Module 2: Biological Membranes

Question 1

The role of membranes

Answer 1

1) Separating cell contents from outside environment.
2) Separating cell components from cytoplasm.
3) Cell recognition and cell signalling
4) Regulating transport of materials in and out of the cell.
5) Some stages of photosynthesis and respiration take place on the membranes of the chloroplasts or mitochondria.

Question 2

Basic membrane structure

Answer 2

All membranes are made up of a phospholipid bilayer.

Phospholipid bilayers are the basic structure of ALL membranes in cells (prokaryotic and eukaryotic and membranes round organelles).

The are usually about 7‐10nm thick.

All membranes are permeable to water ‐ water diffuses through the bilayer. Some contain aquaporins (protein channels which allow water through) which make them much more permeable to water.

Cell surface membranes are partially permeable
membranes ‐ permeable to water and some solutes

Question 3

Phospholipids

Answer 3

• Phosphate group head.
• Fatty acid tail.

The head is hydrophilic- water loving
The tail is hydrophobic- water hating

The phospholipids aren’t bonded to one another but because the hydrophilic head can’t pass through the hydrophobic region easily, the bilayer is quite stable.

The membrane is very fluid because of the lack of bonds ‐ the phospholipids can slide around one another.

Question 4

What happens to phospholipids when put in water?

Answer 4

The head is hydrophilic because the charges on the head are unevenly distributed ‐ this lets it interact with the water molecules easily.

The tails are hydrophobic because the charges on them are evenly distributed ‐ this means they won’t mix with water and will repel the molecules.

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If surrounded completely by water, phospholipids can form a bilayer. The hydrophilic heads point towards the water and hydrophilic tails point towards each other ‐ away from water

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Question 5

The fluid mosaic model of membrane structure

Answer 5

In 1972 Singer and Nicholson put forward their ‘fluid mosaic model’ of membrane structure ‐ it is still the model we accept today.

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Question 6

Glycoproteins and Glycolipids:

Answer 6

Protein + carbohydrate = Glycoprotein

Phospholipid + carbohydrate = Glycolipid

• Glycolipids and glycoproteins are involved in cell signalling that they are ‘self’ to allow recognition by the immune system.
• They are receptors for hormones/drugs
• On the surface of pathogens they are antigens ‐ immune system can recognise them as ‘foreign.’
• Glycoproteins can bind cells together in tissue.

Question 7

Proteins

Answer 7

1) Proteins which span whole bilayer = intrinsic proteins
2) Proteins which are partially embedded inside or outside = extrinsic proteins

Intrinsic proteins for transport ‐ these have to be hydrophobic molecules in order to span the whole bilayer (due to the hydrophobic region):

Channel proteins ‐ pores for small ions or small water soluble molecules to diffuse through membrane.

Carrier proteins ‐ can help larger molecules to diffuse through membrane, or can be used for active transport across membrane.

Question 8

Cholesterol

Answer 8

‐ gives the membranes of eukaryotes more stability.

‐ fits between fatty acid tails and ‘plugs gaps’ ‐ makes the
membrane less permeable (through bilayer) to water
molecules and to ions.

‐ restricts too much movement within phospholipid layer.

Question 9

Actin microfilaments

Answer 9

Actin microfilaments can be closely associated with the membrane and its proteins in order to help anchor them and stop them moving around too much.

Question 10

Membranes and Temperature

Answer 10

Higher temperature = more kinetic energy for molecules = they move faster.

Faster movement of phospholipids and other parts of the membrane makes the membranes more fluid and ‘leaky’. This means they become more permeable and substances that couldn’t normally enter/leave the cell now can.

At very high temperatures, lipids can melt and proteins can denature ‐ this can completely cause the membrane to rupture and release everything that is inside
of it.

Organisms living in very hot or very cold environments need differently adapted molecules in their membranes e.g. cholesterol content, so that their membranes
can function correctly and remain intact at these temperatures.

Question 11

Practical investigation into factors affecting diffusion rates in model cells.

Answer 11

…

Question 12

What is cell signalling?

Answer 12

Cell signalling is when cells communicate with one another by signals to help them work together and coordinate their actions.

This is very complex. Many molecules act as signals ‐ some signal during processes taking place inside cells; other signal from one cell to others.
Cytokines and hormones are examples of cell signals secreted from cells.

To detect signals, cells must have receptors on their surface. These are usually proteins or modified proteins e.g. glycoproteins.

Question 13

Hormone receptors

Answer 13

Cells can communicate with hormones (endocrine signals) which are protein chemical messengers produced in glands and then released into the blood.

Cells with specific receptors for the hormones are called
target cells.

Hormone molecules bind to receptors on their target cell surface membrane. The hormone and receptor have complementary shapes. This binding brings about a response in the target cell.

Question 14

Insulin

Answer 14

Made in: beta‐cells in islets of Langerhans in pancreas.

Made when: blood glucose is too high.

Target cells: insulin receptors on many cells inc. muscle and liver cells.

Effect: cells take up more glucose from blood = reduces blood glucose level.

Question 15

Medicinal drugs

Answer 15

Can be made to be complementary to receptor molecules so they block them.

Beta‐blockers stop heart muscle from increasing heart rate in people at risk of heart attacks.

Some drugs act like natural neurotransmitters that some people can’t make.

e.g. Schizophenics can’t control the levels of dopamine in the brain ‐ the drugs act like dopamine in order to supress the symptoms of the disorder.

Question 16

Poisons

Answer 16

Poisons can bind with receptors.

Toxin from bacterium Clostridium botulinum binds with receptors on muscle fibres and stops them from working ‐ this causes paralysis of the muscles.

The toxin is one of the most lethal neurotoxins known to man but in TINY doses it is used for BOTOX surgery. It paralyses small muscles in the face to reduce wrinkles.

Question 17

Hijacking receptors

Answer 17

Viruses can get inside cells when they bind with their receptors (normally used for signalling).

Human Immunodeficiency Virus (HIV) [causes AIDS] is so dangerous because it can enter the immune system’s cells e.g. T‐lymphocytes [one type of white blood cell].

The HIV virus can lie dormant and the reproduce inside the cell and cause it to burst open and be destroyed. The main symptom of AIDS is a lowered immune system.

Question 18

Functions of glycoproteins

Answer 18

1. Cell signalling ‐ communication between cells to help them work together/coordinate actions.
2. Act as antigens for…
3. cell recognition ‐ recognition as self/non‐self.
4. receptors found on target cells… for hormones/cytokines to trigger reactions/responses in cells.
5. cell adhesion ‐ hold cells together in tissues.
6. form bonds with water molecules to stabilise membrane.
7. receptors on transport proteins.

Question 19

Diffusion

Answer 19

Diffusion is the net movement of molecules from a region of high concentration to a region of lower concentration of that molecule, down a concentration gradient. This is a passive- doesn’t require energy.

No net movement is called equilibrium.

The steeper the concentration gradient, the quicker the rate of diffusion will be.

i.e the bigger the difference between the concentrations in the regions of high and lower concentration then the faster the rate of diffusion.

Cells maintain conc gradients to ensure constant net movement of the molecules they need into the cell/organelle.

Question 20

Osmosis

Answer 20

Osmosis is the movement of water molecules from a region of high concentration to a region of lower concentration of water molecules down the concentration gradient, across a partially permeable membrane. This is a passive process and is effected by the concentration gradient.

Question 21

Active transport

Answer 21

Active transport is the movement of molecules or ions across membranes from a region of low concentration to a region of higher concentration of that molecule against a concentration gradient. Active transport requires energy in the form of ATP.

Question 22

Factors affecting the rate of diffusion

Answer 22

Temperature ‐ higher temp = more kinetic energy for molecules = higher rate of random movement = higher rate of diffusion

Stirring/moving ‐ stirring increases movement of molecules = higher rate of diffusion

Surface area ‐ diffusion across membranes is faster if there is a larger SA to diffuse across. e.g. alveoli

Distance/thickness ‐ thinner membrane = faster diffusion e.g. alveoli

Size of molecule = smaller molecules = faster diffusion

Question 23

Diffusion across membranes

Answer 23

The non‐polar, hydrophobic tails of phospholipids in membranes are a barrier to most substances, but a few can diffuse directly across the bilayer :

• Lipid‐based molecules ‐ are fat soluble so dissolve and diffuse readily across the bilayer e.g. steroid hormones.
• Very small molecules ‐ oxygen and carbon dioxide are small enough to pass between phospholipid molecules. Water and urea molecules are very small but because they are polar (charged) they pass across the bilayer much slower.

Generally, the smaller and less polar a molecule the quicker it will diffuse across a membrane.

Charged particles (ions) can’t diffuse across the bilayer even if they are small; nor can larger molecules. When these substances diffuse across a membrane, they are helped by 2 types of intrinsic proteins ‐ this is known as facilitated diffusion.

Different membranes have different carrier and channel proteins ‐ this means different membranes will allow different substances across ‐ specific to their cell or organelle’s needs.

Question 24

Channel proteins

Answer 24

‐ form pores in membranes.
‐ hydrophilic conditions inside pore.
‐ are specific to certain small‐water soluble molecules or ions e.g. sodium, calcium ions.
‐ allow diffusion in or out of cell/organelle.
‐ can be ‘gated’ which means they can be opened or closed by a signal or change in voltage across the membrane.

Question 25

Carrier proteins

Answer 25

‐ carrier larger molecules e.g. glucose & amino acids ‐
shaped for specific molecules to bind to.
‐ the molecule binding changes the shape of the carrier
protein.
‐ causes the molecule to be carried to and released on
the other side of the membrane.
‐ allow diffusion in or out of cell/organelle.
‐ no energy needed in facilitated diffusion.

Question 26

Examples of active transport

Answer 26

Mineral ions e.g. Mg are in low concs in soil and higher in the root ‐ active transport takes them in to be used to make chlorophyll

Absorbing glucose + amino acids from intestines (low conc) to blood (higher conc).

Calcium is stored in high concs in specialised ER and released to let muscles contract. To relax the muscle, the Ca ions have to be pumped back to the stores quickly by calcium pumps on the membrane of the ER via active transport.

Question 27

The process of active transport across membranes

Answer 27

• Carrier proteins work as pumps.
• Specific and complementary shaped molecule binds
• Molecules can’t diffuse (due to gradient)
• ATP provides the energy needed for the protein
to change shape and transport the molecule
across the membrane.
• Shape change only allows the molecule to fit in the protein on one side of the membrane ‐ it will not fit on the other side ‐ stops transport in the wrong direction.
• Much faster than diffusion
• Can go inside cells/organelles or outside cells ‐ dependant on gradient.

Question 28

Bulk transport

Answer 28

To move large amounts of substances in or out of a cell, a different active process is used.

e.g. ‐ large amounts of insulin into blood from Golgi vesicles in pancreas ‐ phagocytes engulfing pathogens to form a vesicle which then fuses with lysosomes to digest it using enzymes.

Endocytosis ‐ moving in
Exocytosis ‐ moving out

Phago‐ solids
Pino ‐ liquids

1) exophagocytosis ‐ moving solids out
2) endopinocytosis ‐ moving liquids in
3) endophagocytosis ‐ moving solids in
4) exopinocytosis ‐ moving liquids out

Question 29

Describe what is made by exo and endocytosis

Answer 29

Endocytosis ‐ moving in
Exocytosis ‐ moving out

Both endocytosis and exocytosis need energy in the form of ATP in order to occur.
The energy is needed to move the membrane around to form a vesicle in endocytosis and to move vesicles to the
cell surface membrane to fuse in exocytosis.

Exocytosis:
• vesicle moves towards the cell surface membrane
• on microtubles
• using ATP
• the vesicle fuses with the cell surface membrane ‐ requires ATP
• molecules are ejected from the cell via exocytosis

Endocytosis:
• a molecule binds to a receptor
• this causes the cell surface membraneto fold in ‐ invaginates
• this requires ATP
• cell surface membrane fuses with itself
• to form a vesicle
• vesicle e.g. moves through cytoplasm to where it is needed in the cell

Question 30

Describe what is meant by osmosis, in terms of potential

Answer 30

Osmosis is the net movement (diffusion) of water molecules from a region of high water potential to a region of lower water potential, down a water potential gradient across a partially permeable membrane.

Osmosis continues until an equilibrium is reached ‐ water potential on each side of the membrane is equal.

Question 31

Difference between solute, solvent and solution

Answer 31

Solute - solid that dissolves into a liquid
Solvent - Liquid that dissolves a solute
Solution - Liquid containing dissolved solids

pure water ‐ all water molecules are free to move and diffuse = highest water potential
conc. solution ‐ water molecules cluster around the solute
‐ less free water molecules to diffuse = lower water potential
more solute dissolved = lower water potential

Question 32

Water potential

Answer 32

Water potential is the tendency of water molecules in a system to move.

It is shown by the symbol Ψ and is measured in kiloPascals (kPa).

0kPa is the highest possible water potential.

Negative numbers show lower water potential.

Water will move by osmosis towards more negative water potentials.

Question 33

Cells in higher water potential (closer to 0kPa)

Answer 33

• putting cells in a solution of higher water potential than the cell contents will cause water to move in the cell by osmosis
• The cells will swell
• Animal cell surface membranes will burst ‐ the cell is haemolysed
• Plant cells cytoplasms and vacuoles will push the membrane against the cell wall ‐ the wall will stop the cell from bursting or getting larger [osmosis will also stop regardless of the water potential gradient] ‐ the cell is turgid.

Question 34

Cells in lower water potential (more negative)

Answer 34

• putting cells in a solution of lower water potential [conc salt/sugar solution] than the cell contents will cause water to move out of the cell by osmosis.
• Animal cells will shrink and the membrane will wrinkle ‐ the cell is crenated.
• Plant cells cytoplasms and vacuoles will shrink when water is lost and the cell.
surface membrane will pull away from the cell wall ‐ the cell is plasmolysed. The point when the membrane is just about to pull away from the cell wall is called incipient plasmolysis