Osmosis etc Flashcards
What substances can/can’t move through the bilayer?
Can: Urea, O2, CO2 Can’t: Charged ions and larger proteins- glucose, sodium ions, proteins
Describe the phosopholipid bilayer
It is the cell membrane composed of hydrophobic heads and hydrophilic tails. These properties align the phospholipids so that they form a barrier between the internal and external cellular fluid. The bilayer has three functions: 1. Structure- gives the cell its shape 2. Flexibility- allows cells to change shape 3. Barrier- prevents water-soluble substances from entering/exiting the cell The membrane is also studded with various proteins. It IS permeable to lipid-soluble proteins and small, uncharged molecules.
What is Frick’s law and what does it tell us?
It tells us all of the factors that may affect the speed/ease of simple diffusion across a membrane. C- Concentration gradient P- Permeability of the membrane to a certain substance A- Surface Area of the membrane for diffusion If these things increase, diffusion speeds up X- Membrane width MW- Weight of substance If these things increase, diffusion slows
What is osmosis?
When water moves down its activity (concentration gradient). The presence of solutes reduces water activity. Penetrating solutes will diffuse and reduce the water activity gradient so that it is equal on both sides. If non-penetrating solutes occur on one side of a membrane, a water activity gradient can exist! If non-penetrating solutes are higher on one side, that side will have a lower water gradient. Thus, water will move in from the other side across the membrane. SOLUTE HIGH-WATER LOW
Describe osmosis in the presence of non-p solutes.
Water moves from an area with low total solute to an area with high total solute. This equalises both the water and the solute.
What other factors may affect water activity?
Whether an ion dissociates in a solution. This may create a higher osmolarity gradient, and thus a lower water activity gradient. WATER moves from an area of LOW osmolarity (low solute) to HIGH osmolarity (high solute).
What is tonicity?
It is the effect of bathing solutions on cell volume (what happens to the cell in a solution?) Hypotonic: cell swells (less than 300 mOsm) -low osmolarity inside cell so water rushes in with activity gradient Hypertonic: cell shrinks (more than 300 mOsm) -high osmolarity inside cell so water rushes out Isotonic: no change
Describe three characteristics of membrane proteins.
They all have… 1) Specificity: Each carrier protein is specialised to transport one or, at most, a few closely related substances 2) Saturation: There are a limited number of carrier binding-sites in the membrane for a particular substance. These can become full (Tm- transport maximum) 3) Competition: If closely related substances can use the same carrier protein they will compete to use that carrier.
Describe simple diffusion using ion channels.
Channels are an easy way for ions to travel. They come in a few different types: a) ion selective (possibly only letting in Na+, or K+) b) Gated- volatage, ligand or mechanically gated Ungated- always open! These channels are “downhill” only! They only follow the concentration gradient! Basically, when they are open: K+ moves out, Na+, Ca++, Cl- move in.
Describe facilitated diffusion.
Carrier-mediated transport Step 1: Solute to be transported binds weakly to a carrier protein (trans-membrane protein) Step 2: This binding changes the shape of the carrier protein. It “flips” over. Step 3: Transported solute detaches from membrane into area of low concentration Step 4: Carrier protein changes back to original conformation (shape)
Carrier-mediated transport. What is active transport?
This is what happens when the cell needs to move a solute that does not have a favourable concentration gradient. It is “uphill” movement, and so it needs energy! Primary active transport: 1) Carrier protein splits ATP into ADP plus a phosphate group. The phosphate group binds to the carrier, increasing the affinity of its binding site for a particular ion. 2) Ion binds to carrier on low-concentration side 3) This binding changes the conformation of the carrier so that its binding site is exposed to the other side of the membrane. This change in shape also reduces the affinity for the ion in the binding site. 4) Carrier releases the ion to the high-concentration side. The phosphate group is also released. 5) Carrier changes back to original shape after ion is released.
What is the most important example of primary active transport?
The sodium/potassium pump: These are in every cell in the body and maintain the low Na/high K concentration in the cells.
Describe Secondary Active transport.
This is very similar to facilitated diffusion except an extra ion is transferred.
What happens if the cell needs to transport a fairly unusual or large substance for which there is no carrier protein?
It would use vesicular transport. Materials are transferred between ICF and ECF in vesicles, small membraneous sacs that form at or fuse with the cell membrane. This process requires ATP.
Describe the process of endocytosis.
- A trigger primes the membrane (may be a ligand or presence of material) 2. Membrane indents 3. A pouch forms on the membrane 4. The neck of the pouch is sealed off. 5. The vesicle detaches from the membrane.
Describe receptor-mediated endocytosis.
- Substance attaches to membrane receptors in ECF. 2. Membrane pockets inward, containing substance 3. Membrane pinches off forming vesicle containing target molecules. 4. Primary vesicle fuses with a primary lysosome forming a secondary lysosome. 5. This fusion breaks the connection with the receptors and the receptors (ligands) detach. Lysosome also detaches. 6. Membrane and receptors return to the cell membrane. Pinocytosis: Cell drinking -not as selective and not triggered by ligand binding -target is general ECF fluid
What is phagocytosis?
Cell eating. The cell engulfs large objects. This is performed by specialised cells. Ex: macrophages
Describe exocytosis.
Vesicles are created within either the Golgi apparatus or the ER in the cytoplasm. Four types of vesicles are produced: 1. Transfer vesicles- transfer enzymes between ER and Golgi apparatus 2. Lysosomes- contain intracellular enzymes 3. Secretory vesicles- allows cells to secrete products (hormones, eg) 4. Membrane renewal vesicles- contain new/recycled membrane components (phospholipids, proteins, receptors) These vesicles travel to the membrane, fuse and contents are deposited to ECF.
Describe the ER.
Network of intercellular tubules and fluid-filled sacs. Synthesises and stores proteins, carbs and lipids. Detoxifies drugs or toxins Transfers products to the Golgi through transport vesicles for further processing.
Describe the Golgi apparatus.
Produces lysosomes, membrane-renewal vesicles, secretory vesicles and transfer vesicles. Sets of stacked, flattened membraneous discs called sacuules. Processes material from ER into final form. Synthesises enzymes for use within the cell and secretions for release Produces new membrane
What are lysosomes?
They are created in the Golgi and contain digestive enzymes. They bind to vesicles to create secondary lysosmes. They modify or digest the contents of some vesicles.
What are membrane renewal vesicles?
They add news lipids and proteins to the cell membrane. This allows the cell to change its number or types of carrier proteins or receptors thus changing the sensitivity of the cell.
What are secretory vesicles?
They are produced by the Golgi and are specialised for the secretion of enzymes or hormones. Bud off from Golgi –> fuse with cell membrane Secretory vesicles may be unregulated or regulated (stored for release later)
What is a resting membrane potential?
The unequal distribution of a few key ions between the ICF and the ECF and their selective movement through the membrane. Membrane potentials: a) Electrically neutral- there is a balance of ions outside and inside the cell. There is NO potential. b) More positive ions on one side- Potential (This gives the cell the “potential” to do work, or to change) c) Separated charges responsible for potential (Charges attract and begin to line-up along membrane, but the rest is in imbalance. This separates the ions and the remainder of the fluid is electrically balanced/neutral) d) Separated charges accumulate along the membrane The greater the separation of charges across the membrane (the more there are) the larger the potential!!