B8: Photosynthesis

Topics covered: White/Black - Photosynthesis Equation Yellow - Adaptations Blue - Limiting Factor Red - Uses of Glucose made in Photosynthesis Purple - Practicals (Pondweed Practical + Leaf Starch Practical) Green - Light Intensity and Distance

Process of Photosynthesis

Light energy from the Sun enters. Carbon Dioxide enters the leaves in tiny holes called stomata. Green chloroplasts inside the leaf cells makes the reaction happen. Water is carried to the leaves from the roots in tubes called xylem. Oxygen is given out by the leaf. Glucose is made in the leaves, which is converted to starch for the plant to store.

Is photosynthesis endo/exo thermic?

Photosynthesis is endothermic

Why is photosynthesis endothermic?

The products absorb energy from the surrounding area (light energy), meaning the temperature of the surrounding area decreases as the energy was taken in. This makes the area colder.

Balanced Chemical Equation of Photosynthesis

6H20 + 6CO2 —> C6H12O6 + 6O2 (Equation is the opposite of respiration)

Leaf adaptations to get lots of sunlight

There are lots of chloroplasts containing chlorophyll to absorb sunlight. The waxy cuticle is clear and allows light to pass through. Broad leaves provide a large surface area. Chloroplasts are largely found in the upper layers.

Leaf adaptations to get lots of carbon dioxide

Lots of stomata on the underside of leaves. Air spaces in the spongy mesophyll increase the rate of diffusion. Guard cells open and close stomata to regulate gas exchange.

Leaf adaptations to get water

Xylem vessels bring water to the leaf cells.

Limiting Factor

The factor that most prevents the rate of photosynthesis being higher.

Factors affecting rate of photosynthesis

Light Intensity, Temperature, and Carbon Dioxide concentration

Light Intensity explained

As light intensity increases, so does rate of photosynthesis. Rate of photosynthesis then plateaus despite light intensity increasing as another factor becomes limiting.

Temperature explained

As temperature increases so does rate of photosynthesis. Rate of photosynthesis then decreases sharply as temperature increases. At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. Enzymes denature at high temperatures.

CO2 concentration explained

As CO2 concentration increases so does rate of photosynthesis. Rate of photosynthesis then plateaus despite CO2 concentration increasing, as another factor becomes limiting.

Used in cellular respiration

Photosynthesis traps light energy into glucose, and cells break the glucose apart to release the energy when they need it.

Can be used to make Cellulose

Made when lots of glucose is combined. It is a complex carbohydrate that strengthens plant’s cell walls.

Combined to make Starch

Another complex carbohydrate with no specific purpose. Better for long term storage as it is compact and insoluble, so it wont draw water in via osmosis. Can be broken down into glucose when photosynthesis isnt happening (night/winter .etc)

Makes Amino Acids

Happens when Glucose is combined with Nitrate ions from the soil. Amino acids can be combined to make proteins.

Making Oils and Fats

Stored as future energy usages (for seeds in particular)

Why is Glucose essential for Growth?

As Glucose is so essential for plants, their growth rate is often determined by how fast they can photosynthesise.

Risks of testing a leaf for Starch

Bunsen Burner - Burning yourself

Glass Beaker - Sharp glass if broken

Ethanol - Highly flammable

Iodine - Harmful

How can the apparatus be used to investigate of light intensity on the rate of photosynthesis?

Light Intensity is changed by changing the distance of the light. The number of bubbles counted is an indicator of rate in a given time.

Relationship between light intensity and distance

Light intensity is inversely proportional to Distance²

Light Intensity = k/Distance²

Why is Sodium Hydrogen Carbonate (NaHCO3) used?

It releases Carbon dioxide, which will help the leaf photosynthesise.

Two ways plants avoid excessive water loss

Stomata close at night to avoid moisture loss. The waxy cuticle is waterproof to avoid water escaping through the top.

As distance doubles… light intensity … ?

Quarters (x 0.25)

Distance 1 = Light Intensity 1

Distance 2 = Light Intensity 0.25