Cell Structure and Function: Organelles, Endomembrane System, Endosymbiosis, and Cytoskeleton

Cell Structure & Function

Overview of Eukaryotic Cell Structure

Eukaryotic cells contain a variety of specialized structures called organelles that perform distinct functions necessary for cellular life. These organelles are often surrounded by membranes and are suspended in the cytoplasm.

• Organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and chloroplasts (in plants).

• The endomembrane system is a group of membranes and organelles that work together to modify, package, and transport lipids and proteins.

Organelles & Endomembrane System

Nucleus

The nucleus is the control center of the cell, housing most of the cell's genetic material (DNA). It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for molecular transport.

• Nucleolus: Dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Nuclear envelope: Double membrane that separates the nucleus from the cytoplasm.

• Nuclear pores: Allow selective exchange of materials (e.g., mRNA, ribosomal subunits) between the nucleus and cytoplasm.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They are composed of protein and rRNA and can be found in both prokaryotic and eukaryotic cells.

• May be free-floating in the cytoplasm or bound to the rough endoplasmic reticulum (ER).

• Site of translation, where mRNA is decoded to build proteins.

Mitochondria

Mitochondria are the sites of cellular respiration and energy (ATP) production, often called the "powerhouse of the cell." They have a double membrane structure and their own DNA.

• Outer membrane: Smooth and encloses the organelle.

• Inner membrane: Folded into cristae to increase surface area for ATP production.

• Matrix: The innermost compartment containing enzymes, mitochondrial DNA, and ribosomes.

Endomembrane System

The endomembrane system is a network of membranes inside eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, transport vesicles, and the plasma membrane.

Endoplasmic Reticulum (ER)

The ER is a network of membranes attached to the nuclear envelope. It comes in two forms:

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies certain chemicals.

Golgi Apparatus (Golgi Bodies)

The Golgi apparatus acts as the cell's packaging and distribution center. It modifies, sorts, and ships proteins and lipids received from the ER.

• Consists of flattened membranous sacs called cisternae.

• "Receiving" side (cis face) and "shipping" side (trans face).

Transport Vesicles

Transport vesicles are small, membrane-bound sacs that move molecules between organelles or to/from the cell membrane.

• Essential for intracellular transport of proteins, lipids, and waste products.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes. They break down macromolecules, damaged organelles, and foreign substances.

• Formed at the Golgi apparatus.

• Fuse with vesicles to digest contents.

Central Vacuole (in Plant Cells)

The central vacuole is a large, fluid-filled organelle found in plant cells. It stores water, nutrients, and waste products, and helps maintain cell turgor pressure.

Cell Wall (in Plant Cells)

The cell wall is a rigid structure outside the plasma membrane in plant cells, providing structural support and protection. It is composed mainly of cellulose.

• Primary cell wall: Thin and flexible, formed first.

• Secondary cell wall: Thicker and more rigid, formed after the primary wall in some cells.

Chloroplasts and Other Plastids (in Plant Cells)

Chloroplasts are the sites of photosynthesis, converting sunlight into chemical energy (glucose). They have a double membrane and contain their own DNA.

• Thylakoid membrane: Internal system of interconnected sacs where light-dependent reactions occur.

• Stroma: Fluid surrounding the thylakoids, site of the Calvin cycle.

• Grana: Stacks of thylakoids.

• Other plastids include chromoplasts (pigment storage) and leucoplasts (starch storage).

Endosymbiosis & Organelle Origins

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells. Instead of being digested, these bacteria formed a symbiotic relationship with the host cell.

• Both organelles have double membranes, their own DNA, and ribosomes similar to those of bacteria.

• This theory explains the evolutionary origin of mitochondria and chloroplasts.

Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and facilitates cell movement and intracellular transport.

• Microfilaments: Thinnest fibers, composed of actin; involved in cell movement and shape changes.

• Microtubules: Thickest fibers, composed of tubulin; provide tracks for vesicle movement and form the spindle during cell division.

• Intermediate filaments: Provide mechanical support for the cell.

Microfilaments in Movement

Microfilaments are essential for muscle contraction, cell motility, and cytokinesis during cell division.

Microtubules

Microtubules serve as tracks for the movement of organelles and vesicles, and are the main structural components of cilia and flagella.

Flagella and Cilia

Flagella and cilia are hair-like structures that extend from the cell surface and are involved in cell movement or moving substances along the cell surface. Both are surrounded by the plasma membrane and contain a core of microtubules arranged in a characteristic "9+2" pattern.

• Flagella: Usually longer and fewer; propel cells (e.g., sperm, Euglena).

• Cilia: Shorter and more numerous; move fluid or cells (e.g., Paramecium).

Cell Specializations

Examples of Specialized Cells

Cells can differentiate to perform specialized functions, such as absorption (intestinal cells), storage (fat cells), or secretion (stomach gland cells). Each cell type has unique structural adaptations to support its function.

• Intestinal cells: Adapted for nutrient absorption with microvilli.

• Fat cells: Store energy as lipids.

• Stomach gland cells: Secrete digestive enzymes.