B2.1 Membranes and Membrane transport Flashcards
What makes cell membranes essential components of all cells?
Controls entry and exit of substances.
Allows sensitivity and communication.
Separate cytoplasm from external environment.
Separate interior of organelles from each other from cytosol.
Internal membranes in cells that can divide some organelles even further.
Structure of a phospholipid
Almost same as triglyceride, but the third fatty acid is replaced by a phosphate that joins to the 3rd OH by a condensation reaction.
Phosphate head is hydrophilic, fatty acid tails are hydrophilic.
Structures formed by phospholipids when exposed to water
A micelle: circular structure where hydrophilic head faces out, hydrophobic tail faces in.
A bilayer: chain, where hydrophobic tails touch in side and hydrophilic head faces out.
Which phospholipid structure is suited to be a membrane
Bilayer: cells usually exist in aqueous environment, and inside of cell is also aqueous. Bilayer allows outer surface of hydrophilic heads to interact with water.
How does the lipid bilayer act as a barrier?
They separate internal components of cell or organelle from external environment.
Regulates movement of substances in and out:
Non-polar hydrophobic tails in cell membrane act as barrier.
The smaller, non-polar molecules diffuse fastest.
Charged particles cannot diffuse across a membrane at all.
What determines the permeability of the membrane?
The size of molecule (smaller = more likely to pass)
Hydrophobic nature (hydrophobic = more likely to cross than charged molecules)
What are membrane proteins?
Proteins in the membrane:
Can be classed as:
integral proteins
OR
peripheral proteins.
What are integral membrane proteins?
Transmembrane (cross the entire membrane) proteins embedded through both layers of membranes.
Amphipathic molecules (hydrophobic amino acids in core of membrane, hydrophilic amino acids on surface and interior.)
Usually:
Channel proteins: Allow charged particles to enter via diffusion.
OR
Carrier proteins: Involved in both active and passive transport by changing shape.
What are peripheral membrane proteins?
Hydrophilic nature.
Found on surface of membrane.
Interact with hydrophilic regions of integral proteins and with hydrophilic heads of phospholipids.
Function of channel proteins
Allows slightly polar molecules (E.g. water) and some slightly larger molecules to diffuse through.
Define simple diffusion
Passive net movement of particles from an area of high concentration to low concentration on fluids.
Happens straight across membranes: small non-polar molecules.
Define Facilitated diffusion
Passive net movement of particles from an area of high concentration to low concentration using membrane proteins (Channel or carrier proteins)
Charged or polar molecules cross membrane through the integral proteins’ hydrophilic pore, where they bind to the specific protein, which changes the shape, which opens a pathway allowing the molecules to pass through.
Define Active Transport
Movement of particles from an area of low concentration to high concentration using ATP.
In membrane, pump proteins.
Pump proteins allow selective permeability: only transport specific ions or molecules in one direction.
How do Pump proteins transport ions or molecules in membranes
Pump protein in stable conformation is open to one side of membrane.
Has high affinity to particle and binds to it.
ATP hydrolyzed to ADP, releasing energy: changes shape of protein to unstable form: opens it to other side.
Pump releases particle against concentration gradient. Pump goes back to original form.
How does water pass through membranes?
Through aquaporins: channel protein selectively for water.
Aquaporins has a tetrameric structure composed of four monomeric subunits. Each subunit has a water channel.
Very narrow pore; positively charged; only allows water through.
What are 6 functions of membrane bound proteins (with examples)
Transport: Membrane proteins facilitate the movement of substances across the cell membrane. Example: Glucose transporter proteins (GLUTs) move glucose into cells.
Receptor: Membrane proteins act as receptors for signaling molecules, initiating cellular responses. Example: Insulin receptors bind insulin, triggering glucose uptake.
Enzymes: Some membrane proteins function as enzymes, catalyzing biochemical reactions. Example: ATP synthase produces ATP during cellular respiration.
Cell Adhesion: Membrane proteins mediate cell-to-cell interactions and adhesion. Example: Cadherins connect cells in tissues.
Cell Recognition: Membrane proteins act as markers for cell identification and immune responses. Example: MHC proteins present antigens to T cells.
Signal Transduction: Membrane proteins transmit signals from the outside to the inside of the cell. Example: G-protein coupled receptors (GPCRs) relay signals through second messengers.
Explain channel protein specificity for ions
Ion channels are highly selective:
binding sites of hydrophilic amino acid chains lining the channels are highly ion-specific.
Size of pore acts as a size filter.
Explain the two types of Active transport
Direct: energy released by breakdown of ATP directly used to transport molecules across membrane. These transport proteins are called ATPase pumps.
Indirect: movement of one solute down its concentration gradient drives movement of second solute against its own concentration gradient.
Example of active transport
ABC transporters are carrier proteins that pump antibiotics out of the cell.
Types of gates on channel proteins
Ligand-gated
Voltage-gated
Mechanically gated
Structure of glycoproteins
Covalent bonding of oligosaccharides (short carbohydrate chains) to the protein molecules make glycoproteins.
Carbohydrate groups of glycoproteins stick out into extracellular environment.
Structure of glycolipids
Covalent bonding of carbohydrates to lipids results in glycolipids.
Vital part of cell membrane. Amphipathic.
Role of glycolipids and glycoproteins in Cell recognition and Cell adhesion
Cell recognition: They act as markers on cell surface, helps cells of body recognize each other. Also help cells of immune system to recognize foreign cells.
Cell adhesion: Help cells to attach and bind to each other to form tissues.
Compare and contrast the structure, melting point and relative fluidity of saturated and unsaturated fatty acids in lipid bilayers.
Saturated fatty acid: higher melting point - provides stability to membrane.
Unsaturated fatty acid: lower melting point - ensures fluidity of membrane.
Fatty acid composition of phospholipids
Two long fatty acid chains: hydrophobic.
Vary in length and number of double bonds (saturation)
An example of adaptation in membrane composition in relation to habitat
Cold-blooded organisms (e.g. frogs) adapt to lower temperatures by increase the proportion of unsaturated fatty acids in their membrane to regulate fluidity.
Structure of cholesterol
Amphipathic steroid:
Hydrophobic region: four steroid rings and a hydrocarbon side chain.
Hydrophilic region: polar hydroxyl group.
Describe the structural placement of cholesterol within the cell membrane
Polar hydroxyl group interacts with phospholipid head, stiffens that region of bilayer which reduces lateral movement.
Hydrophilic region is more fluid, and interacts with phospholipid tails, preventing the packing of phospholipid tails and increasing fluidity within membrane.
How does temperature affect fluidity?
At low temp, cholesterol prevents fatty acid chains of phospholipids from fitting closely together, preventing freezing.
At high temp, cholesterol stabilizes membrane, reducing fluidity.
How are extremely large substance moved across a cell membrane?
Bulk transport used, requiring membrane to be fluid and move.
Two types of bulk transport:
Endocytosis: into cell
Exocytosis: out of cell.
These are active processes that require energy (ATP).
Process of endocytosis
Cell surface membrane invaginates and eventually engulfs the particles to be taken in. Membrane pinches off to form a vesicle with the ingested particles.
Cell surface membrane’s fluidity allows for this to occur. (e.g. engulfing of pathogens by phagocytosis.) Area of cell membrane decreases
Process of Exocytosis
Reverse of endocytosis. Vesicles (usually formed by golgi apparatus) move towards and fuse with cell surface membrane. Contents of the cell are then released outside of the cell. (e.g. release of hormones) Area of cell membrane increases
Role of Cell-Adhesion molecules (CAMs)
They are glycoproteins that mediate the binding of cells with other adjacent cells or with the extracellular matrix.
What are cell junctions
Connect cells to each other allowing intracellular transport and communication. CAMs essential for forming these junctions.
Different types of cell junctions and their roles
Adhesive junctions: present in epithelial cells and cardiac cells. Facilitate cell-cell adhesion in tissues to ensure structural stability and allow cells to withstand mechanical stress.
Tight junctions: present in epithelial cells. Form tight seal between cells and act as occluding junctions. This barrier prevents the unregulated movement of molecules across the barrier.
Gap junction: found in several cell types. Intracellular channels physically connecting cells for movement of molecules.
