Membranes, Diffusion and Osmosis

Introduction

  • Greeting

  • Reminder about the upcoming exam and announcements

Announcements

  • Nursing Event:

    • Opportunity to listen to a speaker for those interested in nursing

  • Pharmacology Event:

    • Recommended for anyone considering a career in pharmacology

  • Importance of attending these events as opportunities may not arise frequently

Factors Affecting Fluidity of Membranes

  • Importance of fluidity in biological membranes

  • Factors influencing fluidity:

    • Temperature:

    • High Temperature: Cholesterol interferes with the rapid movement of phospholipids, preventing membrane breakdown.

    • Low Temperature: Phospholipids tend to pack together, leading to reduced fluidity.

    • Cholesterol's Role:

    • Acts as a buffer for membrane fluidity by preventing both excessive packing (in cold) and excessive movement (in heat).

Plant Responses to Temperature Changes

  • High Temperature Responses:

    • In plants, phospholipids slow down by:

    1. Extending the fatty acid tail length.

    2. Converting saturated fats to unsaturated fats to allow movement.

  • Low Temperature Responses:

    • To increase movement, plants may:

    1. Shorten the fatty acid tail length.

    2. Convert saturated fats to unsaturated fats.

Cholesterol and Phytosterols

  • Cholesterol in Animals:

    • Integral for maintaining membrane fluidity.

  • Phytosterols in Plants:

    • Present but do not function similarly to cholesterol for membrane fluidity.

Proteins in Membranes

  • General Characteristics of Proteins:

    • Proteins are typically large and may or may not move within the membrane.

  • Chimeras: This term refers to a fused cell derived from different species; an example being human and mouse cell fusion.

    • Experiment Procedure:

    • Proteins from mouse and human cells were observed for their movement across membranes after incubation and freezing, revealing that proteins can move depending on conditions.

  • Cadherin Proteins:

    • Type of cell adhesion molecule; crucial for cell-cell attachment but does not facilitate movement to the extracellular matrix.

Movement and Function of Proteins in Membranes

  • Bilateral Movement of Proteins:

    • Movement is dependent on protein function; some proteins are stationary, while others can move freely.

  • Types of Membrane Proteins:

    • Intrinsic Proteins:

    • Can only be removed by breaking down the membrane (e.g., transmembrane proteins, lipid-anchored proteins).

    • Extrinsic Proteins:

    • Adhere either to the polar heads or surfaces of membrane proteins; removed easily without breaking the membrane by utilizing solutions of opposite charge.

Glycosylation

  • Definition: The addition of oligosaccharides to proteins and lipids; essential for cellular recognition and function.

  • Importance of Glycosylation:

    • Facilitates organ transplant compatibility by recognizing self vs. non-self cells through glycosylated proteins (e.g., immunoglobulins).

    • Glycolipids assist in tissue-specific recognition by aiding in communication between cells of different tissues.

Glycocalyx

  • Definition: The dense layer of carbohydrate trees projecting from glycoproteins and glycolipids; serves protective and signaling functions.

  • Functions of Glycocalyx:

    1. Protects the plasma membrane from physical and mechanical stress.

    2. Aids in the organization of cells during development (gastrulation).

Transport Across Membranes

  • Selective Permeability: Transport proteins regulate which substances enter or exit the cell, featuring:

    • Diffusion: Movement from high to low concentration without energy required.

    • Active Transport: Movement against a concentration gradient requiring energy (ATP).

Types of Diffusion

  • Passive Diffusion: Movement through the phospholipid bilayer without aid from proteins.

  • Facilitated Diffusion: Utilizes transport proteins while remaining energy-free.

    • Types of Transport Proteins:

    • Channels: Allow free flow of ions or molecules, resulting in rapid transport

    • Carriers: Bind and transport specific molecules; generally slower due to the binding process.

Osmosis and Tonicity

  • Osmosis: Movement of water across a semi-permeable membrane, driven by solute concentration differences.

  • Tonicity: Refers to the comparative concentrations of solutes in the external solution versus the cell.

    • Isotonic Solution: Equal solute concentration within and outside the cell.

    • Hypertonic Solution: Higher solute concentration outside the cell, leading to cell shrinkage (crenation).

    • Hypotonic Solution: Lower solute concentration outside the cell, resulting in cell swelling and potential lysis (bursting).

Summary of Key Points

  • Understand the role of cholesterol in membrane fluidity

  • Recognize plant adaptation methods to temperature

  • Differentiate between intrinsic and extrinsic proteins in membranes

  • Grasp the importance of glycosylation for cellular recognition and communication

  • Review transport mechanisms and differentiate between passive and active transport definitions

Concluding Notes

  • Encourage students to engage actively, invite questions, and express concerns about the subject matter.

  • Reinforce the significance of the upcoming exam and continuous study.