2.1.5- Membranes

Membranes- 2.1.5

what is the name of the model used to describe membranes

fluid mosaic model

what is a membrane

(in animals) phospholipid bilayer which separates parts of the cell or cells themselvers

functions of membranes

• barriers between internal cell contents and external environment

• location for reactions e.g. some respiration

• cell signalling

• compartmentalisation

• isolate metabolic reactions and form organelles

how do phospholipids arrange themselves to form a membrane

• hydrophilic heads on the outside and hydrophobic tails on the inside

• this forms a bilayer

what types of molecules cannot enter directly across the membrane and why

• polar/charged/hydrophilic molecules

• they cannot pass through the non-polar interior; repelled from it

why are membranes described as the fluid mosaic model

• fluid= molecules in the membrane are able to move around; are not rigid and are flexible

• mosaic= lots of protein molecules are found inside the membrane that vary in location like a mosaic

what other things does the membrane contain other than phospholipids

• proteins

• cholesterol

what are the two types of proteins found in membranes and what do they mean

• extrinsic proteins= found only on one side of the membrane

• intrinsic proteins= fully embedded in the membrane from one side to the other

examples of intrinsic proteins

• channel proteins

• carrier proteins

• glycoproteins

• glycolipids (though are not proteins)

what are channel proteins

• channel lined with hydrophilic amino acids and water molecules

• means that polar molecules are able to move across the membrane

• only passive, facilitated diffusion

what are carrier proteins

• they change their shape to move substances from one side to the other

• can be passive (when facilitated diffusion) or active (when active transport)

how are intrinsic proteins able to embed themselves fully

• they have AAs with hydrophobic R groups on the surface to interact with non-polar tails

what are glycoproteins and what is their role

• protein with carbohydrate attached

• roles:

  • cell adhesion (attaching to other cells)

  • receptors for chemical signals (e.g. hormones)

what are glycolipids and what are their roles

• phospholipid with carbohydrate attached

• roles:

  • antigens to show they are not bacteria

  • cell recognition

what roles do extrinsic proteins have

• can be receptors e.g. for hormones

• includes cholesterol

• have hydrophilic R groups on the outer surface

diffusion definition

net movement of particles from a region of higher concentration to a region of lower concentration passively (does not require any energy)

what permeability do cell surface membranes have and what does this mean

partially permeable; means some substances can enter but others cannot

factors that affect diffusion

• temperature

• molecule size

• concentration gradient

• surface area

• diffusion distance

how does temperature affect diffusion

• a higher temperature means higher rate of diffusion

• this is b/c there is more kinetic energy so particles move and therefore diffuse faster

• BUT if facilitated diffusion, too high of a temperature causes proteins to denature so rate of diffusion rapidly declines

how does molecule size affect rate of diffusion

• larger the molecule the slower the rate of diffusion

• this is b/c they are too big to fit through the membrane so takes them longer

how does concentration gradient affect rate of diffusion

• higher gradient means a higher rate of diffusion

• this is because a higher gradient means particles move faster from one side to the other, therefore increasing rate

how does SA affect rate of diffusion

• higher SA means higher rate of diffusion

• this is because there is more space for particles to diffuse at the same time so more diffusing at once so faster rate

• for facilitated diffusion this could mean the number of channel/carrier proteins to carry molecules across

how does diffusion distance affect the rate of diffusion

• the longer the distance, the slower the rate of diffusion

• this is because the particles have further to travel to get across so it takes them longer

what are the two types of diffusion and what do they mean

• simple diffusion= diffusion straight across the membrane

• facilitated diffusion= the diffusion of hydrophilic/polar/charged substances which require a protein to carry them across

how do channel and carrier proteins work in facilitated diffusion

• carrier= molecule binds to the carrier protein, which changes its tertiary structure which changes its shape to allow the molecule across to the other side

• channel= central pore which is selective for a specific chemical that it allows to pass

is facilitated diffusion active or passive

passive; no metabolic energy is required

active transport definition

the movement of molecules or ions from a region of low concentration to a region of high concentration across a membrane against the concentration gradient (active process so therefore requires energy)

where does active transport get its energy from

ATP from respiration

how does active transport work; describe and explain the process

1. the molecule being transported attaches to a receptor on a carrier protein

2. inside, ATP binds to the carrier protein, hydrolysing it into ADP and phosphate (P+) ion

3. the phosphate ion then binds to the carrier protein on the outside, changing the tertiary structure of the protein and therefore its shape

4. this transports the molecule to the other side

5. the P+ is then released and recombines with ADP in respiration to reform ATP

are the carrier proteins specific

yes; only work for specific chemicals (and are also different to those in facilitated diffusion)

osmosis definition

the diffusion of water molecules (net movement) from an area of higher water potential to an area of lower water potential across a partially permeable membrane until equilibrium reached; passive process

what are the three types of solution and describe them

• isotonic= water potential inside and out are equal so no net movement

• hypertonic= solution with a higher solute concentration so therefore a lower water potential

• hypotonic= solution with a lower solute concentration so therefore a higher water potential

what is water potential

• the pressure of water molecules on a membrane or contained measured in kPa

what solution has the highest water potential

pure water with 0kPa as there are no solutes; the higher the solute concentration, the lower the water potential

what happens to plant and animal cells when placed inside a hypertonic solution

• animals= water moves out of the cell and it shrivels (in extreme cases this leads to crenation, and the death of the cell)

• plants= water moves out and the cell membrane shrinks from the cell wall (in extreme cases is plasmolysis)

what happens to plant and animal cells when placed inside a hypotonic solution

• animal= water moves in and the cell swells; in extreme cases cytolysis occurs (the bursting of the cell)

• plant= water enters, increasing the hydrostatic pressure, pushing the membrane against the cell wall, becoming turgid

why do animal cells burst in hypotonic solutions but plant cells don’t

• animal cells have a very thin membrane that is unable to handle the increased hydrostatic pressure from the entering water

• plants have a cell wall, which has very high tensile strength and so does not break under the increased hydrostatic pressure of the water

what is bulk transport

form of active transport for molecules that are too big to fit in carrier proteins e.g. enzymes, hormones, bacteria

what are the two types of bulk transport

• endocytosis and exocytosis

describe and explain the process of endocytosis

• is bulk transport into the cell

1. cell surface membrane invaginates (bends inwards)

2. the cell surface membrane then engulfs and fuses to form a vesicle

3. the vesicle then breaks off and goes for processing

what types of molecules would go through endocytosis

• bacteria to be broken down (phagocytosis)

describe the process of exocytosis

• the movement of molecules out of the cell

1. vesicles (usually from the golgi) fuse with the cell surface membrane, transported by the cytoskeleton

2. contents are then released on the other side

examples of molecules that would go through exocytosis

• lipids

• proteins

• etc

what processes require energy in bulk transport

• movement of vesicles along the cytoskeleton

• to change the cell shape to engulf

• fusion of the cell membrane as vesicles form

fluidity definition

the viscosity of the lipid bilayer of a cell membrane; how much it is able to move

permeability definition

the ability for substances to move in and out of the cell membrane through diffusion

factors that affect the fluidity of membranes

• temperature

• solvent

• cholesterol concentration

• type of fatty acid

how does temperature affect fluidity and permeability

• the higher the temperature, the higher the kinetic energy so the phospholipids move more and so increases fluidity

• also increases permeability as there are more gaps when moving more BUT if the temperature is too high then proteins denature and permeability decreases

• lower temperatures mean lower fluidity as less movement and lower permeability as less gaps

how do solvents affect fluidity and permeability

• water is polar and essential for the phospholipid bilayer formation

• other organic solvents are less polar e.g. alcohols and so disrupt the membrane

• this increases fluidity and permeability as the structure is less together so there are more gaps

• this is why alcohol is used in antiseptics; destroys the membranes of bacteria

how does cholesterol concentration affect fluidity of the membrane

• the higher the concentration, the lower the fluidity because cholesterol embedded in the membrane controls fluidity by increasing stability and holding phospholipids together

• in higher temperatures, there is more cholesterol and vice versa

• therefore permeability also decreases as there are less gaps and vice versa

how does the type of fatty acid affect fluidity and permeability

• unsaturated fatty acids means higher fluidity as there are kinks, increasing the gaps between fatty acids so can move more; also means increased permeability

• vice versa for saturated