BIOLOGY SPECIFICATION NOTES
Edexcel Biology Salters-Nuffield A AS/A-Level
Key:
Definitions are in turquoise
Core practicals are in orange
Equations are in green
Topic 2: Genes and Health (GAH)
2.1 i) Know the properties of gas exchange surfaces in living organisms (large surface area
to volume ratio, thickness of surface, difference in concentration). p.60
Living organisms have to exchange substances with their surroundings. This can be like
taking in oxygen from the air, and releasing carbon dioxide. Substances that diffuse in or out
of a cell move down a concentration gradient, which is maintained by constant diffusion.
Smaller organisms that are unicellular have their whole cell membrane as an exchange
surface, but as an organism gets bigger, more exchange needs to take place to meet the
organism’s metabolic needs. Larger organisms have more trouble absorbing substances as
they have a low surface area to volume ratio. For example, a mouse would have a large
surface area but low volume, but an elephant would have a small surface area and high
volume.
Larger organisms can’t only rely on diffusion to fulfill their cell's needs, so they often have
adapted other organs to increase the surface area for exchange. It's also worth noting that
those with a high surface area to volume ratio lose heat, so it's important that heat is not
lost.
i) Understand how the rate of diffusion is dependent on these properties and can be
calculated using Fick’s Law of Diffusion.
The rate of diffusion is dependent on three properties of the gas exchange surface. These
are:
Surface area: rate of diffusion is directly proportional to surface area. As one increases,
the other does.
Concentration gradient: rate of diffusion is directly proportional. The concentration
gradient is essentially the difference in gradients across the gas exchange surface.
Thickness of the gas exchange surface: rate of diffusion is inversely proportional.
When you combine these factors, you get Fick’s Law.
, Rate of diffusion is proportional to… surface area x difference in concentration / thickness of
the gas exchange surface
iii) Understand how the structure of the mammalian lung is adapted for rapid gaseous
exchange.
The alveoli in the lungs provide a large surface area for exchange of gases between the air
and blood. The alveoli is adapted to have a large surface area, numerous capillaries, and
thin walls of both to reduce the distance between the air in the alveolus and blood in the
capillaries. It also maintains a concentration gradient due to the ventilation of the alveoli
and constant flow of blood.
2.2 i) Know the structure and properties of cell membranes.
A cell membrane is composed of a phospholipid bilayer. The structure of a phospholipid is
similar to a triglyceride, which is three fatty acids and a glycerol. In a phospholipid, there are
two fatty acids, a glycerol, and a negatively charged phosphate group replaces the third
fatty acid.
The phosphate head of the phospholipid is polar,
which makes it attract other polar molecules like
water, making it hydrophilic. The fatty acid tails are
nonpolar, and so hydrophobic.
Due to these properties, phospholipids arrange
themselves so that their tails do not come into contact
with water. They may arrange themselves into
micelles (spheres), or a bilayer, which is often
favoured as the tails may be too bulky to fit in a
micelle.
The phospholipid bilayer makes up the fluid mosaic
model.
Edexcel Biology Salters-Nuffield A AS/A-Level
Key:
Definitions are in turquoise
Core practicals are in orange
Equations are in green
Topic 2: Genes and Health (GAH)
2.1 i) Know the properties of gas exchange surfaces in living organisms (large surface area
to volume ratio, thickness of surface, difference in concentration). p.60
Living organisms have to exchange substances with their surroundings. This can be like
taking in oxygen from the air, and releasing carbon dioxide. Substances that diffuse in or out
of a cell move down a concentration gradient, which is maintained by constant diffusion.
Smaller organisms that are unicellular have their whole cell membrane as an exchange
surface, but as an organism gets bigger, more exchange needs to take place to meet the
organism’s metabolic needs. Larger organisms have more trouble absorbing substances as
they have a low surface area to volume ratio. For example, a mouse would have a large
surface area but low volume, but an elephant would have a small surface area and high
volume.
Larger organisms can’t only rely on diffusion to fulfill their cell's needs, so they often have
adapted other organs to increase the surface area for exchange. It's also worth noting that
those with a high surface area to volume ratio lose heat, so it's important that heat is not
lost.
i) Understand how the rate of diffusion is dependent on these properties and can be
calculated using Fick’s Law of Diffusion.
The rate of diffusion is dependent on three properties of the gas exchange surface. These
are:
Surface area: rate of diffusion is directly proportional to surface area. As one increases,
the other does.
Concentration gradient: rate of diffusion is directly proportional. The concentration
gradient is essentially the difference in gradients across the gas exchange surface.
Thickness of the gas exchange surface: rate of diffusion is inversely proportional.
When you combine these factors, you get Fick’s Law.
, Rate of diffusion is proportional to… surface area x difference in concentration / thickness of
the gas exchange surface
iii) Understand how the structure of the mammalian lung is adapted for rapid gaseous
exchange.
The alveoli in the lungs provide a large surface area for exchange of gases between the air
and blood. The alveoli is adapted to have a large surface area, numerous capillaries, and
thin walls of both to reduce the distance between the air in the alveolus and blood in the
capillaries. It also maintains a concentration gradient due to the ventilation of the alveoli
and constant flow of blood.
2.2 i) Know the structure and properties of cell membranes.
A cell membrane is composed of a phospholipid bilayer. The structure of a phospholipid is
similar to a triglyceride, which is three fatty acids and a glycerol. In a phospholipid, there are
two fatty acids, a glycerol, and a negatively charged phosphate group replaces the third
fatty acid.
The phosphate head of the phospholipid is polar,
which makes it attract other polar molecules like
water, making it hydrophilic. The fatty acid tails are
nonpolar, and so hydrophobic.
Due to these properties, phospholipids arrange
themselves so that their tails do not come into contact
with water. They may arrange themselves into
micelles (spheres), or a bilayer, which is often
favoured as the tails may be too bulky to fit in a
micelle.
The phospholipid bilayer makes up the fluid mosaic
model.
