THE CYTOPLASM AND ITS ASSOCIATED STRUCTURES
The cytoplasm includes the area inside the plasma membrane, but outside the nucleus. All of the specialized organelles in the eukaryotic cell sit in the cytoplasm.
free ribosomes
Ribosomes are the sites of protein synthesis in the cell. Free ribosomes are located in the cytoplasm. They are not attached to the ER. Free ribosomes synthesize proteins that are used inside the cell to build structures and catalyze chemical reactions. The instructions for building these proteins is encoded in the DNA.
endoplasmic reticulum
The endoplasmic reticulum is a series of continuous membranes that loop back and forth between the nucleus and plasma membrane. There are two types of endoplasmic reticulum in the cell, the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
rough endoplasmic reticulum
The rough endoplasmic reticulum is studded with ribosomes giving it a rough appearance. It is continuous with the nuclear membrane. This means it extends out directly from the nuclear membrane into the cytoplasm. Look at how the RER above (pink) extends directly from the nuclear membrane (purple). The ribosomes on the rough endoplasmic reticulum make proteins that are exported from the cell.
smooth endoplasmic reticulum
The smooth endoplasmic reticulum is not studded with ribosomes giving it a smooth appearance. The membranes of the smooth endoplasmic reticulum extend from the rough endoplasmic reticulum in the cytoplasm. Look at how the SER (peach) extends directly from the RER (pink) in the figure above. The SER has two functions; it synthesizes lipids and detoxifies harmful substances.
golgi apparatus
The golgi apparatus looks like a stack of flattened pancakes and is located in the cytoplasm of the cell. All of the proteins and lipids synthesized by the RER and SER are sent to the golgi. The golgi sorts, modifies, and packages the products of the RER and SER before sending them to their final destination inside or outside of the cell. The golgi works like a post office. The post office sorts packages by destination (domestic and international). It boxes and labels the packages and then distributes them to their final destination.
Lysosomes
Lysosomes are located in the cytoplasm of the cell. These organelles are packed with digestive enzymes and break down food, wastes, and old or defective organelles. They are like the garbage disposal of the cell.
Vacuoles
The vacuoles in the cytoplasm are used to store food, water and waste products. Plants cells have one large, central vacuole that is important in maintaining turgor pressure. Animal cells have numerous, small vacuoles.
mitochondria
Inside the cytoplasm of all eukaryotic cells are tubular sacs called mitochondria. The mitochondria has a smooth outer membrane, and a highly folded inner membrane. Within these membranes, energy in the form of ATP is produced to fuel cellular activities. This is why the mitochondria is called the “powerhouse” of the cell.
chloroplasts
Chloroplasts are unique to plant cells and photosynthetic protists. Animal cells do not have chloroplasts. The cytoplasm of a typical plant cell has multiple chloroplasts. Here, the energy from sunlight is captured and used to synthesize sugars (glucose). The pigment chlorophyll that captures the energy of sunlight gives the chloroplasts their green color.
plastids
Plastids are a storage organelle unique to plant cells and some photosynthetic protists. They are not found in animal cells. Located in the cytoplasm, the plastids are used to store substances such as pigments and starches. The pigments stored in the plastids of fruits and vegetable cells give them their red, yellow and orange colors when they are ripe. The glucose produced from photosynthesis is stored in the form of starch in the plastids year after year.
cytoskeleton
The cytoskeleton extends through out the cytoplasm. Composed of protein fibers called intermediate filaments, microtubules and microfilaments, the cytoskeleton maintains cell shape, allows the cell to move, and moves structures within the cell. For example, when a cell ingests a food particle by phagocytosis, it extends its plasma membrane around the food particle. The cytoskeleton is what moves the plasma membrane. Proteins and lipids made on the RER and SER are packaged into vesicles and transported to the golgi for modification. The cytoskeleton is what transports the vesicle.
centrioles
A centriole is a short, barrel shaped ring composed of nine microtubules around an empty center. Centrioles are found only in animal cells. They are arranged in pairs that sit perpendicular to one another. They are located in the cytoplasm next to the plasma membrane and serve two functions. They are the base that anchors cilia and flagella to the plasma membrane and they are used during cell division.
