Photosynthesis

Purpose and Reaction• Photosynthesis is the process in which plants use

sunlight to produce glucose which is food for the plant. This is converted and stored as starch (converts solar energy into chemical potential energy, which is then available to other organisms).

* Glucose is modified into other substances eg cellulose for the cell walls

Chloroplasts – Where Photosynthesis Happens

Chloroplasts – Where Photosynthesis Happens

• The chloroplast is the organelle where photosynthesis occurs. Chloroplasts have thin membranes / large surface area for absorption of light.

• The organelle is surrounded by a double membrane (membranes are thin for rapid diffusion, and with a large surface area for absorption of light and other substances). Inside the inner membrane is a complex mix of enzymes and water. This is called stroma and is important as the site of the light-independent reactions. Stroma is a clear fluid, which doesn’t block the light.

• Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum and each of the flattened sacs that make up the granum is called a thylakoid. The large surface area created by this arrangement, allows the maximum amount of light to be captured, and allows the enzymes that control the reaction to have the best opportunity to reach the reactants and maximise the rate of reaction.

Light Dependent vs Light Independent

Parts of a Leaf

Parts of a Leaf

Key: 1) cuticle 2) upper epidermis 3) palisade mesophyll 4) spongy mesophyll 5) lower epidermis 6) stoma 7) guard cells 8) xylem 9) phloem 10) vascular bundle

Functions of the Parts of a Leaf• The epidermis (2 = upper, 5 = lower) is usually transparent (epidermal cells lack chloroplasts) and

coated on the outer side with a waxy cuticle that prevents water loss which would decrease the rate of photosynthesis

• Cells containing most chloroplasts are found near the top of the leaf in (palisade) long cylindrical cells (3), with the chloroplasts close to the walls of the cell. These factors combine and lead to more light being received where it can be used to maximise the rate of photosynthesis. NOT CLOSER TO THE SUN! – just more light intensity. The slight separation of the cells provides maximum absorption of carbon dioxide.

• Beneath the palisade layer is the spongy mesophyll (4). The cells of the spongy layer are more rounded and not so tightly packed. There are large intercellular air spaces. This maximises the rate of diffusion of the gases during photosynthesis. (CO2 entering the leaf and O2 leaving it.)

• Specialised guard cells (7), mainly on the lower leaf epidermis, function to control the movement of gases and water loss through the stomata (6).

• Vacuoles push chloroplasts closer to cell edges (MUST NOT SAY PUSH OUTSIDE THE CELL!). This reduces diffusion distance of CO2 and O2 and may help prevent shading by other organelles?

• The combination of the cell and organelle structure and location collectively helps to keep photosynthesis at its maximum potential rate.

Factors Affecting the Rate of Photosynthesis

Limiting factor: factor that is low in abundance thus lowering the rate of reaction

Temperature affects the rate of photosynthesis. Because reactions are enzyme controlled, photosynthesis has an optimum temperature.

ie. Greater rate of photosynthesis at midday than early morning or evening, and little or no photosynthesis at night; also greater rate in summer compared to winter

Thinking…

Explain the ‘saw tooth’ pattern

Carbon DioxideThe annual saw-tooth pattern in the carbon dioxide graph to the right is produced by growing vegetation that consumes carbon dioxide in the spring and summer, and releases it in the fall and winter. The long-term upward trend results from carbon dioxide that is released by the combustion of fossil fuels.

Limiting Factors: Phytoplankton BloomsSpectacular evidence of what happens when a limiting factor is removed is seen in this image taken from the ESA Enviro Sat. So what’s happening? Well, in marine phytoplankton, iron is often a limiting micronutrient, along with other mineral elements. These tend to sink to the bottom of the ocean, but upwelling currents can push them up to the near-surface, making them available for use and resulting in a population explosion.