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C6

Page history last edited by gerryc 15 years, 5 months ago


C6 - Plant cells are able to make their own food through photosynthesis.

 

Student Outcome C6.1 Understand that plants are designed to get as much sunlight as they can.

 

It's not surprising that early scientists hypothesized that plants ate dirt. They didn't know, as we now do, how energy-rich sunlight is. Still, it seems remarkable that plants have evolved photosynthesis--the ability to harness the sun's energy to produce their own food.

Photosynthesis is the process by which plants transform water and carbon dioxide (a gas that's plentiful in the air) into carbohydrates (sugars and starches), using the energy of sunlight. While sunlight provides the energy needed to drive this reaction, a chemical in the leaves of plants makes the reaction possible. That chemical is a green pigment called chlorophyll. Chlorophyll is found inside the photosynthetic cells of plants, attached to the membranes of small, round structures called chloroplasts. Chlorophyll absorbs light in the red and blue-violet portions of the visible spectrum, and reflects the green portion of the spectrum; this is what gives chlorophyll its characteristic green color.

As remarkable as photosynthesis is, the process is not very efficient. Studies show that prairie grasses in the western United States are some of the most efficient plants at harnessing the sun's energy, but even they capture little more than about 3 percent of the energy that reaches the prairie surface. The rest of that energy is reflected away, absorbed by humidity in the air or by the ground, or simply lost in myriad other ways before the plants can use it.

One of the most critical factors influencing the efficiency of photosynthesis is the amount (intensity and duration) of light that hits a leaf. Generally, the more light that strikes a leaf, the greater the rate of photosynthesis in that leaf. For example, a leaf that is exposed to direct sunlight will photosynthesize at the highest rate, while a leaf directly beneath it and in its shadow will photosynthesize at a much lower rate. Because of this, many plants have evolved leaf and branch structures that minimize overlap and shading, and thus maximize the plant's overall rate of photosynthesis.

 

Source: http://www.teachersdomain.org/resource/tdc02.sci.life.stru.photosynth/

 

Student Outcome C6.2 Recognise and label the major structures found in plant leaves.

This diagram shows close ups of the major organelles found in plants - the chloroplasts.

 

 

In the leaves of most trees, three distinct tissue layers can be discerned in their leaves. These are:

  1. The epidermis with its cuticle and stomata

    The epidermis is the cell layer which covers the outer surface of the leaf.

    There are three three  basic kinds of epidermal cells:

    Ordinary epidermal cells

    The ordinary epidermal cells may show a variety of shapes depending on the tree species and are often covered with a waxy cuticle which is secreted by the protoplast of the epidermal cells. The cuticle varies greatly in its pattern of deposition and is often a characteristic for a particular species.

    Trichomes or hair cells

    Trichomes or hairs cells grow out of the surface of  the epidermis. These may be uni-or multicellular depending on the tree species. Both uni-and multicellular hairs may be branched.

    Some leaves have glandular hairs which have an enlarged cell or group of cells at the end of  a stalk. These cells excrete etherial oils which may leave a stickiness.

    Guard cells

    The guard cells are the specialised cells which are found on either side of a stoma (plural stomata).

    Epidermal cells have long-lived protoplasts and do not contain chloroplasts except for the guard cells. The epidermis provides a protective layer for the other tissues in the leaf. The cuticle prevents the movement of gases though the outer walls of the epidermal cells thus preventing excessive water loss. Movement of gases into and out of the leaf is restricted by the stomata.

     

  2. The mesophyll where most of the chloroplasts are found and photosynthesis takes place.

    The mesophyll of  the leaf consists of parenchyma tissue between the abaxial and adaxial epidermis of the leaf. The mesophyll cells contain chloroplasts and it is in these cells that photosynthesis takes place. In many tree species, two distinct layers of mesophyll cells may be recognised, the palisade mesophyll and the spongy mesophyll.

    The Palisade mesophyll

    The palisade mesophyll consists of one or several layers of  elongated, narrow parenchyma cells with their long axes at right angles to the axis of the leaf and are situated under the adaxial epidermis.  Chloroplasts are especially concentrated in the palisade mesophyll and it is in these cells that much of the photosynthesis in a tree takes place.

    The Spongy mesophyll

    The spongy mesophyll consists of irregularly shaped parenchyma cells which are located between the palisade mesophyll and the abaxial epidermis. Spongy mesophyll cells contain less chloroplast than the palisade mesophyll cells but photosynthesis take place in these cells as well. There are many intercellular air spaces between the spongy mesophyll cells which are interconnected and communicate with the stomata of the abaxial epidermis. This allows the food-producing cells of the leaf (the mesophyll) to access the gases (carbon dioxide - CO and oxygen - O2) which they need for photosynthesis and respiration.

  3. The leaf veins which transport water and inorganic compounds into the leaf, and organic compounds produced by photosynthesis away from the leaf, to other parts of the plant.

    Through the leaf tissues run numerous interconnected veins or vascular bundles of varying sizes.  These veins transport water and dissolved mineral salts into the leaf which are used for the production of organic substances during photosynthesis and related chemical pathways, which take place in the leaf. They are also responsible for transporting these substances away from the leaves to the other parts of the tree where they will be used. The larger veins of  the leaf provide the mesophyll of the leaf with support.

    The larger veins form the midrib and lateral veins of the leaf but there are also numerous small veins which can only be seen with the aid of a microscope.

    The larger veins have xylem with vessels and tracheids, and phloem with sieve tubes and companion cells.  In these larger veins, the xylem is orientated towards the adaxial epidermis. The small veins have fewer vascular cells and no supporting cells. The veins are surrounded by parenchyma cells, through which water and dissolved organic and inorganic substances must pass when entering or leaving the vein. The bundle sheath may or may not contain chloroplasts. The vascular tissue of the leaf is continuous with that of the stem.

Source: http://www.botany.uwc.ac.za/ecotree/leaves/InsideLeaf3.htm#mesophyll

 

This site has a very quick interactive animation where you can label the major parts of the internal structure of a leaf

 

 

Student Outcome C6.3 Able to describe in words the overall chemical reaction of photosynthesis (chemical formula optional).

 

This is what is happening in a leaf: (Source: http://www.butler.edu/herbarium/treeid/photosynthesis.jpg)

 

 

 

This is the word equation: (Source: http://earthguide.ucsd.edu/events/TeacherTECH_2005/equation_photosynthesis.gif)

 

Here is a simple video (aimed at Kindergarten kids I think) which explains photosynthesis. Don't worry about the voice.

YouTube plugin error

 

Go here if none of the above videos work. It is from the HowStuffWorks site.

 

Student Outcome C6.4 Show the movement of the products of photosynthesis throughout the rest of the plant.

 

The plants have a pump! ..

When sucrose is made in the leaves, it is pumped into certain cells where it accumulates at up to 100 times the level found elsewhere in the plant. ..

This produces an extremely hypertonic situation, so water, of course, diffuses into these cells causing a lot of pressure that pushes the sap through the phloem to the destination or "sink" tissues. ..

Here is the overall scheme for the DOWN ward movement:

..... sap's pump

Source: http://www.science-projects.com/sap.htm

 

 

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