Topic 4 Cell membranes and Transport Flashcards
Features of cell surface membrane
- 7nm thick
- partially permeable
- controls movement of substances in and out of the cell
- contains: phospholipid, cholesterol, proteins, glycolipids and glycoproteins
Why is the term ‘fluid mosaic’ used to describe a cell membrane
fluid: components can move freely through diffusion
Mosaic: protein molecules are scattered
Phospholipid structure
- 1 glycerol & 1 phosphate group, 2 fatty acids
- (1 fatty acid in triglyceride is replaced by a phosphate group)
- Hydrophillic head (polar)
- Hydrophobic tail (insoluble, polar)
Phospholipid function
- Provide stability
- maintain fluidity
Cholesterol structure
- small molecule
- hydrophobic tail
- hydrophillic head
- not found in prokaryotes
- fits between phospholipid molecules
Cholesterol function
- Regulates fluidity of membrane
- stabilised the membrane (the hydrophobic layer)
- blocks passage of very small ions through the membrane
Types of membrane protein
- channel proteins
- carrier proteins
Function of membrane proteins
passage for:
- ions
- charged molecules
- polar molecules
- large molecules
Difference between channel and carrier protein
channel protein:
- highly specific
- channel is water filled (moves through diffusion)
- only for facillitated diffusion
- can be gated: open or close
carrier protein:
- highly specific
- conformational change occurs when it interacts with the molecule
- facilitated diffusion/ active transport
- sites open alternately
Function of glycolipid/ glycoprotein
- interacts with water to stabilise membrane structure
- cell to cell adhesion
- cell recognition
- (only glycoproteins) receptor for cell signalling
process of cell signalling
- ligands are secreted from cell
- ligands travel via blood stream to target cells
- ligands binds to cell surface receptors
- receptor activates G protein
- G protein triggers production of secondary messngers by enzymes
- secondary messengers trigger enzyme cascade
- enzyme carry out specific response
Type of ligands
- water soluble ligands (cannot pass through cell membrane)
- lipid-soluble ligans (can directly diffuse across the cell membrane)
What can be done for less permeability?
- more cholesterol=small products can’t pass
- saturated phospholipid
- less transport proteins
What are 5 mechanisms of transport?
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
- endocytosis/ exocytosis
Simple diffusion
Net movement of molecules for a region of high to low concentration down the concentration gradient until equilibrium
- no ATP required (passive)
- occurs across a phospholipid membrane
- lipid soluble, uncharged, small substances
Factors that affect rate of simple diffusion
- Concentration gradient
- Temperature
- Nature of molecules
- surface area to volume ratio
Facillitated diffusion
Diffusion through transport proteins from a region of high to low concentration down the concentration gradient
- involves channel and carrier proteins
- large, soluble, charged, polar molecules
Factors that affect rate of facillitated diffusion
- Concentration gradient
- Temperature
- Number of transport proteins available
- surface area of the membrane
Osmosis
Net movement of molecules for a region of high to low water potential down the water potential gradient until equilibrium
Factors that affect rate of osmosis
- concentration of the solute/ solution
- Pressure
Active transport
Movement of molecules or ions through carrier proteins from a region low to high concentration against the concentration gradient
-involves carrier protein
Endocytosis and exocytosis
endocytosis:
- into the cell
- eg phagocytosis, pinocytosis
exocytosis:
- substances packaged into secretory vesicles
- fuse with the cell surface membrane
- releace contents
