Cellular transport - Biology exam - AS

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The 4 types of cellular transport are: diffusion, osmosis, active transport and co-transport.
Diffusion and osmosis both involve the passive transport of substances and molecules from an
area of high concentration to an area of low concentration. Active transport and co-transport
both involve going against the concentration gradient from a low concentration to a high
concentration. Unlike diffusion and osmosis, it is an active process which involves metabolic
energy in the form of ATP. Comparing and contrasting them, I think it is logical to compare
diffusion with osmosis and active transport with co-transport.
Diffusion is the movement of molecules which are in a liquid or gaseous state. In contrast to
osmosis, it involves the movement of only water across a partially permeable membrane. For
example, with perfume or air freshener, the gas molecules diffuse into the air, spreading the
aroma. For osmosis, water moves into root hair cells. Diffusion also helps in the exchange of
gases during respiration, photosynthesis and transpiration while as osmosis influences the
distribution of nutrients and the release of metabolic waste products.
Facilitated diffusion is similar to diffusion in that both involve the movement of molecules down
their concentration gradient and is carried out without any input of energy. However, in
facilitated diffusion, the movement of molecules will only take place if it is facilitated by a special
protein carrier. An example of facilitated diffusion is the transport of glucose into the cell.
Glucose is too large to diffuse through the membrane but there is a special carrier in the
membrane that can transport glucose.
Active transport and co-transport both use energy and carrier molecules. Active transport is
very selective and involves specific substances while as co-transport involves 2 molecules.
Co-transport also occurs in two ways; antiport and symport. For example, in symport, sodium
ions and specific sugar simultaneously bind to the protein on the outside of the cell. In antiport,
the sodium and sugar bind onto the same protein but in this case, bind on opposite sides of the
membrane and are moved in opposite directions. For active transport, the sodium potassium
pump helps explain how it occurs in relation to co-transport. In the sodium potassium pump, 3
sodium ions bind to the protein channel and ATP provides energy to change the shape of the
channel which in turn drives the ions through the channel. The phosphate molecule from the
ATP sticks to the protein which makes it transfer the ions to the other side. Potassium ions
then bind to a specific shape in the protein which again changes its shape and transfers it across
to the cytoplasm side.
Comparing facilitated diffusion with active transport, one similarity is that they both involve a
carrier protein. Facilitated diffusion occurs down a concentration gradient while active transport
occurs against a concentration gradient which means that facilitated diffusion does not require
metabolic energy while active transport does.

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