Chapter 5 - Plasma Membrane Flashcards
Functions of membranes
• Compartmentalisation
• Isolation of substances e.g. isolating hydrolytic enzymes within the lysosomes, so they don’t escape + break down most of the cellular components
• Site for attachment of enzymes / ribosomes
• Provide selective permeability
• Create concentration gradients
What is compartmentalisation
creating organelles within a cell e.g. Mitochondria
Example of isolation
isolating hydrolytic enzymes within the lysosomes, so they don’t escape + break down most of the cellular components
What organelles have membranes (7)
Lysosomes
Golgi body
ER
Mitochondria
Chloroplast
Vacuole
Nucleus
What organelles do not have membranes
Centrioles
Ribosomes
What is the model of the membrane called
Fluid Mosaic model
Why is the model called fluid
o The phospholipids and proteins can move around via diffusion
o The phospholipids mainly move sideways, within their own layers
o The many different types of proteins interspersed throughout the bilayer move about within it although some may be fixed in position
Why is the model called mosaic
o The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
Main components in the fluid mosaic model
o Phospholipids
o Cholesterol
o Glycoproteins and glycolipids
o Transport proteins - carrier + Channel proteins
What is the cell surface membrane also known as
the phospholipid bilayer
How are phospholipids arranged in the cell surface membrane
tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane
Heads face outwards - hydrophilic sides
What does the cell surface membrane act as a barrier to
most water-soluble substances (due to hydrophobic tails)
What does having a membrane that acts as a barrier most water-soluble substances ensure
ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
Can water go through the membrane
Yes…kinda
Why can water go through the membrane
• Water – very small molecule – can still slowly squeeze its way through, despite being hydrophobic core
Where a lot more water is needed, such as the kidney, how does water quickly get into the cells
• Where a lot of water is needed, aquaporins are present – special channel for water
How can phospholipids be chemically modified to act as signalling molecules
o Moving within the bilayer to activate other molecules (eg. enzymes)
o Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
What is a Micelle
Phospholipids arranged in a spherical form
Why do micelles form
As there are not enough phospholipids to form a full bilayer
Two types of phospholipids
Saturated and unsaturated
What is the difference between saturated and unsaturated phospholipids
• saturated fatty acids – straight tails
• unsaturated fatty acids = double bond = – tails with kinks/bends in them
What increases the fluidity of the membrane
Increasing the number of unsaturated fatty acids / phospholipids
Why does increasing the number of unsaturated fatty acids increase the fluidity of the membrane
• more unsaturated fatty acids = more distance between the tails and thus fewer intermolecular interactions = more fluidity
Function of cholesterol
• regulates the fluidity of the membrane
- allows cells to survive at lower temperatures
- increases strength + stability
How does cholesterol in the membrane allow cells to survive at lower temperatures
• stops phospholipid tails packing too closely together = stopping it from becoming too rigid at low temperatures + crystalising = allowing cells to survive at lower temperatures
How does cholesterol stop the membrane from becoming to fluid at high temps
o Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together= decreasing fluidity
What are extrinsic proteins
proteins which are found on the surface of the plasma membrane.
General function of extrinsic proteins
usually function as enzymes and catalyse chemical reactions inside the cell.
What are intrinsic proteins
proteins which span both bilayers of the plasma membrane.
General function of intrinsic proteins
act as channels or carrier proteins to transport water-soluble molecules.
What are glycoproteins
Proteins Embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.
Function of glycoproteins
Cell Signalling
Recognition sites
play a role in cell adhesion (when they join to form tight junctions in certain tissues) + as receptors for chemical signals.
• When chemical binds to the receptors, it elicits a response from cell. May cause a direct response or set off a cascade of events inside cell. = cell communication/ cell signalling.
Example of cell signalling
• Receptors for neurotransmitters such as acetylcholine at nerve cell synapses – binding of neurotransmitters triggers / prevents impulse in next neurone.
• Receptors for peptide hormones, inc. insulin and glucagon, which affect the uptake/storage of glucose by cells.
What type of proteins are glycoproteins
Intrinsic proteins
What are glycolipids
lipids with attached carbohydrate (sugar) chains
Function of glycolipids
Recognition sites / antigens
can be recognised by cells of immune system as self (of the organism) or non-self (belongs to another organism).
What are extrinsic proteins also known as
Peripheral proteins
Function of transport proteins
create hydrophilic channels to allow ions and polar molecules to travel through the membrane.
Types of transport proteins
Channel proteins
Carrier proteins
Function of channel proteins
Transport polar substances down a concentration gradient
allow the cell to control which substances enter or leave
Function of carrier proteins
Transport molecules against the concentration gradient
allow the cell to control which substances enter or leave
Adaptation of carrier proteins
change shape to transport a substance across the membrane
Can any molecule go down any channel / carrier protein
No = • Each transport protein is specific to a particular ion or molecule
What two factors effect membrane permeability
Temp + solvent concentration
How does temp effect membrane permeability
• Phospholipids in a cell membrane are constantly moving.
• When temp. increased, the phospholipids have more kinetic energy = move more.
• This makes a membrane more fluid, and it will start to lose structure = REVERSIBLE
• Temp. continues to increase = cell will break down completely.
• Loss of structure increases permeability of membrane, making it easier for particles to cross it.
• Carrier and channel proteins in membrane will be denatured at high temps. These proteins → involved in transport across membrane = so as they denature = membrane permeability is affected. = IRREVERSIBILE
What also takes place at increased temps across a membrane e
• Any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)
How does solvent concentration effect membrane permeability
• Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure
Solvents less polar thane water
Less polar - (ex. Alcohols) or non-polar (ex. Benzene).
What does drinking alcohol do to cells
Not strong enough to completely destroy membranes but will still cause damage
Define diffusion
• the net passive movement of molecules from an area of high concentration to an area of low concentration
What happens after a long period of diffusion
Reach equilibrium
How’s does the concentration gradient effecyt rate of diffusion
How does temp effect rate of diffusion
How does SA effect the rate of diffusion
How can you increase the SA of an organelle / organism
Or decreasing it’s size
How does the properties of the molecules or ions dissolving effect the rate of diffusion
Question - how do decrease the fluidity of a membrane
Question - how to increase the fluidity of a membrane
What can no diffuse through phospholipid bilayer
o Large polar molecules such as glucose and amino acids
o Ions such as sodium ions (Na+) and chloride ions (Cl-)
What is facilitated diffusion
When certain molecules require transport proteins to diffuse across a membrane
What two types of molecules enable facilitated diffusion
o Channel proteins
o Carrier proteins
Properties of transport proteins
• They are highly specific (they only allow one type of molecule or ion to pass through)
What are channel proteins
water-filled pores
What do channel proteins allow
charged substances (eg. ions) to diffuse through the cell membrane = does conc Gradient
Most channel proteins are…
Gated
What does gated mean
that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• This allows the channel protein to control the exchange of ions
How do carrier proteins function in facilitated diffusion
the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
Define active transport
net movement of molecules against the concentration gradient, across a cell membrane, using ATP
What does active transport require
ATP + carrier proteins
Carrier proteins can change shape. Can it change shape in facilitated diffusion + why
No - requires energy for it to change shape = ONLY CHANGES SHAPE IN ACTIVE TRANSPORT
In facilitated diffusion molecules move
Down a conc gradient
How does atp release energy
It is hydrolysed
Importance of active transport
• The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
• The absorption of some products of digestion from the digestive tract
• The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
• The loading of inorganic ions from the soil into root hairs
What is diffusion / osmosis / active transport responsible for
the transport of individual molecules or ions across cell membranes
What is endocytosis / exocytosis responsible for
the bulk transport of larger quantities of materials into or out of cells is also possible
Example of bulk transport materials that need to cross the membrane
o Large molecules such as proteins or polysaccharides
o Parts of cells
o Whole cells e.g. Bacteria
• Bulk transport into cells
endocytosis
• Bulk transport out of cells
exocytosis
What do both exocytosis + endocytosis need
Energy
If both exocytosis and endocytosis require energy, what does that make them
Forms of active transport
Define endocytosis
process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it
Two forms of endocytosis
Phagocytosis
Pinocytosis
What is phagocytosis
bulk intake of solid material by a cell
Cells that specialise in phagocytosis are called…
Phagocytes
The vacuoles formed in phagocytosis are called
phagocytic vacuoles
Example of phagocytosis
engulfing of bacteria by phagocytic white blood cells
What is pinocytosis
bulk intake of liquids
Define exocytosis
process by which materials are removed from, or transported out of, cells
How does exocytosis work
• The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
• These vesicles then travel to the cell surface membrane
• Here they fuse with the cell membrane and release their contents outside of the cell
Example of exocytosis
secretion of digestive enzymes from pancreatic cells
Which processes require energy
ALL EXOCTYOSIS + ENDOCYTOSIS + ACTIVE TRANSPORT REQUIRE ENERGY
Define osmosis
• Net movement of water molecules from an area of high water potential, to an area of lower water potential, across a partially permeable membrane
Water potential of pure water
0 kPa (at standard temp. and at pressure, -25 degrees C and 100kPa)
Symbol of water potential
Psi
All solutions have …… water potentials
Negative
If the concentration of a solution increases, what happens to the numerical value of the water potential
more concentrated the solution, the more negative the water potential
What happens if an animal cell is placed in a hypertonic solution
cell water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
• Crenation = fatal
What happens if an animal cell is places in a hypotonic solution
water will enter the cell through its partially permeable cell surface membrane by osmosis
• Cell will gain water until cell membrane is stretched too far
• Lysis = bursts = hypotonic
What happens if a plant cell is placed in a hypertonic solution
water will leave the plant cell through its partially permeable cell surface membrane by osmosis
• As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
• The protoplast gradually shrinks and no longer exerts pressure on the cell wall
• As the protoplast continues to shrink, it begins to pull away from the cell wall
• This process is known as plasmolysis – the plant cell is plasmolysed
What happens if a plant cell is placed into a hypotonic solution
water will enter the plant cell through its partially permeable cell surface membrane by osmosis
• As water enters the vacuole of the plant cell, the volume of the plant cell increases
• The expanding protoplast pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting
• The pressure created by the cell wall also stops too much water from entering and this also helps to prevent the cell from bursting
• When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
What is the protoplast
(living part of the cell inside the cell wall = everything except the cell wall
Why is turgidity important in plant cells
the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
When you are talking about effect of temp on the membrane dont refer to it as the membrane
Refer to it as the bilayer
State two functions of membranes within cells
You shouldnt say semi permeable you should say…
Partially permeable
State what is meant by cell signalling
How to make more reliable and accurate
C
B
C
B
D
C
D
A
Define Plasmolysis
when plant cells lose water after being placed in a solution that has a higher concentration of solutes than the cell does.
