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Revision for AS OCR Biology Topic 2 (Cell Membranes)
Roles of the Cell (Plasma) Membranes:
Controls what substances go into and out of the cell partially permeable membrane.
For cell signaling and cell recognition.
Roles of the Membranes within cell:
Controls what substances go into and out of the organelle.
Membrane around organelles divide the cell into different compartments to make
different functions more efficient.
Membranes around organelles are highly folded to increase surface area and make
chemical reactions more efficient.
Phospholipid bilayer structure
Roles of the components of the cell
Bilayer acts as a barrier between cell component and external environment.
Is fluid so that cell component can move around within the membrane.
Is permeable to nonpolar substances such as oxygen and fatty acids.
Is permeable to small polar substances such as water, ethanol and carbon dioxide.
Is impermeable to large polar molecules and charged particles.
Stabilises the phospholipid bilayer by binding to the polar head and the nonpolar tail
Reduces permeability of polar substances and charged particles.
Stabilises the bilayer by making hydrogen bonds with the surrounding water molecules.
Act as receptors for cell signaling.
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Are transmembrane that act as carrier proteins and channel proteins.
Act as receptors for chemicals made by other cells.
Affect on the structure of phospholipid bilayer and the permeability:
Temp. below 0 degrees:
Phospholipds can't move as they have no energy, membrane is rigid. Increased permeability.
Temp. between 0 degrees and 45 degrees:
Phospholipids have gained kinetic energy and can move around, partially permeable membrane. As
the temp. increases the phospholipids gain more kinetic energy and move around more, this
increases the permeability.
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Releases the molecule on the other side of the membrane.
Channel proteins form pores in the membrane for charged particles to diffuse through. Different
channel proteins facilitate the diffusion of different charged particles.
Active transport uses energy (ATP) to moves substances across a conc. Gradient
Cells take in substanes by endocytosis. Some molecules are too large to be taken in through the
carrier and channel protiens. A cell can surround a substance with a section of its plasma