Comprehensive Notes on Plant Nutrition and Photosynthesis

Plant Nutrition

Focus Points

  • Photosynthesis: Plants create biological molecules through photosynthesis, controlled by enzymes.
  • Key Questions:
    • What food do plants make?
    • What do they use it for?
    • How do they use the properties of different biological molecules?
  • Focus Points:
    • What is photosynthesis?
    • What is chlorophyll and what does it do?
    • How are the products of photosynthesis stored and what are they used for?
    • Why are nitrate and magnesium ions important for plants?
    • What is the effect of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis?

Photosynthesis - Key Definitions

  • Photosynthesis: Plants synthesize carbohydrates from raw materials using light energy.
  • All living organisms need food for:
    • Raw materials to build new cells and tissues.
    • A source of energy, a fuel for vital living processes and chemical changes.
  • Animals take in, digest, and use food for tissue building or energy release.
  • Plants, except for a few insect-eating species, do not ingest food, sourcing raw materials primarily from the soil.
  • Weight gain in plants surpasses the weight loss in the soil, indicating additional raw material sources.
  • Hypothesis: Plants create food from air, water, and soil salts.
  • Carbohydrates (e.g., glucose, C6H{12}O_6) contain carbon, hydrogen, and oxygen.
    • Carbon and oxygen from carbon dioxide (CO_2) in the air.
    • Hydrogen from water (H_2O) in the soil.
    • Nitrogen and sulfur for proteins come from nitrates and sulfates in the soil.
  • Synthesis: Building complex food molecules from simpler substances, requiring enzymes and energy.
    • Enzymes are present in plant cells.
    • Energy for the initial stages comes from sunlight.
  • Photosynthesis: 'photo' means 'light'.
  • Chlorophyll in chloroplasts facilitates photosynthesis by absorbing sunlight and converting light energy to chemical energy.
  • Simple Chemical Equation: In order to keep the equation simple, glucose is shown as the food compound produced.
  • Glucose is quickly converted to sucrose for transportation and stored as starch or converted into other molecules.
  • Chemical Equation for Photosynthesis (Extended Syllabus):
    • 6CO2 + 6H2O \xrightarrow[chlorophyll]{light} C6H{12}O6 + 6O2
  • Remember the number of each type of molecule and the symbols.

The Process of Photosynthesis

  • Photosynthesis details vary among plants, but the hypothesis is supported by experimental testing.
  • Process mainly occurs in leaf cells.
  • Water is absorbed by roots and carried via xylem to the leaf.
  • Carbon dioxide is absorbed through stomata.
  • In leaf cells, carbon dioxide and water combine to make sugar using sunlight energy absorbed by chlorophyll in chloroplasts.
  • Chloroplasts are small, green structures in the cytoplasm of leaf cells.
  • Chlorophyll splits water molecules into hydrogen and oxygen.
  • Oxygen escapes from leaves, and hydrogen joins with carbon dioxide to make sugar.
  • Light energy is converted into chemical energy in carbohydrates.

Plant's Use of Photosynthetic Products

Starch

  • Glucose is converted to sucrose for transport.
  • Excess sugar is turned into starch and stored or changed into other molecules.
  • Starch molecules are added to growing starch granules in the chloroplast.
  • Increased glucose concentration could affect osmotic balance.
  • Starch is relatively insoluble, maintaining cell concentration.
  • Some plants store starch grains in stems or roots.
  • Plants like potato or cassava use tubers to hold starch reserves.
  • Sugar is stored in fruits like grapes (up to 25% glucose and other sugars).

Sucrose

  • Starch breaks down into soluble sucrose.
  • Transferred out of the cell into phloem.
  • Phloem are food-carrying cells in leaf veins.
  • Veins pass sucrose to non-photosynthesizing parts:
    • Growing buds
    • Ripening fruits
    • Roots
    • Underground storage organs

Cellulose

  • Plant cell walls are made of cellulose, which are long chains of glucose.
  • Cellulose forms a tough meshwork around the cell.
  • Cell wall: holds cell contents but is permeable.

Respiration

  • Sugar provides energy through respiration.
  • Oxidation of glucose produces carbon dioxide and water.
  • Released energy is used for chemical reactions like protein synthesis.

Nectar

  • Nectar is made of a mixture of sugars, including glucose, fructose, and sucrose.
  • Insect-pollinated plants produce nectar to attract insects for pollination.

Mineral Requirements

  • Plants need nitrates (NO3–) for amino acids.
  • Amino acids are important for making proteins.
  • Proteins make up enzymes and cytoplasm.
  • Nitrates are absorbed from the soil by the roots.
  • Plants need magnesium ions (Mg2+) to make chlorophyll.
  • Magnesium is acquired in salts from the soil.

Sources of Mineral Elements and Effects of Their Deficiency

  • Nitrates and magnesium ions are mineral salts or elements.
  • Deficiency symptoms appear in soils lacking mineral salts.
  • Slow-growing wild plants may not show deficiency symptoms on poor soils.
  • Fast-growing crop plants show clear deficiency symptoms.
  • Nitrate shortage:
    • Stunted growth
    • Weak stem
    • Yellowing and death of lower leaves
    • Pale green upper leaves
  • Magnesium shortage:
    • Inability to make chlorophyll
    • Yellowing of leaves from bottom upwards (chlorosis)
  • Farmers and gardeners add fertilizers to counter mineral shortages.
  • Plants absorb salts to obtain mineral elements.
  • Potassium nitrate (KNO3) provides potassium (K) and nitrogen (N).
  • Salts are derived from the breakdown of rocks forming the soil.
  • Salts are continually taken up or washed out of the soil.
  • Decomposition of dead plants and animals replenishes salts in the soil.
  • Arable farming removes mineral ions:
    • Ground is ploughed, and crops are removed, so no dead plants decay to replace minerals.
    • Farmers spread animal manure, sewage sludge, or artificial fertilizers.
    • Common fertilizers: ammonium nitrate, superphosphate, compound NPK (nitrogen, phosphorus, potassium).

Water Cultures

  • Water cultures demonstrate the importance of mineral elements.
  • Full water culture contains salts for healthy growth:
    • Potassium nitrate (potassium, nitrogen)
    • Magnesium sulfate (magnesium, sulfur)
    • Potassium phosphate (potassium, phosphorus)
    • Calcium nitrate (calcium, nitrogen)
  • Green plants use these, along with carbon dioxide, water, and sunlight, to stay healthy.
  • Hydroponics:
    • Growing crops in a glasshouse without soil
    • Plants grow in flat polythene tubes, with solution pumped along
    • Increases the yield, eliminates the need to sterilize the soil

Practical Work - Experiments to Investigate Photosynthesis

Controlled experiments use a control to ensure results are due to studied conditions, not other factors.

If comparing plant growth in a house vs. a glasshouse, the difference could be due to extra light or high temperature.

Only one condition (variable) should be altered at a time to compare results with the control experiment.

A hypothesis tries to explain observations.

The equation for photosynthesis is a way of stating the hypothesis.

If photosynthesis occurs, leaves should produce sugars, which rapidly turn into starch.

Testing for starch indicates photosynthesis has taken place.

Experiments test if leaves can make starch without chlorophyll, sunlight, or carbon dioxide. If any are missing, photosynthesis should stop.

Only one variable should be changed at a time along with a control that includes the missing condition.

To test for single variable change, a control is setup with every test.

Destarching is needed to ensure no starch is in the leaf at the beginning of the experiment. Otherwise destarching cannot be done chemically without damaging the leaves.

Destarching involves leaving a plant in darkness for 2–3 days so that starch is turned to sugar and carried away to other parts.

Leaves can be covered in aluminium foil or black card for 2 days to test for starch beforehand.

Testing a Leaf for Starch:

Iodine solution (yellow/brown) mixes with starch (white) to make a deep blue color.

Iodine cannot soak into the leaf, and chlorophyll hides color changes, so the leaf needs to be treated as follows

  • Heat water to boiling point, then turn off the Bunsen flame.
  • Use forceps to dip a leaf in the hot water for about 30 seconds. This kills the cytoplasm, denatures the enzymes, and makes the leaf more permeable to iodine solution.
  • Push the leaf to the bottom of a test tube and cover it with ethanol (alcohol). Place the tube in the hot water. The alcohol has a boiling point of 78°C so it boils, removing most of the chlorophyll.
  • Pour the green alcohol into another beaker, remove the leaf, and dip it into the hot water again to soften the leaf.
  • Spread out the decolourised leaf on a white tile and drop iodine solution on to it. Parts with starch will turn blue; parts without starch will stain brown or yellow.
Is Chlorophyll Necessary for Photosynthesis?
  • Variegated leaf (leaf with patches of chlorophyll) is used because it is not possible to remove chlorophyll without killing the leaf.
  • White part of the leaf has no chlorophyll and is the experiment; the green part is the control.
  • After destarching, expose leaf to daylight for a few hours.
  • Only parts with chlorophyll turn blue with iodine; the white parts stain brown.
  • Starch is only present in parts with chlorophyll, suggesting chlorophyll is needed for photosynthesis.
  • However, the experiment does not rule out other possibilities, like starch being made in the green parts and sugar in the white parts.
Is Light Necessary for Photosynthesis?
  • Cut shape from aluminium foil to make a stencil, and fix it to a destarched leaf.
  • After 4–6 hours of daylight, remove the leaf and test it for starch.
  • Only areas that received light go blue with iodine.
  • Starch has not formed in areas without light, so light is needed for starch formation and photosynthesis.
  • It is possible aluminium foil stopped carbon dioxide from entering, so a control could be designed with transparent material.
Is Carbon Dioxide Needed for Photosynthesis?
  • Water two destarched plants and cover their shoots in polythene bags.
  • Place soda lime in one pot (experiment) and sodium hydrogencarbonate solution in the other (control), to absorb and produce carbon dioxide, respectively.
  • Place both plants in light for several hours and test leaves for starch.
  • The leaf without carbon dioxide does not turn blue; the one with carbon dioxide does.
  • Starch was made in leaves that had carbon dioxide, implying that this gas is needed for photosynthesis.
  • Plastic bag rules out the chance that high humidity or high temperature stops normal photosynthesis.
Is Oxygen Produced During Photosynthesis?
  • Place a short-stemmed funnel over some Canadian pondweed in a beaker of water.
  • Fill a test tube with water and place it upside-down over the funnel stem.
  • Place in sunlight; bubbles of gas should appear from the cut stems and collect in the test tube.
  • Set up a control in a dark cupboard.
  • When enough gas has collected, remove the test tube and put a glowing splint in it.
  • The glowing splint bursts into flames.
  • The relighting of a glowing splint does not prove the gas collected is pure oxygen, but it shows that it contains extra oxygen and this must have come from the plant.
  • The gas is only given off in the light.
  • The composition of the gas in the test tube may not be the same as the composition of the bubbles leaving the plant.

Hypotheses

  • The two sets of procedures in designing an experiment and control are the experiment and the control depend on how the prediction is worded.
  • For example, if the prediction is that ‘in the absence of light, the pondweed will not produce oxygen’, then the control is the plant in the light. If the prediction is that ‘the pondweed in the light will produce oxygen’, then the control is the plant in darkness.
  • As far as the results and interpretation are concerned, it does not matter which is the control and which is the experiment.
  • The results of the four experiments support the hypothesis that starch formation only takes place in the presence of light, chlorophyll and carbon dioxide and oxygen production happens only in the light.
  • Therefore if starch or oxygen production had happened when any one of these conditions was missing, we would have to change our hypothesis about the way plants get their food. However, although our results support the photosynthesis theory, they do not prove it.

Gaseous Exchange in Plants

  • Air contains nitrogen, oxygen, carbon dioxide, and water vapor. Plants and animals exchange the last three gases.
  • Photosynthesis produces oxygen, so plants take in carbon dioxide and give out oxygen in daylight.
  • Opposite of respiration, which plants, like animals, do all the time.
  • Respiration uses oxygen and produces carbon dioxide for energy.
  • During daylight, carbon dioxide produced by respiration is used up by photosynthesis, and oxygen needed by respiration is supplied by photosynthesis.
  • Carbon dioxide is taken in, and excess oxygen is given out when the rate of photosynthesis is faster than the rate of respiration.

How Will the Gas Exchange of a Plant Be Affected by Being Kept in the Dark and in the Light?

  • This investigation uses hydrogencarbonate indicator, which is a test for the presence of carbon dioxide. An increase of carbon dioxide turns it from pink/red to yellow. A decrease in carbon dioxide levels turns the indicator purple.
  • Wash three boiling tubes. First use tap water, then distilled water and finally use hydrogencarbonate indicator (this is because the indicator will change colour if the boiling tube is not clean).
  • Fill the three boiling tubes to about two-thirds full with hydrogencarbonate indicator solution.
  • Place equal-sized pieces of Canadian pondweed in tubes 1 and 2 and seal all the tubes with stoppers.
  • Shine light on tubes 1 and 3 using a bench lamp and place tube 2 in a black box, a dark cupboard or wrap it in aluminium foil .
  • After 24 hours record the colour of the hydrogencarbonate indicator in each tube.

Interpretation

Hydrogencarbonate indicator is a mixture of dilute sodium hydrogencarbonate solution with two dyes called cresol red and thymol blue. It is a pH indicator in equilibrium with carbon dioxide, i.e. its starting colour is because of the acidity produced by the carbon dioxide in the air. An increase in carbon dioxide makes it more acid, so it changes colour from orange/red to yellow. A decrease in carbon dioxide makes it less acidic and causes a colour change to purple.
So, the results show that in the light (tube 1) aquatic plants use up more carbon dioxide in photosynthesis than they produce in respiration.
In darkness (tube 2) the plant produces carbon dioxide (from respiration).
Tube 3 is the control, showing that the presence of the plant causes a change in the solution in the boiling tube.
The experiment can be challenged because the hydrogencarbonate indicator is not a specific test for carbon dioxide; it will change colour when there is any change in acidity or alkalinity. In tube 1 there would be the same change in colour if the leaf produced an alkaline gas like ammonia. In tube 2 any acid gas produced by the leaf would turn the indicator yellow. However, knowledge of leaf chemistry suggests that these are less likely events than changes in the carbon dioxide concentration.

Effects of External Factors on the Rate of Photosynthesis

  • The rate of the light reaction depends on the light intensity.
  • Water molecules in the chloroplasts split faster with brighter light.
  • The dark reaction is affected by temperature.
  • The rate of carbon dioxide combining with hydrogen to make carbohydrate increases as the temperature increases.

Limiting Factors

  • A limiting factor is something present in the environment in short supply that limits life processes.
  • Increasing light intensity speeds up photosynthesis only to a point; beyond that, further increases have a small effect.
  • All available chloroplasts are fully engaged in light absorption, or there might not be enough carbon dioxide or low temperature is limiting the rate of enzyme reactions. So, even if the light intensity increases more, no more light can be absorbed and used.
  • Raising temperature increases the effect of light intensity to a point where temperature or carbon dioxide concentration limits:
  • Temperature, light intensity, or carbon dioxide concentration can limit the effects of the others. Any one of the external factors – temperature, light intensity or carbon dioxide concentration – can limit the effects of the other two.
  • A rise in temperature can speed up photosynthesis, but only to the point where the light intensity limits further increase.
  • The external factor that limits the effects of the others is called the limiting factor.
  • Artificially high carbon dioxide levels in glasshouses increase yields of crops.
  • Glasshouses and polytunnels optimize light, maintain a higher temperature, and reduce the effect of low temperature as a limiting factor.
  • Tropical countries benefit from optimum temperatures and rainfall.
  • Glasshouses and polytunnels allow growers to control water, nutrients, crop damage by pests, and disease.
  • Sometimes, rainfall is too great to benefit the plants.
  • In an experiment in the Seychelles in the wet season of 1997, tomato crops in an open field yielded 2.9 kg m–2 . In a glasshouse, they yielded 6.5 kg m–2 .
  • The idea of limiting factors applies to other processes as well as photosynthesis. Factors that can limit mineral uptake in roots are
    • Absorbing area of roots
    • Rates of respiration
    • Aeration of the soil
    • Availability of carbohydrates from photosynthesis

Effect of Changing Light Intensity on the Rate of Photosynthesis

Method 1

  • Uses bubble production by a pond plant to calculate the rate of photosynthesis.
  • Saturate water with carbon dioxide by stirring in sodium hydrogen carbonate.
  • Cut one end of the stem of a fresh piece of Canadian pondweed using a scalpel blade.
  • Attach a piece of modelling clay or a paper clip to the stem and put, this will help to stop the stem floating into the beaker.
  • Set up a light source 10 cm away from the beaker and switch on the lamp. Bubbles should start appearing from the cut end of the plant stem. Count the number of bubbles for a fixed time (e.g. 1 minute) and record the result. Repeat the count.
  • To improve the method, another beaker of water could be placed between the bulb and the plant. This filters the heat but allows the plant to receive the light.
  • If the bubbles appear too fast to count, try tapping a pen or pencil on a sheet of paper at the same rate as the bubbles appear. Get your partner to slide the paper slowly along for 15 seconds. Then you can count the dots.

Method 2

  • Uses leaf discs from the land plants.
  • Use a cork borer or paper hole punch to cut out discs from a fresh, healthy leaf like spinach, avoiding any veins. The leaves contain air spaces. These make the leaf discs float when they are placed in water.
  • Dissolve a spatula of sodium hydrogencarbonate in a beaker of water. Remove the discs from the syringe and place them in the beaker.
  • Start a stopwatch and record the time taken for each of the discs to float to the surface. Ignore those that did not sink. Calculate an average time for the discs to float.
  • Repeat the method, varying the light intensity the discs are exposed to in the beaker for varying the light intensity produced by a bench lamp.

The Effect of Changing Carbon Dioxide Concentration on the Rate of Photosynthesis

  • When sodium hydrogen carbonate is dissolved in water, it releases carbon dioxide.
  • To set this up, remove the plunger from the 20 cm3 syringe. Place two or three pieces of pondweed, with freshly cut stems facing upwards, into the syringe barrel. Hold a finger over the end of the capillary tube and fill the syringe with distilled water.
  • Replace the plunger and turn the apparatus upside-down. Push the plunger to the 20 cm3 mark, making sure that no air is trapped.
  • Repeat the procedure using the following concentrations of sodium hydrogencarbonate solution: 0.010, 0.0125, 0.0250, 0.0500 and 0.1000 mol dm–3

What is the Effect of Changing Temperature on the Rate of Photosynthesis?

  • Use the methods described in experiments 6 or 7, but vary the temperature of the water instead of the light intensity.

Leaf Structure

Focus Point

  • What are the main structures in a leaf?
  • How are the leaf features adapted for photosynthesis?
  • A broad-leaved plant has a leaf structure that consist of a leaf stalk (midrib), a network of veins branches and a leaf blade.
  • The leaf blade (or lamina) is broad.
  • It is attached to the stem by a leaf stalk.
  • When this goes into the leaf it is called a midrib. A network of veins branches from the midrib.
Adaptation of Leaves for Photosynthesis
  • Biologists say that structure is well suited to its job. Although there are many types of leaf shape, the following statements apply to most leaves:
  • Their broad, flat shape gives a large surface area for the absorption of sunlight and carbon dioxide.
  • Most leaves are thin so the carbon dioxide only needs to diffuse across short distances to reach the inner cells.

Parts of the Leaf and Their Functions

*   Epidermis
Single layer of cells on the upper and lower surfaces of the leaf.
There is a thin waxy layer called the cuticle over the epidermis.
Helps to keep the leaf’s shape. Helps to reduce evaporation from the leaf and prevent bacteria and fungi from getting in.
*   The cuticle is a waxy layer lying over the epidermis that helps to reduce water loss.
*   Stomata and guard cells
Structures called stomata (singular = stoma) in the leaf epidermis. A part of guard cells forms a stoma that surrounds an opening.
Stomata in the epidermi changes its shape when the pressure of water in them changes.
Scientists do not know exactly how the stomata open and close. However, they do know that in the light, the potassium concentration in the guard cell vacuoles increases.
Guard cells are the only epidermal cells containing chloroplasts.
*   Mesophyll
Tissue between the upper and lower epidermis. The palisade cells are usually long and contain many chloroplasts cells are usually long and contain many chloroplasts in the cytoplasm.
The job of the palisade cells is to make food by photosynthesis.
They also contain chloroplasts.
*   Air Spaces Make it easy for carbon dioxide to diffuse . Water evaporates from surface of cells around them.
*   Vascular Bundles (veins) Veins make a network through the leaf. Vascular bundles bundles are made of two different types of tissues, called xylem and phloem.