Organelles

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 

Transcription & Translation

• The nucleus of a cell has DNA, which acts like a blueprint for making components for survival (e.g., enzymes, proteins, etc.)

• Transcription: the double-stranded DNA is unwound, "read" by a (polymerase) enzyme, and a complimentary single-stranded "messenger RNA" (mRNA) is created from this DNA template.

• Translation: The mRNA strand is "read," and an organic compound is constructed from it (e.g., protein, enzyme)

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.

Plastids

Chloroplast

• This organelle is the site of photosynthesis 

• Contains chlorophyll for intercepting light

• 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

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.