2.5 - Biological Membranes Flashcards
What are the roles of membranes within cells and at the surface of cells?
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.
Explain why phospholipids form a bilayer in the plasma membrane.
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.
Why is the cell-surface membrane described as a fluid-mosaic?
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.
What is the role of cholesterol in eukaryotic cells?
Regulates membrane fluidity.
Describe the arrangement of phospholipids, proteins and carbohydrates in the bilayer.
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).
How does the plasma membrane differ between cell types? Use neurones, white blood cells, root hair cells and mitochondria as examples.
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.
How do polar and non-polar molecules pass through the membrane?
Polar molecules require proteins.
Non-polar molecules can diffuse directly through the bilayer.
Define facilitated diffusion.
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.
Compare and contrast simple and facilitated diffusion.
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).
Apart from surface area, concentration gradient and diffusion distance, what other two factors affect the rate of diffusion?
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.
Define osmosis.
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.
What is water potential?
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.
What is the water potential of pure water?
0 kPa.
What is plasmolysis and what causes it?
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.
Why doesn’t a plant cell burst?
When it fills with water the protoplast pushes against the cell wall and makes it turgid.
The cell wall prevents the membrane bursting.
