Cell Membranes

Question 1

Cell-surface Membranes

Answer 1

• Barrier between cell and environment, controlling which substances enter and leave the cell
• Partially permeable
• Membranes are found around some organelles

Question 2

Phospholipids

Answer 2

• The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
• Hydrophobic tails point to the centre of the cell membrane, repelled by water
• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble substances entering and leaving
• Make membrane flexible and self-sealing
• Small, non-polar substances like water and carbon dioxide can diffuse through the membrane

Question 3

Proteins

Answer 3

• Some proteins can completely span the membrane, such as protein channels
• Carrier proteins bind with different molecules to allow them to cross the membrane
• They can also sit in one side of the bilayer and give mechanical support
• They also act as cell receptors in conjunction with glycolipids
• Help cells adhere together
• Allow active transport

Question 4

Cholesterol

Answer 4

• Add strength to membranes
• Lipid present in membranes that binds to hydrophobic tails of the phospholipids, causing them to pack more closely together
• Restricts movement, making membrane less fluid and more rigid
• Has hydrophobic regions so it’s able to create a further barrier to polar substances (to prevent loss of water and dissolved ions from the cell)
• Cholesterol helps maintain shape of animal cells that don’t have cell walls

Question 5

Glycoproteins

Answer 5

• Protein with carbohydrate attached
• Act as recognition sites
• Help maintain stability of membrane
• Allows cells to recognise one another e.g. lymphocytes can recognise an organism’s own cells

Question 6

Glycolipids

Answer 6

• Lipid with a carbohydrate attached
• Act as recognition sites
• Help maintain stability of membrane
• Help cells attach to one another and so form tissues

Question 7

Why some molecules can’t diffue directly?

Answer 7

• Not lipid soluble
• Too large to pass through channel proteins
• Of the same charge as the protein channels so are repelled
• Electrically charged (i.e. polar) so cannot pass through lipid bilayer.

Question 8

Fluid mosaic model

Answer 8

• Basic structure of cell membranes is composed of lipids, proteins and carbohydrates
• Model describes arrangement of molecules in the membrane
• Phospholipid molecules form a continuous, double layer (bilayer) as phospholipids are constantly moving
• Proteins are scattered through bilayer, like a mosaic
• Some move sideways while others are fixed in position
• These include channel, carrier and receptor proteins
• Also bilayer includes glycoproteins, glycolipids and cholesterol

Question 9

How can membrane permeability be affected?

Answer 9

• Phospholipids acquire more kinetic energy so move around more, making membrane more permeable
• Proteins can denature at low temperatures (ice crystals pierce the membrane when it thaws)
• Fatty acids become less compressed
• Proteins can denature at high temperatures, reducing their ability to control transport across the membrane (becoming more permeable to substances and damaging cells)
• Changes in pH away from the optimum can also affect the function of proteins in the membrane
• Organic solvents such as ethanol will damage a membrane and increase its permeability by dissolving lipids held in it.

Question 10

Diffusion

Answer 10

• Movement of molecules or ions from an area of high concentration to an area of lower concentration until the concentrations of the two regions are equal (dynamic equilibrium)
• Passive process where particles diffuse down a concentration gradient
• When particles directly diffuse, known as simple diffusion
• The rate of diffusion depends on surface area of the cell (large means faster rate-microvilli), thickness of the membrane (thin means short diffusuion pathway), strength of the diffusion/concentration gradient (i.e. the difference in concentration) and temperature
• Small, non-polar molecules can diffuse through a membrane (oxygen, CO2, vitamins)

Question 11

Facilitated Diffusion

Answer 11

• Faster than normal diffusion
• Used to transport large and polar molecules (glucose, ions, amino acids) as they would diffuse slowly due to centre of bilayer being hydrophopic (water soluble)
• Happens down a concentration gradient (high to low) and is a passive process
• Facilitated diffusion of large molecules through a carrier protein:
• Molecule binds with carrier protein molecules on the cell surface
• The protein changes shape, in doing so the molecule is transported through to the inside of the cell
• The molecule detaches from the transporter protein and the protein reverts to its original shape
• Alternatively, small polar molecules can diffuse through the pores in the membrane created by channel proteins (they are selective and only open in presence of certain ions)
• The rate of facilitated diffusion is proportional to the concentration gradient and to the number of channels or transporter proteins that are available

Question 12

Osmosis

Answer 12

• Diffusion of water molecules from an area of high water potential to an area of low water potential (through a partially permeable membrane)
• Therefore, cells are affected by the water potential (likelihood of water molecules to diffuse out of or into solution) outside a cell
• Adding solutes to water decreases the water potential (more negative)
• Pure water has a potential of zero
• Water molecules are small and can diffuse easily but large solutes can’t
• Factors that affect osmosis are water potential gradient (rate of osmosis levels off over time), thickness of exchange surface and surface area of exchange surface

Question 13

Isotonic

Answer 13

When 2 solutions have the same water potential

Question 14

Hypotonic

Answer 14

Solutions with a higher water potential compared with the inside of a cell

Question 15

Hypertonic

Answer 15

Solutions with a lower water potential than the cell

Question 16

Active Transport

Answer 16

• Uses energy to move molecules and ions, against a concentration gradient
• Moves solutes from a low to a high concentration
• ATP undergoes hydrolysis to release energy for the reaction
• Involves carrier proteins and co-transport
• Carrier Proteins= molecule attaches to the protein by binding to receptor
• Protein shape changes and this moves molecule across the membrane, releasing it on the other side
• Process is selective as specific subsatnces are transported
• ATP binds to protein and splits which causes protein to change shape
• When phosphate is released, protein shape goes back to normal
• Co-transport= They are a type of carrier protein
• Bind 2 molecules at a time
• Concentration of one molecule is used to move the other molecule against it’s own concentration gradient e.g. sodium ions and glucose
• Factors affecting rate include speed of individual carrier proteins, number of carrier proteins and rate of respiration (availability of ATP)

Question 17

Co-transport and absorption of glucose

Answer 17

• Glucose is absorbed into the bloodstream in the small intestine
• In the mammalian ileum, the concentration of glucose is too low for glucose to diffuse out into the blood
• So glucose is absorbed from the lumen of the ileum by co-transport

1. Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood via sodium-potassium pump

• This creates a concentration gradient-higher concentration of sodium ions in the lumen of ileum than inside the cell
• This causes sodium ions to diffuse from the lumen of ileum into the epithelial cell, down their concentration gradient via sodium-glucose co-transporter proteins
• Co-transporter carries glucose (against concentration gradient) into the cell with sodium
• As a result, the concentration of glucose inside the cell increases
• Glucose diffuses out of the cell, into the blood, down it’s concentration gradient through a protein channel, by facilitated diffusion