2.1.5 Biological Membranes Flashcards
What is the role of membranes?
- Partially permeable barrier between cell/environment, between organelles/cytoplasm and within organelles
- Site of chemical reactions
- Sit of cell communication (signalling)
What is the fluid mosaic model?
The mixture and movement of the phospholipids, proteins, glycoproteins and glycolipids the membrane is made of
What is the role of the phospholipds?
- Align as a bilayer due to hydrophobic tails and hydrophilic heads
- Heads are attracted to water, so face outwards, and tails are repelled by water, so face inwards
What is the role of cholesterol?
- Hydroxyl group aligns with the phosphate heads and remaining portion tucks into fatty acid tail part of membrane
- Cholesterol is a lipoprotein
- It maintains the fluidity of the cell membrane
What is the role of glycolipids and glycoproteins?
- They are lipids and proteins on the cell membrane that have carbohydrate chains protruding
- Form hydrogen bonds with any surrounding water molecules to stabilise the membrane structure
- Used as receptor molecules to bind with hormones/neurotransmitters
What is the role of proteins?
- Can be extrinsic or intrinsic
- Extrinsic proteins provide mechanical support or make glycoproteins
- Function is cell recognition as receptors
- Intrinsic proteins are carrier or channel proteins
How does temperature affect the membrane?
- As temperatures increase, the kinetic energy of the phospholipids increases
- This makes them move faster so the bilayer is more fluid which increases permeability
- High temperatures may denature carrier and channel proteins
What happens to the membrane at different temperatures?
Below 0:
- Membrane is rigid as packed closely together
- Channel and carrier proteins may denature
- Ice crystals may form and pierce the membrane
- Increases permeability
0-45:
- Phospholipids can move freely
- Partially permeable
- As temperature increases, the permeability increases
Above 45:
- Phospholipids start to melt
- Water inside cell expands putting more pressure on membrane
- Channel and carrier proteins denature
- Increases permeability
How do solvents affect the membrane?
- Non-polar solvents can insert themselves into the bilayer
- This pushes phospholipids out of their orderly placement and increases movement
- The disruption of the membrane structure increases fluidity and increases permeability
- Solvents may denature proteins
How do molecules move across the membrane by simple and facilitated diffusion?
Diffusion = the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached
- Small or lipid soluble molecules move via simple diffusion
- Ions and polar molecules move via facilitated diffusion
- Facilitated diffusion uses channel proteins and carrier proteins
- Carrier proteins change shape to allow a specific type of molecule through
- Channel proteins from pores in the membrane
What is Fick’s law?
Rate of diffusion is proportional to (surface area x concentration difference)/length of diffusion path
What is osmosis?
The movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane
What is water potential?
The pressure exerted by water molecules that are free to move in a system (kPa)
- Pure water has a water potential of 0
- Lower water potential is negative
What happens when a cell is in a hypertonic solution?
Hypertonic = water potential is more negative
- Water moves out of an animal cell and it shrinks/crenates
- Plant cell loses water and goes flaccid as vacuole becomes flaccid and cytoplasm stops pushing against cell wall
- Known as plasmolysis
What happens when a cell is in a hypotonic solution?
Hypotonic = water potential is more positive
- Water moves into an animal cells so it expands
- It may burst, known as lysis
- Water enters a plant cell and fills vacuole so plasma membrane pushes against the cell wall
- Plant cell will be turgid
What happens when a cell is placed in an isotonic solution?
Isotonic = water potential is the same
- Same amount of water enters the cell as moves out
- No change or damage
What is active transport?
The movement of molecules into or out of a cell from a low to high concentration using ATP and carrier proteins
How do pumps work?
- Moves molecules against concentration gradient using carrier proteins in the membrane which act as pumps
- Pumps are selective as they have a complementary shape so only certain molecules can bind to the receptor site
- Use metabolic energy and only carry molecules one way across the membrane
- Much faster rate than by diffusion
- The transported molecule no longer fits the carrier so cannot be moved back as the carrier protein changes shape to ensure one way flow
How is ATP used?
- ATP –> ADP + Pi
- ATP binds to the protein and is hydrolysed into ADP and Pi
- Causes the protein to change shape and open towards the inside of the membrane
- Molecule is released on the other side of the membrane
- Pi molecule is then released from protein so the protein can revert back to its original shape
What is exocytosis?
The bulk transport of molecules out of a cell
How does exocytosis work?
- Vesicles move towards the cell surface membrane and fuse with the membrane
- Contents of the vesicle are released outside the cell
- Requires ATP as energy is needed to move the vesicle along the cytoskeleton (type of active transport)
What is endocytosis?
The bulk transport of molecules into a cell
How does endocytosis work?
- Cell surface membrane bends inwards around the molecule, surrounding it to form a vesicle
- Vesicle pinches off and moves within the cytoplasm
- Requires ATP as energy is needed for cell to engulf and change shape around the material (type of active transport)
- Classed as either phagocytosis (solid particles) or pinocytosis (liquid)
Where is phagocytosis common?
Amoeba and white blood cells
How is co-transport involved in the absorption of glucose?
- Transport proteins move glucose across the membrane due to its size
- Glucose is transported with Na+ ions in the Na+-glucose symporter
- Na+ ions move down a concentration gradient
- Na+ ions are actively transported out of the cells by the Na+/K+ pump to ensure a low concentration of Na+ ions inside the cell
- Na+ ions therefore can diffuse into the cell with glucose
- Glucose passes into the blood by facilitated diffusion
