Module 2.5 - Biological Membranes Flashcards
Role of membranes
Separate cell components from cytoplasm
Separate cell contents from outside environment
Hold components of some metabolic pathways in place
Cell recognition and signalling
Regulate transport of materials in and out of cells
Describe the structure of phospholipids
Hydrophilic phosphate head
2 fatty acid/lipid hydrophobic tails
Why is the phospholipid head hydrophilic?
Charges are unevenly distributed so it can interact with water more easily
Why are the phospholipid tails hydrophobic?
Charges are evenly distributed so will repel water molecules
Why is the membrane fluid?
Lack of bonds between phospholipids means they can slide around each other, like a fluid
Why is the membrane mosaic?
Embedded proteins create a mosaic pattern
Thickness of phospholipid bilayers
7-10nm thick
How are membranes partially permeable?
Water diffuses through the bilayer
Some membranes contain aquaporins (protein channels which allow water through) which make them more water permeable
Functions of glycoproteins
Cell signalling (allow recognition from immune system)
Receptors for hormones/drugs
On surface of pathogens are antigens, immune system can recognise them as foreign
Bind cells together in tissues
Functions of glycolipids
Cell signalling (allow recognition from immune system)
Receptors for hormones/drugs
On surface of pathogens are antigens, immune system can recognise them as foreign
Functions of intrinsic proteins
Transport for small water-soluble molecules/ions and larger molecules
Functions of cholesterol
Stability
Plugs gaps between fatty acid tails and makes membrane less permeable to water molecules and ions
Restricts too much movement within phospholipid layer
Function of actin microfilaments
Anchors proteins and stops them moving around too much
Effect of high temperature on membranes
More kinetic energy for molecules so they move faster
Membrane becomes more fluid so more permeable and more substances can enter and exit the cell
How do hormones bind to cells?
Hormone receptors
Cells with these specific hormone receptors are target cells
Hormone molecules bind to receptors on target cell surface membrane as they are complementary in shape
Binding stimulates a response in the target cell
How can medicinal drugs work?
Can be made complementary to receptor molecules so they block them
Some act like natural neurotransmitters that some people can’t make
Beta-blockers stop increasing heart rate for people at risk of a heart attack
How do poisons work?
Bind with receptors
Blocks effect of natural neurotransmitters or hormones
Substances that can simply diffuse across a membrane
Lipid-based molecules (fat soluble so dissolve and diffuse readily across the bilayer e.g. steroid hormones)
Very small molecules (O2 and CO2 are small enough to pass between phospholipid molecules, water and urea are polar so they cross much slower)
Main points about channel proteins
Form pores in membranes
Hydrophilic inside pore
Specific to certain small water-soluble molecules or ions (e.g. sodium, calcium)
Can be gated, so can be opened or closed by a signal or voltage change across the membrane
Main points about carrier proteins
Carry larger molecules (e.g. glucose, amino acids)
Shaped for specific molecules to bind to
Molecule binding changes shape of protein
Causes molecule to be carried to and released on other side of membrane
No energy required
Examples of active transport
Mineral ions from soil t root
Glucose and amino acids from intestines to blood
How does active transport work?
Carrier proteins act as pumps
Complementary shaped molecule binds
Molecule can’t diffuse due to gradient
ATP provides energy needed for the protein to change shape and transport the molecule across the membrane
Shape change only allows the molecule to fit in the protein on one side of the membrane - won’t fit other side - stopping transport in wrong direction
Faster than diffusion
Phago
Solids
Pino
Liquids
