2.1.5 - Biologial Membranes (set A - role of membranes)

Question 1

Give 4 functions of membranes within cells?

Answer 1

• separate the contents from the cytoplasm (act as barriers)
• allowing the cell to change shape
• provide attachment sites for enzymes
• site for biochemical reactions - eg respiration

Question 2

Give 2 functions of membranes regarding to transport within or out of cells?

Answer 2

• formation of vesicles - to transport substances (eg Golgi apparatus)
• allowing selected membranes to move in and out of the cell

Question 3

What is compartmentalisation?

Answer 3

Intracellular membranes (internal membranes) form compartments within the cell such as organelles and vacuoles

• important so specific conditions for reactions can be maintained

Question 4

Explain the importance for compartmentalisation in cells?

Answer 4

vital to a cell as metabolisms include many different and often incompatible reactions - so containing the reactions allows the specific conditions to be maintained

Question 5

What is the plasma membrane?

Answer 5

The cell surface membrane which separates the internal cell environment from its external environment composed of a phospholipid bilayer

• involved in cell-cell recognition and selective transport

Question 6

Outline the structure of a phospholipid bilayer?

Answer 6

• Hydrophilic phosphate heads of phospholipids form both inner and outer surface of membrane
• fatty acid tails of the phospholipids in between,forming a hydrophobic core
• bilayer contains intrinsic and extrinsic proteins

Question 7

Outline the role of the phospholipid bilayer?

Answer 7

forms basic structure of the membrane, and acts as a barrier to most water-soluble substances - non-polar fatty acid tails prevent polar molecules or ions from passing across

• ensures molecules like sugar,amino acids and proteins can’t leak out and water soluble molecules can’t get in

Question 8

Explain two ways phospholipid bilayer can be chemically modified to act as a signalling molecule?

Answer 8

• 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

Question 9

Explain why phospholipid bilayer are well adapted as membranes?

Answer 9

perfectly suited as membranes because the outer surface of hydrophilic phosphate heads can interact with water (cells normally in aqueous environments)

Question 10

Explain the early theories on membranes - what supported them?

Answer 10

seen for the first time following invention of electron microscopy (allowed images to be taken with higher mag and resolution)

• images showed membrane as two black parallel lines - supported early theory that membrane is composed of liquid bilayer

Question 11

Explain the work of Singer and Nicholson regarding membranes - 1972?

Answer 11

They proposed a model (building on earlier lipid-bilayer model) in which proteins occupy various positions in the membrane - model is known as fluid-mosaic model because the phospholipids are free to move within the layer giving the membrane flexibility

Question 12

Explain the fluid-mosaic model as a way of describing membranes?

Answer 12

Forms basis of understanding membranes - Idea phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility

• proteins embedded in the bilayer vary in shape,size and position (they are like tiles of a mosaic)

Question 13

Explain what intrinsic proteins are, describe their structure?

Answer 13

transmembrane proteins embedded through both layers of a membrane - they have amino acids with hydrophobic R-groups on their external surfaces - interact with the hydrophobic core of the membrane, keeping them in place

Question 14

Give three examples of intrinsic proteins?

Answer 14

• channel proteins
• carrier proteins
• glycoproteins

Question 15

Explain what channel proteins are, and the type of movement that is involved?

Answer 15

Provide a hydrophilic channel that allows the passive movement (diffusion) of small, polar molecules and ions down a concentration gradient through membranes - eg oxygen molecules

Question 16

Explain what carrier proteins are the type of movement that is involved?

Answer 16

bind specific molecules and transport these molecules and ions across the membrane by active transport and facilitated diffusion e.g sodium ions

• often involves the shape of the protein changing - when molecule binds

Question 17

What are glycoproteins - what do they do?

Answer 17

Intrinsic proteins with attached carbohydrate chains of varying lengths and shapes - enables them to act as receptor binding sites for hormones ext

• play a role in cell adhesion (hold cell together in a tissue)
• stabilise membrane - form hydrogen bonds with surrounding water molecules
• act as antigens for recognition of cell as self, or non-self

Question 18

Explain the role of glycolipids and glycoproteins as cell markers?

Answer 18

Act as cell markers or antigens, for cell-to-cell recognition

When molecule binds to the protein, a chemical reaction is triggered inside the cell

Question 19

Give and explain two examples of cell signalling?

Answer 19

• receptors for neurotransmitters such as acetylcholine at nerve cell synapses - the binding of the neurotransmitters triggers or prevents an impulse in the next neurone
• receptors for peptide hormones, including insulin and glucagon, which affects the uptake and storage of glucose by cells

Question 20

Explain what glycolipids are?

Answer 20

lipids with attached carbohydrate chains

• act as receptor molecules
• act as cell markers/antigens

Question 21

Explain what extrinsic proteins are - outline what there involved in?

Answer 21

Proteins found on the membrane surface or bound to an intrinsic protein

involved in
- cell signalling
- cell recognition
- act as receptors for molecules (eg hormones)

Question 22

Outline what cholesterol is, give its structure and location?

Answer 22

lipid with a hydrophilic end and a hydrophobic
end - regulates the fluidity of membranes and increases stability

• formula C ₂₇ H ₄₆ O
• positioned between phospholipids in a membrane bilayer

Question 23

Outline how cholesterol stops membrane becoming to rigid, why is this important?

Answer 23

low temperatures - cholesterol increases membrane fluidity by stopping phospholipid tails packing too close together and crystallising

• allows cells to survive at low temperatures

Question 24

Explain the importance of cholesterol regarding the strength of membranes?

Answer 24

Cholesterol increases the mechanical strength and stability of membranes - without it membranes would break down and burst

Question 25

Give the 3 main receptor types regarding glycoproteins and glycolipids?

Answer 25

• signalling receptors for hormones and neurotransmitters
• receptors involved in endocytosis
• receptors involved in cell adhesion and stabilisation

Question 26

Explain what glycoproteins and glycolipids are?

Answer 26

Contain carbohydrate chains that exist on the surface (extrinsically) which enables them to act as receptor molecules - some act as cell markers or antigens for cell-to-cell recognition

The lipids and proteins bind to certain substances at the cells surface

Question 27

Give an example of an extrinsic protein?

Answer 27

Peripheral proteins

Question 28

Explain the presence of proteins in the membrane - outline the location of the different types?

Answer 28

Proteins typically make up 45% by mass of a cell membrane (depends on the cell)

• intrinsic proteins span the whole width of the membrane
• extrinsic proteins are confined to the inner or outer surface of the membrane

Question 29

Give an example of a membrane bound organelle and explain the importance of its membrane?

Answer 29

Lysosomes (found in animal cells) each containing many hydrolytic enzymes that can break down many different kinds of biomolecule - these enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

Question 30

Explain how channel proteins are held in place within the membrane?

Answer 30

Interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins

Question 31

Explain how intrinsic proteins are adapted to remain in place within the membrane?

Answer 31

have amino acids with hydrophobic R-groups on their external surfaces - R-groups interact with hydrophobic core of membrane

• interactions hold protein in position

Question 32

Explain how glycoproteins stabilise the membrane?

Answer 32

form hydrogen bonds with surrounding water molecules Stabilising the membrane

Question 33

Outline the role of intrinsic protein on the extracellular side?

Answer 33

proteins on the extracellular side of the membrane act as receptors for hormones or neurotransmitter, or are involved in cell recognition

• many are glycoproteins

Question 34

Outline the role of intrinsic protein on the cytosolic side - describe a major difference to those on extracellular side?

Answer 34

proteins on the cytosolic side - involved in cell signalling or chemical reactions - can dissociate from the membrane and move into the cytoplasm

Question 35

Outline how cholesterol regulates the membrane at high temperatures?

Answer 35

Stops membrane becoming too fluid by binding to phospholipids hydrophobic tails, causes them to pack closer together

Interactions between hydrophilic ends interacting with the head and hydrophobic end interacting with tails - pulls phospholipids together

Question 36

Explain the role of enzymes and coenzymes in the cell surface membrane?

Answer 36

Some reactions take place in membranes thereby requiring enzymes. E.g. some reactions of respiration take place in the membrane of the cristae of the mitochondria

Question 37

Give the two types of transport proteins?

Answer 37

• channel proteins
• carrier proteins

Question 38

Explain what they are and the role of transport proteins in cell surface membranes?

Answer 38

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

• specific to particular ion or molecule
• allow the cell to control which substances enter or leave

Question 39

Explain what a phospholipid is - give its structure?

Answer 39

molecule with a hydrophilic phosphate head (water-loving and polar) and a hydrophobic( (non-polar) fatty acid tails.

Many of them form the phospholipid bilayer

Question 40

Give three factors which affect fluidity of a membrane - explain what they result in?

Answer 40

• length of the fatty acid side chains - longer chains the lower fluidity
• proportion of the fatty acids which are saturated - high percentage of saturated fats - lower fluidity
• cholesterol content - high cholestrol - lower fluidity

Question 41

Outline the role of receptors in cell surface membranes?

Answer 41

Proteins in the cell surface membrane (e.g. glycoproteins act as receptor binding sites for the messenger molecules e.g. drug/hormone.

• specific site for complementary molecule to bind to

Question 42

Name and explain 3 factors that affect membrane permeability?

Answer 42

• temperature - high temps denature proteins, phospholipids have more kinetic energy and move further apart
• pH - changes tertiary structure of membrane proteins
• solvents - dissolves membranes

Question 43

Outline how temperature can affect membrane structure and permeability?

Answer 43

Higher temperatures results in the phospholipids having more kinetic energy - so they move more, membrane becomes more fluid and starts to loose its structure

• increases permeability of the membrane - easier for particles to cross due to carrier and channel proteins denaturing

Question 44

Outline how solvents can affect membrane structure and permeability - why does water not?

Answer 44

Organic solvents (alcohol) dissolve membranes, disrupting cells - membrane becomes more fluid and permeable

• water is a polar solvent which interacts with charged phosphate heads, which keeps bilayer intact - organic solvents are less polar than water