Cross section of a Chlamydomonas reinhardtii algae cell, a 3D representation
DIC image of Scenedesmus quadricauda with the pyrenoid (central four circular structures) clearly visible.

Not to be confused with pyrena, a nutlet within a drupaceous fruit.

- Pyrenoid

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The name Hornwort also refers to aquatic plants of the genus Ceratophyllum, in the family Ceratophyllaceae

Life cycle of a typical hornwort Phaeoceros. Click on the image to enlarge.
The hornwort Dendroceros crispus growing on the bark of a tree.

In half of the roughly 200 hornwort species, this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food.

Chlamydomonas reinhardtii

Single-cell green alga about 10 micrometres in diameter that swims with two flagella.

C. reinhardtii trajectory, in HSA (culture medium), under red light.

It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an eyespot that senses light.


Enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose.

A 3d depiction of the activated RuBisCO from spinach in open form with active site accessible. The active site Lys175 residues are marked in pink, and a close-up of the residue is provided to the right for one of the monomers composing the enzyme.
Active site of RuBisCO of Galdieria sulphuraria with CO2: Residues involved in both the active site and stabilizing CO2 for enzyme catalysis are shown in color and labeled. Distances of the hydrogen bonding interactions are shown in angstroms. Mg2+ ion (green sphere) is shown coordinated to CO2, and is followed by three water molecules (red spheres). All other residues are placed in grayscale.
Location of the rbcL gene in the chloroplast genome of Arabidopsis thaliana (positions ca. 55-56.4 kb). rbcL is one of the 21 protein-coding genes involved in photosynthesis (green boxes).
Two main reactions of RuBisCo: CO2 fixation and oxygenation.
A 3D image of the active site of spinach RuBisCO complexed with the inhibitor 2-Carboxyarabinitol-1,5-Bisphosphate, CO2, and Mg2+. (PDB: 1IR1; Ligand View [CAP]501:A)
Overview of the Calvin cycle and carbon fixation.
Figure 3. In this figure, each protein chain in the (LS)2 complex is given its own color for easy identification.

Some plants, many algae, and photosynthetic bacteria have overcome this limitation by devising means to increase the concentration of carbon dioxide around the enzyme, including carbon fixation, crassulacean acid metabolism, and the use of pyrenoid.


Type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells.

Chloroplasts visible in the cells of Bryum capillare, a type of moss
Euglena, a euglenophyte, contains secondary chloroplasts from green algae.
Chlorarachnion reptans is a chlorarachniophyte. Chlorarachniophytes replaced their original red algal endosymbiont with a green alga.
Scanning electron micrograph of Gephyrocapsa oceanica, a haptophyte.
The photosynthetic pigments present in their chloroplasts give diatoms a greenish-brown color.
Ceratium furca, a peridinin-containing dinophyte
Karenia brevis is a fucoxanthin-containing dynophyte responsible for algal blooms called "red tides".
Dinophysis acuminata has chloroplasts taken from a cryptophyte.
Chloroplast DNA replication via multiple D-loop mechanisms. Adapted from Krishnan NM, Rao BJ's paper "A comparative approach to elucidate chloroplast genome replication."
Over time, base changes in the DNA sequence can arise from deamination mutations. When adenine is deaminated, it becomes hypoxanthine, which can pair with cytosine. During replication, the cytosine will pair with guanine, causing an A --> G base change.
Transmission electron microscope image of a chloroplast. Grana of thylakoids and their connecting lamellae are clearly visible.
Instead of an intermembrane space, glaucophyte algae have a peptidoglycan wall between their inner and outer chloroplast membranes.
Granum-stroma assembly structure The prevailing model of the granum-stroma assembly is stacks of granal thylakoids wrapped by right-handed helical stromal thylakoids which are connected to large parallel sheets of stromal thylakoids and adjacent right-handed helices by left-handed helical structures. (Based on ).

Some contain pyrenoids.


Process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities.

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.
Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
Light-dependent reactions of photosynthesis at the thylakoid membrane
The "Z scheme"
Overview of the Calvin cycle and carbon fixation
Overview of C4 carbon fixation
Plant cells with visible chloroplasts (from a moss, Plagiomnium affine)
Portrait of Jan Baptist van Helmont by Mary Beale, c.1674
Melvin Calvin works in his photosynthesis laboratory.
The leaf is the primary site of photosynthesis in plants.
Absorbance spectra of free chlorophyll a ( blue ) and b ( red ) in a solvent. The action spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment–protein interactions.

Pyrenoids in algae and hornworts also act to concentrate around RuBisCO.


Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis.

PhotorespirationFrom left to right: chloroplast, peroxisome, and mitochondrion
Oxygenase activity of RuBisCO
Overnight graph of absorbed by a CAM plant
In C2 plants, the mitochondria of mesophyll cells have no glycine decarboxylase (GDC).

In nearly all species of eukaryotic algae (Chloromonas being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the pyrenoid.

Jean Pierre Étienne Vaucher

Swiss Protestant pastor and botanist who was a native of the Republic of Geneva.

Door of the Theses in Wittenberg, Saxony-Anhalt, Germany

He is credited for describing the development of the networks that occur in the cells of Hydrodictyon (water net algae), and for describing the pyrenoid of algae.


Genus of green algae in the family Chlamydomonadaceae.

Estimated accepted genus totals by kingdom - based on Rees et al., 2020

It is closely related to the model green algae, Chlamydomonas, and distinguished mainly through the absence of a pyrenoid.


Genus of green algae consisting of about 150 species all unicellular flagellates, found in stagnant water and on damp soil, in freshwater, seawater, and even in snow as "snow algae".

Drawings of Chlamydomonas caudata Wille.
Cross section of a Chlamydomonas reinhardtii cell
Light micrograph of Chlamydomonas with two flagella just visible at bottom left
Chlamydomonas globosa, again with two flagella just visible at bottom left

The nucleus is enclosed in a cup-shaped chloroplast, which has a single large pyrenoid where starch is formed from photosynthetic products. Pyrenoid with starch sheath is present in the posterior end of the chloroplast.

Chlorella pyrenoidosa

Species of the freshwater green alga in the Division Chlorophyta.

The species name pyrenoidosa refers to the presence of a prominent pyrenoid within the Chlorella chloroplast.