Cell membrane (1st year) Topic 2

Question 1

What is the plasma membrane?

Answer 1

The plasma membrane of the cell separates the internal contents of the cell from its external environment. It only allows specific substances to enter or leave the cell.

It is differentially permeable

Question 2

Mention some functions of the plasma membrane.

Answer 2

1) It separates the contents of the cell from its external environment.
2) Communicates with other cells.
3) Regulates the exchange of substances in and out of the cell between the cytoplasm and its external environment.
4) Chemical reactions occur on it.
5) It acts as a receptor site to recognize external stimuli.

Question 3

What is the plasma membrane made up of?

Answer 3

Phospholipids

Question 4

What are phospholipid molecules made up of ?

Answer 4

Hydrophilic polar phosphate heads and hydrophobic non-polar fatty acid tails.

Question 5

How do phospholipid molecules arrange themselves within the cell?

Answer 5

In a single layer they arrange themselves.The hydrophilic phosphate head spread over the surface of the water, whilst the hydrophobic fatty acid tails arrange themselves away from the water,forming a bilayer.

Question 6

What can we also find on the surface of the plasma membrane?

Answer 6

Glycolipids,glycoprotein,cholesterol, proteins

Question 7

What is a glycolipid?

Answer 7

carbohydrate + lipid
contains polar and non-polar parts
Also contains carbohydrate side chains which is used for cell signalling.

Question 8

What is a glycoprotein?

Answer 8

carbohydrate + protein
can be amphipathic (having both polar and non-polar regions). Also contains carbohydrate side chains used for cell signalling

Question 9

What is cholesterol?

Answer 9

Slightly similar to lipids.
It is slightly polar at one end

Question 10

What can we call the plasma membrane instead of a phospholipid bilayer?

Answer 10

The fluid mosaic model

Question 11

What are micelles?

Answer 11

Micelles are spherical structures that form in water which has its hydrophilic phosphate head molecules pointing outward (water-loving),whilst its hydrophobic fatty acid tails pointing inward (water-hating). These structures are important for the absorption of fats in living organisms. They form if there are phospholipids present in large amounts.

Question 12

What are polar molecules?

Answer 12

Electrons are unevenly distributed, due to the difference in electronegativity between the atoms. Polar molecules never mix with non-polar molecules.

Question 13

What are non-polar molecules?

Answer 13

Electrons are evenly distributed. No electronegativity between the atoms,therefore there are no charges.

Question 14

What is freeze-fracturing?

Answer 14

This technique has allowed us to better understand how proteins fit into the phospholipid bilayer. First cells are rapidly frozen ,then the surfaces are cut with a sharp metal blade, and this allows the insides to be examined.

Question 15

Do phospholipid molecules stay still in the cell?

Answer 15

No,they are constantly moving which means they are fluid.

Question 16

Differentiate between saturated and unsaturated fatty acid tails.

Answer 16

The fatty acid tails in the plasma membrane can be either saturated or unsaturated. When fatty acid tails are unsaturated,they are more fluid and they move more. The more unsaturated a fatty acid tail is,the more fluid it is. Unsaturation means that there is the presence of double bonds. If a fatty acid tail is bent,it is because it is unsaturated.If it is straight it means that it is saturated.

Question 17

Mention the different proteins that can be found on the plasma membrane

Answer 17

Integral (intrinsic) proteins: embedded within the membrane)
Transmembrane proteins (proteins that span the width of the entire membrane
Extrinsic (peripheral) proteins: attached to the membrane’s surface

Question 18

What is the main function of cholesterol in the plasma membrane?

Answer 18

To avoid the phosopholipids from sticking to eachother,therefore making them more fluid. Cholesterol also makes membranes flexible and stable.Also reduces the escape of polar molecules through the membrane.

Question 19

Can the proteins within the membrane have hydrophilic and hydrophobic regions?

Answer 19

Yes,some parts of a protein can have hydrophilic regions and other parts can have hydrophobic regions within the same protein.

Question 20

How can we further classify these types of proteins? Specifically the transmembrane.

Answer 20

Channel proteins and carrier proteins

Question 21

What are carrier and channel proteins?

Answer 21

They are proteins which are responsible in the selective transport of polar molecules and ions across the membrane through facilitated diffusion or active transport.

Question 22

Can you mention some functions of these proteins within the plasma membrane?

Answer 22

They can be used as receptor molecules to act as receptor sites for cell signalling between cells. For example hormones are chemical messengers which circulate in the blood and bind to specific target cells which have the correct antigens.
Proteins also take part in energy transfer systems in process like photosynthesis and respiration.

Question 23

What is an antigen?

Answer 23

A cell identity marker. For example glycolipid or glycoprotein.

Question 24

Draw a diagram of a phosopholipid bilayer,showing cholesterol,protein with labelling

Question 25

Cells need essential substances like glucose,oxygen,amino acids etc and need to get rid of toxins like carbon dioxide. How do these material pass into and out of the cell through the plasma membrane? Mention 4 transport mechanisms.

Answer 25

Diffusion
Osmosis
Active transport
Bulk transport (endocytosis or exocytosis)

Question 26

Which of these 4 transport mechanisms are passive and which ones use energy?

Answer 26

Passive: Diffusion
Osmosis
Energy needed: Active transport and Bulk transport

Question 27

What is an ion?

Answer 27

An ion is an atom which is losing or gaining an electron.Therefore it is a charged particle.

Question 28

Some molecules or ions are small enough to squeeze and pass through the cell membrane by simple diffusion such as water molecules,oxygen and carbon dioxde. Define Diffusion.

Answer 28

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration until an equilibrium is reached. (Molecules are evenly spread out)

Question 29

What is a concentration gradient?

Answer 29

The difference between the 2 regions of diffusion. (The one which has the most particles and the one which has the less particles)

Question 30

What is a solution made up of ?

Answer 30

A solute and a solvent. The solvent dissolves the solute making the solution.

Question 31

Mention 3 factors which affect the rate of diffusion.

Answer 31

1) Rate of diffusion decreases rapidly with distance: diffusion is effective if short distances are present.
2)The greater the surface area,through which diffusion is taking place,the greater the rate of diffusion. (the larger the cell,the smaller the surface area,which means that diffusion wouldnt be as effective.This limits cell size.
3)The steepness of the diffusion gradient: the steeper the gradient,the faster is the rate of diffusion. This is ideal for efficiency.

Question 32

Some molecules and ions like glucose,amino acids,or glycerol,or sugars are too large and are repelled by the hydrophobic region of the membraneand need other methods of transport to enter the membrane.What is facilitated diffusion?

Answer 32

Facilitated diffusion is the movement of molecules from a region of high concentration to a region of low concentration across a membrane, with the help of a transport protein.

Question 33

Therefore which proteins do these substances make use of the cross the membrane?

Answer 33

Channel proteins and carrier proteins. These proteins have a hydrophilic environment which allows the entry of these substances to pass through the membrane by facilitated diffusion.

Question 34

What are the “gated ion channels” known as?

Answer 34

Channel proteins

Question 35

What is their main function? (Gated ion channels)

Answer 35

Transport proteins which have a fixed shape and allow the passage of ions are called ion channels.

The ion channels can exist in either an open or a closed state.

Allow the passage of ions.

Question 36

What is the main function of carrier proteins?

Answer 36

Exist in 2 states : “ping and pong” (the protein rapidly alternates between the 2 shapes ping and pong.
Change their shape often
The binding site faces outwards

Allow the passage of molecules

Question 37

What is osmosis?

Answer 37

Osmosis is a special kind of diffusion which only involves water molecules. Osmosis occurs through a barrier which is semi-permeable,which only allows small water molecules and not other big molecules like sugar molecules.

Question 38

Define osmosis

Answer 38

Osmosis is the movement of water molecules from a dilute solution to a concentrated solution through a semi-permeable membrane

Question 39

What is a dilute solution?

Answer 39

A solution which has a lot of water molecules.

Question 40

What is a concentrated solution?

Answer 40

A solution which has few water molecules.

Question 41

What happens when the dilute solution and the concentrated solution are separated by a semi-permeable membrane?

Answer 41

Water molecules will diffuse from the region where there is a lot of free water molecules to the region where where there is a few water molecules.

Question 42

What do we call a membrane which allows all substances to pass through?

Answer 42

Permeable membrane

Question 43

What do we call a membrane which does not allow substances to pass through?

Answer 43

Non-permeable

Question 44

Define isotonic solution and explain what happens to a plant cell and an animal cell in this type of solution and describe the terms that we give them.

Answer 44

An isotonic solution is a solution in which the concentration of the solutes is the same both inside and outside of the cell.
(50 % solute 50% water) No net effect will be seen (normal)
Animal cell: Normal
Plant cell: Flaccid
Water will diffuse in and out of the cell normally and not net effect will be seen.

Question 45

Define hypertonic solution and explain what happens to a plant cell and an animal cell in this type of solution and describe the terms that we give them.

Answer 45

A hypertonic solution is a solution in which a higher concentration of solute is present than the inside of the cell. This will cause the water inside of the cell to move out,which will cause the cell to shrink,to decrease the higher concentration of solute in the enviroment.
Animal cell: shriveled/it shrinks
Plant cell: Plasmolysed
In plant cells,the water will move out from the vacuole by osmosis,and the cell membrane will detach from the cell wall.
Animal cell: for example in red blood cell Crenation happens. The water in the red blood cell will move out which will cause the volume to decrease and the cell membrane to shrink

Question 46

Define hypotonic solution and explain what happens to a plant cell and an animal in this type of solution and describe the terms that we give them.

Answer 46

A hypotonic solution is a solution in which the solution will have a lower concentration of solute in the outside environment that inside of the cell. This will cause water to move into the cell which causes the cell to swell.
Animal cell: burst/swells/lyses
Plant cell: Turgid
In plant cells water will continue to move into the vacuole but it will not burst due to strong cells walls.
In animal cells for example a red blood cell Haemolysis occurs. This is because water willenter the red blood cell by osmosis which will cause the volume to increase causing the cell to burst with the escape of haemoglobin

Question 47

Define active transport

Answer 47

In active transport,movement of molecules takes place from a region of low concentration to a region of high concentration against a concentration gradient,with the help of a carrier protein and energy from ATP.

Question 48

When does active transport happen?

Answer 48

It happens when a substance in higher concentration should enter the cell the membrane stops it because it is harmful

Question 49

Why is active transport beneficial?

Answer 49

This is because it gives the cell more control over which substances should enter the cell. If diffusion would be the only method by which materials could enter the cell,then the cell would have too many substances in it and would have no control of what the cell needs the most and even harmfoul substances could enter it,so active transport gives the cell more control on what the cell really needs.

Question 50

True or false, the plasma membrane has control on what enter or leaves the cell thanks to active transport even when the substances are present in high concentrations it can allow these substances to not enter (no entry of substances) It is going against the principles of diffusion,this is called active transport

Answer 50

True

Question 51

The cells that carry out active transport do not have a lot of mitochondria,true or false?

Answer 51

False

Question 52

What is the sodium potassium pump?

Answer 52

The sodium potassium pump is a carrier protein which spans the membrane from one side to another. This sodium pump is coupled with a potassium pump which actively moves potassium ions in the cell with the help of ATP. ATP changes into ADP which causes the carrier protein to change shape.

Question 53

How many sodium and potassium ions are exchanged between the inside and the outside of the cell?

Answer 53

3 sodium ions move out and 2 potassium ions move in

Question 54

Why is the sodium potassium pump important?

Answer 54

1) it keeps the electrical activity of nerve cells and muscle cells.
2) Controls the osmotic balance of animal cells
3) Drive active transport of substances like sugars and amino acids

Question 55

What are endocytosis and exocytosis?

Answer 55

These are active processes involving the bulk transport of materials through membranes either into cells (endocytosis) or out of cells (exocytosis)

Question 55

How does endocytosis happen?

Answer 55

Occurs by an infolding or extension of the cell surface membrane to form a vesicle or a vacuole

Question 55

What are the 2 types of endocytosis?

Answer 55

Phagocytosis and Pinocytosis

Question 55

Mention some characteristics of phagocytosis

Answer 55

Material is taken up in solid form.
Some phagocytes specialize in phagocytosis ex:some white blood cells engulf bacteria

Question 56

Mention some characteristics of pinocytosis

Answer 56

Material is taken up in liquid form
Vesicles formed are very small

Question 56

What is exocytosis?

Answer 56

The reverse process of endocytosis.
This is when waste materials are removed from cells and useful chemicals like enzymes are secreted

Question 57

Mention some examples where simple diffusion takes place.

Answer 57

Food is mostly absorbed by the intestine in this way,or when gases move in and out of red blood cells or in leaves

Question 58

Mention some examples where osmosis takes place.

Answer 58

Water moves into amoeba by osmosis or when water moves into or out of plant cells.

Question 59

Mention some examples where active transport takes place

Answer 59

Glucose is absorbed by active transport through the small intestines,and in plants root hair cells have protein carriers that pump substances into the hair cells by active transport.