Chapter 3 Notes: Cell Membrane and Organelles

Plasma Membrane: overview and core components

• The plasma membrane is the boundary of the cell and regulates what enters and leaves the cell; it is selectively permeable.
• Three basic organic components form the core structure of the plasma membrane:
  • Phospholipids (phospholipid bilayer): hydrophilic (polar) heads face the aqueous exterior and interior fluids; hydrophobic (nonpolar) fatty acid tails face inward.
  • Cholesterol: interspersed within the bilayer; contributes rigidity and stability to membrane structure and helps hold components together.
  • Transmembrane (integral) proteins: span the membrane and regulate transport, signaling, and cell interactions.
• Additional components mentioned:
  • Glycolipids and glycoproteins: carbohydrate-containing lipids and proteins on the extracellular surface; function as cell identifiers and participate in cell binding.
• General function of the plasma membrane: maintains intracellular environment, supports cytoskeletal connections, and mediates transport and signaling.
• It has a phospholipid bilayer with hydrophilic heads and hydrophobic tails; acts as a barrier between extracellular fluid and intracellular fluid.
• Image description (concept): a phospholipid bilayer with phosphates heads on the outside, fatty acid tails in the middle; cholesterol molecules interspersed; transmembrane proteins spanning the bilayer; glycolipids and glycoproteins on the surface.

Key terminology for membrane components

• Phospholipid bilayer:
  • Heads: hydrophilic
  • Tails: hydrophobic
• Cholesterol: provides membrane rigidity and organization
• Transmembrane (integral) proteins: channels, receptors, transporters, enzymes, adhesion molecules
• Glycolipids and glycoproteins: cell surface identifiers and mediators of cell–cell binding
• Lateral organization: lipids ~98% of the membrane composition; proteins occupy the rest and mediate function

Membrane transport: permeability and regulation

• The membrane is selectively permeable: some substances diffuse directly through lipids, others require transport proteins.
• Transport categories:
  • Passive processes (do not require ATP): diffusion, osmosis, and filtration.
  • Active processes (require ATP): active transport and vesicular (bulk) transport.
• Facilitated diffusion: carrier- or channel-mediated diffusion that requires assistance for solutes to cross the membrane.
• Examples for understanding:
  • Simple diffusion: movement of solutes down their concentration gradient without energy input.
  • Osmosis: movement of water down its concentration gradient across a selectively permeable membrane.
  • Filtration: selective passage of substances based on size/charge through a membrane (analogy: coffee filter).
• Important concepts affecting diffusion rate:
  • Temperature: higher temperature speeds diffusion; lower temperature slows it.
  • Molecular weight: heavier molecules diffuse more slowly.
  • Concentration gradient: steeper gradients increase diffusion rate.
  • Surface area: larger area increases diffusion rate.
  • Permeability: whether the molecule can cross the membrane directly.
• Quantitative ideas relevant to diffusion/osmosis:
  • Diffusive flux (Fick's first law): J = -D \frac{dC}{dx}
  • Osmotic pressure (van't Hoff): \Pi = i M R T, where $i$ is the van't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.
  • Osmolarity: \text{Osm} = \sumi i Ci

Nucleus and nuclear envelope

• The nucleus is enclosed by the nuclear envelope, which is perforated by nuclear pores that regulate traffic into and out of the nucleus.
• Inside the nucleus (nucleoplasm) you find DNA material and RNA precursors; the nuclear region contains nutrients and nucleotides needed for transcription and RNA processing.
• The nuclear envelope is related in structure to the plasma membrane (both have bilayer membranes and pores for transport).
• Cytoplasm exists outside the nucleus (in the nucleoplasm is inside the envelope), and the cytosol contains enzymes and nutrients.

Endomembrane system: rough and smooth endoplasmic reticulum

• Endoplasmic reticulum (ER) is a membrane-bound organelle with two forms:
  • Rough ER: studded with ribosomes; function is protein synthesis and initial processing; proteins synthesized here are packaged in vesicles for delivery to other organelles or secretion out of the cell.
  • Smooth ER: lacks ribosomes; function includes lipid and steroid synthesis and other metabolic processes (e.g., detoxification activities).
• Ribosomes: sites of protein synthesis; ribosomes can be free-floating in the cytoplasm or attached to the rough ER; they read messenger RNA (mRNA) and work with tRNA to assemble amino acids into polypeptide chains.
• The distinction between rough and smooth ER is an important functional difference, not simply a structural label.

Golgi apparatus (Golgi complex)

• Golgi is involved in packaging and modifying proteins and lipids; it sorts and targets them to their appropriate destinations.
• It forms vesicles that become lysosomes or secretory vesicles that can be released from the cell.
• The Golgi acts as a processing and shipping hub for molecules synthesized in the ER.

Lysosomes and peroxisomes

• Lysosome: contains hydrolytic enzymes that digest proteins, nucleic acids, carbohydrates, phospholipids, and other cellular components; important for recycling cellular material.
• Peroxisome: contains enzymes that generate hydrogen peroxide and are involved in detoxification; helps neutralize free radicals; participates in fatty acid breakdown; works in concert with mitochondria during metabolism.
• Conceptual distinction: lysosome = recycling center; peroxisome = detoxification and fatty acid processing

Mitochondria: the site of cellular respiration

• Mitochondria are the powerhouse of the cell; site of aerobic respiration and ATP production.
• They generate ATP required for cellular activities via oxidative phosphorylation and related pathways.

Cytoskeleton-related surface features: microvilli, cilia, and flagella

• Microvilli: finger-like protrusions on the cell surface that increase surface area to enhance absorption (common in the digestive system, especially the small intestine).
  • Structure: contain actin filaments; increase absorptive capacity of the plasma membrane.
• Cilia: hair-like extensions on the cell surface that beat rhythmically to move fluids and debris along the surface; found in respiratory and digestive tracts to move mucus and other materials.
  • Ciliary motion helps move mucus and debris; dysfunction can contribute to respiratory issues.
  • Example condition mentioned: cystic fibrosis, where mucus clearance is impaired due to ciliary dysfunction, leading to blocked airways.
• Flagellum: a whip-like structure used for cell propulsion; in humans, the primary example is the sperm cell flagellum, enabling movement.

Key transport structures and concepts (specific terms from the lecture)

• Transmembrane proteins can serve multiple roles:
  • Channels: allow specific ions or molecules to pass through (can be gated or open).
  • Receptors: bind external molecules to trigger internal responses.
  • Enzymes: catalyze reactions at the membrane or within the cell.
  • Cell adhesion molecules: help hold cells together.
  • Some proteins act as transporters for specific substances.
• Channels can be gated (open or closed) and regulate movement across the membrane; related to concepts like the sodium–potassium pump discussed in earlier material.
• Membrane transport applications: transport proteins regulate movement of solutes, supporting cell homeostasis and signaling.

Cell identification and immune interaction

• The membrane contains identifiers (glycolipids and glycoproteins) that help cells recognize each other.
• Transmembrane proteins can act as cell identifiers; if a transplanted cell presents foreign markers, the immune system may respond, leading to rejection in some cases.
• This concept is important for understanding immunology, transplantation, and how membranes contribute to cell recognition.

Osmosis, isotonic, hypertonic, and hypotonic solutions (practical outcomes)

• Isotonic: solute concentration is equal on both sides of the membrane; cells remain stable with no net water movement.
• Hypotonic: solution has lower solute concentration outside than inside the cell; water moves into the cell, potentially causing swelling and lysis (cell rupture) if excessive.
• Hypertonic: solution has higher solute concentration outside than inside the cell; water moves out of the cell, causing shrinkage (crenation in red blood cells).
• Real-world analogy for hypertonic/hypotonic effects:
  • Beef jerky analogy: salt draws water out of meat, concentrating solutes and drying it, analogous to water leaving a cell in a hypertonic environment.
• IV fluids in medicine are chosen to avoid lysing patient cells; wrong solutions can cause cells to lyse or crenate.

Practical lab and exam connections (as mentioned in the lecture)

• Upcoming labs: virtual labs on osmosis and facilitated diffusion; electrophoresis.
• Exam focus: the first four chapters serve as review and cover membrane structure and basic organelle functions; emphasis on understanding transport mechanisms and organelle functions.

Summary of central ideas (quick recap)

• The plasma membrane is composed of phospholipids, cholesterol, and transmembrane proteins, with glycolipids and glycoproteins contributing to cell identification and binding.
• Membrane transport includes passive processes (diffusion, osmosis, filtration) and active processes (active transport, vesicular transport); facilitated diffusion requires a carrier or channel.
• Diffusion moves solutes from high to low concentration; osmosis moves water from high to low water activity; both are influenced by temperature, gradient, surface area, and permeability.
• The nucleus and nuclear envelope regulate genetic material transport via nuclear pores; the nucleus houses DNA and RNA precursors.
• The endomembrane system includes rough ER (protein synthesis), smooth ER (lipid synthesis), Golgi (modification/packing), lysosomes (digestive enzymes), and peroxisomes (detox and fatty acid metabolism).
• Mitochondria generate ATP through respiration.
• Microvilli increase surface area for absorption; cilia move fluids; flagella propel sperm.
• Understanding organelle function and membrane transport is foundational for physiology, pathology (e.g., CF), and clinical applications (IV solutions, transplants).

Quick reference: key equations and constants (LaTeX)

• Diffusion flux: J = -D \frac{dC}{dx}
• Osmotic pressure (van't Hoff): \Pi = i M R T
• Osmolarity: \text{Osm} = \sumi i Ci

glossary of terms (high-yield definitions)

• Phospholipid bilayer: two layers of phospholipids forming the basic structure of a cell membrane.
• Cholesterol: lipid that modulates membrane fluidity and stability.
• Transmembrane proteins: proteins spanning the membrane, enabling transport and signaling.
• Glycolipids/glycoproteins: carbohydrate-containing lipids/proteins that function in cell recognition.
• Diffusion: passive spread of solutes down their concentration gradient.
• Osmosis: diffusion of water across a semi-permeable membrane.
• Facilitated diffusion: diffusion of solutes that require a carrier or channel.
• Isotonic/hypotonic/hypertonic: terms describing solute concentration relative to the cell interior.
• Rough ER: ER with ribosomes; protein synthesis and processing.
• Smooth ER: ER without ribosomes; lipid/steroid synthesis and detoxification.
• Golgi apparatus: packaging, modification, and distribution center for proteins and lipids.
• Lysosome: organelle containing enzymes for digestion and recycling.
• Peroxisome: organelle involved in detoxification and fatty acid metabolism.
• Mitochondrion: powerhouse of the cell; ATP production.
• Microvilli: small projections increasing surface area for absorption.
• Cilia: motile projections moving fluids along surfaces.
• Flagellum: long, whip-like structure enabling cell movement (sperm).
• Na+/K+-ATPase: an ATP-dependent pump maintaining ionic gradients across the membrane (example of active transport).