Plasma membranes

define diffusion.

The passive and random net movement of molecules and ions from an area of high concentration to an area of low concentration down a concentration gradient. 

when will diffusion stop?

when equilibrium is reached.

what is equilibrium?

Equilibrium is when the molecules have spread out evenly; when the same number of molecules are on both sides of the membrane; there does not have to be a membrane present e.g. perfume.  

does diffusion require a membrane?

no

does diffusion require energy?

no.

what 3 factors effect diffusion

• concentration gradient. 

• temperature. 

• diffusion distance

how does concentration gradient effect diffusion?

the greater the concentration difference between two areas, the faster the diffusion will occur.  

how does temperature effect diffusion

• the greater the temperature the faster diffusion will occur.  

^ this is because the molecules have more kinetic energy (from the thermal energy) and so will move around more (although their movement is still random) 

how does diffusion distance effect diffusion

the greater the distance, the longer diffusion will take (as molecules have further to travel)  

what are the 2 diffusion calculations, and are they effected by surface area

Distance travelled / time (not affected by surface area) 

Volume filled / time (this IS affected by surface area)  

3 characteristics of diffusion across membranes 

• It is passive transport across membranes 

• It does not require energy 

• It is movement of molecules down their concentration gradient 

3 types of diffusion 

• Simple diffusion (straight through the phospholipid bilayer)  

• Facilitated diffusion using channel proteins 

• Facilitated diffusion using carrier proteins  

 

what are the 5 things facilitated diffusion is depends on?

• temp

• conc gradient 

• membrane surface area

• membrane thickness 

• number of channel or carrier proteins present 

how does temp affect facilitated diffusion

too high temperature (above 40 degrees Celsius) WILL denature channel and carrier proteins  

how does membrane surface area affect the rate of facilitated diffusion?

the greater the surface area, the more molecules that will diffuse across it in a certain length of time – the greater the SA, the faster the rate of diffusion 

how does membrane thickness effect facilitated diffusion?

the greater the thickness the slower the rate of diffusion 

what type of molecules is simple diffusion for?

• diffusion of small molecules 

• diffusion of lipid based molecules 

how does simple diffusion work?

• molecules can simply pass through the membrane because they are too small. 

• lipid based molecules can pass through the membrane because the membrane is a phospholipid bilayer (and lipids are non-polar)

what type of molecules use facilitated diffusion?

• polar molecules and ions (e.g. calcium ions)

• large molecules (e.g. glucose)

How does facilitated diffusion work using channel proteins?

• channel proteins only allow one type of ion through. 

• they are often ‘gated’ (open or closed - like a gate)

what is facilitated diffusion?

When a molecule needs either a channel or a carrier protein to move across a membrane.

How does facilitated diffusion work using carrier proteins?

• allow only one type of molecule through 

• when molecule fits into protein, the carrier protein changes shape

• molecule moves through to the other side of membrane 

• as carrier protein has changed shape, the molecule can’t diffuse back to the other side 

• once the molecule has moved through, the carrier protein changes shape so that another molecule can move through the membrane. 

what is the point of doing repeats?

to identify and remove anomalies before calculating the mean average.  

what do small error bars mean?

• Results are less spread about the mean which means they are more reproducible (reliable). 

• ^increases validity of our results.   

what is the aim of the osmosis PAG?

• To investigate how osmosis affects the mass of the potato cylinder when placed in solutions of different water potentials

• to determine the approximate water potential of the potato cells.

what is the method of the osmosis PAG?

• Prepare samples

  • Cut potato cylinders to the same size using a cork borer.

  • Measure and record their initial mass or length.

• Prepare sucrose solutions

  • Make a range of known concentrations (e.g. 0.0, 0.1, 0.2, 0.4, 0.8, 1.0 mol dm⁻³).

  • Often done using a serial dilution

  • These have different water potentials.

• Incubate the potato samples

  • Place one cylinder into each solution.

  • Leave for a set time (commonly 20–40 minutes).

  • Keep temperature and volume of solution constant.

• Measure final mass/length

  • Remove potato pieces, blot dry to remove surface water.

  • Record final mass or length.

what is the result of the osmosis PAG?

• In low sucrose (high water potential), water moves into potato cells → mass increases.

• In high sucrose (low water potential), water moves out → mass decreases.

• If water potential inside and outside the cells is equal, there is no net change.

• so the more the mass increases the higher the water potential the solution has.

how do we calculate percentage change in mass?

(final mass - initial mass)/ initial mass x100

how to plot a graph to show the results of the osmosis PAG?

Plot:
Percentage change in mass (y-axis) (use error bars after doing standard deviation) 
against
Sucrose concentration (x-axis).

Usually:

• Positive percentage change → water entered (hypotonic solution)

• Negative percentage change → water left (hypertonic solution)

Where the graph crosses 0% change, the solution is isotonic with the potato cells.
This concentration can be used to estimate the water potential of the potato cylinder. 

define osmosis

the random net movement of water molecules from an area of high water potential to an area of low water potential (down a water potential gradient across a partially permeable membrane)

what is net movement?

the overall movement of the molecules.

why is there always movement both ways across the membrane?

Molecules move randomly all the time

what type of membrane can osmosis only occur over?

• Osmosis will only occur across a partially permeable. In nature, this is usually the cell surface membrane.  

• Whenever water enters or leaves a cell, it is moving across the cell surface membrane. 

Is osmosis passive?

Osmosis is passive and so therefore does not require energy.  

what water has the highest water potential?

pure water

what is the highest water potential?

0kPa 

what type of numbers is water potential always in?

negative numbers

which way will water always move in?

Water will move from an area of negative water potential to an area with more negative water potential.

Water will therefore always move from an area of pure water to an area where there are solutes (e.g. glucose, sucrose, ions) in the water. 

the higher the conc of solutes the more ________ the water potential

negative 

what are the 4 factors affecting osmosis?

• temperature

• water potential 

• distance 

• surface area 

how does temp affect osmosis?

the higher the temperature the higher the kinetic energy so the faster rate of osmosis. 

how does water potential affect osmosis?

the steeper the water potential gradient, the faster the rate of osmosis. 

how does distance affect osmosis?

the greater the distance the water molecules have to travel, the slower the rate of osmosis. 

how does surface area affect osmosis?

• the larger the surface area, the greater the rate of osmosis.  

what happens to a plant cell in a dilute solution (has a higher water potential than the cell)?

• water moves in via osmosis 

• creates turgor pressure which is very important for plants 

• cell is turgid. 

what happens to a plant cell in an isotonic solution?

• no osmosis

• water potential in solution is same as inside cell.

what happens to a plant cell in a concentrated solution (solution has a lower water potential then the cell)?

• water leaves the cell via osmosis 

• plant starts to wilt as cells are flaccid. 

what happens to a plant cell in a really concentrated solution (solution has a far lower water potential then the cell)?

• water leaves cell via osmosis 

• plant wilts as cells become flaccid

• Plasmolyed cell - cytoplasm is pulled away from the wall

what is a hypotonic solution?

a solution which has a higher water potential then inside the cell.

what is a isotonic solution?

a solution which has the same water potential as inside the cell.

what is a hypertonic solution?

a solution which has a lower water potential then inside the cell.

what type of terms are hypotonic, hypertonic and isotonic?

comparative

what happens to an animal cell in a hypotonic solution?

• water moves into the cell

• they become turgid and then burst

• they will undergo cell lysis

• they become ‘lysed’

what is lysis?

bursting or breaking of the cell

what happens to animal cells in an isotonic solution?

• no osmosis

• water potential outside = water potential inside

what happens to animal cells in a hypertonic solution?

• water leaves the cell via osmosis

• the cell crenates (shrink, shrivel and deform)

• no enzyme reactions can take place, cell will die

why is it so important that storage molecules (such as starch in plants and glycogen in animals and fungi) to be insoluble?

So they don’t effect the water potential of the cell (they are osmotically inactive). 

what does active transport require?

ATP and carrier proteins

is active transport against the conc gradient?

yes

how does active transport use carrier proteins?

• Shape of carrier protein is complementary to molecule to be transported. 

• ATP binds to carrier protein and is hydrolysed to ADP and P(i). 

• This causes the carrier protein to change shape, releasing the molecule or ion inside the cell.  

• As the carrier protein changes shape, molecules can therefore not move back through the membrane. 

• (The P(i) is then released and recombines with ADP to form ATP and the carrier protein returns to its original shape). 

what are 2 examples of active transport?

exocytosis and endocytosis (they are bulk transport)

what is bulk transport?

• Bulk transport also requires ATP and is therefore another form of active transport.  

• It is for large molecules and whole cells (e.g. bacteria). 

what is exocytosis?

-vesicles containing a molecule move to and fuse will cell surface membrane - remember protein production step by step from cell chapter. 

-molecules (e.g. proteins – hormones) are released outside cell.  

what is endocytosis?

- cell surface membrane surrounds and forms a vesicle around the molecule or cell e.g. phagocytosis 

- vesicle pinches off from membrane and is transported inside cell to organelle (e.g. phagolysosome) 

what do membranes control?

passages of different substances and organelles.

what will happen if membranes lose their structure?

they lose control and cell processes will be disrupted.

what are 2 factors that effect membrane structure?

• temperature 

• presence of solvents 

how does temperature effect membrane structure?

• Phospholipids in cell membranes are constantly moving.  

• When temp increases – phospholipids = move kinetic energy = moves more.  

• Makes membrane more fluid and it begins to lose its structure.  

• If temp continues to increase the cells will eventually break down completely.  

• This loss of structure increases the permeability of the membrane, making it easier for particles to cross it.   

• Carrier and channel proteins in the membrane will be denatured at higher temps. 

• These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.  

how does the presence of solvents effect membrane structure?

• Water is a polar solvent and is essential for forming the phospholipid bilayer.

• Phospholipids have hydrophilic heads that interact with water and hydrophobic tails that orientate away, forming a stable bilayer with a hydrophobic core.

• Organic solvents (e.g., alcohols, benzene) are less polar or non-polar and can dissolve or disrupt membranes.

• Alcohol wipes work because alcohol dissolves bacterial membranes, killing the cells.

• Strong alcohol is toxic to human cells for the same reason.

• Lower concentrations (e.g., alcoholic drinks) don’t fully dissolve membranes but alcohol molecules slip between phospholipids, increasing membrane fluidity and permeability.

• Some cells, such as neurones, rely on intact membranes for proper nerve impulse transmission.

• When alcohol disrupts neuronal membranes, impulses are not transmitted normally, causing the behavioural effects seen after drinking.

what are the functions of a biological membrane?

• Separating cell contents from outside environment. 

• Separating cell components from the cytoplasm.  

• Cell recognition and cell signaling. 

• Holding the components of some metabolic pathways in place e.g. mitochondria and chloroplasts. 

• Regulating the transport of materials into and out of cells: cell surface membranes are selectively permeable. this is because of the channel and carrier proteins. 

what makes up the basic structure of the membrane?

Phospholipid molecules.

what’s a phospholipids structure?

• It is made up of a glycerol molecule, 2 hydrophobic fatty acid tails and a hydrophilic head. 

• One of the fatty acid tails is saturated, the other is unsaturated and has a C=C double bond.  

• Amphipathic = dual nature 

• modified triglycerides, where one of the fatty acids is replaced with a phosphate group. 

• Inorganic phosphate ions are negatively charged, so the phosphate end of the molecule ('the head') is hydrophilic (will interact with water).  

• The fatty acid chains ('tails') are non-polar and therefore repelled by water – hydrophobic

why is it important membranes aren’t just made of a phospholipid

• A phospholipid bilayer on its own would be too fragile to function as a biological membrane. 

• Membranes therefore have other components, as well as the phospholipid bilayer, to make sure that they can function properly.  

 

what’s the fluid mosaic model?

• The fluid mosaic model was proposed by Singer and Nicolson in 1972. 

• This model was put forward because: 
  ^the phospholipids are free to move within the layer (fluid). 

^the proteins embedded in the bilayer vary in shape size and position (mosaic). 

why do the phospholipids create a bilayer?

 

• At the boundary between 2 aqueous environments phospholipids will form a bilayer with the hydrophobic tails face inwards.  

• they rotate this way to avoid or get closer to the water.

how are phospholipids surfactants?

• Because of their hydrophilic/hydrophobic structure, phospholipids  will naturally form a layer on the surface of water – they are surfactants. 

what are intrinsic proteins?

• Proteins that spans the phospholipid bilayer. 

• 3 different types in membranes: channel, carrier and glycoproteins. 

what do channel proteins do and how?

• Provide a hydrophilic channel that allows the passive movement (diffusion) of polar molecules and ions down a concentration gradient through membranes.  

• They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of proteins.  

what do carrier proteins do and how?

• Have an important role in both passive transport (down a concentration gradient) into cells. 

• This often involves the shape of the protein changing. 

what do glycoproteins do and how?

• Embedded in surface cell membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.  

• They also play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals.  

• When the chemical binds to the receptor, it elicits a response from the cell.  

• This may cause a direct response or set off a cascade of events inside the cell signalling.  

• Some examples include: 
  - receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone.  

       -receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.  

• Some drugs act by binding to cell receptors. 

what are extrinsic proteins?

• Extrinsic proteins or peripheral proteins are present in one side of the bilayer. 

• They normally have hydrophilic R-groups on their outer surfaces and interact with polar heads of the phospholipids or with intrinsic proteins.  

• They don’t span the phospholipid bilayer. 

• They can be present in either layer and some move between layers. 

what is cholesterol and what does it do?

• Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid.  

• It regulates the fluidity of membranes.  

• Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.  

• In this way cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.  

what are glycolipids and what do they do?

• Similar to glycoproteins. 

• They are lipids with attached carbohydrate chains.  

• These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism).