Cambridge (CIE) AS Biology Your notes
Movement into & out of Cells
Contents
Diffusion
Osmosis
Active Transport
Endocytosis & Exocytosis
Investigating Transport Processes in Plants
Investigating Diffusion
Surface Area to Volume Ratios
Investigating Surface Area
Estimating Water Potential in Plants
Osmosis in Plant Cells
Osmosis in Animals
Comparing Osmosis in Plants & Animals
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, Diffusion
Your notes
Diffusion & Facilitated Diffusion
Diffusion is a type of transportation
It can be defined as:
The net movement, as a result of the random motion of its molecules or ions, of a
substance from a region of its higher concentration to a region of its lower concentration.
The molecules or ions move down a concentration gradient
The random movement is caused by the natural kinetic energy of the molecules or ions
Simple Diffusion Across a Cell Membrane Diagram
Diffusion across the cell membrane
As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given
sufficient time), where they are evenly spread within a given volume of space
The rate at which a substance diffuses across a membrane depends on several factors:
Diffusion Factors Table
Factor How the factor affects the rate of diffusion
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,'Steepness' of the This is the difference in the concentrations of the substance
concentration of both sides of the surface
Your notes
gradient
If there are more molecules on one side of a membrane than
on the other, at any one moment more molecules will move
randomly across the membrane from that side than from the
other side
A greater difference in concentration means a greater
difference in the number of molecules passing in the two
directions and therefore a faster rate of diffusion
Temperature Molecules and ions have more kinetic energy at higher
temperatures
They move faster, resulting in a higher rate of diffusion
Surface area The greater the surface area across which diffusion is taking
place, the greater number of molecules or ions that can cross
it at any one moment and therefore, faster diffusion occurs
The surface area of cell membranes can be increased by
folding (eg. microvilli in an intestinal epithelium cell or cristae
in a mitochondrion)
As a cell increases in size, the surface area to volume ratio
decreases, which slows down the rate that substances can
diffuse through a cell as the distance required to move
becomes too great
Properties of Large molecules diffuse more slowly than smaller ones as
molecules or ions they require more energy to move
Uncharged and non-polar molecules diffuse directly across
the phospholipid bilayer
Non-polar molecules diffuse more quickly than polar ones
because they are soluble in the non-polar central layer of the
phospholipid bilayer
Facilitated diffusion
Certain substances cannot diffuse through the phospholipid bilayer of cell membranes.
These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)
Because most substances in biology are water-soluble, facilitated diffusion is required
by a large majority of molecules that diffuse across membranes
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, These substances can only cross the phospholipid bilayer with the help of certain
proteins
Your notes
This form of diffusion is known as facilitated diffusion
There are two types of proteins that enable facilitated diffusion:
Channel proteins
Carrier proteins
Channel and carrier proteins are highly specific (they only allow one type of molecule or
ion to pass through)
Channel proteins
Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane
The diffusion of these ions does not occur freely, most channel proteins are ‘gated’,
meaning that part of the channel protein on the inside surface of the membrane can
move in order to close or open the pore
This allows the channel protein to control the exchange of ions
Channel Protein Diagram
A channel protein (open and closed)
Carrier Proteins
Unlike channel proteins which have a fixed shape, carrier proteins can switch between
two shapes
This causes the binding site of the carrier protein to be open to one side of the
membrane first, and then open to the other side of the membrane when the carrier
protein switches shape
The direction of movement of molecules diffusing across the membrane depends on
their relative concentrations on each side of the membrane
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Movement into & out of Cells
Contents
Diffusion
Osmosis
Active Transport
Endocytosis & Exocytosis
Investigating Transport Processes in Plants
Investigating Diffusion
Surface Area to Volume Ratios
Investigating Surface Area
Estimating Water Potential in Plants
Osmosis in Plant Cells
Osmosis in Animals
Comparing Osmosis in Plants & Animals
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, Diffusion
Your notes
Diffusion & Facilitated Diffusion
Diffusion is a type of transportation
It can be defined as:
The net movement, as a result of the random motion of its molecules or ions, of a
substance from a region of its higher concentration to a region of its lower concentration.
The molecules or ions move down a concentration gradient
The random movement is caused by the natural kinetic energy of the molecules or ions
Simple Diffusion Across a Cell Membrane Diagram
Diffusion across the cell membrane
As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given
sufficient time), where they are evenly spread within a given volume of space
The rate at which a substance diffuses across a membrane depends on several factors:
Diffusion Factors Table
Factor How the factor affects the rate of diffusion
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,'Steepness' of the This is the difference in the concentrations of the substance
concentration of both sides of the surface
Your notes
gradient
If there are more molecules on one side of a membrane than
on the other, at any one moment more molecules will move
randomly across the membrane from that side than from the
other side
A greater difference in concentration means a greater
difference in the number of molecules passing in the two
directions and therefore a faster rate of diffusion
Temperature Molecules and ions have more kinetic energy at higher
temperatures
They move faster, resulting in a higher rate of diffusion
Surface area The greater the surface area across which diffusion is taking
place, the greater number of molecules or ions that can cross
it at any one moment and therefore, faster diffusion occurs
The surface area of cell membranes can be increased by
folding (eg. microvilli in an intestinal epithelium cell or cristae
in a mitochondrion)
As a cell increases in size, the surface area to volume ratio
decreases, which slows down the rate that substances can
diffuse through a cell as the distance required to move
becomes too great
Properties of Large molecules diffuse more slowly than smaller ones as
molecules or ions they require more energy to move
Uncharged and non-polar molecules diffuse directly across
the phospholipid bilayer
Non-polar molecules diffuse more quickly than polar ones
because they are soluble in the non-polar central layer of the
phospholipid bilayer
Facilitated diffusion
Certain substances cannot diffuse through the phospholipid bilayer of cell membranes.
These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)
Because most substances in biology are water-soluble, facilitated diffusion is required
by a large majority of molecules that diffuse across membranes
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, These substances can only cross the phospholipid bilayer with the help of certain
proteins
Your notes
This form of diffusion is known as facilitated diffusion
There are two types of proteins that enable facilitated diffusion:
Channel proteins
Carrier proteins
Channel and carrier proteins are highly specific (they only allow one type of molecule or
ion to pass through)
Channel proteins
Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane
The diffusion of these ions does not occur freely, most channel proteins are ‘gated’,
meaning that part of the channel protein on the inside surface of the membrane can
move in order to close or open the pore
This allows the channel protein to control the exchange of ions
Channel Protein Diagram
A channel protein (open and closed)
Carrier Proteins
Unlike channel proteins which have a fixed shape, carrier proteins can switch between
two shapes
This causes the binding site of the carrier protein to be open to one side of the
membrane first, and then open to the other side of the membrane when the carrier
protein switches shape
The direction of movement of molecules diffusing across the membrane depends on
their relative concentrations on each side of the membrane
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