are substances which are used to provide information about the
classification of a particular ubstance. Some commonly
used indicators in the living environment/biology lab include
Litmus (pH) paper, Lugol's (iodine) solution, and Benedict's
Making sense of
observations is important in science. Classification
provides a means of describing objects or organisms by their
common characteristics. A dichotomous key is used to
provide a method of sorting categories of organisms and
specifically identifying particular organisms.
The dissection of
representative plant and animal specimens is a required skill
for the living environment/biology course. Dissection
provides information about the internal and external
characteristics of a specimen in a direct manner which other
techniques can not as directly provide. Flowers, frogs, and
fetal pigs are commonly dissected specimens.
Indicators and Stains
indicator is any substance used to assist in the
classification of another substance. There are many
different kinds of indicators. Some common kinds of
indicators used in Living Environment/Biology will be
The pH Scale
Acids and bases (alkalis) are common
substances studied in science.
The pH scale is used to indicate the relative
strength of an acid of base. The pH scale goes from
0 to 14. A pH of 7.0 is considered to be
neutral. The greater the pH is than 7.0, the more
basic the substance is. The lower the pH is below
7.0, the more acidic a substance is.
Stomach acid has a pH of approximately 2.0.
paper turns red or a shade of red in acids.
Litmus turns blue or a shade of blue in bases.
It is important to place a few drops or a small
amount of the substance to be tested on the
litmus paper when testing it. Do not dip the
litmus paper in the substance to be tested. A
paper which provides a more specific indication
of the pH level of a substance is pH paper.
This paper turns different shades of various
colors which may be compared to a scale to
determine the pH value.
Bromthymol blue is an indicator used to show
the presence of either carbon dioxide in
solution or an acidic solution. Low levels of
carbon dioxide or acid will result in the
bromthymol blue solution remaining blue, while
higher levels of carbon dioxide or acid will
result in the bromthymol solution taking on a
yellow tint. Frequently this indicator is used
in biology labs to indicate photosynthetic
activity (solution turns blue as CO2 is used) or
respiratory activity (solution turns yellow as
CO2 is added to the solution).
Lugol's iodine solution (which is actually
IKI) is a brown solution which turns black in
the presence of starches. The test tube at the
right shows Lugol's (iodine) solution mixed with
a starch suspension.
solution is used to detect the presence of
simple sugars such as glucose. When a simple
sugar is mixed in Benedict's solution and heated
for a short period of time in a test tube, it
goes through a variety of color changes,
eventually ending as an orange-red or brick red
color. The use of Benedict's solution before and
after to detect the presence of the simple sugar
glucose is shown in the pictures on the right.
Very frequently it is helpful to dye certain cell
structures so that they can be seen more clearly.
Chemicals that dye parts of cells for this purpose are
called stains. Two commonly used stains in the
biology laboratory are Lugol's iodine solution and
methylene blue. Lugol's iodine solution is a good
stain to make the nuclei of plant cells stand out more
prominently. It has the unfortunate drawback of killing
the cells it is used on however. Methylene blue is often
used to stain animal cells, such as human cheek cells, to
make their nuclei more observable. It is a vital dye which
does not immediately kill the specimen.
Stains in Biology
plant cells stained with Lugol's iodine solution
so their nuclei are visible.
human cheek cells stained with methylene blue
solution making their nuclei and outlines much
Correctly Staining Specimens
- A specimen is
obtained and placed on the slide with forceps. A
cover slip is then lowered on to the specimen from
an approximately 45 degree angle gently. This
reduces the number of air bubbles the specimen
will have. The student then places a drop or two
of water on the specimen.
- The student places a
drop of stain beside and under one corner of the
- The student places a
towel on the opposite side of the cover slip in
the water beside the cover slip. This will draw
the stain through the entire specimen in a few
seconds without removing the cover slip. This
technique will also remove any air bubbles which
have formed. The stained specimen may now be
observed. Note that this technique can be used to
draw salt water or distilled water into a specimen
having a cover slip over it without removing the
cover slip as well.
dichotomous key is a sequence of steps that allows the
identification of a living thing. The key will consist of a
series of choices that lead the user to the correct name of
a given item. The term dichotomous means that there will
always be two choices in each step of the key until the
organism is correctly identified.
Key Ideas in Dichotomous Key Construction
1. Use constant
characteristics rather than ones that disappear or
vary with the season or other environmental factor.
2. Use characteristics which can be
3. Use quantitative measurements with
an amount or dimension rather than vague terms like
"big" and "small."
4. Precede the descriptive terms with
the name of the anatomical part to which it applies.
to Follow When Using a Dichotomous Key
Always read both choices, even if the first seems to
be the logical.
2. Understand the meaning of the
terms involved in the key.
3. When measurements are given, use a
scale to measure the specimen. Do not guess at a
4. Living things are always variable,
so do not base your organism identification in the
field on a single observation.
a Dichotomous Key to Identify an Organism
The example below
will illustrate the use of a dichotomous key to
identify the unknown creature above.
Steps in the
to Stream Water Animals
With a shell
go to 2
B. Without a
go to 3
creature clearly does not have a shell, so
go to #3.
Shell made of two parts held together by a
B. Shell made of only one
3. A. Body flat,
oval and brown Water Penny
B. Body not exactly like a water
penny go to 4
creature does not have an oval body it
is long, so go to #4.
4. A. With
six jointed legs
go to 5
With more than six jointed legs
go to 12
C. With less than six jointed legs; body
often wormlike go to 14
creature has 6 jointed legs, so go to #5.
With two or three thin, hair-like
tails go to 6
B. Without thin, hair-like tails
go to 7
creature has three thin tails, so go to #6.
6. A. With
one hook at the end of each leg; usually
with three tails, sometimes only two
B. With two hooks at the end of each leg;
two tails Stonefly
organism clearly has three tails and only a
single hook at the end of each leg which
makes it a Mayfly larva.
7 A. Body
with many long, pointed parts go
B. Body not exactly like this
go to 9
8. A. Body
brown or black, often very large
B. Body white, yellow or tan; not
9. A. Body
with hook-like claws at tail end; animal
protected with bits of sand, pebbles or
B. Body without
go to 10
10 A. Body
small, dark, hard
and beetle-like Riffle beetle
B. Body not exactly
go to 11
11 A. With 3 wide tails
B. Without tails, but with three short
12 A. With two large claws and eight legs;
B. Without large
go to 13
13. A. Body flattened side to side; usually
B. Body flattened top to bottom; usually
14. A. Body with very small legs;
usually with a head go to
B. Body without any legs or head
go to 16
15 A. Tail-end of body wider than
Black fly larva
B. Tail-end of body
not wider Midge
16. A Body
brown, plump, and caterpillar-like
Crane fly larva
B. Body not exactly
like this go to 17
17. A. Body with
suckers at each end Leech
B. Body without suckers; small, thin and
We didn't need to
beyond step 6 with the organism we
above, but some
require the use of
many more steps
before its proper identification.
Before beginning a dissection, it is important to
have an understanding of some of the basic directional
terminology associated with the dissection of
specimens. Some of these terms include proximal,
which means toward the body, and distal, which means
to move away from the body. Other important anatomical
directions are indicated below.
safety procedures when working with dissection
tools and specimens is of greatest
importance. Some safety rules to engage in when
dissecting specimens are as follows.
Dissection Safety Rules
- Follow all
instructions given by your teacher.
- Inform your
teacher of any illness as a result of
exposure to chemicals used in specimen
- Avoid contact
with preservative chemicals. Rinse the
specimens completely before dissection.
- Know where
the eye-wash fountain is if needed.
- Wear safety
goggles to prevent the splashing of any
chemicals into the eyes.
mount dissection specimens to dissecting
pan. Do not dissect a specimen while
scalpel or razor blade (safety edged) with
- Always cut
away from your body and away from others.
- Never ingest
- Never remove
specimens or specimen parts from the
classroom -- until the dissection is
completed all parts of the dissection must
remain within the dissecting pan.
dispose of dissected materials.
specimens in as directed by your teacher.
- Clean up the
work area and return all equipment to the
proper place when the dissection is
- Wash hands
after each dissection.
dissection equipment from left to
right is (1.) a teasing or
dissection needle which used to
pull apart muscle tissue, (2)
dissecting scissors which are used
to cut through tissue, and (3) a
scalpel, which is a knife used to
slice through and cut tissue.
Many kinds of flowering plants, such as lilies,
daffodils, or tulips are commonly subjects for
dissection in biology. The
flower is the
plant structure specialized for reproduction in
advanced plants. The processes of meiosis and
fertilization occur in the flower.
Some Key Flower structures
colored parts inside the sepals which
sepals: structures which are usually
green outside the petals which help to
protect the flower
stamen: forms the male
reproductive organ and consists of an anther
and a filament
anther: pollen box in which pollen
grains are formed containing the genetic
material which produces sperm
filament: supports the anther
pistil or carpel: female reproductive
organ which consists of three parts
stigma: found at the top of the pistil,
is often sticky and hairy adapting it to
catch and hold pollen
style: tube-like connection between the
stigma and the ovary
enlarged part of the pistil attached to the
receptacle (stem tip on which the flower
rests) and contains the ovules
small white structures within the walls of
the ovary which produces the plant egg cells
The dissection of animals is important for many
reasons. It helps in the learning about the
internal structures of animals. It also allows
students to learn how organs and tissues are
interrelated. Another purpose of dissection is to
allow the comparison of organisms in terms of
their organs and relative complexities.
many good simulations of dissections may be
observed, it seldom can replace the benefits of
the actual participation in an actual dissection.
common vertebrate organisms dissected in the
living environment lab include the frog and the
fetal pig. Usually the dissection procedure
involves tying the organism down firmly on the
dissection pan, cutting the organism open on its
ventral side (as pictured below), and pinning its
tissues and muscles back to observe its internal
organs. Different teachers may have their own
preferences in terms of their emphasis on the
tissues and organs to be observed in a dissection.
Key Internal Organs of the Frog
thinking and coordination of body activities
pumps blood through the body
stores and begins the chemical digestion of
finishes chemical digestion and absorbs
digested nutrients into the blood
digestive (and other systems)
makes bile, detoxifies poisons, many other
stores bile from liver for release into
small intestine to aid in fat digestion
exchanges gases with the external
environment (aided by the skin in the frog)
filter wastes from the blood
carries wastes to the urinary bladder
stores urine before its release from the
produces hormones like insulin which
regulate blood sugar, produces pancreatic
juice which aids in digestion in the small
makes eggs in female frog
makes eggs in male frog
Frog Internal Anatomy
Virtual Pig Dissection (Earl W. Fleck, Ph.D.
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