Plants and food
Plants make starch
● all plants contain starch
● it is the main storage carbohydrate made by plants
● starch is a good way of storing carbohydrate because it is insoluble, compact and can be broken down easily
● starch is only made in the parts of plants that contain chlorophyll
● can use variegated leaves to test
practical: testing leaves for starch
● test for starch in food by adding a few drops of yellow-brown iodine solution
● if food contains starch, a blue-black colour will be produced
● leaves that have been in sunlight also contain starch
● shouldn’t directly add iodine solution -> the outer waxy surface of the leaf will not absorb the solution & the
green colour of the leaf will hide the colour change
● to test for starch in a leaf -> the outer waxy layer needs to be removed and the leaf decolourised
● this is done by placing the leaf in boiling ethanol
1. set up a beaker of water on a tripod and gauze -> heat until it is near 100°C
2. remove a leaf from a plant and kill it by placing it in boiling water for 30 seconds (stops all the chemical reactions
in the leaf)
3. turn off the Bunsen burner (important because ethanol is highly flammable)
4. place the leaf in a boiling tube containing ethanol -> place the tube inside the beaker of boiling water
5. boiling point of ethanol is lower than that of water -> ethanol will boil for a few minutes, until the water cools
down -> long enough to remove most of the chlorophyll from the leaf
6. when the leaf has turned colourless/ pale yellow -> remove it and wash in cold water to soften it
7. spread out on a tile and cover it with a few drops of iodine solution
8. after a few minutes, any parts of the leaf that contain starch will turn a dark blue-black colour
**only works if the plant has had plenty of light for some hours before the test
**safety precaution: wash your hands after handling the leaves, do not splash boiling water, do not heat the
ethanol directly with any flame, iodine solution will badly stain everything including skin
de-starch
● place it in the dark for 2 or 3 days
● plant uses up the starch stores in its leaves
● de-starched plants are used to find out the conditions needed for the plant to make more starch by
photosynthesis
● placing a plant in a closed container containing soda lime also prevent a plant from making starch
● soda lime absorbs carbon dioxide from the air around the plant
● if the plant is kept under a bright light, but with no carbon dioxide -> it again won’t be able to make starch
where does the starch come from
● three important factors about starch production
- it uses carbon dioxide from the air
- it needs light
- it needs chlorophyll in the leaves
● apart from starch, oxygen gas is also made
● when placing a piece of aquatic plant elodea in a test tube of water under a bright light -> a stream of small
bubbles are collected
● after analysing their contents -> they are found to contain a high concentration of oxygen
● starch is composed of long chains of glucose
● plant does not make starch directly -> first produces glucose -> then joined together in chains to form starch
molecules (polysaccharide)
, ● glucose has the formula C6H12O6
● carbon and oxygen comes from carbon dioxide in the air around the plant
● hydrogen atoms come from another molecule essential to the living plant, water
Photosynthesis
● carbon dioxide + water -> (light) glucose + oxygen
6CO2 + 6H2O -> (chlorophyll) C6H12O6 + 6O2
● the role of the green pigment, chlorophyll, is to absorb the light energy needed for the reaction to take place
● respiration releases energy from the breakdown of glucose
● chemical energy in the glucose came originally from the light ‘trapped’ by the process of photosynthesis
structure of leaves
● leaves are the plant organs that are best adapted for this function
● to able to photosynthesise efficiently -> leaves need to have
- a large surface area to absorb light
- many chloroplasts containing chlorophyll
- a supply of water and carbon dioxide
- a system for carrying away the products of photosynthesis to other parts of the plant
● they also need to release oxygen and water vapour from the leaf cells
● most leaves are thin, flat structures supported by a leaf stalk which can grow to allow the blade of the leaf to be
angled to receive the maximum amount of sunlight
● the outer layer of cells (upper epidermis) have few chloroplasts
● they are covered by a thin layer of a waxy material: cuticle -> reduces water loss by evaporation, acts as a
barrier to the entry of disease-causing organisms (e.g. bacteria and fungi)
● the lower epidermis has many holes/ pores (stomata - a single pore is a stoma)
● the stomata allow carbon dioxide to diffuse into the leaf, to reach the photosynthetic tissues
● allow oxygen and water vapour to diffuse out
● each stoma is formed as a gap between two highly specialised cells (guard cells) -> can change their shape to
open/ close the stoma
● in the middle are two layers of photosynthetic cells: mesophyll
● just below the upper epidermis: palisade mesophyll
● tissue made of long, narrow cells, each containing hundreds of chloroplasts -> main site of photosynthesis
● palisade cells are close to the source of light and the upper epidermis is relatively transparent -> allow light to
pass through to the enormous numbers of chloroplasts that lie below
● below palisade layer is a tissue made of more rounded, loosely packed cells
● there are air spaces between them (spongy mesophyll)
● they also photosynthesise, but have fewer chloroplasts than the palisade cells
● they form the main gas exchange surface of the leaf
● absorbs carbon dioxide and releases oxygen/ water vapour
● the air spaces allow these gases to diffuse in and out of the mesophyll
● water and mineral ions supplied to the leaf by vessels in a tissue called the xylem
● forms a continuous transport system throughout the plant
● water is absorbed by the roots -> passes up through the stem and through veins in the leaves in the
transpiration stream
● in the leaves, the water leaves the xylem and supplies the mesophyll cells
● products of photosynthesis e.g. sugars are carried away from the mesophyll cells by another transport system:
phloem
● phloem supplies all other parts of the plants -> tissues and organs that can’t make their own food receive
products of photosynthesis
● the veins in the leaf contain both xylem and phloem tissue -> branch again and again to supply all parts of the
leaf
Plants make starch
● all plants contain starch
● it is the main storage carbohydrate made by plants
● starch is a good way of storing carbohydrate because it is insoluble, compact and can be broken down easily
● starch is only made in the parts of plants that contain chlorophyll
● can use variegated leaves to test
practical: testing leaves for starch
● test for starch in food by adding a few drops of yellow-brown iodine solution
● if food contains starch, a blue-black colour will be produced
● leaves that have been in sunlight also contain starch
● shouldn’t directly add iodine solution -> the outer waxy surface of the leaf will not absorb the solution & the
green colour of the leaf will hide the colour change
● to test for starch in a leaf -> the outer waxy layer needs to be removed and the leaf decolourised
● this is done by placing the leaf in boiling ethanol
1. set up a beaker of water on a tripod and gauze -> heat until it is near 100°C
2. remove a leaf from a plant and kill it by placing it in boiling water for 30 seconds (stops all the chemical reactions
in the leaf)
3. turn off the Bunsen burner (important because ethanol is highly flammable)
4. place the leaf in a boiling tube containing ethanol -> place the tube inside the beaker of boiling water
5. boiling point of ethanol is lower than that of water -> ethanol will boil for a few minutes, until the water cools
down -> long enough to remove most of the chlorophyll from the leaf
6. when the leaf has turned colourless/ pale yellow -> remove it and wash in cold water to soften it
7. spread out on a tile and cover it with a few drops of iodine solution
8. after a few minutes, any parts of the leaf that contain starch will turn a dark blue-black colour
**only works if the plant has had plenty of light for some hours before the test
**safety precaution: wash your hands after handling the leaves, do not splash boiling water, do not heat the
ethanol directly with any flame, iodine solution will badly stain everything including skin
de-starch
● place it in the dark for 2 or 3 days
● plant uses up the starch stores in its leaves
● de-starched plants are used to find out the conditions needed for the plant to make more starch by
photosynthesis
● placing a plant in a closed container containing soda lime also prevent a plant from making starch
● soda lime absorbs carbon dioxide from the air around the plant
● if the plant is kept under a bright light, but with no carbon dioxide -> it again won’t be able to make starch
where does the starch come from
● three important factors about starch production
- it uses carbon dioxide from the air
- it needs light
- it needs chlorophyll in the leaves
● apart from starch, oxygen gas is also made
● when placing a piece of aquatic plant elodea in a test tube of water under a bright light -> a stream of small
bubbles are collected
● after analysing their contents -> they are found to contain a high concentration of oxygen
● starch is composed of long chains of glucose
● plant does not make starch directly -> first produces glucose -> then joined together in chains to form starch
molecules (polysaccharide)
, ● glucose has the formula C6H12O6
● carbon and oxygen comes from carbon dioxide in the air around the plant
● hydrogen atoms come from another molecule essential to the living plant, water
Photosynthesis
● carbon dioxide + water -> (light) glucose + oxygen
6CO2 + 6H2O -> (chlorophyll) C6H12O6 + 6O2
● the role of the green pigment, chlorophyll, is to absorb the light energy needed for the reaction to take place
● respiration releases energy from the breakdown of glucose
● chemical energy in the glucose came originally from the light ‘trapped’ by the process of photosynthesis
structure of leaves
● leaves are the plant organs that are best adapted for this function
● to able to photosynthesise efficiently -> leaves need to have
- a large surface area to absorb light
- many chloroplasts containing chlorophyll
- a supply of water and carbon dioxide
- a system for carrying away the products of photosynthesis to other parts of the plant
● they also need to release oxygen and water vapour from the leaf cells
● most leaves are thin, flat structures supported by a leaf stalk which can grow to allow the blade of the leaf to be
angled to receive the maximum amount of sunlight
● the outer layer of cells (upper epidermis) have few chloroplasts
● they are covered by a thin layer of a waxy material: cuticle -> reduces water loss by evaporation, acts as a
barrier to the entry of disease-causing organisms (e.g. bacteria and fungi)
● the lower epidermis has many holes/ pores (stomata - a single pore is a stoma)
● the stomata allow carbon dioxide to diffuse into the leaf, to reach the photosynthetic tissues
● allow oxygen and water vapour to diffuse out
● each stoma is formed as a gap between two highly specialised cells (guard cells) -> can change their shape to
open/ close the stoma
● in the middle are two layers of photosynthetic cells: mesophyll
● just below the upper epidermis: palisade mesophyll
● tissue made of long, narrow cells, each containing hundreds of chloroplasts -> main site of photosynthesis
● palisade cells are close to the source of light and the upper epidermis is relatively transparent -> allow light to
pass through to the enormous numbers of chloroplasts that lie below
● below palisade layer is a tissue made of more rounded, loosely packed cells
● there are air spaces between them (spongy mesophyll)
● they also photosynthesise, but have fewer chloroplasts than the palisade cells
● they form the main gas exchange surface of the leaf
● absorbs carbon dioxide and releases oxygen/ water vapour
● the air spaces allow these gases to diffuse in and out of the mesophyll
● water and mineral ions supplied to the leaf by vessels in a tissue called the xylem
● forms a continuous transport system throughout the plant
● water is absorbed by the roots -> passes up through the stem and through veins in the leaves in the
transpiration stream
● in the leaves, the water leaves the xylem and supplies the mesophyll cells
● products of photosynthesis e.g. sugars are carried away from the mesophyll cells by another transport system:
phloem
● phloem supplies all other parts of the plants -> tissues and organs that can’t make their own food receive
products of photosynthesis
● the veins in the leaf contain both xylem and phloem tissue -> branch again and again to supply all parts of the
leaf
