RHS Level 3: Plant taxonomy, structure, and function Q3

-3. Understand photosynthesis, respiration, water/solute relations in the plant and their relevance to horticulture.

3.1 Describe the process of photosynthesis.

Describe the structure of the chloroplast to include: stroma,thylakoids, grana.



Note: diagram is included for clarity, no mention of needing to know it.

A chloroplast is an organelle where photosynthesis takes place. The chloroplasts are filled with stroma, a colourless liquid, and with grana which are a stack of thylakoids. A thylakoid is a membrane bound compartment containing chlorophyll which absorbs sunlight for photosynthesis.  Photosynthesis begins in the grana and then continues in the stroma. Chloroplasts also contain starch grains and ribosomes.

Describe the process of photosynthesis: The interception of light by chlorophyll (wavelengths absorbed, PAR). The light dependent reaction (splitting of water to release oxygen and electrons, production of ATP and NADPH).


Photosynthetically Active Radiation (PAR)

Light wavelength is the distance between wave peaks that we perceive as colour. The range of wavelengths in photosynthesis is PAR, it is between 400 and 700nm (red+blue light). Chlorophyll absorbs this red and blue light, then each photon of light becomes excited which in turn excites an electron, which in turn excites another electron and so on, creating a chain of reactions known as the electron transport chain.

dependent and independent


Light dependent

  • In the thylakoid membranes,  light is absorbed by the chloroplasts.
  • Water (H2O) is broken down by electrons that are energized by the sunlight. The oxygen (O2) in the H2O is released as a waste product and the hydrogen (H) converts the NADP+ to NADPH.
  •  ADP (Adenosine Di-Phosphate) becomes ATP (Adenosine Tri-Phosphate) by the addition of P (phosphorus). ATP is an energy molecule that is used to create the sugars in the next stage.

The light independent reaction/dark reaction (carbon dioxide fixation, use of ATP, NADPH and electrons from the light reaction to produce sugar (glucose)).

Light independent (aka Calvin cycle)

  • This happens in the stroma and the energized electrons from the light dependent reactions provide the energy for the following processes.
  • The NADPH become NADP again. The ATP becomes ADP +phosphate (P) again.
  • Carboyhydrate (C6 H12 O6) is formed from: Hydrogen (H) originally came from water and then stored as NADPH, released during the light independent reaction. Carbon (C) comes from the carbon dioxide (CO2) , and oxygen (O) which comes from water and air.

The light dependent stage feeds the light independent stage feeds and vice versa.

Significance of enzymes in photosynthesis: catalysts, temperature dependency.

Carbon fixation is catalysed by the enzymes RuBiSCo, and ATPase.

Enzymes have an optimum temperature and the higher and closer to that temperature the plant gets, the faster the enzyme works. However, above that temperature the enzymes slow down again, as they are denatured and finally they stop working.

ATPase catalyses the reaction to change ATP to ADP plus a phosphate ion, this releases energy for other chemical reactions (see above for light dependent and independent reactions for where this happens).

RuBiSCo is responsible for fixing carbon (C) in the light independent stage so that it can be converted to complex sugars (C6 H12 O6)

Specify differences in photosynthetic efficiency in: C3, C4 and CAM plants in outline using NAMED plant examples.


C3 – occurs in  85% of all plants, for example rice or Helianthus annuus. It’s slightly inefficient in that 25% of carbon fixation is lost due to RuBiSCo which causes an oxidation reaction, this is known as photorespiration – whereby oxygen reacts with the carbon to form carbon dioxide which escapes as gas.

C4 – occurs in 3% of all plants, most of which are angiosperms. It has evolved in plants growing in areas with low light or water. For example Zea mays, Cyperus rotundus. It has one extra step before the calvin cycle that reduces photorespiration and loss of carbon fixation. It does this by moving the process to a special part of the leaves that contain little oxygen (which is needed to fix the carbon), leaves with this area are known as Kranz leaves.

CAM – occurs Clusia spp., Sedum spp., some bromeliads, Orchids, and cacti. CAM plants (crassulacean acid metabolism) are very efficient in their use of water, an adaptation needed by xerophytic plants. In CAM plants the stoma are closed during the day, so that water cannot escape, however this also means that gases cannot enter or leave. During the night the stoma open, so that CO2 can enter. CO2 is needed during the day, so it is converted to malate at night and stored until needed.

3.2 Explain how the rate of photosynthesis can be manipulated.

Describe how the following factors influence the rate of photosynthesis and explain how they can be manipulated: Light: light intensity (light compensation point, light saturation point); supplementary and replacement lighting; choice of lamp in relation to wavelength. Carbon dioxide: levels; ventilation; CO2 enrichment methods. Temperature: temperature range, heating, shading, ventilation, damping down. Water and mineral nutrients: the need for adequate levels of irrigation and essential elements for chlorophyll formation (nitrogen, magnesium and iron).

The main limiting factors in photosynthesis are light, temperature and CO2 concentration.



Light compensation points and light saturation point

Light compensation point  – the points on the light curve when the rate of photosynthesis matches the rate of respiration.

Light saturation point – the point at which increasing the light levels no longer increase the photosynthesis rate.

Light limits the light dependent phase of photosynthesis, and that in turn stops the light-independent phase. As light intensity increases the rate of photosynthesis will increase given that other factors are sufficient. However, there is a point of light intensity at which chlorophyll is damaged and photosynthesis stops.

Supplemental light is usually used during the winter, but if growers want to maximise crops then it can be used all year round.

More info here on lighting for the greenhouse.

Light can be supplemented with:

High Intensity Discharge Lamp (HID)s – either Metal Halide (MH) lamps or High-pressure sodium lamps (HPS).

All HIDs are good at lighting a large area without taking up much space. This is important, because lamps that take up a large area will block natural sunlight when the lamps aren’t switched on. HIDs generate a lot of heat, which is an advantage in the winter, but potentially a problem in summer.

MH – create white light, and this is better for some crops.

HPS – create yellow/orange light. They are more efficient and last 3x longer than MH

Fluorescents – cover a large area, more efficient than HIDs. The light wavelength is the least effective for photosynthesis. Inexpensive, widely available and don’t generate so much heat. However, the light is not as intense as HIDs and the units are large, so block out the sun.

LEDs – these have become more popular recently. They are long lasting and give off the least heat. They have a similar spectrum to sunlight, with the highest wavelength in red and blue light, ideal for photosynthesis. They take up small area and are efficient to run. The initial cost is greater, although worth it in the long run.

Carbon Dioxide

Good article where much of this info comes from

CO2 is in the air at the very low percentage of 0.04%, and increasing this percentage increases photosynthesis rate, until a maximum level of carbon fixation is reached. Above 1% and it becomes toxic to plants, causing necrosis and malformation. It can also cause humans to become dizzy.

The simplest form of increasing CO2 is by having vents open, so that as plants use up CO2 it is replenished from otuside. Disadvantages are that levels won’t get higher than 0.04% and if it is too cold or windy outside, vents should not be open. The opposite issue occurs with artificially created CO2 – that air escapes, and air with lower CO2 gets in from outside.

CO2 is heavier than air, so must come from above.

Natural gas, propane or kerosene all give off Co2, but also heat (which may be an advantage, but not automatically so). If fuels happen to be contaminated, then that can harm the plants. They also generate moisture. Both moisture and heat may lead to moulds growing on plants (botrytis)

Propane gas burners – CO2 generators, good for larger areas.

Paraffin heaters – for smaller areas.

Exhale bags filled with mycelium (fungus) give off CO2. These are a cheap, but not hugely effective method.

Liquid carbon dioxide – more expensive, but pure, doesn’t create heat or moisture and is more exact, with a timer and regulator. However, it requires special tank for storage. For smaller areas, simple canisters can be used.

Organic materials – eg manure, coir – give off CO2 while breaking down, this is a simpler way, requires no additional cost or machinery, but only lasts for a month, and can’t be regulated like a machine .


The light dependent stage of photosynthesis is not affected by temperature, but the light independent stage is. The rate of photosynthesis is driven by enzymes (catalysts to the reaction) which have an optimum temperature. The higher the temperature, the better the catalysts work, until they reach optimum.

The optimum temperature for enzymes is 25-35°C, minimum is 5°C max is 45°C. However, different plants have other temperature needs, so a plant is not automatically happier in that temperature range.

Increasing heat

Electric – cheaper to install than gas and requires less maintenance, but daily costs are higher. Also easier to control effectively.

Gas – cheaper to run than electricity.

Paraffin – requires ventilation to release the gases given off, only useful for small scale and crops that require low temperatures.

Ground source heat pumps – are more environmentally sustainable, but expensive to install.

Hot water pipes –requires a complex water system and a lot of water. This is usually considered the most efficient for large areas since the heat spreads evenly throughout the greenhouse.

Biofuel  – sustainable plant based fuels (although their production is harmful to the environment). Require less maintenance, but costly.

Another aspect to be considered when heating, is insulation so that heat is not lost.

Reducing heat

Damping down – a quick, cheap way of cooling a room, plus increasing humidity (depending on the plants, that may or may not be useful).

Shading can be used to reduce heat, but will also reduce sunlight, although if there is a danger of scorch, that is a good thing.

Here’s an article on ventilation and shading

Water and Mineral Nutrients

Note: there is nothing specified as to whether this means outside or inside irrigation, and both are quite different. Light and heat are usually only supplemented inside, so maybe it’s assumed this is the same for irrigation. For this reason, I will talk about watering inside, but include a few articles on outside irrigation, so you can decide for yourself.

Irrigation is essential in a protected environment, and necessary for crop maintenance even outside during dry periods.

Forms of irrigation:  overhead, drip, manual and capillary matting.

Overhead – sprinkler systems, not very efficient (water goes everywhere), all plants are watered the same, and not usually used with feed. Not used with plants in pots, only those on inside/outside beds or large crop areas.

Drip – pipes lead directly to plants (from a main hose). Very efficient and can be used with feed. However it is tricky to install and once installed it’s even more tricky to move the plants around. All plants will be watered the same.

Manual – slow, using labour, however it means that each plant can be watered according to its needs. Often used with feed, and the feed can also be changed part way through for different requirements.

Capillary matting – polythene mats, with an absorbant layer on the bottom and perforated layer on top. The mats are kept permanently saturated. Expensive to set up, but plants are never overwatered because they soak up what they need. Can’t be used with feed.


Nitrogen is a macro nutrient, common in most fertilisers. Levels of nitrogen are shown in the ratio N:P:K (N=nitrogen, P=phosphorus, K=potassium). For example a high nitrogen feed  would be 6:4:4. Magnesium and iron are micro nutrients and require more specialized fertilizers. Iron (Fe) is found in ferrous sulphate, usually a applied as a foliar feed. Magnesium (Mg) is found in epsom salts which will raise Mg without increasing pH, lime will increase Mg and alkalinity.

Article on how to add magnesium to soil

Article for outside irrigation

Article on soaker hose

3.3 Describe the processes of respiration. Structure of the mitochondrion to include: matrix, cristae. Describe aerobic respiration: Glycolysis (sugar (glucose) split to form pyruvate, production of ATP in the cytoplasm).

Aerobic Respiration

Aerobic respiration = glucose breaking down to CO2 +H2O + 38 ATP (energy)

Note: I’ve included more detail than the syllabus requires, that only asks for the glycolysis part and the structure of the mitchondrion. I’ve included the rest for completeness and it may be useful to be aware of it.

1. Glycolysis – 1 molecule of glucose (6C) splits to form 2 molecules of pyruvate (3C). ATP is created, and NAD+ is converted to NADH. This occurs in the cytoplasm.

2. Pyruvate oxidation – The pyruvate is converted to acetyl CoA in the mitchondria matrix, CO2 and NADH is created.

3. Citric acid cycle/ krebs cycle – The acetyl CoA goes through chemical processes which result in ATP, NADH, FADH2 being created and CO2 released. This occurs in the mitochndrial matrix.

4.Oxidative phosphorylation – More ATP is produced in the mitochondria, NADH and FADH2 return to being NAD+ and FAD. Water is formed.

Energy output up to 38 ATP for EACH glucose molecule.


Mitochondrion – consists of two mebranes, an outer one that covers and contains the organelle, and an inner one that is folded over many times into cristae. The outer layer has pores to allow ions and small proteins through. The cristae is folded to create a large surface area. The matrix is a gel like fluid inside the cristae. The mitochondria also contain ribosomes (that syntehsize proteins), mitochondrial DNA and enzymes.

Anaerobic Respiration

Anaerobic respiration = glucose breaking down to ethanol (toxic) and CO2+ 2 ATP

This consists of the same first two stages, both glycolysis and pyruvate oxidation occur. However the next two stages do not happen in anaerobic respiration. Instead a process called fermenation occurs. In fermentation, zymase (an enzyme not used in aerobic respiration) is used to produce ethanol. Ethanol is toxic and will eventualy kill the plant. Anearobic respiration only produces two molecules of ATP, unlike aerobic respiration’s 38.

Anaerobic equation: C6H12O6 + zymase >> 2CO2+2C2H5OH + 2ATP released

Summary. Processes in  RED CAPITALS, products in blue lower case:

Glucose > GLYCOLOSIS > pyruvate > FERMENTATION > lactic acid + ethanol

Describe examples of horticultural situations leading to anaerobic respiration.

Anaerobic respiration is used intentionally when making alcohol, a natural by-product of anaerobic respiration in some plants. For example, barley is fermented to make beer.

Anaerobic respiration is also used by plants as a way of surviving in bad conditions, but it will eventually kill the plant. Some conditions that leads to this are:

  • Waterlogged or compacted soils in which oxygen can’t be accessed.
  • Fruits and vegetables continue to respire after picking. If packaged with too little oxygen, they will respire anaerobically, which causes spoiling.
  • Germinating seeds use anaerobic respiration because they don’t have the ability to take in oxygen yet.

3.4 Explain the movement of water and solutes through the plant.

Explain how water and solutes enter, move through, and leave the plant. To include: apoplast, symplast, endodermis, Casparian strip, transpirational pull, root pressure, capillary action, guttation.



How water enters and moves through the root:

  • Water is drawn into the root hairs by osmosis
  • Water moves through the cortex through the symplast pathway or the apoplast pathway
  • The apoplast pathway ­– water moves through the space outside the plasma membrane. Plant materials can diffuse freely through it, except when blocked – eg by the casparian strip in the roots. CO2 needs to be made soluble in the apoplast before it can travel through the symplast.
  • The symplast pathway – water moves through the cytoplasm around the vacuoles of adjacent cells.
  • Most water travels by the apoplastic pathway, but is then stopped by the casparian strip.
  • Water is pushed across the root by root pressure.

How water moves up the stem:

  • Transpirational pull –  because water evaporates from the surface of the leaves, more water moves to the surface to replace it, and more follows on behind.
  • Capillary action – this is when water can move upwards through a narrow tube despite gravity. It occurs because adhesion of water to the walls of the capillary is stronger than the cohesion between the water molecules. However, the cohesion is what keeps the water flowing.

Endodermis –   this is a single layer of cells around the cortex. It controls water movement into the plant.

Guttation – Transpiration does not usually occur at night because stomata are closed. This leads to a build up of pressure – root pressure – and causes droplets of water to appear on leaf tips or margins in the early morning – guttation. This happens most often in tropical climates.

Describe the significance of the following terms in relation to water movement in the plant: diffusion, osmosis, mass flow, capillarity, adhesion and cohesion.

Diffusion – this is the method by which oxygen in solution in the soil moves into roots, from high concentration to low. Also it enable carbon dioxide and oxygen to move in and out of the leaves through stoma.

Osmosis – this enables water to move into roots from a less concentrated solution (ie where there is more water) to a higher concentrated one. Osmosis is also how water moves across the leaves to the surface where it evaporates.

Mass flow – this is the movement of disolved nutrients through the phloem or water through the xylem. Nutrients are moved from the source to the sink (ie from where the nutrients are stored to where they are needed.) This direction changes throughout the year because sugar-needs change.

Capillarity (capillary action) – how water moves from the roots to the xylem (description above).

Adhesion – water molecules adhere to celluose molecules in walls of xylem

Cohesion – sticking together of water molecules so they form a continuous stream from leaves to roots.

Explain how environmental factors affect the rate of transpiration: temperature, water availability, relative humidity, wind speed.

  • An increase in atmospheric temperature leads to an increased rate of evaporation, decreased relative humidity outside the leaf and increased water potential gradient. Therefore  transpiration increases. (ie when it’s hot, moisture content around the leaf is reduced, so more water is drawn up the plant to replace it).
  • A decrease in water available to the plant leads to water stress which leads to closure of stomata therefore transpiration decreases.
  • A decrease in humidity leads to drier surroundings, gives a steeper water potential gradient leads to an increase in transpiration.
  • In still air, water that has transpired from a plant forms a vapour around the leaf and water potential gradient from inside to outside is slightly less. Transpiration decreases. (ie humidity increases around the leaf so more water is not drawn up the plant to replace it).

Describe the uptake and distribution of mineral nutrients in the plant: nutrients from soil solution/foliar feed, distribution in the xylem and phloem, active uptake against concentration gradient into cells by membrane carriers.

carrier proteins

Membrane carriers transporting proteins against the concentration gradient

Xylem transports water and mineral salts from the roots up to other parts of the plant, while phloem transports sucrose and amino acids between the leaves and other parts of the plant. Because the minerals need to move against the concentration gradient (ie from a low concentration to a high concentration) when moving from the soil to the root hairs, the plant uses active transport. This where membrane carriers in the membrane bind to the mineral on the soil side and then release it on the other side of the membrane inside the root.

Describe transport of sucrose in the phloem: mass flow hypothesis, phloem loading and unloading.


Mass flow hypothesis is the most likely explanation for how sap (sugar and water) moves through the phloem, but nobody is certain this theory is correct. The movement can be in either direction, unlike water moving through the xylem, which only moves from the roots and up the stem.

The sap moves from sugar sources (where sugar is stored) to sugar sinks (where sugar is needed), although where the sinks and sources are changes depending on need and time of year.

Mass flow hypothesis, the stages:

The movement of sap is called translocation.

  1. Phloem loading entails a sieve tube element picking up the sugar solution by active transport. This requires energy from ATP and is controlled by companion cells that are connected to the sieve tube elements.
  2. The sugar increases the concentration in the sap causing water to move into the sieve tube element from the xylem by osmosis.
  3. This creates pressure that pushes sap down the phloem to the sink.
  4. Once the sink is reached, the sugar is unloaded.
  5. The sap now has a lower concentration, so water leaves the phloem by osmosis, lowering the turgor pressure and stopping the movement.
  6. That water is drawn into the xylem by transpirational pull.

Describe the concept of sources and sinks in relation to: plant organs; seasonal changes.

During the growing season (spring, summer) the roots are sugar sources and the growing points are sugar sinks, because the leaves need sugar to grow. During dormancy, the leaves become the sugar source and the roots become the sinks, as sugar is stored.

3.5 Explain the interrelationships between photosynthesis, respiration and transpiration and describe their control in horticultural situations.

Describe how photosynthesis, respiration and transpiration are interrelated when optimising plant yield and/or quality in the following horticultural situations: outdoor planting; intensive glasshouse production; post harvest storage (manipulation of temperature and water loss, controlled and modified atmospheres, climacteric/non climacteric behaviour).

Outdoor planting:  light and carbon dioxide can’t be controlled in outside planting (but are free) water can be added. Transpiration is increased by high winds, high temperature, low humidity and light levels. Wide spacing of plants lowers humidity and consequently increases transpiration, it can also increase access of sunlight to plants. To maximise photosynthesis, plants need positioning in the ideal sunlinght for those plants (some plants prefer shade).

Intensive glasshouse production: carbon dioxide is usually the limiting factor (ie what limits photosynthesis), water must be supplied and light can be increased.

It’s possible to increase CO2 during the day (especially in the case of C3 plants). Humidity can be increased to reduce transpiration (to avoid wilting), but excess humidity can cause botrytis or other fungi to grow. Spacing plants further apart increases photosynthesis, by allowing them access to more light. However, this also increases transpiration, because the water can evaporate more easily, so more water will be needed required.

Post harvest storage: the ideal conditions to prolong storage life is a low temperature and O2 levels, and high CO2, this is to reduce respiration (respiration increases when temps and oxygen levels are high). Fruits transpire after harvest, but without connection to the plant, they can no longer replace that water, so humidity must be kept high to reduce transpiration. However, if too high humidity will encourage the growth of botrytis (mould).

So increasing CO2, decreasing O2, keeping the temperature cool, with humidity not too high or low maintains a good quality of fruit.

Modified atmosphere means the fruit product is sealed, but not manipulated.

In a controlled atmosphere the atmosphere is constantly monitored and adapted.

The effects of ethylene/nitrogen levels and humidity on fruit storage – ethylene starts the fruit ripening process and as the fruit ripens, more is produced which speeds up the process.

Climacteric – some fruits and vegetable continue to ripen after harvest, these are climacteric. Climacteric fruits are often picked before fully ripe, so they can store for longer and then ethylene is controlled.

Non climacteric fruits don’t ripen once picked, so must be picked at the ideal stage to eat.

Article on fruit storage

Good PDF on water loss.

Good article on respiration in fruits

3.6 Explain how plants are adapted to different habitats.

Explain how plants are adapted for: low light levels (shade plants – to include differences in leaf surface area, chlorophyll density, palisade layer); anaerobic conditions (bog plants – to include aerenchyma, pneumatophores); reduced water supply (xerophytes – to include xeromorphic adaptations in Pinus). To include ONE NAMED plant example for EACH adaptation.

Low light levels

Sun and shade leaves can exist within the same plant.


Codiaeum variegatum

Red/purple chlorophyll – to absorb different frequency of light, eg Codiaeum variegatum.

Broader, thinner leaves – to catch more sunlight, eg Monstera deliciosa.

Shade leaves – horizontal leaves; low saturation point; low compensation point; less rubsico produced, more chlorophyll; photosynthesis is limited.




Anaerobic Conditions

Aerenchyma – spaces or air channels in leaves, stems and roots that allows exchange of gases between shoot and root eg in Phaseolus vulgaris.

Pneumataphores – special aerial  roots, either grow down from stem or up from typical roots, they move air from where the plant can access it to where it is needed. Surface is covered in lenticels eg Black mangrove, Avicennia germinans.

Adapted stomata – that can discharge excess water through guttation eg Zantedeschia aethiopica

Carnivorous plants  trap insects for nitrogen which are unavailable in waterlogged soil. eg Drosera spp.

Prop roots for stability eg Ficus elastica.

Other adaptations:

  • Excreting of excess salt eg Spartina.
  • Capable of growing in acidic conditions eg Sphagnum.




Hairy leaves to reduce air flow and increase humidity


Hairy stems – to reduce air flow eg Opuntia erinacea

Spines instead of leaves – to reduce surface area and therefore transpiration. eg Opuntia chlorotica

Increased thickness of cuticle – a thick, waxy surface to the leaf eg Lophophora williamsii


Leaves reduced to spines and thick stems for water storage

Water is stored in bulbs and fleshy stems – eg Saxifraga tridactylites

Other adaptations without examples: Fewer stomata, sunken stomata. Dormant during drought conditions

Xeromorphic adaptations in Pinus

  • Lateral root system with sinker roots – greater surface area to soak up water
  • Tightly assembled needle structure – slows air movement and evaporation
  • Needles covered with waxy cuticle to reduce transpiration