1.4 Flashcards
How do multicellular organism signal between cells?
Multicellular organisms signal between cells using extracellular signalling molecules
Extracellular signalling molecules examples
Steroid hormones
Peptide, hormones
Neurotransmitters
Describe the general process of extracellular signalling
A cell releases a signal molecule that is responded to by another cell (target cell).
The target cell must have receptor molecules (proteins) with a binding site for the specific signal molecule.
When binding occurs, it changes the conformation of the receptor, which brings about the response in the target cell
Describe, hydrophilic signalling
Most signal molecules are hydrophilic, and are therefore unable to cross the plasma membrane directly; instead they bind to cell-surface receptors which generate one or more signals inside the target cell
Describe, hydrophobic signalling
Some small signal molecules diffuse across the plasma membrane and bind to receptors inside a target cell, either in the cytoplasm or in the nucleus.
Many of the small signal molecules are hydrophobic and nearly insoluble in aqueous solutions and therefore are transported in the bloodstream and other extracellular fluids after binding to carrier proteins, from which the associate before entering the cell.
Describe various cell types and the signals they produce
Different cell types produce specific signals that could only be detected and responded to by cells with the specific receptor. Signalling molecules may have different affects on different target cell types, due to differences in the intracellular signalling molecules and pathways that are involved.
Describe Response to signals in multicellular organisms
In a multicellular organism, different cell types may show a tissue-specific response to the same signal
Explain diffusion of hydrophobic signalling molecules
Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors.
What are the receptors of hydrophobic signalling molecules?
The receptors for hydrophobic signalling molecules are transcription factors.
What are transcription factors?
Transcription factors are proteins that when bound to DNA can either simulates or inhibit initiation of transcription
Hydrophobic signalling molecule examples
Steroid hormones
Oestrogen
Testosterone
Where do steroid hormones bind
Steroid hormones bind to specific receptors in the cytosol or the nucleus
Describe the hormone receptor complex
The hormone receptor complex moves to the nucleus where it binds to specific sites on the DNA and affects gene expression
Explain hormone response elements
The hormone-receptor complex binds to specific DNA sequences called hormone response elements. Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.
Describe the binding of hydrophilic signalling molecules
Hydrophilic signalling molecules, bind to transmembrane receptors, and do not enter the cytosol
Hydrophilic signalling molecule examples
Peptide hormones
Neurotransmitters
Describe what happens to transmembrane receptors as a ligand binds
Transmembrane receptors change conformation when the ligand binds to the extracellular face. The signal molecule does not enter the cell. This change in conformation alters the behaviour of the cell.
What do transduced hydrophilic signals often involve?
Transduced hydrophilic signals often involve G-proteins or cascades of phosphorylation by kinase enzymes
What do G-proteins do?
G proteins relay signals from activated receptors (receptors that have found a signalling molecule) to target proteins such as enzymes and ion channels
What do phosphorylation cascades allow for?
Phosphorylation cascade allow more than one intra cellular signalling pathway to be activated
Describe a phosphorylation cascade
Phosphorylation cascade involve a series of events with one kinase activating the next in the sequence and so on. Phosphorylation cascades can result in the phosphorylation of many proteins as a result of the original signalling event
How is the action of hormones on receptors described?
The target cells for signalling molecules have specific receptors at their surface, so the action of the hormones is highly specific
Describe what happens when insulin binds to its receptor?
Binding of the peptide hormone insulin to its receptor results in an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.
Describe the process of insulin in the control of blood glucose
- Binding of insulin to its receptor in the membranes of fat and muscle cells causes a conformational change in the Receptor.
- The start of phosphorylation cascade inside the cell, which eventually leads to GLUT4 containing vesicles being transported to the cell membrane.
- The GLUT4 glucose transporters now allow glucose to pass across the plasma membrane and enter the cell.
