World of Plants

Making Food

G Davidson

Food Webs and Plants• Animals get their food by eating other

living things.• They either eat plants, or eat animals which

have eaten plants.• Plants have to make their own food.• When energy in the form of food passes

from a plant to an animal and then on to another animal, it forms a food chain.

• When we link up all the food chains in an environment we form a food web, e.g.

Woodland webHAWK

FOX

BLACKBIRDS

BLUETIT

INSECTS

FRUITS & SEEDS GRASS TREE BARK ROOTS

RABBITS

SQUIRREL

WOODLICEBEETLES

If some of the plants in this web were to disappear it would have a major effect on other organisms.

Plants and Light• Plants manufacture their own food in a

process called PHOTOSYNTHESIS.

• When a plant carries out photosynthesis it combines simple substances to make more complex substances like glucose and starch.

• Photosynthesis is carried out in the leaves of a plant.

PHOTO SYNTHESIS

(Light) (To build)

PhotosynthesisPhotosynthesis

CarbonDioxide

+ Water Glucose + Oxygen

CO2 H2O C6H12O6 O2

Light energy

Raw Materials Products

Photosynthesis

Photosynthesis CANNOT work without:

• Carbon dioxide

• Water

• Chlorophyll

• sunlight

If any of the above are in short supply they are said to be “Limiting Factors”.

Photosynthesis

We can check for evidence of photosynthesis by testing a leaf for starch.

1. Boil the leaf in water (kills cells)

2. Boil the leaf in alcohol (removes chlorophyll)

3. Dip leaf in hot water (softens the leaf)

4. Put leaf in iodine solution (test for starch)

PhotosynthesisSo what happens to the sugar made in the leaves?

It can be used in a number of ways

SugarSugarUsed as buildingMaterial (e.g. cellulosein cell walls

Used immediatelyFor energy

Stored as starch And Converted back when Sugar is needed byPlant.

Photosynthesis

If a plant is converting carbon dioxide and water to carbohydrate, then it’s weight should increase.

If a plant is dried out and weighed, it’s weight increases during a day.

This is done by taking discs from a leaf at different times of the day, drying them and weighing them.

Leaf Surfaces

• Leaves take in carbon dioxide from the air.• In exchange they give out oxygen produced

during photosynthesis.• These gases must be able to get in and out

of a leaf.• Leaves have special structures which allow

gas exchange.• These structures are called STOMATASTOMATA.

Stomata

• Tiny pores on the surface of a leaf which allow gases in and out.

• They also allow water to escape.

• Each pore is a STOMA and is surrounded by 2 GUARD CELLS.

Stomata

Leaf Surfaces

• A leaf is very thin and flat.• We know it has stomata on it’s surface.• Since it’s job is photosynthesis, there must

be reasons for it’s design.• Inside the leaf there are a number of layers,

all with different jobs.• However, they all help to make

photosynthesis as quick and efficient as possible.

Leaf Surface

Leaf Surface

Water Transport in Plants

• The transport system of a plant does 2 jobs.

• One is to carry materials needed for photosynthesis.

• The other is to carry materials produced by photosynthesis.

Water Transport in Plants

• Water and minerals have to be carried from the roots to the leaves.

• In the leaves water is used in photosynthesis.

• Tubes, in the stem, called XYLEM vessels carry the water and minerals.

• If a plant is cut and placed in red dye, it is easy to see where the xylem vessels are.

Transport in Plants• The other job of the plant’s transport system is to

carry food from the leaves to every part of the plant.

• Tubes called the PHLOEM carry the food.

• The xylem and phloem are usually found close together in bundles called VASCULAR BUNDLES.

• The position of the vascular bundles in roots and stems is not the same.

• In most plants the xylem and phloem are found in the centre of the root, whereas in a stem they are found round the outside.

Transport in Plants

Epidermis

Cortex

Xylem

Phloem

Root – Cross SectionRoot – Cross Section

Transport in PlantsStem – Cross sectionStem – Cross section

Phloem

Xylem

Epidermis

Structure of the Xylem• Xylem cells are dead and have no cell contents.

• The end walls have disintegrated

• The side walls have become inlaid with a tough waterproof substance called LIGNINLIGNIN

• Lignin is arranged in the side walls usually as rings or spirals.

• A number of these cells laid end to end forms a rigid pipe.

• This adds considerable support to the plant

Structure of Xylem

In this case the In this case the lignin is spirallignin is spiral

Structure of the Phloem

• Phloem contains 2 types of cell• Sieve tubes• Companion cells• In sieve tubes the end walls are perforated

(sieve plates)• Strands of cytoplasm pass through the pores

and allow contact with the next cell.• The companion cells lie next to the sieve

tubes.

Structure of the Phloem

Elodea Bubbler• When green plants photosynthesise they produce

oxygen.

• In aquatic plants, this oxygen is given off as bubbles which are easily seen.

• The Canadian pondweed, Elodea, has large spaces in its stem.

• When the stem is cut, and the plant is placed in strong light, bubbles of oxygen can be seen coming from the cut end.

• The number of bubbles produced every minute, i.e. the rate, gives us an idea of how fast the plant is photosynthesising.

Elodea bubbler

Elodea Bubbler

Light Intensity No. of Bubbles / 3 minutes

10 0

20 6

30 15

40 26

50 35

60 38

70 38

Typical Set of Results

Elodea Bubbler

Limiting Factors

• Photosynthesis does not just depend on light, carbon dioxide and a suitable temperature, but also the amounts of each available.

• If they are in short supply, they cut down or LIMIT the rate of photosynthesis.

• Light, water, carbon dioxide and temperature can act as “Limiting Factors”.“Limiting Factors”.