Biology 11: Topic 2: Transport across the membranes

The plasma membrane of a cell has many functions:
-separate internal and external environment
-protect the cell
-a place for recognition sites
-controls transport processes
As such, it is rigid, yet elastic and selectively permeable (this means it lets some things cross through it, but others cannot)

Structure of the plasma membrane
- 7-9nm thick (under an electron microscope it only looks like 2 lines)
- organic solvents (eg. alcohol, ether, chloroform) can cross it, since it is made of lipids
- in 1959, the unit membrane theory was created, which stated that all membranes (surface and inner) share a basic structure. This is true. It also stated that it was 2 protein layers (for elasticity) sandwiching a phospholipid bilayer (for strength). This was incorrect.

- 13 years later (1972) a technique called "freeze-fracture" was used to visualize the membrane and it showed that the unit membrane theory was WRONG! So a new theory called the fluid mosaic model was created (and so far still seems right!).

It states that there is:
-a phospholipid bilayer in which the phospholipids can move (hence the fluid part of the name)
-cholesterol (steroid lipid) is found interspersed in the bilayer. Cholesterol is more rigid than phospholipids, so it provides stability.
-globular membrane proteins are embedded in the membrane, but they don't form a continuous layer. Different proteins have different functions, some are transport molecules (they create channels through the bilayer, or they carry certain substances across the bilayer), some are enzymes, some are receptor molecules. Proteins can also move sideways in the membrane. Proteins provide the mosaic part of the fluid-mosaic model.
-carbohydrates are found on the outside only. They form receptor sites and are attached to proteins or lipids (glycoprotein, glycolipid).

Image from http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/graphics/fluid_%20mosaid_model.jpg

Transport Processes
- homeostasis maintains constant conditions in a living organism. Membrane control is essential for this.

Passive transport
This is movement across the membrane without any energy input. There are 3 forms: diffusion, facilitated diffusion and osmosis

Diffusion:
- particles are in constant motion (Brownian or heat-kinetic motion)
-the overall effect of this random motion over time is diffusion (or the movement of substances from an area of high concentration to an area of lower concentration, thus resulting in equal concentration in both areas)
-for this to occur across the membrane, the membrane must be permeable to the substances (apolar particles and small molecules like H2O, O2 and CO2)

Facilitated diffusion:
-a transport protein helps the molecule to pass through the membrane - these proteins usually form channels through the membrane (some may have gates to limit diffusion) and some are carrier proteins
-changed ions or polar particles can only move across the membrane with the help of a protein.

Osmosis:
-diffusion of H2O molecules (a special kind of diffusion)
We can observe osmosis by placing cells into different solutions:
An isotonic solution has the same concentration as a cell, so no visible changes occur (this is called dynamic equilibrium)
A hypotonic solution is less concentrated than the cell, so in this case the cell swells and bursts (or in the case of a plant cell, the cell walls prevent bursting, so there is an increase in turgor pressure)
A hypertonic solution is more concentrated than the cell, so the cells shrivels up (in plants this also happens and the cell membrane pulls away from the cell wall, this is called plasmolysis).

Image from http://access.mmhs.ca/docs/Science/MMHS%20Web%20Folder/Kamla/Image130.gif

Active Transport

- transport of a substance against the concentration gradient

-requires energy

-must use transport proteins and ATP

-a substance binds the transport protein and energy is released from ATP to allow the protein to change shape or move and release the substance on the other side of the membrane.

Example: Na+/K+ pump - this is especially important in nerve and muscle cells

Image from http://bio100.nicerweb.com/doc/classbio1151/Locked/medi/ch07/07_16SodiumPotassiumPump.jpg

Endocytosis and Exocytosis
- this occurs when large particles (even whole cells) move across the membrane.
Endocytosis occurs when something is moved into the cell. The membrane engulfs and encloses the particle - this forms a vacuole or vesicle.
- 3 types:
phagocytosis: engulfing a very large, solid particle
pinocytosis: engulfing liquid droplets or small particles.
receptor-mediated endocytosis: engulfing specific substances which bind to receptors found on the surface of the cell membrane.