1. Chapter 7: Membrane Structure & Function
Since we learned about endomembrane system, let’s continue to study membrane more details.
What is basic biomolecules forming membrane? Phospholipid, right.
Because of the properties of phospholipid, membrane has very interesting characteristics.
We will learn about membrane structure and function. In addition, how molecules go across membrane in this chapter.
I. Membrane Structure & Function
II. Selective Permeability of Membranes

2. I. Plasma Membrane Structure
– boundary that ________ living cells
from their nonliving surroundings.
- apprx. 8 nm thick
composed chiefly of ________ and ______
surrounds the cell and controls chemical traffic in/out of cell
is selectively ________!
Plasma membrane, in the other word is cytoplasmic membrane, which surrounds outside of cells. So, plasma membrane separates cellular components from the outside. It is mainly composed with phospholipids and inserted proteins. This membrane protects cells and also controls chemical traffic in and out of cells. This chemical passing is not random, it has selective permeability controlling chemicals passing. This selective permeability is main characteristic of membrane, which cause different environments between inside and outside of cells.
Enables cells to maintain internal environment different from external environment

3. - composed of ________ phospholipids
Phospholipid bilayer
- composed of ________ phospholipids
Major components of membrane is phospholipids. Phospholipids have polar hydrophilic head with phosphate group and hydrophobic tails of fatty acids. That’s why it is amphipathic. Membrane forms bilayer of phospholipids by hydrophobic interaction between fatty acids. So polar hydrophilic head sticks out to aqueous environment and fatty acid tails bind inside.

4. Membrane Structure (________ Model)
amphipathic membrane proteins embedded in phospholipid bilayer
held together by weak _______________
give membrane ‘fluidity’ similar to salad oil
phospholipids & proteins can _____ laterally (2 um / sec)
So, 1972, SJ Singer and G Nicolosn revised the membrane model and proposed fluid mosaic model. Here, the proteins are dispersed and individually inserted into the phospholipid layers. Due to the amphipathic properties of membrane proteins, hydrophilic regions protrude out from the bilayer and exposed to water. The hydrophobic part of proteins remained inside of phospholipid tails and hold together by hydrophobic interaction. This is fluid mosaic model. Hydrophobic interaction is not a strong bond, it is very weak bonds so the membrane has fluidity like salad oil. Because of this fluidity, phospholipid and protein can be drift laterally. Especially phospholipids can move 2 um/sec. Transverse drift is possible but it is very rare because of hydrophobicity of the fatty acid tails.

5. Membranes must be fluid to work properly !
- solidification causes changes in permeability and enzyme deactivation
How do cells control membrane fluidity ?
1. ___________ fatty acid tails
enhance fluidity
kinks at C=C hinder close packing
2. Adding _______ makes membrane:
The fluidity of membrane is very important charateristics for its function. If the membrane is solidified, membrane cannot be permeable anymore and also some of enzymes located on membrane cannot be functioned. Solidification on membrane is occurring by compacting phospholipid closely. Decreasing temperature can cause this packing and leads to solidify membrane. However, cell has mechanisms to control membrane fluidity. If the phospholiipds of membrane are rich with unsaturated fatty acids. It enhances the fluidity because double bond are kinked and leads to loose the packing of phospholipids. Steroid cholesterol has very important roles for membrane fluidity in animal cells. Cholesterol is wedged between phospholipids and decrease fluidity at higher temp. and increase fluidity at low temps.
Decreases fluidity at ______ temps by restraining phospholipid movement
Increases fluidity at _____ temps by preventing close packing of phospholipids

6. Proteins in Plasma Membrane
mosaic of proteins ‘_____’ in a fluid lipid bilayer
proteins determine a membrane’s specific function:
Two types
1. ______ proteins (‘transmembrane’, or embedded)
2. ________ proteins (bound to surface of membrane)
So membrane has mosaic structure of combination of protein and phospholipids. This proteins determine most of membrane’s specific functions. There are two different types of proteins associated with membrane. Integral protein and peripheral proteins. Integral proteins penetrate the hydrophobic core of lipid bilayer, means “transmembrane protein”. Peripheral proteins are not embedded in membrane instead it loosely binds to the surface of membrane or on the integral proteins. Some of membrane proteins are held in place by cytoskeleton, which supports the shape of cells. Toward the outside of membrane, some of proteins have glycoprotein. Some of them has extracellular matrix fibers to support the attachment between cells.

7. Some Functions of Membrane Proteins
So let’s talk about the functions of membrane proteins.
Transporter (passive and active), enzymes, signal transduction, intercellular joining. Cell to cell recognition by glycoprotein.

8. Synthesis of membrane and it components
Synthesis of membrane proteins and lipids in the ___
Carbohydrate modification in the _________
Transport of membrane components to the plasma membrane
Fusion of ______ with the membrane
Where is the membrane components made from? Endoplasmic reticulum is the answer. The vesicle generated from ER transfers to Golgi and then vesicle released out from Golgi apparatus. The vesicle is fused to cytoplasmic membrane and add to the portion to membrane. Because of this fusion, internal and external layer of membrane have different carbohydrate, protein and lipid.

9. Passive Transport vs. Active Transport
Selective Permeability of Membranes
Passive Transport vs. Active Transport
So diffusion and facilitated diffusion are passive transports. They don’t require energy to transport molecules across the membrane. The major forces of transporting molecules are concentration gradients.
However, active transport is against the concentration gradient by pumping out molecules using eneergy.

10. I. Passive Transport: diffusion and facilitated diffusion
1.1. Diffusion : net movement of a
substance down
a __________________.
Solutes diffuse from ____ to ____ concentration.
Continues until a dynamic _________ is reached.
____ requirement for ________ expense (passive)
Examples:
uptake of O2 by cell performing respiration
elimination of CO2 from cell
Major function of membrane is transporting molecules. There are two different type of transport. Passive transport and active transport depend on the requirement of energy. Passive transport does not require energy, typical examples are diffusion and osmosis.
Diffusion is net movement of substance along a concentration gradient, which increase entropy in universe. Diffusion continues until it reaches equilibrium with surrounding. Thru phospholipid membrane, only small molecules with hydrophobic property can be diffuse in. examples are oxygen uptake and co2 emission from the cell.

11. Diffusion of solutes across a membrane
Each dye diffuses down its own ___________ gradient.
These two models show diffusion dynamics in one solute suspension vs. two solute suspension. One solute diffuses from one side to the other side until it reaches equilibrium. When two solutes are present, both of solutes pass each other until both of them reach to equilibrium.

12. Osmosis
Diffusion (passive transport) of _____ across a selectively permeable membrane, (the membrane are __________ for solute but for water).
Direction of water movement is determined by the difference in total _____ concentration, regardless of type or diversity of solutes.
Water moves always from _____ concentration solution to ______concentration solution.
Osmosis is another passive transport of water in response to differences in solute concentration. If the membrane is not allowed to pass solute, water moves against the concentration gradient of solute. So water moves from hypotonic solution to hypertonic solution. Hypotonic is diluted and hypertonic is concentrated with solutes. If two solution is isotonic, the same concentration, water diffuse in both direction at the same rate, there is no net movement.

13. Water balance of living cells
Tonicity : the ability of a solution to cause a cell to gain or lose water
Isotonic : __________ of water across the membrane (same).
Hypertonic : the cell ______ water to its environment (more).
Hypotonic : the cell ______ water from its environment (less).
Water balance inside of living cell is very important. For example, in animal cells, if the cells are in hypertonic solution, all the water takes out from the cell and cell becomes shrink. In the other hand, cells are in hypotonic solution, cells will take up more water and eventually burst. Both of the case, the animal cells cannot be survived. So it is very important to maintain isotonic conditions fro animal cells. In the case of plant cells, it is different from animal cells because of the presence of cell wall, which is very rigid structure. Plant cells prefer to have hypotonic condition than isotonic condition. Plant cell can remain firm with more uptake of water, which provide better condition to deliver nutrient. In other hand, if the plant cells are isotonic condition, the plant become wilt. Of course, hypertonic conditions are dangerous for plant cells as well because it loose all the fluid inside. Regulating the amount of water inside cells calls osmoregulation.

14. Questions
An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to sucrose.
Glucose?
Fructose?
Hypotonic/Hypertonic?
Water?
Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18

15. 1.2. Facilitated diffusion
_______ transport
Transport ______ speed the movement of molecules across the plasma membrane.
Channel protein and Carrier protein
Channel protein : aquaporins, ion channels
Carrier protein
Passive transport also occurs with membrane proteins. These are transporter protein, which mediate facilitated diffusion. There are two different models for facilitated diffusion. One is auaporin, which is water channel protein. Massive transport is mediated by diffusion related to concentration gradients. The second one is gate channels, which requires some signals either chemical or electonical to open the channels. In this model, once solute comes in the channel, it change its conformation to transport solute to inside. This has very selective specificity depending on which molecules bind to the channels.

16. 2. Active Transport Requires the cell to expend _______
Transport proteins pump molecules across a membrane _________ their concentration gradient.
“uphill” transport
Maintain steep ionic gradients across the cell membrane (Na+ , K+ , Ca++ , Mg++ , Cl-)
How is it done?
using transport proteins and ____!
Active transport requires energy and transport molecules against the concentration gradient, which is uphill transport. This requires energy so this is endergonic process. Examples are ionic gradients across the membrane.

17. An Example of Active Transport: The Sodium-Potassium Pump
One of typical example of active transport is sodium and potassium pump. The cell has tendency to lower sodium concentration inside and want to have potassium inside. So, Three sodium ions bind to the pump and then ATP phosporylation gives phosphate group. Phosphorylation causes protein conformation and open the other side of channel. The conformation changes push out sodium ion and offer binding for potassium ion. Two potassium ions bind and phosphate group is released, which change the protein conformation again. It opens inside channel and release potassium ions. And then the cycle goes again. You may notice that three sodium ions were transferred out while two potassium ions were in. These difference cause voltage across the membrane.

18. Passive Transport vs. Active Transport
So diffusion and facilitated diffusion are passive transports. They don’t require energy to transport molecules across the membrane. The major forces of transporting molecules are concentration gradients.
However, active transport is against the concentration gradient by pumping out molecules using eneergy.