5 - Plasma Membranes

Front 1

Glycocalyx

Back 1

Glycoprotein joined with a carbohydrate chain – for cell recognition/ receptors so cells group together to form tissues.

Front 2

Other than as carrier proteins state 2 functions of membrane-bound proteins

Back 2

enzymes, receptors.

Front 3

Glycoprotein

Back 3

cell signalling telling cells that the membrane is part of the body.

Front 4

ATP

Back 4

energy currency – 32 made from 1 glucose molecule

Front 5

Explain why the model for membrane structure is known as the fluid mosaic model

Back 5

Fluid – phospholipid molecules can more freely so it makes it a fluid – not a solid.

Mosaic – proteins are distributed throughout the membrane un-evenly and in a mosaic structure.

Model – the agreed structure is based upon experiment ands chemical evidence so it is classed as a model.

Front 6

How can polar and non-polar molecules pass through the membrane

Back 6

non-polar molecules enter as they can diffuse directly through the bilayer à passive, require no energy. Polar molecules require proteins to help them pass through the membrane.

Front 7

Peripheral

Back 7

inside or outer membrane.

Front 8

Integral proteins

Back 8

extend through both layers.

Front 9

Extrinsic

Back 9

halfway through/only on one side of the membrane

Front 10

Intrinsic

Back 10

all the way through.

Front 11

Extracellular

Back 11

outside

Front 12

Intracellular

Back 12

inside

Front 13

Aqueous solution

Back 13

water

Front 14

How can enzymes get in the membrane without help of proteins?

Back 14

has to be small enough and non-polar/soluble/fat-soluble.

Front 15

Transmembrane proteins

Back 15

proteins thatwedge themselves in the bilayer and are free to move around.

Front 16

Why can't water touch the tails of a phospholipid in membranes?

Back 16

membranes form a hydrophilic bilayer so the water wont touch the tails as they are hydrophobic.

Front 17

what is the average size of a membrane in animals?

Back 17

7-8nm.

Front 18

Compartmentalisation

Back 18

the formation of separate membrane-bound areas in a cell. It is vital to a cell as metabolism includes many different and often incompatible reactions. Containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions e.g. chemical gradients to be maintained.

Front 19

Partially permeable plasma membrane

Back 19

separates cell from its external environment. Membranes are formed from a phospholipid bilayer the hydrophilic phosphate heads form the inner and outer surface of the membrane, sandwiching the hydrophobic fatty acid tails in the middle which forms a hydrophobic core. Cells normally exist in aqueous environments, the inside of the cell and organelles are also often in aqueous solution which makes the bilayer a good structure as the hydrophobic heads interact with the water.

Front 20

Fluid mosaic model fluidity

Back 20

phospholipids are free to move within the layer (they are fluid), giving the membrane flexibility

Front 21

Intrinsic proteins

Back 21

also known as integral protein, are transmembrane protein that embedded through both layers of the membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place. Channel and carrier proteins are intrinsic proteins.

Front 22

Channel proteins

Back 22

provide a hydrophilic channel that allows the passive movement of polar molecules and ions held down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core pf the membrane and the hydrophobic R-groups on the outside of the proteins.

Front 23

Carriers proteins

Back 23

have an important role in both passive (down a gradient) and active transport (against a concentration gradient) into cells. This often involves the shape of the protein changing.

Front 24

Glycoprotein

Back 24

branching carbohydrate potion of a protein which acts as a recognition site for chemicals such as hormones. Intrinsic proteins embedded in the cell surface membrane with an attached carbohydrate chains of varying length and size. They play a role in cell adhesion and receptors for chemical signals. Cell signalling (cell communication) - when the chemical binds to the receptor, it elicts a response from the cell. This may be a direct response or cause a cascade of events inside the cell.

Front 25

Examples of cell signalling

Back 25

e.g.1 receptors for neurotransmitters at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone. E.g.2. receptors for peptide hormones, including insulin and glucagon, which affect the uptage and storage of glucose by cells. E.g.3. some drugs act by binding to cell receptors such as beta blockers which are used to reduce the response of the heart to stress.

Front 26

Examples of intrinsic proteins

Back 26

glycoproteins, glycolipids, channel and carrier proteins.

Front 27

Glycolipid

Back 27

also known as cell markers or antigens. acts as a recognition site e.g for cholera toxins. Lipids with attached carbohydrate (sugar) chains, they can be recognised by cells of the immune system as self (part of the organism) or non-self (of cells belonging to another organism.

Front 28

Cholesterol

Back 28

for stability/flexibility. A lipid with a hydrophilic end and and a hydrophobic end like a phospholipid. It regulates the fluidity of membranes. Cholesterol molecules are positioned between the phospholipids in the phospholipid bilayer with the hydrophobic end reacting with the tails and hydrophilic end reacting with the heads, pulling them together. This means cholesterol adds stability to membranes without making them too ridged. They also prevent the membrane becoming to solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Front 29

Extrinsic proteins

Back 29

also known as peripheral proteins are present in one side of the bilayer. They normally have hydrophilic R-groups on their surface that react with the hydrophilic heads of the phospholipids or with intrinsic proteins. They can be present in either layer and some more between layers. E.g. cholesterol.

Front 30

Temperature

Back 30

phospholipids in a cell membrane are constantly moving. When temperature is increased, the molecular kinetic energy will increase do the phospholipids in the membrane will vibrate more, so they are more unstable. This makes the membrane more fluid and it beginsto lose it’s structure. It will become more ‘leaky’ so it will allow more molecules through than usual. Once a certain temperature is reached the membrane breaks and the cell will break down completely. This loss of structure increases permeability of membrane making it easier for particles to cross it, carrier and channel proteins will be denatured.

Front 31

Adapted to temperature

Back 31

some organisms have adaptes to live in environments with extreme temperatures, and so have membranes that are more resistant to heat. One wy of increasing resistance to heat is by increasing the amount of cholesterol in the membrane.

Front 32

What effect might a solvent such as ethanol have on membrane permeability?

Back 32

ethanol will cause membrane disintegration as it forms temporary bonds with the phospholipid heads in the membrane. This causes the phospholipids to move out of place so large gaps form in the membrane allowing pigment to leak out. Ethanol also affects bonding in the membrane proteins leading to denaturation causing further gape in the membrane. Cholesterol, important in membrane structure, is soluble in ethanol so once it is removed, larger gaps allow increased permeability of molecules such as betalain.

Front 33

Ethanol

Back 33

can dissolve the membrane, membrane becomes more permeable. Cells cant keep alcohol out because they are non-polar. Non-polar can enter, polar cant.

Front 34

Ethanol in use

Back 34

alcohols are used in antiseptic wipes as alcohols dissolve the membranes of bacteria, killing them and reducing the risk of infection. Pure or very strong alcohols are toxic as they destroy cells in the body. Less cocentrated alcohols DON’T DISSOLVE MEMBRANES BUT DO DAMAGE CELLS.

Front 35

pH

Back 35

Hydrogen (H+) bonds are broken/affected so tertiary strustue changes and denatures proteins.

Front 36

What are three things that can effect permeability of a cell membrane

Back 36

heat, pH, ethanol.

Front 37

State 4 limitations of membrane permeability PAG

Back 37

not washing all of the pigment from the beetroot surface, pigment being present in different amounts in different parts of the beetroot, the beetroot cylinders being left in the water for slightly different lengths of time, different amounts of drying/patting an insufficient temperature range, age of beetroot may vary if more than one is used per student/group, might not have equilibrated to the temperature in the time allowed.

Front 38

How does membrane permeability pag work?

Back 38

Beetroot cells contain betalain, a red pigment that gives them their distinctive colour, becauseof this they are useful for investigating the effects of temperature on membrane permeability. When beetroot cells membranes’ are disrupted, the red pigment is released and the surrounding solution is coloured. The amount of pigment released into a solution is related to the disruption of the cell membranes.

Front 39

Membrane permeability PAG

Back 39

5 test tubes labelled with the temperatures 20 – 60 *C.

Add 10cm3 (100ml) distilled water.

Place the test tubes in their corresponding water bath for 5 minutes to give them time to equilibrate to the correct temperature.

Collect 10 beetroot cylinders and trim them all to 30mm using a knife and ruler on a white tile.

Add 2 cylinders to a tube in each temperature and leave for 15 minuites.

Rinse the cylinders under a running tap and pat dry using paper towels.

Label cuvettes with the same temperatures as before.

Remove the tubes from the water bath, carefully swirl once and use the forceps to remove the cylinders.

Pour the remaining liquid into the cuvette with the corresponding temperature labelled on it. You should now have 5 cuvettes showing different intersiteies of pigment in the water.

Use the colourimeter to measure the absorption for each temperature.

Front 40

Some cells of Canadian pondweed were broken open using a liquidiser and some of the chloroplasts were released intact. what would happen to an intact chloroplast if it were then placed into distilled water with a water potential of 0?

Back 40

water enters the cell by osmosis, cell bursts (cell membrane will burst) as it has no cell wall

Front 41

a leaf of Canadian pondwater, which had been kept out of water for a short time, was seen to have wilted. Explain in terms of water potential what would happen to its cells if the leaf was then placed in distilled water with a water potential of 0 (pure water).

Back 41

water will enter cell by osmosis down a concentration gradient from an area of higher to lower concentration so the cell is filled. water pushes membrane against the cell wall. it will reach equilibrium.

Front 42

suggest a range of concentration for the sucrose solutions to find the solute of rhubarb cells

Back 42

0-1 mol

Front 43

list the chemical elements present in protein, lipid, carbohydrate

Back 43

protein - C, H, O, N, S

lipid - C, H, O, P

carbs - C,H,O

Front 44

list the chemical elements present in proteins, lipids and carbohydrates

Back 44

proteins - C, H, O, N, S

lipids - C, H, O, P

carbs - C, H, O

Front 45

suggest a range of concentration for sucrose solutions for trying to find he solute of rhubarb cells

Back 45

0-1 mol

Front 46

difference between bulk transport and osmosis

Back 46

• bulk transport occurs with vesicles and it is where large groups of molecules are all transported together at the same time

• osmosis doesn't use vesicles, it is the diffusion of water particles from an area of higher concentration to lower concentration, down a concentration gradient

Front 47

differences between structure of cell surface membrane at 20°C and 80°C

Back 47

proteins will start to change shape and denature making the cell lose it's function while it is all intact and working well at 20. holes on membrane breaks down membrane

Front 48

describe two differemces between active transport and facilitated diffuson

Back 48

• active transport go 's against a concentration gradient while facilitated diffusion goes down a concentration gradient

• active transport is an active process so it requires energy, while facilitated diffusion is passive so it doesn't require energy.

Front 49

function of cholesterol, protein and phospholipid bilayer

Back 49

• cholesterol - regulates marine fluidity

• protein - cell signalling

• phospholipid bilayer - selectively permeable, only lets specific molecules enter the cell

Front 50

some cells of Canadian pondweed were broken open using a liquidiser and some of the chloroplasts wee released intact. what would happen to an intact chloroplast if it were then placed into distilled water with a water potential of 0 (distilled water).

Back 50

water enters cell by osmosis, cell bursts (membrane bursts) as it has no cell wal

Front 51

a leaf of Canadian pondweed, which had been kept out of water for a short time, was seen to have wilted. explain, in terms of water potential, what would happen to its cells if the leaf was then placed in distilled water with a water potential of 0 (pure water)

Back 51

water will enter cell by osmosis down a concentration gradient from an area of higher concentration to an area of lower concentration so cell is filled. water pushes membrane against cell wall, will reach equilibrium.

Front 52

Plasma membranes

Back 52

Membranes are the structures that separate the contents of cells from their environment. They also separate the different areas within cells (organelles) from each other and the cytosol. Some organelles are divided further by internal membranes.

Front 53

Compartmentalisation with membranes

Back 53

The formation of separate membrane-bound areas in a cell is called compartmentalisation. Compartmentalisation is vital to a cell as metabolism includes many different and often incompatible reactions. Containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.

Front 54

Membrane structure

Back 54

All the membranes in a cell have the same basic structure. The cell surface membrane which separates the cell from its external environment is known as the plasma membrane. Membranes are formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane. Cells normally exist in aqueous environments. The inside of cells and organelles are also usually aqueous environments. Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.

Front 55

Cell membrane theory

Back 55

Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution. Images taken in the 1950s showed the membrane as two black parallel lines – supporting an earlier theory that membranes were composed of a lipid bilayer. In 1972 American scientists Singer and Nicolson proposed a model, building upon an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane. The model is known as the fluid-mosaic model because the phospholipids are free to more within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size and position (in the same way as the tiles of a mosaic). This model forms the basis of our understanding of membranes today.

Front 56

Membrane proteins

Back 56

Membrane proteins have important roles in the various functions of membranes. There are two types of proteins in the cell-surface membrane – intrinsic and extrinsic proteins.

Front 57

Intrinsic proteins

Back 57

Intrinsic proteins, or integral protein, are transmembrane proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place. Channel and carrier proteins are intrinsic proteins. They are both involved in transport across the membrane. Channel proteins provide a hydrophilic channel that allows the passive movement 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 the proteins. Carrier proteins have an important role in both passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells. This often involves the shape of the protein changing.

Front 58

Glycoproteins and cell signalling

Back 58

Glycoproteins are intrinsic proteins. They are embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes. Glycoproteins 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. This process is known as cell communication or cell signalling.

Front 59

Examples of cell signalling

Back 59

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. For example, beta blockers are used to reduce the response of the heart to stress.

Front 60

Glycolipids

Back 60

Glycolipids are similar to glycoproteins. They are lipids with attached carbohydrate (sugar) chains. These molecules are called cell markers or antigen 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).

Front 61

Extrinsic proteins

Back 61

Extrinsic proteins or peripheral proteins are present in one side of the bilayer. They normally have hydrophilic R-groups on their outer surface and interact with the polar head of the phospholipids or with intrinsic proteins. They can be present in either layer and some move between layers.

Front 62

Cholesterol

Back 62

Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid. It regulated the fluidity of membranes. Cholesterol molecules are positioned between phospholipids in 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 adds stability to membranes without making them too rigid. The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Front 63

Sites of chemical reactions

Back 63

Like enzymes, proteins in the membranes forming organelles, or present within organelles, have to be in particular positions for chemical reactions to take place. For example, the electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration. The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts.

Front 64

What happens when membranes lose their structure

Back 64

Membranes control the passage of different substances into and out of cells (and organelles). If membranes lose their structure, they lose control of this can cell processes will be disrupted. A number of factors affect membrane structure including temperature and the presence of solvents.

Front 65

Factors affecting membrane structure – Temperature

Back 65

Phospholipids in a cell membrane are constantly moving. When temperature is increased the phospholipids will have more kinetic energy and will move more. This makes a membrane more fluid, and it begins to lose its structure. If temperature continues to increase the cell 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 temperatures. These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.

Front 66

Solvents in the phospholipid bilayer

Back 66

Water, a polar solvent, is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping to keep the bilayer intact.

Front 67

Factors affecting membrane structure – solvents (why alcohols affect membranes)

Back 67

Many organic solvents are less polar than water for example alcohols, or they are non-polar like benzene. Organic solvents will dissolve membranes, disrupting cells. This is why alcohols are used in antiseptic wipes. The alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection. Pure or very strong alcohol solutions are toxic as they destroy cells in the body. Less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but still cause damage. The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane.

Front 68

Factors affecting membrane structure – solvents (how they affect the membrane)

Back 68

The increase in kinetic energy of the phospholipids disrupts the structure of the membrane, creating gaps and making it more permeable. When the membrane is disrupted it becomes more fluid and more permeable. Some cells need intact cell membranes for specific functions, for example, the transmission of nerve impulses by neurones (nerve cells). When neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal. This also happens to neurones in the brain, explaining the changes seen in people’s behaviour after consuming alcoholic drinks.

Front 69

Effects of temperature and solvents on beetroot cells

Back 69

Beetroot cells contain betalain, a red pigment that gives them their distinctive colour, because of this they are useful for investigating the effects of temperature and organic solvents on membrane permeability. When beetroot cells membranes are disrupted the red pigment is released and the surrounding solution is coloured. The amount of pigment released into a solution is related to the disruption of the cell membranes.

Front 70

Investigating membrane permeability in beetroot cells

Back 70

To invertigate the effect of temperature on the permeability of vell membranes a student carried out the following procedure. Five small pieces of beetroot of equal size were cut using a cork borer. The beetroot pieces were thoroughly washed in tunning water, they were then placed in 100ml of distilled water in a water bath. The temperature of the water bath was increased in 10*C intervals. Samples of the water containing the beetroot were taken five minutes after each temperature was reached. The absorbance of each sample was measured using a colourimeter with a blue filter. The experiment was done three times, each time with fresh beetroot pieces and a mean calculated for each temperature.

Front 71

Movement requiring energy

Back 71

The exchange of substances between cells and their environment or between membrane bound compartments within cells and the cell cytosol is defined as either active (requiring metabolic energy) or passive. All movement requires energy. Passive movement, however, utilise energy from the natural motion of the particles, rather than energy from another source. This topic will focus on passive transport methods.

Front 72

Diffusion

Back 72

Diffusion is the net, or overall, movement of particles (atoms, ions or molecules) from a region of higher concentration to region of lower concentration. It is a passive process, and it will continue until there is a concentration equilibrium between the two areas. Equilibrium means a balance or no different in concentrations.

Front 73

Why does diffusion occur

Back 73

Diffusion happens because the particles in a gas or liquid have kinetic energy (they are moving). This movement is random and unequal distribution of particles will eventually become an equal distribution. Equilibrium doesn't mean the particle stop moving, just that the movements are equal in both directions. Particles move at high speeds in a constantly colliding, which slows down their overall movement. This means that over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place. For this reason, cells are generally microscopic - the movement of particles within cells depends on her diffusion and a large cell would lead to slow rates of diffusion. Reactions would not get the substrates they need quickly enough, or ATP will be supplied to slowly to energy-requiring process.

Front 74

Factors affecting rate of diffusion: Temperature

Back 74

the higher the temperature the higher the rate of diffusion. This is because the particles have more kinetic energy and move at higher speeds.

Front 75

Concentration differences

Back 75

the greater and the difference of concentration between two regions the faster the rate of diffusion, because the overall movement from the higher concentration to the lower concentration would be larger.

Front 76

Concentration gradient

Back 76

A concentration difference is said to be a concentration gradient, which goes from high to low concentration. Diffusion takes place down a concentration gradient. It takes a lot more energy to move substances up a concentration gradient. So far diffusion in the absence of a barrier on membrane has been considered. This is simple diffusion.

Front 77

Rate of diffusion and surface area

Back 77

The rate of diffusion can be calculated in two ways - by distance travelled/time and volume filled/time. Distance travelled/ time is not affected by changes in surface area, whilst volume/time varies depending on the surface area. A student used different sized agar blocks to investigate how the rate of diffusion was affected by surface area. The agar used to make the blocks contained the indicator phenolphthalein with turns pink in the presence of an alkali. The agar blocks were immersed in a solution of sodium hydroxide for ten minutes. The blocks were removed and distance the sodium hydroxide had diffused was measured with a ruler.

Front 78

Diffusion across membranes

Back 78

Diffusion across membranes involves particles passing through the phospholipid bilayer. It can only happen if the membrane is permeable to the particles - non-polar molecules such as oxygen (O2) diffuse through freely down a concentration gradient. The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions), so they cannot easily pass through. Polar molecules, such as water (H,O) with partial positive and negative charges can diffuse through membranes, but only at a very slow rate. Small polar molecules pass through more easily than larger ones. Membranes are therefore described as partially permeable.

Front 79

The rate at which molecules or ions diffuse across membranes is affected by: surface area

Back 79

the larger the area of an exchange surface, the higher the rate of diffusion thickness of membrane the thinner the exchange surface, the higher the rate of diffusion.

Front 80

Facilitated diffusion

Back 80

As you have learnt, the phospholipid bilayers of membranes are barriers to polar molecules and ions. However, membranes contain channel proteins through which polar molecules and ions can pass. Diffusion across a membrane through protein channels is called facilitated diffusion. Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion.

Front 81

Facilitated diffusion and carrier protein

Back 81

Facilitated diffusion can also involve carrier proteins, which change shape when a specific molecule binds. In facilitated diffusion the movement of the molecules is down a concentration gradient and does not require external energy. The rate of facilitated diffusion is dependent on the temperature, concentration gradient, membrane surface area and thickness, but is also affected by the number of channel proteins present. The more protein channels, the higher the rates of diffusion overall.

Front 82

Investigations into the factors affecting diffusion – Why dialysis tubing is used as a substitute membrane

Back 82

As you have already seen, cell membranes are highly complex structures involved in the active and passive transport of ions and molecules. The hydrophobic hydrocarbon core of the membrane is a barrier to ions and large polar molecules, but it allows the passage of non-polar molecules. Cells are too small and cell membranes too thin to use in practical investigations, so dialysis tubing is used as a substitute membrane. This model enables us to investigate the effects of temperature and concentration on the rate of diffusion across membranes.

Front 83

Investigations into the factors affecting diffusion – dialysis tubing (model cell)

Back 83

Dialysis tubing is partially permeable, with pores a similar size to those on a partially permeable membrane. This means that small molecules like water can pass through it, but larger molecules like starch cannot fit through the pores. The tubing is therefore a barrier to large molecules. A model cell can be simulated by tying one end of a section of tubing, filling with a solution and then tying the other end. The 'cell' is then placed into another solution. The solutions could contain different sizes, or concentrations, of solute molecules.

Front 84

How testing for rates of diffusion works

Back 84

The changes in concentration of solute molecules, both inside and outside the model cells, can be measured over time. Rates of diffusion across the tubing can then be calculated. Glucose is a small molecule which can cross the tubing. Benedict's solution is used to test for the presence of glucose, and can also be used to estimate concentration. Starch molecules are large and will not cross the tubing. lodine is used to test for the presence of starch. Water is a small molecule which will pass through the tubing while other solutes such as sucrose will not. Model cells can be placed in solutions with different solute concentrations. The rates of osmosis can be calculated using changes in volume or mass of the model cells over time. Rates of diffusion at different temperatures can also be calculated using a water bath to change the temperature of the model cell. Other variables such as concentration must then be kept constant

Front 85

Need for active transport

Back 85

Diffusion, by its nature will ultimately result in concentration gradients being reduced until particles (atoms, molecules or ions) in the different regions reach equilibrium. However, many biological processes depend on the presence of a concentration gradient, for example, the transmission of nerve impulses. To maintain this concentration gradient, particles must be moved up it as a rate faster than the rate of diffusion. This is an energy-requiring process called active transport.

Front 86

Active transport

Back 86

Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. The process requires energy and carrier proteins. Energy is needed as the particles are being moved up a concentration gradient, in the opposite direction to diffusion. Metabolic energy is supplied by ATP. The process is selective – specific substances are transported by specific carrier proteins. Carrier proteins span the membranes and act as ‘pumps’. The general process of active transport is described below – in this example transport is from outside to inside a cell.

Front 87

Steps of active transport

Back 87

The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell. On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate. Binding of the phosphate molecule to the carrier protein causes the protein to change shape – opening up to the inside of the cell. The molecule or ion is released to the inside of the cell. The phosphate molecules released from the carrier protein and recombines with ADP to form ATP. The carrier protein returns to its original shape.

Front 88

Bulk transport

Back 88

Bulk transport is another form of active transport. Large molecules such as enzymes, hormones, and whole cells like bacteria are too large to move through channel or carrier proteins, so they are moved into and out of cell by bulk transport.

Front 89

Endocytosis (bulk transport)

Back 89

the bulk transport of material into cells. There are two types of endocytosis, phagocytosis for solids and pinocytosis for liquids – the process is the same for both.

Front 90

Exocytosis (bulk transport)

Back 90

the reverse of endocytosis. Vesicles, usually formed bt the golgi apparatus, move forwards and fuse with the cell surface membrane. The contents of the vesicle are then released outside of the cell. The cell surface membrane first invaginates (bends inwards) when it comes into contact with the material to be transported. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell. For example, vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes.

Front 91

ATP for movement during bulk transport

Back 91

Energy in the form of ATP is required for movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of cell membranes as vesicles form or as they meet the cell- surface

Front 92

Osmosis is a particular type of diffusion

Back 92

specifically the diffusion of water across a partially permeable membrane. As with all types of diffusion it is a passive process and energy is not required.

Front 93

Water potential

Back 93

A solute is a substance dissolved in a solvent (for example water) forming a solution. The amount of solute in a certain volume of aqueous solution is the concentration. Water potential is the pressure exerted by water molecules as they collide with a membrane or container. It is measured in units of pressure pascals (Pa) or kilopascals (kPa). The symbol for water potential is the Greek letter psi.

Front 94

Pure water

Back 94

Pure water is defined as having a water potential of 0kPa (at standard temperature and atmospheric pressure -25*C and 100kPa). This is the highest possible value for water potential, as the presence of a solute in water lowers the water potential below zero. All solutions have negative water potentials – the more concentrated the solution the more negative the water potential.

Front 95

Net movement and equilibrium of water

Back 95

When solutions of different concentrations, and therefore different water potentials, are separated by a partially permeable membrane, the water molecules can move between the solutions but the solutes usually cannot. There will be a net movement of water from the solution with the higher water potential (less concentrated) to the solution with the lower water potential (more concentrated). This will continue until the water potential is equal on both sides of the membrane (equilibrium).

Front 96

Effects of osmosis on plant and animal cells

Back 96

The diffusion of water into a solution leads to an increase in volume of this solution. If the solution is in a closed system, such as a cell, this results in an increase in pressure. This pressure is called hydrostatic pressure and has the same units as water potential, kPa. At the cellular level this pressure is relatively large and potentially damaging.

Front 97

Osmosis in Animal cells

Back 97

If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell. All cells have thin cell-surface membranes (around 7nm) and no cell walls. The cell-surface membrane cannot stretch much and cannot withstand the increased pressure. It will break and the cell will burst, an event called cytolysis. If an animal cell is placed in a solution that has a lower water potential than the cytoplasm it will lose water to the solution by osmosis down the water potential gradient. This will cause a reduction in the volume of the cell and the cell-surface membrane to ‘pucker’, referred to as crenation. To prevent either cytolysis or crenation, multicellular animals usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic). In blood the aqueous solution is blood plasma.

Front 98

Osmosis in an animal cell

Back 98

Net movement of water when water potential of external solution is higher than cell solution – water enters cell

Front 99

Osmosis in an animal cell

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Net movement of water when water potential of external solution is equal to cell solution – water constantly enters and leaves, but at equal rates

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Osmosis in an animal cell

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Net movement of water when water potential of external solution is lower than cell solution – water leaves cell

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Osmosis in an animal cell

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state of cell when water potential of external solution is higher than cell solution – swells and bursts

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Osmosis in an animal cell

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state of cell when water potential of external solution is equal to cell solution – no change

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Osmosis in an animal cell

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state of cell when water potential of external solution is lower than cell solution - shrinks

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Osmosis in plant cells

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Like animal, plant cells contain a variety of solutes, mainly dissolved in a large vacuole. However, unlike animals, plants are unable to control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water. Plants cells have strong cellulose walls surrounding the cell-surface membrane. When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls. This pressure against the cell wall is called turgor. As the turgor pressure increases it resists the entry of further water and the cell is said to be turgid. When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis. This leads to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall – the cell is said to be plasmolysed.

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Osmosis in a plant cell

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Net movement of water when water potential of external solution is higher than cell solution – water enters cell

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Osmosis in a plant cell

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Net movement of water when water potential of external solution is equal to cell solution – water constantly enters and leaves, but at equal rates

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Osmosis in a plant cell

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Net movement of water when water potential of external solution is lower than cell solution – water leaves the cell

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Osmosis in a plant cell

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condition of protoplast when water potential of external solution is higher than cell solution – swells and becomes turgid

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Osmosis in a plant cell

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condition of protoplast when water potential of external solution is equal to cell solution – no change

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Osmosis in a plant cell

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condition of protoplast when water potential of external solution is lower than cell solution – plasmolysis, contents shrink

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Effect of solutions with different water potential investigation in plant cells

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Pieces of potato or onion can be placed into sugar or salt solutions with different concentrations, and therefore different water potentials. Water will move into or out of cells depending on the water potential of the solution relative to the water potential of the plant tissue. As the plant tissue gains or loses water it will increase or decrease in mass and size, and vice versa.

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Effect of solutions with different water potential investigation in plant Animal cells

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Eggs can be used to demonstrate osmosis in animal cells. A chickens egg is not exactly a single cell, but with the shell removed a single membrane-bound structure remains and it will behave in the same way as a cell when placed in solution of varying water potentials. To investigate osmosis, eggs without their shells are placed in different concentrations of sugar syrup. Over time, osmosis takes place and there will have been a net movement of water into or out of the eggs, depending on the concentration of the syrup they were in. (if the egg is hard boiled for easier handling this will damage the membrane.