Chapter 2 - Plasma Membranes

What is a plasma membrane?

a cell surface membrane

What are some of the functions of membranes?

• partially permeable

• involved in cell signalling

• provide attachment sites for enzymes and other molecules involved in metabolism

• allow electrical signals to be passed

• allow compartmentalisation

Why is it important that membranes allow compartmentalisation?

It allows cellular compartments to have different conditions required for optimum reaction rates, as the membrane acts as a barrier

Explain why phospholipids form a bilayer in plasma membranes

• phospholipids have a polar phosphate group head, which is hydrophilic and will face the aqueous solution

• the fatty acid tails are non-polar and hydrophobic so will move away from the aqueous solution

• as both the tissue fluid and the cytoplasm are aqueous, phospholipids form 2 layers with the hydrophilic tails facing inwards and the hydrophobic phosphate groups facing outwards

Which part of a phospholipid is polar and hydrophilic?

phosphate heads

Which part of a phospholipid is non-polar and hydrophobic?

fatty acid tails

True or false? Phospholipids act as a barrier to most water-soluble substances, such as dissolved glucose and amino acids

True

Why is the structure of the plasma membrane often called the “fluid mosaic model”?

• ‘fluid’ as the phospholipids are free to move laterally within the layer

• ‘mosaic’ due to the scattered pattern of intrinsic and extrinsic proteins in the membrane

• ‘model’ as the agreed structure is based on experimental and chemical evidence

True or false? Proteins (both intrinsic and extrinsic) cannot move through the phospholipid bilayer

false - they can move depending on the number of phospholipids with unsaturated fatty acids in their tails

What is an intrinsic or integral protein?

a protein that is embedded in both layers of the phospholipid bilayer

What is an extrinsic or peripheral protein?

a protein that is present in only one layer of the phospholipid bilayer

How are intrinsic proteins fixed in position in the phospholipid bilayer?

The hydrophobic R-groups in their amino acids interact with the hydrophobic core of the membrane

How are extrinsic proteins held in place?

The hydrophilic R-groups interact with the polar heads of the phospholipids or with intrinsic proteins?

True or false? All extrinsic proteins can move between the layers of the phospholipid bilayer

false - only some can

What type of molecules can diffuse directly through a membrane?

small, non-polar, fat soluble molecules

Give 2 examples of transport proteins?

channel and carrier proteins

What characteristic of molecules means they need a transport protein to transport them through a membrane?

polar

Fill in the blanks and choose the correct words: channel proteins are (p)____ in the membrane that provide a hydrophilic/hydrophobic channel to allow the passive/active of small/large or polar/non-polar molecules through membranes down a ___ ___. Some may be (g)____ to control movement by opening and closing.

pores, hydrophilic, passive, large, polar, concentration gradient, gated

What are carrier proteins?

proteins that change shape to carry molecules across a membrane

True or false? Carrier proteins always require energy

false - some are passive

Are glycoproteins intrinsic or extrinsic proteins?

intrinsic

What are the carbohydrates chains (of varying lengths) that protrude from glycoproteins and glycolipids called?

glycolax

On a diagram, how can you tell the difference between a glycolipid and a glycoprotein?

the glycolax (carbohydrate chain) of a glycoproteins is usually more highly branched

How do glycoproteins help to stabilise the cell membrane?

they form hydrogen bonds with the water molecules surrounding the cell

Give 3 functions of glycoproteins

• help stabilise the membrane 

• involved in cell adhesion (for building cell tissues)

• act as cell receptors for cell signalling, neurone transmission, hormones, and drugs

What is the function of glycolipids?

• immune recognition - act as cell identity markers for antigens allowing the immune system to recognise its own cells and distinguish them from pathogens, preventing self destruction

• receipts for drugs and hormones 

What does amphiphilic mean?

a molecule is both hydrophobic and hydrophilic

Is cholesterol an amphiphilic molecule?

yes

How is cholesterol positioned within a membrane?

it is found in small amounts positioned in between the phospholipids with the hydrophilic end interacting with the phospholipid heads and hydrophobic end with tails, pulling them together

Choose the correct word: cholesterol - at a high temperature membranes becomes more/less fluid, so more/less stable, as cholesterol releases/binds/modifies the phospholipids together, so they are packed more loosely/tightly.

less, more, binds, tightly

How does cholesterol affect membrane fluidity at low temperatures?

cholesterol maintains the membranes fluidity by preventing the phospholipid molecules from grouping too closely together.

What are the 2 functions of cholesterol in cell membranes?

• regulate fluidity

• increase mechanical strength (less likely to burst)

Label parts A, G and F

A - phospholipid head, fatty acid tail, G - intrinsic channel protein, F - extrinsic protein

Label the parts B, C and D

B - glycolipid, C - protein component of glycoprotein, D - carbohydrate chain of glycoprotein

Label Part E

cholesterol

What 2 factors can affect membrane permeability?

temperature and solvents

How does a temperature below 0ºC affect membrane permeability?

• the phospholipids don’t move as they have little energy, so they are packed closely together causing the membrane to become rigid increasing permeability

• proteins denature increasing permeability

• ice crystals may pierce the membrane, making it very permeable when it thaws

How does a temperature between 0-45ºC affect membrane permeability?

phospholipids have increasing kinetic energy so move around more, meaning they are less tightly packed together so permeability increases

How does a temperature above 45ºC affect membrane permeability?

Fill in the blanks: the bilayer starts to ___, ____ permeability. Water inside the cell ___ putting more ___ on the cell surface membrane. Proteins begin to ___, disrupting the membrane (s)___ so it is no longer an effective (b)___ and allows substances pass through freely.  This process is (i)___

break down, increasing, expands, pressure, denature, structure, barrier, irreversible

How do organic solvents affect the permeability of the phospholipid bilayer?

they increase the permeability by dissolving the phospholipids in the membrane

Give an example of an organic solvent

alcohols

Even though water is a solvent, why does it benefit the phospholipid bilayer?

it has hydrophilic interactions with the phospholipid heads of the phospholipids which help to keep the membrane intact

Describe the method that a student could use to investigate the affect of temperature on membrane permeability of beetroot cubes/cylinders, which contain a red pigment called betalain

1. cut 5 equal sized beetroot cubes/cylinders using a scalpel or cork borer

2. place 5 (labelled) test tubes containing distilled water in thermostatically electronic water baths from 20-60ºC and allow to equilibrate to temperature

3. rinse the cylinders under a tap and pat them dry

4. take the temperature of the water in the test tubes and add a beetroot cylinder/cube

5. leave for 15 mins

6. remove the tubes from the water bath and decant the liquid into cuvettes

7. measure the absorption of each cuvette on a colorimeter

A student uses this method to investigate the affect of temperature on membrane permeability of beetroot cylinders:

1. cut 5 equal sized beetroot cylinders using a scalpel or cork borer

2. place 5 (labelled) test tubes containing distilled water in thermostatically electronic water baths from 20-60ºC and allow to equilibrate to temperature

3. rinse the cylinders under a tap and pat them dry

4. take the temperature of the water in the test tubes and add a beetroot cylinder

5. leave for 15 mins

6. remove the tubes from the water bath and decant the liquid into cuvettes

7. measure the absorption of each cuvette on a colorimeter

Explain the importance of step 3.

it removes excess betalain pigment from the outer damaged cells

A student uses this method to investigate the affect of temperature on membrane permeability of beetroot cylinders:

1. cut 5 equal sized beetroot cylinders using a scalpel or cork borer

2. place 5 (labelled) test tubes containing distilled water in thermostatically electronic water baths from 20-60ºC and allow to equilibrate to temperature

3. rinse the cylinders under a tap and pat them dry

4. take the temperature of the water in the test tubes and add a beetroot cylinder

5. leave for 15 mins

6. remove the tubes from the water bath and decant the liquid into cuvettes

7. measure the absorption of each cuvette on a colorimeter

Describe and explain the results that the student would expect to find.

Higher absorbance at higher temperatures, as the denaturation of proteins in the membrane and increased kinetic energy of the phospholipids increases membrane permeability, so more red betalain pigment leaks out of the beetroot cells

Why is cell signalling important?

allows cells to detect and respond to changes in their internal and external environments

What are the 3 cell signalling pathways? Briefly explain them

1. endocrine signalling - cells overs large distances using signalling molecules

2. paracrine signalling - local scale

3. autocrine signalling - within a cell or to cells of the same type

What is the name of the ‘message molecule’ sent in cell signalling, and give an example?

signals e.g. hormones

How can cells detect signals?

cells must have receptors with complementary shapes to the signal

True or false? cell signalling molecules can be intracellular or extracellular (inside or outside the cell)

true

Describe the process of cell signalling (hint: there are 5 stages)

1. stimuli

2. receptors - usually found on plasma membranes and receive signal

3. transducers - convert the stimulus into a signal (chemical) that can be passed from the outside to inside of the cells through transduction

4. amplifiers - increase intensity of cell signal

5. intracellular response

What cells produce insulin and where are they found?

beta cells in the pancreas

Fill in the blanks: when the ____ detects high blood glucose levels, ___ ___ in the pancreas secrete the hormone ___ into the blood, which binds to liver, fat and muscle cells. This causes the cells to open more glucose channels in the plasma membranes so more glucose enters the cell and is converted to ___, reducing blood sugar levels.

pancreas, beta cells, insulin, glycogen

Fill in the blanks: many drugs have been developed so they are complementary to certain cell signalling receptors such as beta-blockers, which block ____ so signalling molecules cannot instigate a ___. Other drugs mimic natural signalling molecules, which the patient’s body cannot produce

receptors, response

What is diffusion?

the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient until there is a concentration equilibrium (they are evenly distributed)

Fill in the blank: (B)_____ (m)____ causes particles to randomly move around due to kinetic energy and bump into each other

Brownian motion

Why is diffusion a passive process?

it doesn’t require energy

True or false? when diffusion reaches equilibrium across a membrane the particles no longer move across it

False - the particles still move, just equally in both directions, so there is no net movement

What characteristics of molecules mean they can diffuse straight through a membrane?

small and non-polar

Give an example of a molecule that can diffuse straight across a membrane?

water, oxygen, carbon dioxide, steroid hormones, small ions

How do larger, charged particles diffuse through membranes? (just the name of the process)

facilitated diffusion

Give an example of a molecule that diffuses across membranes by facilitated diffusion

glucose and amino acids

What is facilitated diffusion?

the diffusion of large, polar molecules and ions across cell membranes with the help of certain proteins, which are highly specific

Is facilitated diffusion an active or a passive process?

passive - molecules are moving down a concentration gradient

What are the two types of protein that carry out facilitated diffusion?

channel and carrier proteins

Fill in the blanks and choose the correct words: Channel proteins are intrinsic/extrinsic proteins which contain a ____ (a hydrophilic/hydrophobic channel) which has a ___ shape. They may be (g)___ - ___ cause the proteins to change shape closing one side and opening the other

intrinsic, pore, hydrophilic, specific, gated, ions

Fill in the blanks: carrier proteins are intrinsic/extrinsic ___proteins with a ___ shape. The molecule binding to the protein causes a ___ shape change in the protein which delivers the molecule to the other side of the membrane.

intrinsic, glyco-, specific, conformational

Rank in terms of speed for diffusion (slowest to fastest): diffusion, facilitated diffusion by a carrier protein and facilitated diffusion by a channel protein

diffusion, channel, carrier

List the factors that affect the rate of diffusion

temperature, concentration gradient, surface area, size of molecule, distance/thickness, stirring/moving

How does the size of the molecule affect the rate of diffusion?

smaller molecules require less kinetic energy to move, so move and therefore diffuse faster. They are also move likely to be able to diffuse straight through the membrane, which is faster than larger molecules diffusing by facilitated diffusion

Fill in the blanks to compare active and passive transport:

• in passive transport ___ isn’t needed but it active transport it is needed in the form of ___

• passive transport transports molecules from a ___ to ___ concentration ___ a concentration gradient

• active transport carries molecules ___ a concentration gradient, and requires ____ ____

energy, ATP, high, low, down, against, carrier proteins

True or false? active transport can occur if diffusion isn’t quick enough and cannot meet the needs of the cell

true

What is active transport?

the movement of molecules and ions through a cell from a region of lower concentration to a region of higher concentration, against a concentration gradient using energy from respiration/in the form of ATP

True or false? active transport for a specific molecule can happen both ways across a cell membrane

false - it only allows a one-way flow

In active transport, what do the molecules or ions bind to inside the carrier proteins and why is this possible?

bind to receptors due to their complementary shape

Fill in the blanks to describe the process of active transport: 

1. the molecule/ion binds to the ____ of the carrier protein

2. ATP also binds and is ___ into ___ and a ___

3. the binding of the ____ causes the carrier protein to ___ ____

4. the molecule/ion is ___

5. the phosphate molecule is ____ and recombines with ___ to form ___

6. the carrier proteins return to its ___ ___

1. receptors

2. hydrolysed, ADP, phosphate

3. phosphate, change shape

4. released

5. released, ADP, ATP

6. original shape

Give an example of a use of active transport

• reabsorption of useful molecules and ions into blood after filtration into kidney tubules 

• absorption of glucose and amino acids from digestion into the blood

• absorption of mineral ions by the roots of plants

• exchange of sodium and potassium ions in nerve impulses

What is bulk transport?

the movement of large quantities of materials into and out of cells

True or false? Bulk transport is a type of diffusion

false - it is a type of active transport

Do diffusion, osmosis and active transport transport large amounts of molecule or ions?

no - they only transport individual molecules or ions 

Give an example of a substance that is moved by bulk transport?

hormones, enzymes, phagocytes

What is endocytosis?

the bulk transport of materials into cells

Compare the 2 types of endocytosis

phagocytosis is the bulk intake of solids by a cell whereas pinocytosis is the bulk intake of liquids

What type of cell specialises in phagocytosis?

phagocytes

When phagocytes intake a substance, what are the vacuoles formed?

phagocytic vacuoles

What is the term for the pinocytosis of a very small amount of a substance?

micropinocytosis

True or false? both phagocytosis and pinocytosis work in the same way

true

Fill in the blanks for the process of endocytosis: The cell surface ____ and the membrane ___ the material until the membrane ____, forming a ___. The vesicle ____ ___ and moved into the cytoplasm where it is further ____ by the cell

invaginates, enfolds, vesicle, pinches off, processed

What is exocytosis?

the process by which materials are removed from/transported away from the cell

Fill in the blanks for the process of exocytosis: (s)___ vesicles are transported from the ___ ___ along (m)____ in the ____. The vesicle becomes linked and pulled into contact with the ___ ___ _____, with which the (p)___ ___ begin to merge. In (c)___ (f)___ the vesicle membrane fully ___ with the cell surface membrane, which creates a (f)___ (p)____ allowing the contents of the vesicle to leave the cell

secretory, Golgi apparatus, microtubules, cytoskeleton, cell surface membrane, phospholipid bilayers, complete fusion, fuses, fusion pore

exocytosis

endocytosis

What is osmosis?

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane until the water potential is in equilibrium

What is water potential?

the pressure exerted by water molecules as they collide with a membrane or container

Fill in the blanks: water potential is a measure of the water molecule potential for ___ in a solution and the ___ created by water molecules.

movement, pressure

What is the symbol for water potential?

Ψ (Greek letter psi)

What are the units of water potential?

pascals (Pa)

What is the water potential of pure water under standard conditions of temperature and pressure?

0

What are the standard conditions of temperature and pressure?

25ºC and 100kPa

What happens to the water potential in the presence of a solute?

it lowers as water molecules cluster around the solute particles, so are no longer free to move