Cell Membranes, Transport Of Small And Large Molecules Lectures 2.1-2.3 Flashcards
What are cell membranes?
-selectively permeable barriers
-maintain constant internal environment and enclose cell contents
-very thin
Phospholipid molecule:
-has a dynamic structure so components are free to move around
*amphipillic: both hydrophilic and hydrophobic parts
*polar head: negatively charged phosphate group
*non polar tail: no charge
*phospholipid: “self-assembles” to create the bilayer. They migrate but rarely flip
What are the two different types of tails?
Saturated and Unsaturated
Saturated: single bonds only
Unsaturated: at least one double bond
Double bond creates a kink in the fatty acid chain of the tail
What do higher concentrations of saturated fatty acids cause?
Reduces cell membrane fluidity
What do higher concentrations of unsaturated acids cause?
They promote membrane fluidity
What does concentrations of unsaturated phospholipids affect?
It affects membrane fluidity
What are the two types of carbohydrates?
*glycolipids
*glycoproteins
Three major roles of carbohydrates
-maintain membrane stability and cell protection and aids in cell adhesion and recognition.
Tell me about cholesterol:
-is a lipid (approx 20% of the membrane lipid is cholesterol)
-has polar and non polar regions
-wedges itself between the phospholipid tails
-is able to migrate and flip between the membrane layers
Proteins:
-proteins are able to serve different functions such as:
Enzymes, carrier proteins, channel proteins, receptors, cell adhesion and recognition.
Two main types of proteins?
Integral and peripheral
Integral proteins:
-firmly inserted into the membrane
-span the bilayer of the membrane
-transmembrane protein is hydrophobic
-extracellular and cytosolic portions are hydrophilic
-carrier proteins and channels are integral proteins
Peripheral proteins:
-loosely attached to the membrane
-can be removed from the membrane with minimal disruption
What is membrane fluidity affected by?
-extreme temperatures; either hot or cold
-concentration of unsaturated fatty acids
-cholesterol levels
Membrane fluidity: what does a low temperature cause?
Less kinetic energy, phospholipids pack together and membranes have less fluid
Membrane fluidity: what does a high temperature cause?
More kinetic energy, phospholipids move further away and membranes have more fluid
Membrane fluidity: what does having cholesterol cause?
It maintains fluidity by preventing lipids from getting too close together or too far apart from each other.
Red blood cells?
Equal amounts of lipid, protein and a small amount of carbohydrate
Nerve cells?
Higher amounts of lipid in the cell membrane (80%)
Passive transport?
-moves down a concentration gradient
Three types: simple diffusion, facilitated diffusion and osmosis
-uses inherent kinetic energy
Active transport?
-moves against a concentration gradient
-has directionality
-requires a specific carrier protein
Two types: primary and secondary
-needs external energy source
Simple diffusion?
-small, uncharged non polar molecules
Properties:
-no metabolic energy is required
-uses kinetic energy
-the rate of diffusion is proportional to the concentration gradient
Facilitated diffusion?
-small polar molecules; H2O, glucose etc
-uses specific proteins
-the rate of diffusion is proportional to the concentration gradient
Simple diffusion and facilitated diffusion kinetics:
Simple diffusion= linear relationship between concentration and rate of diffusion
Facilitated diffusion= the rate of diffusion reaches a plateau since the proteins become fully occupied
What is osmosis?
-net movement of water down a concentration gradient
-water is highly polar however some small amounts can ‘sneak’ through the membrane
-water moves freely and reversibly via a specific protein channel; aquaporins
-bulk movement of water is facilitated diffusion
Primary active transport:
-movement of Na+ and K+ uses the Na+/K+/ATPase pump
-maintains ion concentration differences inside the cell
-hydrolysis of ATP results in= phosphorylation of the pump —> results in conformational change of pump
-causes transport of 3Na+ out and 2K+ into the cell
Secondary active transport (co-transport):
movement of two substances at the same time;
*A substance against its concentration gradient—> active transport
*Ion movement down its concentration gradient—> usually facilitated diffusion
-energy used for secondary active transport comes from the electrochemical gradient across the membrane.
What are the two types of secondary active transports?
Symport;
*transported substances move in the same direction
*Na+/Glucose transporter
-Antiport;
*Transported substances move in the opposite direction
-Na+/Ca2+ exchanger- uses the Na electrochemical gradient to transport Ca2+ out of the cell.
What is endocytosis?
Moves large molecules into the cell
Three types: phagocytosis, pinocytosis and receptor- mediated
What is exocytosis? And what is it used for?
Moves large molecules out of the cell
*Used for: hormone secretion, neurotransmitter release and mucous secretion
What are vesicles? And what are they made up of?
-vesicular transportation of large molecules
*bubble-like membranous sacs
*made up of phospholipid bilayer containing fluid
*transport bubble
*protects substances being transported
Endocytosis- phagocytosis
ingestion of large particles by specialised cells (phagocytes -> immune system)
-macrophages
-certain white blood cells
*used for removing bacteria and debris
*when bacteria binds to a phagocyte the cell membrane wraps itself around the particle= results in a phagosome containing the particle
Endocytosis- pinocytosis
-occurs in most cells
*process used to take in extracellular fluid with dissolved substances
*droplet enters the cell and fuses with an endosome (sorting vesicle)
Endocytosis- receptor mediated
*main mechanism for specific uptake of macromolecules
*very selective method
*uptakes substances (hormones, cholesterol) found in small amounts
*receptors for this are specific membrane proteins
Exocytosis
*substances are being moved out the cell—> hormones and waste products
*process is stimulated by a cell-surface signal that results in these steps:
• substance is enclosed in a vesicle
• Transported to plasma membrane
• Vesicle attaches to the ‘docking’ sites on the membrane
• Fuses with membrane and ejects substances
