BICD 110: Cell Biology Notes - Dr. Amy Kiger

BICD 110: Cell Biology - Dr. Amy Kiger

Course Overview

  • Course Title: Cell Biology

  • Instructor: Dr. Amy Kiger

  • Date: September 29, 2025

  • Topics Covered: Biomembranes, Membrane Transport

  • Important Info:

    • Check registration and student resources.

    • Reference Canvas Modules for lecture information.

Announcements

  • Access materials for "Lecture 1A" and "Lecture 1B" online.

  • Not all slides will be presented in class; use posted slides as a guide.

  • Assignments Due:

    • iClicker (BB)—today; begins for credit on Thurs, 10/2.

    • Problem Set 1—past due; penalties start next week.

    • Canvas Questions 1—posted by class Thursday, due 5 PM Friday, 10/3.

    • Problem Set 2—posted on Thursday, due 10 AM Monday, 10/6.

  • Get into the routine of keeping up with the week's lectures.

Office Hours and Sections

  • Dr. Kiger's Office Hours: Wednesdays 2:00-3:00 PM, NSB 6109, as listed on Canvas.

  • Sections: Monday Sections have started!

Course Questions?

  • Students are encouraged to ask questions regarding the course content or logistics.

I. Cell Biology Methods

Cell Fractionation

  • A technique to purify and study sub-cellular structures.

Cell Microscopy

  • Essential for imaging and visualizing cells.

A. Fluorescence (Light) Microscopy

  • The capability to visualize more markers using labels with different wavelengths.

  • Important note: "you only see what you probe for!"

B. Phase-Contrast Microscopy

  • Enhances contrast in unstained cells; useful for viewing cells and structures.

C. Various Types of Microscopy

  • Fluorescence Microscopy: Shows specific fluorescently labeled molecules in stained cells.

  • Scanning Electron Microscopy (SEM): Provides a 3D image of the surface.

  • Transmission Electron Microscopy (TEM): Displays ultrastructure of organelles in thin sections.

    • Requires fixed cells cut into very thin sections with heavy metal stains for contrast based on electron densities.

II. Biomembranes

Overview

  • Key Functions of Cellular Membranes:

    1. Isolate biochemical processes, creating optimal environments for activity and allowing regulation in time and space.

    2. Act as a selective permeability barrier.

    3. Facilitate cellular fluidity.

Composition and Properties of Biomembranes

  • Eukaryotic cell membranes are dynamic structures.

  • Fluid Mosaic Model: Approximately 50% lipid and 50% protein make up the membrane structure, providing both a barrier and fluidity.

Lipid Bilayer Structure

  • Phospholipid Bilayer:

    • Composed of amphipathic lipids with hydrophilic polar head groups and hydrophobic nonpolar tails.

    • Spontaneous assembly into two leaflets, creating a sealed structure.

    • The bilayer is a selective barrier; stable due to weak chemical interactions including ionic and hydrogen bonds and Van der Waals interactions.

A. Asymmetric Nature of Biomembranes

  • Orientation maintained across two leaflet faces with specific roles.

  • The cytosolic face always facing the inside of the cell while the extracellular face interacts with the outer environment.

Major Classes of Membrane Lipids

I. Phosphoglycerides

  • The most abundant phospholipids, contributing to bilayer functions.

II. Sphingolipids

  • Found in high abundance in plasma membranes, consisting of sphingosine, fatty acid chains, and possible head groups.

III. Sterols (Cholesterol)

  • Present in mammalian cell membranes (up to 50% in plasma membranes), providing stability and maintaining fluidity by preventing tight packing between lipid tails.

Lipid Synthesis

  • Phospholipids are synthesized in the cytosol, starting from acetate and transported by fatty acid binding proteins to integrate into the endoplasmic reticulum (ER) membrane.

    • Present in asymmetric arrangements with flippases facilitating translocation, generating asymmetry and preventing overload on the cytosolic leaflet.

Variations in Lipid Composition

  • Different compartments have unique lipid compositions affecting their functions.

    • For example, phosphatidylserine (PS) is predominantly found in the cytosolic leaflet of the plasma membrane, playing a role in cellular signaling and interactions.

Membrane Proteins

  • Classification:

    • I. Integral Membrane Proteins: Include single-pass and multipass transmembrane proteins.

    • II. Lipid-Attached Proteins: Exteriorly (e.g., GPI) and cytosolic attachments (e.g., acylation, prenylation).

    • III. Peripheral Membrane Proteins: Often associated with alpha-helices within the membrane.

Fluidity of Phospholipid Bilayer

  • Properties of the lipid bilayer result in fluid-like characteristics with rapid lateral movement.

    • Various mechanisms exist to reduce the mobility of membrane proteins.

A. FRAP (Fluorescence Recovery After Photobleaching)

  • A technique to measure protein and lipid movement within the membrane, quantifying lateral motion.

III. Membrane Transport

Selective Permeability

  • Membranes demonstrate selective permeability, allowing certain molecules to cross while restricting others, which is crucial for cellular homeostasis and function.

Transport Gradients

  • Electrochemical Gradients: The flow of substances across membranes based on concentration differences and charge.

  • Movement down a gradient (from high to low concentration) does not require energy (ATP), whereas movement up a gradient requires work and energy.

Methods of Transport

  1. Diffusion:

    • Passive flow of molecules from high to low concentration, requiring no ATP.

    • Influenced by size, charge, and hydrophobic or hydrophilic properties.

    • Examples: O2, CO2, some hormones and drugs.

  2. Facilitated Transport:

    • Involves proteins to assist diffusion, includes channels like aquaporins for water.

  3. Active Transport:

    • Requires energy to move substances against their concentration gradient, utilizing pumps and transporters or carried out via vesicle fusion.

A. Osmosis

  • The diffusion of free water across a membrane, unaffected by solute concentration that cannot pass through.

    • Osmotic Pressure: Determines the direction of water flow and cell integrity.

    • Cells can burst in hypotonic solutions (where outside solute concentration is lower) or shrivel in hypertonic solutions (higher outside solute concentration).

B. Tonicity Effects on Cell Size

  • Cells maintain equilibrium in isotonic solutions, where concentrations across the membrane are equal, leading to no net water movement.

Conclusion

  • Upcoming Topics: Further discussions on membrane transport mechanisms will continue in Lecture 1B, building on the concepts introduced in this session.