Membrane Structure and Function - Phoenix College 181/Adobe... · 2019. 8. 27. · Membrane...
Transcript of Membrane Structure and Function - Phoenix College 181/Adobe... · 2019. 8. 27. · Membrane...
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Membrane Structure and Function
Eukaryotic Cell: Neuron
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Membrane Structure and Function
All cells have a plasma or cell membrane, which contains the cell. Scanning electron micrograph (SEM) of adipocytes (Ad)
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Membrane Structure and Function
Prokaryotic Cells: Bacteria
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The Formation of Cell Membranes is Crucial to Life
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Functions of the Cell Membrane
• Contains the cell• Regulates the traffic
of molecules and substances in and out of the cell (semi-permeable membrane)
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Cell Membrane Structure
Pancreatic Secretory Cell: TEM of Basal and Apical Parts
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Major Components of the Cell Membrane
• The major constituents of the cell membrane are proteins and lipids
• Membrane proteins and lipids are arranged in a particular fashion, both contributing to containing the cell and to selectively allowing or blocking the traffic of certain substances through the cell
• Such arrangement of molecules provides fluidity to the cell membrane
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Major Components of the Cell Membrane: Lipids
• Phospholipids are amphipathic molecules (with hydrophobic tails and a hydrophilic head)
• One of the phospholipidtails exist mostly in a trans configuration, providing more fluidity to the membrane
• Cholesterol is a rigid molecule that makes membranes less fluid
Cholesterol
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Major Components of the Cell Membrane: Lipids
One of the phospholipid tails exist mostly in a trans configuration, providing more fluidity to the membraneCholesterol is a rigid molecule that makes membranes less fluid
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How Are Phospholipids Organized in the Cell Membrane?
• Phospholipids constitute two mirror-image oriented layers — the lipid bilayer
• The hydrophilic heads are exposed to the high-content water regions, while the hydrophobic tails constitute a barrier impenetrable to almost all substances
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hydrophobic tails: repel water: phospholipids: lipid bilayer
mak
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mak
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50%50%
1. makes sense2. makes no sense
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Major Components of the Cell Membrane: Membrane Proteins
• Membrane proteins are embedded in the fluid matrix of the lipid bilayer
• More than 50 types of proteins have been found in the plasma membrane. Membrane proteins determine most of the membrane specific functions
• Transport proteins, enzymes and receptor proteins (membrane proteins that interact with other cells or molecules) include the vast majority of membrane proteins
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Major Components of the Cell Membrane: Organization
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Major Components of the Cell Membrane: Organization
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Functions of the Cell Membrane
• Contains the cell• Regulates the traffic
of molecules and substances in and out of the cell (semi-permeable membrane)
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Traffic of Substances Across the Plasma Membrane
• Selective
• Bidirectional
• Depending Upon Differences of Concentration Inside and Outside of the Cell
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Traffic of Substances Across the Plasma Membrane
• Selective: only a few molecules can go through the lipid bilayer. Transport proteins mostly determine what substances cross the cell membrane, as they carry out the majority of membrane transport
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Traffic of Substances Across the Plasma Membrane
• Bidirectional: only a few molecules can go through the lipid bilayer. Transport proteins determine what substances cross the cell membrane. Transport can occur in/out or out/in
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Traffic of Substances Across the Plasma Membrane
• Depending Upon Differences of Concentration Inside and Outside of the Cell: Osmosisand diffusion are the two main processes by which molecules move across the cell membrane
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___ control the traffic of molecules through the cell membrane
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1. Membrane proteins2. Lipids3. Membrane proteins
and lipids
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Traffic of Substances Across the Plasma Membrane: Osmosis
• Osmosis is the movement of water and some small molecules through a semi-permeable membrane from areas of low concentration of solutes to areas of high concentration of solutes
• Why does water move in that particular direction?
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Traffic of Substances Across the Plasma Membrane: Osmotic Shock
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Traffic of Substances Across the Plasma Membrane: Osmosis
• Draw a situation where the extracellularenvironment is such that water flows out of the cell
• Is the extracellularenvironment hypo-, hyper-, or isotonic?
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In this situation, will water flow out of the cell?
Yes No
50%50%
1. Yes2. No
[NaCl] = 0.03 mg/ml
[NaCl] = 1.1 mg/ml
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Traffic of Substances Across the Plasma Membrane: Diffusion
• Diffusion is the movement of substances from an area of high concentration of solutes to an area of low solute concentration (down to a concentration gradient)
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Traffic of Substances Across the Plasma Membrane: Diffusion
• Draw a situation where a molecule of NaCl will enter the cell. Assume that a transport protein is needed
• Is the extracellularenvironment hypo-, hyper-, or isotonic?
• Direction of water?
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In this situation, will a molecule of NaCl enter the cell?
Yes No
50%50%
1. Yes2. No
[NaCl] = 1.9 mg/ml
[NaCl] = 1.1 mg/ml
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Traffic of Substances Across the Plasma Membrane: Facilitated
Diffusion• Facilitated diffusion is a
protein-mediated passive(no energy required) diffusion of molecules across the cell membrane
• Transport proteins carry out facilitated diffusion; facilitated diffusion is very selective, as each transport protein transports just one type of molecule
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Traffic of Substances Across the Plasma Membrane: Active Transport
• Active transport is a protein-mediated transport of molecules across the cell membrane against a concentration gradient (low to high solute concentration areas). It requires a boost of energy (ATP) to occur. As facilitated diffusion, is very selective
• Glucose is actively transported through the plasma membrane of intestinal cells
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Bulk Transport of Substances Across the Plasma Membrane: Exocytosis and
Endocytosis
Pancreatic Secretory Cell: TEM
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Types of Endocytosis: Phagocytosis
In phagocytosis (“cell eating”), a cell engulfs a particle or another cell through the emission of pseudopodia, and packs it into a vacuole. The contents of the vacuole is digested after the vacuole fuses with a lysosome
Phagocytosis of erythrocytes (Er) by blood macrophages (Ma).
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Types of Endocytosis: Pinocytosis
• In pinocytosis (“cell drinking”), the cell takes in droplets of extracellular fluid into small vesicles. Many molecules enter the cell dissolved in the droplets in a non-specific manner
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Types of Endocytosis: Receptor-Mediated Endocytosis
• Receptor-mediated endocytosisrequires of specific receptor proteins located in the cell membrane. Cell receptors interact with the molecule to be transported into the cell through a ligand — a molecule that binds specifically to the receptor
• Receptor-mediated endocytosis is highly specific. Human cells use receptor-mediated endocytosis to take in cholesterol. Some viruses (i.e. HIV virus) enters the cell through receptor-mediated endocytosis
• Mutations in receptor proteins involved in receptor-mediated endocytosisusually block the entrance of substances meant to be transported by this process (i.e. natural HIV immunity, familial hypercholesterolemia)
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The arrows point to a process of:
rece
ptor-m
ediat
ed ...
exoc
ytosis
phag
ocyto
sis
33% 33%33%
1. receptor-mediated endocytosis
2. exocytosis3. phagocytosis
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The arrows point to a process of:
rece
ptor-m
ediat
ed ...
exoc
ytosis
phag
ocyto
sis
33% 33%33%
1. receptor-mediated endocytosis
2. exocytosis3. phagocytosis