Leaf photosynthesis and respiration

Leaf photosynthesis and respiration

The leaf between photosynthesis, respiration and colored pigments

The leaf is the plant organ that contains the highest number of regulating functions of plant life: responsible for the synthesis of sugars, the production of oxygen, transpiration, guttation, the wonderful autumn colors of some species and important source of food for animals and humans.

It is essential for all gardeners to know the leaf in depth: is it not the first revealing of plant malaise, with its wilting, drying out and the symptoms of the most dangerous pathologies?

If the trunk talks about the history of the plant and its growth over time, the leaf talks about the present of the plant and its life at that moment.

Without getting lost in the maze of botany, we propose an analysis that has as its subject the leaf organ in general and its functions.


A leaf consists of a lamina furrowed by ribs, which converge in the petiole, which is a sort of connection with the hydraulic system of the branches.

Not all leaves have the same structure, some have modified laminae or have no petiole and still others have enlarged parts that make them very different from the classic leaflet that a child can draw on a school notebook.

In the leaf there are integumental tissues, which protect it from external agents and also have the function of regulating gas exchanges. Then there is a special tissue called the chlorophyll parenchyma in which photosynthesis takes place.

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In all the green parts of the plant and in particular in the leaf there is chlorophyll, a pigment able to capture the light rays and exploit their energy:

6 molecules of water + 6 molecules of carbon dioxide are transformed into a molecule of glucose with the release of 6 molecules of oxygen, in the most famous chemical process in the world, chlorophyll photosynthesis (from the Greek photo = light and synthesis = construction).

6 CO2 + 6 H2O → C6H12O6 + 6 O2

The plant is composed entirely of substances that derive from glucose: cellulose, starch and everything that can be useful to the plant is created in the leaf.


It is the process by which the loss of water vapor from the leaf surface occurs. From the roots the water travels in conductive tissues that carry it up to the leaves, which act as a kind of valve for the plumbing of the plant. The vapor escapes through the stomata and the quantity at the outlet is directly proportional to the opening of the stomata.


It is the reverse process of photosynthesis, it uses sugars and oxygen to create energy, it occurs in every part of the plant not exposed to light, not only in the green ones. Breathing takes place therefore also in the leaves; consuming sugars produces energy necessary for the plant.

Breathing consumes oxygen, so a plant at night or in general in poorly lit environments removes oxygen from the atmosphere; hence the scientific explanation of the popular admonition that suggests not placing plants in bedrooms.

Respiration, photosynthesis, transpiration and guttation regulate the life of our plants: the health of a plant and its growth depend on the plant-environment relationship, atmospheric humidity, precipitation, solar radiation and root conditions influence the aforementioned processes, determining the development values.

An example :

a tomato plant, grown in pots, received abundant irrigation and nitrogen fertilization, consequently there was an excessive development of the epigeal part and in particular of the foliar apparatus.

In the summer following a period of drought, the plant closes the stomata to minimize water losses and consequently slows down the photosynthetic activity, decreasing the production of glucose.

Breathing on the other hand does not stop, because it is independent of the photosynthetic activity and continues to consume the sugars of the plant. Summarizing and simplifying, the plant consumes more than it produces, creating an imbalance that will result in less fruit production.

If the drought persists, having a plant with a very luxuriant epigeal part, will therefore be a disadvantage.


When transpiration is unable to expel water vapor due to excessive atmospheric humidity, the leaves directly expel water drops with the guttation process. This also happens when the roots absorb too much water.

Leaf photosynthesis and respiration: Pigments

Why are the leaves green, in some species red, or variegated and change their color as the seasons change?

The coloring of the leaves is linked to the presence of 4 pigments which, in addition to performing particular functions, color the leaf surface.

chlorophyll = green

anthocyanins = red

carotenes = orange

xanthophylls = yellow

In the autumn season with the gradual disappearance of chlorophyll due to the decrease in photosynthetic activity, the other pigments prevail, hence the different shades of the autumn foliage.

Video: Transportation in Plants (October 2021).