Life On Earth – The Main Source Of Energy The ultimate source of nearly all energy for life on Earth is the Sun. The energy that powers our muscles, allows birds to fly, and drives the many chemical reactions that are characteristic of living cells, originates from the Sun. The Nature of Photosynthesis The light energy from the sun must first be transformed into chemical energy before it can be used by living organisms. ENERGY FROM THE SUN The process that transforms light energy into chemical energy and uses this to manufacture organic food is called photosynthesis. Autotrophs and Heterotrophs The living organisms that inhabit our planet can be classed as either autotrophs or heterotrophs. Autotrophs are organisms that are capable of synthesising their own complex organic food molecules from simpler inorganic ones. The term autotroph means “self-feeding” and organisms in this group make use of an external, non-living supply of energy to drive their “self-feeding” way of life. The vast majority of autotrophs harness the energy of sunlight to manufacture their own food during the process of photosynthesis. These are the photoautotrophs ENERGY FROM THE SUN Photoautotrophs (1) Photoautotrophs capture the sun’s energy and use it to convert simple inorganic molecules, such as carbon dioxide and water, into complex, energy-rich organic food. The various species of green plants in this woodland are photoautotrophs and manufacture their own food by the process of photosynthesis: 6CO2 + 6H2O Carbon dioxide + Water Light Light Energy C6H12O6 + 6O2 Glucose + Oxygen Energy Photoautotrophs (2) There is a huge diversity of photoautotrophs in the many ecosystems. Diatoms – microscopic algae Giant Redwoods of California The various species of photoautotrophs range in size from the Giant Redwoods of California to the microscopic algae (Diatoms) that are the major producers of food for marine animals. Chemoautotrophs (1) Light is not the only source of energy utilised by autotrophs. Various species of bacteria have evolved mechanisms for manufacturing their own food through utilising the energy contained in certain inorganic molecules. These are the chemoautotrophs ENERGY FROM inorganic molecules Chemoautotrophic bacteria obtain the energy they need for food manufacture by oxidising inorganic molecules such as ammonia and hydrogen sulphide. Chemoautotrophs (2) Species of chemoautotrophic bacteria known as the nitrifying bacteria play an essential part in the nitrogen cycle. Nitrifying bacteria oxidise ammonium and nitrite ions to nitrates. NH4+ NO2 energy The energy released from these oxidation reactions is used by the bacteria to manufacture their own food. NO3 energy The nitrates are absorbed by green plants and the nitrogen is incorporated into nitrogen-containing organic compounds. Chemoautotrophs (3) In habitats devoid of light such as caves and deep ocean beds, these archaean and bacterial organisms are the primary producers that supply the energy that supports an entire community of organisms. A dark cave - chemosynthetic bacteria supply energy to support a community of organisms. The deep ocean bed - devoid of light. Chemoautotrophs - Extremophiles Most of the Archaea are tolerant to extreme environmental conditions, i.e. they are extremophiles. Halophiles (thrive in high levels of salt), piezophiles (thrive under high pressure), thermophiles (thrive in high temperatures) and acidophiles (thrive in acidic conditions), etc. In the deepest parts of the oceans (no sunlight), where tectonic plates meet, there may be cracks in the ocean floor into which cold sea water seeps. The water is super-heated, mixes with sulfur compounds and may be released via hydrothermal vents. Chemosynthetic bacteria and Archaea convert the heat, methane, and sulfur compounds (particularly hydrogen sulfide) into energy by chemosynthesis. ‘Black smoker’ Chemoautotrophs – Supporting An Ecosystem AN OCEAN BED FOOD CHAIN IN THE PACIFIC OCEAN H2S (Hydrogen Sulphide from the earth’s core) Archaea and Chemosynthetic bacteria (primary producers) barnacles clams anemones mussels small worms Eaten by large crabs and fish All of the living things are extremophiles as they have to thrive in high temperatures (thermophiles) and high pressures (piezophiles). Chemoautotrophs - Thermophiles The optimum growth temperature range for extreme thermophiles is between 80ºC and 115ºC. Archaea produce the bright colours in the hot springs in Yellowstone Park USA. Some Archaea can fix nitrogen at 90°C. A strain of Methanopyrus kandleri can even reproduce at 122°C (252°F) - the highest recorded temperature tolerated by any living thing. Heterotrophs (1) The majority of living species are unable to carry out either photosynthesis or chemosynthesis. These are the heterotrophs Energy from already manufactured organic materials Heterotrophs must consume already manufactured organic materials as a source of energy for their life activities. Heterotrophs are ultimately dependent upon the autotrophs for their supply of organic food. Energy is transferred from autotrophs to heterotrophs through the food chain. Heterotrophs (2) Caterpillars obtain organic food material from the products of photosynthesis locked up in photosynthesising plants. Caterpillars are a source of food for small birds, which are in turn eaten by larger birds and other carnivores. The energy locked up in the organic material of this autotrophic producer is passed along the food chain. General Structure of a Leaf (Dicotyledon) The palisade layer of cells located close to the upper epidermis are the principal photosynthetic cells of the leaf. A section through the leaf from upper to lower surface reveals a characteristic arrangement of tissues. Palisade cells are packed with chloroplasts. A single chloroplast (the site of photosynthesis). The Chloroplast (1) The chloroplast, the site of photosynthesis, is surrounded by an envelope of two membranes and contains a jelly-like matrix called the stroma. Envelope Stroma The Chloroplast (2) Many of the sugar molecules formed during photosynthesis are stored as starch and, starch grains can be found growing close to the grana. Envelope Stroma Thylakoids Circular DNA molecule Starch Grain Lipid droplet Ribosomes Many of the thylakoids are stacked to form grana. A single granum The stroma also contains a circular DNA molecule, numerous ribosomes and lipid droplets. The Chloroplast (3) The photomicrograph (false colour) below details chloroplast structure as viewed with a transmission electron microscope (TEM). Chloroplast envelope Stroma containing numerous small ribosomes Lipid droplet Starch grain A single granum Inter granal thylakoid (connects different grana) Leaf Structure This typical dicotyledonous sycamore leaf is the major photosynthetic organ for the sycamore tree. Dicotyledonous leaves are structurally adapted for their photosynthetic role. THE MAJOR PHOTOSYNTHETIC ORGAN OF GREEN PLANTS How are Leaves Adapted for Photosynthesis? Most leaves are wide and flat which gives them a large surface area for the absorption of light energy and gaseous exchange. Most leaves are also thin, which allows light to penetrate right through them. Leaves have a dense network of veins containing xylem vessels which carry the water needed for photosynthesis up from the roots. lamina The midrib and stalk of a leaf connect its xylem and phloem vessels with those in the stem. Dicotyledon Leaf - Internal Structure (1) waxy cuticle upper epidermis Layer of palisade mesophyll cells – main photosynthetic layer. xylem vessel Spongy mesophyll layer layer of irregular shaped cells with numerous air spaces. substomatal air space lower epidermis stoma guard cell thin cuticle Dicotyledon Leaf - Internal Structure (2) Drag each label to the correct position on the diagram to produce a correctly labelled section of a leaf... Click here to view the animation… Dicotyledon Leaf - Internal Structure (3) palisade cells upper epidermis xylem stoma and guard cells spongy mesophyll cells phloem intercellular air spaces parenchyma (packing tissue) lower epidermis Section of midrib area of a typical dicotyledonous leaf. Dicotyledon Leaf - Internal Structure (4) leaf vein palisade cells upper epidermis guard cell spongy stoma mesophyll cells Section of intercellular air spaces lower epidermis lamina area of a typical dicotyledonous leaf. Dicotyledon Leaf - Adaptations For Photosynthesis (1) The colourless flattened cells of the epidermis and the transparent protective waxy cuticle readily allow light to pass through the leaf surface to the photosynthetic tissue. The closely packed palisade cells with their numerous chloroplasts maximise light absorption for photosynthesis. Numerous transport tissues permeate the leaf structure allowing water to be efficiently delivered to the photosynthetic cells. The extensive network of air spaces in the spongy mesophyll layer provides for an easy passage of gases to and from the palisade cells and an efficient gas exchange system via the stomata. Stomata are the sites of gas exchange and their opening and closing is controlled by specialised epidermal cells called guard cells – such regulation allows for efficient gas exchange while, at the same time, reducing water loss as a result of transpiration. Dicotyledon Leaf - Adaptations For Photosynthesis (2) The leaf is very thin and presents a large surface area for maximising light absorption and gas exchange. According to Fick’s Law: Rate of diffusion = surface area x difference in concentration thickness of the diffusion barrier The large surface area presented by the leaves of dicotyledons and the large differences in concentration for O2 and CO2 between the leaf interior and the external environment, produce a high value for the top line of this equation. The thinness of the leaf, providing short diffusion paths for gases from the environment to the photosynthesising cells of the palisade layer, produces a low value for the bottom line of the equation. The consequences of these leaf features allow for a rapid rate of diffusion of CO2 gas to the photosynthesising cells – a feature essential for efficient photosynthesis. More About the Palisade Cells Palisade cells are well suited to their role as the principal photosynthesising cells of the leaf. The column-shaped nature of the cells allows for close packing of the palisade layer close to the surface of the leaf, and each cell possesses numerous chloroplasts. The chloroplasts are able to move freely around the cytoplasm. vacuole containing cell sap cell wall chloroplasts cell surface membrane The chloroplasts have the ability to orientate themselves in positions that maximise light absorption in dim light and reduce potential damage when light intensities are very high. nucleus A Palisade Cell More About the Palisade Cells Dim light In dim light, chloroplasts tend to aggregate at the top surface of the cell and to orientate themselves so as to display a large proportion of their surface area to the incoming light rays. Intense light In intense light, chloroplasts aggregate at the lower end of the cell and orientate themselves in a vertical position - this reduces the chances of damage to the chloroplasts through bleaching. The Stomata – Structure (1) Stomata are the sites of gas exchange between the leaf and the environment. Stomata are the pores that allow for exchange of gases, and their size is regulated by specialised cells called guard cells. epidermal cells guard cells containing chloroplasts closed stoma open stoma Stomata are mostly found in a leaf’s lower epidermis although they may be present, in fewer numbers, in the upper epidermis. The large number of stomata, together with the regulation of their size by guard cells, allows for efficient gas exchange at the interface between leaf and the environment. This facilitates rapid uptake of CO2 for use by the photosynthetic cells during daylight hours. The Stomata – Structure (2) thick, inelastic inner wall of guard cell stomatal pore thin, elastic outer wall of guard cell epidermal cell Epidermis of Tulip leaf showing stomata (x400) The Stomata – Structure (3) mesophyll cell below the stoma Scanning electron micrograph (SEM) of a single stoma - false colour. Adaptations of Sun and Shade Plants The leaves of many typical sun and shade plants show adaptations that allow them to thrive in their respective environments. Normal Plant Shade Plant thick cuticle reduces transpiration losses in more intense light thin cuticle more than one palisade layer with smaller chloroplasts thicker leaf - greater distance between upper and lower epidermis single palisade layer with larger chloroplasts to maximise light absorption thinner leaf - shorter distance between upper and lower epidermis Autotrophs - Summary Autotrophs manufacture their own food using simple inorganic sources and, in doing so, provide the resources on which heterotrophs depend. Approximately 600 billion tonnes of carbon dioxide is converted into organic food by the autotrophs each year, with 400 billion tonnes of oxygen being released into the environment. All life is centred around the activities of the autotrophs, with photosynthesis being the central process for powering our own existence.
