Biological Membranes Flashcards
Role of cell surface membrane/membranes within cells ?
- creates enclosed space separating internal environ from external environ/ organelles and cytoplasm (COMPARTMENTALISATION)
- form partially permeable barrier between cell and environment , between cytoplasm and organelles etc
- site of chemical reactions (membrane within cells)
- sites of cell signalling
What is the fluid mosaic model?
Explains how biological membranes are arranged to from cell membranes
Role and structure of phospholipids ?
HYDROPHOBIC TAIL - forms hydrophobic core (inner part of membrane)
HYDROPHILIC HEAD - forms outer part of membrane
Role : acts as a barrier to water soluble substances - non polar hydrophobic tail prevent polar molecules from passing across
- ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell
How can phospholipids act as signalling molecules ?
- moving within bilayer to activate other molecules
- being hydrolysed, releasing water soluble molecules that bind to specific receptors in cytoplasm
Role of Cholesterol?
STABILITY OF MEMBRANE
→ controls fluidity of membrane :
- stops phospholipid tails packing too close tgt - stops it from being too rigid at low temps, so cells survive at low temps
- At high temps, hydrophobic region of cholesterol bind to phospholipid tails - causing phospholipids to pack closer tgt —> less fluid
Makes membrane more impermeable - cholesterol fill in all gaps
Increases mechanical strength/stability of membrane
Role of glycoplipids and glycoproteins ?
Stabilise membrane by forming hydrogen bonds with surrounding water molecules
- Contain carbohydrate chains that exist on surface , so can act as RECEPTOR molecules for cell signalling
- some act as ANTIGENS for cell to cell recognition
- site where drugs/hormones and antibodies bind
Role of transport proteins ?
Create hydrophilic channels to allow ions/polar molecules to travel through membrane
- allow cell to control what substances enter/leave
2 types :
CHANNEL proteins
CARRIER proteins - change shape to transport substance
Each transport protein is specific to a particular ion or molecule
How does temperature effect permeability of cell membranes ?
At low temps : phosphilids don’t have a lot of energy so can’t move around - packed very closelytgt/rigid
- channel proteins and carrier proteins deform= more permeable
Ice crystals may pierce membrane making it more permeable when it thaws
- phospholipids can move around more as have more energy/not packed tightly tgt- partially permeable
- temp increases, lipids are more FLUID - INCREASING PERMEABILITY - any diffusion happens at higher rate
- changes in fluidity are reversible
At high temps , protein denature- disrupts structure of membranes - more permeable
- denaturation is irreversible
- phospholipid bilayer starts to melt =permeable
Water in cell expands and puts pressure on membrane
How does solvent concentration effect permeability of membranes ?
Increase permeability , as they dissolve lipids in membranes , so loses structure
What is diffusion?
The movement of a substance from high conc to low conc , due to random motion of molecules/ions
- down thee conc gradient
What factors effect rate of diffusion?
Steep conc grad : difference in conc of the substance on two sides
Temperature : molecule/ions have more kinetic energy , so move faster = increased rate of diffusion
Surface area : faster rate of diffusion , bc more particles can pass at one time
Thickness of surface - thinner diffusion distance, faster rate of diffusion
Properties of molecules/ions : larger molecules diffuse more slowly as require more energy to move
- non polar molecules diffuse quicker than polar molecules - SOLUBLE in non polar bilayer
What is facilitated diffusion?
Using carrier/channel proteins to diffuse through the phospholipid bilayer
- as larger polar molecules/ions cannot pass through phospholipids bilayer by normal diffusion - not soluble in non polar phospholipid bilayer
Function of carrier proteins in facilitated diffusion?
- move large molecules into/out cell down the conc grad
- Large molecule attaches to carrier protein
- Protein changes shape
- Releases molecule on opposite side of membrane
Function of channel proteins in facilitated diffusion?
They form pores in the membrane for charged particles to diffuse through
- different channel proteins facilitate diffusion of different charged particles
- channel proteins are ‘GATED’ - part of channel protein can move to open/close the pore
What is active transport ?
Movement of molecules/ions through cell membrane from lower conc to region of higher conc using ENERGY from respiration
- against conc grad
-requires carrier proteins - energy is used to change protein shape
Process of active transport ?
Molecule attaches to carrier protein , and protein changes shape
- molecule released on other side membrane
Energy used from ATP to move solute against conc grad
What is endocytosis ?
process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it
- bulk transport into cells
2 forms of endocytosis ?
PHAGOCYTOSIS : bulk intake of solid material by a cell
PINOCYTOSIS : bulk intake of liquids
What is exocytosis and the process?
process by which materials are removed from, or transported out of, cells (the reverse of endocytosis)
- Substances to be released (enzymes, hormones or cell wall building materials) packaged into secretory vesicles formed from Golgi body
- vesicles travel to cell surface membrane and fuse with cell surface membrane/release contents outside cell or into plasma membrane
USES ATP
What is osmosis?
the diffusion of water molecules from a dilute (higher water potential) solution to a more concentrated (lower water potential) solution across a partially permeable membrane
- down water potential gradient
Meaning of hypotonic , isotonic and hypertonic in animal cells?
HYPOTONIC : solution has higher water potential (dilute) than cell (water moves into cells -> cell bursts)
ISOTONIC : solution with same water potential as cell
HYPERTONIC : solution with lower water potential (concentrated ) than cell (water moves OUT cells , so shrinks )
What happens if cell is hypotonic /hypertonic in plant cells ?
HYPOTONIC : water moves into cell , so vacuole swells
- vacuole/cytoplasm push against cell wall —> cell is TURGID
- TURGID - provide support/strength to plant
HYPERTONIC : water moves out cell —> becomes FLACCID
Cytoplasm/membrane pull away from cell wall - plasmolysis
PRACTICAL : permeability of cell membrane
- Cut 5 equal pieces of beatroot/rinse to remove pigment caused by cell damage when cutting
- Place pieces in 5 test tubes , each with 5cm cubed of water
- Place each test tube in water bath at different temps for same length of time
- Remove pieces from liquid
- Use colorimeter to measure absorbance —> higher the permeability , more pigment released , higher absorbance
PRACTICAL : investigate water potential
- Prepare sucrose solutions by serial dilutions (0M, 0.2M , 0.4M , 0.6M, 0.8M, 1.0M)
- Use cork borer to cut potatoes into same sized pieces
- Measure mass of the potatoes (when in groups of 3)
- Place each group in one solution /leave for 20 mins
- Remove/pat dry with paper towel
- Weigh each group again/calculate % mass change /plot graph
PRACTICAL : investigate diffusion in model cells
- Make agar jelly with phenolpthalein/sodium hydroxide
- Fill beaker with HCl /use scalpel to cut out cubes of jelly /add to acid
- Cubes will turn colourless , as acid diffuses into jelly/neutralises NaOH
Can use this to investigate factors like :
SUFRACE AREA : cut the jelly into different sized cubes /Time how long it takes to go colourless
CONC GRAD : use different concs of HCL /Tim how long it takes to go colourless
TEMPERATURE : put HCL into test tubes , in different water baths /put jelly /time how long to takes to go colourless
Role of cell membrane receptors?
- receptor proteins have specific shapes,which only messenger molecules with a complementary shape can bind to
- cells that respond to particular messenger molecules are called TARGET CELLS
Why can’t polar/large molecules and ions move through membrane by normal diffusion?
Cannot diffuse through fatty acid tails
Why does the ‘fluid mosaic model’ describe cell membranes as ‘fluid’ and ‘mosaic’ ?
Fluid becuase :
- phospholipids/proteins can move around via diffusion
- phospholipids move sideways,within their own layers
- many different types of proteins interspersed throughout the bilayer move about within it
‘mosaics’ because:
The scattered pattern produced by the proteins looks like a mosaic when viewed from above
Why is active transport important?
- reabsorbtion of useful molecules/ions into blood
- loading of inorganic ions from soil into root hairs
Edocytlsis process?
Some substance too large to be taken into cells by carrier proteins
(Proteins,lipids, carbs)
- cell surrounds substance with section of its plasma membrane
- membrane pinches off to form vesicle inside cell
USES ATP
Examples of the role of cell membrane receptors in cell signalling?
GLUCAGON :
Hormone released when not enough glucose in blood
- bind to receptors on liver cells - cause liver cells to break down glycogen to glucose
ANTIHISTAMINES (drug):
Antihistamines block histamine receptors on cell surface - prevent histamine from binding to the cell/ stop inflammation
