Biology Paper 2 - 2.5 Biological Membranes

Partially permeable barrier.
Separates cell components from external environment.
Regulates transport in/out of cell.
Antigens for cell recognition.
Localisation of enzyme pathways.
Cell signalling.
Receptors for communication and signalling.
Site of chemical reactions.

Phospholipids are polar.
One layer has its hydrophilic heads pointing inwards, interacting with water in the cell.
Other layer has its hydrophilic heads pointing outwards, interacting with water outside of the cell.
Hydrophobic tails on both layers point inwards to the centre of the membrane.

Fluid - individual phospholipid molecules can move relative to one another, gives flexible structure that is constantly changing shape.
Mosaic - proteins embedded in the bilayer vary in size, shape and pattern.

Regulates membrane fluidity.

Phospholipids make up the bilayer so are throughout it.
Proteins are randomly spread out along the bilayer; integral proteins span membrane.
Carbohydrates and lipids are attached to proteins (glycoprotein and glycolipids).

Neurone - myein sheath, flattened cells wrapped around neurone. 20% protein, 76% lipid.
White blood cell - protein receptors on plasma membrane to recognise antigens of foreign cells.
Root hair cell - carrier proteins for active transport of nitrate ions into cell.
Inner mitochondrial membrane - differs from outer membrane. 76% protein, 24% lipid. Site of electron carriers and ATP synthase - needed for aerobic respiration.

Polar molecules require proteins.

Non-polar molecules can diffuse directly through the bilayer.

The net transport of a substance across a biological membrane from an area of higher concentration to an area of low concentration by means of a carrier molecule.

Simple: Passive (no ATP required), small molecules, lipid soluble, no carrier proteins, occurs down concentration gradient, lesser selective, diffuses directly through bilayer.
Facilitated: Passive (no ATP required), large molecules, ions, water soluble, carrier proteins, occurs down concentration gradient, selective (1 molecule for 1 carrier protein).

Temperature - an increase in temperature means an increase in kinetic energy, vice versa for decrease.
Size of diffusing molecule - small ions/molecules diffuse more quickly than larger ions/molecules.

The net movement of water molecules from an area of high concentration (of water) to a low concentration (of water) across a partially permeable membrane.
The net movement of water molecules from an area of high or less negative water potential to an area of low more more negative water potential across a partially permeable membrane.

A measure of the tendency of water molecules to diffuse from one area to another.
As a solute dissolves, the solvent becomes more negative and can dissolve less; the water potential is lowered.
The lower the water potential, the more negative the value in kPa.

0 kPa.

The pulling away of the plant cell membrane from the cell wall, as a result of the movement of water out of the cell by osmosis, due to the water potential of the external solution being higher than that of the cell.

When it fills with water the protoplast pushes against the cell wall and makes it turgid.
The cell wall prevents the membrane bursting.

The bursting and swelling of an animal cell, as the result of movement of water into the cell by osmosis, due to the water potential of the external solution being higher than the cell.

The shrinking of a cell, as the result of movement of water into the cell by osmosis, due to the water potential of the external solution being higher than the cell.

Hypertonic - has a higher concentration of solute than the cell.
Hypotonic - has a lower concentration of solute than the cell.
Isotonic - has the same concentration of solute as the cell.

The net movement of a solute against a concentration gradient using carrier proteins and energy, in the form of ATP.

Changes confirmation/shape of the carrier protein to carry molecule against concentration gradient.

Bulk movement of large molecules into cell.
Substance presses against plasma membrane.
Plasma membrane extends around substance.
Substance is enclosed in a vesicle.

Membrane bound vesicle moved to cell surface membrane.
Vesicle fuses with cell surface membrane.
Contents released from vesicle and cell.

Temperature - increase/decrease of kinetic energy of membranes and substances passing through, relate temperature change to change in rate.
Increase in temperature = increase in kinetic energy = increase in permeability.
Decrease in temperature = decrease in kinetic energy = decrease in permeability.
At high temperatures proteins denature and drop out of bilayer, permeability increases.
Cytoskeleton denatures at high temperature, permeability increases.
Cholesterol - regulates fluidity in response to temperature change.