Osmosis etc

Question 0

What substances can/can’t move through the bilayer?

Answer 0

Can: Urea, O2, CO2 Can’t: Charged ions and larger proteins- glucose, sodium ions, proteins

Question 1

Describe the phosopholipid bilayer

Answer 1

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.

Question 2

What is Frick’s law and what does it tell us?

Answer 2

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

Question 3

What is osmosis?

Answer 3

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

Question 4

Describe osmosis in the presence of non-p solutes.

Answer 4

Water moves from an area with low total solute to an area with high total solute. This equalises both the water and the solute.

Question 5

What other factors may affect water activity?

Answer 5

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).

Question 6

What is tonicity?

Answer 6

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

Question 7

Describe three characteristics of membrane proteins.

Answer 7

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.

Question 8

Describe simple diffusion using ion channels.

Answer 8

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.

Question 9

Describe facilitated diffusion.

Answer 9

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)

Question 10

Carrier-mediated transport. What is active transport?

Answer 10

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.

Question 11

What is the most important example of primary active transport?

Answer 11

The sodium/potassium pump: These are in every cell in the body and maintain the low Na/high K concentration in the cells.

Question 12

Describe Secondary Active transport.

Answer 12

This is very similar to facilitated diffusion except an extra ion is transferred.

Question 13

What happens if the cell needs to transport a fairly unusual or large substance for which there is no carrier protein?

Answer 13

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.

Question 14

Describe the process of endocytosis.

Answer 14

1. 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.

Question 15

Describe receptor-mediated endocytosis.

Answer 15

1. 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

Question 16

What is phagocytosis?

Answer 16

Cell eating. The cell engulfs large objects. This is performed by specialised cells. Ex: macrophages

Question 17

Describe exocytosis.

Answer 17

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.

Question 18

Describe the ER.

Answer 18

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.

Question 19

Describe the Golgi apparatus.

Answer 19

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

Question 20

What are lysosomes?

Answer 20

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.

Question 21

What are membrane renewal vesicles?

Answer 21

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.

Question 22

What are secretory vesicles?

Answer 22

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)

Question 23

What is a resting membrane potential?

Answer 23

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!!

Question 24

What is the membrane potential?

Answer 24

The separation of ions across the membrane and the relative difference between cations (+) and anions (-) in the ICF and ECF. Separated ions can be used to form work, and this energy is measured in Volts of miniVolts. (ICF tends to be negative because anions can’t get out of the cell as they are too big)

Question 25

What do we need for a membrane potential?

Answer 25

An ion concentration gradient (potassium/sodium pump would help this!) A selectively permeable membrane (with plenty of leak channels) The resting membrane potential is a balance between the electrical gradient, the concentration gradient and the permeability of the cell membrane for various ions. Basically: -a relatively large diffusion outwards of K+ (this makes the ICF more negative) -no diffusion of A- across to ECF -relatively small diffusion of Na+ into the ICF (neutralises some of the negativity created with K+ leaving) Resting potential= -70mV

Question 26

What is the membrane potential/equilibrium?

Answer 26

Membrane potential (Vm) is always closest to the equilibrium potential of the ion for which the cell is most permeable. Lots of K+ leaving the cell- membrane is most permeable to THIS ion. SO the Vm is closest to the Ek (-90 mV) (EnA= +60 mV)

Question 27

What is an action potential?

Answer 27

When specialised cells actively change their membrane potential through a trigger. This change leads to a change in ion permeability therefore affecting the membrane potential. 1. Membrane permeability for sodium changes. An electrical AND concentration gradient for sodium move sodium into the cell 2. Sodium rushing in changes the membrane potential, creating a signal. If generator potentials are large enough and reach the threshold potential, they will activate the opening of voltage-gated Na Channels (vgsc). This propogates until all sodium channels are opened, creating an action potential.

Question 28

What are local potentials? Where do they start?

Answer 28

A signal is carried in through the dendrites (input zone) to the axon hillock (trigger zone). It then travels down the axon (conducting zone) until it reaches the axon terminalis (output zone). From here it may synapse onto an interneuron or onto a muscle. Generator potentials occur in the input zone (cell body). They are activated by a trigger or a stimulus. This stiumulus changes the cell’s permeability to ions (usually sodium). When the threshold potential is reached, vgsc open.

Question 29

What are receptor potentials?

Answer 29

A stimulus changes the permeability of the membrane of a receptor cell. This triggers exocytosis of a chemical messenger from the receptor cell. This chemical messenger diffuses across to the afferent neuron fibre, activating chemical-messenger gated channels for sodium. Sodium rushes in. Often, the chemical messenger is histamine and the trigger is a nauseous stimuli (like a chemical in the air). The histamine released attaches to receptors on the afferent neuron, changing the permeability. If enough histamine is released, threshold potential is achieved and vgsc open.

Question 30

What is a synaptic potential?

Answer 30

Synaptic potentials: generated locally, underneath the receptors of the receiving cell. These potentials are generated by the opening of transmitter-sensitive receptors (transmitter is often acetylcoleine). Sodium is the main ion passing through the channels, but they are not selective channels. This is due to the high electrical and concentration gradient. There is a presynaptic terminal head that contains neurotransmitter molecules in synaptic vesicles. Through exocytosis, the neurotransmitters diffuse across the synaptic cleft to the synaptic membrane of the postsynaptic neuron where it binds to a receptor site. This opens chemical messenger- gated ion channels, causing ions to move across electrical and concentration gradients, creating an AP.

Question 31

What is a faster type of synaptic transmission?

Answer 31

Some transmissions do not involve a change in ion composition. This type acts on a receiving cell without an electrical change. Transmitter acts on a protein. That protein is linked to a second messenger underneath. That G-protein acts as a catalyst for second messengers. G-protein is activated and produces second messengers which go on to release calcium from internal stores of to activate gene expression. This can contract a muscle without a local potential. When?? Cardiac muscle and smooth muscle. Noradrenaline in smooth muscle binds to proteins with a g-preotein. ATP is converted cyclic AMP (check?)