biological membranes

Question 1

role of membranes within cells

Answer 1

-provide internal transport system
-selectively permeable to regulate passage of molecules into or within organelles
-provide reaction surface
-isolate organelles from cytoplasm for specific metabolic reactions.

Question 2

Functions of membranes on the surface of cells

Answer 2

Isolates cytoplasm from extracellular environment
Selectively permeable to regulate transport of substances
Involved in cell signalling/cell recognition

Question 3

Describe the fluid mosaic model of membranes

Answer 3

Phospholipid molecules form a continuous double layer(bilayer). Fluid-phospholipid are constantly moving =membrane has flexible shape

Mosaic= extrinsic and intrinsic proteins of different sizes and shapes are embedded. Some proteins have a carb attached -glycoproteins and some have a lipid attached- glycolipids. It also has cholesterol molecules in the bilayer

Question 4

Draw the phospholipid bilayer

Question 5

What’s three factors that affect membrane permeability

Answer 5

Temp
Solvents
Ph

Question 6

How can substances move across the plasma membrane

Answer 6

Osmosis
Diffusion
Active transport

Question 7

How do phospholipids form the bilayer

Answer 7

They have a head and tail. The head is hydrophilic so it attracts water the tail is hydrophobic it repels water.
The molecules automatically arrange themselves in a bilayer where the heads face out towards the water on either side of the membrane
The centre of the bilayer is hydrophobic so it doesn’t allow water soluble substances/hydrophilic molcules (ions and polar molecules)to diffuse through it, it acts as a barrier to dissolved substances.
Fat soluble substances/hydrophobic molcules dissolve in bilayer and easily pass through

Question 8

Why can’t water soluble substances go past the hydrophobic centre

Answer 8

Because of the hydrophobic tails being non polar and water soluble substances being polar

Question 9

Why can water molcules pass through the cell membrane

Answer 9

Because even though it’s polar it’s extremely small

Question 10

Function of cholesterol in phospholipid bilayer

Answer 10

Gives the membrane stability.
Cholesterol molecules have a hydrophilic group on one end, this can attract the the polar hydrophilic heads on the phospholipid molecules. The rest of the cholesterol molecule is hydrophobic and non polar which can attract the hydrophobic tails of the phospholipids.
So because it fits between the phospholipids and binds to the tails it packs the molcules more closer together increasing strength of cell surface membrane.
This makes the membranes less fluid more rigid preventing the sideways movement of phosphilipids and other molecules.
And this prevents membrane from being too fluid in warm conditions and too rigid in cool conditions.
Also by packing spaces between phospholipids it reduces movement of water soluble substances across cell surface membrane.

Question 11

Where is cholesterol molecules found

Answer 11

Lipid found In all cell membranes except bacterial

Question 12

What’s the functions of proteins in the bilayer

Answer 12

Controls what enters and leaves the cell
Extrinsic proteins - Binding sites/receptors
Antigens
Bind cells together
Involved in cell signalling

Intrinsic proteins - electron carriers(photosynthesis and resp)
Channel proteins (allow small charged particles through with facilitated diffusion)
Carrier proteins(transport larger molcules and charged particles across membranes by facilities diffusion and active transport)

Question 13

What’s the function of glycoproteins and glycolipids

Answer 13

Stabilise the membrane by forming hydrogen bonds with surrounding water molecules.
Some GP allow cells to attach to each other to form nervous tissue
Act as receptors for messenger molecules in cell signalling and is a binding sites for drugs antibodies and hormones.
They can act as antigens - cell surface molcules involved in self recognition and the immune response

Question 14

What are glycolipids and glycoproteins

Answer 14

Carbs attached to lipids and carbs attacged to proteins

Question 15

How does temp affect membrane permeability

Answer 15

It affects how much the phospholipids in the bilayer can move.
Temps below 0= phospholipids don’t have much energy so can’t move much .they’re packed close and rigid but channel and carrier proteins in the membrane denature(losing structure and function) increasing permeability of membrane. I’ve crystals may form and pierce the membrane making highly permeable when it thaws

Temps between 0-45- phospholids can move around and aren’t packed tight tg so the membrane is partially permeable. As the temp increases the phospholipids move more because they have more energy this increases permeability of membrane.
Temps above 45- Pbilayer starts to melt and break down and membrane becomes more permeable. Water inside cell expands putting pressure on membrane. Channel proteins and Carrier proteins denature so they can’t control what enters or leaves the cell- inc permeability of membrane

Question 16

How to investigate membrane permeability

Answer 16

Colorimetry-
1-cut 5 equally sized pieces of beetroot and rinse the pieces to remove an pigment released
2-add 5 pieces to diff test tubes containing 5cm of water and use a measuring cylinder/Pipette to measure the water
3- placed each test tube in a water bath at diff temps for same length of time
4- remove pieces of beetroot from tubes leaving just coloured liquid
5- turn on colorimeter and leave for 5 mins for it to stabilise and then set it up with a blue filter
6- add distilled water to a cuvette and put it into the colorimeter and calibrate the machine to zero.
7- use a pippete to transfer a sample of liquid from first test tube to clean cuvette
8-put cuvette in colorimeter and read and record absorbable
9- repeat 7-8 for the other 4 liquids
The higher the absorbable reading the more pigment released so the higher the permeability of membrane

Question 17

What is diffusion

Answer 17

The net movement of particles from an area of higher conc to an area of low conc.

Question 18

How does diffusion work

Answer 18

Particles will diffuse down a concentration gradient until particles are evenly distributed between both concentration where equilibriums reached and there is no overral net movement.

Question 19

Does diffusion need energy?

Answer 19

No it’s a passive process no metabolic energy needed. It can easily diffuse across plasma membrane as long as particles in membrane can move freely.

Question 20

Factors that affect rate of diffusion

Answer 20

Conc-gradient-the greater this is the greater the rate of diffusion

Particle charge/size-because of the cell membranes hydrophobic core charged particles will not be able to diffuse through but some molecules like water can because of its size. The smaller the molecule the faster the rate of diffusion

Temp-in warmer conditions particles have more Ke and diffuse faster

SA-larger the Sa the faster ROD as there’s more space for molecules to diffuse across a membrane

Distance - greater distance the slower ROD. Particles have to travel more

Question 21

Hydrophilic substances are needed for many processes in the cell so how do they get by the cell membrane?

Answer 21

Through facilitated diffusion

Question 22

What happens in facilitated diffusion

Answer 22

In facilitated diffusion hydrophilic substances diffuse across cell membrane via carrier proteins or channel without interacting with the hydrophobic core of the bilayer
Carrier proteins move large molcules into or out the cell down the conc gradient. First a large molecule attached to carrier protein in membrane
Then the tertiary structure of protein changes shape.
This change on shape brings the chemical across membrane where it’s now released.
Channel proteins- form pores in membrane for smaller ions and polar molcules to diffuse though down conc gradient.The pore is lined with hydrophilic AA and contains water. Hydrophilic substances can pass through channel. These are very specific to the chemical that passes through so only certain ones can pass though each type of PC.They may not always be open and may need to get triggered, either by a chemical binding to the channel or a change in voltage across membrane.

Question 23

Similarities between facilitated diffusion and diffusion

Answer 23

Both have particles moving from a high concentration to a low concentration down a conc gradient
They’re both passive as well

Question 24

How to investigate diffusion (conc gradient)

Answer 24

With agar jelly- same consistency to cytoplasm of real cell- and phenolphthalein (ph indicator) it’s pink in alkali solutions and colourless in acidic. So if you place cubes of agar jelly in phenolphthalein and an alkali(NaOH) it will turn colourless as the acid diffuses into agar jelly neutralising NaOH.
1) make up some agar jelly with Phnelohtlanr and dilute NaOH.
2)prepare 5 test tubes containing Hcl in inc conc
3)cut out 5 equal sized cubes from agar jelly
4) put one of the cubes in test tube and time how long it take to turn colourless
5) do the same for rest
The cube in highest conc of HCL to go go colourless fastest as it has highest concentration gradient

Question 25

How to investigate diffusion based on SA and temp

Answer 25

Cut the agar jelly into different sized cubes this times and work out SA:V ratio. And time how long it takes to go colourless. Cube w the highest Sa:V will.

Temp-prepare agar jelly and cut into equal sized cubes and prepare several boiling tubes w same conc of HCl and put it into water baths of diff temps. Then put agar jelly into each and time how long it takes to go colourless. Highest temp would go fastest

Question 26

What is osmosis

Answer 26

The net movement of water molecules form an area of high water potential to an area of low water potential across a partially permeable membrane.

Question 27

What’s the highest water potential and what can adding solutes do to it

Answer 27

0 KPa(pure water)
dilute the water potential into negatives so the more negative a solution the stronger the concentration of solutes

Question 28

When does the net movement of water become zero

Answer 28

When the water potential between both concentrations are equal and water molecules are evenly spread out and have reached equilibrium.

Question 29

Why does the cytoplasm of all animal cells have a negative alter potential

Answer 29

Contains a number of solutes dissolved in water these solutes could be mineral ions and glucose

Question 30

What is the same as the water potential of cytoplasm and whats this mean

Answer 30

The water potential of blood plasma in RBC
Osmosis isn’t taking place as water potential is same outside and inside but water molcules are still moving across membrane but rate of movement inwards and outwards is the same so no net movement of molcules.
If two solutions have same water potential then they’re isotonic.

Question 31

What happens when we put an animal and plant cell into pure water and what does that make the solution

Answer 31

Water potential outside cell is higher than inside so water molcules will move inside cell, this movement of water molcules causes pressure inside the cell to increase rupture and burst.
In plant it will increase pressure inside and cause cell to become turgid but not burst due to the inelastic cell wall
This makes the solutions hypertonic ( higher water potential than inside)

Question 32

When an animal cell and plant cell is placed In a solution with a lower water potential then what happens

Answer 32

Water will move out of cell by osmosis. Solutions with lower water potentials are hypertonic. If animal cell placed in hypertonic solutions it’s gna shrink.
If plant cell placed in a hypertonic solution it’s Gna become flaccid. It w experience plasmolysis ( cytoplasm and plasma membrane will eventually pull away from cell wall)

Question 33

How to Investigate the effect of water potential on plant cells

Answer 33

1-prepare sucrose solutions of 0.0,0.2,0.4,0.6,0.8,1.00
2-use cork bored to cut potatoes into identically sized cylinders 1cm in diameter and measure the mass of each
3-place the cylinders into each solution and leave for 20 mins for each
4- remove cylinders and pat it dry and weigh mass again
5- calc % change
Potato cylinders will gain water with solutions with a higher water potential and lose water in solutions with a lower water potential

Question 34

How to investigate the effect of water potential on animal cells

Answer 34

1- make up NaCl solution of diff conc 0.2,0.4,0.6,08,1.0
2- take your deshelled eggs and carefully pat them with absorbent paper to remove excess moisture
3-weigh each eggs w mass balance
4-place each egg in diff beaker covered with Nacl and leave eggs for same amount of time
5-remove eggs dry and weigh again.
6-Calc % change

Question 35

What’s active transport and how’s it work

Answer 35

It uses energy to move molcules and ions across plasma membranes against a concentration gradient.
molecule attaching to the receptor site of carrier protein and then an ATP molecule binds to carrier protein and ATP undergoes hydrolysis making phosphate and a molecule of ADP the phosphate attaches to carrier protein causing it to change shape. this moves the molecule across the membrane from a region of lower concentration to a region of higher concentration releasing it on the other side.phosphate now leaves carrier protein causing it to return to its original shape and ADP and phosphate will reform atp during resp
It’s an active process so this metabolic energy is used from ATP to move solute against its concentration gradient

Question 36

What happens in endocytosis

Answer 36

A cell can surround a substance with a section of its plasma membrane then the membrane pinched off to form a vesicle inside the cell containing the ingested substance - and this substance has been taken in by endocytosis
This is active so it uses atp for energy

Question 37

When do molcules under go endocytosis

Answer 37

When molecules are too large to be taken into a cell or when cells want to take larger substances into the cell eg WBC bring in dead organisms and microorganisms by endocytosis to destroy them

Question 38

What happens in expcytosis

Answer 38

Substances made by cell( hormones,digestive enzymes, lipids) need to be released and this is done by exocytosis.
Vesicles containing these substances pinch off from the sacs of Golgi and move towards plasma membrane. The vesicles fuse with the cell surface plasma membrane and release contents outside the cell ( some substances like membrane proteins in steered straight into plasma membrane).
This is an active process and requires atp for energy