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HELPFUL REVIEWS FOR BENCHMARK
ORGANIC COMPOUND OVERVIEW:
Organic compounds are compounds that contain one or more atoms of carbon covalently
bonded to atoms of other elements, most notably hydrogen, oxygen, phosphorus,
nitrogen, or sulfur. However, not all carbon-containing compounds are classified are
organic. Inorganic carbon compounds include carbon dioxide, carbon monoxide, metal
cyanides, carbonates and bicarbonates of metal ions, and a few others.
Many organic compounds are important for living organisms. Examples of organic
compounds can be found in carbohydrates (which include polysaccharides), lipids,
nucleic acids (including DNA and RNA as polymers), vitamins, enzymes, and amino
acids (monomer building blocks of proteins and peptides). Polysaccharides include
starches in animals and animals, as well as cellulose in plants.
DIGESTIVE SYSTEM REVIEW:
In the digestive system, both physical and chemical changes occur as food digests. When
food first enters the digestive system in the mouth, it changes physically through the
process of chewing. Food then physically changes into smaller pieces that are easy to
swallow. Most of the physical change (mechanical digestion) is done by chewing. Saliva
mixes with food to moisten it making it easier to swallow. Digestive enzymes are present
in saliva beginning the process of chemically changing the food (chemical digestion).
Food passes from the mouth to the stomach through the esophagus. The walls of the
esophagus produce mucus, which lubricates the food making it easier to move. Next,
food enters the stomach where the breakdown of protein in food begins. The muscles in
the stomach churn the food contributing to mechanical digestion (physical change). Food
is also mixing with digestive juice containing mucus, pepsin (an enzyme that digests
proteins) and hydrochloric acid. These help to break food down chemically. As food
leaves the stomach and passes into the small intestine, the food becomes a thick liquid
called chyme. Muscles move food through the small intestine by peristalsis. Most
chemical digestion occurs in the small intestine. Food mixes with several digestive juices
produced by the small intestine, liver and pancreas.
Once food is digested, it absorbs into the bloodstream in the small intestine. This is where
nutrients pass through all body parts. When food enters the small intestine, the pancreas
releases enzymes that help digest starch, proteins and fats. The liver produces bile, which
helps break apart large clusters of fat into tiny droplets making it easier for enzymes to
digest. The last part of the digestive system is the large intestine. Undigested food
entering the large intestine contains lots of water and minerals. After the water and
minerals are reabsorbed by the large intestine, feces remain as waste until it is eliminated
from the body.
As food is digested, larger molecules break down chemically into smaller molecules with
the help of digestive enzymes.
WORK OVERVIEW:
Work might happen when an object moves a certain distance under force. Work here
means mechanical work, and it is measured in units called joules. Note that the force and
distance here have to be in the same directions. That is, the force should be in the
direction in which the object moves.
Work is linked with certain quantity of energy change, and does not change based on how
the work is finished. For example, there are different ways to accomplish work, such as
making use of inclined planes, ramps or simple machines. However, when the force
required is reduced by these versatile methods, the other factor in the Work formula,
distance, does increase. Thus the total amount of energy or work remains the same.
Friction happens between the contacting surface and a moving object. This acts as a
resister to prevent the relative motion between the surface and the object. When we move
an object, the direction of this friction force is opposite to the direction where we push
the object. Hence this resisting force needs to be overcome in order to successfully move
the object. Forces affect motion. There are lots of examples you may already notice in
your daily life. The process of seedling growth demonstrates a greater upward force than
the gravity, which is working in the opposite direction. This is the reason that the plants
can break through the soil.
The turgor pressure of the water in the cytoplasm must balance the force from outside
the cell. Plants can stand upright and still due to the effect of this turgor pressure. The
combined pressure is so strong that it could break other hard objects, such as concrete.
Plants have the ability to sense the gravity and grow in the opposite direction of gravity.
A demonstration example is if plants are turned over, they still can recognize the
downward pull of gravity. This makes them grow in a geotropic way so their stems go
upward and roots go downward.
WATERSHED OVERVIEW:
A watershed, also known as a drainage basin, is the area of land that funnels all of its
surface water and groundwater into a body of water, such as a stream, river, lake, aquifer
or ocean. Areas of high ground separate watersheds from each other. On the large scale,
the area of land bordered by the Rocky Mountains on the west and the Appalachian
Mountains on the east is a watershed that drains all of its water into the Gulf of Mexico.
Large watersheds, such as this one, are made up of thousands of smaller watersheds.
Harris County has seven different watersheds, which help to drain all of the county’s
surface water into the Gulf of Mexico. The city of Houston is located within the Brazos
Watershed. All of the surface water in Houston eventually drains into the Brazos River,
which then drains its water into the Gulf of Mexico.
The water that is held underneath Earth’s surface is called groundwater. Surface water
can become groundwater by infiltrating, or sinking, below the surface. Groundwater can
be stored in and move through layers of rock known as aquifers. A rock layer is a good
aquifer if it has high porosity and high permeability. High porosity means that there are
many pores, or spaces, in the rock where water can reside. High permeability means that
these pores are well connected, allowing water to flow through the rock. For example, a
cupful of gravel has high porosity and high permeability, so water will flow through it
quickly. Relative to gravel, a cupful of sand has lower porosity and permeability, and
therefore, water will flow through it more slowly than through gravel.
One of the ways we get drinking water is by drilling wells into aquifers and pumping the
water to the surface. Unfortunately, we can deplete our drinking water supplies if we
pump water out faster than rain can infiltrate the ground and replenish it. During
summers in Texas, we can experience droughts severe enough that our aquifers cannot be
replenished fast enough. In order to maintain groundwater during droughts, local
governments try to regulate usage by limiting the amount of water we pump out. Also,
when we pump out too much water, the pore spaces in the rocks, which are used to hold
water, shrink in size. This shrinkage leads to subsidence, or the sinking of Earth’s
surface. If Earth’s surface sinks, our buildings and homes can be damaged (for example,
cracked foundations).
Human activities can also lead to the pollution of Earth’s groundwater and surface
waters. Water pollution is either point source or non-point source. If water pollution is
point source, we can trace back to the source of pollution (for example, a factory that is
releasing chemicals directly into a river). If water pollution is non-point source, there is
not an apparent single source of pollution. Examples of non-point source pollution
include runoff of the following pollutants into storm drains: fertilizer, pesticide,
herbicide, pet waste, and oil leakage from cars.
Water pollution can impact our health as well as the health of the environment. If there
are chemicals in our drinking water, they can lead to illness. Similarly, polluted waters
can hurt the environment. Fertilizers that are applied on farms can be washed into our
streams and rivers. If too much fertilizer is applied, eventually these contaminants drain
into larger bodies of water (lakes and oceans). In order to ensure we have enough clean
drinking water for future generations, it is important that we use our water in a
sustainable manner and that we stop water pollution.
Due to the connected processes of weathering, erosion, and deposition, Earth’s surface is
constantly changing. Weathering is the process of rock breaking down into smaller
pieces. Erosion moves weathered pieces of rock to another place. After material has been
eroded and is no longer being moved, it is deposited in a new location. In addition, once
this material has been eroded, fresh rock is exposed, which can then also be weathered.
These processes not only shape Earth’s surface (i.e. topography), but also influence soil
characteristics. Soil is made up of broken down rocks, produced by weathering and
erosion, as well as of organic matter produced by organisms.
Weathering is the process of either chemically or physically breaking down rock. If a
rock is chemically weathered, it is chemically altered or dissolved. Air and water are
agents of chemical weathering. When carbon dioxide in the air dissolves in rainwater, it
acts as a mild acid and degrades the rocks that it contacts. Chemical weathering can
weaken rocks and make them more susceptible to physical weathering, but these two
processes do not always occur together. If a rock is physically weathered, it is broken into
smaller pieces through mechanical processes. For instance, the movement of the crust due
to plate tectonics can cause fractures in rocks, breaking them into smaller parts. Frost
wedging is another physical weathering process where water seeps into cracks in rock,
then freezes and expands, which makes the cracks larger. As this process is repeated over
a winter or over years, the cracks get bigger and bigger until the rock breaks into multiple
pieces. In addition, organisms cause both chemical and physical weathering. Bacteria and
algae produce acid, and when they live in cracks in rocks, this acid acts as a chemical
weathering agent. Trees and animals can act as physical weathering agents. Tree roots
can grow into rocks and break them apart. We can see this when we walk over sidewalks
that have been cracked by the trees growing next to them. Animals can also break rocks
through burrowing into them. Once rocks have been broken into smaller pieces, or
sediment, either through chemical and/or physical weathering, they can then be moved to
a new location.
Erosion is the process of moving rock or sediment from one area to another. There are
several different forces that can act as agents of erosion. These include water, wind, ice,
and gravity. The strength of water, wind, and ice varies. The more powerful these
erosional agents, the bigger the pieces of rocks they move. For example, a slight breeze
can blow dirt and sand around, but hurricane-force winds can move boulders. Typically,
water in rivers moves sediment from upstream to downstream and the water usually
deposits that sediment into bays, gulfs, or oceans. Wind moves sediment along Earth’s
surface and is an important erosional agent in flat areas, such as in deserts or on the
plains. Ice is one of the most powerful erosional agents. A glacier can drag large boulders
under its ice and deposit the boulders hundreds to thousands of miles away. Gravity
moves rocks and sediments downhill. Landslides and mudslides are erosional events
where rocks and sediments are moved from a high elevation to a low elevation (due to
gravity). Eroded materials are deposited when gravity overcomes the force of the
erosional agent. For example, in a river, sediments deposit when the force of the moving
water can no longer overcome the force of gravity.
BIODIVERSITY OVERVIEW:
On the microscopic scale, there are also environmental variations that differentiate types
of habitats. Microhabitats exist in gardens and in the spaces between rocks, for example.
A variety of organisms live in these microhabitats, where they interact with different
biotic and non-biotic factors (resources) such as water, food, and shelter.
Biodiversity is a shortened term for biological diversity, and it can be observed and
measured on a large scale — such as in the ocean or a national park or on a smaller scale,
such as in a microhabitat. In a certain geographical region, the variety of life and the
intricate interactions between them indicate the level of diversity present in an area. A
variety of genes, different kinds of species, the ecosystems and the relationship between
all of these factors form biodiversity. The stability of the ecosystem is tightly linked to its
diversity. Simply put, the lower the biodiversity of an ecosystem, then the more
vulnerable it is to environmental change and imbalance. If an ecosystem has a higher
biodiversity, which means more genetic diversity, more varieties of species and so on,
then the ecosystem can withstand more environmental stress. Thus, if certain genes or
species are lost permanently, then the biodiversity is reduced. With the same
environmental stress, this ecosystem will have less ability to withstand. The biodiversity
of an ecosystem depends on those factors that limit the ability of the habitat to sustain a
population.
SUCCESSION OVERVIEW:
Following a major disturbance, such as natural disaster, a progression of re-building
occurs. Weeds, small insects, and other pioneers will move into the disturbed area first.
This literally lays the foundation for other species to move into the area, and the progress
continue. This is referred to as ecological succession.
There are also different kinds of successions. Generally, if the development happens on a
bare site, which has not been occupied before, it is considered as primary succession. A
succession which begins on the rock exposed by geologic activity would be this kind. On
the other hand, if the community develops and proceeds on a site from which another
community was removed, it is secondary succession, as it develops on the existing soil.
DICHOTOMOUS KEYS OVERVIEW:
Biologists have identified more than 1.5 million different species on Earth. This is only a
fraction of what scientists believe the total number could be — anywhere from 5 million
to 100 million. Because of this abundance and diversity, scientists organize species with
similar characteristics into groups based on their structure, function, and relationships.
This is known as taxonomy or taxonomic classification. Organisms can be classified into
groups based on their cellular structure, whether they have defined nuclei or not
(eukaryotes versus prokaryotes) or if their entire body is made up of one cell (unicellular)
or many cells (multicellular). Scientists can also look at how organisms function to help
classify them. For example, organisms that make their own food are known as autotrophs,
while organisms that need to consume other organisms in order to get the nutrients they
need to survive are known as heterotrophs.
How organisms reproduce is another way we can separate organisms into smaller groups.
There are two main types of reproduction: asexual, in which one parent passes copies of
its genes to its offspring, or sexual, in which two parents combine their genes and pass it
on to their offspring.
ADAPTATIONS AND NATURAL SELECTION OVERVIEW:
Through natural selection, organisms in a population that are best adapted to their local
environment increase in frequency relative to less well-adapted organisms over a number
of generations. This difference in survival and reproduction is not due to chance. The
genetic variation within a population leads to some individuals surviving and reproducing
more successfully than others.
An example of natural selection can be seen with the Galapagos finches. A few million
years ago, one species of finch migrated to the rocky Galapagos from the mainland of
Central or South America. From this one migrant species would come many species of
finch evolving from the single ancestor. The ancestral finch was a ground-dwelling, seedeating finch. Selection pressures exerted by the ecological niches pushed the populations
in various directions. On various islands, finch species have become adapted for various
diets, including seeds, insects, flowers, leaves, etc. This process has resulted in many
types of finches that differ mainly in their sizes and the shape of their beaks.
Selective breeding is the process of breeding plants and animals for particular genetic
traits. Breeding is done intentionally in attempts to produce offspring with desirable
characteristics or improved traits. Because selective breeding often times uses inbreeding
techniques, it decreases the genetic variety in the gene pool. Through this process,
however, animals can be bred for domestication. Human are able to breed animals for
various domestic reasons such as food production or commodities (wool, cotton, etc.),
help with types of work or transportation, scientific research, or for pets.
The internal structures of many organisms have adaptations that allow specific functions.
The gills in fish are respiratory organs that extract dissolved oxygen from water, and
excrete carbon dioxide. When a fish breathes, it takes in water and forces the water
through the gill openings so that it passes over the gills to the outside. Gases are
exchanged through the thin walls in gills allowing blood to carry oxygen to other parts of
the body. Carbon dioxide passes from the blood through the thin gill tissue into the water.
BODY SYSTEMS OVERVIEW:
The human body is made up of several organ systems that work together as one unit.
Each system depends on the others, either directly or indirectly, allowing the body to
function normally and maintain homeostasis.
The circulatory system consists of the heart, blood, and blood vessels. Its primary
function is to transport needed substances, such as oxygen, to cells and tissues, and carry
waste products away from cells and tissues. Blood carries oxygen from the lungs to the
body cells and also transports the glucose cells use to produce energy. The circulatory
system also picks up waste from cells such as carbon dioxide. When CO2 passes from
cells into the blood, the circulatory system carries it to the lungs where it is exhaled. The
circulatory system also transports disease-fighting blood cells helping the body systems
restore homeostasis.
The primary function of the respiratory system is to supply the blood with oxygen in
order for the blood to deliver oxygen to all parts of the body. Through breathing, the body
inhales oxygen and exhales carbon dioxide. The respiratory system is specifically
designed to maintain the gas exchange relationship between the body and the
environment. Through inhaling, oxygen and other gases pass through the mouth and
nose, through the pharynx, trachea, and branching into the left and right bronchi. This
exchange system keeps the cell well supplied with oxygen and removes carbon dioxide
wastes from the body.
The skeletal system, made up of all the bones in the body, has several functions. The
skeleton is the framework of the body as it supports the softer tissues and provides points
of attachment for most skeletal muscles. It provides protection for many of the body’s
internal organs, reducing risk of injury to them. For example the vertebrae protect the
spinal cord, the skull protects the brain, and the rib cage protects the heart and lungs. The
skeletal system assists with movement of the body. Skeletal muscles are attached to
bones, therefore when these muscles contract, they cause the bones to move. Bone tissues
store several minerals such as calcium and phosphorus. Bones release these minerals into
the blood as needed by the body.
The muscular system comprises over 600 muscles categorized into one of three types:
skeletal, smooth, and cardiac muscle. Muscles help the body with movement, both
internally and externally. Skeletal muscles are attached to bones and are voluntary and
aid in motility. The inside of many internal organs, such as the walls of the stomach and
blood vessels, contain smooth muscles, which are involuntary. Cardiac muscles are
striated and involuntary. These muscles do not tire and can contract repeatedly. The
muscular system also provides the body with strength, shock absorption, maintaining
shape and posture, and heat production. It helps with respiration and plays a role in the
digestive process by peristalsis allowing food to move through the digestive system.
The digestive system has three main functions. It first breaks down food into smaller
molecules that the cells of the body can use. Then, the molecules are absorbed into the
blood and carried throughout the body. Finally, wastes are eliminated from the body.
Digestion is the process by which the body breaks down food into smaller nutrient
molecules. In mechanical digestion, food is physically broken down into smaller pieces,
such as chewing by the teeth. In chemical digestion, chemicals produced by the body
break foods into smaller molecules. For example, digestive enzymes in the mouth and
stomach break down larger compounds into their smaller components. After food is
digested, the molecules are transported throughout the body. Absorption is the process by
which nutrients pass through the walls of the digestive system into the blood (such as in
the small intestine). Materials that are not absorbed are eliminated from the body as
waste.
The excretory system is the system in the body that collects wastes produced by cells and
removes wastes from the body. It also eliminates unused by-products excreted from cells,
eradicates harmful chemical build-ups, and maintains a steady, balanced chemical
concentration. Waste is excreted through a number of different ways. Carbon dioxide is
removed from the body through the lungs. Sweat is excreted through sweat glands in the
skin. The liver helps break down certain wastes before they can be excreted. For
example, urea, which comes from the breakdown of proteins, is produced by the liver. It
converts part of the hemoglobin molecule from old blood cells into substances such as
bile, which break down fats during digestion. The kidneys, one of the major organs of the
excretory system, help filter wastes from the blood, forming the liquid waste urine. The
large intestine not only removes solid waste but collects water from the waste that can be
reused.
The function of the reproductive system is to produce new individuals of the same type
through reproduction. Unlike other organ systems, the reproductive system has
significant differences for males and females. The male reproductive system is
responsible for the production of the male gamete, or sperm, for fertilization of the ovum
and for production of the hormone testosterone. The function of the female reproductive
system is to produce eggs, and, if an egg is fertilized, to nourish a developing embryo
until birth. The female system also produces hormones such as estrogen, which triggers
the development of some adult female characteristics.
The integumentary system consists of the skin, hair, nails, nerves, and glands acting as a
barrier to protect the body from the outside world. It also functions to retain necessary
body fluids, maintain steady body temperature, eliminate wastes, and protect against
disease. As it works with other body systems, the integumentary system helps maintain
homeostasis in order for the body to function properly. The skin, the largest and one of
the most important organs of the body, has several functions including thermoregulation,
detecting sensations such as touch, pressure, temperature, production of vitamin D, and
healing from cuts and burns.
The nervous system has two divisions that work together – the central nervous system
and the peripheral nervous system. The central nervous system consists of the brain and
spinal cord and is the control center of the body. The peripheral nervous system consists
of all the nerves located outside of the central nervous system. The brain is the part of the
central nervous system that controls most functions in the body. The spinal cord is the
column of nerve tissue that links the brain to most of the nerves in the peripheral nervous
system. The peripheral nervous system consists of a network of nerves that branch out
from the central nervous system and function to connect it to the rest of the body.
Impulses from this network of nerves travel through the spinal cord to get to the brain.
The brain directs a response that usually travels from the brain, through the spinal cord,
and then to the peripheral nervous system.
The endocrine system is a collection of glands that secrete different hormones for the
various functions and chemical reactions that occur in the body. The main function of the
endocrine system is to maintain a stable internal environment, or homeostasis. It regulates
metabolism, growth, development, and tissue functions. The hypothalamus is a collection
of specialized cells located in the lower central part of the brain and is the main link
between the endocrine system and the nervous system. It controls the pituitary gland by
stimulating or suppressing the hormone secretions. The pituitary gland is one of the most
important parts of the endocrine system. For example, one pituitary hormone signals the
thyroid gland to produce hormones. Other pituitary hormones control body activities
directly. Growth hormone regulates growth from infancy to adulthood. Another pituitary
hormone directs the kidneys to regulate the amount of water in the blood.
STRUCTURE AND FUNCTION IN PLANT AND ANIMAL CELLS OVERVIEW:
Cells are the structural and functional units common to all living organisms. A cell is the
smallest unit of life that is classified as a living thing. Some organisms are unicellular,
meaning they consist of only a single cell. Most bacteria are unicellular. Other organisms,
including humans, are multicellular, consisting of many cells. For example, humans have
about 100 trillion cells.
There are two distinct types of cells: prokaryotic cells (e.g. bacterial cells) and eukaryotic
cells (e.g. plant or animal cells). The main difference between the two is a well-defined
nucleus surrounded by a membranous nuclear envelope that is present in only eukaryotic
cells. Both types of cells share many common features. The genetic information is stored
in genes. Proteins serve as the main structural material. Ribosomes are used to synthesize
proteins. And a cell membrane controls what substances enter and leave the cell. The
primary difference between the two types is that prokaryotes lack a true nucleus.
Multicellular organisms, such as plants and animals, have various levels of organization
within them. Although individual cells can perform specific functions, they become
dependent on one another and can work together for the good of the entire organism. The
first level, cells, is the simplest level as cells are the basic structural and functional units
in living things. Examples include blood cells, bone cells, root cells, etc. The next level,
tissues, are made up of cells that are similar in structure and function and which work
together to perform a specific activity. Humans, for example, have four basic tissues:
connective, epithelial, muscle, and nerve. Organs are made up of tissues that work
together to perform a specific activity. Examples of this level are the heart, brain, skin,
etc. The fourth level is the organ systems. These are groups of two or more organs that
work together to perform a specific function for the organism. Examples in the human
body include the circulatory, nervous, skeletal, muscular, integumentary, endocrine,
digestive, immune, reproductive, excretory, and respiratory systems. The final level of
organization is the organism. Organisms are entire living things that carry out all basic
life processes. Organisms take in materials, release energy from food, release wastes,
grow, respond to the environment, and reproduce. Although an organism can be made up
of only one cell such as bacteria, most organisms, including plants and animals, are
usually made up of organ systems performing these functions. The levels of organization
from smallest to largest are:
cells → tissues → organs → organ systems → organisms
The cell membrane is an important structure present in all cells. It has many functions
including regulating the entry and exit of molecules in and out of the cell, maintaining the
boundaries of the cell, supporting its contents, and maintaining proper cell to cell contact.
The structure of the cell membrane includes a phospholipid bilayer. The hydrophilic parts
are on the outside and the hydrophobic parts point inwards towards each other. This
prevents the entry of polar solutes and is one of the main factors responsible for
regulating what enters and exits the cell. Integral membrane proteins present in the cell
membrane structure have many important functions. They serve as receptors for the cell
and can act as carriers for active transport of substances in and out of the cell.
The cell wall is a structure that is present only in plant cells which surrounds the cell
membrane. This is a special characteristic that helps distinguish plant from animal cells.
The cell wall is composed of polysaccharides including cellulose, lignin, protein, certain
lipids, and water for example. The cell wall has many functions for a plant cell. It
determines the shape of the cell, helping monitor the rate and direction of growth of the
cell. Since it is rigid in nature, the cell wall provides strength and support to plant cells.
Cell walls maintain turgor pressure, giving the plant rigidity. It is semi-permeable,
allowing exchange of substances in and out of the cell. The cell wall also provides
protection for the cell, and is the first line of defense for the cell from pathogens or
harmful microorganisms.
The nucleus is a highly specialized spherical structure that is the control center of a
eukaryotic cell. The nucleus controls the hereditary characteristics of an organism and is
responsible for protein synthesis, cell division, growth, and differentiation. Proteins and
RNA are stored in the nucleolus. Ribosomes are produced in the nucleolus as well. As the
control center of the cell, the nucleus stores all the chromosomal DNA of an organism.
The region between the cell membrane and the nucleus of a cell is the cytoplasm. It is a
gel-like substance which is part of the cell and holds important organelles. The cytoplasm
is the site for many biochemical reactions necessary for maintaining life. It is the place
where cell expansion and growth take place. Cell reproduction, protein synthesis,
glycolysis, and cytokinesis are some of the vital functions that are carried out in the
cytoplasm.
The cell has many organelles present in the cytoplasm that have specific vital functions.
The mitochondria are rod-shaped organelles made up of a double layered membrane. The
primary function of mitochondria is the production of energy in the form of ATP and the
regulation of cellular metabolism.
Chloroplasts are organelles found only in plant cells that carry out the process of
photosynthesis in which light energy is converted into chemical energy. Chloroplasts
contain chlorophyll, giving plants a green color. They are present in each and every part
of the plant including stems and even fruits. They also play an important role in storage
of energy and synthesis of metabolic substances.
Vacuoles are large, round, water-filled sacs in the cytoplasm that are the storage areas of
the cell. Plant cells usually have one large vacuole taking up a large amount of space. At
times, it can occupy more than 90% of the plant cell space. It is selectively permeable and
helps maintain the pH and ionic concentration of the cell by regulating what travels in
and out of the vacuole. Vacuoles store food and other materials needed by the cell. Most
of the water in plant cells is stored in vacuoles.
A single cell is a system, made up of many individual parts that work together, much the
same way that an entire organism is a system made up of many structures with specific
functions. A cell’s organelles and other structures work to carry out functions necessary
to sustain life. A cell requires various substances and energy for growth, as does any
living organism.
The cell uses many different chemical reactions to survive. These processes create a large
amount of products that the cell needs to remove. This waste includes byproducts of
reactions, toxic substances, and many of the parts of the cell that break down over time
and need to be eliminated. Lysosomes are organelles that help break down part of the cell
that are worn out and help get rid of waste products that are created by different parts of
the cell. They contain a number of different enzymes that break down waste, similar to
enzymes in the digestive systems of organisms. The cell has specialized pumps that use
energy to move the smaller amounts of waste product out of the cell. Exocytosis through
the plasma membrane allows the cell to get rid of large quantities of waste products all at
once. The process in which a cell removes waste is similar to the processes of the
excretory systems of many organisms.
CELL THEORY BACKGROUND:
The cell theory is an explanation of the relationship between cells and living organisms.
It states that all living organisms are composed of cells, cells are the basic unit of
structure and function in living things, and that cells arise from pre-existing cells. This
theory holds true for all living things, unicellular or multicellular. Modern variations of
the theory include the ideas that energy flow occurs within cells, hereditary information
(DNA) is passed on from cell to cell, and all cells have the same basic chemical
composition.
To carry out their day to day functions, cells require energy. The ultimate source of this
energy is the sun. Some organisms can trap energy directly from the sun, storing it away
in the bonds of organic molecules such as glucose through photosynthesis, and organisms
which are capable of photosynthesis are called autotrophs. Organisms which are not
capable of photosynthesis are called heterotrophs, and must acquire their energycontaining organic molecules through their diet instead. To convert the energy stored in
organic molecules into a form that is usable, both autotrophs and heterotrophs must take
large molecules and break them down, and then recapture the energy released in the
process and store it the bonds of smaller, easier to use molecules.
INHERITED TRAITS OVERVIEW:
Students have previously been introduced to the basic concepts of heredity by examining
and being aware of observable traits, such as eye color in humans or shapes of leaves in
plants. Such shared characteristics are different from learned behaviors, such as table
manners or learning a language. Students have likely also explored the basic concept of a
cell and that it contains a nucleus. They may even be aware that each human cell has 46
chromosomes, with all of a person’s DNA organized into two sets of 23 chromosomes.
Chromosomes contain the DNA for these traits and that traits, such as eye color, are
passed from one generation to the next by each parent contributing a set of chromosomes
to an offspring. This is why children look similar to their parents. Furthermore, which set
of chromosomes gets inherited from each parent is random. This is why siblings born
from separate pregnancies look similar but not identical, and why identical twins are just
that, because they actually do both carry the same inherited sets of chromosomes.
Essentially, the DNA provides the instructions or recipe for “building” an offspring,
using the blueprint provided by the combination of the two individual parents.
Heredity is not merely observed within single species, however. Mapping the human
genome, as well as that of other species, has provided insight into how different species
are related to each other. Not only have mammals inherited traits such as mammary
glands and hair from a common ancestor, for example, but also about 75% of known
human disease genes have a recognizable match in the genome of fruit flies. This infers
that humans and fruit flies also share some common ancestry.
A genotype is the genetic makeup of an organism, while the phenotype is a description of
how that genotype is expressed in the organism’s morphology and physiology.
Furthermore, a genotype for a trait often includes two variations that are referred to as a
dominant allele and a recessive allele. When both a dominant allele and a recessive allele
are present for a trait, the dominant allele will mask the recessive allele’s expression of
the trait. Only when two copies of the recessive allele are present – one from each parent
– is the recessive form expressed. This concept is especially easy to understand when
examining phenotypic traits that are controlled by single genes. The ability to roll your
tongue or the presence of a Widow’s Peak hairline are examples of dominant expression
of traits that scientists believe are controlled by a single gene. If a person’s DNA that
controls hairline shape contains both the dominant allele (Widow’s peak hairline) and the
recessive allele (straight hairline), or a heterozygous state, then the person’s phenotype
will show a Widow’s Peak hairline. Individuals who have two recessive alleles, or a
recessive homozygous state, for the trait will have a straight hairline.
However, not all traits are controlled by single genes. Most inherited traits are controlled
by a combination of multiple genes. This fact makes genetic research especially
complicated when trying to figure out how defects and risks for disease are configured
into a person’s genotype. Surveying generational data, ongoing work on mapping
genomes, and other studies continue to further our understanding of how heredity works
and how medical professionals can predict, and possibly curb, health risks.
Wild animal and plant populations, of course, also demonstrate how traits are inherited.
There are eight genetic lineages of felines, for example. Lions, leopards, panthers,
servals, cheetahs, pumas, and mountain lions, all share genetic traits inherited from a
common ancestor. Genetic similarities can easily be observed between cat species,
including teeth, nose, hair, feet, and tail characteristics. Wild populations can suffer,
however, when their numbers are reduced. Inbreeding can occur, which results in low
genetic variation and often causes what are typically recessive, deleterious traits to show
up in the phenotypes in successive generations of offspring. Such growing homozygosity
in recessive traits is observable in the Florida Panther, for example. Abnormal phenotypic
traits include kinked tails and severe birth defects.
The principles of inheritance are also studied and applied in domestication of wild
species. Artificial selection, or selective breeding, has produced a variety of livestock
breeds and plant types that boost human population survival and growth, while pet breeds
provide comfort and companionship. For hundreds of generations, humans have bred
together two individuals with desirable traits in order to enhance those desirable traits in
their offspring. Unfortunately, also due to the principles of inheritance and the nature of
chromosomes and their contained DNA, not all of the desirable traits can be teased from
non-desirable traits. A hybrid plant that may produce a high seed yield may also have a
higher vulnerability to disease, for example. Desirable traits in a dog breed may also be
accompanied by a higher risk of hip dysplasia.
ASEXUAL VS. SEXUAL REPRODUCTION OVERVIEW:
In asexual reproduction there is only one parent and the offspring looks uniform (exactly
like the parent).
In sexual reproduction there are two parents and the offspring look diverse (different)
because they carry characteristics from both parents.
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