Organelles
Organelles are specialized or organized structures, within a living eukaryotic cell, which allows for healthy functioning
Cell Wall
Plant cells have a rigid wall surrounding the plasma membrane; cellulose is principal structural component
Protects cell; provides structure
Primary (1º) cell wall: flexible, due to impregnation of pectin in a cellulose matrix
Secondary (2º) cell wall: usually rigid, due to impregnation of lignin in cellulose matrix
Plasmodesmata: strands of cytoplasm between cells that allow dissolved substances to travel
Above: Cell walls (and chloroplasts) of a moss
Nucleus
Serves as the information processing and administrative center of the cell
Stores the cell's hereditary material, or DNA, bound up in chromosomes
Coordinates the cell's activities, which include growth, metabolism, protein synthesis, and reproduction (cell division)
Nucleolus
Site of synthesis of ribosomal RNA (rRNA)
Crucial to making proteins from messenger RNA (i.e. Translation)
Discovery of nucleus
Nucleus was discovered by Robert Brown in 1831 while observing orchid epidermis and pollen
He also discovered "Brownian Motion" while observing pollen grains suspended in water
Brown noticed that they were jiggling, but couldn't understand why.
Einstein in 1905 provided an explanation... the pollen was being hit by unseen water molecules moving around.
Above: Allium (onion) nuclei, in purple
Plastid
Chloroplast
This organelle that is the site of photosynthesis
Contains chlorophyll for intercepting site
Thylakoid: Double-membrane “coins” that contain chlorophyll
Grana: Stack of thylakoids
Stroma: Colorless fluid matrix containing enzymes
Chloroplasts that have not been exposed to light are called etioplast
Chloroplasts possess their own DNA
The endosymbiont hypothesis proposes that these structures were originally free-living (cyano)bacteria, that fused with an early cell to become the chloroplast
Chromoplast
Plastid that contains non-chlorophyll pigments
Used by plants for attraction or warning
e.g. Provide coloration in the cells of flower petals and/ or pulpy fruit to attract animal pollinators and dispersers, respectively
Leucoplast
Plastid that is colorless, and contains plant storage materials
Amyloplast: Starch-containing leucoplast
Elaioplast: Oil-containing leucoplast
Proteinoplast: Protein-containing leucoplast
Above: Types of plastids found in plants
Above: Chloroplasts in plant cells
Above: Amyloplasts in cells of a potato tuber, stained with iodine to make the starch visible
Plasma or Cell Membrane
It is a thin, outer boundary of the living part of the cell
The membrane encloses the cellular contents of all living cells
It regulates the passage of molecules in and out of the cells.
Ribosome
All living cells contain ribosomes (including bacteria)
They are tiny, 0.02μm or 2nm in size, and ellipsoidal in shape
Tiny organelles composed of approximately 60% RNA and 40% protein
Assemble proteins under the instructions of DNA
Vacuole
Each plant cell has one large, central vacuole which plays an important structural role
The vacuole is filled with cell sap, which maintains osmotic pressure in the cell
The vacuole also stores compounds, such as water-soluble pigments like anthocyanins
Dictyosomes
Plants cells contain smaller Golgi Apparatus-type vesicles, which are called dictyosomes
They are sacs roundish in shape but appear flattened
The dictyosomes function in the collection, packaging, and delivery of the cell's chemical products
They also modify proteins and fats
Dictyosomes can be built into the endoplasmic reticulum, where they prepare materials for export outside of the cell.
Above: Golgi apparatus in a plant parenchyma cell
Endoplasmic Reticulum (ER)
The ER is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell
Connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm
The ER connects cells via the plasmodesmata
Rough Endoplasmic Reticulum: ER lined with ribosomes and involved in protein synthesis
Smooth Endoplasmic Reticulum: ER lack ribosomes, and is involved in the transportation of other materials through the cell
Mitochondria
Mitochondria are oblong in shape and very small in size (1.0-3.0 μm)
These structures are numerous and found in the cytoplasm of (almost) all eukaryotic cells
Mitochondria are organelles that have two membranes
Mitochondria possess their own DNA
The endosymbiont hypothesis proposes that these structures were originally free-living bacteria, that fuse with an early cell to become the mitochondrion
Through a process called "respiration", the mitochondria break down carbohydrates (sugars), to provide energy to the cell
Carbohydrates + Oxygen --> Caron Dioxide + Water
Above: Diagram of a mitochondrium
Additional Resources
Scientists discover first nitrogen-fixing organelle (Phys.org 11Apr2024)
└Coale et al. (2024) Nitrogen-fixing organelle in algae
└Massana (2024) The nitroplast: a nitrogen-fixing organelle
First eukaryotes found without a normal cellular power supply (Science 12May2016)