Membranes, lipids and transporters Flashcards
What are membranes composed of?
Although different functions occur in different cells, all cell membranes have the following same structures:
• Thin film
• Lipid bilayer fluid
• Proteins, functional (transport, catalyzing reactions and supporting structures)
What are lipids?
• Organic molecules
• Non-solvent (don’t like water)
• Diverse – Examples: fat, waxes, sterols, fatty acids
* The lipids in the membranes can’t exist on their own, they’re attached to phosphates and so are referred to as “phospholipids”
* Phospholipids in the membranes are amphipathic: It is when there are both hydrophobic and hydrophilic. Phospholipids have hydrophobic tails, and hydrophilic heads
Where are the proteins within biological membranes?
Fluid mosaic model:
Proteins bobbing in fluid lipid bilayer, hydrophobic protein part inside bilayer, hydrophilic protein part outside bilayer. This bilayer is fluid i.e, it isn’t rigid
Integral proteins:
The inside layer is made of hydrophobic parts, while the exterior is hydrophilic. Since proteins are made up of amino acids, the hydrophobic/ non-polar amino acids will be in the middle structure of the protein.
These proteins are across the membrane.
Peripheral functions:
Peripheral proteins are found on the surface of the membrane, not deeply embedded and can disconnect without affecting the structure of the membrane.
They may act as receptors, enzymes, or cytoskeletons giving a cell its shape, and movement facilitation through three main components: micro filaments, intermediate filaments, and micro tubules
What are functions of membrane proteins?
• Transport: solutes across membrane
• Enzymatic activity: catalyze series of reactions
Signal transduction: transfer information from external environment into cell e.g. hormones
What is Co-transport?
- Co transport is when two substances are simultaneously transported across a membrane by one protein. An example is Na+ and glucose.
- Na+ follows down the concentration gradient, and glucose follows Na+.
- Glucose is against the concentration gradient.
- Moving a molecule against its concentration gradient requires energy. * This energy in co-transport is supplied by the molecule moving along its concentration gradient
What are the steps in intercellular signalling?
• Cells communicate via chemical signals
• These chemical signals are in the form of proteins or other molecules. They are known as ligands, which is a term for molecules that bind to other molecules
• These ligands are produced and secreted by the sending cell into the extracellular space
• In order to be a target cell/ responding/ receiving cell, the cell must have the right receptor for that signal
* When a ligand binds to a receptor, the receptors shape and activity changes, triggering a change within the cell
Distinguish between direct, endocrine, paracrine and autocrine signaling, and describe them
Direct
* In cells, gap junctions exist; they connect cells together. They are water-filled channels which allow small signaling molecules to diffuse between the two cells
Endocrine
* Cells transmit signals over long distances using circulatory system = hormones
Paracrine
- Cells next to each other can communicate through the chemical release of ligands which can diffuse through the short space between the cells
- Neurotransmitters
Autocrine
* A cell signals itself, releasing a ligand that binds to receptors on its own surface
Describe G- Protein coupled receptors (GPCR’s).
• GPCR’s have seven transmembrane alpha helices (seven green thingos)
• They interact with G proteins which are heterotrimeric; 3 sub units
• G proteins are specialized proteins. They are able to bind guanosine triphosphate and guanosine diphosphate
• The alpha and gamma subunits are bound to the membrane by lipid anchors. Beta sub unit is not
• In its inactive form, the alpha subunit is bound to a GDP. In its active form, it is bound to GTP
• When a ligand bind to the GPCR, the GPCR undergoes a conformational change
• When GPCR undergoes a conformational change, it causes the alpha helix to change its GDP to GTP
• When the alpha unit binds to the GTP, it disassociates from its beta and gamma sub units
• The alpha subunit is now able to regulate target proteins
• The target proteins relay a second messenger
* The GTP is hydrolyzed (loses one phosphate) into GDP, stopping the cycle
Describe enzyme- linked receptors.
- Extracellular domain (where ligand binds to receptors) and an Intracellular domain (within the cell, it is the part of the receptor that can act as an enzyme)
- When a ligand binds to the extracellular domain, the catalytic activity of the intracellular domain is activated
- Intracellular proteins go and dock onto the enzymes of the receptor
- The signal will be relayed by these intracellular proteins into the cells interior
What are second messengers?
They are small, non-protein molecules that pass along a signal initiated by the binding of a ligand (the “first messenger”) to its receptor.
Describe the 3 types of second messengers.
Calcium ions Ca+:
In most cells, Ca+ levels are low. For signalling purposes, Ca+ may be stored in compartments such as the E.R. Ligands will bind to ligand-gated Ca+ channels, allowing Ca+ into the cell. Once in the cell, calcium 2+ ions binds to proteins with complementing binding sites. When the ion binds, it changes the proteins shape and thus function
cAMP:
Cyclic adenosine monophosphate is a small molecule made from ATP. Enzymes called adenylyl cyclase converts ATP into cAMP. The cAMP activates a protein called protein kinase (PKA) which adds phosphates to target proteins, changing its structure and function
Inositol triphosphate IP3 diacylglycerol DAG:
When a ligand binds to the GPCR, it activates it, which in turn activates phospholipase. The phospholipase is an enzyme which then separates pip 2 into IP3 and DAG. IP3 is freed, and goes and binds to the IP3 Ca+ channel. The Ca+ channel is opened, allowing Ca+ to travel into the cytosol. The increased level of Ca+ along with DAG, activate Protein Kinase C, it will begin to phosphorylate target proteins
List the functions of biological membranes
• A barrier between the inner and outer surface
• Boundaries among organelles / inner compartments
• Protect the cells
• Control the movement of substances into and out of cells
• Regulate the composition within individual cells
• Control the flow of information between cells (recognizing or sending signals) – Capture and release energy (e.g. mitochondria)
• Cell adhesion
* Synthesize steroids
Structure and function of plasma membrane
• Amphipathic – possessing both hydrophilic and hydrophobic/ lipophilic properties
• Selective permeability – some molecules are allowed in while others are kept out
• Permeable to lipid-soluble substances (e.g. O2)
• Impermeable to charged molecules (e.g. ions)
• Allow cell to interact with watery environment but remain water resistant
* Movement can occur by passive transport or active transport
Differentiate the different classes of membrane transport proteins
Channels: These channels may be water filled pores, and can be specific or non-specific. Channels can be:
• Ligand gated: molecules bind to receptors
• Voltage-gated: open in response to a voltage for ions (depolarization, making cell less negative inside)
Carrier proteins
• Carrier proteins are transport proteins that are only open to one side of the membrane at once
• They are often designed this way because they transport substances against their concentration gradient
* Being open to both sides of the membrane simultaneously might allow these substances to simply flow back along their concentration gradient, canceling out thecarrier protein’s work
* To accomplish their work, carrier proteins typically use energy to change shape
Define isotonic/hypotonic/hypertonic solutions
Isotonic:
• Concentration of dissolved substances (water) is the same in cell as outside of cell
• The same OP
Hypotonic:
• Concentration of dissolved substances (water) is lower in the cell than out of cell
• OP low
• Water moves into cell
Hypertonic:
• Concentration of water is higher in cell than out of cell
• OP high
• Water moves out of cell
