Premed · Premed · General Biology I
Lecture 7: Cell Organelles and the Endomembrane System
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and function of the nucleus, including the nuclear envelope and nucleolus
- Trace the flow of materials through the endomembrane system (ER, Golgi, lysosomes, vesicles)
- Distinguish between rough and smooth endoplasmic reticulum
- Explain the roles of the Golgi apparatus in protein modification, sorting, and secretion
- Describe the functions of lysosomes, peroxisomes, and vacuoles
- Explain the function of mitochondria and chloroplasts in energy conversion
Lecture Content
I. The Nucleus
The nucleus is the largest organelle in most eukaryotic cells, typically 5 to 10 micrometers in diameter, and it serves as the cell's command center. It houses the vast majority of the cell's DNA and is the site of DNA replication, transcription, and the assembly of ribosomal subunits. The nuclear envelope is a double membrane studded with nuclear pores--large protein complexes of approximately 125 megadaltons that regulate traffic between the nucleus and the cytoplasm. Small molecules diffuse freely through the pores, but large molecules such as proteins and RNA require nuclear localization signals (NLS) and transport proteins called importins and exportins for active, energy-dependent passage.
Within the nucleus, the nucleoplasm is a gel-like interior that contains chromatin--DNA complexed with histone proteins. Chromatin exists in two functional states: euchromatin, which is loosely packed and transcriptionally active, and heterochromatin, which is densely packed and largely silent. During cell division, chromatin condenses into visible chromosomes. The nucleolus is a dense, non-membrane-bound region within the nucleus that is the site of ribosomal RNA synthesis and the assembly of ribosomal subunits. It contains rRNA genes, nascent rRNA transcripts, and ribosomal proteins imported from the cytoplasm. The nucleolus disassembles during mitosis and re-forms afterward.
II. Ribosomes
Ribosomes are not membrane-bound organelles but molecular machines responsible for protein synthesis. Each ribosome consists of two subunits made of ribosomal RNA and proteins: eukaryotic ribosomes are 80S (with 60S large and 40S small subunits), while prokaryotic ribosomes are 70S (50S plus 30S)--the latter also being found in mitochondria and chloroplasts, reflecting their endosymbiotic origin. Free ribosomes float in the cytoplasm and synthesize proteins destined for use within the cell--cytoplasmic enzymes, nuclear proteins, and mitochondrial proteins, for example. Bound ribosomes are attached to the rough endoplasmic reticulum and produce secretory proteins, membrane proteins, and lysosomal enzymes. Importantly, free and bound ribosomes are structurally identical; what determines their location is the presence of a signal sequence on the mRNA being translated, which directs the ribosome to the ER membrane.
III. The Endomembrane System
The endomembrane system is a network of interconnected and communicating membranes that compartmentalize the cell, enabling specialized biochemical processes to occur in defined locations. Its components include the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the plasma membrane. Materials move between these compartments via transport vesicles that bud from one membrane and fuse with another.
Rough and Smooth Endoplasmic Reticulum
The endoplasmic reticulum is an extensive network of membrane-enclosed tubules and flattened sacs (cisternae) that is continuous with the outer nuclear membrane. It occupies a large fraction of the cell's total membrane and is divided into two functionally distinct regions.
The rough ER (RER) is studded with ribosomes on its cytoplasmic surface, giving it its characteristic "rough" appearance. The function of the rough endoplasmic reticulum is synthesis, folding, and N-linked glycosylation of secretory proteins, membrane proteins, and lysosomal enzymes (with chaperones such as BiP). The rough ER also operates a quality control system: misfolded proteins are identified and targeted for degradation through the ER-associated degradation (ERAD) pathway. Cells that secrete large amounts of protein--pancreatic acinar cells producing digestive enzymes, or plasma cells churning out antibodies--have particularly extensive rough ER.
The smooth ER (SER) lacks ribosomes and has a more tubular appearance. Functions of the smooth endoplasmic reticulum include lipid synthesis (phospholipids, cholesterol, and steroid hormones), detoxification by cytochrome P450 enzymes, carbohydrate metabolism (glucose-6-phosphatase in hepatocytes), and calcium storage in the sarcoplasmic reticulum of muscle. Protein synthesis on bound ribosomes is NOT a function of the smooth endoplasmic reticulum; that is the defining job of the rough endoplasmic reticulum. Chronic alcohol consumption or drug use induces proliferation of smooth ER in hepatocytes.
Golgi Apparatus
The Golgi apparatus consists of a stack of flattened, membrane-enclosed cisternae--typically three to eight per stack--and exhibits a clear structural and functional polarity. The cis face (receiving side) faces the ER and receives transport vesicles laden with newly synthesized proteins and lipids. Materials move through the medial compartments and exit from the trans face (shipping side), which faces the plasma membrane.
Within the Golgi, proteins and lipids undergo further modification--additional glycosylation, phosphorylation, sulfation, and proteolytic cleavage. The Golgi is also the cell's central sorting and packaging facility: proteins are tagged with molecular addresses that determine their final destination. Lysosomal enzymes, for instance, are marked with a mannose-6-phosphate tag that directs them to lysosomes. Secretory vesicles destined for release from the cell are packaged at the trans face. Whether materials move through the Golgi by vesicular transport between stable cisternae or by cisternal maturation (where the cisternae themselves progress from cis to trans) has been debated; current evidence supports elements of both models.
Lysosomes
Lysosomes are membrane-bound vesicles that function as the cell's digestive system. They contain approximately fifty different hydrolytic enzymes (acid hydrolases)--proteases, lipases, nucleases, glycosidases, and phosphatases--all of which work optimally at the acidic pH of about 5 that is maintained inside the lysosome by H+ ATPase proton pumps in the lysosomal membrane. The surrounding cytoplasm has a pH of approximately 7.2, providing a safety margin: if a lysosome leaks, its enzymes are far less active at the higher pH, limiting damage.
Lysosomes carry out intracellular digestion of materials brought in by endocytosis--phagosomes fuse with lysosomes to degrade engulfed bacteria or debris. Through autophagy, cells can digest their own damaged or superfluous organelles: an autophagosome encloses the target organelle and then fuses with a lysosome. Lysosomes also participate in apoptosis, or programmed cell death. Genetic deficiencies in lysosomal enzymes cause a class of devastating disorders known as lysosomal storage diseases: Tay-Sachs disease (deficiency of hexosaminidase A, leading to lipid accumulation in neurons), Gaucher disease (deficiency of glucocerebrosidase), and Pompe disease (deficiency of acid maltase, causing glycogen accumulation) are among the best-known examples.
IV. Peroxisomes
Peroxisomes are small, membrane-bound organelles (0.1 to 1 micrometer) that contain oxidative enzymes generating hydrogen peroxide (H2O2) as a byproduct. The enzyme catalase rapidly decomposes H2O2 into water and oxygen, protecting the cell from oxidative damage. Peroxisomes carry out beta-oxidation of very-long-chain fatty acids, detoxification of harmful substances (including alcohol in liver cells), and the synthesis of bile acids and plasmalogens (a component of myelin). Unlike the organelles of the endomembrane system, peroxisomes grow by fission and import their proteins directly from the cytoplasm. Zellweger syndrome, a severe neurological disorder, results from defective peroxisome assembly.
V. Vacuoles
Vacuoles are membrane-bound compartments that serve diverse functions depending on the cell type. The central vacuole of plant cells is often enormous, occupying up to 80% of the cell's volume. Bounded by the tonoplast membrane, it stores water (generating turgor pressure that supports the plant), sequesters waste products and toxic compounds, stores pigments such as anthocyanins, and can house defensive chemicals. Contractile vacuoles in freshwater protists pump out excess water to prevent the cell from bursting in its hypotonic environment. Food vacuoles, formed by phagocytosis, fuse with lysosomes for digestion of ingested material.
VI. Mitochondria
Mitochondria are double-membrane organelles and the primary sites of aerobic cellular respiration--the process by which cells extract energy from organic molecules and convert it to ATP. The smooth outer membrane contains porins that make it permeable to small molecules. The intermembrane space between the two membranes plays a key role in the proton gradient that drives ATP synthesis. The highly folded inner membrane forms structures called cristae that greatly increase the surface area available for the electron transport chain and ATP synthase--the molecular machinery of oxidative phosphorylation. The inner membrane is impermeable to most molecules, requiring specific transporters for passage. The innermost compartment, the mitochondrial matrix, contains enzymes of the citric acid cycle, the mitochondrion's own circular DNA (approximately 16,500 base pairs in humans, encoding 13 proteins, 22 tRNAs, and 2 rRNAs), and 70S ribosomes.
Most mitochondrial proteins are actually encoded by nuclear DNA, synthesized on cytoplasmic ribosomes, and imported into the mitochondrion. The number of mitochondria per cell varies dramatically by cell type: metabolically active liver cells may contain 1,000 to 2,000, while mature red blood cells have none. Mitochondria exhibit maternal inheritance--they are passed almost exclusively from mother to offspring--making mitochondrial DNA a powerful tool for tracing maternal lineages.
VII. Chloroplasts
Chloroplasts, found in plant and algal cells, are the double-membrane organelles where photosynthesis takes place--the conversion of light energy into chemical energy stored in organic molecules. A structure found in plant cells but NOT in animal cells is the chloroplast; plant cells also have a cellulose cell wall and a large central vacuole, which animal cells lack. Like mitochondria, they have a smooth, permeable outer membrane and a more selective inner membrane. The fluid-filled interior, the stroma, contains the enzymes of the Calvin cycle, the chloroplast's own circular DNA (approximately 150,000 base pairs, encoding about 100 proteins), and 70S ribosomes. Within the stroma lie the thylakoids--flattened membrane sacs that house chlorophyll and the photosynthetic pigments. Thylakoids are stacked into structures called grana (singular: granum), the sites of the light-dependent reactions, and are interconnected by stroma lamellae. Chloroplasts are found predominantly in the mesophyll cells of leaves, where they are positioned to capture maximum sunlight.
<image>A side-by-side comparison of a mitochondrion and a chloroplast. Left: Mitochondrion cross-section showing the smooth outer membrane, the folded inner membrane forming cristae, the intermembrane space, and the matrix containing circular DNA and 70S ribosomes. The electron transport chain complexes and ATP synthase are shown embedded in the inner membrane. Right: Chloroplast cross-section showing the outer membrane, inner membrane, stroma (with circular DNA and 70S ribosomes), thylakoid membranes organized into grana (stacks), and stroma lamellae connecting grana. Photosystems and ATP synthase are shown in the thylakoid membrane.</image>
Exam presentation: describes function golgi apparatus
Exam questions use this wording. Which of the following best describes the function of the Golgi apparatus? Lecture objective: Explain the roles of the Golgi apparatus in protein modification, sorting, and secretion.
Why this matters
Understanding the endomembrane system is crucial for the MCAT, particularly when analyzing protein trafficking pathways and cellular disease mechanisms. Questions frequently test the movement of proteins from synthesis to their final destination, as well as the roles of specific organelles in processes like secretion, digestion, and detoxification. Clinically, many genetic disorders, such as lysosomal storage diseases, directly result from malfunctions in this system, manifesting in a wide range of debilitating symptoms.
A picture in words
Imagine the cell as a bustling factory. The nucleus is the CEO's office with all the blueprints (DNA). Ribosomes are assembly lines. The ER is the main production floor for raw materials (proteins and lipids). The Golgi apparatus is the packaging and shipping department, sorting and labeling products. Lysosomes are the recycling and waste disposal units, breaking down old or damaged goods, while vesicles are the delivery trucks moving everything around.
Checkpoint
How do proteins destined for secretion differ in their initial synthesis location from proteins destined for the cytoplasm?
Proteins destined for secretion begin synthesis on ribosomes bound to the rough endoplasmic reticulum, while cytoplasmic proteins are synthesized on free ribosomes in the cytoplasm. This difference in location is dictated by a signal sequence on the nascent polypeptide chain.