Biology 12 Unit 2

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Plasma Membrane Structure & Function Text Ref: Pg 81-91

https://www.youtube.com/watch?v=moPJkCbKjBs The Functions of Membranes 1) Membranes Define the Boundaries of a Cell

Membranes are a highly controlled barrier between the intracellular fluid within cells and the extracellular fluid outside of cells.

Essentially, membranes function to keep desirable substances within the cell and undesirable substances out.

2) Compartmentalization

To compartmentalized specialized cell organelles so they can carry out their function without affecting other cell processes.

E.g. Mitochondria carry out complex biochemical reactions to produce the ATP cells need to function. If mitochondria were not compartmentalized, these reactions could interfere with other cellular processes.

3) Transport

Membranes regulate the movement of substances into and out of cells and their organelles.

Nutrients, ions, gases, water, and other substances are taken up by the cell, and various products and wastes must be removed.

4) Detection and Transmission of Signals

Cell membranes receive information from the extracellular environment in the form of electrical or chemical signals.

These signals trigger certain chemical and physical responses from a cell.

E.g. Various hormones and neurotransmitters bring messages to the cell that cause changes in either the nature or rate of cellular activities.

5) Cell-to-Cell Communication

Membranes provide a means of communication between adjacent cells.

Although many textbooks depict cells as separate entities, many cells in multicellular organisms are in contact with one another via direct connections.

Every cell within our body is bound by a highly specialized phospholipid bilayer called the plasma membrane. Essentially, the plasma membrane is a highly controlled barrier between the intracellular fluid within cells and the extracellular fluid outside of cells that keeps desirable substances within the cell and keeps the undesirable substances out. In addition to the plasma membrane, cells have various intracellular membranes that compartmentalize cell organelles. Compartmentalization is necessary so that organelles like the mitochondria can carry out complex biochemical reactions in an isolated environment within the cell. Without such compartments, life could not exist. As you will see, the unique structure of a cell’s membrane also allows it to play a dynamic role in many other important cellular activities. In this unit we will examine the molecular structure of membranes and the role that membranes play in the life of a cell. We will also examine how cells transport solutes across membranes.

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The Fluid-Mosaic Model of Membrane Structure

The plasma membrane is typically depicted as a “fluid” phospholipid bilayer with a mosaic of specialized proteins, lipids and carbohydrates dispersed in it

The fluidity of the plasma membrane allows cells to change shape and size as needed

Components of the Plasma Membrane

1) Phospholipids

the hydrophobic nature of phospholipids they spontaneously arrange themselves into a bilayer

The hydrophobic (water hating) non-polar tails of each layer face each other on the inside of the double membrane

The hydrophilic (water loving) polar heads face the outside

At normal body temperature, the phospholipid bilayer of the plasma membrane has the consistency of olive oil

Hydrophilic Heads

Hydrophobic Tails

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2) Glycolipids

Have a structure similar to phospholipids EXCEPT the hydrophilic head is made of a variety of sugars joined to form carbohydrate chains

Function: cell recognition, provide cell with energy

3) Cholesterol

Found ONLY in animal cell plasma membranes (other related steroids are found in plant cells)

Cholesterol molecules are NOT typically found in the membranes around cell organelles.

Approximately 20-50% of the total lipids in a plasma membrane are cholesterol molecules.

In high temperatures cholesterol reduces plasma membrane fluidity and permeability to most biological molecules

In cold temperatures, cholesterol molecules increase membrane fluidity and prevent membranes from freezing

In cold temperatures membranes don’t freeze because cholesterol molecules prevent close packing of the hydrophobic tails of the phospholipids and stop crystals from forming

Proteins

Proteins make up approximately 50% of the membrane by mass.

Most proteins are free to drift laterally through the lipid bilayer and can be inserted or removed as needed

Cholesterol

Homeostatic Imbalance

People who suffer from moderate to severe burns experience water filled blisters and in severe cases, entire layers of skin peel off. In these patients, the cell membranes of their skin cells have become so damaged they are not able to regulate what flows in and out of these cells. As

a result, fluids from tissues flood in and the cells swell and become detached from one another.

Glycolipid

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Did You Know?

Every individual has their own unique MHC, a type of glycoprotein, called the major histocompatibility complex. People who need organ transplants have to find a donor who has the same type of MHC or else their body will reject the organ. People who have the same MHC proteins are said to be “a match”. If the recipient of the transplant organ has a different MHC protein than the donor, the organ is rejected. The recipient’s white blood cells, the cells responsible for maintaining immunity, attack the cells in the donor organ until they are destroyed.

Also, a person’s blood type (A, B, O, or AB) is due to the presence of particular glycoproteins

in the membrane of red blood cells.

Classes of Membrane Proteins

1) Peripheral proteins

Not embedded directly in the membrane but have a variety of mechanical functions:

o Anchor proteins – attach to the cytoskeleton filaments inside cells and the extracellular matrix outside of cells

o Anchor proteins also help maintain cell shape and play a role in cell movement by manipulating the cytoskeletal filaments that are attached to them

o Other peripheral proteins link neighboring cells together and act as the “glue” that holds the cells of tissues together

2) Glycoproteins

Glycoproteins – have short polysaccharide chains (carbohydrates) attached to them that stick out of the external surface of the cell.

These carbohydrate chains play an important role in cell to cell communication and the recognition of certain molecules such as bacteria, fungi and viruses

If a foreign invader is recognized by the cell it will trigger an immune response

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3) Enzymatic Proteins

Some are involved in catalyzing specific chemical reactions within the body

4) Receptor Proteins

Act as chemical messengers for hormones and other chemicals on the outside of the cell and relay messages to the inside of the cell

Each receptor protein has a receptor site with a very specific shape that binds only one kind of molecule i.e. growth hormone or insulin

When a molecule binds to a receptor protein it changes shape and elicits a cellular response

5) Carrier/Channel Proteins

All carrier proteins are integral proteins, fully embedded in the membrane (stick out on both the inside and outside of the cell)

Involved in transporting molecules through membranes

Did You Know?

People who live in third world countries often have contaminated water. One of these contaminants is Cholera, a type of bacteria responsible for causing severe diarrhea. Cholera bacteria release a toxin that interferes with the proper functioning of a specific enzyme, and as a result sodium ions and water leave intestinal cells and the individual becomes severely dehydrated. If the bacterial infection is not treated the person who has it will die. Many children die of this disease every year.

Homeostatic Imbalance

A faulty carrier protein for glucose molecules may be the cause of diabetes mellitus in some people. Patients with these faulty carrier proteins experience an accumulation of glucose in the bloodstream because it cannot be transported into cells and used to make ATP. Excess blood glucose is then filtered out of the blood into urine.

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Channel proteins are another type of integral protein that creates a channel or passage through a membrane. SMALL molecules such as water and ions diffuse down their concentration gradients through these channels.

Homeostatic Imbalance

People who suffer from Cystic Fibrosis have faulty chloride (Cl-) channels in their plasma membranes. Normally, these chloride ions can move freely across the plasma membrane, however as a result of faulty channel proteins the chloride ions do not move in and out of the cell as they should. As a result thick mucus collects in airways and in pancreatic and liver ducts. Eventually, the alveoli (air sacs) within the lungs get plugged up by this mucus and people with this disease literally drown in their own bodily fluids.

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Transport Across Cell Membranes

Passive Transport

Passive forms of transport DO NOT require ATP (energy) to transport molecules across a selectively permeable membrane

Solutes ALWAYS move down their concentration gradient from an area of high concentration to a lower concentration.

There are 3 types of passive transport:

1) Diffusion

2) Osmosis

3) Facilitated transport 1) Diffusion

The movement of molecules from a high concentration to a lower concentration

Molecules will move down their concentration gradient until equilibrium is achieved and they are distributed equally on both sides of the membrane

The chemical and physical properties of the membrane limit the types of molecules that can diffuse through it

The plasma membrane is selectively permeable: some substances can move across the membrane and some cannot. For example, macromolecules such as complex carbohydrates or proteins are too large to squeeze through the spaces between the phospholipids and so they cannot move freely through the membrane. Polar molecules and ions (molecules that carry a + or – electrical charge) cannot get past the nonpolar tails of the phospholipids and are also unable to squeeze through. However, there are a number of molecules that are able to cross the membrane in a variety of ways. Molecules that are able to pass through a selectively permeable membrane do it through passive or active processes.

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Transport Across the Plasma Membrane: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/

Factors Affecting the Rate of Diffusion

2) Osmosis

Is the diffusion of WATER through a selectively permeable membrane

The concentration of a solute (molecules dissolved in water) on one side of a selectively permeable membrane will determine the direction of the NET MOVEMENT of water

Even though water molecules are polar, they are small enough to fit between the phospholipids in the plasma membrane.

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Tonicity

The tonicity of a solution will determine the direction WATER moves through the membrane and can affect the shape and volume of a cell

When discussing tonicity you ALWAYS make reference to the CONCENTRATION OF A SOLUTE IN A SOLUTION

Hypotonicty (“hypo” = “less dissolved stuff”)

When a SOLUTION has a LOWER concentration of solute than the cell (solution has more water) it is said to be hypotonic

If a cell is placed in a hypotonic solution, water will enter the cell

The NET FLOW of water is from outside the cell to inside the cell

As a result the volume of the cell increases and a force or pressure is exerted on the membrane. This is called osmotic pressure.

Cells can burst (lyse) if the osmotic pressure exerts a force greater than what the cell membrane can withstand

When red blood cells burst due to osmotic pressure it is called hemolysis

Hypertonicy (“hyper” = “more dissolved stuff”)

When a SOLUTION has a HIGHER concentration of solute than the cell (has less water) it is said to be hypertonic

If a cell is placed in a hypertonic solution the NET MOVEMENT of water will be OUT of the cell i.e. down the concentration gradient.

As a result the cell’s cytoplasm shrinks

When red blood cells shrink it is called crenation

Here is another look at what happens when you have 2 solutions with different solute concentrations on either side of a semi-permeable membrane.

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Isotonic Solution (“Iso” the same as)

Once the solute concentration is the same on both sides of the membrane, there is no NET movement of water in or out of the cell and the solution is said to be isotonic

** Molecules NEVER become static; they are always in constant motion. Therefore, when equilibrium is reached both solvent and solute molecules are still moving across the membrane, however, there is NO NET movement of either one in any direction.

In an isotonic solution the NET amount of water entering the cell is equal to

the amount leaving it E.g. Intravenous medications administered in hospitals are isotonic to red blood cells. Both the medication and red blood cells have a sodium chloride concentration of 0.9%. This means there is no NET movement of water in or out of cells. * Draw 3) Facilitated Transport

Molecules that are not lipid soluble such as glucose and amino acids CANNOT simply diffuse through a plasma membrane

Carrier proteins (integral proteins) embedded in the membrane “help” these molecules across through a process called facilitated transport

Carrier proteins speed up the rate at which a solute crosses a membrane

Because facilitated transport is a passive process, molecules always move down the concentration gradient from an area of high concentration to an area of lower concentration

Active Transport

REQUIRES ATP and carrier proteins to transport molecules across a selectively permeable membrane

Molecules involved in this process are moved AGAINST their concentration gradients from areas of low concentration to areas of high concentration

Up to 40% of a cells energy supply is used in active transport.

Cells involved in active transport have many mitochondria to supply ATP

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The goal of this type of transport is to accumulate ions or molecules on one side of a membrane

E.g. The sodium-potassium pump

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

1. Carrier proteins embedded in the membrane have an initial conformation that allows them to take up 3 sodium ions (3 Na+) on the inside of the cell.

2. ATP attaches to the carrier protein.

3. One of the high energy bonds between phosphate groups on ATP is broken and the free phosphate stays attached to the carrier protein. Energy released when the bond between phosphate groups is broken, allows the carrier protein to change its conformation (shape) and it transports the 3 sodium ions through the membrane to the outside of the cell.

4. The new conformation of the carrier protein allows it to accept 2 potassium ions (K+) on the outside of the cell.

5. The phosphate group is released from the carrier protein and the carrier protein changes conformation again. As a result 2 potassium ions are transported into the cell.

Once the potassium ions are released the process can start again because the carrier protein is in the correct conformation to accept 3 new sodium ions.

Some other examples of active transport that occur within the body include:

Iodine is collected inside the cells of the thyroid gland.

Nutrients are completely absorbed from the gut by the cells lining the digestive tract.

Sodium ions (Na+) can be almost completely re-absorbed from urine by cells in kidney tubules.

Exocytosis http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP11203

Exocytosis is a cellular process where cells eject waste products or chemical transmitters (such as hormones) stored in membrane bound vesicles from the interior of the cell

Phospholipids and proteins in the membrane of vesicles carrying molecules packaged by

the Golgi apparatus fuse with the plasma membrane of the cell so they become one.

This is how most molecules are secreted from eukaryotic cells.

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Endocytosis “Cellular Eating”

During endocytosis cells take in substances by forming vesicles

A portion of the plasma membrane envelops the molecules to be transported

The membrane then pinches off to form an intracellular transport vesicle that can deliver the contents elsewhere in the cell

There are 2 types of endocytosis:

1) Phagocytosis

2) Pinocytosis

Phagocytosis “Cellular Eating”

A process of cellular ingestion by which the plasma membrane folds inward to

bring LARGE substances such as carbohydrates, proteins, or foreign invaders into the cell

In our bodies specialized white blood cells called macrophages destroy bacteria and worn-out red blood cells by “eating” them or engulfing them

The vesicle that results can be delivered to a lysosome where its contents can then be digested

Single celled organisms such as amoebas feed this way

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Pinocytosis “Cellular Drinking”

Occurs when solutes dissolved in a solution are to be transported into the cell

The liquid contents of the pinocytoic vesicle are then delivered into the cytoplasm of the cell.