Solutions, osmosis and escape from the cell!

Solution: a solution contains solute (substance dissolved into a solvent to form a solution) and solvent: liquid that dissolves solute (to form a solution).

Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration until all particles are evenly distributed. All particles diffuse independently of each other. Diffusion is a passive process and does NOT require an input of energy such as ATP. All particles in liquid and gaseous states can diffuse.

Factors that affect the rate of diffusion in a liquid or a gas:

Concentration gradient:

The greater the concentration gradient, the faster the rate of diffusion. Concentration

gradient is the difference in concentration between an area of high concentration and

an area of low concentration.

Temperature:

As temperature increases so does the rate of diffusion because particles move faster.

Diffusion occurs more quickly in a gas than in a liquid because the particles are moving

more quickly and there is more space between particles.

Particle size:

The smaller the particles the faster the rate of diffusion. Smaller particles can fit between

other particles more easily and they move more quickly at any given temperature because

it takes less energy for them to move than large particles.

Particle shape:

Some particles are more streamlined than others and can fit between other particles more

easily.

Electrical charge:

Non-polar molecules diffuse more readily in non-polar solutions and polar molecules

diffuse more readily in polar solutions. Polar and non-polar molecules do not mix well so

they do not diffuse readily in each other.

Factors that affect the rate of diffusion across a membrane:

All of the above factors plus:

Number of pores/carrier proteins/channel proteins:

Most cells can vary the number and type of pores, carrier proteins or channel proteins that are present at any one time. The more openings that are present for a particular ion or molecule, the more of that substance can cross the membrane.

Pressure:

The greater the difference in pressure the greater the rate of diffusion. For example the greater the osmotic pressure the more water will diffuse (osmose) across the membrane.

Pressure gradients act the same way as concentration gradients.

Hormonal effects:

Hormones such as insulin can affect whether or not a substance can access a carrier protein to cross a membrane. If insulin is present glucose can enter the cell.

Lipid solubility:

Small non-polar molecules can diffuse directly across the cell membrane because they are lipid soluble.

Cyclosis:

In some cells the cytoplasm moves in a cyclic path. This is called cyclosis or cytoplasmic streaming. Because the movement of the cytoplasm removes particles away from the membrane it serves to increase the concentration gradient across the membrane.

Membrane permeability:

One cell may be permeable to a particular molecule and another cell may not be permeable to that molecule. A cell may also vary its permeability to a substance depending upon need.

Describe osmosis and osmotic pressure.

Osmosis is the movement of water from an area of higher pressure to an area of lower pressure across a selectively [differentially] permeable membrane. 

Or, differently put-- it’s the diffusion of water -- into and out of cells.

Osmotic pressure is the pressure generated by the flow of water across a semi permeable membrane. This pressure is created by the solute of the solution.

We dunk a cell into a solution. 

The solution can be one of three types of solutions.

Isotonic solution: the concentration of solute and water is the same both inside and outside of the cell.

Hypotonic solution: lower concentration of solute, higher concentration of water than the cell. 

Hypertonic solution: this solution has a higher concentration of solute and lower concentration of water than the cell.

The solution can cause the following effects:

An isotonic solution causes neither a shrinking nor swelling of the cell because it is a balanced solution. 

Hypertonic and hypotonic solutions can have devastating effects to a cell.

Hypotonic environment causes Lysis:  Osmotic pressure builds up when the water surrounding the cell moves to the area of least resistance, which is inside the cell. The cell explodes from the pressure. 

Hypertonic environment causes Plasmolysis: when a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall because the large central vacuole loses water. Plasmolysis is the shrinking of the Plant cell's cytoplasm due to osmosis. 

 

Hypertonic environment causes Crenation: The cell’s water rushes out to the hypotonic solution because of osmotic pressure. The cell shrinks like a raisin.

Describe and be able to differentiate between facilitated transport and active transport.

Facilitated transport:  passive transfer of materials into or out of a cell along a concentration gradient by a process that requires a carrier.

Active transport: Transfer of a material into or out of a cell from a region of lower concentration to an area of higher concentration by a process that requires a carrier and an expenditure of energy.

Both processes require a carrier and both processes facilitate transport into or out of the cell. Both processes move material along a concentration gradient. However, Active transport requires an expenditure of energy and facilitated transport does not.

Describe endocytosis, including pinocytosis and phagocytosis, and contrast it with exocytosis.

Endocytosis: brings material INTO the cell. A portion of the plasma membrane invaginates to form a vesicle around the material. Then, the vesicle pinches off inside the cell.

Three kinds of endocytosis:

1. Phagocytosis: when the material coming into the cell is large- like a molecule of food or another cell (when old red blood cells are absorbed) this is common in unicellular organisms like amoebas that engulf their food.

2. Pinocytosis: happens when vesicles form around liquid or very small particles.

3. Receptor mediated endocytosis: makes use of receptor proteins in the plasma membrane. A specific substance binds to receptors that then gather in one location before endocytosis happens.

Exocytosis: brings material OUT of the cell. The [intracellular] vesicle fuses with the plasma membrane so the contents can be released outside of the cell.

CELL WALL

let's do cell walls all in ONE BIG FELL SWOOP, shall we? since the test is on Monday and there's lots of midterm review to do ;)
BTW, rocked the last test- 93%.



I'm loving this cell membrane unit. It makes sense to me, builds on what we've learned before. I find it fascinating how all the pieces work together and form a wall that holds all organelles in and anchors cytoskeleton.

In all things I love about biology, the plasma membrane really interests me. It's a true work of scientific engineering. It’s made of mostly of phospholipids and embedded proteins. These proteins can do a lot of things, like help channel things in and out of the cell, help with cell recognition or be enzymatic proteins. This is all covered later.

With all of this content, the structure of the fluid mosaic membrane is still fluid, with a texture similar to light oil. The proteins are scattered throughout the sea of phospholipids and create the mosaic texture that earns the phospholipid bilayer the term the ‘fluid mosaic membrane’.  

Remember our phospholipid. He has a polar head and nonpolar tails. He’s made from two fatty acids on one phosphate molecule. These nonpolar tails are hydrophobic while the head is hydrophilic. This is how the plasma membrane spontaneously arranges itself with the heads pointing to the outsides of the membrane and the tails facing each other.

Here is a generic picture that I take no credit for but really- I could have drawn it ;) 

Within this membrane of phospholipids, a molecule similar to a phospholipid resides. It’s a glycolipid. It is made of two hydrophobic tails with a head made of sugars joined into a straight or branching carbohydrate chain.  Even though it reduces the permeability of most biological molecules, cholesterol is also found in the plasma membranes of animals. Plants have similar steroid molecules in their membranes.   

The cell isn’t symmetrical on the outside and inside. While the phospholipid bilayer sounds like it would be symmetrical, the outside is coated with carbohydrates from glycolipids and glycoproteins. As well, the outside has proteins that anchor to an intracellular matrix. 

The inside has protein anchorings for the cytoskeletal filaments. 

 

Proteins are found throughout the membrane. 

Here are a few kinds.  

Integral proteins usually have a hydrophobic quality to them. Most of these integral proteins are also glycoproteins meaning they have an attached carbohydrate chain that floats out from the outside of the cell membrane.  

Peripheral proteins hang out on the outside of the membrane (either the cytoplasmic or external side) sometimes anchored by covalent bonds, other times, just resting on the surface of the cell lightly tethered with non-covalent interactions and are prone to shifting around when the cell moves or is shaken or even has a pH shift.

 Channel protein:

Shaped like a tube. It’s shaped to let particular kinds of ions pass at will across the plasma membrane.

Carrier protein:Selectively interacts with specific molecules or ions so they can cross the plasma membrane.

Cell recognition protein: The carbohydrate chains and glycolipids and glycoproteins are the cell’s fingerprints. These vary from person to person and species to species. These cells are why organ transplants are difficult to match since the body’s immune system can sense that the cells of the foreign organ do not have the same ‘fingerprint’ as the body’s original cells.

Receptor protein:

These are shaped in such a way that they can only bind to a certain substance i.e. growth hormones.

Enzymatic protein: Catalyzes specific reactions for example, the protein adenylate cyclase is involved in ATP metabolism.

The cell membrane is selectively [differentially] permeable. 

Some things can move across the membrane and others can’t. 

Ions and other charged molecules can’t cross because they can’t get through the hydrophobic layer. Macromolecules are just too big to pass through. 

Oxygen, Carbon Dioxide and Water can easily pass through because they are noncharged and small enough.