrons move from carrier to carrier within the inner membrane of
the mitochondria, eventually being "pulled" to oxygen at the end of the chain. There the oxygen and electrons
combine with hydrogen ions, forming water. Each transfer in the chain releases a small amount of energy. This
energy is used to pump hydrogen ions across the membrane from where they are less concentrated to where
they are more concentrated. This pumping action stores potential energy in much the same way as a dam stores
potential energy by holding back water. The energy stored by a dam can be harnessed to do work (such as
generating electricity) when the water is allowed to rush downhill, turning giant wheels called turbines.
Similarly, your mitochondria have protein structures called ATP synthases that act like miniature turbines.
Hydrogen ions pumped by electron transport rush back "downhill" through the ATP synthase. The ATP
synthase uses the energy from the flow of H ions to convert ADP to ATP. This process can generate up to 34
ATP molecules per original glucose molecule.
+
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3.7.3
Question: Understanding the biology of plants underlies a scientific understanding of ecology. List three
reasons why plants are important to most other organisms, including humans.
An organism such as a plant that makes its own food is called an autotroph. Starting with inorganic molecules,
autotrophs make organic molecules. For example, plants use the sun's energy to convert water and carbon
dioxide into sugars (photosynthesis). Autotrophs are also called producers because they produce the organic
molecules that serve as food for the organisms in their ecosystem. Flowering plants provide nearly all the food
that supports human life. All fruit and almost all vegetable crops are flowering plants(angiosperms). Corn, rice,
wheat, and the other grains are fruits of grass species. In addition to feeding humans, plants (such as grains) are
the main food source for domesticated animals such as cows and chickens.
Plants are also harvested for furniture, medicines, perfumes, decorations, and fibers for clothes (such as cotton).
So far, only a tiny fraction of more than 280,000 known plant species (including non-flowering plants) have
been explored for potential uses. For example, almost all of the human food supply is based on the cultivation
of only about two-dozen species. And, while more than 120 prescription drugs are currently extracted from
plants, researchers have so far investigated fewer than 5000 plant species as potential sources of new medicines.
Certain human activities are threatening plant species, sometimes before their potential uses are even known.
The tropical rain forest, which is losing plant species at the fastest rate of all Earth's ecosystems, may be a
medicine chest of healing plants that could become extinct before they are even discovered.
3.7.4
Question: Animals vary. This variation is important in understanding the function of animals in farming,
medical research, and biotechnology. Explain what types of animals are most commonly used in medical
research that is focused on the development of human treatments and why.
Mammals (specifically rodents) are most commonly used in medical research that is focused on the
development of human treatments. These types of animals are because they are most anatomically and
physiologically like humans and also reproduce at a fast rate.
Mammals are endothermic vertebrates that possess mammary glands and hair. Internally, all mammals have
lungs, even aquatic mammals such as whales and dolphins. All mammals also have a muscular diaphragm that
separates the lungs and heart from the rest of the body cavity. The diaphragm aids in breathing. Mammals are
endothermic and have a four-chambered heart with two separate circuits of blood flow. Mammals also
reproduce sexually by internal fertilization. Most mammals give birth to young, but one group, monotremes,
which lays eggs.
Most mammals are called placental mammals because the embryo completes its development while protected
within the mother's uterus. Inside the uterus, an organ called the placenta provides the embryo with nutrients
and oxygen and removes waste products. The embryo is also bathed in fluid contained by a protective
membrane, the amnion.
There are more about 2,000 species of rodents, including squirrels, beavers, rats, porcupines, and mice.
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4.2.1
Question: Describe how geological time is used to understand the earth’s past.
The geologic time scale (see figure below) organizes Earth's history into four distinct ages known as the
Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. These eras are divided into shorter time spans called
periods. Periods are divided into epochs. The boundaries between eras are marked in the fossil record by a
major change (or turnover) in the forms of life. For example, the beginning of the Paleozoic Era (the start of the
Cambrian period) is marked by the appearance of a diversity of multicellular animals with hard parts. Fossils of
these animals are absent in rocks of the Precambrian Era. The boundaries between eras and between some
periods are also marked by widespread extinctions. For example, many of the animals that lived during the late
Paleozoic Era became extinct at the end of that era.
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