Eukaryotic cells are distinguished by membrane-enclosed organelles. The largest is the nucleus, but we'll focus first on the mitochondria. Mitochondria are unique, consisting of two membranes, and are believed to have originated from a prokaryotic organism (bacteria) that entered a larger cell billions of years ago. This symbiotic relationship led to both cells benefiting and working together.

Mitochondria resemble bacteria in shape and size, like E. coli, and possess two membranes (similar to gram-negative bacteria). They have their own circular genome and ribosomes, which are similar in size and structure to bacterial ribosomes, further supporting the endosymbiotic theory. This origin story, where a bacterium was endocytosed but not digested, is not unlike what happens with certain pathogens today, like listeria.

Mitochondria are crucial for generating most of the cell's ATP through cellular respiration, including the Krebs cycle. ATP production relies on creating a proton gradient across the inner mitochondrial membrane. The outer membrane is smooth and permeable to small molecules, while the highly folded inner membrane strictly regulates molecular passage and maximizes surface area for ATP synthesis.

ATP production by mitochondria is analogous to a battery, utilizing a gradient (proton gradient) to generate energy. High-energy electrons from nutrients are used to pump protons into the intermembrane space, creating a gradient. The flow of these protons back into the matrix through ATP synthase drives ATP production. This process combines active and passive transport mechanisms.

Mitochondria contain their own circular genome, which is expressed and translated within mitochondrial ribosomes. This genome is primarily coding and lacks introns, similar to bacterial genes. Mitochondrial DNA is inherited maternally (from the mother), making it a valuable tool for tracing maternal lineages, as seen in studies like Neanderthal sequencing. While the mitochondrial genome is small, many proteins needed for mitochondrial function are coded by nuclear chromosomes, highlighting their symbiotic relationship.

Mitochondrial DNA is relatively unprotected and unrepaired compared to nuclear DNA, leading to higher damage rates. Major defects in mitochondrial function are often lethal during embryonic development. However, certain mitochondrial diseases can affect various body parts and contribute to cancer and aging, often linked to apoptosis (programmed cell death). Leber hereditary optic neuropathy is a notable mitochondrial disease causing vision loss due to mutations in mitochondrial genes.

The nucleus is the largest organelle in animal cells, housing the cell's DNA packed into chromosomes. Its evolution is partially unclear, with recent discoveries of giant viruses that share similarities, suggesting a possible 'engulfment' origin. The nuclear envelope, consisting of two connected membranes with large pores, regulates the passage of molecules like RNA and proteins. The presence of the nuclear membrane allows for post-transcriptional gene regulation, such as splicing.

Ribosomes are assembled in the nucleolus and then exit to the cytoplasm. They exist as free ribosomes in the cytosol or membrane-bound ribosomes on the rough endoplasmic reticulum (ER). Free ribosomes produce proteins for use inside the cell or for import into other organelles like mitochondria and the nucleus. Membrane-bound ribosomes produce proteins destined for secretion, integration into membranes, or delivery to other organelles like lysosomes.

The endomembrane system begins with the ER. The rough ER, covered in ribosomes, is involved in protein translation, folding, and processing. The smooth ER, lacking ribosomes, is crucial for lipid and steroid hormone synthesis. Proteins from the rough ER are transported via vesicles to the Golgi apparatus.

The Golgi apparatus is the cell's 'post office,' receiving, modifying, and sorting proteins. It consists of flattened membrane sacs called cisternae, organized into cis, medial, and trans regions. Proteins move through these cisternae, undergoing enzymatic modifications (e.g., glycosylation). Finally, in the trans-Golgi network, proteins are packaged into vesicles and sent to their appropriate destinations, maintaining their membrane orientation throughout the process.

Lysosomes are membrane-bound organelles originating from the Golgi apparatus. They contain about 50 different hydrolytic enzymes, capable of degrading almost every type of molecule (nucleotides, proteins, lipids). These enzymes are activated by the low pH (around 5.5) inside the lysosome, providing strict control over their destructive activity. Lysosomes are vital for housekeeping functions, such as removing worn-out organelles (e.g., mitochondria) through autophagy, and for immunity by degrading phagocytosed bacteria.

Lysosomes also play a role in programmed cell death (apoptosis), essential for embryonic development (e.g., forming fingers). They recycle cells and are involved in less organized cell death, such as necrosis, which occurs due to sudden damage like lack of oxygen during a cardiac ischemia. In such cases, lysosomes can contribute to the destruction of affected cells, sometimes causing more damage than initially intended.