Chapter 5: Plasma Membranes Flashcards
What are the roles of plasma membranes?
- partially permeable so they can control movement of substances in the organelle
- barrier between organelles and the cytoplasm making chemical reactions more efficient
Compartmentalisation allows different environments within the cell to be maintained e.g. chemical gradients - sites of chemical reactions with folding to increase SA
- sites of cell communication (cell signalling)
- smaller membranes within organelles to separate contents
- transport of substances between different areas of the cell
What makes up the bilayer and how is it adapted to its function?
Phospholipids
- hydrophilic phosphate head (polar)
- glycerol molecule
- 2 hydrophobic fatty acid chains - saturated/unsaturated (non-polar)
Bilayer arranged so that the phosphate heads interact with the aqueous environment
Hydrophobic core makes the bilayer semi permeable
Fluid mosaic model
Model of the plasma membrane structure
- fluid: kinks in the fatty acid tails allow the phospholipids to space out to give flexibility and permeability
- mosaic: extrinsic and intrinsic proteins of various shapes and sizes embedded throughout
What is the role of cholesterol
Regulates fluidity and mechanical stabilty
Stops crystallisation phospholipids
- cholesterol molecules found between phospholipids bind to the hydrophobic tails, causing them to pack closer together which restricts movement
Makes membrane more rigid
What is the role of glycolipids?
- lipid with a carbohydrate chain attached
- Act as cell markers/antigens for cell recognition by the immune system to detect foreign cells
- Recognition site e.g. for cholera toxins
What is the role of glycoproteins?
Intrinsic proteins
Cell adhesion - cells join together forming tight junctions in certain tissues
Cell Signalling - receptors for neurotransmitters at nerve cell synapses
Or receptors for peptide hormones
Binding of some drugs
What are the 2 types of proteins and what is the differences between them?
Intrinsic proteins:
- transmembrane (embedded through both layers of a membrane)
- eg. Channel and carrier proteins
- amino acids with hydrophobic R-groups on external surface keep them in place
Extrinsic proteins:
- present only on one side of the bilayer
- have hydrophilic R groups and interact with the polar heads or intrinsic proteins
What is the role of Channel Proteins?
Provide a hydrophilic channel for passive movement of polar molecules and ions down a concentration gradient
(Facilitated diffusion)
What is the role of Carrier Proteins?
Active and passive transport
Often involves the shape of the protein changing
How does temperature effect membrane permeability?
Increasing temperature:
- increases kinetic energy
- more movement of phospholipids and molecules
- more fluidity
- if temperature continues to increase, cell will break down completely
- carrier and channel proteins denature
Decreasing temperature:
- very low kinetic energy
- could lead to crystalisation of the bilayer
- permeability at low temperature determined by proportion of saturated to unsaturated fatty acids (saturated fatty acids more compressed, more rigid membrane)
How do solvents effect membrane permeabilty?
Organic solvents dissolve membranes and disturb cells as it is less polar than water
Increased fluidity and permeability
Eg. Alcohol:
- non-polar alcohol molecules can enter membrane
- its presence between the phospholipids disrupts the membrane
How can beetroot cells help investigate permeability? Method?
Pigment betalain is released when the membrane is disrupted - easy to observe
- Cut 5 pieces - equal sizes
- Wash thoroughly then placw in 100ml distilled water
- Place in different water baths - temperatures with 10⁰ intervals
- Take samples from each
- Calibrate colorimeter using distilled water
- Put through colorimeter using a filter of a complementary colour (e.g. blue) to find absorbance
- Repeat 3 times to find the mean
What is the definition of diffusion?
The net movement of paticles from a region of high concentration to a region of low concentration
Passive process (down a concentration gradient)
- across a partially permeable membrane
Small non-polar substances e.g. O2 or CO2 pass directly through phospholipid bilayer
What factors effecting diffusion?
Temperature
- higher temperature leads to more kinetic energy of particles
- increased diffusion rate
Concentration
- higher concentration gradient leads to a higher diffusion rate
Surface Area
- higher surface area increases rate of difusion
Thickness
- greater thickness decreasee rate of diffusion
Charge of particles
- charged particles cannot diffuse across plasma membrane
Size of particles
Facilitated diffusion
Diffusion across a selectively permeable membrane through channel proteins or carrier proteins
Allows large/ polar molecules / ions to pass through e.g. glucose
Passive process - does not require ATP (down a concentration gradient)
Active transport
Net movement of molecules / ions from a region of low concentration to a region of high concentration (against a concentration gradient)
Requires ATP
- Happens in carrier proteins e.g. sodium potassium pump, glucose in villi in small intestine, ions from soil into plant root hair cells
- bulk transport
Active transport in carrier proteins
Selective process
From outside to inside:
- Molecule/ion being transported binds to receptors on carrier protein outside the cell
- ATP binds to channel protein inside the cell and hydrolysed -> ADP and phosphate
- Phosphate molecule binds to carrier protein to change shape and open up to inside of cell
- Molecule/ion released to inside cell
- Phosphate molecule is released and recombines with ADP -> ATP
- Carrier protein returns to original shape
Bulk transport
Form of active transport of large molecules - too large to fit through channel/carrier proteins e.g. hormones
- Endocytosis
- Exocytosis
Requires ATP
Endocytosis
Bulk transport of material into cells
Phagocytosis- for solids
Pinocytosis - for liquids
1. Part of cell surface membrane envelopes particles it comes into contact with
2. fuses with it to form a vesicle
3. Pinches off and moves into cytoplasm for further processing
Exocytosis
Bulk transport of material out of cells
- vesicle containing particles go be exported forms in cytoplasm
- Vesicle travels to plasma membrane and fuses with it
- Contents are released to outside of the cell
Osmosis
Net movement of water molecules from a region of high water potential to a region of low water potential Across a partially permeable membrane Passive process (does not require ATP) Down a concentration gradient - isotonic Solutions with same osmotic pressure
Water potential
Pressure exerted by water molecules when they collide with a membrane or container (kPa or Pa)
Pure water has the highest possible water potential of OkPa
High water potential (low solute concentration)- less negative
Low water potential (high solute concentration)- more negative
Osmosis in animal cells
in solution of higher water potential - hypotonic
- water moves into cell, increasing hydrostatic pressure
- will swell and burst (lysis)
In solution of lower water potential - hypertonic
- water leaves cell, reduces volume
- cell will ‘pucker’ (crenation)
Osmosis in plant cells
in solution of higher water potential - hypotonic
- water moves into cell, increasing hydrostatic pressure
- will push membrane against cellulose cell walls, eventually resist further entry of water (turgid)
In solution of lower water potential - hypertonic
- water leaves cell, reduces volume of cytoplasm
- pulls cell-surface membrane away from cell wall (plasmolysed)
