Post on 31-May-2020
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Cell Biology
The Plasma Membrane
recall…Fluid Mosiac Model • S.J. Singer • Semipermeable membrane • “fluid” portion is double layer of
phospholipids (=phospholipid bilayer) • “mosaic” portion is the proteins and
carbohydrates embedded in the membrane
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recall…Role of the Cell Membrane • Isolate the cytoplasm and organelles
from external environment • Regulate the exchange of substances • Cell communication • Cell identification
1. Phospholipids
• Primary structural building block of the plasma membrane
Membrane Composition:
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• Phospholipids contain a hydrophyllic head and non-polar hydrophobic tail
• Hydrogen bonds form between the heads and the watery environment inside and outside the cell
• Hydrophobic interactions force tails to face inward • Phospholipids are not bonded together which make the
double layer fluid
2. Cholesterol (eg. Lipoprotein) • An important structural element
• Provides strength and stability to the membrane • Makes membrane less fluid
Image: Cholesterol within the animal cell membrane
3. Glycolipid • Phospholipid with a CHO attached • Provide energy • Cellular recognition markers for specific elements
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4. Glycoprotein • Protein with a CHO attached
• Important for cell-cell interactions • Important for white blood cell recognition • Mucin (secreted in respiratory and digestive tracts)
• protective coating, water retention capacity
5. Proteins
• Serve many functions depending on structure and location
• May be integral (embedded) or peripheral (on the inside surface of membrane)
Cell Cytoplasm
Extracellular Environment
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Embedded Proteins Channel Proteins
• Form small openings for molecules to diffuse through Carrier Proteins
• Binding site on protein surface selects certain molecules and allows them to pass through
Receptor Proteins • Molecular triggers that set off cell responses (release of
hormones)
Cell Recognition Proteins • ID tags to identify certain cells to the body’s immune system
Enzymatic Proteins • Carry out metabolic reactions
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Transport Across a Cell Membrane What controls how substances Move thru the membrane?
• Size of the molecule (selective permeability) • Charge of the molecule
What moves easily thru the membrane?
• Water • Carbon dioxide
• Oxygen
What does NOT move easily thru the membrane? • Large molecules such as glucose and proteins • molecules or atoms with charges, such as ions
(sodium, potassium, calcium)
PASSIVE TRANSPORT • Does NOT require chemical energy from the cell • Depends on permeability of the cell
• Consider characteristics of the membrane (lipids, proteins, etc)
Three main types: 1. Diffusion 2. Facilitated transport 3. Osmosis
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1. Simple Diffusion
• Movement of molecules from an area of HIGH concentration (more solute) to LOW concentration (less solute)
• These molecules follow their concentration gradient • Cells are able to dispose of O2, CO2, and H2O without
energy
n http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
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2. Facilitated Diffusion/Transport
• Movement of a molecule from an area of [HIGH] to [lower] with with aid of proteins (carrier or channel proteins)
• works as a facilitating pathway for larger molecules or charged ions (amino acids, glucose, Na+ and K+)
• Cell membranes have specific proteins for each molecule
n http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
• Several factors determine rate of diffusion: • Number of molecules in cell (mol) • Permeability of a particular molecule • Width of cell membrane • Concentration of diffusing molecule (mole/cm3) • Temperature (rate ↑’s as temp ↑’s)
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3. Osmosis
• Diffusion of WATER through a semi-permeable membrane
• Water moves from high H2O potential and low [solute] to low H2O potential and hi [solute]
Given: A - 100% water B - 90% water
Question:
What direction does the water flow?
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Given: A - 100% water B - 90% water
Question:
What direction does the water flow? Water flows from A --> B
Given: A - 1% glucose B - 20% glucose
Question:
What direction does the water flow?
What direction does the glucose flow?
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Given: A - 1% glucose B - 20% glucose
Question:
What direction does the water flow? Water flows from A --> B
What direction does the glucose flow? Glucose flows from B --> A
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Types of Solutions: 1. Hypertonic • Hyper = more ; tonic = solute • Thus, a cell in a solute rich environment
• Water flows outward from region of high [water] to low [water] (or low to high [solute])
• Cell undergoes plasmolysis (fluid loss) and crenate (shrivel)
2. Hypotonic • Hypo = less ; tonic = solute • Thus, a cell in a solute poor environment
• Water flows inward from region of high [water] to low [water] • Cell undergoes cytolysis (cell breaks) and bursts
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3. Isotonic • Iso = equal ; tonic = solute • Thus, a [solute] both equal within and outside the cell
• Water flows in and out of cell with net zero movement • Equilibrium
n http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
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ACTIVE TRANSPORT • Requires chemical energy (ATP) • Involves the movement of molecules “uphill” against a
concentration gradient • Proteins involved are called pumps because they are used to
‘pump’ substances across the concentration gradients Eg. Sodium (Na) Pump
Types:
1. Endocytosis (entering the cell) • Phagocytosis
• Pinocytosis
2. Exocytosis (exiting the cell)
BULK TRANSPORT: Endocytosis and Exocytosis
• A form of active transport (uses ATP) • When particles are too big or you are moving in bulk
ENDOCYTOSIS (endo=inside; cyte=cell)
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Endocytosis: Phagocytosis • Engulf large particles
Endocytosis: Pinocytosis • engulf very small particles or liquids • “sipping”
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EXOCYTOSIS (exo=outside; cyte=cell) • Export of a secretory vesicle or vacuole that will
migrate to membrane’s inner surface and fuse with membrane
• Contents are forced out of the cell • Opposite of endocytosis
• Examples: peptide hormones, steroid hormones, wastes
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Test Yourself 1. Describe in your own words, the fluid mosaic model of a
plasma membrane. 2. How is active transport different from facilitated
transport? 3. Distinguish between phagocytosis and pinocytosis