AS Biology Chapter 4: Cell Membranes and Transport

Introduction

• Lecture by Dr. Demi on cell membranes and transport.
• Encouragement to check out previous videos for a comprehensive understanding.
• Focus on:
  • Structure of phospholipids.
  • Identifying proteins in the cell membrane.
  • Role of cell membrane components and cell signaling.

Structure of Phospholipids

• Phospholipid: molecule with a phosphate head (hydrophilic) and two hydrophobic tails.
• Arrangement in cell membrane forms a phospholipid bilayer:
  • Phosphate heads outside.
  • Lipid tails inside.
• Function: Regulates passage of substances through the membrane.

Fluid Mosaic Model

• Discovered by Singer and Nicholson.
• Cell membrane: not solid; has oily consistency.
• Proteins scattered throughout.
• Components include:
  • Phospholipids
  • Carbohydrates
  • Glycoproteins
  • Cholesterol

Cell Membrane Components

• Proteins:

  • Channel and Carrier Proteins: Key in transport functions.
  • Integral (Intrinsic) Proteins: Embedded within the membrane.
  • Peripheral (Extrinsic) Proteins: Attached to the outer or inner surface.

• Cholesterol:

  • Has hydrophilic heads and hydrophobic tails.
  • Regulates membrane fluidity:
    • Prevents close packing of phospholipids at low temperatures.
    • Prevents excessive fluidity at high temperatures.
  • Less present in plants; absent in prokaryotes.
  • Provides mechanical stability.

• Glycolipids and Glycoproteins:

  • Glycolipids: Lipids with carbohydrates.
  • Glycoproteins: Proteins with carbohydrates.
  • Function as receptors for cell signaling and markers for immunity.

Cell Signaling

• Receptors:

  • Signaling Receptors: Recognize and bind messenger molecules to elicit a response.
  • Endocytosis Receptors: Bind molecules to facilitate engulfing by the cell.

• Cell Communication:

  • Ligands: Small signaling molecules.
  • Pathway: Sending cell releases ligand → binds to receptor on target cell → causes a cellular response.

• Membrane Signaling Process:

  • Signal arrives at receptor → receptor changes shape → activates G protein → activates second messenger → response (e.g., enzyme activation).

Types of Signals

• Hydrophobic Signals: e.g., steroid hormones like estrogen; can pass through the cell membrane.
• Hydrophilic Signals: Cannot pass through cell membrane; requires surface receptors.

Conclusion

• Importance of understanding cell membrane structure and function for cellular processes like signaling and transport.
• Encouragement to review material and ask questions for clarification.

• Final Note: For any confusion, watch the video again or reach out with questions. Share the video with peers and subscribe for more content.