Cell Structures and Organelle Functions in Eukaryotic Cells

Cell Wall

Cell wall: A secreted outer layer on the external surface of the plasma membrane. Its functions include providing shape and rigidity to the cell (especially in plant cells), maintaining osmotic balance, uniting adjacent cells to allow fluid exchange and cellular communication, and serving as a barrier against pathogens.

Plasma Membrane

Plasma membrane: A thin shell that surrounds and separates the cytoplasm from the external environment. The cell contents therefore have a different composition than the outside environment. The plasma membrane regulates exchange with the exterior and is primarily formed by lipids and proteins.

Cytoplasm

Cytoplasm: Found in both prokaryotes and eukaryotes. It occupies the space between the nucleus (in eukaryotes) and the plasma membrane. The cytoplasm is composed of three main components: the cytosol, the cytoskeleton, and various organelles.

Cytosol

Cytosol: An aqueous fluid containing a large quantity of dissolved substances (glucose, amino acids, minerals, proteins). It is the site of several important metabolic reactions such as glycolysis and glycogen synthesis.

Cytoskeleton

Cytoskeleton: Present in eukaryotes and formed by a network of protein filaments that serve as the cell's internal support. The cytoskeleton determines cell shape, organizes internal organelles, and enables cellular movement.

Ribosomes

Ribosomes: Non-membranous complexes composed of RNA and proteins. Ribosomes can be free in the cytoplasm or associated with the rough endoplasmic reticulum (RER) or the nuclear membrane. They are involved in protein synthesis.

Centrioles

Centrioles: Structures formed by stable microtubules. They are related to cellular division and to the formation of cilia and flagella. Centrioles are composed of protein (microtubules formed of tubulin). They are present in many eukaryotes but are generally absent in higher plants. Their functions include organizing cilia, flagella, and the spindle apparatus during cell division.

Cilia and Flagella

Cilia and flagella: Mobile appendages on the cell surface. Cilia are short and numerous, while flagella are fewer and long. Their function is to move the cell through a liquid medium or to move extracellular fluid over the cell surface.

Lysosomes

Lysosomes: Membrane-bound vesicles present in eukaryotic cells whose interior contains many hydrolytic enzymes (hydrolases) capable of digesting most biological macromolecules. Their primary function is cellular digestion and recycling of macromolecules.

Peroxisomes

Peroxisomes: Membrane-bound organelles present in eukaryotic cells. Their membrane may originate in part from the endoplasmic reticulum, and they contain oxidative enzymes. Their functions include:

• Oxidative reactions: Various oxidases oxidize organic compounds; during these reactions electrons are transferred to oxygen, forming hydrogen peroxide (H2O2), which is then degraded within the peroxisome.
• Detoxification: Peroxisomes contain enzymes that remove or neutralize toxins within the cell.

Endoplasmic Reticulum (ER)

Endoplasmic reticulum (ER): Part of the endomembrane system, consisting of irregular membranous sacs and tubules. There are two main types of ER.

Rough Endoplasmic Reticulum (RER)

RER: Located near the nucleus and has ribosomes attached to its outer membranes. The RER is involved in protein synthesis and in the glycosylation of proteins.

Smooth Endoplasmic Reticulum (SER)

SER: More distant from the nucleus and lacks attached ribosomes. The SER functions include detoxifying organic compounds (notably in the liver) and synthesizing steroid hormones in the gonads and adrenal cortex, as well as aspects of lipid metabolism.