Transport across cell membranes Flashcards
What are the molecules which form the structure of the cell membrane?
Phospholipids Proteins Cholesterol Glycolipids Glycoproteins
What is the structure of phospholipids? (in relation to membranes)
Form a phospholipid bilayer
Hydrophilic head point outwards (outside) the membrane which attracts water
Hydrophobic tails face inwards and point to the centre which repel water
What are the functions of phospholipids?
Lipid soluble molecules move across the membrane via the phospholipids (enter/leave)
Prevent water soluble substances entering/leaving
Makes membrane flexible/self-sealing
Where are proteins located in phospholipid bilayer?
Interspersed throughout
Some don’t extend completely across
Some span completely across, forming protein channels to allow water-soluble ions to diffuse across + some are carrier proteins that bind to ions/molecules like glucose and change shape to allow molecules across the membrane
What are the functions of proteins?
Provide structural support
Act as channels to transport water soluble molecules
Allow active transport through carrier proteins across the membrane
Form cell surface receptors for identifying cells
Help cells join together
What is the function of cholesterol in the phospholipid bilayer?
Reduce movement of molecules -> e.g phospholipids
Make membrane less fluid at high temperatures
Prevent leakage of water and dissolved ions from cell
Provide strength
What is the structure of glycolipids?
Carbohydrate covalently bonded with a lipid
What is the function of glycolipids in the phospholipid bilayer?
Act as recognition sites
Maintain stability
Form tissues by helping cells attract to one another
What is the structure of glycoproteins?
Carbohydrates attached to proteins on outer surface of membrane
What is the function of glycoproteins?
Act as recognition sites
Allow cells to recognise each other -> lymphocytes can recognise own cells
Why can’t molecules freely diffuse across the cell membrane?
Not soluble in lipids so cannot pass through phospholipid bilayer
Too large to pass through channels
Same charge as protein channels so are repelled
Polar so cannot pass through the non polar hydrophilic tails in bilayer
What is the fluid mosaic model?
How molecules are combined to form the phospholipid bilayer
Fluid- phospholipid molecules can freely move + has a flexible structure which changes shape constantly
Mosaic- proteins within the membrane are different shapes, sizes
What is the definition of simple diffusion? + what type of process is it?
Net movement of molecules or ions from a region of high concentration to a region of low concentration until evenly distributed
Passive
Down a concentration gradient
What factors affect the rate of diffusion?
Concentration gradient
Size of molecule
Temperature
How do protein channels work?
Allow specific water-soluble molecules to pass through
Therefore there is control over the entry and exit of ions
Ions bind to the protein which causes it change shape, allowing it to close one end of the protein but open the other
How do carrier proteins work?
When a molecule that is specific to the protein present, it binds to the protein causing it to change shape so the molecule is released to the inside of the membrane
What are the differences between simple and facilitated diffusion?
Simple
- Passes directly through phospholipid bilayer
- Protein channels are not required
- Small/non polar molecules
- Slower
Facilitated
- Does not pass directly through phospholipid bilayer
- Protein channels are required
- Large/polar molecules
- Faster
What is the definition of osmosis?
The passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane
What is the highest value of water potential?
Pure water = 0
More negative the value -> lower water potential
What factors affect the rate of osmosis?
Surface area
Water potential gradient
Thickness of exchange surface
Diffusion distance
What factors affect the rate of facilitated diffusion?
Surface area Concentration gradients (until the number of proteins is the limiting factor as all are in use/saturated) Number of channel/carrier proteins
What is the definition of active transport?
The movement of molecules or ions into or out of the cells from a low to high concentration using ATP or carrier proteins
Describe the direct active transport of a single molecule
Molecule binds to receptor site of carrier protein
Inside of cells/organelle, ATP binds to protein causing it to split into ADP and a phosphate - this results in the protein changing shape to open on the other side of membrane + release molecule
Phosphate molecule is released so protein reverts back to original shape + phosphate and ADP recombine to from ATP in respiration
What is a sodium potassium pump?
Sodium ions are actively transported out of cell + potassium ions are actively transported in from surroundings
How is the rate of movement across membranes increased?
Microvilli-> Larger surface area
Increase number of protein channels/carrier proteins
What is the role of diffusion in absorption?
Greater concentration of glucose/amino acids as carbs and proteins are being digested continuously
- > this creates a concentration gradient where glucose moves down (by facilitated diffusion) from ileum to blood
- > concentration gradient between ileum + blood which increases rate of facilitated diffusion
- > glucose is constantly moving around the body through the blood to supply cells
How does co-transport of glucose take place?
- Sodium ions are actively transported out of epithelial cell by sodium potassium pump into blood using a carrier protein
- This creates a higher conc of sodium ions in lumen of small intestine than inside epithelial cells
- Sodium ions diffuse into epithelial cells through co-transport protein down conc gradient + also carry amino acid/glucose molecules with them
- Glucose/amino acids pass into the blood plasma by facilitated diffusion
