Channel 4 Learning


Green Plants

Background Information

The Diversity of Plants

The natural world includes a phenomenal variety of green plants. All of these organisms have the ability to make complex carbohydrates from simple compounds of carbon dioxide and water. To do this they need light, energy and the essential catalyst, chlorophyll. Oxygen gas is produced as a waste product. Photosynthesis has gradually transformed the Earth’s atmosphere from a mixture of water vapour, carbon dioxide, hydrogen sulphide, methane and nitrogen to the present situation where 21% of the atmosphere is oxygen.

The adaptations that plants have made to environmental conditions have influenced the development of the ecosystems found on the Earth today. Animals are fundamentally dependent on plants, not only for food but also for oxygen, shelter, camouflage and perching sites. The plant community becomes a vital part of the habitat for the animal community. Ecosystems, such as the Tropical Rainforest, can be classified according to their dominant types of vegetation.

More and more people are becoming aware of the importance of preserving the diversity of plant species. As well as the aesthetic argument for preserving biodiversity, there is also the argument that plants by their very diversity present us with a genetic library with information about millions of potentially useful chemical compounds.

History of Ideas about Photosynthesis

In the eighteenth century, significant developments were made in our understanding of the atmosphere and the way in which living things both depend upon and modify it. At this time many amateur scientists believed that the bulk of plant material originated in soil.

Jan Baptista van Helmont (1579-1644) carried out a famous experiment by growing a willow tree in a pot for five years. At the end of this period the tree had increased in mass by 74kg but the mass of the soil had changed little. Van Helmont believed that water was the source of the extra mass.

Joseph Priestley performed another celebrated experiment in 1772 when he kept a mouse in a jar of air until it collapsed. He found that a mouse kept with a plant would survive. Together with Antoine Lavoisier he demonstrated the importance of oxygen as the gas in air vital for animal life and for burning.

Jan Ingenhousz (1730-1799) took Priestley’s work further and demonstrated that it was light, rather than heat, that plants needed to make oxygen. Ingenhousz was mistaken in believing that the oxygen made by plants came from carbon dioxide.

The modern understanding of photosynthesis recognises two stages, the light and the dark stage. In 1937 Robert Hill began the work which demonstrated that chlorophyll can split water molecules releasing oxygen as a by-product. This process requires light. In the 1950s Melvin Calvin followed a complex series of reactions which convert carbon dioxide to glucose. Light is not needed during this stage of photosynthesis.

Getting Plants to Grow Fast

Plants do not always photosynthesise as quickly as they can. In the early morning the rate of photosynthesis will be limited by the amount of light available. In this situation light is known as the limiting factor. On a bright day the rate may reach a maximum before midday and even as the day gets brighter no improvement in the rate of photosynthesis occurs. In this situation something else is the limiting factor:

  • carbon dioxide concentration
  • temperature

Commercial growers need to manipulate the environmental conditions in their glasshouses to achieve the most economically productive results. In Britain this means increasing the temperature. An increase in temperature of 10oC will double the rate of photosynthesis. At temperatures above 40oC the rate begins to decrease because enzymes are denatured.

If a boiler that burns fossil fuels is used to heat the greenhouse, carbon dioxide from the exhaust gases can be added to the atmosphere. If plants have enough light, increasing carbon dioxide concentration can cause significant improvements in the rate of photosynthesis.

Water is not normally regarded as a limiting factor because a lack of water will have such devastating effects on a plant. However, a good supply of water is essential for healthy growth and humid air reduces water loss from the leaves.

Using the Products of Photosynthesis

Some of the glucose formed in photosynthesis is used as a source of energy. Plant cells can break down the glucose by respiration, just like animal cells. Most of the glucose is stored for later use but it cannot be stored in the form of glucose. If it were stored as glucose, osmosis would affect the water content of the cell.

The simplest solution to this problem is to store the glucose as starch. Starch is a polymer formed by linking glucose molecules end to end. Starch is insoluble and can easily be stored as grains in the cytoplasm. A small amount of potato stained with iodine reveals these grains when viewed under a microscope.

Another important polymer made from glucose is cellulose. This is a fibrous polymer that forms a network of fibres in the cell wall. It gives the plant structural strength and is the dietary fibre so important to many animals. Cellulose is very difficult to digest. Ruminants, such as cows, use symbiotic bacteria to help them digest grass.

The glucose can also be converted into a range of oils and protein molecules as well as other sugars such as fructose (in fruits) and sucrose (in sugar cane).

The Structure of a Leaf

The shape of a leaf is often determined by the need to absorb light:

  • large surface area and thin blade means that most of the leaf cells are exposed to light
  • leaves are arranged so that they do not overlap or leave space in between them
  • where plants have to conserve moisture, leaf shape may be very different, e.g. cactus spikes

All leaves have the same general structure made up of layers:

Layer

Description

Function

waxy cuticle

waterproof layer of wax which makes the leaf shiny

prevents moulds growing on the leaf surface and water vapour from escaping

upper epidermis

thin, flat cells

protective upper layer

palisade layer

tall thin cells, tightly packed with a lot of chloroplasts which make the upper surface of the leaf dark green

most photosynthesis takes place here

spongy layer

rounded cells with air space in between and few chloroplasts

oxygen and carbon dioxide can diffuse through the air spaces

lower epidermis

thin, flat cells and guard cells surrounding stomata (pores)

allow oxygen and carbon dioxide to diffuse in and out of the leaf

Leaf Adaptations

Leaves vary in colour, size and shape depending on the environment in which the plant is found.

Where light is scarce:

  • Leaves have a large area to collect as much light as possible
  • Leaves are dark green, sometimes with a range of colours, to collect the remaining light that has filtered through other leaves
  • The arrangement of leaves on the stem avoids overlapping and covers the largest possible area

Where the light is bright:

  • Leaves can be reduced in size to prevent water loss

Where water is in short supply:

  • Leaves may be fat with a thick waxy cuticle to prevent water loss
  • Leaves can be reduced to from spines with very little surface for water loss

Where water is plentiful:

  • Pointed leaves with deep grooves down the midrib allow water to run off
  • The cuticle is very waxy to prevent moulds growing into the leaf
  • Leaves can be very broad

Leaves defend themselves from animals in a number of ways. Some leaves have thick cuticles and tough fibrous cells to make them difficult to eat. Many plants use a range of chemical poisons to deter both mammal and insect grazers. Bracken produces an acid that can cause stomach cancer as well as insect moulting hormones that disrupt the life cycle of any insect that eats it.

Transporting Water

Water gets into the roots by the process of osmosis. The root hairs found near the tips of roots increase the surface area over which osmosis can take place.

Vital mineral salts are absorbed from the soil water at the same time. These will allow the plant to make a wide range of compounds from the glucose produced during photosynthesis, e.g. nitrates are used to make protein.

From the root, water and minerals move up the stem in vessels called xylem. These are continuous tubes of dead cells joined like sections of drainpipe. Branches of the xylem are found in the midrib * and veins of every leaf. The xylem vessels have holes in their side walls so that water can pass out into the leaf.

* midrib: the main vein of a leaf, running down the centre of the blade.


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