IGCSE Biology 4.2 Plant nutrition

Define photosynthesis as the fundamental process by which plants manufacture carbohydrates from raw materials using energy from light.

Plants use water from the soil, carbon dioxide from the air and energy from sunlight to make carbohydrates & oxygen.

Green plants absorb light energy using chlorophyll in their leaves.

They use it to react carbon dioxide with water to make a sugar called glucose.

The glucose is used in respiration, or converted into starch and stored.

Oxygen is produced as a by-product.

Explain that chlorophyll traps light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent storage.

Chloroplasts turn some of the sugar they make into starch, as this is a good way to store it.

Starch is not soluble in water, and must be converted into other substances before it can be translocated.

State the word equation for the production of simple sugars and oxygen.

carbon dioxide + light energy ( in the presence glucose + oxygen

of chlorophyll )

State the balanced equation for photosynthesis in symbols

6CO2 + 6H2O light (chlorophyll ) C6H12O6 + 6O2

Investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.

Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide.

chlorophyllPhotosynthesis occurs in chloroplasts.Chloroplasts are located in the palisade mesophyll tissue of leaves.Chlorophyll is the pigment that absorbs light energy that is used for

photosynthesis.Chloroplasts can move around inside cells. Chloroplasts need a good supply of water, carbon dioxide and sunlight in order to carry out photosynthesis efficiently.

chloroplasts as seen through a microscope

a leaf section as seen through a microscope

chlorophyllA variegated plant is used to test for the relationship between the green

pigment in plants & starch production.

1. A destarched variegated plant was left in the sunlight for four hours.2. A leaf is removed and tested for starch.

The green parts of the leaf contain starch. The white do not.

3. This suggests that chlorophyll is needed for starch production.

light A leaf usually has a large surface area, so that it can absorb a lot of light.

Its top surface is protected from water loss, disease and weather damage by a waxy layer.

The upper part of the leaf is where the light falls, and it contains a type of cell called a palisade cell.

This is adapted to absorb a lot of light.

It has lots of chloroplasts and is shaped like a tall box.

Testing a leaf for starch.If you want to see if starch has been produced in a plant, you have to start with a plant that does not have starch.1. Test the plant for starch.2. Place the plant in the dark for 2 or 3 days.3. Test again for starch.

Using iodine to test for the presence of starch.Iodine solution ( an amber colored liquid ) turns dark ( blue/black ) in the presence of starch.If leaves are to be tested for the presence of starch, the cell walls in the

leaf must be broken, and the chlorophyll removed.The cell walls must be broken because to make it easier for substances to leave or enter the cell.The chlorophyll must be removed so that the color change in the iodine can be

seen.

light Showing how light is needed to make starch.1. Two leaves of a de-starched plant are to be tested for the presence of

starch.2. One is completely covered with aluminum foil, the other with clear plastic wrap.3. The plant is put in the sun for 4 hours, and both leaves are tested for

starch.The leaf in the plastic contains starch.The leaf in the foil does not.

This suggests that light is needed for the production of starch.carbon dioxideSometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesize if there is

insufficient carbon dioxide.

Investigate and state the effect of varying light intensity on the rate of photosynthesis (e.g. in submerged aquatic plants).Increasing the light intensity will boost the speed of photosynthesis.

However, once the light intensity has reached a certain level, giving the plant more light has no effect. The extra light cannot be used for

photosynthesis because the plant is photosynthesizing as fast as it can.Describe the intake of carbon dioxide and water by plants.intake of carbon dioxidePlants get carbon dioxide from the air through their leaves. Only about 0.04% of the atmosphere consists of carbon dioxideCarbon dioxide molecules diffuse through the stomata, through the air spaces between the spongy mesophyll cells, through the cell walls, cell membranes and cytoplasm of the spongy and palisade cells and into the chloroplasts.

The carbon dioxide diffuses through small holes in the underside of the leaf called stomata. (One of these holes is called a stoma. The plural is stomata.)

The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to reach the other cells in the leaf.

This also allow the oxygen produced in photosynthesis to leave the leaf easily.

intake of water

Plants need water: to help keep cells turgid, so that soft parts of the plant ( petals &

leaves ) are held in shape. to be used as a solvent; substances need to be dissolved in water

inside cells so that metabolic reactions can take place. for photosynthesis

Plants get their water from the soil.Water is taken up by root hairs and moves across the root into the xylem.In the xylem it flows up through the root and stem into the leaves.Inside the leaf, the water moves out of the xylem vessels, and some of it goes into the chloroplasts in the mesophyll cells.Most of the water turns to water vapor and evaporates.The water passes through the leaf and out the stomata in a process called transpiration.The movement of water along this pathway ( from soil to the atmosphere ) is called the transpiration stream.

root hairs

Plants get the water they need for photosynthesis through their roots.

The roots have a type of cell called a root hair cell - these project out from the root into the soil. Roots have a big surface area and thin walls,

which allow water to pass into them easily by osmosis. Corn root with root hairs for uptake of

water & nutrients from the soil. Note that root cells do not contain chloroplasts, as they are normally in the dark and cannot photosynthesize.

The water absorbed by the root hair cells passes through the plant in xylem tubes, and eventually reaches the leaves. If a plant does not absorb enough water, it will wilt or go floppy. Without water it may also not photosynthesise quickly enough, and it may die.

Turgid plant that's watered regularly & a flaccid plant without enough water.Identify and label the cuticle, cellular and tissue structure of a

dicotyledonous leaf, as seen in cross-section under the light microscope and describe the significance of the features of a leaf in terms of functions, to include:

• distribution of chloroplasts – photosynthesis,• stomata and mesophyll cells – gas exchange• vascular bundles (xylem and phloem) – transport and

support.

name of tissue

how these structures help it perform its function more efficiently function

palisade

mesophyll

packed with chloroplasts ( contain chlorophyll);

regular shaped, closely packed cells form continuous layer for efficient

absorption of sunlight

photosynthesis

spongy mesophyll cells

irregular shaped, loosely arranged cells provides large surface areafor efficient gas exchange

gas exchange

vascular

bundles

phloem – live cellsxylem – dead cells

arranged end-to-endhollow

walls contain ligninno cytoplasm, cell membrane, nucleus

phloem –transport glucose

xylem –transport of

water,

dissolved minerals;support

Describe the importance of:• nitrate ions for protein synthesis,• magnesium ions for chlorophyll synthesis.

vascular bundle

Explain the effects of nitrate ion and magnesium ion deficiency on plant growth.

Plants need to take in a number of elements to stay alive.

The most important are: carbon hydrogen oxygen

Plants get hydrogen and oxygen from water in the soil, and carbon and oxygen from carbon dioxide and oxygen in the atmosphere.

Water and carbon dioxide are used to synthesize food during photosynthesis.

Oxygen is used to release energy from food during respiration.

In addition to these three elements, plants need a number of minerals for healthy growth.

These are absorbed through the roots as mineral ions dissolved in the soil water.

Two important mineral ions needed by plants are: nitrate - for making amino acids, which are needed to make

proteins magnesium - for making chlorophyll

If a plant does not get enough minerals, its growth will be poor. It will suffer from deficiency symptoms:deficient in nitrate - it will suffer from stunted growthdeficient in magnesium - it's leaves will turn yellow

TOMATO PLANT

Nitrogen deficiency

TOMATO PLANT

Magnesium deficiency

Plants and minerals

Plants use minerals from the soil to build the complex molecules they need to survive and grow.

Poor plant growth may be due to a deficiency of one or more minerals.

Tests can be done to establish which mineral is deficient or a general purpose NPK fertilizer added.

Plants need to build the complex molecules they need from minerals (elements and simple compounds), which they get from the soil.

Animals on the other hand get their complex molecules ready made from their food – they need very few minerals to supplement their diet.If a plant shows poor growth it may be due to a lack of one or more minerals in the soil. It's possible to do chemical tests to find out which minerals are deficient, but it's often more convenient to add a general fertiliser.

This is sometimes called NPK fertiliser because it contains the elements nitrogen (N), phosphorus (P) and potassium (K).

These elements, plus magnesium (Mg), are the minerals that plants need in the greatest quantities.

Uses of different minerals

Mineral Use Deficiency symptoms

Explanation

Nitrate ions (NO3)

Building proteins and growth

Poor growth and yellow leaves

All amino acids contain nitrogen. Amino acids are the building blocks of protein.

Phosphate ions (PO4)

Respiration and growth

Poor root growth and discolored leaves

A component of DNA molecules and cell membranes

Potassium ions (K+)

Respiration and photosynthesis

Poor flower and fruit growth, discolored leaves

Must be present for photosynthesis and respiration enzymes to work

Magnesium ions (Mg2+)

Photosynthesis Yellow leaves Chlorophyll molecules contain magnesium ions. It's the magnesium that makes chlorophyll green.

How minerals are absorbedThe concentration of minerals in the soil is very low.

They dissolve in water and move around the soil in solution.

Root hair cells are adapted to absorb the water out of soil by osmosis – they have a large surface area, thin walls and are close to the xylem cells used for transporting water up the plant.

Minerals cannot be absorbed by osmosis (which is the movement of water only) or diffusion (because the minerals are in very low concentration).

The root hair cells have carrier molecules on their surface that pick up the minerals and move them into the cell against the concentration gradient.

This requires energy, and is called active transport.

Describe the uses, & the dangers of overuse, of nitrogen-containing fertilizers.

As crops grow they remove nutrients, such as compounds of nitrogen, potassium and phosphorus, from the soil.

These crops are harvested & taken away from the fields where they were grown. If this process is repeated for a number of years the supply of nutrients in the soil is exhausted. The land becomes less fertile, so plants grow poorly & produce only a small yield.Farmers use fertilizers to put back into the soil the nutrients that have been removed by growing crops.Fertilizers are chemicals used by farmers to help their crops grow well. Rain can wash fertilizers off the fields and into rivers. This causes water plants to overgrow and block out the light.Other plants die as a result. They use up oxygen as they rot away, and fish and other animals suffocate. This process is called eutrophication.