BIOLOGICAL MEMBRANES (2.1.5) Flashcards
describe the function of the plasma membrane
- partially permeable membrane between cell and environment
- involved in cell recognition (immune system)
- involved in cell communication/cell signaling
describe the functions of membranes within cells
- divide cell into compartments (makes the organelles functions more efficient)
- can form transport vesicles of substances within cells (e.g. proteins to Golgi Apparatus)
- site of chemical reactions
state the 6 components of the membrane
- phospholipids
- channel proteins (intrinsic)
- carrier proteins (intrinsic)
- glycoproteins (extrinsic)
- glycolipids
- cholesterol
state and explain the function of phospholipids in the membrane
- arrange themselves into a bilayer
- phosphate hydrophilic head face outwards, towards water on either side of membrane
- hydrophobic tails face inwards
what substances does the bilayer allow through and why?
- stops water-soluble substances (e.g. ions, polar molecules) through, due to its hydrophobic centre
- allows lipid-soluble substances (e.g. vitamins) to dissolve and pass directly through
describe the structure of a phospholipids
- hydrophilic, polar phosphate head
- 2 hydrophobic, non-polar fatty acid tails
describe and explain the role of intrinsic proteins
- span both bilayers of the plasma membrane
- due to hydrophobic amino acids on outside, allowing them to interact with hydrophobic phospholipid tails
- channel proteins lined with hydrophilic R groups allowing the diffusion of polar molecules down the concentration gradient
describe and explain the role of extrinsic proteins
- found on one side of bilayer
- act as receptors during cell signaling
- act as antigens
- GLYCOPROTEINS - carbohydrate attached to a protein
- GLYCOLIPIDS - carbohydrate attached to a phospholipid
describe cholesterol and their roles
- lipid
- binds to hydrophobic tails, so increases rigidity
- creates further barrier to polar substances
describe the Fluid Mosaic model
- “Fluid” bilayer, due to phospholipids that are constantly moving
- “Mosaic”, due to proteins, cholesterol, glycolipids, glycoproteins scattered throughout
explain what is meant by diffusion
- passive overall movement of particles from a region of high concentration to an area of low concentration
- occurs with small, non-polar molecules
- continues until equilibrium
state what is meant by equilibrium
when particles move equally in both directions
why does diffusion occur with small, non-polar molecules
- must be small enough to pass through gaps between the phospholipids
- must be non-polar, to avoid being repelled by the hydrophobic interior of the phospholipid bilayer
explain what is meant by facilitated diffusion
- transport of larger molecules (e.g. glucose) and charged particles and ions through the use of carrier and channel proteins
state why facilitated diffusion occurs with large, charged particles
- large molecules must pass through proteins, as they cannot pass the bilayer fast enough
- charged particles would be repelled by hydrophobic interior of bilayer
- proteins are lined with hydrophilic amino acids, so interact with the polar molecules
explain why transport proteins are described as selectively permeable
they are specific to one molecule or ion
state the 5 factors that affect rate of diffusion
- temperature
- concentration
- membrane surface area
- membrane thickness
- number of transport proteins present (FACILITATED ONLY)
describe what is meant by active transport
- overall move to of ions or molecules from an area of low concentration to an area of high concentration
- process requires carrier proteins and energy
describe the stages of how carrier proteins work during active transport
span the membrane and act as pumps
1. molecule attaches to carrier protein
2. protein changes shape and moves molecule to the other side of the membrane
3. carrier transport releases molecule on the other side of
explain why active transport is an “active” process
- requires metabolic energy supplied by ATP
- needed to move particles against its concentration gradient
describe what is meant by bulk transport
form of active transport for molecules/substances that are too large to move through transport proteins (e.g enzymes, hormones)
state 2 types of bulk transport
- endocytosis
- exocytosis
describe process of endocytosis
- bulk transport of MATERIAL INTO CELLS
1. plasma membrane invaginates when coming into contact with the material
2. membrane enfolds the material until membrane fuses, forming a vesicle
3. vesicle pinches off
4. vesicle moves into the cytoplasm to transfer the material to be processed within the cell
(e.g. bacteria to lysosomes inside cell)
describe process of exocytosis
bulk transport of material OUT OF CELL
1. substances are enclosed in vesicles which pinch off from the Golgi Apparatus and move towards the plasma membrane
2. vesicle fuses with the plasma membrane and releases their contents to the outside of the cell
(e.g. digestive enzymes, hormones must be released)
why is energy required for bulk transport?
endocytosis and exocytosis require ATP, due to the involvement of
- movement of the cytoskeleton
- changing cells shape (to engulf material)
- the fusion of cell membranes as vesicles form OR as they meet the membrane
define osmosis
the net movement of water molecules from a high water potential to a low water potential across a partially permeable membrane
define water potential
likelihood of water molecules to diffuse into or out of a solution ψ
what are the units of water potential?
- kPa
- water molecules collide with the membrane, so is a measure of pressure
which substance has the highest water potential and why?
- pure water, ψ = 0kPa
- due to highest possible concentration of water molecules
describe relationship between concentrated solutions and water potential
more concentrated solutions have lower water potentials than less concentrated solutions
define hypertonic
when the solution outside the cell has a lower water potential (more -) than the solution inside the cell
define isotonic
when the solution outside the cell has an equal water potential to the solution inside the cell
define hypotonic
when the solution outside the cell has a higher (less -) than the solution inside the cell
describe what happens to an animal cell when placed in a hypertonic solution
- H2O moves out of the cell
- cell loses volume and shrinks.
describe what happens to an animal cell when placed in an isotonic solution
- there is an equal movement of H2O molecules into and out of the cell, so no net movement
- no change in cells shape
describe what happens when an animal cell is places in a hypotonic solution
- H2O molecules move into the cell
- cell swells and bursts (cytosis)
describe what happens to a plant cell when placed in a hypertonic solution
- H2O molecules leave the cell
- protoplast pulls away from the cell wall
- cell becomes plasmolysed
describe what happens to a plant cell when placed in an isotonic solution
- water molecules enter and leave the cell at an equal rate
- no change to the cell
explain what happens to a plant cell when placed in a hypotonic solution
- H2O moves into the cell
- protoplast swells, exerting pressure on the cell wall
- cell becomes turgid
why is it important for plant cells to be turgid?
- turgidity results in strength that allows plants to stand upright to catch sunlight
- if cells do not receive enough water, plants will wilt
why do plant cells not burst when placed in a hypotonic solution
due to cell walls inelasticity, pressure can build up inside making the cell firm
why do animal cells burst when placed in a hypotonic solution
due to their lack of cell wall
state the factors that effect membrane permeability
- type of solvents
- temperature
explain how the type of solvent can affect membrane permeability
- some dissolve lipids in the membrane, resulting in loss of structure
- increasing concentration of the solvent increases membrane permeability
explain how cold temperature effects membrane permeability
BELOW 0DEGREES C
- phospholipids have little KE, packed close
- membrane is rigid
- transport proteins denature, increasing permeability
- ice crystals can form and pierce membrane, increasing permeability
explain how temperature of 0-45degrees C affects membrane permeability
- phospholipids have increased KE
- not tightly packed, can move more
- membrane is partially permeable
explain how temperatures above 45DEGREES C can affect membrane permeability
- phospholipid bilayer starts to melt
- water inside expands, puts pressure on cell membrane
- transport proteins denature, no control over substances entering/leaving
- membrane permeability increases
State the purpose of cell signalling
Used during communication to respond to environmental changes
Describe the process of cell signalling
1 - cell releases messenger molecule (e.g. Hormone)
2 - molecule travels to another cell (e.g. through blood)
3 - binds to receptor on cells membrane and is detected
4 - triggers a change in the cell
Describe how proteins act as membrane-bound receptors
- have specific shapes
- allow only complementary messenger molecules to bind to them
Describe how drugs use cell signalling to work
- bind to receptors and either
- trigger a response or
- block the receptor from working
