Biological membranes Flashcards
define the term simple diffusion
diffusion is the met movement of particles (atoms, molecules or ions)from a region of higher concentration to a region of lower concentration. it is a passive process and will continue until there is an equilibrium between the two areas. there is no membrane!
define the term facilitated diffusion.
the same as simple diffusion - a net movement of particles from an area of higher to an area of lower concentration - except this happens over a membrane or partially permeable membrane. it can involve the use of protein channels or carrier proteins.
define the term osmosis.
the net movement of water molecules across a partially permeable membrane from one solution to a more concentrated one with a lower water potential. it is a passive process.
define the term passive transport
no energy from the cell is used in the process of transport
define the term active transport.
the movement of molecules or ions across membranes, using ATP to drive protein pumps within the membrane. happens against the concentration gradient.
define the term endocytosis
a form of bulk transport into the cell
define the term exocytosis
a form of bulk transport out of the cell
define the term phagocytosis.
when solid materials are moved into or out of the cell by bulk transport
define the term pinocytosis.
when liquids are moved into or out of the cell by bulk transport.
what is the difference between a carrier protein and a channel protein?
channel proteins are non polar channels through the membrane. carrier proteins are shaped to allow one specific molecule through and can be opened and closed.
list the factors affecting rate of diffusion.
- temperature
- concentration gradient
- stirring/moveing
- surface area of membrane
- thickness of membrane
- size of the molecule/s
explain how substances can move across a membrane by simple diffusion.
small molecules such as gases can fit through the gaps in between lipid molecules so can diffuse down the concentration gradient. this is also true of lipid-soluble molecules that can diffuse without a protein. simple diffusion occurs when molecules are small, non polar and lipid soluble.
which types of molecules can travel using simple diffusion?
- small
- non-polar
- lipid-soluble.
Explain how substances can move across a membrane by facilitated diffusion.
Facilitated diffusion occurs when molecules cannot pass directly through the membrane bilayer so must use a protein. Channel proteins allow smaller ions to pass through using a hydrophilic channel. Carrier proteins allow much larger molecules such as glucose to pass through the membrane. These proteins are often specific to certain molecules or ions.
Describe the process of endocytosis.
When materials are bulk transported into the cell. In phagocytosis (solid materials) the cell surface membrane surrounds and engulfs a material forming a vehicle with the outer membrane
Describe the process of exocytosis.
When a vehicle within the cell merges with the cell surface membrane, expelling its contents outside the cell
Which transport mechanisms require ATP?
Active transport
Endocytosis
Exocytosis
Why is it easier for an oxygen molecule to diffuse across the membrane than it is for a water molecule?
Oxygen is not a charged/ polar molecule so it is not affected by the polar regions of the phospholipids in the membrane. Oxygen is also a smaller molecule than water.
Why is it easy for steroid hormones to cross the cell membrane?
Because they are lipid based molecules so will dissolve in the bilayer
Explain why ions are only able to cross the membrane via a protein channel.
Because they are charged. This means that they cannot dissolve in the bilayer and cannot get past the charged phosphate heads of the lipids. The protein channel is a non polar channel through the membrane that doesn’t repel them.
State the five roles of membranes within and on the surface of cells.
- separating cell contents from the outside environment
- separating cell components from the cytoplasm
- cell recognition and cell signalling
- holding the components of metabolic pathways in place
- regulating transport of materials in and out of cells
Define compartmentalisation and state why it’s useful.
When membranes form around organelles such as the mitochondria or chloroplasts.
It is useful to the cell as it stops potentially harmful substances from getting into the cytoplasm. It can also be useful to some organelles that use the membrane (photosynthesis occurs in the membrane of a chloroplast)
Define the term partially permeable.
Permeable to some substances - usually those useful to the cell. And non-permeable to others.
Define the term cell signalling
When cells communicate with one another using molecules on the membrane that act as signals
Define the term phospholipid bilayer
The main component of the membrane, it consists of of phospholipids joining together in two layers. The tails of the phospholipids are hydrophobic so will collect on the inside of the bilayer. The heads are hydrophobic so form the top and bottom towards the water.
Explain what is meant by fluid and mosaic in terms of the fluid mosaic model.
Fluid - the phospholipids give the membrane freedom to move and fluidity
Mosaic - the appearance of the proteins in the membrane
Define the term glycoprotein
A protein in the membrane with a carbohydrate molecule attached to its surface
Define the term phospholipid.
A lipid molecule with a phosphate head and a fatty acid tail
Define the term cholesterol
A small molecule within the membrane that regulates fluidity
Define the term glycolipid
A lipid molecule with a carbohydrate molecule attached to it
Define the term channel protein
Allow the movement of substances that are too large or hydrophilic to fit through the membrane. They are proteins with a hydrophilic channel through the centre
Define the term carrier protein
Proteins within the membrane that actively move substances across. They are involved in active transport and are likely specific to one molecule. They use ATP to do this
Define the term intrinsic protein
Proteins that span the width of the membrane
Define the term extrinsic protein
Proteins confined to the outer or inner surfaces of the membrane
Define the term receptor protein
Allow recognition by the immune system
Define the term cell adhesion
When two cells are anchored to each other usually by a glycolipid or glycoprotein
Explain how cholesterol effects membrane fluidity.
It sits between phospholipids and holds them together. When hot, cholesterol stops the phospholipids from spreading too far apart and stops them from binding too close together when cold.
Explain the importance of membrane bound proteins in chemical reactions
They regulate the amount of chemicals present in the cell and get rid of waste when the reaction is complete.
Explain how the binding of a molecule to a membrane bound glycoprotein can cause effects within the cell.
When a molecule binds to a receptor it causes a response within the cell. Enzymes within the cell attach to the other end of the protein. These then release a second messenger molecule that travels through the cell. This response is them amplified and travels to the relevant areas.
Describe the effect of temperature on the permeability of the membrane
Hot - phospholipids have more kinetic energy so move further apart. The membrane becomes more permeable.
Cold - the membrane loses fluidity due to the lack of kinetic energy so the membrane becomes less permeable
Describe the effect of solvent concentration on membrane permeability
Solvents such as ethanol disrupt cell membrane ps so the higher the concentration of solvent the more permeable the membrane becomes.
Define the term solvent
The liquid a substance dissolves in
Define the term solute
The substance that can dissolve
Define the term solution
A solute dissolved in a solvent
Define the term water potential
A measure of the tendency of water molecules to diffuse from one place to another
What is the unit for water potential?
kPa
State the water potential of pure water
0kPa
Explain why water potential cannot have a positive value.
Pure water is 0kPa and adding any sort of solute decreases the water potential so it can only go down.
Define the term water potential gradient
From an area of of high concentration to an area of lower concentration
Define the term net movement of water
The overall direction in which water molecules move
Describe what happens when a animal cell is put into a solution of higher water potential than the cell
Water moves in by osmosis making the cell burst open. It is Haemolysed
Describe what happens to a plant cell when placed in a solution of higher water potential than the cell
Water moves into the cell by osmosis but is prevented from bursting by the cell wall. It is turgid
Describe what happens when an animal cell is placed in a solution of lower water potential
Water moves out of the cell by osmosis, making it wrinkle, the cell is crenated
Describe what happens when a plant cell is placed in a solution of lower water potential
Water moves out of the cell by osmosis. The cell membrane pulls away from the cell wall. It is plasmolysed
Write the equation for the water potential of a cell
Water potential of cell = solute potential + pressure potential.
