www.soran.edu.iq Cell and Molecular Biology Behrouz Mahmoudi
Cell Membrane Structure and Functions 1
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www.soran.edu.iq Membrane Function and structure Membranes
organize the chemical activities of cells. The outer plasma
membrane forms a boundary between a living cell and its
surroundings Exhibits selective permeability Controls traffic of
molecules in and out Internal membranes provide structural order
for metabolism - Form the cell's organelles - Compartmentalize
chemical reactions 2
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www.soran.edu.iq Fluid Mosaic Model of the PM A membrane is a
mosaic Proteins and other molecules are embedded in a framework of
phospholipids A membrane is fluid Most protein and phospholipid
molecules can move laterally 3
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www.soran.edu.iq Macromolecules found in Membranes Lipids
Phospholipid bilayer Cholesterol Proteins Transport proteins
Receptor proteins Recognition proteins Carbohydrates
Oligosaccharides on glycoproteins 4
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Functions of Membrane Macromolecules Lipids Phospholipid
bilayer Forms boundary to isolate cell contents from environment
Restricts passage of hydrophilic substances across the membrane
Cholesterol Increases bilayer strength, flexibility Reduces
membrane fluidity Reduces permeability to water- soluble substances
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www.soran.edu.iq Functions of Membrane Macromolecules Proteins
Transport proteins Regulate movement of water soluble substances
Channel proteins have pores that allow passage of ions and small
water-soluble molecules Carrier proteins bind to molecules and
change shape for delivery across membrane 7
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www.soran.edu.iq Functions of Membrane Macromolecules Proteins
Receptor proteins Docking site for molecules outside the cell
Trigger internal cellular response Recognition proteins
Identification tags Oligosaccharides aid in cell-cell recognition
Cell-surface attachment sites 8
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www.soran.edu.iq Fluid Mosaic Model of Membrane Structure
Within the phospholipid bilayer are many different types of
embedded proteins and cholesterol molecules whose presence spawned
the term mosaic. From scanning electron microscope images, it was
observed that the embedded molecules can move sideways throughout
the membrane, meaning the membrane is not solid, but more like a
fluid. 9
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www.soran.edu.iq Passive Transport A process that does not
require energy to move molecules from a HIGH to LOW concentration
DDiffusion FFacilitated Diffusion OOsmosis 10
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www.soran.edu.iq Diffusion is the movement of small particles
across a selectively permeable membrane like the cell membrane
until equilibrium is reached. These particles move from an area of
high concentration to an area of low concentration. outside of cell
inside of cell 11
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www.soran.edu.iq Osmosis is the diffusion of water through a
selectively permeable membrane like the cell membrane Water
diffuses across a membrane from an area of high concentration to an
area of low concentration. Semi-permeable membrane is permeable to
water, but not to sugar 12
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www.soran.edu.iq Facilitated Diffusion is the movement of
larger molecules like glucose through the cell membrane larger
molecules must be helped Proteins in the cell membrane form
channels for large molecules to pass through Proteins that form
channels (pores) are called protein channels outside of cell inside
of cell Glucose molecules 13
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www.soran.edu.iq Hypertonic Solutions: contain a high
concentration of solute relative to another solution (e.g. the
cell's cytoplasm). When a cell is placed in a hypertonic solution,
the water diffuses out of the cell, causing the cell to shrivel.
Hypotonic Solutions: contain a low concentration of solute relative
to another solution (e.g. the cell's cytoplasm). When a cell is
placed in a hypotonic solution, the water diffuses into the cell,
causing the cell to swell and possibly explode. Isotonic Solutions:
contain the same concentration of solute as another solution (e.g.
the cell's cytoplasm). When a cell is placed in an isotonic
solution, the water diffuses into and out of the cell at the same
rate. The fluid that surrounds the body cells is isotonic. 15
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www.soran.edu.iq Active Transport Active transport is the
movement of molecules from LOW to HIGH concentration. Energy is
required as molecules must be pumped against the concentration
gradient. Proteins that work as pumps are called protein pumps. Ex:
Body cells must pump carbon dioxide out into the surrounding blood
vessels to be carried to the lungs for exhale. Blood vessels are
high in carbon dioxide compared to the cells, so energy is required
to move the carbon dioxide across the cell membrane from LOW to
HIGH concentration. outside of cell inside of cell Carbon Dioxide
molecules 17
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www.soran.edu.iq Endocytosis Active process for movement of
large molecules and organisms Substance is taken in by vesicle
formed from cell membrane Phagocytosis: solid substance in vesicle
Pinocytosis: liquid droplets in vesicle; The uptake of
extracellular fluids such as hormones and enzymes by the body cells
Receptor-Mediated Endocytosis: incoming substance binds to receptor
Iron Uptake, Cholesterol Uptake, 20
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www.soran.edu.iq Exocytosis Substance is expelled after being
enclosed in a vesicle within the cell Used to move large molecules
out of the cell 22