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Biology
Topic 2
Gas Exchange, Cell Membranes & Transport
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Properties
of Gas Exchange Surfaces
Surface area to volume
ratio
Diffusion
pathway
Concentration
gradient
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Surface area to volume ratio (SA:V ratio)
The
surface area
of an organism in relation to its
volume
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As the overall size of the organism increases, the
surface area
becomes
smaller
in comparison to the organism's volume, and the organism's surface area: volume ratio decreases
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Single
-celled organisms
Have a
high
SA:V ratio which allows the exchange of substances to occur by simple
diffusion
The large surface area allows for maximum absorption of
nutrients
and gases and removal of
waste products
The small volume within the cell means the
diffusion distance
to all organelles is
short
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Large
multicellular organisms
Have evolved
adaptations
to facilitate the exchange of substances with their environment
The gas exchange systems are adapted to
increase
the surface
area
available for the exchange of gases
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Adaptations to increase surface area
Alveoli
increase the surface area of mammalian lungs
Fish gills have structures called
lamellae
which provide a very
large
surface area
Leaves have a
spongy mesophyll
layer within which a large area of leaf cell surface is exposed to the
air
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Diffusion
pathway
The diffusion distance across an
exchange surface
is very
short
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Concentration gradient
The difference in
concentration
of the
exchange substances
on either side of the exchange surface
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A greater difference in concentration means a
greater
rate of
diffusion
as the gas molecules move across the exchange surface
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The continued movement of exchange substances
away
from the exchange surface mean that a
concentration gradient
is maintained
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Fick
's Law of Diffusion
Relates the rate of
diffusion
to the concentration gradient, the
diffusion
distance and the surface area
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Fick
's Law of Diffusion
1. Rate = P x A x ((
C1
-
C2
)÷ T)
2. P =
Permeability
constant
3. A =
Surface area
4. C1 - C2 = Difference in
concentration
5. T =
Thickness
of exchange surface
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The lungs of
air-breathing
animals provide an ideal exchange surface for the
diffusion
of gases
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Mammalian
lungs
Located in the
thorax
Enable
efficient
gas exchange
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Structures
of the mammalian lung
Trachea
Bronchi
Bronchioles
Alveoli
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Trachea
Tube that allows air to
travel
to the
lungs
Contains
C-shaped
rings of
cartilage
Lined with
mucus
and
cilia
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Bronchi
Similar structure to
trachea
but
thinner walls
and smaller diameter
Cartilage rings
are full
circles
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Bronchioles
Narrow
,
self-supporting
tubes with thin walls
Larger
ones have
elastic
fibres and smooth muscle
Smaller ones have
no
smooth muscle but
elastic
fibres
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Alveoli
Groups located at ends of bronchioles
Alveolar
wall is a single layer of flattened epithelium
Surrounded by extensive
capillary
network
Lined with
moisture
to facilitate gas diffusion
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Membranes
are
vital
structures found in all cells
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Cell surface membrane
Creates an enclosed space separating the
internal
cell environment from the
external
environment
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Intracellular
membranes
Form compartments within the cell such as the
nucleus
,
mitochondria
, and endoplasmic reticulum
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Membranes
Control the exchange of
substances
from one side to the other
Act as an
interface
for communication
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Phospholipids
Consist of a
glycerol
molecule, a
phosphate
group, and two fatty acid tails
Phosphate head is
polar
(hydrophilic)
Lipid tails are
non-polar
(hydrophobic)
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Phospholipid
bilayer
Forms the basic
structure
of the cell membrane
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Phospholipids
Molecules that form the basic
structure
of cell membranes
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Representations
of phospholipids
Simple representation of
phosphate
head and
lipid
tails
Chemical structure showing
glycerol
,
phosphate
group, and ester bonds
Diagrammatic
representation of
chemical
structure
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Phospholipid monolayer
Phospholipids spread over surface of water with
hydrophilic phosphate
heads in water and
hydrophobic fatty acid tails
sticking up away from water
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Phospholipid
bilayer
Two-layered
structures that form in
sheets
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Phospholipid bilayers
form the basic
structure
of the cell membrane
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Components
of cell membranes
Phospholipid bilayers
Proteins
Cholesterol
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Intrinsic (integral) proteins
Embedded in the
membrane
with their precise arrangement determined by their hydrophilic and
hydrophobic
regions
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Extrinsic
(peripheral) proteins
Found on the
outer
or
inner
surface of the membrane
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Cholesterol
Regulates
membrane fluidity by increasing fluidity at
low
temperatures and stabilising the membrane at higher temperatures
Increases
the mechanical strength and
stability
of membranes
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Glycolipids
and glycoproteins
Present on the surface of the cell, aid
cell-to-cell
communication
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Fluid
mosaic model
Describes the scattered pattern and fluid
movement
of components within the
phospholipid bilayer
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The cell membrane is
partially
permeable
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Small, non-polar molecules
Can pass through the
gaps
between the
phospholipids
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Large, polar molecules
Must pass through specialised membrane proteins called
channel proteins
and
carrier proteins
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The distribution of proteins within the
membrane
gives a
mosaic
appearance
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