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A Level Biology > 2.5 Biological Membranes > Flashcards

2.5 Biological Membranes Flashcards

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1
Q

What does a cell surface membrane create

A

An enclosed space separating the internal cell environment from the external environment

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2
Q

What do intracellular membranes form

A

Compartments within the cell, such as organelles including the nucleus, mitochondria and RER and vacuoles

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3
Q

How do membranes control the exchange of materials

A

They are partially permeable

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4
Q

What different ways can substances cross membranes

A

Diffusion, facilitated diffusion, osmosis and active transport

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5
Q

How do membranes play a role in cell signalling

A

Acting as an interface for communication between cells

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6
Q

What does fluid mosaic model of membranes explain

A

How biological membranes are arranged to form cell membranes

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7
Q

How does the fluid mosaic model describe cell membranes as ‘fluid’

A

The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position

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8
Q

How does the fluid mosaic model describe cell membranes as ‘mosaics’

A

The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

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9
Q

What are the 4 main components included in the fluid mosaic model

A

Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins

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10
Q

What do phospholipids form

A

The basic structure of the membrane (the phospholipid bilayer)

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11
Q

Phospholipid head

A

Hydrophilic (water loving)- attracts water

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12
Q

Phospholipid tail

A

Hydrophobic (water hating)- repels water

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13
Q

How is phospholipid bilayer formed

A

Head faces out towards water on either side of the molecule
The centre of the membrane is hydrophobic

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14
Q

What does phospholipid bilayer act as

A

barrier to most water-soluble substances

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15
Q

What does phospholipid bilayer ensure

A

water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

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16
Q

How can phospholipids be chemically modified to act as signalling molecules

A

Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

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17
Q

What does cholesterol do

A

increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)

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18
Q

How does cholesterol increase fluidity of the membrane

A

Stops there phospholipid tails packing too closely together

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19
Q

How does cholesterol work stabilise the cell membrane at higher temperatures

A

Stopping the membrane from becoming to fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
The impermeability of the membrane to ions is also affected by cholesterol

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20
Q

Role of cholesterol (strength)

A

increases the mechanical strength and stability of membranes (without it membranes would break down and cells burst)

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21
Q

How are glycolipids and glycoproteins able to act as receptor molecules

A

Contain carbohydrate chains that exits on the surface

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22
Q

Role of glycolipids and glycoproteins

A

Bind with certain substances at the cell’s surface

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23
Q

Three main receptor types

A

Signalling receptors for hormones and neurotransmitters
Receptors involved in endocytosis
Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell

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24
Q

Cell adhesion

A

The process by which cells interact and attach to neighbouring cells through specialised molecules (glycoproteins and glycolipids) on the outer layer of the cell surface membrane

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25
Q

What do transport proteins do

A

create hydrophilic channels to allow ions and polar molecules to travel through the membrane

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26
Q

Two types of transport proteins

A

Channel (pore)
Carrier proteins

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27
Q

Transport protein specificity

A

Each transport protein is specific to a particular ion or molecule

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28
Q

What do transport proteins allow

A

Transport proteins allow the cell to control which substances enter or leave

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29
Q

What factors affect permeability of cell membranes

A

Temperature
Solvent conditions

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30
Q

What major components in cells membranes are affected by temperature

A

Proteins and lipids

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31
Q

What happens to lipids as temperature increases

A

Lipids become more fluid

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32
Q

What does increased fluidity lead to in the cell membrane

A

increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through

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33
Q

How do higher temperatures affect any diffusion taking place through the cell membrane

A

any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)

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34
Q

Are changes in membrane fluidity reversible and why

A

Yes-If temperatures decrease, the lipids will return to their normal levels of fluidity)

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35
Q

How can temperature cause denaturation in the cell membrane

A

At a certain temperature (often around 40°C) many proteins (including those in cell membranes) begin to denature
This disrupts the membrane structure, meaning it no longer forms an effective barrier

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36
Q

What happens when the protein in the cell membranes begin to denature

A

This disrupts the membrane structure, meaning it no longer forms an effective barrier
As a result, substances can pass freely through the disrupted membrane

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37
Q

Is proteins denaturing in the cell membranes reversible

A

No it is irreversible

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38
Q

How does solvent concentration affect permeability of cell structure

A

Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure

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39
Q

Why do we use beetroot to investigate how different factors affect membrane structure and permeability

A

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

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40
Q

Why do we rinse beetroot pieces

A

To remove any pigment released during cutting

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41
Q

Investigating the effect of temperature on membrane permeability method

A

Using a scalpel, cut five equal-sized cubes of beetroot

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

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42
Q

Why do beetroot pieces have to equally sized

A

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

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43
Q

General pattern of effect of temperature on membrane permeability practical

A

as temperature increases, membrane permeability also increases

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44
Q

Why does higher temperature increase permeability of membrane

A

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

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45
Q

How does volume of water inside the cells expanding increase the permeability of the membrane

A

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

46
Q

How to improve beetroot practical

A

conduct several repeats, using different parts of the beetroot and find a mean

47
Q

Limitations of beetroot practical

A

Some parts of beetroot tissue have more pigment in their cells than others

The beetroot pieces may not be identical in size and shape

48
Q

Diffusion

A

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration (down a concentration gradient)

49
Q

What is the random movement of molecules or ions in diffusion caused by

A

the natural kinetic energy of the molecules or ions

50
Q

What happens as a result of diffusion

A

molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space

51
Q

Factors affecting affect diffusion

A

Steepness of concentration gradient, temperature, surface area and properties of molecules or ions

52
Q

How does steepness of concentration gradient affect diffusion

A

This is the difference in the concentration of the substance on the two sides of the surface
A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion

53
Q

How does temperature affect rate of diffusion

A

Molecules and ions have more kinetic energy at higher temperatures
They move faster, resulting in a higher rate of diffusion

54
Q

How does surface area affect rate of diffusion

A

The greater the surface area, the greater the number of molecules or ions that can cross at one moment

55
Q

How do properties of molecules or ions affect rate of diffusion

A

Large molecules diffuse more slowly than smaller ones as they require more energy to move
Uncharged and non-polar molecules diffuse directly across the phospholipid bilayer
Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipids bilayer

56
Q

Facilitated diffusion

A

The passive movement of molecules down a concentration gradient (high to low) across a membrane, and it involves special carrier and channel proteins

57
Q

Why is facilitated diffusion important

A

Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)

58
Q

Channel proteins

A

Water-filled pores which allow charged substances (ions) to diffuse through the cell membrane

59
Q

How do channel proteins control the exchange of ions

A

The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore

60
Q

How do carrier proteins differ to channel proteins

A

Carrier proteins can switch between two shapes

61
Q

What does carrier proteins being able to switch between two shapes mean

A

the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape

62
Q

What is the net diffusion (movement) of molecules or ions into or out of a cell

A

down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

63
Q

How can rate of diffusion be investigate

A

timing the diffusion of ions through different sized cubes of agar

64
Q

Agar cube rate of diffusion investigation practical method

A

The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)

65
Q

Agar cube rate of diffusion investigation measurements

A

The time taken for the acid to completely change the colour of the indicator in the agar blocks
The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)

66
Q

Active transport

A

the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

67
Q

What does active transport require

A

carrier proteins (each carrier protein being specific for a particular type of molecule or ion)

68
Q

How does active transport use carrier proteins differently to facilitated diffusion

A

The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane

69
Q

How is the energy required for active transport provided

A

The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy

70
Q

Why is active transport important

A

The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
The absorption of some products of digestion from the digestive tract
The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
The loading of inorganic ions from the soil into root hairs

71
Q

Examples of bulk transport of larger quantities of materials

A

Large molecules such as proteins or polysaccharides
Parts of cells
Whole cells eg. bacteria

72
Q

Endocytosis

A

Bulk transport into cells

73
Q

Exocytosis

A

Bulk transport out of cells

74
Q

What type of process in endocytosis and exocytosis

A

These two processes require energy and are therefore forms of active transport

75
Q

What is the process of endocytosis

A

the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it

76
Q

Two forms of endocytosis

A

Phagocytosis and pinocytosis

77
Q

Phagocytosis in terms of endocytosis

A

This is the bulk intake of solid material by a cell
Cells that specialise in this process are called phagocytes
The vacuoles formed are called phagocytic vacuoles

78
Q

Example of phagocytosis

A

the engulfing of bacteria by phagocytic white blood cells

79
Q

Pinocytosis

A

This is the bulk intake of liquids
If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis

80
Q

Process of exocytosis

A

materials are removed from, or transported out of, cells (the reverse of endocytosis)

81
Q

How does exocytosis happen

A

The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
These vesicles then travel to the cell surface membrane
Here they fuse with the cell membrane and release their contents outside of the cell

82
Q

Example of exocytosis

A

the secretion of digestive enzymes from pancreatic cells

83
Q

Osmosis

A

the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane down a concentration gradient

84
Q

Partially permeable

A

allows small molecules (like water) through but not larger molecules (like solute molecules)

85
Q

Water potential

A

the tendency of water to move out of a solution

86
Q

Water potential of a dilute solution

A

High

87
Q

Water potential of a concentrated solution

A

Low

88
Q

What potential of pure water

A

The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

89
Q

Osmosis in animal cells

A

Animal cells can lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are severe

90
Q

Animal cells losing water

A

If an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)
Water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up

91
Q

Cremation

A

The loss of water in an animal cell which is usually fatal for the cell

92
Q

Animal cells gaining water

A

If an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential
The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created

93
Q

Lysis

A

The disintegration of a cell by rupture of the ell wall or membrane
occurs when the cell is in a hypotonic environment

94
Q

Hypotonic environment

A

the solution outside of the cell has a lower solute concentration than the inside of the cell

95
Q

Isotopic environment

A

the solution outside of the cell has the same solute concentration as the inside of the cell

96
Q

Movement of water in animal cells in isotonic environment

A

The movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells

97
Q

Osmosis in plant cells

A

Like animal cells, plants cells can also lose and gain water as a result of osmosis
As plant cells have a supporting cell wall, the results of this loss or gain of water on the cell are less severe than in animal cells

98
Q

Plant cells losing water

A

If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis

99
Q

Process of plant cells losing water

A

As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
The protoplast gradually shrinks and no longer exerts pressure on the cell wall
As the protoplast continues to shrink, it begins to pull away from the cell wall

100
Q

Protoplasm

A

All o the contents of a bacterial or plant cell except for the cell. The living parts of the cell

101
Q

Plasmolysis

A

The process of contraction or shrinkage of the protoplasm of a plant cell and is caused due to the loss of water in the cell

102
Q

Plant cells gaining water

A

If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell

103
Q

Process of plant cells gaining water

A

As water enters the vacuole of the plant cell, the volume of the plant cell increases
The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting
The pressure created by the cell wall also stops too much water from entering and this also helps to prevent the cell from bursting
When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid

104
Q

Why is turgidity important for plants

A

as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight

105
Q

What happens if plants do not receive enough water

A

the cells cannot remain rigid and firm (turgid) and the plant wilts

106
Q

Investigating water potential using potato cylinders method

A

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

107
Q

What does a positive percentage change in potato mass indicate

A

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

108
Q

What does a negative percentage change in mass of potato indicate

A

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)

109
Q

What happens when potato loses mass

A

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

110
Q

What happens in mass of potato has neither decreased or increased

A

it means there was no overall net movement of water into or out of the potato cells
The solution that this particular potato cylinder was in had the same water potential as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient and therefore no net movement of water into or out of the potato cells

A Level Biology (10 decks)

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