Transcript Plasma membrane

I. Plasma Membrane Structure
Plasma membrane
– Boundary that separates living cells
from their nonliving surroundings.
- Apprx. 8 nm thick
- Composed chiefly of lipids and proteins
- Surrounds the cell and controls chemical traffic in/out of cell
- Is semi-permeable
Enables cells to maintain internal environment
different from external environment
Phospholipid bilayer
-Composed of 2 layers of phospholipids
-Heads are hydrophillic
-Tails are hydrophobic
Membrane Structure (Fluid
Mosaic Model)
• Membrane proteins embedded in
phospholipid bilayer
• Give membrane ‘fluidity’ similar to salad
oil
• Phospholipids & proteins can drift laterally
(2 um / sec)
Membranes must be fluid to work properly !
- solidification causes changes in permeability and enzyme deactivation
How do cells control membrane fluidity ?
1. Unsaturated hydrocarbon tails
2. Adding cholesterol makes membrane:
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Decreases fluidity at low temps by restraining phospholipid movement
-
Increases fluidity at high temps by preventing close packing of
phospholipids
So, how do the plant overcome the winter?
winter wheat
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Increase the percentage of
cholesterol in phospholipids
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Prevents membrane from
solidifying in cold weather
Proteins in Plasma Membrane
- Mosaic of proteins ‘bobbing’ in a fluid lipid bilayer
- Proteins determine a membrane’s specific function:
Two types
1. Integral proteins (‘transmembrane’, or embedded)
2. Peripheral proteins (bound to surface of membrane)
Some Functions of Membrane Proteins
Transport – protein provides channel across
membrane for particular solutes
Enzymatic activity – proteins may be enzymes
that catalyze steps in metabolic pathway
Signal transduction – protein is a receptor for
chemical messenger (hormone). Conformational
change in protein relays message to inside of cell
Intercellular joining – membrane proteins of
adjacent cells join together for strength
(epithelium)
Cell-cell recognition – glycoproteins act as I.D.
tags that are recognized by other cells (e.g. RBCs)
Regulating Traffic Across Membranes
II. Passive Transport: Diffusion and Facilitated diffusion
Diffusion : net movement
of a substance down
a concentration gradient.
•
Solutes diffuse from high to low concentration.
•
Continues until a dynamic equilibrium is reached.
•
No requirement for energy expense (passive)
•
Examples:
-
Uptake of O2 by cell performing respiration
-
Elimination of CO2 from cell
Diffusion of solutes across a membrane
Each dye diffuses down its own concentration gradient.
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Facilitated diffusion
Passive transport
Transport proteins speed the movement of molecules
across the plasma membrane.
Channel protein and Carrier protein required
a) Channel protein :
aquaporins, ion
channels
b) Carrier protein
Osmosis
•
Diffusion (passive transport) of
water across a selectively
permeable membrane
•
Direction of water movement is
determined by the difference in
total solute concentration,
regardless of type or diversity of
solutes.
•
Water moves always from high
concentration solution to low
concentration solution.
Water balance of living cells
•
Tonicity : the ability of a solution to cause a cell to gain or lose water
- Isotonic: no net movement of water across the membrane (normal).
- Hypertonic : the cell loses water to its environment (crenation).
- Hypotonic : the cell gains water from its environment (lysis).
Questions
An artificial cell consisting of an aqueous solution
enclosed in a selectively permeable membrane has just
been immersed in a beaker containing a different
solution. The membrane is permeable to water and to
the simple sugars glucose and fructose but completely
impermeable to sucrose.
1. Glucose?
2. Fructose?
3. Hypotonic/Hyp
ertonic?
4. Water?
Active Transport
•
Requires the cell to expend energy: ATP
•
Transport proteins pump molecules across a membrane
against their concentration gradient.
•
“Uphill” transport
•
Maintain steep ionic gradients across the cell membrane
(Na+ , K+ , Ca++ , Mg++ , Cl-)
Na+
Na+
Na+
Na+
Na+
Na+
Na+
inside
Na+
Na+ Na+
outside
An Example of Active Transport: The Sodium-Potassium Pump
Passive and Active Transport
More examples of active transport
• Exocytosis
– Removing large
particles out of the
cell with a vesicle
• Endocytosis
– Ingesting large particles
– Pinocytosis: “Cell drinking”
– Phagocytosis: “Cell eating”
Protein Synthesis
• The process of using DNA to form proteins
• Involves two steps:
– Transcription
– Translation
Genetic Information
• Uses 2 main forms of genetic information:
– DNA Deoxyribonucleic Acid
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Double stranded
Sugar: Deoxyribose
Stays in the nucleus
Bases: A T G C
– RNA Ribonucleic Acid
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Single stranded
Sugar: Ribose
Can leave the nucleus
Bases: A U G C
Transcription
• DNA unwinds
• One strand of the double helix is used as a
template
• Nucleotides line up along the DNA and
form a copy, called mRNA
• Once completed, DNA winds back up and
mRNA leaves
• mRNA must be spliced before it leaves the
nucleus ( immature RNA)
– Enzymes remove noncoding areas called
introns, and coding regions called exons are
spliced back together
– The result is a shorter, coding strand of mRNA
– Every 3 bases on mRNA is a codon
Codons
• Codes for amino acids
• 64 codons can code for 20 different amino
acids
Translation
• mRNA binds to a ribosome
• tRNA binds to ribosome along the codon and
reads which amino acid it codes for
• tRNA finds the specific amino acids
• For every codon, the tRNA brings the amino acids
• Amino acids link together forming a proteins
• Peptide bonds link each amino acid together.