Transport Across Cell Membranes Flashcards
How are plasma membrane often described as?
- fluid mosaic model
Why are plasma membranes described as fluid mosaic model?
- due to the mixture of movement of the phospholipids, proteins, glycoproteins and glycolipids it is made of
Are plasma membranes partially permeable?
- yes
Why do the phospholipids arrange as a bilayer?
- hydrophilic heads being attracted to water and face outwards and the hydrophobic tails being repelled by water and face inwards
What does the head of the phospholipid contain?
- phosphate group which is negatively charged
- glycerol molecule
Why is the phospholipid head hydrophilic?
- it is negatively charged
Why are the fatty acid chains of phospholipids hydrophobic?
- do not have charge
What is the role of cholesterol in the phospholipid bilayer?
- restricts the lateral movement of other molecules in the membrane
- makes membrane was fluid at high temperatures and prevents water and dissolved ions leaking out of the cell
Why would happen if phospholipid bilayer was too fluid?
- gaps between phospholipids increase and evoke too large
- water and dissolved ions can leak in or out of the cell
- could cause dehydration or cells to burst
Integral (intrinsic)proteins
- protein carriers or channel proteins involved in the transport of molecule across the membrane
Peripheral (extrinsic) proteins
- provide mechanical support or are connected to proteins or lipids to make glycoproteins and glycol lipids
- function is cell recognition as receptors
Describe structure of peripheral (extrinsic) proteins
- proteins that do not extend completely across the membrane
Describe the structure of integral (intrinsic) proteins
- proteins that span across from one side of the bilayer to the other
Protein channels
- forms tubes that fill with water to enable water soluble ions to diffuse
Carrier proteins
- bind with larger molecules such as glucose and amino acids and change shape to transport them to the other aid of the membrane
Examples of proteins in the phospholipid bilayer
- peripheral membrane (extrinsic) protein
- integral membrane (intrinsic) protein
- glycoprotein
- channel protein
What is a glycoprotein?
- protein with a carbohydrate chain attached to
What is a glycolipid?
- lipid with a carbohydrate chain attached
Which molecules can diffuse through the phospholipid bilayer?
- lipid soluble substances (some hormones)
- very small molecules (carbon dioxide, oxygen and water)
Which molecules cannot pass through the phospholipid bilayer by simple diffusion?
- water soluble (polar) substances (sodium ions)
- large molecules (glucose)
What are the four key types of transport?
- Simple diffusion
- Facilitated diffusion
- Active transport
- Osmosis
Simple diffusion
- the movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached
- does not require ATP
Which energy is required for simple diffusion and what are the requirements?
- kinetic energy
- must be lipid double and small
Facilitated diffusion
- the movement of molecules from an area of higher concentration to an area of lower concentration with the use of protein channels or carriers to transport molecules
How do protein channels transport molecules in facilitated diffusion?
- form tubes filled with water which enables water soluble ions to pass through the membrane
How do carrier proteins transport molecules in facilitated diffusion?
- carrier proteins bind with a molecule such as glucose which causes a change in the shape of the protein, shape change enables the molecule to be released to the other side of the membrane
What energy is required for facilitated diffusion and what are the requirements?
- no ATP required
- movement of ions and polar molecules
- molecule should be complementary to carrier protein
Osmosis
- movement of water from an area of higher water potential to n area of lower water potential across a partially permeable membrane
What is water potential?
- pressure created by water molecules and is measured in kPa
What does pure water have a water potential of?
- 0
What happens when solutes are dissolved in pure water?
- water potential becomes negative
Isotonic solution
- when water potential is the same in the solution and the cell within the solution
Hypotonic solution
- when water potential of a solution is more positive (closer to 0)
Hypertonic solution
- when the water potential of a solution is more negative than the cell
What happens in animal/plant cells if they are placed in a hypotonic solution such as pure water?
- as animal cells do not have cell wall, pressure will cause the cell to burst
- plant cells do not burst as they have a strengthened cell wall and become turgid
What happens in animal/plant cells if they are placed in a hypertonic solution?
- both will shrink and become shrivelled due to large volumes of water leaving the cell by osmosis
Active transport
- the movement of molecules and ions from an area of lower concentration to an rates of higher concentration using ATP and carrier proteins
How do the carrier proteins work in active transport?
- act as pumps to move substances across the membrane
Why is active transport very selective?
- only certain molecules can bind to the carrier proteins to be pumped
Why does active transport require ATP and carrier proteins?
- against the concentration gradient
Which molecules can bind to the receptor site on carrier proteins for active transport?
- ATP bind to potion on the inside of the membrane and is hydrolysed into ADP and Pi causing the proton to change shape and open towards the inside of the membrane
- causes the molecule to be released in the other side of the membrane
- Pi molecule is released from the potion and results in the protein reverting to its original shape
What is the formula to calculate the volume of solution?
- V1 x C1 = V2 x C2
What is the rearranged formula to find out the volume of stock concentration needed?
- V1 = V2 x C2 / C1
What does C1 stand for?
- stock concentration (starting concentration given)
What does C2 stand for?
- concentration of working solution (what you’re making up)
What does V1 stand for?
- volume of stock concentration needed
What does V2 stand for?
- volume of total solution
Describe the steps to work out the volume of total solution
- Work out the volume of stock concentration needed using above formula
- Then you can work out the volume of distilled water as you know the total volume of
solution
Describe the steps for active transport?
- Transport through carrier proteins spanning cell membrane
- Molecule binds to receptor complementary in shape on the protein
- ATP binds to carrier protein from inside of the cell and is hydrolysed into ADP + Pi
- Causes carrier protein to change shape and release the molecule to the other side
- Phosphate ion is released and the protein returns to its original shape
Describe co transport of glucose or amino acids with sodium ions in the ileum
- Sodium ions are actively transported out of the epithelial cell into the blood
- Reduces sodium up concentration in the epithelial cell
- Sodium ions then diffuse from the lumen down their concentration gradient into the epithelial cell
- The proton the sodium ions diffuse through is a co transported protein so either glucose or amino acids attach and are transported into epithelial cell against their concentration gradient
- Glucose then moves by facilitated diffusion from the epithelial cell to the blood
Why is there no build up of glucose in the blood in co transport?
- concentration of glucose is lower than in the epithelial cells because the blood flows and carries away absorbed glucose
What is the function of microvilli in the epithelial cell?
- increase surface area for co transporter proteins
What are the conditions to absorbed glucose from the lumen to the gut?
- there must be a higher concentration of glucose in the lumen impaired to the epithelial cell for facilitated diffusion
Why is active transport and co transport both required for absorption of glucose from the lumen to the gut?
- there is usually more glucose in the epithelial cells so require both forms of transport
