Cell membranes/Transport

Question 1

describe the role of phospholipids in membranes

Answer 1

1 glycerol
2 fatty acid
1 phosphate group - highly charged - polar - hydrophilic region
hydrophobic region consisting of fatty acids and glycerol

because phospholipids contain both a hydrophilic region and hydrophobic region, if they are placed in water they arrange themselves in a certain way

The hydrophilic part of the phospholipid molecules (the phosphate groups) can interact with water
However, the hydrophobic parts (the fatty acids) are buried in the centre away from the water

This structure is called a phospholipid bilayer
The phospholipid bilayer is the basis of all cell membranes - includes the cell surface membrane and the membranes that surround organelles such as lysosomes and mitochondria

Question 2

functions of membranes in cells

Answer 2

they act as barriers e.g. between the internal contents of the cell/cytoplasm and the external environment OR between the contents of an organelle and the cytoplasm

Sometimes membranes separate one part of an organelle from another part e.g. in mitochondria

membranes can also be a location for chemical reactions e.g. some of the stages in respiration

membranes are involved in cell signalling

Question 3

because the cell-surface membrane contains a hydrophobic centre, what does it mean for hydrophobic molecules trying to pass through

Answer 3

because the cell-surface membrane contains a hydrophobic centre, Hydrophobic molecules such as steroid hormones can easily pass through the membrane

Question 4

because the cell-surface membrane contains a hydrophobic centre, what does it mean for hydrophilic molecules trying to pass through

Answer 4

because the cell-surface membrane contains a hydrophobic centre,

the hydrophobic centre prevents hydrophilic, water soluble molecules from easily passing through

This is because hydrophilic substances are polar - they have a charge
These substances cannot easily pass through the non-polar region of the membrane

Question 5

can water molecules pass through the cell membrane

Answer 5

Water molecules can pass through the cell membrane, even though water molecules are polar.
This is because water molecules are extremely small

However, this does take place at a slow rate

Question 6

what is the cell-surface membrane based on

Answer 6

the cell-surface membrane is based on the phospholipid bilayer

Question 7

Other parts which make up the cell-surface membrane

Answer 7

Phospholipid bilayer

within bilayer there are :

large no. of Membrane protein molecules - some of these proteins are only on one side of the membrane, whereas other proteins span the membrane from one side to the other

cholesterol - contain a hydrophobic region and hydrophilic region (at one end)
The polar hydrophilic group can attract the polar head groups on the phospholipid molecules

Rest of cholesterol molecule is non-polar and hydrophobic - this part of the cholesterol molecule can attract the non-polar fatty acids in the phospholipids

Because cholesterol interacts with phospholipids, it increases the strength of the cell-surface membrane. This makes the membrane more stable and less likely to get damaged

Cholesterol reduces the sideways movement of phospholipids and other molecules in the membrane
This helps to control the fluidity of the membrane

This prevents the membrane from becoming too fluid under warm conditions and too rigid under cool conditions

By packing the spaces between phospholipids, cholesterol helps to reduce the movement of water-soluble chemicals across the cell-surface membrane.

Question 8

describe the fluid-mosaic model of the cell surface membrane

Answer 8

The structure of the cell-surface membrane is called the fluid mosaic model

The word fluid is used because the phospholipid molecules can move around within each layer. This means that the membrane is flexible and can change shape

The word mosaic is used because the membrane is studded with protein molecules. The arrange of these proteins varies - like tiles in a mosaic

Question 9

describe structure of triglyceride and phospholipids

Answer 9

1 glycerol
3 fatty acid
non polar molecule
hydrophobic
non-soluble in water

1 glycerol
2 fatty acid
1 phosphate group - highly charged - polar - hydrophilic region
hydrophobic region consisting of fatty acids and glycerol

Question 10

state the different types of proteins found in cell-surface membranes

Answer 10

intrinsic (integral) proteins
extrinsic (peripheral) proteins

Question 11

describe intrinsic proteins

Answer 11

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

because they pass right through the lipid bilayer, intrinsic proteins have hydrophobic amino acids outside on the surface of the protein

These hydrophobic amino acids can interact with the hydrophobic fatty acid tails in the phospholipid bilayer

Question 12

state and describe the two types of intrinsic proteins

Answer 12

Protein channels are intrinsic proteins - these contain a channel running through the centre. This channel is lined with hydrophilic amino acids and is filled with water molecules.

Protein channels allow water-soluble molecules and ions to diffuse through.

Carrier proteins are intrinsic proteins. These can change their shape or position to transfer molecules or ions from one side of the membrane to the other

Question 13

what are extrinsic proteins

Answer 13

Extrinsic proteins do not span the membrane
They are found on one side of the membrane or the other

Sometimes extrinsic proteins are attached to intrinsic protiens

ontop/below phosphate group
or in one region of phospholipid bilayer

Question 14

functions of extrinsic proteins

Answer 14

Some play a structural role within the membrane

Some can act as enzymes

Some are receptors for other molecules such as hormones

Question 15

What do many membrane proteins (both extrinsic and intrinsic) also have attached to their surface

Answer 15

Many membrane proteins also contain a carbohydrate molecule attached.

These are called glycoproteins

Question 16

Functions of glycoproteins within the cell

Answer 16

Some glycoproteins allow cells to attach to each other to form tissues such as nervous tissue

Other glycoproteins play a role in the immune system presenting antigens to T cells.

Other glycoproteins act as receptors for hormones

Question 17

Where can carbohydrate molecules also be found attached to

Answer 17

Carbohydrates can also be found attached to phospholipid molecules - called glycolipids

Question 18

what is meant by a glycolipid

Answer 18

Carbohydrates can also be found attached to phospholipid molecules - called glycolipids

Question 19

describe the function of glycolipids

Answer 19

Glycolipids are often used when cells come in contact with each other
The glycolipids on the surface of one cell can be recognised by another cell
This can determine whether cell come into contact.

Glycolipids can also act as antigens for example in determining your blood group

Answer 20

particles have kinetic energy since they are moving randomly
colliding constantly with each other

Question 21

no concentration gradient means

Answer 21

concentration of gas is same everywhere

Question 22

what is diffusion

Answer 22

diffusion is the net (overall) movement of particles from a region of higher concentration to a region of lower concentration (down the concentration gradient)

net movement - overall movement

particles move randomly in all directions
however during diffusion, more particles are moving in one direction than the other

e.g. co2 in cell

conc. gradient for co2
co2 molecules will diffusion from region with a higher conc. (inside the cell) to a region with lower conc. (outside cell)
eventually co2 concentration is the same inside and outside
equilibrium has been reached - no net movement of particles therefore diffusion has stopped

Question 23

what type of process is diffusion

Answer 23

diffusion is a passive process

doesn’t require metabolic energy to take place

metabolic energy means energy released by respiration

cell membrane = partially permeable
some chemicals can easily through the membrane while others cannot

Question 24

factors affecting rate of diffusion

Answer 24

concentration gradient - the greater the concentration gradient, the greater the rate of diffusion.

particle charge/size
because cell-membrane contains a hydrophobic core, hydrophilic substances charged particles such as ions/polar molecules will not be able to diffuse through the membrane

however uncharged molecules e.g. oxygen can diffuse rapidly across the membrane

size - smaller particles will diffuse faster than larger ones

temp - particles have greater E.K. and diffuse faster in warmer conditions than colder ones
not an issue for mammals and birds which maintain a constant body temperature

SA of the membrane - diffusion takes place more rapidly if the membrane has a larger SA compared to a smaller one

distance that diffusion takes place over - greater distance, slower rate of diffusion - why membranes are so thin

Presence of carrier/channel proteins in the membrane
facilitated diffusion

Question 25

explain how water can diffuse across membranes

Answer 25

Water can diffuse across membranes - despite it being polar
Because water molecules are very small

Question 26

which type of substances cannot diffuse across cell membrane

Answer 26

Hydrophilic substances (polar molecules + ions) cannot diffuse across cell membrane# - since cell membrane contains a hydrophobic fatty acid core

problem - many of these substances required for processes inside the cell

Question 27

describe facilitated diffusion

Answer 27

In facilitated diffusion, hydrophilic substances diffuse across the cell membrane via protein molecules (carrier proteins or protein channels)

These protein molecules allow the hydrophilic substances to cross the membrane without interacting with the hydrophobic centre of the phospholipid bilayer

TYPE OF DIFFUSION
CHEMICAL STILL MOVES FROM REGION OF HIGHER CONC. TO A REGION OF LOWER CONC.

Question 28

types of protein molecules involved in facilitated diffusion and uses

Answer 28

Carrier protein - have a binding site for a specific chemical
when a chemical binds it causes the tertiary structure of the carrier protein to change
This change in the tertiary structure brings the chemical across the membrane, where the chemical is now released

Protein channels - protein with central pore
pore is lined with hydrophilic amino acids and contains water
hydrophilic substances can pass through the channel from one side of the membrane to the other

Protein channels are selective for the chemical that can pass through - means only certain chemicals can pass through - each type of protein channel

while some types of protein channels are always open, other protein channels only open in response to a certain trigger - e.g. chemical binding to protein channel or change in the voltage across the membrane

Question 29

describe what is meant by active transport

Answer 29

in active transport, carrier molecules in the membrane transport a chemical from a region of lower concentration to a region of higher concentration (against the concentration gradient)
Active transport requires metabolic energy which is provided by the molecule ATP

ACTIVE PROCESS REQUIRING METABOLIC ENERGY

Question 30

describe role of ATP in active transport

Answer 30

during active transport, the molecule/ion to be transported attaches to a receptor site on the carrier protein

Takes place on the side of the membrane where the chemical is at a lower concentration

A molecule of ATP then binds to the carrier protein

Next, the ATP molecule undergoes hydrolysis producing phosphate and a molecule of ADP

The phosphate attaches to the carrier protein and causes it to change shape

This shape change causes the carrier protein to transport the molecule or ion to the other side of the membrane where it is released

Phosphate now leaves the carrier protein, causes it to return to its previous shape

ADP and phosphate will later reform ATP during respiration

Question 31

key points about active transport

Answer 31

Uses a lot of ATP
So we find lots of mitochondria in cells which carry out a lot of active transport

carrier proteins used in active transport are specific
Each carrier protein will only transport one type of molecule/ion

carrier protein different in facilitated diffusion and active transport

Question 32

two types of active transport

Answer 32

direct active transport - protein molecule directly moves the molecule/ion against conc. gradient

cotransport

Question 33

what is meant by cotransport

Question 34

describe the role of cotransport in the ileum

Answer 34

role of ileum - to absorb the molecules produced by digestion includes the sugar glucose

glucose is absorbed from lumen of ileum into epithelial cells lining the lumen
glucose molecules then move from epithelial cell to the bloodstream

After Digestion, there is a high conc. of glucose in the lumen of the ileum
glucose molecules move down the concentration gradient by facilitated diffusion into the epithelial cells lining the ileum
Takes place via carrier proteins
these glucose then move into the bloodstream and are carried away - by same process

PROBLEM: rate of facilitated diffusion depends on the size of concentration gradient for glucose
As glucose moves into the epithelial cells, this concentration gradient falls
Means that facilitated diffusion cannot absorb all of the available glucose

So as well as facilitated diffusion, glucose is also absorbed by active transport

Two stages to this process:
1st stage involves a carrier protein called the sodium-potassium pump
Using ATP, the sodium potassium pump actively transport sodium ions out of the epithelial cells into the bloodstream
At the same time, it actively transports potassium ions into the epithelial cell

Now there is a very low conc. of sodium ions inside the epithelial cells

The lumen of the ileum contains a relatively high conc. of sodium ions

Now there is a concentration gradient for sodium ions between the lumen of the ileum and the interior of the epithelial cells

In the membrane of the epithelial cells there is a protein called the sodium-glucose cotransporter

Sodium ions can diffuse through this protein down the conc. gradient into the epithelial cell

As sodium ions diffuse down their concentation gradient, the sodium glucose cotransporter also transports glucose molecules into the cell

this means that the glucose molecules are being transported against their conc. gradient

Glucose is being active transported into the cell

ensures glucose is transported at a fast rate

energy for this glucose transport comes indirectly from the concentration gradient of the sodium ion

Epithelial cells - lots of mitochdonria
provide ATP needed for the sodium-potassium pump
The membrane of epithelial cells is folded into a large number of microvilli
increases SA of the membrane
provides more space/greater SA for membrane proteins needed for facilitated diffusion and active transport

Glucose molecules which are absorbed, are rapidly carried away by the bloodstream
this means that there is a steep conc. gradient between the epithelial cells and the blood

So facilitated of glucose into the bloodstream takes place rapidly