2.5 biological membranes Flashcards
explain the fluid mosaic model
the fabric of the cell membrane is a phospholipid bilayer, with proteins floating in it making a mosaic pattern
what is the role of the inner membrane of chloroplasts?
the THYLAKOID contains chlorophyll and is the location for some of the reactions in photosynthesis
what is the role of the inner membrane in mitochondira?
CRISTAE provides a large surface area and localises the enzymes needed for some of the reactions in areaobic repiration
what is the role of membranes WITHIN CELLS?
separates organelle contents from the cell cytoplasm so they can carry out their functions
what is the role of the plasm membrane at the SURFACE OF CELLS?
- separates cell components from the external environment
- regulates transport of materials
- can release chemicals
- contains enzymes
- has antigens
- has receptors to recieve chemical signals
- site for chemical reactions
what are 3 ways that substances can pass through membranes?
- very small molecules DIFFUSE between structural molecules
- some substances DISSOLVE in the lipid layer
- other molcules pass through special protein channels by CARRIER PROTEINS
Why are cell membranes partially permeable?
they dont allow all types of molecules to pass through them
what is the fluid mosaic model?
the theory of cell membrane structure with proteins e,bedded in a sea of phospholipids
what is a glycolipid?
a lipid/phospholipid with a chain of carbohydrate molcules attatched
what is a glycoprotein?
a protein with a chain of carbohydrate molecules attatched
what is a plasma membrane?
the cell surface membrane
What are 2 roles of glycoproteins in the cell membrane?
- antigens can bind to the antibodies
- receptors for cell signalling
Who proposed the fluid mosaic model?
Singer and Nicolson in 1972
Why is a membrane ‘fluid’?
The phospholipids can move within the bilayer
What is the ‘mosaic’ in the fluid mosaic model?
The proteins scattered within the bilayer make a mosaic pattern
What is the role of cholesterol in cell surface membranes?
Cholesterol makes the membrane more stable and rigid
What are intrinsic proteins?
Proteins that span the width of the cell surface membrane
What are examples of intrinsic proteins?
- protein channels
- carrier proteins
What are examples of extrinsic proteins?
- receptors
- enzymes
What are extrinsic proteins?
Proteins that are on one side of the membrane
What is cholesterol?
A type of lipid that gives mechanical stability and flexibility
What is compartmentalisation?
The separation of cell organelles from their surroundings with membrane
What do cells need to diffuse into them?
- oxygen
- glucose
What do cells need to diffuse out of them?
- water
- carbon dioxide
What do cells export?
- enzymes
- hormones
What is a passive process?
A process that uses kinetic energy rather than metabolic energy (ATP)
What are aquaporins?
Specific water channel proteins needed if there is a high rate of water movement
What is diffusion?
The movement of particles from an area of high concentration to an area of low concentration
What is facilitated diffusion?
The movement of particles from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels or carriers
How do ions move across the membrane?
They can’t diffuse directly so use facilitated diffusion
How do small polar molecules move across the membrane?
They diffuse slowly
How do small non-polar molecules move across the membrane?
They diffuse rapidly
Explain the effect that a higher percentage of cholesterol would have on a plasma membrane
More cholesterol means the phospholipids can move less so the membrane becomes more rigid
Describe what would happen to the structure of the cell membrane if it was exposed to temperatures below 0°C
The phospholipids will move closer together so that it is more rigid and less permeable. Ice crystals may form and proteins may denature
Suggest an adaptation you might see on a cell membrane of a neurone cell
There may be lots of carrier channels for the movement of cations
Why can low temperatures reduce the speed at which nerve impulses are conducted?
Cations will have less kinetic energy so they move more slowly across the cell membrane, reducing the speed of nerve impulses
Explain why phospholipids form a bilayer in plasma membranes
Phospholipids have a polar phosphate group which is hydrophilic, whilst the non-polar fatty acids are hydrophobic, so they form a bilayer when surrounded by a aqueous solutions in the cell
what factors affect the rate of simple diffusion?
- temperature
- surface area
- diffusion distance
- concentration gradient
- size of diffusing molecule
how does simple diffusion work?
when there is a high concentration of a certain molecule, they will bump into each other as they randomly move, this causes them to spread further from eachother. When the molecules are evenly dispersed there is no net diffusion
how do fat-soluble molecules pass through the bilayer?
they can dissolve in the lipid bilayer
why are water molecules a special case?
they are polar and insoluble so the bilayer shouldf be inpenetrable BUT water is present in such high concentrations that direct diffusion occurs.
how is concentration gradient maintained?
molecules entering the cells then pass into organelles and are used for metabolic reactions, maintaining the concentration gradient
e.g.
- oxygen is used in the mitochondria for aerobic respiration
- carbon dioxide in palisade mesophyll cells are used in the chloroplast for photosynthesis
how does temperature affect the rate of simple diffusion?
as temperature increases, molecules have more kinetic energy so rate of diffusion increases
how does diffusion distance affect the rate of simple diffusion?
the thicker the membrane the slower the rate of diffusion
how does surface area affect the rate of simple diffusion?
more diffusion can take place across a larger surface area
how does size of diffusing molecule affect the rate of simple diffusion?
smaller ions or molecules diffuse more rapidly than larger ones
how does concentration gradient affect the rate of simple diffusion?
the steeper the gradient, the faster the rate of diffusion
why are ions insoluble in the bilayer?
they cant interact with the hydrophobic tails of the phospolipid bilayer
how do ions travel across the membrane?
they diffuse through water filled protein channels embedded in the membraneh
how is glucose transported across the membrane?
they bind to transmembrane carrier proteins, which allows them to pass out on to the other side of the membane
what protein channels do neurone plasma membranes have?
channels specific to sodium ions or potassium ions
what protein channels are in the epithelial cells lining your airway?
chloride ion channels
What is endocytosis?
The bulk transport of molecules too large to pass through a cell membrane, even via channel or carrier proteins, INTO a cell.
What is exocytosis?
Bulk transport of molecules too large to pass through a cell membrane, even via channel carrier proteins OUT of a cell.
What is an example of active transport?
Root hair cells use active transport to absorb ions from the soil.
What is the energy from the hydrolysis of ATP do to a carrier protein?
The energy helps the carrier protein change its shape so, it carries the ion from one side of the cell membrane to the other.
What are two types of endocytosis?
- phagocytosis
- pinocytosis
What does phagocytosis transport?
Solid matter.
What does pinocytosis transport?
Liquids.
What is ATP needed for an endocytosis?
To provide energy to form the vesicles and move them using molecular motor proteins along cytoskeleton threads into the cell interior.
What is an example of exocytosis?
At the synapses, when neurotransmitters are released from the pre-synaptic bulb.
What is the process of exocytosis?
- A membrane bound vesicle containing the substance to be secreted is moved towards the cell surface membrane
- The cell surface membrane and the membrane of the vesicle fused together
- the fused site opens releasing the contents of the Secretory Vesicle.
What is the process of the stomata opening?
Potassium ions are actively transported into the guard cells. This lowers the water potential, so water enters the cell by osmosis.
What is water potential?
The measure of tendency of water molecules to diffuse from one region to another.
What is the unit for measuring water potential?
KPa
Explain what happens to the water potential of a solution when more solute molecules are added.
The water potential of the solution will become more negative as the solid molecules bind to the solvent molecules.
Explain what happens if the aqueous solutions either side of a plasma membrane have different water potentials.
The water molecules will move from an area of high water potential to a lower water potential across the plasma membrane by osmosis.
Explain what happens if the aqueous solutions either side of a plasma membrane have the same water potential.
There will be no net movement of water molecules.
What will happen if a cell is placed in pure water?
Water molecules will move into the cell.
What will happen if a cell is placed in a concentrated sugar solution?
Water molecules will move out of the cell
What will happen to a plant cell if it is placed in pure water?
The water will move in by osmosis down a potential gradient.
The cell will become turgid as the membrane pushes against the wall.
What will happen to a plant cell if it is placed in a concentrated sugar solution?
Water moves out of the cell by osmosis, down a water potential gradient.
- The cell membrane pulls away from the cell wall , the cell is plasmolized.
What will happen to an animal cell if it is placed in a concentrated sugar solution?
Water moves out of the cell by osmosis down the water potential gradient.
The animal cell shrinks and appears wrinkled. It is crenated.
What will happen to an animal cell if it is placed in pure water?
Water moves into the cell by osmosis, down a wharf potential gradient.
The animals cell bursts open. It is cytolysed
What factors can affect membrane permeability?
- temperatures
- solvents
What happens to the fatty acids when the temperature drops?
Saturated fatty acids become compressed, but when the unsaturated fatty acids become compressed, their tails push adjacent phospholipid molecules away. This maintains the fluidity of the membrane.
How does cholesterol act as a buffer to temperature changes in the membrane?
Cholesterol prevents the phospholipid molecules from packing together, too closely, so the fluidity of the membrane is maintained.
What effect does increasing temperature have on membrane permeability?
The phosphor lipids have more kinetic energy, so they move around more, which increases the membrane’s fluidity and permeability.
What can increasing membrane fluidity affect?
- the infolding of the plasma membrane during phagocytosis
- cell signalling
- the way membrane embedded proteins are positioned and may function.
How the high temperatures affect proteins in the membrane?
The proteins denature as the high temperatures cause the molecules to vibrate, so the bonds break, changing the tertiary structure.
Why may holes appear in the plasma membrane?
The proteins in the plasma membrane become denatured alongside the cytoskeleton threads, which causes the membrane to fall apart and become more permeable.
What could happen to membrane embedded enzymes if there was a change in temperature?
The enzymes were no longer function as they become denatured, changing the shape of their active site.
How do you solvents effect for phospholipids?
Organic solvents such as acetone and ethanol were damaged cell membranes as they dissolve the phospholipids
What is it called when an animal cell bursts?
Cytolysed
What is it called when an animal cell shrivels?
Crenated
Describe the role of the cholesterol in cell surface membranes in the human body.
Cholesterol binds to phospholipid fatty acid tails, increasing the packing of the membrane, therefore reducing the fluidity of the membrane
