Cell Membranes - Harder Flashcards
Roles of cell surface membranes
- Partially permeable to control which substances enter and leave the cell.
- Site of cell signalling.
Roles of membranes within cells
- Forms vesicles to transport substances around cells.
- Provide barrier between cytoplasm and organelles.
Describe fluid-mosaic model
Fluid - components free to move in phospholipid bilayer.
Mosaic - Components such as cholesterol, phospholipids, glycoproteins, glycolipids scattered throughout.
How thick is cell membrane?
5-10 nm
List components of cell surface membrane
- Proteins - peripheral, integral, glycoprotein
- Phospholipids
- Cholesterol (animal cells)
Why is phosphate head hydrophilic?
It is polar - faces outwards
Why are fatty acid tails hydrophobic?
They are non-polar
Role of cholesterol
Restricts movement of phospholipids, which prevents membrane being too fluid when warm.
Controls fluidity, so prevents membrane becoming too solid when cold.
Function of membrane proteins
- Receptors
- Channels for facilitated diffusion
- Pumps for active transport
Role of phospholipid bilayer
- Allows lipid soluble substances to enter/leave cell.
- Doesn’t allow water soluble/polar substances to enter/leave cell.
Role of glycoLIPIDS
- Act as recognition sites.
- Adhesion - helps cells attach to each other to form tissues.
Role of glycoPROTEINS
- Act as recognition sites
- Receptors for cell signalling
- Adhesion - helps cells stick together to form tissues.
- Act as antigens
Cell signalling process
- Signal molecule released by exocytosis.
- Glycoproteins act as receptors
- Receptors are specific site for signal molecules.
- Shape of receptor and signal molecule are complementary.
- Attachment of signal molecule to receptor causes change inside cell.
- Cell surface membrane allows entry of some lipid-soluble signal molecules.
Effects of high temperature on membrane structure
- KE of phospholipids increase.
- Phospholipids move more.
- Bilayer becomes more fluid and begins to lose structure.
- Increases number of gaps in membrane (becomes more permeable).
- Proteins in membrane could denature.
Effects of organic solvent on membrane
Organic solvents dissolve membranes and can disrupt and break down cells
What investigation looks into the effects of temperature on membrane permeability?
Beetroot PAG
How could beetroot PAG investigate the effects of different solvents on membrane permeability?
Keep temp constant.
Use range of organic solvents
Diffusion definition
Net movement
High to low conc
Down concentration gradient
Why do some substances diffuse through cell membrane at different rate?
centre of cell membrane is hydrophobic due to fatty acid tails.
What type of substances can diffuse rapidly across membrane?
Hydrophobic substances e.g. steroid hormones
What type of molecules cannot diffuse across membrane?
Hydrophilic molecules e.g. polar molecules, ions
Why can water diffuse across membrane, even though it’s polar?
It’s a very small molecule
How do polar molecules and ions pass through membrane?
Facilitated diffusion
What type of proteins are involved in facilitated diffusion?
Carrier & channel proteins
Both intrinsic proteins
What do carrier proteins do?
Have binding site for specific chemical.
When chemical binds, causes tertiary structure of carrier protein to change.
This change brings chemical across membrane, where it is released.
What do channel proteins do?
Protein with central pore.
Central pore lined with hydrophilic amino acids and contains water.
Hydrophilic substances can pass through one side of membrane to the other.
They are selective about type of chemical that can pass through.
Some types are always open, some only open when triggered e.g. type of chemical binds to channel i.e. neurotransmitter.
Describe facilitated diffusion
Net movement
High to low concentration
Through protein channels or using carrier proteins that change shape when molecules bind.
Name of protein channels for water
Aquaporins
Define water potential
Pressure exerted by molecules as they collide with a membrane or container.
Measured in kPa
Define osmosis
Net movement of water molecules
From area of high to low water potential
Across partially permeable membrane
Effect of putting animal tissue in conc solution
Solution has higher water potential than tissue.
Water moves out via osmosis.
Cell becomes crenated (wrinkled)
Effect of putting plant tissue in conc solution
Solution has lower water potential than plant cells.
Water moves out of cell via osmosis.
Cells become flaccid
Cells become more plasmolysed
Effects of putting animal cells in dilute solution
Water moves into animal cells via osmosis
Since no cell wall, cells swell and burst
Effect of putting plant tissue in dilute solution
Water moves into cells via osmosis.
Since cell wall is strong, cell becomes turgid
Define active transport
Net movement
Low to high concentration
Requires energy from ATP
Explain how fluidity of cell membrane can be used to transport substances
By endocytosis and exocytosis
What is endocytosis?
Plasma membrane encloses solid particles.
Membrane pinches off (requires ATP).
Vesicles formed.
Vesicle breaks away from plasma membrane and is free to move inside cell.
What is exocytosis?
Vesicles move to plasma membrane (requires ATP)
Vesicles fuse with plasma membrane.
Vesicles release contents to outside of the cell.
Membrane returns to original shape.
How are vesicles moved around in cell?
Microtubules of cytoskeleton.
Provide pathways for movement.
Vesicles along microtubules.
Process requires ATP
How do larger substances enter and leave cell?
Endocytosis and exocytosis (bulk transport)
How do polar substances enter cell?
Facilitated diffusion through channel proteins and carrier proteins
Can also be active transport through carrier proteins.
