C5 Plasma membranes

Question 1

What is a plasma membrane known as and why?

Answer 1

Fluid mosaic model
Fluid = components can move freely along the membrane
Mosaic= components of different shapes and sizes can fit together

Question 2

What are the top and bottom of a plasma membrane called?

Answer 2

-external = tissue fluid
-internal = cytoplasm

Question 3

What components does a plasma membrane contain ?

Answer 3

• lipids (phospholipids and cholesterol
• proteins
• carbohydrates

Question 4

What does the phospholipid bilayer contain?

Answer 4

Phosphate heads - hydrophilic
Fatty acid tails - hydrophobic
Glycerol molecule in the middle
Two layers hence jus called a bilayer

Question 5

General functions of a plama membrane:

Answer 5

1) Compartmentation
2) site of chemical reactions
3) acts as a barrier

Question 6

What does compartmentation in plasma membranes do?

Answer 6

• provides specific conditions for a certain reaction to occur

Question 7

Example of compartmentation in plasma membranes

Answer 7

EG: When within a cell they want to keep their cell organelles in a specific area
EG: Mitochondrial membrane keeps all the mitochondrial DNA and protein within for respiration to occur

Question 8

Explain the site of chemical reactions in plasma membranes:

Answer 8

Plasma membranes hold proteins/enzymes (crucial for photosynthesis and respiration) that are involved in certain chemical reactions

Question 9

Explain acts as a barrier in plasma membranes:

Answer 9

• It is partially permeable
• Only allows small, lipid-soluble/non-polar substances to diffuse across

Question 10

What are the two different types of proteins found in plasma membranes ?

Answer 10

1) intrinsic - imbedded in the membrane
2) extrinsic - exists on the surface of the membrane

Question 11

Examples of intrinsic proteins

Answer 11

1- carrier proteins
2- channel proteins

Question 12

What is a carrier protein used for ?

Answer 12

A carrier protein is used to transport molecules across the membrane , carrier proteins require energy as they change shape

Question 13

What processes are carrier proteins involved in?

Answer 13

1) mainly active transports
2) sometimes passive transport (which is facilitated diffusion)

Question 14

Difference between passive and active transport

Answer 14

In Active transport the molecules are moved across the cell membrane, pumping the molecules against the concentration gradient using ATP (energy). In Passive transport, the molecules are moved within and across the cell membrane and thus transporting it through the concentration gradient, without using ATP (energy)

Question 15

Type of passive diffusion

Answer 15

facilitated diffusion

Question 16

What does facilitated diffusion transport in in plasma membranes

Answer 16

-polar molecules
-hydrophillic molecules

Question 17

What does active transport transport in plasma membranes?

Answer 17

Metal ions

Question 18

What is a channel protein?

Answer 18

a transmembrane protein that moves substances without binding to them and without spending energy.

Question 19

What is a carrier protein?

Answer 19

A carrier protein is a membrane protein that moves solutes across the membrane by creating conformational changes in the protein

Question 20

What does “specific mean”?

Answer 20

you can have a specific channel protein for one specific molecule
eg: aquaporins - water

Question 21

What is the plama membrane known as in terms of permeability ?

Answer 21

before it is known as partially permeable, but with carrier and channel proteins it is now as selectively permeable

Question 22

What do the branches represent?

Answer 22

• carbohydrate chains (sugar chains)

Question 23

What do the carbohydrate chains act as?

Answer 23

• receptors
  -antigens

Question 24

What can a cholesterol chain be attached to?

Answer 24

can be attached to a protein or a phospholipid

Question 25

What is the protein called when it is attached to a carbohydrate chain?

Answer 25

glycoprotein

Question 26

What is the phospholipid called when it is attached to a carbohydrate chain?

Answer 26

glycolipid

Question 27

how can a carbohydrate chain act as a receptor or antigen?

Answer 27

receptor= receives certain EG: growth hormones
antigen = for cell recognition

Question 28

large part of the channel protein vs smaller part

Answer 28

larger part of the channel protein is the hydrophillic amino acids wheras the smaller parts are the hydrophobic amino acids

Question 29

what does the cholesterol do in the plasma membrane?

Answer 29

Helps to stabilise the membrane
Helps to regenerate the fluidity
(makes the membrane rigid or not rigid)
centre part = hydrophobic

Question 30

What factors affect membrane structure ?

Answer 30

1- solvents
2- temperature

Question 31

How does temperature affect the plasma membrane?

Answer 31

Between all the structure in the plama membrane, there are weak intermolecular forces which stabilise the entire structure and holds phospholipids in roughly the same place because it is a fluid

Increasing temperature means phospholipid will vibrate more and will eventually break together, permeability will increase

Protein carriers and channels can also denature

Question 32

How can solvent affect the structure of a plasma membrane ?

Answer 32

Certain solvents, such as alcohols or detergents, can dissolve the lipid bilayer and disrupt the structure of the membrane, leading to increased permeability.

EG: Alcohol wipes

Question 33

Practical investigations into factors affecting membrane structure and permeability

Answer 33

You can investigate how these different factors affect membrane structure and permeability using beetroot
Beetroot cells contain a dark purple-red pigment
The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell

Question 34

What is a colorimeter ?

Answer 34

A colorimeter is a machine that passes light through a coloured liquid sample and measures how much of that light is absorbed (and therefore gives an indication of how much of the colour is present in the solution)
A colour filter is used in the light path to ensure that the correct wavelength of light is used to measure the optical density of the specific pigment in the solution (e.g. the beetroot pigment called betalain)
The colorimeter must be zeroed before each colorimeter tube (called a cuvette) is inserted. This can be done using distilled water in a cuvette

Question 35

Method to work out how temperature affects permeability?

Answer 35

Using a scalpel, cut five equal-sized cubes of beetroot
The pieces must have the same dimensions so that they all have equal surface areas and volumes, as these factors could affect the rate at which the pigment leaks out
A cork borer can also be used, as long as the cores are cut to the same length
You should also use a digital balance to check that all pieces have the same mass
Rinse the beetroot pieces
To remove any pigment released during cutting
Add the beetroot pieces to five different test tubes, each containing the same volume of water (e.g. 5cm3)
Put each test tube in a water bath at a different temperature (e.g. 10℃, 20℃, 30℃, 40℃, 50℃) for the same length of time
The time should be long enough to allow the pigment to diffuse into the water (e.g. around 30 minutes)
Remove the beetroot pieces, leaving just the coloured liquid in the five test tubes
Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability

Question 36

In general results

Answer 36

The general pattern you would expect to see is that as temperature increases, membrane permeability also increases
As temperature increases, the phospholipids within the cell membrane move more because they have more energy
Increased movement means the phospholipids are not as tightly packed together, increasing the permeability of the membrane
At high temperatures, the phospholipid bilayer may even start to melt and breakdown, further increasing the permeability of the membrane
In addition, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell. These factors also increase the permeability of the membrane
Temperature also affects the conformation (3D shape) of proteins as at high temperatures the intermolecular forces between amino acids are broken which affects the protein’s specificity and function
If experimenting with temperatures below 0oC, membrane permeability may also be increased (once the cells have thawed again)
Increased permeability can be caused by channel or carrier proteins deforming at these low temperatures
Ice crystals that form can also pierce the cell membrane, making it highly permeable

Question 37

What is the definition of diffusion ?

Answer 37

Diffusion is the net movement of particles down the concentration gradient through a partially permeable membrane

Question 38

What is the definition if facilitated diffusion ?

Answer 38

facilitated diffusion is diffusion across protein channel on a plasma membrane it is selective permeability

Question 39

What are the factors that affect diffusion rate?

Answer 39

1- Higher temperature
2- concentration difference
3- SA to V ratio
4- Diffusion distance

Question 40

Why is diffusion enough for unicellular organisms ?

Answer 40

Unicellular organisms have a very high SA to V ratio hence diffusion is enough

multicellular systems need a double circulatory system

Question 41

What is the definition of active transport?

Answer 41

Movement of particles against the concentration gradient using energy /ATP involving protein carriers
EG: RHC and mineral ions

Question 42

What is bulk transport?

Answer 42

Active transport of large molecules and who cells such as :glucose and bacterial cells

Question 42

Examples of bulk transport

Answer 42

Endocytosis and exocytosis

Question 43

What is endocytosis and exocytosis ?

Answer 43

Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane, and bringing it into the cell.

Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell

Question 44

How do we investigate water potential?

Answer 44

It is possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate the water potential of the plant tissue itself
The most common osmosis practical of this kind involves cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required)

Question 45

Method of investigating different water potentials

Answer 45

The required number of potato cylinders are cut (one for each of the solutions you are testing – or more than one per solution if you require repeats)
They are all cut to the same length and, once blotted dry to remove any excess moisture, their initial mass is measured and recorded before placing into the solutions
They are left in the solutions for a set amount of time (eg. 30 minutes), usually in a water bath (set at around 30o)
They are then removed and dried to remove excess liquid
The final length and mass of each potato cylinder is then measured and recorded
The percentage change in mass for each potato cylinder is calculated

Question 46

What does a positive percentage change in mass suggest

Answer 46

A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution had a higher water potential than the potato
The gain of water makes the potato cells turgid, as the water exerts turgor pressure (or hydrostatic pressure) on the cell walls – the potatoes will feel hard

Question 47

What does a negative percentage change suggest

Answer 47

A negative percentage change suggests the opposite, that is, the solution had a lower water potential than the potato
The potato cylinder in the strongest sucrose concentration will have decreased in mass the most as there is the greatest concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)
More water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the potato cylinder – the potato cylinders will feel floppy