In the early $${}1880$$s, German botanist Andreas Schimper ($${}1856$$–$${}1901$$) became the first to describe the chloroplasts of plant cells. He identified their crucial role in starch formation during photosynthesis and noted that chloroplasts divided independently of the cell's nucleus.

Andreas Schimper

Based on the ability of chloroplasts to reproduce independently, Russian botanist Konstantin Mereschkowski ($${}1855$$–$${}1921$$) suggested in $${}1905$$ that chloroplasts may have originated from ancestral photosynthetic bacteria living symbiotically inside a eukaryotic cell. He proposed a similar origin for the nucleus of plant cells. This represented the first articulation of the endosymbiotic hypothesis to explain how eukaryotic cells evolved from ancestral bacteria.

Konstantin Mereschkowski

The modern endosymbiotic theory states that mitochondria and chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host. Mounting genetic evidence, particularly from DNA sequencing, supports this theory by demonstrating similarities between these organelles and bacteria.

Endosymbiotic Theory

A multi-membrane organelle where photosynthesis occurs in eukaryotes

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- Chloroplast
- Photosynthetic pigment
- Photosystems
- Light-harvesting complex
- Reaction center

Features of Photosynthesis

Eukaryotes have membrane-bound organelles, each with their own structure and function. Organelles commonly found in eukaryotes include:
- Nucleus
- Endoplasmic Reticulum
- Golgi Apparatus
- Lysosome
- Vacuole
- Mitochondria
- Chloroplast
- Peroxisomes

Organelles Commonly Found in Eukaryotes

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Chloroplast

A pigment molecule that absorbs light energy for conversion into chemical energy

Photosynthetic pigment

Units of photosynthetic membranes within photosynthetic membranes that each have a light-harvesting complex and a reaction center

Photosystems 

A group of proteins and pigments that may be excited by absorbing light energy and then transfer said energy between pigment molecules until it reaches a reaction center

Light-harvesting complex

A protein complex with a pigment molecule that oxidizes when excited by the light-harvesting pigment, losing an electron (the chemical energy product of the photosynthetic pathway)

Reaction center

The nucleus is a membrane-bound organelle that serves as the defining feature of eukaryotic cells. It houses the DNA genome within a complex nuclear membrane, controls all cellular activities, and plays an essential role in reproduction and heredity.

Nucleus

The Golgi apparatus is an organelle within the endomembrane system that consists of a series of stacked, flat membranous disks. It receives proteins and lipids from the endoplasmic reticulum and uses enzymes to modify them, frequently by adding carbohydrate components to produce glycoproteins, glycolipids, or proteoglycans.

The Golgi Apparatus

Lysosomes are membrane-bound organelles belonging to the endomembrane system that contain digestive enzymes. They function to break down external particles, such as food, microorganisms, and immune complexes, as well as internal debris like damaged organelles, while keeping these enzymes compartmentalized to protect the cytoplasm.

Lysosomes

Vacuoles are formed off of the plasma membrane in a process called phagocytosis.

Vacuole

The mitochondria is responsible for aerobic respiration in eukaryotes. It contains two lipid membranes which are the inner and outer membranes. The inner membrane is the site for the electron transport chain for aerobic respiration. The mitochondrial matrix is also an important site for metabolic enzymes and the mitochondrial DNA.

Mitochondria

Peroxisomes are membrane-bound eukaryotic organelles that produce hydrogen peroxide to break down molecules such as uric acid, amino acids, and fatty acids. They contain the enzyme catalase to degrade hydrogen peroxide, and they function alongside the smooth endoplasmic reticulum (SER) in lipid biosynthesis, while protecting the cytoplasm from oxidative damage through compartmentalization.

Peroxisome

The endoplasmic reticulum (ER) is a network of membranous tubules and flattened sacs called cisternae, all bounded by a single lipid bilayer. The ER is continuous with the nuclear envelope, and it is divided into two structurally and functionally distinct regions—the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER)—which serve as sites for synthesizing different cellular molecules.

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