B2.1 Membranes and membrane transport Flashcards
What is the purpose of a membrane?
- They separate the cytoplasm and internal environment of a cell from the external environment
- They also separate the interior of organelles from each other and from the cytosol
primary function of the cell membrane?
it’s semi-permeable, so it acts as a selective barrier + regulates the passage of specific substances in and out of the cell (ex. diffusion, osmosis, and active transport)
- allows sensitivity + communication
- allows internal conditions to be different from external conditions
what are lipid bilayers, what does it do, and where are they found?
- they are the fundamental structural component of cell membranes
- it regulates the movement of substances
- found in almost all biological membranes (ex. plasma membrane)
what do phospholipids naturally form in water?
they naturally form continuous sheet-like bilayers
describe phospholipids
- it is amphipathic (because it has both hydrophilic and hydrophobic regions)
- phosphate head = hydrophilic
- fatty acids = hydrophobic
how are phospholipids able to form membranes?
it can form membranes because:
- the majority of the molecule is insoluble, BUT the phosphate head is hydrophilic
When exposed to water, phospholipids form one of two structures: ____ OR ___
micelle OR bilayer
KEY:
- in each structure, the hydrophilic heads face the water, and the hydrophobic tails point inwards away from the water
why are phospholipid bilayers ideally suited for membranes?
because it allows the outer surfaces of the hydrophilic heads to interact with water
what is the permeability of the membrane is dependent on?
- Size of the molecule (smaller molecules are more likely to pass)
- Hydrophobic nature (hydrophobic molecules are more likely to cross the membrane)
what can diffuse directly through the lipid bilayer?
Very small molecules (ex. oxygen and carbon dioxide)
Generally, the smaller and less polar a molecule, the _______ it will diffuse across a cell membrane.
Generally, the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane
how do / can these diffuse across a cell membrane?
- Small, non-polar molecules
- Small, polar molecules
- Charged particles (ions)
- Small, non-polar molecules : rapidly diffuse (ex. oxygen and carbon dioxide)
- Small, polar molecules : can diffuse, but much slower (ex. water and urea)
- Charged particles (ions like Na+) : cannot diffuse, no matter how small they are
examples of mostly impermeable particles? (low permeability)
glucose, sucrose
they are:
- large, uncharged polar molecules
examples of particles with high permeability?
steroids, oxygen, carbon dioxide
they are:
- non polar
- lipid-soluble
examples of impermeable particles?
Na+, K+, H+
they are:
- ions
examples of mostly permeable particles?
water, urea, ethanol
they are:
- small, uncharged polar molecules
what are membranes composed of?
lipids, proteins, + small amounts of carbohydrates (in the form of glycolipids and glycoproteins)
What are the most abundant and diverse lipids?
Membrane phospholipids
Explain why the hydrophobic core of a lipid bilayer has low permeability to large molecules
Large molecules face difficulty crossing the lipid bilayer due to their size
The fatty acid tails in the membrane are tightly packed, so large molecules would have to disrupt the membrane structure to pass through
–> This requires a lot of energy to be used + is potentially damaging to the membrane, so large molecules are generally blocked
Explain why the hydrophobic core of a lipid bilayer has low permeability to hydrophilic particles
Hydrophilic particles = polar or charged and can form hydrogen bonds with water
BUT! The hydrophobic core of the lipid bilayer is non-polar and made of fatty acid tails, which cannot interact favorably with hydrophilic molecules
–> This creates a high energy barrier, making it difficult for polar molecules like ions and glucose to pass through the membrane without help from transport proteins
what is simple diffusion? describe.
the movement of molecules down a concentration gradient
(high –> low concentration)
- it eventually results in equilibrium (equal concentration in both regions)
- it is a passive process (does not involve the expenditure of energy by cells)
What is an example of simple diffusion into a cell?
during gas exchange:
Oxygen diffuses from red blood cells (where concentration is high) into body cells (where concentration is low) to be used in respiration
This oxygen is carried to tissues, where it diffuses from erythrocytes (where oxygen concentration is higher) to metabolically active cells (where oxygen concentration is lower)
What is an example of simple diffusion out of a cell?
during gas exchange:
Carbon dioxide diffuses from body cells (where its concentration is high) into the blood (where its concentration is lower) to be transported to the lungs
Then, the carbon dioxide is carried to the lungs, where it diffuses from the blood to the alveoli (down the concentration gradient)
impact of concentration gradient on the rate of diffusion across a lipid membrane?
- Higher concentration gradient = faster rate of diffusion
- Lower concentration gradient = slower rate of diffusion
impact of particle size on the rate of diffusion across a lipid membrane?
The rate of diffusion is inversely proportional to molecule size
smaller = higher rate of diffusion
impact of polarity on the rate of diffusion across a lipid membrane?
More polar = slower rate
Nonpolar = faster rate
impact of the charge of molecules on the rate of diffusion across a lipid membrane?
a charge on a molecule significantly hinders its ability to diffuse across a lipid membrane
greater charge = lower rate of diffusion
membrane proteins can be classified as:
- integral proteins
- peripheral proteins
location of integral and peripheral proteins in the membrane?
- integral proteins : embedded in the lipid bilayer
- peripheral proteins : found on the surface of the membrane
about integral proteins?
- embedded in lipid bilayer
- difficult to isolate (BC extraction involves disrupting the bilayer)
- amphipathic molecules
- most of them = transmembrane proteins (they extend across the membrane)
how did integral proteins end up being embedded in the lipid bilayer?
they are embedded because:
- Their hydrophobic regions interact with the hydrophobic interior of the lipid bilayer
+
- Their hydrophilic regions face the aqueous environments inside and outside the cell
quite strongly anchored to the membrane
about peripheral proteins?
- hydrophilic in nature
- do not have hydrophobic regions
- found on the surface of the membrane
- only interact with the hydrophilic regions of the integral proteins (sometimes with the hydrophilic heads of the phospholipids)
–> SO, it is easier to remove these molecules from biological membranes then it is to remove integral proteins
not as strongly anchored to the membrane as integral proteins BUT some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring it to the membrane surface
both integral and peripheral are _____ oriented across the lipid bilayer
both integral and peripheral are asymmetrically (unevenly) oriented across the lipid bilayer
2 examples of integral proteins? + state what they are involved in / what they do
- Channel proteins: allows charged particles to enter via diffusion
- Carrier proteins: involved in both active and passive transport by changing shape
Outline six functions (with examples) of membrane bound proteins
1. Transport
- Membrane proteins facilitate the movement of molecules in and out of the cell
- EX: Channel proteins that form channel for the passage of molecules
2. Recognition
- Membrane proteins help in cell–cell recognition acting as ‘name tags’ for the cells
- EX: Glycoproteins on the surface of cells help the immune system recognize self and non-self cells (+ also helps in blood type recognition)
3. Receptors
- Membrane proteins act as receptors for chemical signals and are binding sites for molecules like hormones and neurotransmitters
- EX: Beta-adrenergic receptors which bind adrenaline and trigger intracellular responses
4. Enzymes
- Membrane proteins show enzymatic activity (can act as enzymes) and catalyse reactions
- EX: glucose-6-phosphatase (a membrane-bound enzyme found in the endoplasmic reticulum) regulates blood glucose levels
5. Cell Adhesion
- Membrane proteins act as “molecular glue” by mediating binding interactions, allowing cells to attach to each other or their surrounding environment
- EX: Cadherins = involved in the formation of adherens junctions, holding adjacent cells together
6. Cell Motility
- Membrane proteins facilitate adhesion, signaling, and transport, ultimately enabling cells to move and interact with their environment
- EX: Integrins help the cell attach to the extracellular matrix and can interact with the cytoskeleton
define osmosis + describe
osmosis = the diffusion of water across a selectively permeable membrane from lower to higher solute concentrations
*osmosis continues until the concentration becomes the same on both sides of the membrane
*once this happens, although the random movement of water molecules continues, there is nonetmovement of water
Predict the direction of water movement during osmosis based upon differences in solute concentration
Water moves from an area of lower solute concentration (higher water concentration) to an area of higher solute concentration (lower water concentration)
what are aquaporins (AQP)s?
a type of channel protein (+ therefore a type of integral protein)
it can also be referred to as a tetrameric protein because it has four subunits
IMPORTANT: aquaporins are bidirectional (water can flow in either direction, either into or out, of the cell depending on the conc. gradient
structure of aquaporin proteins?
- composed of four monomeric subunits (since it is a tetrameric protein)
- each subunit has a water channel
–> SO, an aquaporin molecule has four identical water channels - the water channels = lined with specific hydrophilic side chains (of amino acid residues)
function of aquaporin proteins?
- the water channels in these proteins allow the passage of water molecules but not of ions
–> SO, water molecules pass in a single file through the channels
they permit the rapid movement of water in and out of the cell by forming hydrophilic channels that span across the membrane
the the number of aquaporins is determined by?
the volume of water that needs to be transported across the cell membrane
function of transmembrane proteins?
to transfer molecules across the membrane
*it’s also a type of integral protein
define facilitated diffusion
the movement of a molecule down its concentration gradient with the help of specialised transport proteins (carrier and channel proteins) across the cell membrane
*still a passive process (no energy required)
what are channel proteins?
transmembrane proteins that assemble to form channels for the passage of molecules down their concentration gradient without the need for energy input
structure + function of channel proteins?
- typically embedded in the lipid bilayer
- has hydrophilic regions that line the inside of the pore (allows polar or charged molecules to pass through)
- has hydrophobic regions to anchor it to the bilayer + selective binding sites that ensure specific molecules are transported
FUNCTION:
to provide a pathway for the transport of specific molecules across the cell membrane, which would otherwise be impermeable to them due to their size or charge
outline the specificity of channel proteins for ions
for ions, there are ion channels (a. type of channel protein), tiny pores that act as pathways for ions
–> these channels = highly selective + usually, different channels are needed for transporting different ions
why are ion channels so selective?
BECAUSE:
- the binding sites of the hydrophilic amino acid side chains lining the channel are highly ion-specific
- the size of the pore acts as a size filter
why are ion channels in facilitated diffusion gated?
because their channels often open or close in response to specific stimuli (ex. changes in voltage + mechanical forces like pressure)
*When the gates are open, the ions pass through the pore down the concentration gradient
*In a closed state, the pore is plugged, preventing the passage of the ion
what are porins?
they are another example of channel protein (tend to be less specific + larger)
types of gates on channel proteins?
- Voltage-gated channels
- Ligand-gated channels
- Mechanically-gated channels
- Temperature-gated channels
give one example of facilitated diffusion through a protein channel
- transport of ions through ion channels
ex: in nerve cells, voltage-gated sodium channels allow Na+ ions to flow into the cell during an action potential. This movement = passive (doesn’t need energy)
what are carrier proteins?
transmembrane transport proteins that play an important role in facilitated diffusion
structure of carrier proteins?
- have highly specific sites for the solute to be transported
function of carrier proteins?
the carrier protein will bind to the solute molecules, undergo a conformational change, and transfer the molecules to the other side of the membrane
example of a carrier protein?
GLUT (glucose transporter)
it is a carrier protein that helps in the transport of glucose into the red blood cell down its concentration gradient
summarize the 3 mechanisms of diffusion
1. simple diffusion
- direct movement of small, non-polar molecules across the phospholipid bilayer of the cell membrane
- EX: Lipid-soluble molecules like steroid hormones
2. facilitated diffusion - carrier
- used for larger or polar molecules that cannot diffuse directly through the membrane
- carrier proteins bind to the specific molecule, change shape, and transport the molecule across the membrane
- Highly selective — each carrier is specific to a particular molecule
- EX: glucose, amino acids
3. facilitated diffusion - channel
- channel proteins (water-filled pores in the membrane) allow certain ions or molecules to pass
–> allowing the movement of charged or polar substances
- Channels are often gated — they open or close in response to stimuli
- EX: transporting water through aquaporins in a process called osmosis
similarities of the 3 methods of diffusion?
- Passive process (no energy required)
- Movement is down the concentration gradient (from high to low concentration)
active transport is used when ____
when molecules need to be transported from a region of their lower concentration to a region of their higher concentration (against their concentration gradient)
what happens during active transport?
since the transport is ‘uphill’, energy is required during active transport.
SO, the transport of the molecules is coupled with an energy-releasing or exergonic reaction like the breakdown of ATP
define active transport
The net movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration
what are the transport proteins used during active transport called?
they are often called pumps because they move the molecules against their concentration gradient
Active transport helps to?
- take up essential nutrients
- remove waste materials from the cell into the extracellular fluid
- maintain the right concentrations of ions in the cells
processes that use active transport? (give 3 examples)
- uptake of glucose and amino acids in the small intestine
- excretion of hydrogen ions and urea by kidneys
- absorption of mineral ions by plant roots
what are the two types of active transport?
- Direct active transport
- Indirect active transport (cotransport)
describe what happens during direct active transport
during direct active transport:
- the energy released by an exergonic reaction (like the breakdown of ATP) is used to directly transport molecules across the cell membrane
- because the energy is derived by the hydrolysis of ATP, these transport proteins are called ATPases or ATPase pumps
describe what happens during indirect active transport
- also called “cotransport”
during indirect active transport:
- the movement of one solute down its concentration gradient drives the movement of the second solute against its concentration gradient
Compare active transport using a pump protein to facilitated diffusion using a channel protein
- Direction of Movement
- Energy Requirement
- Transport Protein Type
- Mechanism
- Example
Explain one example of active transport of molecules into and out of cells through a protein pump
Sodium-Potassium Pump
- an example of direct active transport
- pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell
- this happens against the concentration gradients of both ions
- pump uses energy from ATP hydrolysis to change its shape and move the ions across the membrane
- this helps maintain the correct electrochemical gradient, which is needed for nerve impulse transmission and muscle contraction
structure of pump proteins?
- they are integral membrane proteins
–> SO, they are embedded within the phospholipid bilayer of the cell membrane and span across it
- this structure allows them to interact with both the inside (cytoplasm) and outside (extracellular fluid) of the cell
- they all have specific binding sites for the molecule(s) being transported
- For ATPase Pumps, they have an ATP-Binding Site on the cytoplasmic side of the pump, where where ATP binds and is hydrolyzed to release energy
function of pump proteins?
they actively move ions or molecules against their concentration gradient by using energy input
- Molecule or ion binds to a specific site on the pump protein
- ATP is hydrolyzed into ADP + phosphate, releasing energy
- The pump protein uses this energy to change shape (conformational change)
- The molecule is transported across the membrane and released on the other side
- The pump returns to its original shape, ready to repeat the process
define selective permeability
it means the membrane does not allow the free movement of all molecules and is permeable only to certain molecules
is the cell membrane selective during simple diffusion?
NO! because:
- during simple diffusion, the movement of molecules is based on only concentration gradient, size, and polarity (hydrophobic or hydrophilic) or charge of molecules
- SO, the permeability of the membrane depends on the physical properties of the molecules
–> this lets any molecules that fit these criteria, whether they are useful or harmful, pass through the cell membrane –> MEANING it is not selective
are facilitated diffusion and active transport selective?
YES! because:
- they involve proteins that transport molecules from one side of the membrane to the other
Outline how channels and pumps in the membrane allow for selective permeability
Channels and pumps in the membrane are selective due to their ability to recognize specific molecules or ions
- Channel proteins provide a passageway for specific molecules or ions to flow through the membrane.
–> They are selective because they have a specific size, shape, or charge that only allows certain molecules or ions to pass through
- Pump proteins actively transport molecules against their concentration gradient, using energy (ATP)
–> They are selective because they only transport specific ions or molecules based on their binding sites that fit particular shapes or charges
In addition to phospholipids and proteins, most membranes also consist of small amounts of _____
In addition to phospholipids and proteins, most membranes also consist of small amounts of carbohydrates
- These carbohydrates are either linked to lipids – forming glycolipids, or linked to proteins – forming glycoproteins
glycolipids are ____ molecules
glycolipids are amphipathic molecules
–> they are often restricted to the external surface of the cell membrane
–> their carbohydrate groups are polar and extend into the extracellular environment, whereas the non-polar lipid component lies embedded in the bilayer
outline the structure of glycolipids
- they are formed from the covalent bonding of carbohydrates to lipids
- their carbohydrate groups are polar and extend into the extracellular environment
- their non-polar lipid component lies embedded in the bilayer
what are glycolipids are classified into?
- glycoglycerolipids
- glycerol-based lipids
outline the structure of glycoproteins
- they are formed from the covalent bonding of oligosaccharides to the protein molecules
- the carbohydrate groups of the glycoproteins often protrude into the extracellular environment
functions of glycoproteins and glycolipids? (3)
- cell recognition
- they act as ‘markers’ on the cell surface and help cells of the body recognise each other + also help the immune system recognise foreign cells
- cell adhesion
- they help cells to attach and bind to other cells to form tissues
- cell signalling
- they act as receptors for enzymes and other molecules helping in cell signalling
what is the glycocalyx + what does it do?
- it is the sticky layer formed by the carbohydrate groups of the glycolipids and glycoproteins that protrude from the cell surface
- it helps to protect the cell surface + aids in cell signalling, cell adhesion and cell–cell recognition
label this two-dimensional fluid mosaic model of membrane structure
KNOW HOW TO DRAW THIS, TOO!
what does the fluid mosaic model state?
states that:
- the lipid bilayer is fluid
–> + that the fluidity of the membrane depends on the nature of the fatty acids+ the amount of cholesterol
- the proteins (both integral and peripheral) are embedded in the fluid bilayer
what does the fluid nature of the membrane mean?
means that:
- most of the lipids and proteins are able to move laterally, parallel to the membrane surface
which structures are responsible for the mosaic part of the bilayer?
peripheral and integral proteins
