7_Cell_Interactions_Lecture

Cell & Molecular Biology

Cell Interactions Lecture

• Presented by: Pearson

Learning Objectives

• Understand how cells attach to each other.

• Identify differences between prokaryotic and eukaryotic extracellular structures.

• Define extracellular matrix (ECM) and its components.

• Describe different types of junctions holding cells together.

• Explain how a signal gets into a cell and how the cell responds.

• Understand major cell signaling pathways.

Cell–Cell Interactions

• Cells in multicellular organisms communicate and cooperate, forming an interdependent community.

The Cell Surface

• Plasma Membrane: Composed of a phospholipid bilayer with proteins.

  • Membrane proteins regulate transport and signaling.

  • They attach to cytoskeletal elements and ECM structures.

• Extracellular Matrix (ECM): Provides support and signaling for the cell.

Extracellular Structures Overview

• Organism Comparison:

  • Bacteria:

    • Cell Wall: Yes (Peptidoglycan)

    • Scaffolding Material: Sugar base

  • Plants:

    • Cell Wall: Yes (Cellulose, Pectin)

    • Scaffolding Material: Strong carbohydrate-based compounds

  • Animals:

    • Cell Wall: No

    • Scaffolding Material: Collagen, Polysaccharides

Bacterial Support Structures

• Bacterial cell walls are primarily composed of peptidoglycan polymers linked by peptide bonds.

  • This structure makes bacteria easier to treat with antibiotics.

Primary Cell Wall in Plants

• Primary Cell Wall: Defines plant cell shape, counters turgor pressure.

  • New cells secrete polysaccharide cellulose into a crisscross network.

  • Gelatinous polysaccharides like pectin keep the wall moist.

The Extracellular Matrix in Animals

• Animal cells secrete a fibrous composite known as ECM for structural support and cell connectivity.

• Fibrous Component: Mostly collagen arranged into fibrils.

• Ground Substance: Composed of proteoglycans providing a rubber-like consistency to tissues like cartilage.

ECM Variability by Tissue Type

• The amount and composition of ECM vary across tissue types:

  • Elastin: Allows lung tissue to stretch.

  • Bone: Has a high ECM content.

  • Skin: Contains minimal ECM.

Integrins and ECM Connection

• Integrins are membrane proteins binding ECM components to the plasma membrane.

  • They anchor the cytoskeleton to the ECM, signaling cells they are anchored.

Communication Between Cells

• Direct Connections: Essential for maintaining structure and function of tissues.

• Cell-Cell Attachments: Materials and structures bind adjacent cells.

Types of Junctions

Tight Junctions

• Form waterproof seals between adjacent animal cells using membrane proteins to create a watertight connection.

Desmosomes

• Strong cell-cell attachments composed of linking proteins (cadherins) that prevent tearing and provide structural integrity.

  • Cytoskeletal intermediate filaments reinforce the desmosomes.

Selective Adhesion

• Cadherins: Link proteins that allow specific cell attachment to similar type cells, forming tissues.

Gap Junctions

• Form channels between adjacent animal cells for small molecule flow, enhancing communication and coordination.

Plasmodesmata in Plants

• Gaps in plant cell walls called plasmodesmata connect the cytoplasm and cell membranes of adjacent cells.

Cell-Cell Signaling

• Distant cells communicate via signaling molecules:

  • Neurotransmitters: Facilitate local signaling.

  • Hormones: Secreted by cells, circulate, and act on targeted distant cells.

Steps of Cell-Cell Signaling

1. Signal reception

2. Signal transduction (including amplification)

3. Cellular response (including signal deactivation)

Signal Reception

• Ligands: Molecules binding to receptor proteins to deliver messages and activate them.

  • Responsive only in cells with the appropriate receptors.

  • Examples: Steroid hormone receptors, G-protein coupled receptors, enzyme-linked receptors.

Signal Transduction - Amplification

• Extracellular signals converted to intracellular signals, amplified to elicit large responses through second messengers or phosphorylation cascades.

Second Messenger Role

• Small molecules/ions diffuse rapidly in cells, produced quickly in large quantities, e.g., Ca2+, cAMP.

Phosphorylation Cascade

• Involves repeated activation of kinases that phosphorylate and activate the next kinases, causing larger cellular responses.

Cellular Response

• Responses include changes in gene expression and protein activities.

  • Example: Drought response in plants involving abscisic acid and guard cells for gas exchange.

Insulin Signaling Example

• In response to high blood glucose, insulin is secreted allowing glucose uptake into cells, reducing blood glucose levels.

Steroid Hormone Receptor Action

1. Lipid-soluble molecules diffuse across the plasma membrane.

2. Receptors located in the cytoplasm undergo conformational change to signal responses by altering gene expression.

Membrane-Bound Receptor Signaling

1. Lipid-insoluble signals require recognition at the cell surface.

2. Signal transduction via second messengers or phosphorylation cascades activates cellular responses.

G-Protein Coupled Receptors

• G proteins are peripheral membrane proteins regulated by guanine nucleotides (GTP/GDP).

  • Activated by exchanging GDP for GTP; deactivated when GTP is hydrolyzed.

Enzyme-Linked Receptors

1. Activation occurs via ligand binding.

2. Kinases initiate phosphorylation cascades affecting cellular activities.

Signal Deactivation

• Cells utilize phosphatases to turn off signals, maintaining sensitivity to signaling changes.

Summary of Cell-Cell Signaling Steps

1. Signal Reception

2. Signal Transduction

3. Cellular Response

4. Mechanisms include changes in gene expression or activity due to receptor binding and downstream signaling pathways.

Regulation of Signal Reception

• Receptors can dynamically change in number and affinity, altering signaling sensitivity.

• Beta-blocker drugs can block receptor-ligand interactions.