PhotosynthesisPhotosynthesisPhotosynthesisPhotosynthesis

Learning Outcomes• Explain absorption, transmission and reflection of light by a leaf.

Revision from S.G.• What is the word equation for photosynthesis?

• What is the pigment called that absorbs light energy and where in a cell do you find it?

• What type of energy is this light energy changed in to?

• What are some of the different tissues in a leaf called?

Structure of a leafThe epidermis protects the leaf and is transparent to let light through.

Palisade mesophyll cells are closely packed to absorb maximum light. They contain many chloroplasts. Most photosynthesis takes place in the palisade cells.

Structure of a leafSpongy mesophyll also captures light and makes food. Spongy mesophyll cells have air spaces between them to allow movement of gases.Veins contain xylem (top part of vein) for water transport and phloem (lower part of vein) to take away sugar.

Photosynthesis•During the process of photosynthesis, green plants use the light energy, carbon dioxide and water to make carbohydrates (e.g. glucose).

•Light energy is “fixed” by the green pigment called chlorophyll which is found in the chloroplasts.

Learning Outcomes• Explain the absorption, transmission, and reflection of light by a leaf.

• Describe the absorption spectrum of leaf pigments.

• State that for chlorophyll a and b absorption occurs in the blue and red light regions of the spectrum.

• State that the accessory pigments xanthophyll and carotene absorb light from other areas of the spectrum and pass energy on to chlorophyll.

Fate of light

• Light hitting a leaf can be:– Absorbed– Reflected – Transmitted

• Most of the light that hits a leaf is absorbed. Of this absorbed light only a small part is used in photosynthesis. The rest is converted to heat and lost.

Light and green leafSunlight 100%

5% transmitted

83 % absorbed

12% reflected

NB - only a very small 4% of absorbed light is used in photosynthesis

Light•Light is a form of energy

•When a beam of white light is passed through a prism, it splits in to different colours. This is known as the spectrum.

•Each colour of the spectrum has a different wavelength

The Spectrum

–Blue light has the shortest wavelength (450nm)

–Red light has the longest wavelength (650nm)

Fate of Light• When light strikes a green leaf much of it is absorbed but only 4% of this is used in photosynthesis.

• The rest of the absorbed light is converted to heat and lost.

• Most leaves appear green in colour because they reflect and transmit green light.

Black bands = where light energy is absorbed by the leaf. Therefore it can’t travel through the prism

Colour = where light is not absorbed (it is either reflected or transmitted).

Photosynthetic Pigments

• There are 4 photosynthetic pigments found in a green leaf:

– Chlorophyll a– Chlorophyll b– Carotene– Xanthophyll

• These pigments can be separated by paper chromatography.

Photosynthetic Pigments

• Chlorophyll a is the principal pigment as it is the only pigment that directly brings about photosynthesis.

• The other accessory pigments absorb light and pass the energy on to chlorophyll a.

• Chlorophyll a absorbs mainly in the red and blue regions.

Photosynthetic pigments

• Chlorophyll a contains the element magnesium.

• Chlorophyll b , carotene and xanthophyll are described as accessory pigments.

• This shows the percentage absorption of light by photosynthetic pigments.

• Since each pigment absorbs different wavelengths of light it means a larger range of wavelengths can be absorbed than if there was only one pigment.

Absorption Spectrum

Learning Outcomes• State the relationship between the action spectrum and the absorption spectrum, in relation to photosynthetic pigments and the rate of photosynthesis.

Absorption and Action spectrum

• An action spectrum shows the effectiveness of different wavelengths of light at bringing about photosynthesis.

Action spectrum of chlorophyll a

•The action spectrum and absorption spectrum follow similar patterns.

•Certain wavelengths are used in photosynthesis.

•Chlorophyll a and b absorb the red and blue regions of the spectrum

•Carotenoids absorb the green regions

Learning Outcome• Give an account of the detailed structure of a chloroplast, locating the sites for both the light dependent and independent stages of photosynthesis.

Chloroplasts• A chloroplast is bound by a double membrane and contains a liquid area known as the stroma and layers of stacked membranes called the grana.

Grana• Grana (singular granum) are a coin-like stack of flattened sacs containing photosynthetic pigments.

• They have a large surface area of the pigments to absorb light energy.

• The light dependent stage occurs at the grana.

Chloroplasts• Stroma is the colourless “background” material.

• Carbon fixation occurs here.

• There is no chlorophyll but it contains enzymes and starch grains.

Learning Outcomes• Describe the photolysis of water (part of the light dependent stage), identifying the products and substrates involved.

• Describe how ATP is formed in the light-dependent reaction

• State the importance of ATP and hydrogen formation in the light dependent stage.

Photosynthesis• Photosynthesis is the process by which glucose is synthesised through the reduction of carbon dioxide.

• The energy needed for this process comes from light energy.

• Photosynthesis consists of 2 parts: A light dependent (photochemical) stage and a temperature dependent (thermochemical) stage.

Light dependent stage

• The light-dependent stage occurs in the grana of chloroplasts.

• Light enegry is trapped bychlorophyll and converted into chemical energy.

• This light energy is used to split water molecules into hydrogen and oxygen. This is called photolysis of water.

Photolysis

• The oxygen is released as a by-product.

• The hydrogen combines with a hydrogen acceptor called NADP to form NADPH2.

• At the same time, chlorophyll makes energy available for the regeneration of ATP from ADP + Pi (phosphorylation).

Photolysis

• The hydrogen held by NADPH2 and the energy held by ATP are needed for use in the second stage of photosynthesis- Carbon fixation.

Learning Outcome• Describe the calvin cycle (the light independent stage), identifying its main substrates and products involved.

• Give an account of the fixation of carbon dioxide in the Calvin cycle, identifying the particular roles of RuBP and GP.

Carbon Fixation• This stage occurs in the stroma of the chloroplasts.

• It consists of several enzyme controlled chemical reactions, which take the form of a cycle (the Calvin cycle).

Carbon Fixation• A molecule of carbon dioxide enters the chloroplast by diffusion and joins with a molecule of 5-Carbon ribulose biphosphate (RuBP) in the stroma to form a 6-Carbon molecule.

• This molecule is unstable and rapidly splits into 2 molecules of 3-Carbon glycerate-3-phosphate (GP).

CO2 + 5-C RuBP 6C 2 X 3-C GP

Carbon Fixation• During the next stage in the cycle, GP is converted to a 3-Carbon sugar using the hydrogen temporarily bound to NADP and some of the energy held in ATP.

• When 2 molecules of this 3C sugar combine in an enzyme controlled series of reactions, glucose is formed.

Carbon Fixation• Photosynthesis is said to be a reduction reaction because 3C GP is reduced to form a 3C sugar. It is said to be reduced because hydrogen is added to it.

Calvin Cycle

Regenerating RuBP• The 3 carbon sugars are not all used to make complex products.

• Some are used to regenerate RuBP (the carbon dioxide acceptor). This also requires energy from ATP.

Rate of Photosynthesis

• The rate of photosynthesis can be measured by the following:

– Evolution of oxygen per unit time– Uptake of carbon dioxide per unit time

– Production of carbohydrate (as increase in dry mass) per unit time.

Limiting Factors• A limiting factor is a factor which holds up a process because it is in short supply.

• Limiting factors in photosynthesis are:– Carbon dioxide concentration– Light intensity– Temperature

Limiting factors

Limiting Factors

Limiting factors

P temperatureQ carbon dioxide/CO2R Light intensity

Elodea Bubbler• The number of oxygen bubbles produced each minute can be used to measure the rate of photosynthesis.

• You can change the light intensity etc and see the effect it has on the rate of photosynthesis.