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Higher Biology. Photosynthesis. Mr G Davidson. Pigments and Light. White light is a form of radiant energy it travels in the form of waves. These waves always travel in straight lines. Pigments and Light.

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Higher biology l.jpg

Higher Biology

Photosynthesis

Mr G Davidson


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Pigments and Light

  • White light is a form of radiant energy

    • it travels in the form of waves.

  • These waves always travel in straight lines.

G Davidson


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Pigments and Light

  • White light can be split into its coloured component if it is passed through a glass prism:

    • Red

    • Orange

    • Yellow

    • Green

    • Blue

    • Indigo

    • Violet.

G Davidson


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Pigments and Light

  • Each colour of the spectrum has a different wavelength

  • The wavelength of light is measured in nanometres (nm)

  • 1 nm = 10-9m (1/1,000,000,000th metre)

    • red the longest – around 700 nm

    • violet the shortest – around 400 nm

G Davidson


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Photosynthesis

  • Visible white light from the sun is the source of energy for photosynthesis.

  • Pigments in the leaf absorb light

G Davidson


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Photosynthesis

  • When it strikes a green leaflight is absorbedby these pigments

  • A small percentage of this is used in photosynthesis.

  • The remainder of the light is either reflectedfrom the leaf or transmitted through the leaf.

G Davidson


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Photosynthesis

  • Most leaves are green because they reflect green light and transmit green light.

  • Each of the pigments present in a leaf absorbs light of certain wavelengths therefore if a plant has different pigments, it can absorb a wider range of wavelengths of light.

G Davidson


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Absorption Spectrum

Wavelength (nm)

G Davidson


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Photosynthesis

  • The pigments found in plants depend largely on where the plant is to be found e.g. shady plants need more pigment than canopy plants

  • Different sea weeds have a different range of pigments because of the reducing light intensity at greater depths of water.

G Davidson


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100 % Sunlight

12 % reflected

Only a small percentage of absorbed light is used in photosynthesis

83 % absorbed

5 % transmitted

Fate of Light Striking a Leaf

G Davidson


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Granum

Double unitmembranes

Lamella

Stroma

Chloroplasts

G Davidson


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Chloroplast

G Davidson


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Chloroplasts

  • The light trapping pigments are to be found in the chloroplasts of plant cells.

  • Chloroplasts are relatively large organelles found in the cytoplasm, each one surrounded by a double unit membrane.

  • Inside the chloroplast are large starch grains which act as food stores.

G Davidson


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Chloroplasts

  • The main components of the chloroplast are grana which are interconnected by lamellae.

  • The grana are composed of flattened discs containing the pigments, and are the site of the light reaction.

  • The stroma is the site of carbon fixation and is the fluid part in the chloroplast.

G Davidson


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Carbon dioxide

CO2

water

H2O

glucose

C6H12O6

oxygen

O2

Light Pigments

Chemistry of Photosynthesis

  • In the process of photosynthesis, raw materials carbon dioxide and water are assembled to make organic food molecules producing oxygen as a waste product.

G Davidson


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Chloroplasts

  • Photosynthesis occurs in two stages.

  • The first stage requires light and is called the light reaction or photolysis.

  • The second stage is a temperature dependent series of enzyme controlled reactions called carbon fixation or the Calvin Cycle.

G Davidson


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Photolysis

  • This is the trapping of sunlight energy by photosynthetic pigments to release chemical energy.

  • The light energy splits water into hydrogen and oxygen, the oxygen then being released as a by-product.

  • The hydrogen then combines with a hydrogen acceptor NAD to produce NADH2.

G Davidson


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Photolysis

  • At the same time, energy is also made available to convert ADP + Pi into the high energy molecule ATP.

  • This is a process known as photophosphorylation.

  • The NADH2 and ATP made by photolysis are passed to the carbon fixation stage.

  • Photolysis occurs on the grana of the chloroplasts.

G Davidson


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Sunlight

CO2

NADH2

Water

Photolysis

Carbon

Fixation

Oxygen

Photo-

phosphorylation

ATP

Sugar

Photolysis

G Davidson


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Calvin Cycle

  • This is also referred to as carbon fixation.

  • This stage is a series of enzyme controlled chemical reactions in the form of a cycle.

G Davidson


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Calvin Cycle

  • As the carbon dioxide enters carbon fixation, it combines with a 5 carbon ribulose biphosphate (RuBP) to form an unstable 6 carbon compound which immediately splits into two molecules of 3 carbon glycerate phosphate.

G Davidson


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1C

5C

6C

Glucose

CO2

RuBP

ADP + Pi

ATP

6 Carbon compound

Triose phosphate

6C

3C

NAD

NADH2

Glycerate phosphate

ADP + Pi

ATP

3C

Calvin Cycle

G Davidson


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Calvin Cycle

  • Two molecules of glycerate phosphate are then converted via a triose phosphate into 6C sugar, using hydrogen and energy – both provided by photolysis.

G Davidson


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Calvin Cycle

  • Two molecules of triose phosphate are then used to make one molecule of 6 carbon glucose, which are then built to form starch or cellulose.

G Davidson


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Calvin Cycle

  • Not all triose phosphate molecules are used to make sugar, some are required to continue the cycle i.e. take it back to RuBP.

  • In doing this 5 molecules of 3 carbon triose phosphate become 3 molecules of 5 carbon RuBP using energy from ATP and so the Calvin cycle is allowed to continue.

G Davidson


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Limiting Factors

  • Chemical reactions such as photosynthesis can be speeded up or slowed down depending on certain conditions.

  • A condition which is able to limit the rate of a reaction is usually called a limiting factor.

G Davidson


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Elodea Bubbler

  • In the Elodea bubbler experiment we can alter light intensity, CO2 concentration and temperature, one at a time to show that they are all limiting factors.

  • The rate of photosynthesis is measured by counting the bubbles of oxygen released in a specific time.

G Davidson


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0.5% solution of sodium bicarbonate to provide a supply of CO2

Constant temperature

Elodea Bubbler

G Davidson