Cell Organelles and the Plasma Membrane Study Guide

Pedagogical Strategy and Learning Objectives

• Study notes should focus on the specific sections and learning objectives highlighted for the chapter rather than reading the entire textbook.

• Learning objectives are categorized by active verbs that indicate the expected level of cognitive engagement and critical thinking:

  • Describe is considered a lower level of thinking, generally involving the recall of facts or definitions without heavy critical analysis.
  • Explain, Predict, Compare, and Contrast require higher-level critical thinking, asking for the application of foundational knowledge to solve problems or analyze scenarios.
  • Assess requires evaluating potential consequences, such as what happens to a cell when an organelle ceases to function correctly.

• In visual study materials, a target sign icon identifies slides that directly address a specific learning objective, serving as a visual cue for high-priority information.

Cellular Diversity and The Cell as a Pharmaceutical Target

• Cells are not identical. Even within the human body, cells vary significantly in function, such as brain cells, heart cells, and muscle cells.

• The specific organelle content and the types of enzymes present within a cell are dictated by that cell's unique biological function.

• The cell is the basic unit of a living organism. The human body contains between $30$ and $40$ trillion cells.

• In pharmaceutical sciences, the cell is viewed as a drug target. Medications often target specific processes inside the cell or must navigate through cellular membranes to reach their site of action and produce a therapeutic effect.

Structure and Function of the Plasma Membrane

• The plasma membrane is the protective layer surrounding the cell, holding components inside and acting as a barrier to the outside environment. It controls the entry and exit of substances via a controlled manner.

• Fluid Mosaic Model: This describes the membrane as a dynamic, fluid-like structure composed of a mosaic of different moving molecules, including lipids, cholesterol, proteins, and carbohydrates.

• Phospholipid Bilayer:

  • Assembles into two layers to separate aqueous (water-friendly) environments inside and outside the cell.
  • Polar Head: Composed of a phosphate molecule with a slight charge. It is hydrophilic (water-loving) and points outward toward the water.
  • Hydrophobic Tails: Composed of non-polar fatty acids that repel water and hide on the inside of the bilayer.

• Cholesterol:

  • Made in the liver, cholesterol regulates the fluidity and permeability of the membrane.
  • Structure: Possesses a polar head ($OH$ group) and a hydrophobic tail.
  • Function: It interpolates between phospholipids to prevent them from packing too tightly, thereby maintaining necessary fluidity. The concentration of cholesterol must be optimal to maintain the balance of the membrane.

• Carbohydrates:

  • Attached to the exterior of the membrane as glycolipids (attached to lipids) or glycoproteins (attached to proteins).
  • Roles include cell recognition (the "signature" of the cell), cell-to-cell adhesion, and protection from the immune system.

Factors Maintaining Membrane Fluidity

• The composition of the membrane is asymmetric (the lipids vary) and dynamic (things can pinch off or move in and out).

• Two primary components maintain fluidity:

  1. Cholesterol: Prevents phospholipids from packing too tightly.
  2. Cis-unsaturated Fatty Acids: The presence of a double bond in the "cis" configuration creates a "kink" or bend in the fatty acid tail. This physical bend prevents tight packing between adjacent phospholipids, increasing fluidity.

Clinical Application: Ethanol and Membrane Fluidity

• Ethanol is an amphipathic molecule, meaning it has both a polar (hydrophilic) end and a non-polar (hydrophobic) end.

• Mechanism of Action: Because of its structure, ethanol can interpolate itself into the phospholipid bilayer of neuronal cells in the Central Nervous System (CNS).

• Effect: Small and flexible, ethanol disrupts the balance of the membrane. It alters the fluidity, which in turn prevents integral membrane proteins and receptors from moving or interacting optimally.

• Symptomatic Result: This disruption to neuronal protein function contributes to symptoms like short-term memory loss, confusion, and disorientation observed in high-level ethanol consumption.

Clinical Pearl: Spur Cell Anemia

• In some cases of liver disease, an excess of membrane cholesterol can occur.

• This results in red blood cells taking on a spiny appearance, known as spur cell anemia.

• These malformed cells cannot move through the body optimally and may require treatments as significant as a liver transplant.

Classification of Membrane Proteins

• Integral Proteins:

  • By definition, these are inserted into the membrane or span the entire bilayer from one side to the other.
  • They often function as ion channels or transport pumps to move large or charged molecules across the membrane.

• Peripheral Proteins:

  • These do not span the membrane. They are associated with the membrane surface via weak electrostatic interactions.
  • They interact with the polar heads of phospholipids or with integral proteins.
  • Functions include involvement in the electron transport chain, cell signaling, communication, and structural support.

Prokaryotic vs. Eukaryotic Cells

• Similarities:

  • Both are enclosed by a plasma membrane.
  • Both contain cytoplasm (the internal liquid environment).
  • Both contain DNA as genetic material.
  • Both contain ribosomes for protein synthesis.

• Differences:

  • Nucleus: Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells (like bacteria) do not.
  • Organelles: Eukaryotic cells contain membrane-bound organelles (mitochondria, Golgi, etc.) to compartmentalize complex functions. Prokaryotic cells lack these.
  • Size: Eukaryotic cells are significantly larger and more complex.
  • DNA Structure: Prokaryotes usually have one circular DNA chromosome; eukaryotes have multiple linear chromosomes.

The Nucleus and the Central Dogma

• Nucleus: The largest organelle in animal cells, containing approximately $99\%$ of the cell's DNA. It is surrounded by a double-membraned nuclear envelope with pores that allow the movement of messenger RNA (mRNA), ribosomes, and proteins.

• Central Dogma of Molecular Biology: Describes the pathway of genetic information: $DNA \rightarrow RNA \rightarrow Protein$.

  • Replication: DNA making a copy of itself. Occurs in the nucleus.
  • Transcription: DNA being transcribed into RNA. Occurs in the nucleus.
  • Translation: mRNA being translated into protein. Occurs in the cytosol via ribosomes.

• Composition:

  • Chromatin: The combination of DNA and proteins (specifically histones) packaged inside the nucleus.
  • Nucleolus: The specific site within the nucleus where ribosome synthesis and processing occur.

Mitochondria: The Powerhouse

• Function: The primary site of ATP (adenosine triphosphate) production through cycles like the Krebs cycle and the electron transport chain.

• Structure: Features two membranes (inner and outer).

• Genetic Material: Mitochondria contain their own DNA ($0.1\%$ to $1\%$ of total cellular DNA), encoding $13$ proteins involved in energy production. Mutations in mitochondrial DNA lead to mitochondrial diseases characterized by muscle weakness and extreme fatigue.

Lysosomes: Digestion and Recycling

• Function: Act as the cell's digestive and recycling center. They break down unwanted material, infectious bacteria, and yeast. They participate in tissue remodeling (e.g., the formation of fingers during development or the regression of mammary glands after weaning).

• Enzymes: Contain hydrolytic enzymes (proteases, glycosidases, lipases) that break large structures into building blocks for reuse.

• Internal Acidic Environment:

  • Lysosomal enzymes work best at a pH of $5.5$.
  • The physiological pH of the cell is approximately $7.2$.
  • The low pH is maintained by ATP-dependent pumps that move hydrogen ions ($H^+$) into the lysosome.
  • If a lysosome ruptures, the enzymes become less efficient in the neutral pH of the cytosol, providing a safety mechanism for the cell.

Endoplasmic Reticulum (ER) and Golgi Complex

• Rough ER:

  • Studded with ribosomes, giving it a rough appearance.
  • Function: Synthesis of proteins and post-translational modifications (PTMs).
  • PTMs involve chemically altering a protein after it is made (e.g., adding a sugar to make a glycoprotein, or adding a phosphate group to activate it).

• Smooth ER:

  • Lacks ribosomes.
  • Function: Synthesis of hydrophobic molecules (lipids, steroid hormones).
  • Drug Metabolism: Contains Cytochrome P450 enzymes, which are critical for breaking down medications. Damage to the smooth ER can lead to increased drug toxicity because the body cannot effectively metabolize the drug.

• Ribosomes:

  • Non-membrane-bound organelles responsible for translation (mRNA to protein).
  • Can be "free" in the cytosol or "bound" to the Rough ER.

• Golgi Complex:

  • Acts as the "postal service" or shipping center of the cell.
  • Function: Modifies proteins (sulfation, phosphorylation, carbohydrate adhesion) and sorts/distributes them to lysosomes, the cell surface, or for secretion.

Peroxisomes and the Cytoskeleton

• Peroxisomes: Involved in oxidative reactions and detoxification. They break down very long-chain fatty acids into shorter ones to prevent damage to the cell.

• Cytoskeleton: Provides structure and movement.

  • Microtubules and Microfilaments: The "muscles" of the cell. They are dynamic polymers involved in moving vesicles and cellular cargo over long distances, and in cell surface movement.
  • Intermediate Filaments: The "bones" of the cell. They are stable, low-dynamic structures that provide scaffolding and structural support.

Clinical Case: Gout and Vesicle Transport

• Gout is caused by the deposition of urate crystals in the joints.

• To treat this, a researcher might propose a drug to disrupt the transport of cellular inflammatory cargo.

• A drug that targets microtubules would be most effective at disrupting long-distance vesicle transport because microtubules are the primary structures used for moving vesicles through the cell.

Pharmacist Patient Care Process (PPCP)

• Biochemistry and cell biology assist in the first two stages of the PPCP:

  1. Collect: Gathering information from labs, genetics, patient history, and family.
  2. Assess: Using clinical thinking to analyze the data (e.g., identifying that fatigue may be a mitochondrial issue).

• The full process involves: Collect \rightarrow Assess \rightarrow Plan \rightarrow Implement \rightarrow Follow-up (Monitor and Evaluate).

• The entire process is person-centered, accounting for social factors, affordability, and individual biological responses.