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Cell Biology ١
Lecture 5
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Cell Biology ٢
The plasma membrane
Each cell must interact with its environment in a number of ways.
Each cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, mineral ions, etc.) from the
environment, maintain water balance with its surroundings, and remove waste materials
from the cell.
The plasma membrane separates a cell
from its environment.
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Cell Biology ٣
The plasma membrane
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Cell Biology ٤
The plasma membrane functions
1.
The phospholipid bilayer provides the cell with a structure that separates the outside from the inside of the cell.
2.
The integrity of the membrane is necessary for life functions.
Because of the nature of the phospholipid, many molecules cannot move across the membrane without help.
3.
Maintains the cell's environment by regulating materials that enter or leave the cell.
4.
The plasma membrane is differentially, or selectively,
permeable. Some materials enter and leave easily throughthe membrane, some with the assistance of membrane
molecules, and some prohibited. Provides mechanisms for cell-to-cell communication.
5.
Provides mechanisms for a cell to recognize "self" versus "non- self" (foreign materials), important to the immune system,
development and defense of the organism through genetically
unique cell recognition markers
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Cell Biology ٥
The Fluid Mosaic Membrane Structure
1.
The structure and function of a membrane depends of its molecular composition.
2.
The foundation of the membrane is its 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 reticulum and 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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Cell Biology ٦
Membrane Phospholipids
1. Phospholipids have both hydrophilic (polar) and hydrophobic (non polar) regions (in other words, they are amphipathic).
2. The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the "charged" phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they can form hydrogen bonds with surrounding water molecules.
3. The phospholipid molecules of a membrane provide for its physical integrity
.
Exterior
Cytoplasm Phospholipid Bilayer
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Cell Biology ٧
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 (or flip-flopping) is less common.
Unsaturated/Saturated
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Cell Biology ٨
Phospholipid Movements
Cholesterol, found in membranes of many animal cells, reduces fluid movement of the phospholipids, helping to maintain membrane integrity.
The saturation of fatty acids affects membrane fluidity – the more saturated, the less movement.
Membranes will also solidify as temperature decreases, reducing function.
The saturation of fatty acids will affect the temperature
at which the membrane "solidifies" (just as it does with
fats and oils).
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Cell Biology ٩
Membrane Proteins
1.
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 of the proteins are at the interior and exterior aqueous interfaces of the membrane.
3.
This orientation is important to how 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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Cell Biology ١٠
Protein Mobility
Many proteins within the membrane are mobile; studies of fused mouse and human cells show that proteins from the two cells are intermixed within an hour of fusion
Membrane proteins are divided into two categories, integral and peripheral,depending on their location
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Cell Biology ١١
Membrane proteins
Integral (Transmembrane) Proteins
1. Proteins that go through the membrane are called integral or transmembrane proteins.
2. They have hydrophobic (non-polar amino acids with alpha helix coiling) regions within the interior of the membrane and
hydrophilic regions at either membrane surface
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Cell Biology ١٢
Peripheral Proteins
Are attached to the surface of the membrane, often to the exterior hydrophilic regions of the transmembrane proteins.
On the interior surface, peripheral proteins typically are held in position by the cytoskeleton.
On the exterior, proteins may attach to the extracellular matrix.
Peripheral proteins help give animal cell membranes strength.
The different proteins contribute to the "sidedness" of membranes so that the interior and exterior sides of
membranes have different properties that affect membrane function.
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Cell Biology ١٣
Anchoring proteins
Other proteins have non-polar α helix regions that fix the protein into specific regions of the phospholipid bilayers. Such proteins 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 of a membrane.
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Cell Biology ١٤
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 by providing a hydrophilic channel or pore.
Transport proteins have binding sites that attract specific molecules. Most of our ions, amino acids, sugars and other small nutrient molecules are moved through transport proteins.
When a molecule binds to the carrier protein, the protein shape changes moving the substance through the membrane. This process may require energy (ATP), and the ATP complex is then a part of the transport protein.
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Cell Biology ١٥
Membrane Protein Functions
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 for the 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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Cell Biology ١٦
Membrane Protein Functions
3- Signal Transduction (Receptor) Proteins
Signal transduction proteins have attachment sites for chemical messengers, such as hormones.
The signal molecule, when it attaches to the receptor promotes a conformational change that relays the message into the cell to trigger some cell activity.
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Cell Biology ١٧
Membrane Protein Functions
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.
they permit cells to adhere to each other.
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Cell Biology ١٨
Membrane Carbohydrates
Glycoproteins and glycolipids are also important to membrane 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 for cell-to-cell adhesion
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