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AQA Biology A level Membranes Essay
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The importance of membranes in the functioning of cellsThe structure of the cell membrane can be described as a fluid mosaic. The term ‘mosaic’
indicates that several different components make up the cell membrane, like tiles in a mosaic.
These components are: phospholipids, each made of a hydrophilic head and a hydrophobic tail,
that arrange themselves to form a bilayer with all the heads facing outwards towards the
cytoplasm and the tissue fluid and all the tails sandwiched inside, away from the water ;
cholesterol, a lipid that lodges itself in between phospholipids and decreases the fluidity of the
membrane; proteins immersed in the bilayer that serve a variety of purposes such as channels,
carriers, receptors, enzymes etc. The term ‘fluid’ indicates that these components do not occupy a
fixed position but can diffuse laterally and switch position with each other.
The structure of the membrane is important to explain how diffusion happens in and out of cells.
Due to the hydrophobic tails in the phospholipid bilayer, water soluble substances such as ions or
polar molecules cannot diffuse across the bilayer without involvement of proteins. However, small
and uncharged molecules like oxygen and CO2 can simply diffuse across the phospholipid bilayer
down their concentration gradients. Small ions, charged and polar molecules need to use
membrane proteins to cross the membrane, using facilitated diffusion. Ions diffuse down their
concentration gradient through channels, pores filled with water that are specific to a certain ion.
Other water soluble molecules such as glucose will diffuse through a carrier, an integral
membrane protein with a binding site complementary to glucose. The carrier alternatively opens
on either side of the membrane, thus allowing glucose to bind on the side of higher concentration
and be released on the side of lower concentration. Like other forms of diffusion, carriers do not
require energy so no hydrolysis of ATP is needed.
The membrane of the thylakoids in chloroplasts is important during the light dependent stage of
photosynthesis. Here photosynthetic pigments, including chlorophyll A, are immersed in the
membrane forming two photosystems. Each pigment absorbs a photon at a specific wavelength
causing an electron to become excited and jump to a higher energy level. When the electron
returns to its resting energy level it emits energy which is transmitted to another pigment by
resonance energy transfer, again causing an electron to become excited and so on until the
energy reaches the molecule of chlorophyll A which is central to the photosystem. Each time the
energy is transferred between pigments a little energy is lost as heat according to the second law
of thermodynamics, this ensures that the energy reaching chlorophyll A will not damage it, so
overall the accessory pigment have a photoprotective effect. They also make it possible to absorb
a wider spectrum of wavelengths which can be used for photosynthesis. When chlorophyll A
becomes excited, an electron in its Mg ion jumps to a higher energy level and is lost by
chlorophyll, this is photoionisation of chlorophyll. Chlorophyll will get an electron back from a
molecule of water, causing the photolysis of water which produces O2 (2H2O 🡪 4H+ + 4e- + O2).
The electron lost by chlorophyll is accepted by a primary acceptor and then passed down an
electron transport chain, a series of redox proteins embedded in the thylakoid membrane. Each
electron transporter is reduced by the electron and oxidised when it passes it on, and uses the
energy of the electron to pump protons inside the thylakoid space. After the electron transport
chain the low energy electron is accepted by another photosystem where chlorophyll A has been
ionised by a photon of light. The electron lost by this photosystem (PSI)is passed down a shorter
ETC and eventually to the enzyme dehydrogenase that uses it to reduce NADP to NADPH, a high
energy electron carrier that can be used as a reducing agent. The protons accumulating in the
thylakoid space can diffuse out through the ATP synthase complex that uses the potential energy
of the proton gradient to synthesise ATP from ADP and Pi.
The ATP and NADPH produced in the LDR are important for the LIR. They diffuse in the stroma
reaching the enzymes and substrates involved in the Calvin cycle. Both NADPH and are needed
to reduce 3-phosphoglycerate to triose phosphate, NADPH providing high energy electrons and H
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