1.4.1 Photosynthesisers as biomass producers

Where photosynthesis happens

1. Photosynthesis is the process in which light energy is used to build glucose from carbon dioxide and water, with oxygen released as a by-product.
2. It is carried out by plants, by algae and by some bacteria, and it is the only process on Earth that brings a large amount of new chemical energy into living things.
3. In a plant it takes place inside the chloroplasts, so only cells that contain chloroplasts can photosynthesise.
4. Chloroplasts are most tightly packed into the palisade mesophyll cells just below the upper surface of a leaf, where the most light arrives.
5. Inside each chloroplast the green pigment chlorophyll is spread over stacks of flattened membranes, which spreads it out so that as much light as possible is absorbed.
6. Chlorophyll absorbs light most strongly at the red and blue ends of the visible spectrum.
7. It reflects green light rather than absorbing it, and that reflected light is what reaches your eye, which is why leaves look green.
8. Chlorophyll is a pigment and not an enzyme, so its job is to absorb light energy rather than to speed a reaction up.
9. A cell with no chlorophyll cannot photosynthesise at all, which is why the white parts of a variegated leaf make no glucose.

Reactants, products and the equations

1. The reactants of photosynthesis are carbon dioxide and water, and the products are glucose and oxygen.
2. Carbon dioxide diffuses into the leaf from the air through the stomata, which are the pores on the underside of the leaf.
3. It then diffuses through the air spaces of the spongy mesophyll and into the mesophyll cells that hold the chloroplasts.
4. Water is absorbed from the soil by the root hair cells and carried up to the leaf in the xylem.
5. Oxygen diffuses out through the same stomata, and any that the plant does not use for its own respiration is released into the air.
6. The word equation is carbon dioxide + water $\to$ glucose + oxygen.
7. Light and chlorophyll are written above the arrow rather than on the left, because they make the reaction happen without being used up in it.
8. The balanced symbol equation is $6C{O}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$.
9. Check the balancing by counting atoms: each side has $6$ carbon, $12$ hydrogen and $18$ oxygen atoms.
10. The eighteen oxygen atoms on the right are split between the $6$ in the glucose and the $12$ in the six molecules of oxygen gas.
11. The glucose made is used in several ways: some is respired straight away, some is converted to starch for storage, some to cellulose for new cell walls, and some to lipids for seeds.
12. Glucose is also combined with nitrate ions taken up by the roots to build the amino acids the plant needs for its proteins.

The two stages of the reaction

1. Photosynthesis is not a single step; it happens in two stages, and only the first of them needs light directly.
2. In the first stage, light energy absorbed by chlorophyll is used to split water molecules apart.
3. Splitting the water releases hydrogen, which is held ready for the second stage, and oxygen, which is the by-product that leaves the leaf.
4. Splitting water needs a large input of energy, and that is exactly what the absorbed light provides.
5. In the second stage, the hydrogen is joined with carbon dioxide to build glucose.
6. The second stage does not use light itself, so it depends on the first stage to keep supplying it with hydrogen.
7. This is why the whole process stops in the dark: with no light there is no hydrogen, so no glucose can be assembled.
8. It also explains why the oxygen released comes from the water rather than from the carbon dioxide.

Photosynthesis as an endothermic reaction

1. An endothermic reaction is one that takes energy in from its surroundings, so energy has to be supplied before it will happen.
2. Photosynthesis takes in light energy, which means it cannot run at all unless light is being absorbed.
3. The bonds in glucose and oxygen hold more energy than the bonds in carbon dioxide and water, so energy has to be put in to make the change.
4. The light energy absorbed is therefore transferred into chemical energy stored in the glucose, where it stays until something respires it.
5. Respiration is the opposite, an exothermic reaction that releases the energy the glucose is holding, so the two processes run in opposite directions.
6. A plant carries out both at once, photosynthesising only in the light but respiring day and night.

Producers, biomass and the rest of life

1. A producer is an organism that makes its own food by photosynthesis, and plants, algae and photosynthetic bacteria are all producers.
2. Producers sit at the start of almost every food chain, because they are the only organisms that do not have to eat something else.
3. The glucose they make is used to build new plant material, and the mass of that material is called biomass.
4. Biomass is measured as dry mass, meaning the mass left after all the water has been driven off, because the amount of water in a living thing varies from hour to hour.
5. Only about one or two per cent of the light energy falling on a plant ends up stored in its biomass, because much of the light misses the chloroplasts, passes straight through or is the wrong wavelength.
6. Small as that fraction is, it is the energy that supports almost all life on Earth.
7. When a herbivore eats a plant, the biomass and the energy stored in it pass along the food chain to the next organism.
8. Photosynthetic organisms in the oceans, particularly the microscopic algae of the plankton, produce a large share of the world's biomass and oxygen.
9. Fossil fuels are the remains of organisms that photosynthesised long ago, so even the energy in coal and oil arrived through this one reaction.

Investigating photosynthesis in the laboratory

1. Glucose is difficult to detect in a leaf, because it is used and moved almost as soon as it is made.
2. A plant converts surplus glucose into starch, which stays put in the leaf, so testing a leaf for starch is the practical way of finding out whether it has photosynthesised.
3. Iodine solution is the reagent used, and it turns from orange-brown to blue-black where starch is present.
4. Before any experiment the plant has to be destarched by leaving it in a dark cupboard for one or two days.
5. Destarching works because the plant keeps respiring in the dark and uses up or moves away the starch already stored in its leaves.
6. Any starch found after the experiment must then have been made during the experiment itself, which is what makes the result valid.
7. Once the starch test itself is reliable, you can find out which conditions a leaf actually needs by depriving one leaf of one condition at a time.

1.4.2 Factors affecting the rate of photosynthesis

Measuring the rate of photosynthesis

1. The rate of photosynthesis is the amount of product a plant makes in a given time, and it is what changes when the conditions change.
2. The most convenient thing to measure is the oxygen given off, because it is a gas and comes off a water plant as visible bubbles.
3. The rate can be worked out as $rate=\frac{volume\; of\; oxygen}{time}$, giving units such as ${cm}^3{min}^{-1}$.
4. Counting bubbles per minute is quicker but less precise, because bubbles vary in size and a large one carries far more gas than a small one.
5. Two other measures are possible: the volume of carbon dioxide used up, and the increase in dry mass of the plant over a longer period.
6. Whichever measure is used, the plant is also respiring, so what you actually record is the net effect of photosynthesis minus respiration.
7. In bright light photosynthesis is so much faster than respiration that this correction hardly matters, but in dim light it does.
8. Pondweed such as Elodea or Cabomba is used because it lives underwater, so the oxygen it releases collects as bubbles instead of escaping into the air.

The effect of light intensity

1. Light supplies the energy for the first stage of photosynthesis, in which water is split to release the hydrogen the second stage needs.
2. Raising the light intensity means more energy is absorbed by the chlorophyll each second, so water is split faster and glucose is built faster.
3. In the dark the rate is zero, because no water is being split, and the only gas exchange you can measure is the plant respiring.
4. At low light intensities the rate is roughly proportional to the light intensity, so doubling the intensity roughly doubles the rate.
5. As the intensity keeps rising the line begins to level off, because light is no longer the factor holding the rate back.
6. Light intensity is hard to measure directly in a school laboratory, so the distance from a lamp is used instead.
7. Light spreads out as it travels, so the inverse square law applies: $I\propto \frac{1}{d^2}$, where $d$ is the distance from the lamp.
8. Because the distance is squared, doubling the distance cuts the intensity to a quarter, not to a half.
9. Plotting the rate against $\frac{1}{d^2}$ rather than against $d$ is therefore what reveals the real relationship.

The effect of carbon dioxide concentration

1. Carbon dioxide is a raw material, and it is used in the second stage of photosynthesis where it is joined with hydrogen to build glucose.
2. Raising the carbon dioxide concentration means more molecules reach the chloroplasts each second, so more glucose can be assembled.
3. The air contains only about $0.04\%$ carbon dioxide, which is a very small proportion, so carbon dioxide is often the factor holding the rate back on a warm bright day.
4. The rate rises steeply as the concentration is increased from a low value, because every extra molecule that arrives can be used.
5. The line then levels off, because the chloroplasts cannot process carbon dioxide any faster once light or temperature has become the limit.
6. Carbon dioxide has to reach the mesophyll cells by diffusion through the stomata, so anything that closes the stomata also slows photosynthesis.
7. A plant short of water closes its stomata to save it, which cuts the carbon dioxide supply and is one reason drought reduces growth.

The effect of temperature

1. Every step of photosynthesis is controlled by an enzyme, so temperature affects the rate in the same way it affects any enzyme-controlled reaction.
2. Raising the temperature gives the enzymes and their substrates more kinetic energy, so they move faster and collide more often.
3. More frequent collisions with enough energy to react means more successful reactions per second, so the rate rises.
4. The rate keeps rising until the optimum temperature is reached, which is the temperature at which the process runs fastest.
5. Above the optimum the rate falls sharply, because the enzymes begin to denature as the bonds holding their folded shape are broken.
6. A denatured enzyme has an active site that no longer fits its substrate, so it stops catalysing its step and the whole pathway slows.
7. For most plants the optimum is somewhere around $25{}^{\circ }C$ to $30{}^{\circ }C$, and denaturation becomes serious above roughly $40{}^{\circ }C$ to $45{}^{\circ }C$.
8. At low temperatures nothing is denatured, so the rate is slow but recovers as soon as the plant is warmed again.
9. That difference is what makes the temperature graph asymmetrical: it climbs to a peak and then drops away steeply, rather than levelling off.

The shape of a single-factor graph

1. A graph of rate against light intensity or against carbon dioxide concentration has the same two-part shape.
2. The first part is a steep straight rise, and along that stretch the factor on the horizontal axis is the limiting factor.
3. The gradient of that straight stretch tells you how much extra rate each extra unit of that factor buys.
4. The second part is a plateau, where the line runs flat and adding more of that factor changes nothing at all.
5. A plateau means the factor on the axis has stopped being the limiting one, and something else is now holding the rate back.
6. The point where the line stops rising and starts to flatten is therefore the point at which the limiting factor changes over.
7. A graph of rate against temperature looks different, rising to a peak at the optimum and then falling, because the enzymes are being denatured rather than simply working at full stretch.
8. Reading the rate from any of these graphs means finding the value on the vertical axis, and finding a rate of change means taking a gradient.

1.4.3 Limiting factors in photosynthesis

What it means for a factor to be limiting

1. Photosynthesis needs light, carbon dioxide and a suitable temperature all at once, and it can only run as fast as the least favourable of them allows.
2. The limiting factor is whichever of them is furthest from its ideal level at that moment, because it is the one setting the ceiling on the rate.
3. Only one factor is limiting at any given moment, and increasing any of the others has no effect at all while it stays that way.
4. Increasing the limiting factor does raise the rate, and it keeps raising it until a different factor becomes the one in shortest supply.
5. Once that happens, further increases in the first factor stop working and the second factor has to be raised instead.
6. This is why speeding photosynthesis up in practice usually means raising two or three factors together rather than one on its own.
7. On a bright but cold spring morning there is plenty of light, so temperature is the limiting factor and warming the plant is what raises the rate.
8. In a warm, well-lit glasshouse the plant has light and warmth to spare, so carbon dioxide is usually the limiting factor because air holds so little of it.
9. Deep inside a dense crop canopy the lower leaves are shaded by the ones above, so light is limiting for them even at midday.

How the limiting factor changes

1. The limiting factor is not fixed; it changes as conditions change, so the same plant can be held back by different things at different times.
2. Just before dawn there is no light at all, so no photosynthesis happens and the plant is only respiring.
3. Through the early morning light intensity climbs quickly while the air is still cool, so light is limiting first and then temperature takes over.
4. By the middle of a summer day both light and temperature are high, so carbon dioxide becomes the limiting factor.
5. On a hot afternoon a plant short of water may close its stomata to reduce water loss, which cuts off the carbon dioxide supply and slows the rate further.
6. Late in the evening light intensity falls away again and light becomes limiting for the second time that day.
7. Across the seasons the pattern repeats on a longer scale, with temperature and day length limiting growth in a British winter and carbon dioxide limiting it on a bright summer day.
8. This is why a crop grown outdoors in the United Kingdom puts on most of its biomass between late spring and late summer.

Reading graphs with more than one curve

1. A graph that shows the interaction of two factors carries more than one curve, and each curve was measured at a different fixed level of the second factor.
2. A graph of rate against light intensity, for instance, might carry one curve at $0.04\%$ carbon dioxide and a second at $0.4\%$ carbon dioxide.
3. Along the steep part of any curve, the factor plotted on the horizontal axis is the limiting factor, because raising it raises the rate.
4. Where a curve flattens into a plateau, the factor on the horizontal axis has stopped being limiting and the second factor has taken over.
5. The height of a plateau tells you how much the second factor is allowing, so the curve measured with more carbon dioxide plateaus at a higher rate.
6. The plateau on the upper curve also begins at a higher light intensity, because the plant can keep making use of extra light for longer before carbon dioxide runs short.
7. To name the limiting factor at any labelled point, look at whether the curve is still rising there or has already levelled off.
8. If it is still rising, the factor on the axis is limiting; if it has levelled off, the factor that differs between the curves is limiting.
9. Two curves that lie on top of one another over the first stretch and only separate later show that the second factor was not limiting at low values of the first.

Controlling the factors to raise crop yield

1. A commercial grower controls all three factors at once inside a glasshouse or polytunnel, so that none of them is holding the crop back for long.
2. Artificial lighting extends the working day and keeps the rate up in dull weather and through a British winter.
3. Heaters and thermostats hold the temperature near the optimum, and ventilation stops it climbing past the point where enzymes start to denature.
4. Carbon dioxide enrichment raises the concentration well above the $0.04\%$ in ordinary air, which is often the single largest gain available.
5. A paraffin or propane burner is a common way of doing this because it raises the temperature and releases carbon dioxide at the same time, dealing with two factors from one piece of equipment.
6. A faster rate of photosynthesis means more glucose, so more biomass, so a larger yield of fruit or leaf that the grower can sell.
7. Controlling the conditions also lets a crop be grown out of season, when the price it fetches is higher.
8. None of it is free, though, because lamps, heat and carbon dioxide all cost money, and the extra crop has to be worth more than the extra cost.
9. A grower therefore aims for the most profitable rate rather than the fastest possible one, and will let a factor stay slightly limiting if removing that limit costs more than the extra crop earns.
10. This is a straightforward comparison to make: if enrichment costs an extra £900\pounds 900£900 per week and lifts the crop's value by £1400\pounds 1400£1400 per week, it is worth doing.