Understanding How Photosynthesis Works, A Comprehensive Guide for Gardeners

Photosynthesis is a fundamental process used by plants, algae, and cyanobacteria (blue-green algae) to absorb the energy of sunlight and convert it into stored chemical energy. They do this by using carbon dioxide (CO₂) from the air, and water (H₂O) from the soil, to produce the sugar glucose (C₆H₁₂O₆). In this process, oxygen (O₂) is released as a byproduct.

The balanced chemical equation for the process of photosynthesis can be written as follows:

6CO₂ (carbon dioxide)+ 6H₂O (water) — sunlight –> C₆H₁₂O₆ (glucose) + 6O₂ (oxygen)

Through the process of photosynthesis, energy from the sunlight is captured and stored in the chemical bonds of glucose molecules, which serve as a primary energy storage molecule in plants.

Glucose (C₆H₁₂O₆) is a carbohydrate, and as the name alludes, it’s a molecule comprised of carbon, hydrogen and oxygen. Carbohydrates contains hydrogen and oxygen in the same ratio as water, which is 2:1.

Plants can use the glucose produced by photosynthesis in the following ways:

• The glucose can be used immediately to provide the plant with energy through respiration. In the process of respiration, glucose is reacted with oxygen within cells, which breaks down the glucose to produce carbon dioxide and water, and to release energy (in the form of an energy-carrying molecule known as ATP or adenosine triphosphate). It’s basically the opposite of the photosynthesis chemical reaction.
• Some of the glucose can be changed into starch and stored in all parts of the plant, ready to be converted back into glucose when it’s needed at a later point in time.

Living organisms, such as plants, that have the remarkable ability to produce their own food using energy from external sources, such as sunlight, are known as autotrophs, (auto, meaning ‘self‘ and troph, meaning ‘feeding‘. Ecologically, they are known as producers, an they play a pivotal role in capturing and storing energy in chemical bonds which serves as the foundation of every food web in every ecosystem.

The Chemistry of Photosynthesis Explained

Photosynthesis is a complex biochemical reaction that takes place in the green parts of plants, primarily in their leaves.

Within plant cells are microscopic organelles (cellular structures) known as chloroplasts where photosynthesis takes place. Plant chloroplasts are round, oval or disk-shaped organelles typically around 2 to 10 μm (micrometers) long and have a distinctive green color due to the presence of chlorophyll, the pigment responsible for capturing light energy during photosynthesis. Although chloroplasts are small and not visible to the naked eye, the collective mass presence gives plants their green coloration.

The number of chloroplasts in plant cells can vary depending on the type of plant cell, and can contain anywhere from a few to several hundred chloroplasts. Be aware that the diagram below only shows a single chloroplast in the plant cell for the sake of clarity and tidiness!

This remarkable process of photosynthesis that occurs in the chloroplasts of plant cells is the driving force behind the production of oxygen and the foundation of the food chain that supports all life on the planet.

We can simplify the chemistry of photosynthesis by breaking it down to make it easier to understand.

To begin with the basics, the process of photosynthesis involves several key components, including sunlight, water, carbon dioxide, and chlorophyll – the green pigment responsible for capturing light energy.

How this happens can be explained in the tree steps below:

Step 1. Light Absorption – The process of photosynthesis begins when chlorophyll molecules absorb sunlight to capture energy. Sunlight is made up of all colours of light in the rainbow combined together. Chlorophyll primarily absorbs blue and red light while reflecting away green light that it can’t use as efficiently, which gives plants their characteristic green color.

The range of light that plants use most efficiently for photosynthesis is known as the PAR (Photosynthetically Active Radiation) which is in the range of 400-700 nm. We can see from the graph below that photosynthesis is the most efficient at 650nm, which is in the red light range, and this wavelength is important for biomass growth when plants are at the flowering stage.

The next highest peak of the graph where photosynthesis operates at a high efficiency is at 450nm, which is in the blue light range, and this wavelength is important for biomass growth mainly when plants are at the vegetative stage (growth of leaves and stems), but is also essential during the flowering stage.

The absorbed light energy is then used to power a series of chemical reactions that occur within the chloroplasts. These reactions collectively make up two main stages, the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).

Step 2. Light-Dependent Reactions – During the light-dependent reactions, water molecules are split into oxygen and hydrogen ions using the energy from absorbed sunlight. Oxygen (O₂) is released as a byproduct, contributing to the oxygen-rich atmosphere that supports life on Earth. The energy-rich hydrogen ions (H⁺) are used to create an energy carrier molecule called ATP (adenosine triphosphate) and another molecule called NADPH (nicotinamide adenine dinucleotide phosphate).

Step 3. Calvin Cycle (Light-Independent Reactions) – The energy carrier molecules ATP and NADPH produced in the light-dependent reactions fuel the Calvin cycle, a series of reactions that occur in the stroma of the chloroplast. During this cycle, carbon dioxide (CO₂) from the atmosphere is combined with the hydrogen ions (H⁺) and energy from ATP and NADPH to produce glucose (C₆H₁₂O₆) – a simple sugar that serves as the primary energy source for plants.

What we’ve explained is a general overview of the chemistry of photosynthesis, but plants have developed different approaches to photosynthesis in order to help them better adapt to their different growing environment, which we’ll explore next.

What Are the C3, C4 and CAM Photosynthetic Pathways Used by Plants?

Plants have evolved three different types of photosynthetic pathways, (the C3, C4, and CAM plant photosynthetic pathways) that they use to capture and convert carbon dioxide (CO₂) into organic (carbon-containing) compounds such as sugars.

These pathways are adaptations to different environmental conditions, especially variations in temperature and water availability.

C3 Photosynthesis

The majority of plants utilise the C3 photosynthetic pathway, named after the three-carbon compound that’s initially formed during carbon fixation. In this pathway, carbon dioxide is directly incorporated into a three-carbon compound, which is then used to generate sugars through the Calvin cycle.

While efficient under normal conditions, the C3 pathway can be limited by water availability and high temperatures due to increased water loss through transpiration. C3 plants are adapted for moderate temperature and moisture conditions but are less efficient in hot and dry environments.

Most plants on Earth use the C3 photosynthetic pathway. This includes many crops and common garden plants, such as:

• Wheat (Triticum spp.)
• Rice (Oryza sativa)
• Soybeans (Glycine max)
• Peas (Pisum sativum)
• Sunflowers (Helianthus annuus)
• Spinach (Spinacia oleracea)

C4 Photosynthesis

Plants that employ the C4 photosynthetic pathway have evolved to overcome some of the limitations of the C3 pathway. They create a separation of carbon fixation and sugar production processes. In this pathway, carbon dioxide is initially captured by a four-carbon compound in specialized mesophyll cells. This compound is then shuttled to bundle sheath cells, where it releases carbon dioxide for the Calvin cycle.

C4 plants have evolved a more efficient carbon fixation mechanism, and typically use water more efficiently, making them well-suited to hot and dry environments. They therefore can thrive in environments with high temperatures and intense sunlight, and are often found in tropical and subtropical regions with such conditions.

Some examples of C4 plants include:

• Maize (Zea mays)
• Sugarcane (Saccharum officinarum)
• Sorghum (Sorghum bicolor)
• Switchgrass (Panicum virgatum)
• Bermuda grass or couch grass (Cynodon dactylon)

CAM Photosynthesis (Crassulacean Acid Metabolism)

CAM plants have evolved a unique photosynthetic pathway with a temporal (timing) separation of carbon fixation and sugar production.

• At night, they open their stomata to minimize water loss through transpiration, and capture carbon dioxide which is stored as an organic acid.
• During the day, the stomata are closed to reduce water loss, and the stored carbon dioxide is released to support the Calvin cycle.

This strategy allows CAM plants to survive in extremely arid conditions, as they can effectively utilize stored carbon dioxide for photosynthesis without excessive water loss. CAM plants are common in arid regions and are highly efficient at conserving water, making them well-suited for desert environments.

Some examples of CAM plants include:

• Pineapple (Ananas comosus)
• Agave (Agave spp.)
• Aloe Vera (Aloe barbadensis miller)
• Jade plant (Crassula ovata)
• Cacti (various species)
• Sedum (Sedum spp.)

In Which Leaf Cells Does Most Photosynthesis Occur?

The various cells in a plant leaf perform different functions, and in this section we’ll look at the roles they perform, including the process of photosynthesis.

The Leaf Epidermis and Cuticle

The epidermis is the outermost layer of cells of a plant leaf, covering the upper and lower surfaces. It’s usually compromised of a single layer of tightly-packed cells which regulates the exchanges between the plant and its environment, such as the loss of moisture, the exchange of gases (carbon dioxide and oxygen), and the transmission (admission) of sunlight for photosynthesis.

Additionally, the epidermis secretes a protective waxy cuticle made of the compound suberin, a biopolymer which serves as a protective barrier, and forms gastight and watertight layer that provides insulation or protection from the surroundings and reduces evaporation of water from the leaf tissue. The upper cuticle layer may be thicker than the lower one, and cuticle layers tend to be thicker in drier climates.

Some plants have hairy leaves, and these epidermal hairs serve various functions, such as discouraging herbivores, limiting the effects of wind, and trapping a layer of moisture to reduce evaporative water loss from the leaf.

The Mesophyll Layer and Photosynthesis

The next layer of plant leaf cells is the mesophyll (which means “middle leaf”) and includes the tissues which make up most of the leaf interior. In most plants, the majority of the photosynthesis is carried out in the mesophyll layer, so most are made of thinner-walled parenchyma cells or collenchyma cells with chloroplasts.

Parenchyma cells are simple, thin walled, undifferentiated living cells which form a large majority of many plant tissues, while collenchyma cells which also form a majority of plant tissue, are living elongated cells with irregular cell walls whose flexibility allows them to provide structural support without inhibiting the growth of the plant.

In flowering plants and ferns, two different layers of cells make up the mesophyll:

• The palisade mesophyll in the upper layer of the leaf is comprised of columnar cells that contain many chloroplasts are responsible for capturing most of the sunlight and carrying out most of the photosynthesis.
  
  These cells have slight but precise separations between them to maximise the surface area for exchange of carbon dioxide by diffusion, and capillary movement of water to provide the materials required for photosynthesis.
  
  There may be as many as five layers of palisade cells in leaves that are exposed to high levels of sunlight, while leaves that are shaded may contain one layer only.
  
• The spongy mesophyll in the lower layer of the leaf is made up of more rounded and loosely packed cells that contain fewer chloroplasts, and have larger air spaces between them.
  
  This layer of cells is closely associated with the stomata (breathing pores mainly on the underside of the leaf), and the larger air spaces allow for more efficient diffusion of oxygen, carbon dioxide and water vapour through the stomata when they are open.

The Role of Leaf Stomata

Since the waxy cuticle on the leaf if watertight and gastight, it limits water loss, but also inhibits the uptake of carbon dioxide and release of oxygen.

Stomata (which means “little mouths”) are tiny breathing pores on the surfaces of plant leaves, mainly on the underside (lower epidermis). The opening and closing of stomata allow for the exchange of gases such as water vapor, carbon dioxide, and oxygen, between the plant and its environment.

Daytime Process

Stomata typically open during the day, in the presence of light, when photosynthesis occurs, and the plant needs to take up carbon dioxide for the process. When the stomata are open, water vapor is also released through the stomata in a process called transpiration.

Transpiration is the process by which water is absorbed by plant roots, moves through the plant, and then evaporates from aerial parts such as leaves, stems, and flowers into the atmosphere.

• Plants absorb water from the soil through their root hairs. The roots create a concentration gradient by actively transporting mineral ions into the root cells, causing water to move into the roots by osmosis.
• Once water is absorbed by the roots, it moves upward through the plant’s vascular system, which is facilitated by capillary action (cohesion of water molecules to each other and adhesion of water molecules to the walls of the plant vascular tissue).
• When the water reaches the leaves, if the stomata are open, the water molecules in the leaf cells can move to the leaf surface and evaporate into the surrounding air. This process is called transpiration.

Night-time Process

Stomata usually close or partially close during the night, because in the absence of light, photosynthesis is not actively occurring, and the plant conserves water by reducing water vapour loss of through transpiration.

The opening and closing of stomata are controlled by specialized cells called known as guard cells, which surround each stoma. The turgor pressure within these guard cells changes in response to various environmental factors, including light intensity, humidity, and the plant’s water status, influencing the degree of stomatal opening or closure.

It’s important to note that CAM (Crassulacean Acid Metabolism) plants we discussed earlier, which are adapted to survive in hot, harsh arid environments, open their stomata at night instead.

• CAM plants perform photosynthesis at night to reduce water loss and fix carbon dioxide by storing it as organic acids, such as malic acid, which can be utilized during the day when their stomata are typically closed. This process, known as carbon fixation, involves the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) which catalyses the conversion of atmospheric carbon dioxide into an organic compound.

How Do Plants With Coloured Leaves Photosynthesise?

As mentioned earlier, the green color of plant leaves is a result of the pigment chlorophyll. This pigment absorbs light, with a preference for the red and blue wavelengths, while it absorbs significantly less green light. Instead, it reflects green light, allowing it to reach our eyes, creating the perception of green foliage.

In some plants with colourful or variegated leaves, the green colour of chlorophyll that is actually present is hidden by pigments of other colours, such as carotenoids that are yellow and orange, or anthocyanins that are responsible for red, purple and blue colours.

Respiration in Plants, the Opposite Process of Photosynthesis

Respiration in plants is the vital process through stored energy, primarily in the form of glucose, is transformed into accessible energy in the form of the energy transport molecule adenosine triphosphate (ATP).

The typical form of respiration in plants is aerobic respiration, which occurs in the presence of oxygen. It involves the breakdown of glucose, resulting in the production of ATP, carbon dioxide, and water, which is essentially the reverse of the photosynthesis process which captures the energy in the first place and makes it available to be released later on through respiration when needed.

The process of respiration occurs within the mitochondria of plant cells, and supplies plants with the energy for various cellular activities and physiological functions to support plant growth, maintenance and reproductive processes.

References

• CK-12 Foundation. (n.d.). Leaf Structure and Function – Advanced ( read ) | Biology. https://www.ck12.org/biology/Leaf-Structure-and-Function/lesson/Leaf-Structure-and-Function-Advanced-BIO-ADV/
• Biology 2E, the cell, Photosynthesis, Overview of photosynthesis. (n.d.). OpenEd CUNY. https://opened.cuny.edu/courseware/lesson/644/overview
• Dehigaspitiya, P., Milham, P., Ash, G.J. et al. Exploring natural variation of photosynthesis in a site-specific manner: evolution, progress, and prospects. Planta 250, 1033–1050 (2019). https://doi.org/10.1007/s00425-019-03223-1
• Light absorption for photosynthesis. (n.d.). http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/ligabs.html