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3 Membrane Biophysics
Bachelor of Science in Biology (BSBiol)
Saint Louis University (Philippines)
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Membrane Biophysics
Biological Membrane: Structure and Composition
The plasma membrane is a fluid lipid bilayer embedded with proteins. The most abundant membrane lipids are phospholipids. Phospholipids have a polar head containing a negatively charged phosphate group (hydrophilic) and two non-polar fatty acids tail (hydrophobic). These two-sided molecules assemble into a double layer called phospholipid bilayer when in contact with water. The outer surface of the bilayer is exposed to the extracellular fluid (ECF), and the inner surface is in contact with the intracellular fluid (ICF).
Membrane Structure and Function
The fluid mosaic model of plasma membrane describes its structures as mosaic components. It is mainly phospholipids, cholesterol (for animals), proteins, and carbohydrates that make it fluid. The fluidity of the phospholipid enables many membrane proteins to float freely throughout the lipid bilayer.
Membrane Protein
Channels - enables small water-soluble ions to pass through the membrane.
Carriers - transfer larger, specific substances that cannot cross on their own.
Receptors - those with sites that recognize and bind with specific molecules in the cell’s environment.
Docking-marker acceptors - bind lock-and-key fashion with the docking markers of secretory vesicles.
Enzymes - control specific chemical reactions in and out of the cell.
CAMs (cell adhesion molecules) - protrude from the outer membrane surface and form loops or other appendages that the cells use to grip each other or grasp the connective tissue fibers between cells.
Self-identity markers are important in the cells’ ability to recognize cells of the same type and cell-to-cell interactions.
Membrane Permeability
Molecular size – smaller molecular size makes it easier for the molecule to travel across the membrane.
Polarity – or the relative solubility of the molecule, non-polar molecules can easily travel the membrane.
Charge – uncharged molecules are easier to pass through the membrane.
Types of Membrane Transport
- Unassisted membrane transport
a. Passive diffusion – occurs when solute moves from a region of high concentration to a region of low concentration through a membrane in the absence of transport protein. Passive diffusion is affected by different factors.
Chemistry of solute – molecular weight and polarity of the solute.
Difference in concentration gradient – the higher difference in a concentration gradient, the faster molecule movement.
Temperature – higher kinetic energy of molecules increases diffusion rates.
b. Osmosis – the movement of water across semi-permeable membranes to balance solute concentration from hypotonic to a hypertonic solution.
- Assisted membrane transport
a. Facilitated diffusion – the process of passive transport of molecules or ions across a biological membrane via channel or carrier proteins.
- Channels – transmembrane proteins that form an open passageway for the diffusion of ions and molecules across the membrane.
o Ion channels- such as sodium channel, potassium channel, and calcium channel. o Ligand-gated channels – channel opens in response to binding of ligand.
o Voltage-gated channels – channel opens in response to changes in the amount of electric charge.
- Transporters or carriers – bind solutes in a hydrophilic pocket and undergo a conformational change that switches the exposure of the pocket to the other side of the membrane.
o Pumps – uses ATP to transport molecules against their gradient. These are used to transport organic molecules such as sugars, amino acids, and nucleotides.
- Differences between channels and carriers:
- A carrier is not open simultaneously to both the extracellular and intracellular environments.
- Carriers have binding sites, but channels do not.
Intercellular Communication and Signal Transduction
Types of Intercellular Communication
- Direct Intercellular Communication – involves physical contact between the interacting cells by three mechanisms:
a. Gap junctions – minute tunnels that bridge the cytoplasm of neighboring cells in some tissues where small molecules and ions are directly exchanged between interacting cells without ever entering the ECF.
b. Transient direct link-up of surface markers – specialized markers on the surface membrane of some cells allow them to directly link up transiently and interact with certain other cells that have compatible markers for transient interactions.
c. Nanotubes – long tubes with internal actin-filament support surrounded by a plasma membrane that permits contiguous contact between two cells.
- Indirect Intercellular Communication – involves the use of intercellular chemical messengers, also known as a ligand, that should bind with the target-cell receptors specific to it. There are six categories of chemical messengers:
a. Paracrines – local chemical messengers distributed by simple diffusion, making the effect be exerted only on the neighboring cells in the immediate environment of their site of secretion.
b. Neurotransmitters – used by neurons that communicate directly with their target cells by releasing signal molecules in response to electrical signals. c. Hormones – long-range chemical messengers specifically secreted into circulation by endocrine glands in response to an appropriate signal.
d. Neurohormones – hormones released by neurosecretory neurons into the circulatory fluid and then distributed to the target cells.
e. Pheromones – chemical signals released into the environment, usually by glands, and travel through the air or water to sensory cells in another animal.
The Three Stages of Cell Signaling
- Reception – a chemical signal is “detected” when the signaling molecule (ligand) binds to a receptor protein located at the cell’s surface, causing it to change shape. There are three major types of receptors in the plasma membrane – the G protein-coupled receptors (GPCRs), receptor tyrosine kinase, and ion channel receptors. a. G protein-coupled receptors (GPCRs) – work with cytoplasmic G proteins. Ligand binding activates the receptor, which activates a specific G protein, which activates yet another protein, thus propagating the signal.
b. Receptor tyrosine kinases (RTKs) – react to the binding of signaling molecules by forming dimers and then adding phosphate groups to tyrosines on the cytoplasmic part of the other monomer making up the dimer. Relay proteins in the cell can then be activated by binding to different phosphorylated tyrosines, allowing this receptor to trigger several pathways at once.
c. Ligand-gated ion channels – open or close in response to binding by specific signaling molecules, regulating the flow of specific ions across the membrane.
- Transduction – cascades of molecular interactions relay signals from receptors to target molecules in the cell. At each step in a signal transduction pathway, the signal is transduced into a different form, which commonly involves a shape change in a protein.
a. Protein Phosphorylation and Dephosphorylation – phosphorylation cascades, in which a series of protein kinases each add a phosphate group to the next one in line, activating it. Enzymes called protein phosphatases remove the phosphate groups. The balance between phosphorylation and dephosphorylation regulates the activity of proteins involved in the sequential steps of a signal transduction pathway.
b. Small Molecules and Ions as Second Messengers – second messengers, such as the small molecule cyclic AMP (cAMP) and the ion Ca2+, diffuse readily through the cytosol and thus help broadcast signals quickly. Many G proteins activate adenylyl cyclase, which makes cAMP from ATP. Cells use Ca2+ as a second messenger in both GPCR and RTK pathways.
The tyrosine kinase pathways can also involve two other second messengers, diacylglycerol (DAG) and inositol trisphosphate (IP3). IP3 can trigger a subsequent increase in Ca2+ levels.
3 Membrane Biophysics
Course: Bachelor of Science in Biology (BSBiol)
University: Saint Louis University (Philippines)
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