Plasma Membranes

Chapter 5

describe the relationship between polar and hydrophobic/hydrophilic

• polar phosphate heads= hydrophilic

• non-polar fatty acid tails= hydrophobic

describe the discovery that the Davson-Danelli model was incorrect

• it was found that proteins are distributed throughout the membrane in a mosaic pattern

• found that the membrane is fluid

• fluid-mosaic model for plasma membrane structure

describe the fluid mosaic model for plasma membranes

• proteins can move freely through the lipid bilayer

what is the movement of proteins dependant on in the membrane

the number of phospholipids with unsaturated fatty acids in the bilayer

what can diffuse through the phospholipid bilayer

• fat- soluble organic molecules

• polar molecules- (require proteins)

describe intrinsic proteins

• completely span the phospholipid bilayer

• main transport system of the membrane

• can form channels, carrier proteins or active pumps

describe extrinsic proteins

• on surface of bilayer or partially embedded in it

• provide mechanical support

• act as cell receptors

what are channel proteins

pores in the membrane that let ions diffuse through

describe carrier proteins

• change shape to let specific molecules through

describe glycoproteins

• carbohydrate molecules attached to membrane proteins

• can allow cell adhesion

• act as receptors for hormones and neurotransmitters

describe glycolipids

• carbohydrates attached to phospholipid molecules

• act as cell markers or antigens

describe cholesterol

stabilises the membrane and regulates its fluidity

describe membrane structure and permeability at 0 degrees

• permeability increases due to proteins unfolding and becoming deformed

• lower energy→ cannot move around as much

• if temperatures are low enough for ice crystals to form, they can puncture the membrane

• rigid → phospholipids are tightly packed together

describe membrane permeability and structure at 0-45 degrees

• partially permeable

• as temperature increases, components gain kinetic energy

• more fluid the membranes are, more substances it will allow through it

describe membrane structure and permeability above 45 degrees

• permeability rapidly increases

• proteins in membrane become denatured and begin to unravel

• water inside cytoplasm starts to expand, puts pressure on the cell membrane, creates gaps within the bilayer

describe membrane permeability with small hydrophobic molecules

permeable

describe membrane permability with small uncharged polar molecules

mostly permeable 

describe membrane permeability with large uncharged polar molecules

mostly impermeable, requires transport molecules

describe membrane permeability with ions

completely impermeable, requires transport proteins

what is passive transport

• movement of molecules or ions across membranes down the concentration gradient

• no energy requires

what is active transport

• movement of molecules or ions against the concentration gradient

• requires ATP

what is diffusion

net movement of particles down a concentration gradient; from a region of high concentration to a region of low concentration

what factors effect the rate of diffusion

• temperature

• concentration

what molecules can easily diffuse

• small molecules

• non-polar molecules

• lipid-based molecules (e.g. hormones)

• molecules in high concentrations

What is ficks law

rate of diffusion is proportional to: SA x difference in concentration/ length of diffusion path (membrane thickness)

what happens in diffusion if there is a small SA: vol ration

cell cannot get enough raw materials fast enough

what is facilitated diffusion

• passive process

• carrier proteins or protein channels allow molecules through

• specific molecules can fit into carrier

• carrier flips to allow the molecule to pass through the membrane

how do slightly polar molecules travel through membranes

diffuse through hydrophilic channels in channel proteins

describe channel proteins

• pores in the membrane

• can be specific to one molecule and/or gates

• channel is water filled with allows polar molecules through

• use for diffusion of sodium, calcium and potassium ions into/out of neurones

what does it mean for channel proteins to be gates

can be open or closed under certain condition

describe carrier proteins

• specific to particular molecules

• when a molecule binds to a protein, the protein changes shape to allow the molecule through it

• used for glucose and amino acids

• can also be used for active transport

what are the differences of carrier proteins in active transport to those used in facilitated diffusion

• they only work one way

• they use ATP

• they are specific to certain molecules/ions

• they carry molecules against the concentration gradient

give some examples of active transport

• uptake of glucose and amino acids in the small intestines

• absorbtion of mineral ions by plant roots

• excretion of hydrogen ions and urea by the kidneys

• exchange of sodium and potassium ions in the neurones and muscle cells

describe how carrier proteins move molecules across membranes

1. carrier proteins takes in molecules from outside plasma membrane

2. glucose molecules bind to carrier proteins- ATP attaches to plasma membrane on the inside of the cell

3. protein changes shape- other side opens and allows molecules to exit into the inside of the cell

what are the two types of bulk transport

• endocytosis

• exocytosis

describe endocytosis

• bulk transport of materials into the cell

• split into phagocytosis (solids) and pinocytosis (liquids)

describe exocytosis

bulk transport of materials out of the cell

what is ATP required for

• movement of vesicles along the cytoskeleton

• changing the shape of cells to engulf materials

• fusion of cell membranes as vesicles form or as they meet the cell surface membrane