Mini Lab: Roots
Carrot Dissection
First of all, I need to correct a misconception about the equation for photosynthesis. We’ve been using the following equation:
sunlight, chlorophyll
6 CO2 + 6 H2O ——————————————-> 6 O2 + C6H12O6 (glucose)
Let’s take a closer look at the equation above and outline the misconceptions
· Chlorophyll alone is insufficient for plant photosynthesis. Many other enzymes and organic compounds are required. “Chloroplasts” is a better requirement.
· Glucose is not the major photosynthetic product. There is virtually no free glucose produced in photosynthesis..7 The most common product is starch or sucrose, and students often test leaves for starch. Starch is approximated as (C6H10O5)n, where n is in the thousands.8
· The six water molecules consumed per glucose molecule generated underestimates the water required. Much larger amounts are transpired to keep the stomata open. Without open stomata, photosynthesis is limited by lack of carbon dioxide. Submerged aquatic plants require large amounts of water for their aquatic environment.
· Drawing a single arrow wrongly implies that photosynthesis occurs in one step. Many small arrows should be used.
· The biology teaching literature contains much information on photosynthesis, yet it often has minimal discussion of mineral nutrient uptake by plants.9 To counter this problem, “mineral nutrients” should be added to the equation. Most essential mineral nutrients play a role in photosynthesis.
More correctly it should be:
chloroplasts, light, mineral nutrients
H2O + CO2 àààààààààààààààààààààààO2 + (C6H10O5)n [starch]
water for transpiration or an aquatic environment
Resource: http://www.actionbioscience.org/education/hershey.html
Now, let’s get to the lab
Carrots: Specialized Root Systems
The edible part of the carrot is the root system. Root systems that consist mainly of a single large root are said to have a "tap root"; in contrast, those root systems that have many branches of similar size are said to have "fibrous roots". Plants such as carrot that have a single large tap root use this large underground structure for nutrient storage.
Carrot
Despite its prominent root, the carrot has a very short, inconspicuous stem. You can see little of the carrot shoot system except the leaves that project from the tiny stem.
Let's consider the large root and small stems we find on carrots in the grocery store in terms of the changes that occur in the life of a carrot plant. When carrots are grown from seed, the seedlings form both root and shoot systems as we would expect. The leaves of the shoot system grow quickly and use sunlight to make sugars through the process of photosynthesis. Much of this sugar is transported to the root, where it is stored. The accumulation of sugar in the root causes it to expand greatly, forming the tap root we find in grocery stores. Carrots in natural environments rather than gardens live for more than one year. At the end of their first year of growth, the leaves die and the tiny stem becomes dormant. Next year, in the spring, the sugars in the root are moved upward into the stem, which begins to grow. The stem extends upward and, ultimately, forms flowers and fruits. Flowering and fruiting often requires tremendous energy, and carrots store sugars in their roots during the first year of growth to make them readily available to be broken-down for energy when the flowers and fruits expand during the second year. /
Carrot stem
In our gardens, we harvest carrots as a crop after their first year of growth in order to obtain the storage roots that are filled with nutrients. If left in the ground until the second year of growth, we would find shriveled roots from which all the nutrients had been extracted.
Carrots show how parts of both the root and shoot system can modified. They have a large tap root that functions as a storage structure and a highly reduced stem that consists of little more than photosynthetic leaves, at least during their first year of growth.
Carrots also provide an example of a strategy that plants have for resource storage and use. Carrots are biennials (each lives for two years) that make sugars and store them during their first year of growth and then use them during the second year for reproduction. Alternative strategies are used by other plants.
Names: ______Set ______
Carrot Root Dissection
Procedure:
1) Examine the whole carrot including its stems and leaves. Record your observations in individual sentences. Take note to how and where the stem(s) attach to the carrot root. What internal parts do you expect to be located in this region? Cut off a couple of stems at an angle. Record what you see.
2) Now, very carefully, cut off the leaf stalks leaving the carrot stem attached to the root. Examine the stem at the area where it meets with the root. Discuss what you see with another student. Next, very carefully, cut down lengthwise through the middle of the stem and into the carrot root a distance of about 2-3 cm. Then cut out this portion of the carrot so it roughly looks like the carrot in the photo below, only without the leaf stalks. Write down the tissue types you see from the outside toward the middle of the root starting with the epidermis.
3) Next slice off that remaining portion so you have a cross section view. You now should have a carrot without its leaf stalks, stem, and 2-3 mm of the root. Set the top half portion you just cut off aside, you won’t need it. Then, make a simple drawing of the cross-section and label its various internal parts.
4) Place the carrot root on the paper plate and with your scalpel, very carefully remove a thin slice cross-section of the carrot (about 0.5 mm in width). You know it’s thin enough when you hold it up to the light and you can easily make out the various internal parts you identified in #3 above. Look at the slice using the magnifying lens. Discuss what you see with another student. Check your drawing of the cross section and be sure you have all the parts labeled.
5) Set this thin slice aside and get the rest of the carrot. You will now make a longitudinal cut (cut in half lengthwise, see photo below) from the top of carrot to the bottom. Be careful to make sure you divide the carrot in two equal parts, including the small root tip (if present) at the end of the carrot.
6) Make a drawing of your divided carrot (your carrot should look like the picture above). Looking at diagrams of a taproot system you already have in your possession label your drawing with the following: external structures (lateral roots, epidermis), and internal structure (cortex, stele, endodermis, pericycle, xylem, phloem, cambium and casparian strip). In addition label the zone of cell division, zone of elongation, zone of differentiation, the root cap, and the apical meristem.
Review of procedures and structures from 1-7 given above
Held the crosswise slice of carrot up to light and was able to see all the internal parts. Used the magnifying lens to examine both the thin cross-section and the lengthwise slice of carrot. Looked at both the inner and outer parts of each section. Drew and labeled the parts seen. You should now have a labeled drawing of a cross-section and a lengthwise section of carrot. You also recorded information and answered all the questions in the procedure section of this lab.
NOTES
1. The outside layer of the root is the epidermis. Root hairs grow from the epidermis (or periderm) a source of protection for the root. The secondary roots arise from the meristematic layer between the xylem and phloem layers called the cambium.
Conclusions:
1. What is the function of the root hairs?
2. What is the name and function of the outer ring of cells of the root?
3. What is the green portion at the top end of the carrot?
4. What is the name and function of the thicker layer of cells next to the epidermis?
5. What two cell types are found in the vascular cylinder?
6. What is the function of these (#5) cells?
Discussion:
1. Why are taproots used as food more often than fibrous roots?
2. When transplanting plants, why is it so important to move a lot of soil with it instead of just the main root itself?
3. If you were to place a carrot in food coloring for 24 hours to allow transpiration to occur. What tissues of the root would have the most food coloring in it? Why?