Module 2 - Plasma Membranes

What can the structure of cell membranes be described as?

fluid-mosaic model - fluid - phospholipids form bilayer in which phospholipids molecules are constantly moving - membrane is 7nm thick
mosaic - proteins of differents sizes + shapes embedded in phospholipid layer in a pattern
partially permeable + can adjust permeability
phospholipids + proteins move around in lipid layer so structure is fluid 

What are the key components of cell membranes?

phospholipid bilayer - made of phospholipids with hydrophilic heads + hydrophobic tails - although glycolipids also scattered
cholesterol - add stability
proteins - can be extrinsic or intrinsic
glycoproteins + glycolipids - carbohydrates attached to proteins or lipids

What is the phospholipid bilayer?

each phospholipids molecule made of hydrophilic polar head + 2 hydrophobic non-polar tails - causes the phospholipids to naturally arrange themselves into bilayer so heads facing outwards towards water + tails facing inwards away from water - creates hydrophobic centre in bilayer so water-soluble substances cannot pass through - however lipid-soluble substances can dissolve in bilayer + pass directly through membrane - small non-polar substances can pass through
most stable structure to adopt - hydrophobic tails attract eachother further stabalising structure

What is the function of cholesterol?

provides stability + maintains suitable level of fluidity in membrane - consists of hydrophilic + hydrophobic region - hydrophobic region binds to phospholipids fatty acid tails causing them to pack more closely together - reduces fluidity as restricts movement of molecules - adjacent fatty acids could crystalise making membrane more rigid so presence of cholesterol between fatty acid chains prevents this happening keeping membrane fluid

What is the function of intrinsic protein?

embedded through both sides of phospholipid bilayer also known as transmembrane protein - these proteins include channel (form passageway where water +polar substances can pass through via f diffusion) + carrier proteins (change shape to move substance from one side to another) which transport large molecules and ions across membrane - transport water-soluble charged particles through membrane as cannot get through membrane

What is the function of extrinsic proteins?

including peripheral present on only one side of phospholipid bilayer - these proteins provide support to membrane or may be involved in cell signalling

What is the function of glycoproteins?

cell adhesion - attachment of cells to one another
cell recognition - allows cells to recognise one another
cell signalling - communication between cells acting as receptors for certain molecules (hormones) + triggering change in cell
proteins have short carb chain attach - can form hydrogen bonds with water to stabalise membrane structure 

What is the function of glycolipids?

same as glycoproteins functions
lipids with sort carb chain attached 
both form surface antigens where immune system can identify the cell as belonging to body (or foreign if transported into another body)

What are the roles of cell membranes?

cell-surface membranes - surround cells to act as barries between the cell and its environment,controlling which substance enters and leaves the cell
membrane around organelles - surround organelles to act as a barrier between organelle + cytoplasm,dividing the cell into different compartments (compartmentalisation)
both are partially permeable meaning they let some molecules pass through but not others

How does temperature affect cell membranes?

at low temperatures (below 0) - phospholipids do not have much kinetic energy + are packed closely together to form a rigid cell membrane decreasing the permeability of membrane
at medium temperatures (0-40) - as temperature increases phospholipids have more kinetic energy so move faster + aren’t packed closelt together increasing permeability of membrane
at high temperatures (above 40) - phospholipid bilayer breaks down channel + carrier proteins denature,meaning cannot control what enters or leaves the cell as gaps appear in membrane this increases permeability of membrane as no longer effective barrier - is irreversible

How are solvents linked to cell membranes?

solvents are liquids that have the ability to dissolve substances - when cells placed in solvents such as ethanol phospholipids dissolve causing membrane to make it more permeable - increasing concentration of solvent will further increase permeability of cell membrane - organic solvents dissolve lipids damaging cell membrane leading to increased permeability

How do fatty acids affect permeability at low temperature?

determined by proportion of saturated to unsaturated fatty acids - saturated fatty acids are compressed at low temperatures but kinks in unsaturated fatty acids counteract this by pushing against neighbouring phospholipids - membrane with higher proportion of unsaturated acids will be more permeable than one with a higher proportion of saturated fatty acids at same temperatures 

What is simple diffusion?

net movement of particles from area higher concentration to area of lower concentration + eventually reach equilibrium - does not require energy so a passive process
some molecules such as oxygen + carbon dioxide can diffuse into cells via simple diffusion as the molecules are small (can pass through spaces between phospholipids + non-polar (can dissolve in hydrophobic core of cell membrane) - movement in solid restricted as confined to vibrations in fixed positions so very little diffusion occurs therfore more diffusion occurs in liquids and gas as particles move randomly

What is facilitated diffusion?

large or polar molecules can cross cell membranes by this process - involves two types of protein - carrier proteins + channel proteins - both passive + no metabolic process - each protein is highly specific allowing only 1 or 2 types of molecules to pass through 

How do carrier proteins work?

mainly transport large molecules which are specific to substance being transport across the cell membrane
steps - large molecule attaches to carrier protein - causes the carrier protein to change shape - carrier protein releases the molecule on opposite side of the membrane 

How do channel proteins work?

mainly transport ions and smaller polar molecules across the cell membrane - these proteins form pores in cell membranw which ions can travel through - can be open and closed to regulate flow 

What are the factors affecting rate of diffusion?

1.temperature - at higher temperatures particles have more kinetic energy + diffuse faster
2.concentration gradient - steeper the concentration gradient the faster the rate of diffusion
3.thickness of membrane - particles travel shorter distances through thin exchange surface so diffuse faster
4.surface area - larger surface areas mean more particles can cross the membrane at once making diffusion faster
5.number of proteins - more of these proteins the faster the rate of facilitated diffusion

What is active transport?

movement of particles from an area of lower concentration to an area of higher concentration - process requires energy from respiration in form of adenosine triphosphate (ATP) - energy needed as against concentration gradient so is an active process
cells where active transport takes place will have high concentration of mitochondria - each carrier is specific + only transports certain substances - glucose actively transported across the walls of villi in small intestine - root hair cells in plants also use this to obtain certain ions from the soil

What proteins are involved in active transport?

involves the use of carrier proteins to transport molecules or ions across membranes
steps - molecule/ion binds to carrier protein - ATP binds to carrier protein - hydrolysis of ATP to ADP and phosphate (Pi) causes carrier protein to change shape releasing molecule/ion on opposite side of membrane - phosphate is released from carriers protein causing carrier protein to return to original shape ready for use again 

What factors affect rate of active transport?

1.temperature - at higher temperatures particles have more kinetic energy + travel faster - respiration also increases with temperature but very high temperature denature carrier proteins decreasing active transport
2.thickness of membrane - particles travel shorter distances through thin exchange surfaces so travel faster
3.number of carrier proteins - more of these proteins the faster the rate of active transport
4.rate of respiration - more respiration the more ATP available for active transport

What is bulk transport?

large molecules such as enzymes or hormones cannot be transported through channel or carrier proteins so instead moved into and out of cells using another form of active transport known as bulk transport

What are two examples of active transport/bulk transport?

endocytosis - transports materials into cells
exocytosis - transports materials out of cells 

What is endocytosis?

large molecules at once are transported into cells - cell-surface membrane engulfs materials to form a vesicle which is then moved into cytoplasm where materials can be processed 
has two forms - phagocytosis - uptake of solid materials + pinocytosis - uptake of liquid materials 

What is exocytosis?

large molecules or many molecules ar once are transported out of cells - vesicles (mostly formed by golgi apparatus) move towards and fuse with cell-surface membrane where materials are released outside the cell

What are solutions and water potential?

solutions are mixtures made up of a solute dissolved in a solvent 
water potential Ψ (measure of tendency of water to move from one area to another) is pressure exerted by water molecules on membrane surrounding a solution + measured in kiloPascals (kPa)
high water potential - means solution has a high water concentration (so little solute dissolved in it) low water potential - means solution has a low water concentration (so lots of solute dissolved in it)
pure water has a water potential of 0 kPa then value decreases (becomes more negative) as more solute is added 

What is osmosis?

net movement of water molecules across partially permeable membrane from an area of higher water potential to an area of lower water potential
water molecules are small + can diffuse directly through cell membrane whereas large solute molecules cannot
water molecules diffuse down a water potential gradient until water potential is equal on both sides of membrane (equilibrium)

What is hypotonic,isotonic and hypertonic solution?

hypotonic - solution has a higher water potential than the cell
isotonic - solution has same water potential as the cell
hypertonic - solution has lower water potential than the cell 

What happens in animal cells due to osmosis?

hypotonic - water molecules move into the cell + cell swells and burst - lysis
isotonic - no net movement of water into or out of cell - cell stays the same size
hypertonic - water molecules move out of the cell - cell shrinks - crenated - fatal

What happens in plant cells due to osmosis?

unlike animal cells,plant cells do not burst due to presence of a cell wall
hypotonic - water molecules move into the cell - cell swells + becomes turgid
isotonic - no net movement of water into or out of the cell - cell stays same size
hypertonic - water molecules move out of the cell - cell shrinks + becomes plasmolysed

What factors affect rate of osmosis?

1.temperature - at higher temperatures water molecules have more kinetic energy + diffuse
2.water potential gradient - steeper the gradient the faster the rate of osmosis 
3.thickness of membrane - water molecules travel shorter distances through thin exchange surfaces so diffuse faster
4.surface area - larger surface areas mean more water molecules can cross the membrane at once making osmosis faster 

What membranes do chloroplasts and mitochondria have?

both surrounded by two membranes where outer membrane has pores that allow small molecules to pass through easily - inner membrane contains many proteins so can perform important functions of photosynthesis + respiration