SBI 3U: TRANSPORT MECHANISMS WITHIN VASCULAR PLANTS WORKSHEET
PART A: LEAVES
Green leaves the major sites of photosynthesis b/c they contain chlorophyll, the green pigment needed to capture light energy. Leaves must also
be able to obtain carbon dioxide from the air and water to use to make sugars; leaves must not dry out in bright sun, hot dry air and must survive
against herbivores. There are lots of different shapes, sizes and textures of leaves due to these issues. In general, leaves are divided into 2
categories. Simple leaves have a single, undivided blade (the broad part of the leaf) and Compound Leaves have a blade divided into 2 or more
leaflets.
LEAF BLADES
Xylem and phloem tissue continue from the stems into the leaves, forming part of the vascular bundles. These bundles are called VEINS. The veins
in a leaf branch out extensively so that the xylem and phloem are close to the mesophyll cells. Remember, xylem carries water and dissolved
minerals form the soil, and phloem transports the products of photosynthesis, glucose, in the leaf mesophyll cells, to all parts of the plant. The
conducting tissues found in the leaf are continuous with those in the stem and root. (See PICTURE on slide 5 from “plants – day 2 ppt”)
Venation is the pattern of veins in the blade of a leaf. The vein consists of vascular tissues which are important in the transport of food and water;
in new veins branch and rebranch throughout the whole leaf, the leaf has net venation ; if new leaf veins tend to run from the leaf stalk to the leaf
tip without joining one another, leaf is said to have parallel venation.
Questions:
1.
Which picture is showing net venation, and which is showing parallel venation?
A.___________________
2.
B. _______________________
What is the major difference between a single leaf and a compound leaf? Draw and label this difference.
PART B: ROOTS
Roots are classified into 2 major types: taproot= root systems where the primary root remains predominant, though very small secondary roots
are present and fibrous root= root systems whose primary roots have disintegrated and were replaced by other smaller roots. See the picture on
the left hand side below.
(Pneumatophores in a Kenyan Swamp)
Some plants have roots that evolved special structures or habits that allow plants to survive in special situations – in waterlogged, oxygen-poor
soils (swamp), roots may have specialized extensions called pneumatophores which grow up and out of the water and function to supply oxygen to
the root tissues below. See the picture on the right side above. This specialized ‘breathing’ root developed in some plant species that grow in
waterlogged or strongly compacted soils, e.g. mangroves. The aerial part of the root contains many pores, enabling gas exchange – oxygen enters
by osmosis - with the atmosphere. Internally, a well developed system of intercellular spaces allows gases to diffuse throughout the submerged
portion of the roots.
Water Transport in Roots
Root hairs absorb essential minerals from the soil by active transport, a movement process that requires energy. Once inside, the minerals are
moved to the center of the root where the wax-coated endodermis (a layer of cells around the vascular bundle in the roots) prevents them from
diffusing back out to the soil. This concentration of minerals creates xylem sap that is hypertonic relative to soil water outside the root. As a result,
the hypotonic soil water enters the root by osmosis. Energy is not required for water entry but is needed for mineral transport (active transport).
This accumulation of water and dissolved minerals creates pressure and this pressure pushed the sap up the xylem. This pressure is called ROOT
PRESSURE. Root pressure is able to push water up to only a maximum of a few meters in some plants. However, there are many trees, such as
giant redwoods, that are over 100 m tall!! Also, in many plants there is no measurable root pressure. Clearly, root pressure cannot entirely
account for xylem sap movement.
Questions:
1.
What is the difference between a taproot and a fibrous root?
2.
Why are pneumatophores so important to certain plants?
3.
How does root pressure form within the root?
PART C: STEMS
So, you know that root pressure cannot entirely account for xylem sap movement. What are other factors that push xylem sap up the plant? Well,
you need to know that water clings readily to a variety of non-oily surfaces. This phenomenon is known as adhesion. This principle is
demonstrated every time you dry yourself with a towel. Water clings more readily to the fibre of the towel than to your skin, which is oily
compared to the towel. When a drop of water on your skin comes into contact with the towel, it ‘flows’ and adheres to the towel fibres. This same
principle accounts for the movement of water into sponges even if the movement of molecules is against gravity, as it is in the xylem. Water
adheres to the inner walls of the xylem, creating a ‘pulling’ force on the column of water molecules.
The ability of water to stick to itself is known as cohesion. Because of the hydrogen bonding between adjacent water molecules, very strong forces
of attraction are created between water molecules. If you have ever tried to pry apart 2 wet microscope slides, you will have noticed how tightly
they stick together. The water between them slides acts like glue! A xylem vessel may be very long, but it does not contain much water because it
has a small diameter. The cohesion of water molecules in this long cylinder results in the water column holding together continuously from the
ground to the top of the highest leaves! This narrow column of water is incredibly strong; it has the same strength as steel wire of the same
diameter! This is more than enough to support a column of water in the tallest trees. If a ‘pull’ at the top of this water column were exerted, there
would be enough cohesive strength to draw water all the way from the roots to the top of the plant. The ‘pull’ required comes from the leaves via
a process called transpiration pull.
What is this transpiration pull? Water evaporates from the inner leaf cell walls into the air spaces and out through the stomata if they are open.
This water loss creates a reduced pressure in the leaves and water flows upwards from the roots where there is a relatively higher pressure. This
lost water is replaced by bulk flow from the roots. When water moves upwards from the roots, the cell membranes of the root cells allow more
water and dissolved minerals to enter by osmosis and active transport, respectively.
FIGURE 16.28 IN ADDISON WESLEY
Questions:
1.
What does water transport in the xylem depend on within the plant? Breifely describe each of these factors. Does water transport
within the plant depend upon energy of any sort or does it occur passively?
A Canadian Product Derived from Xylem Sap
In North America, the xylem spa of the sugar maple tree is put to use. In early spring, when the days are warm and the night temperatures fall
below freezing, sugar maples begin to convert starches, stored in the roots, into sugars and send them up the xylem to the branches to the
developing leaf and flower buds. To make maple syrup, holes are tapped into the xylem to collect the clear xylem sap. Traditionally, a special
metal tube was driven into a sugar maple tree and a bucket was hung on the tube. Now, plastic tubing is usually connected to the metal tubes and
the sap is drawn by a pump to a central location. Maple syrup producers then boil this sap down to remove most of the water. About 50 L of sap
are needed to produce one litre of pure maple syrup!!
2.
Speculate why sugar maple trees demonstrate the importance of plants to the growth of Canadian society. Why is this xylem sap, better
known as maple syrup, so important to our society?
PART D – LEAVES AGAIN!
Use your notes “Plants – Day 2” ppt as well as the information given below to answer the following questions:
Carbon dioxide from the atmosphere is one of the raw materials used by the chloroplasts to manufacture sugars. In general, the stomata are open
during daylight hours to allow in the carbon dioxide needed for photosynthesis and to allow water vapour out. Oxygen, a by-product of
photosynthesis, is also released through the open stomata into the atmosphere. Since photosynthesis can’t happen at night, carbon dioxide is not
needed, so the stomata are usually closed at night to prevent excess water loss. Plants are also able to close their stomata during the day,
depending on environmental conditions. When it’s too hot or when there is a lack of water in the soil, plants can close their stomata until
conditions improve.
Each stomata is surrounded by 2 guard cells. The guard cells are specialized, kidney-shaped epidermal cells containing chloroplasts. The closing
and opening of stomata are regulated by the levels of water and carbon dioxide in the guard cells during a complex process.
Most of the water entering the leaves is lost through the process of transpiration. As each water molecule evaporates during transpiration, it
‘pulls’ on the adjacent water molecules. These molecules in turn pull on the next, and so on, in an unbroken cohesive chain all the way to the root
hairs. The evaporation of liquid head energy, which comes form the environment around the plant. Thus, the plant uses heat energy from its
surrounding to pull water from the roots to the top of the plant. Xylem sap movement (water and minerals in the plant) is dependent of the rate of
transpiration. If the air temperature is low, in a northern climate, the rate of evaporation is low. But if the temperature is high, this rate is also
high. That is, the faster water evaporates from the leaves, the faster sap will rise.
1.
Explain why most plants have stomata mainly on the lower surface of their leaves.
2.
Why are guard cells important to stomata?
3.
Predict the environment in which you would expect to find plants with several layers of palisade mesophyll cells densely packed with
chloroplasts. Explain why.