CELS191 Lecture 3: Plasma Membrane & Organelles

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• Dr. Rebecca Bird
  • Anatomical Science Education, First Year Experience, Student Success, Educational Technology
  • Contact: rebecca.bird@otago.ac.nz
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Lecture Overview: Cell Structure and Diversity

• Lecture Title: Plasma Membrane & Organelles
• Objectives:
  • Describe the structure and function of the plasma membrane.
  • Explain the role of membrane proteins.
  • Outline mechanisms of substance transport across the plasma membrane.
  • Discuss the importance of organelles and sub-cellular compartments.
  • Identify key organelles in eukaryotic cells.

The Need for Cells

• Cells must:
  • Manufacture cellular materials
  • Obtain raw materials
  • Remove waste
  • Generate required energy
  • Regulate these processes

Plasma Membrane

• Acts as a semi-permeable barrier:
  • Separates the cell from its external environment.
  • Regulates the movement of substances in and out of the cell.

Importance of the Plasma Membrane:

• Provides conditions necessary for cell function.
• Allows passage of essential substances like oxygen and nutrients.
• Limits the maximum size of cells, favoring a higher surface-to-volume ratio in smaller cells.

Structure of the Plasma Membrane

• Comprised of a phospholipid bilayer:
  • Hydrophilic heads face outward; hydrophobic tails face inward.
• Membrane fluidity is affected by:
  • Saturation: Saturated phospholipids are tightly packed; unsaturated are more fluid.
  • Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.
  • Cholesterol: Stabilizes the membrane's fluidity.

Membrane Proteins

• Thousands of membrane proteins perform essential functions:
  • Signal Transduction: Relaying environmental messages into the cell.
  • Cell Recognition: Often involves glycoproteins.
  • Intercellular Joining: Forms connections between cells.
  • Linking Cytoskeleton & ECM: Connects to structures outside the cell.
  • Membrane Transport: Aids movement of molecules across the membrane.

Mechanisms of Membrane Transport

• Substances cross membranes through different mechanisms:
  • Passive Transport (no energy):
  • Moves down concentration gradient.
  • Examples: Diffusion, Facilitated Diffusion (via channels/carriers), Osmosis (via aquaporins).
  • Active Transport (requires energy):
  • Moves against concentration gradient using ATP (e.g., sodium-potassium pump).
  • Co-Transport (indirect active transport):
  • One substance's transport powers another's movement against the gradient.

Organelles and Their Roles

• Organelles provide special conditions for cellular processes:
  • Separate incompatible processes.
  • Allow concentration gradients and packaging of substances for transport.

Key Organelles in Eukaryotic Cells:

• Animal Cells: Nucleus, Endoplasmic Reticulum, Golgi apparatus, Lysosome, Mitochondrion.
• Plant Cells: Chloroplast, Central vacuole, in addition to the above.

Summary of Lecture 3

• Plasma Membrane: Dynamic, semi-permeable, containing diverse proteins.
• Membrane Transport: Essential for crossing the plasma membrane, governed by molecular properties.
• Organelles: Specialized compartments enhancing cellular function.

Revision Questions

1. Why are organelles membrane-bound?
2. What is the plasma membrane structurally composed of?
3. Which molecule stabilizes membrane fluidity?
4. Contrast passive and active transport mechanisms.
5. Identify molecules that can passively diffuse across the plasma membrane.
6. Explain the process of co-transport.
7. List key functions of membrane proteins.