Intro to plant nutrition Leaf structures and functions –External –Internal The role of guard cells How CO 2 and water enter the leaves Topic Overview Week 4: Leaf Structures & Functions
The basics & importance of photosynthesis Test for starch in leaves of plants Factors affecting the rate of photosynthesis The limiting factors of photosynthesis 2 stages of photosynthesis Uses of glucose by the plants Topic Overview Week 5: Photosynthesis
Aims of Today’s Lesson At the end of this lesson, you should be able to Identify and label the cellular and tissue structure of a leaf Describe the significance of these features in terms of their functions: –Distribution of chloroplasts in photosynthesis –Stomata and mesophyll cells in gaseous exchange –Vascular bundles in transport Outline the intake of CO 2 and water by plants
4 main features 1.Lamina –Large flat surface ensures maximum amount of sunlight absorbed –Thin lamina facilitates rapid diffusion of gases into and out from the inner cells 2.Petiole –Holds lamina away from the stem to obtain max. sunlight and air for leaf 3.Network of veins –Carry water, minerals and food to and fro 4.Leaf arrangement –Dispersed (away from each other) ensures optimal absorption of sunlight External Structure of a Leaf
3 layers 1.Upper Epidermis 2.Mesophyll 3.Lower Epidermis The Main Components Let’s examine what is inside each of this layer!
Upper Epidermis Single layer of closely packed epidermal cells Covered by a waxy layer called the cuticle –Prevent excessive water evaporation –Protects the enclosed leaf tissue Transparent – allows sunlight to penetrate No chloroplasts
Palisade Have 1 or 2 layers of long cylindrical cells that are closely packed Contain numerous chloroplasts Spongy Cells are irregular in shape Numerous large intercellular air spaces between them Fewer chloroplasts Palisade vs. Spongy Mesophyll
Large intercellular air spaces present in the spongy mesophyll allows for rapid diffusion of gases Photosynthesis still takes place in this layer, but to a lesser extent Presence of vascular bundles – xylem & phloem Note! All cells in the palisade and the spongy mesophyll are covered with a thin film of moisture – for CO 2 to dissolve in Spongy Mesophyll
Like the upper epidermis –Single layer of closely packed cells –Also has a waxy layer of cuticle (thinner) to prevent excessive water loss –No chloroplasts Contains many tiny openings/pores called the stomata (singular: stoma) Important in gaseous exchange Lower Epidermis
Stomata Tiny openings in the lower epidermis Allows gaseous exchange to take place Enclosed by two cells – guard cells –Bean-shaped –Contain chloroplasts
Guard Cells In most cases, stomata open in the light and close in the dark Guard cells are the only cells in the lower epidermal layer capable of making glucose So what makes them open and close?
In the presence of light… Photosynthesis takes place Potassium ions are pumped into the guard cells via active transport Decrease in water potential in guard cells Water from epidermal cells enters guard cells via osmosis Guard cells become turgid, swollen and more curved pull the stomata open Opening of Stomata
In the absence of light… Potassium ions diffuse out from the guard cells Increased water potential in guard cells Water leaves the cells by osmosis Guard cells become flaccid stomata close Closing of Stomata
Veins or Vascular Bundles Contains xylem and phloem Xylem transports water and mineral salts from the roots to the leaves Phloem transports sucrose and amino acids from leaves to the other parts of the plant
How does CO 2 enter the leaf? 1.At night, carbon dioxide produced is stored in the air spaces of the leaf 2.In the day, rate of photosynthesis > rate of respiration 3.CO 2 is rapidly used up for photosynthesis 4.Concentration of CO 2 in leaf decreases –generates a diffusion gradient
5.Atmospheric CO 2 diffuses into the leaf via the stomata 6.Carbon dioxide dissolves in the film of moisture 7.After which, it diffuses into the cells for photosynthesis How does CO 2 enter the leaf?
Summary How does the external structures of leaf help in carrying out photosynthesis? How many layers of cells are there in a leaf? How does each layer help in carrying out photosynthesis effectively? What is the purpose of the thin film of moisture that coats most cells of the leaf? How do the stomata open and close? Which cells are responsible for the opening and closing of the stomata?
Origin of Our Energy How do we get our energy? Recall the food you had for your breakfast or your dinner last night? What is the similarity between the food that you and your classmates had eaten?
The truth is our energy stems primarily from green plants Why green plants in particular ? Green plants have the capability to make and store their own food food producers Can you recall why? Origin of Our Energy
Raw Materials and Conditions required for Photosynthesis 1.Sunlight 2.Chlorophyll 3.Carbon Dioxide 4.Water
Products of Photosynthesis 1.Food in the form of glucose 2.Oxygen 3.Water Overall equation: Chlorophyll
Plants do not use all the light that illuminates them!
How to test for evidence of photosynthesis? Glucose is made during photosynthesis Excess glucose is converted to starch Presence of starch can be tested by conducting an iodine test Release of O 2 (for aquatic plants)
Test for Starch http://www.footprints-science.co.uk/Starch.htm
Experiments on Photosynthesis Questions asked: 1.Is sunlight required for photosynthesis? 2.What about chlorophyll? 3.And carbon dioxide? 4.What gas is emitted during photosynthesis? Chlorophyll
Experiment 1 Does photosynthesis require sunlight? Test for the importance of sunlight for plants to carry out photosynthesis Main emphasis is to destarch the plants for 2 days (why?) Cover a portion of the leaf from sunlight and place the plant in sunlight for a few hours (control?) Test for starch
Test for the necessity of chlorophyll for plants to carry out photosynthesis Use plant with variegated leaves Destarch and place plant in sunlight Test for starch and note the distribution of stain Experiment 2 Does photosynthesis require chlorophyll?
Test for the necessity of carbon dioxide for plants to carry out photosynthesis 2 setups (Plants in both setup must be destarched) One set up is a control, the other does not have carbon dioxide Place in sunlight and remove after a few hours Test for starch Experiment 3 Does photosynthesis require carbon dioxide?
Experiment 3 Setup Does photosynthesis require carbon dioxide?
Test for gas that is given off during photosynthesis Use of freshwater plants Dissolve sodium hydrogen carbonate in water (why?) Place in sunlight until gas bubbles appear Test for gas (how?) Experiment 4 What gas is emitted during photosynthesis?
Experiment 4 Setup What gas is emitted during photosynthesis?
How do you remember the details? All three except one experiment require plants to be destarched All plants are subjected to an iodine test Remember the control setup has the basic setup
Why do we test for starch in leaves instead of glucose? How do we test for starch? What are the steps taken to test for starch in a leaf? When is there a need to destarch plants? What is the minimum period of time that a plant needs to be destarched? Summary