2.4a Cells: Membranes

20/09/2012 § Leave a comment

EDITED: Now with accompanying DBQs!

Alright, we’re off to start a new unit that links perfectly to the previous one: membranes! As we know, membranes run rampant in all cells. Seems as though this is the focus for the next two weeks – really, just two weeks. We have a test in two weeks, so heads up!

The most important part of a membrane is what it’s made of: phospholipids. Phospholipids have two specific parts to them – the first being the two tails made of hydrogen and carbon. These tails are hydrophobic, meaning they are not attracted to water. The second part is called the phosphate head, smaller so than the two hydrocarbon tails, and hydrophilic, which means it is negatively charged and attracted to water.

These relationships to water mean that, when mixed with water, phospholipids will arrange themselves automatically to become layers where the hydrophobic tails stick together while the hydrophilic phosphate heads face the water that is attracted to them. This arrangement creates the phospholipid bilayers, which is the basic structure of a cell membrane. They’re very stable because of the bonds that form between the surrounding water and the phosphate heads but are a collectively strong unit because there are so many tails that interact with each other. Because membranes are adjoined and continuous (very long), it’s hard to break them as you would have to break an entire length and many, many tail interactions for the membrane to give.

Besides the phospholipid bilayer, a membrane is also made up of different proteins. The integral proteins are embedded into the phospholipid and have a solid hold in the system. Peripheral proteins, less so, as they are only loosely attached to the surface of the membrane. On the outside surface of phospholipid bilayers (the surface that faces the extracellular fluids and away from the cytoplasm) are glycoproteins that have short chains of sugar molecules. For what exactly, I’m not quite sure but that’s what we have Mr. Ferguson for!

Actually, let’s backtrack, because we just might know what these proteins are for – six functions of proteins in the membrane (which could possibly include glycoproteins) are the following:

  • hormone binding sites (e.g. insulin receptors)
  • enzymes with an external active site (e.g. in the small intestine)
  • cells sticking together to form tight groups of cells in tissues and organs
  • communication between cells (e.g. receptors for neurotransmitters)
  • provide channels in which hydrophilic (water-loving) particles can travel through by facilitated diffusion (more on that later!)
  • provide pumps through which ATP moves particles across the membrane

Alright, diffusion. This is fitting because we’ve just finished a lab on diffusion and osmosis – two very similar concepts. Diffusion is the random and passive movement of particles from an area of high concentration to an area of lower concentration. Think of when that frilly girl in class stands in her corner of the classroom and sprays on some perfume. It might take a few minutes but eventually, the smell will hang over the entire class because the molecules, after being sprayed out, are the region of high concentration themselves. They spread throughout the classroom, which is the region of lower concentration to find an equilibrium – a state of balance between the molecules. A different example of diffusion is when oxygen and carbon dioxide movie into and out of cells.

Simple diffusion is when particles pass between phospholipids in the membrane and can only occur if the phospholipid bilayer is permeable to that particular particle (that’s a lot of Ps!). The molecules that can move through a membrane are small polar molecules that have both positive and negative charges on their surface. These charges are what help them travel first through the phosphate heads (negatively charged and hydrophilic) and then through the centre of the membrane (hydrophobic and attracted to each other). Basically, smaller particles can pass through more easily than large particles. Simple diffusion occurs when the molecule concentration on one side of the membrane is higher than the other side. This is called a concentration gradient.

Briefly the textbook goes over the difference of independent and dependent variables and the relationship between them. To clarify (because we all need clarification every now and then) the independent variable, typically graphed on the x-axis, is the factor of which you can choose the different levels (like our different molarities in the sucrose lab). The dependent variable, typically graphed on the y-axis, is the value of something else that is affected by the independent variable. The independent variable controls the dependent variable (most of the time) and usually – more often than not – there is a relationship between the two variables.

We move on now to facilitated diffusion, which is how larger particles can move through a membrane. In every membrane, different protein channels are produced and inserted into the membrane for certain particles to go through them. The channels are like specially shaped locks and the only things that can go through it are the keys, those special particles. Facilitated diffusion (facilitated – controlled – to make easier) helps to maintain which particles enter a cell because the cell knows what protein channels to make in order to accept certain particles. Movement of facilitated diffusion is passive and doesn’t require energy – it just happens.

Ah, and now for osmosis. Osmosis is the passive movement of water molecules across a (partially) permeable membrane, from a hypotonic solution to a hypertonic solution. Like diffusion, osmosis applies to the previously finished sucrose lab and deals with the movement of particles through membranes. Unlike diffusion, osmosis focuses on the movement of water molecules through membranes. Water is a solvent, a liquid in which particles dissolve, and these dissolved particles are solutes. Since movement of water molecules is passive, sometimes the overall movement is nothing; there is no net movement. But if there is more movement from one side to another, then there is a net movement; there is osmosis. Once again, no energy is required for osmosis to occur – it just happens.

Specifically in plants, osmosis can cause plasmolysis. This happens when a plant is submerged in a highly concentrated solution (like a salt or sugar solution) and water leaves the plant (because the external solution is hypertonic while the plant is more hypotonic – and osmosis is the movement of water molecules from a hypotonic solution to a hypertonic solution!). When the water leaves, the cytoplasm and plasma membrane are pulled away from the cell wall. That, children, is plasmolysis and it sounds bad for the plant.

THIS IS PURELY FOR ME:

It’s a shorter unit this time, but for this unit test, we’ll be facing an onslaught of nine essay questions! These questions are…

  1. Draw a diagram to show the structure of a cell membrane. 5 marks but should be easy enough, considering that we’ve just covered it here.
  2. Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. 9 marks and I’m hoping we’ll be going over this in class!
  3. Explain the role of vesicles in transportation of materials within cells. 8 marks
  4. Describe the process of active transport. 4 marks
  5. Outline the ways in which substances move passively across membranes. 5 marks
  6. Distinguish between active and passive movements of materials across plasma membranes, using named examples. 4 marks and I’m thinking that this involves what we’ve learned here about diffusion, osmosis and facilitated diffusion.
  7. Outline, with an example, the process of exocytosis. 5 marks
  8. Explain the reasons for cell division in living organisms. 8 marks
  9. Outline the processes that occur in a cell during interphase, including those needed to prepare for mitosis.

I’ve bolded the questions that make sense and that I’d be okay to even attempt to answer. So far it seems that we’ve only covered a small portion of the essay questions but it is only the first blogpost. Bring it on, Membranes.

DATA BASED QUESTIONS

These are a work-in-progress. Hey, not everyone’s perfect.

Now everyone is perfect. Click away to see the .pdf file for DBQ pages 27, 31-33.

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