water potential review - chisd · passive transport is the movement of a substance across a...
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Water Potential Review
Which way will the water flow?
Ψs = -7
Ψs = -4
Chapter 7 Review
Membrane Structure and Function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Selective Permeability
The plasma
membrane exhibits
selective
permeability
It allows some
substances to
cross it more
easily than others
Cellular membranes are fluid mosaics of lipids and
proteins
Phospholipids (fluid part)
Are the most abundant lipid in the plasma membrane
Are amphipathic, containing both hydrophobic and
hydrophilic regions
Proteins (mosaic part) can be
Transmembrane (integral)
Peripheral
Fluid Mosaic Model
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Lipids
Phospholipid structure
Consists of a hydrophilic “head” and hydrophobic
“tails”
CH2
O
P O O
O
CH2 CH CH2
O O
C O C O
Phosphate
Glycerol
Fatty acids
Hydrophilic
head
Hydrophobic
tails
–
CH2 Choline + N(CH3)3
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Lipids
The structure of phospholipids
Results in a bilayer arrangement found in cell
membranes
Hydrophilic
head
WATER
WATER
Hydrophobic
tail
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Lipids
Saturated fatty acids
Have the maximum number of hydrogen atoms
possible
Have no double bonds
Are typically solid at room temperature
Stearic acid
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Unsaturated fatty acids
Have one or more double bonds
Are typically liquid at room temperature
cis double bond
causes bending
Oleic acid
Lipids
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How fluid?
The type of hydrocarbon tails in phospholipids
Affects the fluidity of the plasma membrane
Fluid Viscous
Unsaturated hydrocarbon tails with kinks
Saturated hydro-
Carbon tails
Membrane fluidity
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How fluid?
The steroid cholesterol
Has different effects on membrane fluidity at
different temperatures
Cholesterol within the animal cell membrane
Cholesterol
Thinking Question
Would you expect an amoeba that lives in a
cold climate to have a higher or lower
amount of saturated hydrocarbon chains in
its phospholipids compared to a species that
lives in a warm climate?
Thinking Question
Would you expect an amoeba that lives in a
cold climate to have a higher or lower
amount of saturated hydrocarbon chains in
its phospholipids compared to a species that
lives in a warm climate?
Hint: saturated hydrocarbons are MORE fluid
at room temperature.
Thinking Question
Would you expect an amoeba that lives in a cold climate
to have a higher or lower amount of saturated
hydrocarbon chains in its phospholipids compared to a
species that lives in a warm climate?
Fluid Viscous
Unsaturated hydrocarbon tails with kinks
Saturated hydro-
Carbon tails
Membrane fluidity
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Fluid-Mosaic Model
Membrane proteins (mosaic part) are
dispersed and individually inserted into the
phospholipid bilayer
Phospholipid
bilayer
Hydrophilic region
of protein
Hydrophobic region of protein
Membrane Carbohydrates
Cell-cell recognition
Is a cell’s ability to distinguish one type of neighboring cell from another
Short carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cells
These carbohydrates mediate cell-cell recognition
Crossing the Membrane
Cells need to control the exchange of material with their environment
Hydrophobic molecules (non-polar)
Are lipid soluble and can pass through the membrane rapidly
Hydrophilic molecules (polar, ionic)
Do not cross the membrane rapidly
Need help of transport proteins
Passive vs. Active Transport
Passive transport is the movement of a substance across a membrane with no energy investment
Involves the process of simple diffusion
Or facilitated diffusion
Active transport requires an energy input
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Passive Transport
Simple Diffusion
Is the tendency for molecules to move from
areas of high concentration to areas of low
concentration
Small, hydrophobic molecules can move
across the membrane this way Molecules of dye Membrane (cross section)
Net diffusion Net diffusion Equilibrium
(a)
Facilitated Diffusion
In facilitated diffusion
Transport proteins speed the movement of
molecules across the plasma membrane
Does NOT require energy input (no ATP)
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Facilitated Diffusion
Transport proteins
Provide corridors that allow a specific molecule
or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein Solute
CYTOPLASM
(a)
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Special Case of Facilitated Diffusion Osmosis
Is the diffusion of water across a selectively permeable
(semipermeable) membrane
Lower
concentration
of solute = Higher
concentration of
water
Higher
concentration
of solute = Lower
concentration of
water
Water moves from an area of higher
water concentration to an area
of lower water concentration
s
Tonicity
Tonicity Is the ability of a solution to cause a cell to gain or lose water
If a solution is isotonic The concentration of solutes is the same as it is inside the cell
There will be no net movement of water
If a solution is hypertonic The concentration of solutes is greater than it is inside the cell
The cell will lose water
If a solution is hypotonic The concentration of solutes is less than it is inside the cell
The cell will gain water
Tonicity Diagram
5% NaCl 5% NaCl 10% NaCl 20% NaCl
Isotonic
Hypotonic
Hypertonic
Hypotonic
Hypertonic
Water
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Osmosis in Cells
Water balance in cells without walls
Such as animal or protist
Hypotonic solution Isotonic solution Hypertonic solution
H2O H2O H2O H2O
Lysed Normal Shriveled/Crenation
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Osmosis in Cells
Water balance in cells with walls
Such as plant cells
H2O H2O H2O H2O
Turgid (normal) Flaccid Plasmolyzed
Hypotonic solution Isotonic solution Hypertonic solution
Thinking Question
Imagine you are in the hospital recovering
from an operation and require intravenous
fluids. Why should you be concerned about
the concentration (tonicity) of the solution
in the IV bags?
Active Transport
Active transport
Moves substances against their concentration
gradient from low to high
Requires energy, usually in the form of ATP
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Review: Passive and Active Transport Compared Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
ATP
Movement of Macromolecules
Transport of large molecules across the plasma membrane occurs by exocytosis and endocytosis
In exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents
In endocytosis
The cell takes in macromolecules by forming new vesicles from the plasma membrane
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EXTRACELLULAR
FLUID Pseudopodium
CYTOPLASM
“Food” or
other particle
Food
vacuole
1 µm
Pseudopodium
of amoeba
Bacterium
Food vacuole
PINOCYTOSIS
0.5 µm Plasma
membrane
Vesicle
Endocytosis
Three types of endocytosis
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RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Ligand
Coat protein
Coated
pit
Coated
vesicle
Plasma
membrane
Coat
protein
Key Points of Chapter 7
Cellular membranes are fluid mosaics of lipids and proteins
Membrane structure results in selective permeability
Passive transport is diffusion of a substance across a membrane with no energy investment
Active transport uses energy to move solutes against their gradients
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis