Lecture 5 Cell Biology

8/8/2019 Lecture 5 Cell Biology

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Plasma membrane= Cell membrane

SYSTEMS

ORGANS

TISSUES

CELL


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IntroductionWhat dose the cell need to live?

Each cell needs to obtain:

oxygen

other nutrients (carbohydrates, amino acids,

lipid molecules, mineral ions, etc.) from theenvironment

maintain water balance with its surroundings

remove waste materials from the cell

The plasma membrane separates a cell from itsexternal environment


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Functions of plasma membrane

1.The phospholipid bilayer separates the outside from the insideof the cell.

2.Maintains the cell's environment by regulating materials thatenter or leave the cell.

3.The plasma membrane is differentially, or selectively,permeable. Some materials enter and leave easily through themembrane, some with the assistance of membrane molecules,and some prohibited. Provides mechanisms for cell-to-cellcommunication.

4.Provides mechanisms for a cell to recognize "self" versus "non-self" (foreign materials), important to the immune system


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The Fluid Mosaic Membrane Structure

1. The structure and function of a membrane depends on its molecular

composition.

2. The membrane is formed from a phospholipid bilayer, with a

number of associated proteins.

3. Membranes also contain carbohydrates (glycoproteins and

proteoglycans) and glycolipids.

4. The resultant membrane structure (proteins scattered throughout the

fluid phospholipid layers) resembles a mosaic, hence the name "fluid

mosaic membrane".

5. Membrane molecules are manufactured in the endoplasmic reticulumand distributed by Golgi vesicles.

6. The orientation of membranes is determined at the manufacturing

site. Molecules on the inside of the ER and Golgi vesicles become

exterior membrane molecules.


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Membrane Phospholipids

1. Phospholipids have bothhydrophilic (polar) andhydrophobic (non polar) regions (in other words, they areamphipathic).

2. The fatty acid "tails" of the two phospholipid layers areoriented towards each other so that the hydrophilic"heads", which contain the "charged" phosphate portion,face out to the environment as well as into the cytoplasmof the cell's interior, where they can form hydrogen bondswith surrounding water molecules.

3. The phospholipid molecules of a membrane provide forits physical integrity.


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Phospholipid Movements A membrane is held together, for the most part, by hydrophobic

interactions within the phospholipid bilayer.

Because individual phospholipid molecules are not bonded to

each other, a membrane is flexible, or "fluid", particularly to

lateral movement of the fatty acids. Phospholipid molecules easily move along the plane of the

membrane; reversing exterior interior position (orflip-flopping)

is less common.


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Phospholipid Movements

Factors that a

ffect phosphol

ipid

s movement:

(1) Cholesterol, found in membranes of many animal cells, reducesfluid movement of the phospholipids, helping to maintainmembrane integrity.

(2) The saturation offatty acids affects membrane fluidity the

more saturated, the less movement.(3) Membranes will also solidify as temperature decreases,

reducing function.

The saturation of fatty acids will affect the temperature at whichthe membrane "solidifies" (just as it does with fats and oils).

Unsaturated/Saturated


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Membrane Proteins1. Interspersed throughout a membrane's phospholipid layer are a

number of amphipathic proteins.

2. The hydrophobic regions of the proteins are within the fatty

acid regions of the phospholipids and hydrophilic regions are at

the interior and exterior aqueous interfaces of the membrane.

3. This orientation is important to the membrane proteins function.

4. The membrane is also associated with a network of supporting

cytoskeletal filaments, some of which help shape the cell and

some help anchor proteins within the membrane.


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Protein Mobility

Many proteins within the membrane are mobile;studies of fused mouse and human cells show thatproteins from the two cells are intermixed within anhour of fusion

Membrane proteins are divided into two, dependingon their location

Integral

peripheral


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Membrane proteins

Integral (Transmembrane) Proteins1. Proteins that go through the membrane are called

integral or transmembrane proteins.

2. Theyhave hydrophobic (non-polar amino acids

with alpha helix coiling) regions within the interior

of the membrane and hydrophilic regions at either

membrane surface

hydrophilic regions

hydrophilic regions

hydrophobic regions


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Peripheral Proteins

Are attached to the surface of the

membrane, often to the exteriorhydrophilic regions of thetransmembrane proteins.

On the interiorsurface, peripheralproteins typically are held in

position by the cytoskeleton.

On the exterior, proteins may attachto the extracellular matrix.

Peripheral proteins help give

animal cell membranes strength. The different proteins contribute to

the "sidedness" of membranes sothat the interior and exterior sidesof membranes have different

properties that affect its function.


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Anchoring proteins Other proteins have non-polar helix regions that fix the protein

into specific regions of the phospholipid bilayers. Suchproteins are

called anchoring proteins.

The protein receptors at neuromuscular junctions on muscle cells

are anchored proteins.

Anchor proteins can attach to the fibrous network of the

cytoskeleton to give shape and strength to some cells.

Some membrane lipid regions, called lipid rafts, are also

specialized to help anchor proteins within a specific region.


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Membrane Protein Functions

1-Transport Proteins Transport Proteins are transmembrane proteins that serve as carriers

for specific substances that need to pass through the membrane byproviding a hydrophilic channel or pore.

Transport proteins have binding sites that attract specific molecules.M

ost of our ions, amino acids, sugars and other small nutrientmolecules are moved through transport proteins.

When a molecule binds to the carrier protein, the protein shapechanges moving the substance through the membrane. This processmay require energy (ATP), and the ATP complex is then a part of the

transport protein.


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2- Enzymatic Proteins

Many enzymes are embedded in membranes, which attract reacting

molecules to the membrane surface.

The active site of the enzyme will be oriented in the membrane forthe substrate to bind.

Enzymes needed for metabolic pathways can be aligned adjacent to

each other to act like an assembly line for the reactions, minimizing

the need for intermediates to diffuse through the cytoplasm of the

cell.


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3- Signal Transduction (Receptor) Proteins Signal transduction proteins have attachment sites

forchemical messengers, such as hormones.

The signal molecule, when it attaches to thereceptor promotes a conformational change thattransmits the message into the cell to trigger somecell activity.


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4-Attachment Proteins Attachment proteins attach to the cytoskeleton or extracellular

matrix to help maintain cell shape (particularly for animal cells)

5-Recognition (Identity) Proteins

Glycoproteins serve as surface receptors for cell recognition and

identification. They are important to the immune system.

6-Cell Adhesion (Intercellular Joining) Proteins

Special membrane proteins are responsible for the cell junctions(tight junctions, desmosomes and gap junctions.

theypermit cells to adhere to each other.


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Membrane Carbohydrates

Glycoproteins and glycolipids are also important tomembrane structure and function.

Glycolipids function as recognition signals for cell-to-cell

interactions.

Glycoproteins, with their oligosaccharides portions, are critical

for a cell to be recognized by other cells and by protein

molecules, and forcell-to-cell adhesion

Glycoprotein complexwith long Polysaccharide

Collagen fiber

Intgrine


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Function of Cell MembraneGood Luck

Dr Mona


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The cell membrane consists of three classes of amphipathic lipids:

phospholipids, glycolipids, and cholesterols. The amount of each

depends upon th

e type of cell, but in th

e majority of casesphospholipids are the most abundant.[5] In RBC studies, 30% of

the plasma membrane is lipid.

The fatty chains in phospholipids and glycolipids usually contain

an even number of carbon atoms, typically between 16 and 20.

The 16- and 18-carbon fatty acids are the most common. Fattyacids may be saturated or unsaturated, with the configuration of

the double bonds nearly always cis. The length and the degree of

unsaturation of fatty acid chains have a profound effect on

membrane fluidity[6]

as unsaturated lipids create a kink,preventing the fatty acids from packing together as tightly, thus

decreasing the melting temperature (increasing the fluidity) of the

membrane.


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Most membrane proteins must be inserted in some

way into the membrane. For this to occur, an N-

terminus "signal sequence" of amino acids directs

proteins to the endoplasmic reticulum, which insertsthe proteins into a lipid bilayer. Once inserted, the

proteins are then transported to their final

destination in vesicles, where the vesicle fuses with

the target membrane.

[edit]

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Lecture 5 Cell Biology