Topics 2.9 & 8.3 Photosynthesis

Why photosynthesize? Ultimately, all living organisms receive energy from the Sun. Photosynthetic organisms (plants, algae, bacteria, and some protists) capture light energy and use it to convert inorganic CO2 into organic carbon, i.e. glucose (C6H12O6) or other simple sugars (CH2O).

The light energy from the Sun becomes stored energy in molecules of simple sugars. In the process of cellular respiration, glucose molecules are broken down and chemical energy in the form of ATP is harvested. Since heterotrophs cannot synthesize their own glucose, they obtain glucose from autotrophs/producers.

Photosynthesis The conversion of light energy into chemical energy

Cellular respiration is a catabolic process since it breaks down organic molecules for energy. On the other hand, photosynthesis is an anabolic process and focuses on the establishment of chemical bonds to produce organic compounds. It is essentially the reverse of cellular respiration.

Equation for Photosynthesis: From the atmosphere From the Sun 6 CO2 + 6 H2O + Light Energy → [CH2O] + 6O2 A variety of simple sugars may be formed, though glucose (C6H12O6) is one of the most common. From the surrounding environment

The Leaf Leaves are specialized for photosynthesis. They regulate the flow of gases and capture light energy for photosynthesis. The structure and arrangement of leaves maximizes the surface area exposed to sunlight and limits the distance gases need to travel.

Maple leaves are thin and broad with a large surface area Pine leaves are thin and narrow. A single needle does not provide a sizable surface area, but a branch of needles do.

3-D Cross-Section of a Leaf A transparent colourless layer that allows light to pass through to the mesophyll cells Protects the leaf from excessive absorption of light and evaporation of water Where most of the photosynthesis takes place (abundant in chloroplasts). A system of vessels that transport water, minerals, and carbohydrates within the plant. Regulates the exchange of gases in the atmosphere Photosynthetic epidermal cells that create microscopic openings called stomata.

Stomata Guard cells control the size of a stoma by changing their shape in response to water movement by osmosis in the cells. When a stoma is open, its guard cells are turgid (swollen). When a stoma is closed, its guard cells are flaccid (limp).

The direction of osmosis follows the diffusion of K+ (potassium ions) across the guard cell’s membrane. The passive diffusion of K+ is coupled with the active transport of H + through proton-pumps.

H+ are pumped out of guard cells K+ diffuses into guard cells H2O diffuse into cells by osmosis Guard cells swell and open  CO2 enters stoma

In general, stomata are open in the daytime and closed at night. When the Sun comes out in the morning, it activates receptors in the guard cell membranes, stimulating proton pumps that drive protons out of the cells. As the concentration of sucrose in guard cells decrease in the evening, water moves out of the cells and the stomata close.

Chloroplasts Photosynthesis takes place within the chloroplasts of plant cells. The structure of the chloroplast is adapted to its function in photosynthesis

Unstacked thylakoids that connect grana Double membrane The colourless, protein-rich (enzymes) semiliquid material that fill the interior space Membrane-bound sacs that stack on top of one another

Photosynthesis occurs partly within the stroma and partly with the thylakoid membrane.

The chloroplast has been has been photosynthesizing rapidly, then there may be STARCH GRANULES in the stroma

The thylakoid membrane contains light-gathering pigment molecules (i.e. chlorophyll) and electron transport chains which are essential for photosynthesis. The structure of the thylakoid system within the chloroplast increases the surface area of the thylakoid membrane, increasing the efficiency of photosynthesis.

You need to be able to annotat a diagram of the chloroplast to inidcate the adaptations of a chloroplast to its function (page 399) We will come back to this after we finish photosynthesis