Photosynthesis Flashcards

All 28 cards in this deck

What is the word equation for photosynthesis?

carbon dioxide + water $\rightarrow$ glucose + oxygen (light energy needed)

Which structure in a plant cell is the site of photosynthesis?

The chloroplast.

What is the role of chlorophyll in photosynthesis?

It is the green pigment in chloroplasts that absorbs light energy.

Why is photosynthesis described as an endothermic reaction?

It takes in (absorbs) energy from the surroundings — light energy — to make the products.

What is a producer in a food chain?

An organism that makes its own food by photosynthesis, e.g. a plant or alga.

True or false? Photosynthetic organisms are the source of the biomass in almost all food chains.

True. Producers make glucose, which becomes the biomass that all other organisms feed on.

True or false? Oxygen is a reactant in photosynthesis.

False. Oxygen is a product; the reactants are carbon dioxide and water.

What is a limiting factor of photosynthesis?

The factor in shortest supply that is holding back (limiting) the rate of photosynthesis.

Name the three limiting factors of photosynthesis.

Light intensity, carbon dioxide concentration and temperature.

What is the effect of increasing carbon dioxide concentration on the rate of photosynthesis?

The rate increases, until carbon dioxide is no longer the limiting factor and the rate levels off.

Why does the rate of photosynthesis fall at temperatures above the optimum?

The enzymes controlling photosynthesis become denatured (their active sites change shape).

Why does the rate of photosynthesis increase as temperature rises up to the optimum?

More collisions occur, so enzymes catalyse more reactions (more enzyme–substrate complexes form).

On a multi-line graph of rate of photosynthesis against light intensity, what does a higher plateau for one line show?

That another factor (e.g. temperature or $CO_2$ concentration) was increased, so it is less limiting.

True or false? Raising the temperature always increases the rate of photosynthesis.

False. Above the optimum the enzymes denature and the rate falls.

True or false? Only one factor can limit the rate of photosynthesis at any one time.

True. The rate is limited by whichever factor is in shortest supply at that moment.

What is the relationship between light intensity and the rate of photosynthesis at low light levels?

Directly proportional — as light intensity increases, the rate increases.

Why does a graph of rate of photosynthesis against light intensity level off?

Light intensity is no longer the limiting factor — another factor, e.g. temperature or $CO_2$, is limiting.

How is light intensity related to the distance from a light source?

Inversely proportional to the square of the distance (the inverse square law).

What is the formula for relative light intensity using the inverse square law?

relative light intensity $= \dfrac{1}{d^2}$, where $d$ is the distance from the lamp.

What are the steps to find the relative light intensity at a given distance from a lamp?

e.g. a lamp 2 cm from the plant

1. Note the distance: $d = 2$ cm
2. Square it: $d^2 = 4$
3. Take the reciprocal: $1/4 = 0.25$ (arbitrary units)

True or false? Doubling the distance from a lamp halves the light intensity.

False. It quarters the light intensity, because intensity $\propto 1/d^2$.

True or false? The rate of photosynthesis is directly proportional to the distance from a light source.

False. The rate falls as the distance increases, because light intensity is inversely proportional to the square of the distance ($1/d^2$).

In the core practical, how is the rate of photosynthesis measured using pondweed?

By counting the number of oxygen bubbles released per minute (or measuring the volume of gas collected).

What are the steps to investigate the effect of light intensity on photosynthesis using pondweed?

e.g. lamp placed at 10, 20, 30 cm

1. Place pondweed in sodium hydrogencarbonate solution and set the lamp at the first distance, e.g. 10 cm
2. Leave to acclimatise, then count oxygen bubbles per minute
3. Repeat at each distance, keeping temperature and $CO_2$ the same, and calculate means

Which variables must be controlled in the light intensity and photosynthesis core practical?

Temperature, carbon dioxide concentration, and the species/mass of plant used.

Why is a heat shield (glass tank of water) used between the lamp and the plant in this core practical?

To absorb heat from the lamp so temperature stays constant and does not act as a variable.

How do you calculate the rate of photosynthesis from a bubble count?

e.g. 30 bubbles counted in 2 minutes

Divide the number of bubbles by the time: $30 \div 2 = 15$ bubbles per minute.

True or false? Counting bubbles is a less reliable measure of photosynthesis than measuring the volume of gas.

True. Bubbles vary in size and are easily miscounted.