Cell communication Premedical Biology

Plasma membrane half-fluid mosaic of lipids and proteins, it consists of double layer of phospholipids and incorporated proteins controls traffic into and out of the cell is selectively permeable, it allows sufficient passage of oxygen and nutrients, and elimination of wastes

Phospholipids are amphipatic molecules Lipids and proteins Proteins are embeded or attached to surface.

The fluidity Is due to the presence of unsaturated hydrocarbons, which increase fluidity and cholesterol (animal cells), which reduces fluidity; helps stabilize the membrane

Proteins in membrane determine many of the membrane‘s specific functions Integral proteins – transmembrane proteins Peripheral proteins – are not embeded in the lipid bilayer

Transport

Active transport is the pumping of solutes against concentration gradients „uphill“ It is the major mechanism of ability of cell to maintain internal concentrations of small molecules that differ from concentrations in environment.

Cell communication – function of proteins Direct contact between membranes through molecules on the cell surface = cell junctions = intracellular joining = cell-cell recognition (glycolipids and glycoproteins)

Intracellular junctions (joining) Cell walls of plant cells perforated with channels called plasmodesma. In animals are intracellular junctions. Tight junction Desmosomes Gap junctions Adhere, interact and communicate

Cell-cell recognition - by carbohydrates (linked to proteins and lipids), which helps to sort cell into tissues and organs in embryo and helps to recognize and reject the foreign cells in the immune system Carbohydrates are usually short branched oligosaccharides.

Attachment with ECM Plant cells (some Protists, prokaryotes, fungi) are encased by cell walls Animal extracellular matrix – ECM with glycoproteins: Collagen fibers are embedded in network of proteoglycans. Fibronectins bind to receptor protein called integrins in plasma membrane. Integrins bind to microfilaments (cytoskeletal pr.) on cytoplasmatic side.

Signal transduction pathways

Cell communication: Local and long-distance signaling

Cell communication and signal transduction function of protein Cell responds to external signals. Signal molecule (ligand/first messenger) binds to a receptor protein in membrane and causes change of its shape (enzyme). On internal side is the signal transformed into the cascade of molecular interactions (second messengers). The signal leads to regulation of transcription in nucleus or some cytoplasmic activities.

Cell signaling 1.Reception: target cell detects a signaling molecule coming from outside 2.Transduction: change of the receptor protein, initiating process of cellular response 3.Response: cellular activity: catalysis, rearrangement of the cytoskeleton, activation of genes…

Reception Signaling molecule + receptor Receptor or protein associated with it gets activated and is able to transfer the signal inside the cell. G protein associated receptors – on/off Tyrosine kinase receptors have enzymatic activity and catalyze transfer of phosphate groups Ion channel receptors - gate open or close

G-protein-linked receptors G-protein-linked Receptor Plasma Membrane Enzyme G-protein (inactive) CYTOPLASM Cellular response Activated enzyme Activated Receptor Signal molecule Inactivate enzyme Segment that interacts with G proteins GDP GTP P iP i Signal-binding site Figure 11.7 GDP

Receptor tyrosine kinases Signal molecule Signal-binding site CYTOPLASM Tyrosines Signal molecule  Helix in the Membrane Tyr Dimer Receptor tyrosine kinase proteins (inactive monomers) P P P P P P Tyr P P P P P P Cellular response 1 Inactive relay proteins Activated relay proteins Cellular response 2 Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated dimer) 6 ATP 6 ADP Figure 11.7

Ion channel receptors Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Figure 11.7 Gate closed Ions

Intracellular receptors for steroid and thyroid hormones, nitric oxide

Transduction Multistep pathway (cascade) of activation of proteins by addition or removal of phosphate groups or it starts by formation of small molecules or ions that act as second messengers. Phosphorylation and dephosphorylation of proteins acts as molecular switch. Protein kinases transfer phosphate groups from ATP to protein.

A phosphorylation cascade

Multiple steps of signal transduction greatly amplify the signal. In each step the number of activated products is much greater than one step before. Multiple steps also provide different points, at which the response can be regulated and also provide a specifity of cell signaling and coordination.

Small molecules and ions as second messengers Nonprotein molecule, which can be spreaded by diffusion – cAMP and calcium ions Proteins are sensitive to cytosolic concentration of the molecule.

Second messengers: calcium ions Neurotransmitters, growth factors, hormones induce cell response via signal transduction pathways that increase the concentration of calcium ions. Responses: muscle contraction, secretion of substances, cell division Second messengers: inositol triphosphate and diacylglycerol

Responses: End of the pathway may occur in the nucleus or in the cytoplasm = change of transcription or cytoplasmic activities. Response in nucleus is the regulation of gene activity - transcription factors

Nuclear response to a signal

Cytoplasmic response to the signal

Campbell, Neil A., Reece, Jane B., Cain Michael L., Jackson, Robert B., Minorsky, Peter V., Biology, Benjamin- Cummings Publishing Company, 1996 –2010. Thank you for your attention

Repetition 5. Molecular base of DNA inheritance 1. Describe the molecular structure of DNA 2. What is hybridization? 3. Describe the process of DNA replication in eukaryotic cells 4. What are the types of chromatin? 5. What are telomeres and what is their function? 6. How does the human karyotype look like? 7. What are the types of human chromosomes? 8. What is NOR? 9. Describe spiralisation of DNA, what does it serve for? 10. What nucleotides do you know and where you could find them?