Chapter 6 lecture pt.1

Cell Membranes & Transport

Overview

• Author: P. Pearson

Learning Objectives

• Understand the difference between diffusion and osmosis.

• Identify molecules that can cross the membrane freely.

• Describe the structure and function of transmembrane proteins.

• Distinguish between passive and active transport with examples.

Membranes Review

Plasma Membrane

• Composed of a phospholipid bilayer.

• Acts as a selective barrier, allowing certain substances to pass based on permeability influenced by conditions and composition.

How Substances Move Across Lipid Bilayers: Diffusion and Osmosis

Definitions

• Solute: Molecules dissolved in a solution.

• Solvent: Liquid that dissolves the solute.

• Passive Transport: Movement of substances across a membrane without outside energy input.

Diffusion

Characteristics

• Defined as the spontaneous movement of molecules in response to concentration gradients, moving from high to low concentrations.

• Achieves equilibrium when molecules are evenly distributed throughout the solution, despite continuous motion.

Osmosis

Process

• A special case of diffusion involving the movement of water across lipid bilayers.

• Water moves from areas of low solute concentration (dilutive) to high solute concentration, diluting higher concentrations of solute.

Tonicity of Solutions

Types

• Hypertonic: Outside solution has a higher solute concentration than inside the cell, causing cell shrinkage.

• Hypotonic: Lower solute concentration outside the cell, leading to swelling or bursting.

• Isotonic: Equal solute concentrations inside and outside, maintaining cell size.

Membrane Composition

Lipids and Proteins

• Membranes are not only composed of phospholipids but also contain significant amounts of proteins, contributing to the overall function.

Fluid-Mosaic Model of Membrane Structure

Concept

• The model illustrates that proteins harden the fluid structure of the membrane, forming a dynamic mosaic with phospholipids and proteins interspersed throughout.

Proteins and Membrane Function

Characteristics

• Proteins can insert into the membrane and are often amphipathic, having both hydrophilic and hydrophobic regions.

Functions of Membrane Proteins

• Transport: Facilitate movement across the membrane.

• Enzymatic Activity: Act as enzymes.

• Signal Transduction: Function in cellular signaling processes.

• Cell-Cell Recognition: Mediate interactions between cells.

• Intercellular Joining: Connect adjacent cells.

• Attachment: Anchor the cytoskeleton and extracellular matrix (ECM).

Facilitated Diffusion

Mechanism

• Occurs through channel or carrier proteins without energy. It involves movement from high to low concentration and is selective based on channel protein types.

Types of Channel Proteins

• Different channel proteins allow specific ions or small molecules to pass through, for example, Aquaporins facilitate water passage.

Ion Channel Proteins

Mechanism

• Allow specific ions to cross membranes, establishing electrochemical gradients, impacting processes such as muscle contraction and neuron signaling.

Gated Channels

Function

• Gated channels control when substances can pass based on external signals, such as ligand binding or membrane voltage changes.

Carrier Proteins

Functions

• Carrier proteins can selectively transport solutes across the membrane, changing their conformation to facilitate diffusion.

Active Transport Basics

Definition

• Moves substances against their concentration gradient (low to high), requiring energy, typically derived from ATP.

Sodium–Potassium Pump

Function

• An active transport mechanism utilizing ATP to move Na+ and K+ ions against their concentration gradients.

Secondary Active Transport

Concept

• Uses electrochemical gradients established by primary active transport to move additional molecules against their gradient, facilitating diverse cellular functions.

Summary of Transport Mechanisms

Passive vs. Active

• Passive Transport: Through carriers and channels.

• Active Transport: Via pumps requiring energy input.

Plasma Membranes

Functionality

• Enable internal cell environments to diverge from external surroundings, achieved through selective permeability and specific protein functionalities.

Learning Objectives for Endomembrane System

• Discuss sorting signals for protein destinations.

• Explain molecular movement within the endomembrane system.

• Describe functions of lysosomes.

Nuclear Transport

Overview

• Involves passage of large enzyme suites necessary for RNA production through nuclear pore complexes that control entry into the nucleus.

Nuclear Localization Signal (NLS)

Importance

• A 17-amino-acid-long signal required for inbound traffic to the nucleus, critical for protein functions associated with DNA transcription and ribosome assembly.

Endomembrane System: Overview

Functionality

• Integrates various cellular compartments, allowing proteins to be actively imported with precise delivery pathways, guided by specific "zip codes" within protein structures.

Endomembrane System Pathway

Process

• Proteins are synthesized in the rough endoplasmic reticulum (RER) and transported to the Golgi apparatus through vesicles for modification and sorting.

Golgi Apparatus Function

Mechanism

• Proteins enter from the cis face and undergo modifications as they traverse different stages, enhancing their functionality and preparing them for transport to their destinations.

Destination Tagging

Mechanism

• Proteins leave the Golgi apparatus bound with molecular tags, which direct their transport vesicles to the correct cellular locations.

Exocytosis & Endocytosis

Definitions

• Exocytosis: Release of materials from the cell via vesicle fusion with the plasma membrane.

• Endocytosis: Uptake of materials into the cell through vesicle formation from the plasma membrane.

Recycling Materials in the Lysosome

Pathways

• Breakdown of large molecules for recycling through three primary mechanisms: Receptor-mediated endocytosis, Phagocytosis, and Autophagy.

Lysosomal Recycling Strategies

Overview

• Ensures that materials are effectively processed and utilized by the cell, involving proton pumps and hydrolytic enzymes sourced from the endomembrane system.