Membrane Structure, Synthesis and Transport Flashcards
What is membrane structure?
Refers to the arrangement and composition of the lipid bilayer that forms the outer boundary of cells and separates them from their environment.
What is the main component of membranes?
phospholipid bilayer
What is the structure of the phospholipid molecule?
Generally consists of:
- 2 fatty acid “tails”
- a “head” consisting of a phosphate group
(slide 3)
What are phospholipids?
Phospholipids are ‘amphipathic’ molecules that contain:
- a hydrophobic, water-avoiding, non-polar region (the tails)
- a hydrophilic, water-seeking, polar region (the head)
What is the fluid-mosaic model?
A model that describes the structure of the cell membrane as a mosaic of lipid, protein, and carbohydrate molecules in a fluid-like arrangement.
What do membranes contain besides lipids?
Membranes also contain proteins and carbohydrates
Why is a membrane considered a mosaic?
A membrane is considered a mosaic because it is composed of a variety of different types of lipid, protein, and carbohydrate molecules.
Why is a membrane described as fluid?
Because the lipids and proteins can move relative to each other within it
Are the two leaflets of the lipid bilayer symmetrical?
No, the two leaflets (inner and outer faces of the bilayer) are asymmetrical, with different types of each component.
What are the proteins associated with membranes?
- integral or intrinsic membrane proteins
- peripheral or extrinsic membrane proteins
What are integral or intrinsic membrane proteins?
Proteins that are physically embedded within the membrane and span the entire phospholipid bilayer
What are the 2 types of integral or intrinsic membrane proteins?
- transmembrane proteins
- lipid-anchored proteins
What are transmembrane proteins?
Type of integral membrane protein that:
- spans the entire phospholipid bilayer
- has regions that face both the extracellular & intracellular environments
What are lipid-anchored proteins?
Type of integral membrane protein that:
- have an amino acid covalently attached to a lipid in a membrane
What are peripheral or extrinsic membrane proteins?
Proteins that are bound to projecting regions of integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids.
How can computer programs predict when a protein will be a transmembrane protein?
Computer programs can use algorithms to:
- analyse the protein sequence
- predict whether it has the characteristics necessary to span a lipid bilayer (e.g. hydrophobic & hydrophilic regions)
What percentage of all genes may encode transmembrane proteins?
20-30% of all genes may encode transmembrane proteins
In which domains of life are transmembrane proteins found?
Transmembrane proteins are found throughout all domains of life, including archaea, bacteria, and eukaryotes.
What is the function of many transmembrane proteins?
The function of many transmembrane proteins remains unknown
What is Transmission Electron Microscopy (TEM) used for in visualising membranes?
TEM is used to visualise membranes in biological samples by thin sectioning and staining them with heavy-metal dyes
What is the process of staining membranes in TEM?
- biological sample is stained with heavy-metal dyes
- which bind tightly to the polar head groups of phospholipids (but not to the fatty acyl chains)
- results in the membranes appearing as dark lines resembling railroad tracks
What part of the phospholipids do the heavy-metal dyes bind to?
The heavy-metal dyes bind tightly to the polar head groups of phospholipids.
What is Freeze Fracture Electron Microscopy?
Freeze Fracture Electron Microscopy is a specialised form of TEM used to analyse the interior of the phospholipid bilayer.
What is the process of Freeze Fracture Electron Microscopy?
- sample is frozen in liquid nitrogen & fractured with a knife
- due to weakness of the central membrane, leaflets separate into the P face (Protoplasmic face next to the cytosol) and the E face (Extracellular face)
What information can be obtained through Freeze Fracture Electron Microscopy?
This technique can provide significant detail about the form and organisation of membrane proteins, including their orientation and distribution within the membrane.
What is the P face in Freeze Fracture Electron Microscopy?
Protoplasmic face, which is the face next to the cytosol
What is the E face in Freeze Fracture Electron Microscopy?
Extracellular face, which is the face facing the external environment
Are membranes fluid or rigid?
Membranes are semifluid
What does it mean for a membrane to be semifluid?
A semifluid membrane means that:
- most lipids can rotate freely around their long axes
- move laterally within the membrane leaflet
Does “flip-flop” of lipids occur spontaneously in a membrane?
“Flip-flop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously
What is the enzyme responsible for transporting lipids between leaflets in a membrane?
flippase
Does flippase require ATP to transport lipids between leaflets in a membrane?
Yes, flippase requires ATP to transport lipids between leaflets in a membrane
What are lipid rafts?
Lipid rafts are
- groups of lipids
- that associate strongly with each other
- to form a distinct unit
- within the larger sea of lipids in the membrane
How do lipids in a lipid raft differ from the rest of the membrane?
The lipid composition of a raft is different than within the rest of a membrane.
- e.g. a lipid raft might have a higher conc of cholesterol and a unique set of membrane proteins
Do all membranes contain lipid rafts?
No, not all membranes contain lipid rafts
What are some factors that affect membrane fluidity?
- length of fatty acyl tails
- presence of double bonds
- presence of cholesterol
How does the length of fatty acyl tails affect membrane fluidity?
Shorter acyl tails are less likely to interact, which makes the membrane more fluid
How does the presence of double bonds affect membrane fluidity?
The presence of double bonds:
- creates a kink in the fatty acyl tail
- making it more difficult for neighboring tails to interact
- making the bilayer more fluid
How does cholesterol affect membrane fluidity?
Cholesterol tends to
- stabilise membranes & reduce their fluidity at high temperatures
- while increasing their fluidity at low temperatures
What did the Larry Frye and Michael Edidin experiment demonstrate?
The Larry Frye and Michael Edidin experiment demonstrated the lateral movement of membrane proteins
What was the setup of the Frye and Edidin experiment?
- mouse and human cells were fused
- temperature was either lowered to 0°C or raised to 37°C
- mouse membrane protein ‘H-2’ was fluorescently labeled
What was the result of the Frye and Edidin experiment at 0°C?
At 0°C, the fluorescent label stayed on the mouse side of the fused cell
Q: What was the result of the Frye and Edidin experiment at 37°C?
At 37°C, the fluorescent label moved over the entire fused cell
Can all integral membrane proteins move laterally within the membrane?
No
- depending on cell type, 10–70% of membrane proteins may be restricted in their movement
- due to being bound to components of the cytoskeleton
- or being attached to molecules outside the cell
How do eukaryotes synthesise lipids?
- eukaryotes synthesise lipids by working together with the cytosol and endomembrane system
- fatty acid building blocks are made by enzymes in the cytosol or taken into cells from food
- this process occurs at the cytosolic leaflet of the smooth ER
What do the cytosol and endomembrane system synthesise in eukaryotes?
The cytosol and endomembrane system work together to synthesize lipids in eukaryotes.
Where are the fatty acid building blocks for lipid synthesis made?
The fatty acid building blocks for lipid synthesis are made by enzymes in the cytosol or taken into cells from food
Where does the process of synthesizing lipids occur in the Endoplasmic Reticulum (ER)?
The process of synthesizing lipids occurs at the cytosolic leaflet of the smooth ER
How do lipids in the ER membrane diffuse into other membranes?
Lipids in the ER membrane can diffuse laterally into other membranes, such as the nuclear envelope
How are lipids transported to different organelles?
Lipids are transported, via vesicles, to different organelles, such as:
- the Golgi apparatus
- lysosomes
- vacuoles
- plasma membrane
What are lipid exchange proteins?
Proteins that
- extract lipids from one membrane
- insert them into another
- allowing for the transfer of lipids between different membranes
How are transmembrane proteins synthesized?
Most transmembrane proteins are directed to the ER membrane first, via co-translational insertion
What happens to transmembrane proteins after they are synthesized in the ER?
From the ER, transmembrane proteins can be transferred via vesicles to other membranes of the cell
Where does the synthesis of transmembrane proteins occur?
The majority of transmembrane proteins are synthesized in the ER membrane
What is glycosylation?
Glycosylation is the process of covalently attaching a carbohydrate to a protein or lipid
What is a glycolipid?
A glycolipid is a carbohydrate attached to a lipid
What is a glycoprotein?
A glycoprotein is a carbohydrate attached to a protein
What is the function of attached carbohydrates in glycosylation?
Attached carbohydrates can:
- serve as recognition signals for other cellular proteins
- often playing a role in cell surface recognition
- also help protect proteins from damage
How can carbohydrates be added to proteins?
2 different ways:
- N-linked glycosylation
- O-linked glycosylation
What is N-linked glycosylation?
N-linked glycosylation is the attachment of a carbohydrate to a N atom in the side chain of the amino acid asparagine.
- occurs in the endoplasmic reticulum
What is O-linked glycosylation?
O-linked glycosylation is the addition of carbohydrates to an O atom in the side chain of the amino acids serine or threonine.
- occurs only in the Golgi apparatus
What is the selective permeability of the plasma membrane?
The plasma membrane is selectively permeable, which means it allows some ions and molecules to pass through but not others.
What is the purpose of selective permeability?
Selective permeability ensures that:
- essential molecules can enter the cell
- metabolic intermediates remain within the cell
- waste products can exit the cell
What are the ways in which molecules move across membranes?
There are two main ways in which molecules move across membranes:
- passive transport
- active transport
What is passive transport?
Passive transport is
- the movement of molecules across a membrane without the input of energy
- occurs down a gradient, from areas of higher concentration to lower concentration of the solute
What are the two types of passive transport?
The two types of passive transport are:
- passive diffusion
- facilitated diffusion
What is passive diffusion?
Passive diffusion is the diffusion of a solute through a membrane without interaction with a transport protein
What is facilitated diffusion?
Facilitated diffusion is the diffusion of a solute through a membrane with the aid of a transport protein
What is active transport?
Active transport is the movement of molecules:
- across a membrane that requires energy
- occurs up a gradient, from areas of lower concentration to higher concentration of the solute
What is the phospholipid bilayer barrier?
The phospholipid bilayer barrier refers to
- the two layers of phospholipids that make up the plasma membrane of cells
- which act as a barrier to hydrophilic molecules and ions due to their water-repelling interior
What factors affect the rate of diffusion of solute molecules through the phospholipid bilayer barrier?
The rate of diffusion of solute molecules through the phospholipid bilayer barrier depends on the chemistry of the solute and its concentration
What molecules have high permeability through the phospholipid bilayer barrier?
Gases and small uncharged molecules
What molecules have moderate permeability through the phospholipid bilayer barrier?
Water and urea
What molecules have low permeability through the phospholipid bilayer barrier?
Polar organic molecules
What molecules have very low permeability through the phospholipid bilayer barrier?
Ions, charged polar molecules, and large molecules
Sum up the range of permeability of molecules through the phospholipid bilayer barrier
high permeability = gases and small uncharged molecules
moderate permeability = water and urea
low permeability = polar organic molecules
very low permeability = ions, charged polar molecules, and large molecules
Provide an example of how the phospholipid bilayer barrier selectively allows molecules to pass through
- diethylurea diffuses 50 times faster through the bilayer than urea, despite being larger
- due to its nonpolar ethyl groups
- this demonstrates how the bilayer selectively allows nonpolar molecules to pass through more easily than polar molecules
What is the concept of maintaining gradients in cells?
Cells maintain gradients by keeping a relatively constant internal environment that differs from their external environment
What is a transmembrane gradient?
A transmembrane gradient is
- a concentration difference of a solute across a membrane
- where the concentration is higher on one side of the membrane than the other
What is an ion electrochemical gradient?
An ion electrochemical gradient is a gradient that combines:
- an electrical gradient, which is due to the ionic charge
- a chemical gradient, which is due to differences in solute concentration
What is an isotonic solution?
An isotonic solution is one in which
- the concentration of solutes is equal on both sides of a membrane
- resulting in equal water concentration
What is a hypertonic solution?
A hypertonic solution is one in which the concentration of solutes is
- higher (and the water concentration lower) on one side of a membrane
- resulting in water moving from an area of higher to lower concentration
What is a hypotonic solution?
A hypotonic solution is one in which the concentration of solutes is
- lower (and the water concentration higher) on one side of a membrane
- resulting in water moving from an area of higher to lower concentration
What is osmosis in relation to membrane transport?
Osmosis is the movement of water through
- a selectively permeable membrane
from an area of higher water concentration to an area of lower water concentration
How can water movement through osmosis affect the size of a cell?
Water movement through osmosis can cause the cell
- shrink as water leaves the cell
- swell as or enters the cell
What is osmotic pressure?
Osmotic pressure is the tendency for water to move into a cell
- due to differences in solute concentration between the cell and its surrounding environment
- it is the pressure required to stop the net movement of water
- through a selectively permeable membrane
How does osmosis in animal cells affect their size and shape?
Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape.
What is crenation?
Crenation is the shrinkage of a cell when it is placed in a hypertonic solution
What is osmotic lysis?
Osmotic lysis is the swelling and bursting of a cell when it is placed in a hypotonic solution
How does osmosis in plant cells affect their size and shape?
The plant cell wall prevents major changes in cell size
What is turgor pressure?
The pressure that is created by the influx of water into a plant cell
- pressure pushes the plasma membrane against the cell wall and maintains the cell’s shape and size
What is plasmolysis?
Plasmolysis is when the volume of the plant cell shrinks when water exits the cell
- as water leaves, the plasma membrane pulls away from the cell wall
- resulting in the cell losing its turgor pressure and wilting
How does osmosis occur more quickly in some cells?
Osmosis occurs more quickly in cells with transport proteins that allow the facilitated diffusion of water
What was the protein called, that was abundant in red blood cells, bladder, and kidney cells, when it was first identified by Peter Agre and colleagues?
‘Channel-forming Integral Membrane Protein, 28kDa’ (CHIP28)
What is the current name of the protein that Peter Agre and colleagues identified?
Aquaporin, since it forms a channel that allows water to pass through the membrane
What are transport proteins?
Transport proteins are transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes
How many classes of membrane transport proteins are there and what are they based on?
- two classes
- based on the way in which they move molecules across membranes
What are the two classes of membrane transport proteins?
- channels
- transporters
What are channels in transport proteins?
Transmembrane proteins that form an open passageway for the direct diffusion of ions or molecules across the membrane.
Are most channels gated?
Yes, most channels are gated
e.g. Aquaporins
What are transporters (aka carriers) in transport proteins?
Transmembrane proteins that transport solutes across the membrane via conformational (shape) change
What is the principal pathway for uptake of organic molecules in transporters?
Through transporters in transport proteins
What are the three types of transporters in transport proteins?
Answer: The three types of transporters in transport proteins are uniporter, symporter or cotransporter, and antiporter
- uniporter
- symporter or cotransporter
- antiporter
What does a uniporter transporter do?
Uniporter transporter: transports a single molecule or ion type
What does a symporter or cotransporter transporter do?
Symporter or cotransporter transporter: transports two or more ion or molecule types in the same direction
What does an antiporter transporter do?
Antiporter transporter: transports two or more ion or molecule types in opposite directions
What is active transport in transport proteins?
Movement of a solute across a membrane
- against its gradient
- from a region of low to higher concentration
- energetically unfavorable
- requires the input of energy
What is primary active transport?
Primary active transport is a type of active transport that uses - a ‘pump’
- & directly uses energy to transport solute
What is secondary active transport?
Secondary active transport is a type of active transport that uses
- a pre-existing gradient of another molecule to drive transport
What is the role of ATP-driven ion pumps in transport proteins?
Generate ion electrochemical gradients
What is Na+/K+-ATPase?
Na+/K+-ATPase is a type of ATP-driven ion pump that
- actively transports Na+ & K+ against their gradients
- using the energy from ATP hydrolysis
How many Na+ ions are exported for every 2 K+ ions imported into the cell by Na+/K+-ATPase?
3 Na+ ions are exported for every 2 K+ ions imported into the cell
What type of pump is Na+/K+-ATPase?
Na+/K+-ATPase is an antiporter, which means ions move in opposite directions
What is an electrogenic pump?
An electrogenic pump is a type of pump that exports one net positive (+) charge.
- e.g. Na+/K+-ATPase
What is the role of electrochemical gradients in transport proteins?
Electrochemical gradients:
- aid in the function of ion & molecule transport
- production of energy intermediates
- osmotic regulation
neuronal signaling
muscle contraction
- bacterial swimming
What are exocytosis and endocytosis?
The bulk movement of material across membranes
- used to transport large molecules
- such as proteins & polysaccharides which are not free to diffuse across the membrane
What is exocytosis?
Exocytosis is the process of moving material out of the cell (think ‘exit’) using vescicles
What is endocytosis?
Endocytosis is the process of moving material into the cell (think ‘in’)
- invaginates (folds inwards) to form a vesicle
