14
votes
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Why can't H3O+ ions pass through aquaporins?
This question has been directly addressed by the paper The Mechanism of Proton Exclusion in the Aquaporin-1 Water Channel. I think it's a pretty good one too! I paste the abstract below:
Aquaporins ...
- 6,192
7
votes
Accepted
Difference between going against and going down a concentration gradient
"Concentration" is "how much stuff is there someplace?" "Concentration gradient" is "how much is concentration changing from point A to point B?"
Imagine a terrain where concentration is represented ...
- 44k
7
votes
Accepted
Difference between protein channels, protein carriers and protein pumps?
No, carriers are not the same as pumps. Carriers may or may not carry out active transport and pumps always use energy. Carriers, for example, can make use of the concentration gradient of a certain ...
- 52.3k
6
votes
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How do lipid-soluble substances diffuse through the cell membrane?
See this paragraph and image from The Cell: A Molecular Approach. 2nd edition.:
During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then ...
- 14.1k
6
votes
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Membrane Permeability to Pyruvate
Pyruvate is negatively charged and quite polar, which makes it unfavourable to diffuse directly through any membrane. The outer mitochondrial membrane contains porins, which allow small molecules, ...
- 17.7k
6
votes
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Why do ion leak channels exist?
Check the parameters that go into the Goldman equation... it shows that permeability of ions is as important as their concentrations. If the permeability is zero, there is no potential. Because other ...
- 44k
5
votes
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How does carbon dioxide diffuse into red blood cells?
The red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in lipids, such as those of a bilayer.The effect of 4,4′-diisothiocyanato-stilbene-2,...
- 181
4
votes
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Is co-transport within a cell active or passive transport?
Co-transport, also called secondary active transport, is a form of active transport because it is using the energy from one concentration gradient to transport another molecule across the membrane ...
- 17.7k
4
votes
Accepted
Which complexes physically pump protons in photosynthesis?
General Approach
Although I am not an expert in this area, as an ex-chemist I find this topic one of the most fascinating and fundamental in chemical biology. I have constructed an answer starting ...
- 24.3k
4
votes
Accepted
How are lipid-coated mRNA-based vaccines transported into cells for expression?
I can't give an authoritative answer on this because my PhD work was based on mRNA delivery using peptides instead of lipids, but many of the concepts are the same. I also don't have time to provide ...
- 5,318
4
votes
Examples of passive membrane transport proteins that only transport in one direction and their mechanism
Voltage-gated proton channels are passive transmembrane transport proteins that will only transport protons in one direction. These channels will be open when there is a lower pH in the cytoplasm, ...
- 349
4
votes
Accepted
Are channel proteins considered a type of integral protein or peripheral protein?
Channel proteins are located on the peripheral surface of the cell, however, they have the function of integral proteins. Is my understanding correct?
No.
In this case of nomenclature, the context/...
- 6,192
3
votes
What is the purpose of co-transport?
Would it not then make more sense for the latter substance to simply be actively transported to its destination, as opposed to involving another substance which has no net movement at the end of the ...
- 1,515
3
votes
Accepted
Can Oxaloacetate cross the outer mitochondrial membrane?
In general the outer mitochondrial membrane is thought to be basically permeable (through porins) to small molecules such as OAA. As is typical in biology, the situation may actually be more complex --...
- 737
3
votes
Accepted
Valve-like layers example in nature?
I don't know any-such continuous-membrane-material that can do so. But yes certain membrane-transport-proteins can do so.
Carrier proteins are normally highly specific about in which-direction to ...
- 2,659
3
votes
Why do cell membranes let small non-polar molecules through but won't let small polar molecules through?
The plasma membrane consists of hydrophobic and hydrophillic characteristics. Towards the outsides, they are hydrophillic, so they can create bonds with water. The insides are hydrophobic, allowing no ...
- 55
3
votes
Does osmosis require a protein channel?
Normally, epithelial cells of mucous membranes express a chloride ion channel that allows movement of Cl- down its electrochemical gradient and out of the cell. This increase in extracellular ...
- 17.7k
3
votes
Accepted
Can an emulsion enter a cell?
Under the right conditions, emulsions of lipids (fatty compounds) and water can cross cell membranes. If the emulsion is prepared correctly, a structure known as a liposome forms, which is essentially ...
- 5,675
3
votes
Are receptors integral membrane proteins or peripheral membrane proteins?
All the receptors I know about are integral, transmembrane proteins. It would certainly be possible for a receptor domain to exist on a peripheral protein that interacted with a transmembrane protein, ...
- 44k
3
votes
Accepted
How does exercising/starved muscle import glucose (released by liver)?
GLUT4 is the major glucose transporter in muscle even in exercise. However, during exercise, it is translocated to the cell surface by contraction-triggered, insulin-independent mechanisms, such as ...
- 1,297
2
votes
Accepted
How can a polar molecule pass through polar channels of proteins in the cell membrane?
The molecule won't "stick" to the polar amino-acids for at least three reasons :
The amino-acids might not all be on the same plan, so their is always a force pulling the molecules transiting trough ...
- 158
2
votes
Accepted
Are Na+/K+ active pumps and K+/Na+ "Leak" channels same as given in the figure?
Below is the structure of a Na+/K+ ATPase, i.e., the sodium/potassium pump shown on the left in your figure:
(image from here)
Compare to the structure of a potassium leak channel (the type of ...
- 44k
2
votes
Accepted
COPI/COPII proteins and kinesins/dyneins
Interesting question. The term anterograde refers to movement in the forward direction. In the context of vesicular trafficking, anterograde refers to (1) movement from the site of protein synthesis ...
- 17.7k
2
votes
Transport mechanism in absorption of monosaccharides in small intestine
From my initial reading on this, the SGLT1 does not itself directly use energy in the form of ATP. https://en.wikipedia.org/wiki/Sodium-glucose_transport_proteins It appears the uptake of the sugars ...
- 1,351
2
votes
Accepted
Can the Na+/K+ pump backwards to generate ATP?
All enzymes can theoretically catalyze the reverse reaction.
Researchers have driven the Na+/K+ ATPase to synthesize ATP with artificial ion concentrations:
We have studied the apparent affinity ...
- 17.7k
2
votes
Are cells electrically neutral or charged to setup the membrane potential across them?
It isn't neurons that are charged, it is their membranes. From the distance of other cells, they "see" both the charges inside and outside the membrane, so there isn't any substantial repulsion on ...
- 44k
2
votes
Accepted
How does 'phosphorylation of glucose' maintain concentration gradient in membrane transport (facilitated diffusion)?
Sort of a "magic trick" that biology does.
With facilitated transport, the movement is passive. That is, you have a protein that certain molecules/ions can pass through, but you aren't doing any ...
- 44k
2
votes
Accepted
Exocytosis of synaptic vesicles
That sentence is located in a paragraph titled "Stages 5-7: Endocytosis and recycling": it's talking about recycling exocytosed membrane which is necessary for making vesicles for further exocytosis.
...
- 44k
2
votes
Accepted
Giving a sign to Pressure Potential, Osmotic Potential And Other Similar Quantities
I think you already know the sign convention in calculating the water potential: the addition of solutes lowers the potential (negative vector), while an increase in pressure increases the potential (...
- 954
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