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Revision summary B3a
A solution of pure water is separated from a concentrated sugar solution, by a partially permeable
membrane. The molecules will flow both ways because water molecules move about randomly all the
Leaves are adapted to maximise the amount of carbon dioxide that gets to their cells, by:
The underneath of the leaf is an exchange surface covered in little holes called stomata.
This lets the carbon dioxide diffuse in through.
Oxygen and water vapour also diffuse out of the stomata.
The size of the stomata are controlled by guard cells. These close the stomata if the plant is
losing water faster than its being replaced by the roots. Without the guard cells, the plant
would soon wilt.
Flattened shape increases the area of exchange surface, making it more effective.
The walls of the cells inside the leaf increases the area of this exchange surface. The air
spaces inside the leaf increase the area of this surface to there's a higher chance for carbon
dioxide to get into the cells.
The main substance that diffuses out of leaves is water vapour.
The conditions the evaporation of water needs from leaves to happen most quickly in are hot, dry
and windy conditions.
Ventilation is breathing in and breathing out. When you breathe in, your intercostal muscles and
diaphragm contract, the thorax volume increases, which decreases the pressure and draws air in.
When you breathe out, your intercostal muscles and diaphragm relax, the thorax volume decreases,
which increases the pressure, so air is forced out.
Modern ventilators work by pumping air into the lungs. This expands the ribcage- when they stop
pumping; the ribcage relaxes and pushes air back out of the lungs.
The alveoli are specialised to maximise the diffusion of oxygen and CO2. They have:
An enormous surface area
A moist lining for dissolving gases
Very thin walls
A good blood supply
The structure of a villus is ideal for its job, because they have a single layer of surface cells and a very
good blood supply to assist quick absorption.
It's shaped like this because it gives
the plant a big surface area for
absorbing water and mineral ions
from the soil. Most of which is
absorbed by the plant is absorbed
by the root hair cells.
Two main differences between
active transport and diffusion:
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Active transport occurs against the concentration gradient, whereas diffusion doesn't.
Active transport needs energy from RESPIRATION.
Phloem tubes transport food substances, made in the leaves to growing regions and storage organs
of the plant. The transport goes in both directions.
Xylem tubes carry water and minerals from the roots to the stem and leaves in the transpiration
The transpiration stream
Transpiration is caused by the evaporation and
diffusion of water from inside the leaves.…read more
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Artificial blood products can help keep people alive in an emergency. When someone loses a lot of
blood, their heart can still pump the remaining red blood cells around as long as the volume of their
blood can be topped up. Artificial blood is a blood substitute, and is safe as long as no air bubbles get
into the blood. It can keep people alive if they lose 2/3 of their red blood cells.
The heart can be replaced with artificial parts.…read more
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Dialysis has to be done regularly to keep the concentrations of dissolved substances in the blood at
normal levels and to remove waste substances.
In a dialysis machine the person's blood flows alongside a selectively permeable barrier, surrounded
by dialysis fluid. It's permeable to things like ions and waste substances but not big molecules like
the membranes in the kidney.
The dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood.…read more