2.1.5 Biological Membranes Flashcards
What model and why is the plasma membrane described as?
Fluid mosaic model
- ‘Fluid’ = phospholipids are constantly moving around
- ‘Mosaic’ = proteins are scattered throughout
- ‘Model’ = best representaion based on evidence available - may be updated if new information is discovered
What components make up the plasma membrane?
- phospholipids
- glycoproteins
- glycolipids
- cholestrol
- intrinsic proteins
- extrinsic proteins
Describe the structure and function of phospholipids
- form a bilayer - hydrophobic fatty acid tails face inwards, hydrophilic phopshate heads face outwards
- allows lipid-soluble and non polar substances to enter + leave the cell
- prevents large molecules, water-soluble molecules, and polar substances from entering + leaving the cell
Describe the structure and function of glycoproteins
- proteins with carbohydrate molecules attached
- cell adhesion + receptors for cell signalling (neurotransmitters and hormones)
Describe the structure and function of glycolipids
- phospholipids with sugar molecules attached
- act as recognition sites and antigens - allows the body to detect which cells are self or non-self
Describe the structure and function of cholesterol
- lipid which slots in between the phospholipid tails
- increases packing of the membrane - regulating stability + fluidity
Describe the structure and function of intrinsic proteins
- proteins which span both bilayers of the membrane
- act as channels or carrier proteins & transport water-soluble molecules
Describe the structure and function of extrinsic proteins
- proteins found on the surface of the membrane
- examples are enzymes
Describe the structure and function of channel and carrier proteins
Channel Proteins
- transport of ions by facilitated diffusion
- act as “pores” in the membrane + allow specific ions through
- Can be gated so they only open/close in specific circumstances
Carrier Proteins
- facilitated diffusion of large molecules
- only allow specific large molecule to pass through
- changes shape and transfers the molecule to the other side of the membrane
- active transport of water soluble molecules and charged ions using ATP
What is the thickness of the plasma membrane?
7nm
Jobs of membranes outside + inside cells
Inside
- produce different compartmenmts in cells
- sites of chemical reactions
- provide attachment sites for enzymes
- provide attachment sites for pigments
- form vesicles for the transport of proteins
- allow cellular compartments to have different conditions
Outside
- seperates cell contents from the external environment
- cell signalling
- cell rocignition
Define cell signalling
Processes that lead to communication and coordination between cells, so that they can work together to trigger a response
Explain cell signalling
- Receptor in plasma membrane picks up signals - in form of a hormone or chemical mediator binding to the receptor
- signals secreted from cell and travel in blood
- binding brings about actions within cell
Explain key aspects of hormone receptors
- protein and glycoprotein receptors stick out target cells
- specific shape
- shape is complementory to shape of specific signalling molecule
- hormone binds to receptor - cell responds in particular way
What does cell signalling allow?
- communication between cells
- cells to coordinate actions
- trigger reponses inside cells
- cell recognition+ identification
Explain cell signalling in the context of controlling high blood glucose levels
- insulin hormoner releasedwhen blood sugar increases
- insulin receptors on liver + muscle cells bind insulin - cells take in more glucose to reduce blood glucose concentration
- insulin receptors have specific shape - complementory to shape of insulin molecule
Define and explain key aspects of diffusion
The net passive movement of molecules down a concentration gradient from a higher to lower concentration
- passive - doesn’t require ATP
- will continue until equilibrium between two areas
-faster over short distances - slower over long
- happens in gases + liquids - particles have kinetic energy
What molecules diffuse accross the phospholipid bilayer?
- lipid soluble e.g. steroid hormones
- small + non polar e.g. CO2, O2
- small + polar e.g. H20
What factors affect the rate of diffusion and why?
- temperature = more kinetic energy
- concentration gradient = steeper - faster rate of diffusion
- stirring motion = more kinetic energy
- surface area = more space for molecules to move accross
- diffusion distance - smaller distance to move
- size of molecule - larger molecules diffuse smaller than smaller ones
Why can’t large or polar membranes, as well as charged ions cross a membrane?
Can’t pass througfh phospholipid bilayer - require carrier + channel proteins for facilitated diffusion
Key aspects of facilitated diffusion
- there is a concentration gradient
- no ATP required
- protein + carrier proteins present
- polar molecules + ions have kinetic energy
Explain facilitated diffusion in channel + carrier proteins
Channel Proteins
- act as pores
- some only allow specific ions through e.g. Na+ Ca2+
- can be gated
Carrier Proteins
- only allow specific large molecules through e.g. glucose
- when molecule binds, it changes shape and transfers it to the other side
Define and explain key aspects of active transport
The movement of molecules or ions against a concentration gradient, from a lower to high concentration. using ATP and a transport/carrier protein
- ATP required
- only carrier proteins required
- moves substances against concentration gradient
- selective process
How is one way flow ensured in active transport?
- ATP changes shape of carrier proteins
- energy changes shape and releases molecule
