Membrane Transport Flashcards
Compare between intercellular and intracellular
Exchange within the cell itself, i.e. the intracellular environment
Exchange between the surrounding intercellular medium
Types of macrotransfer
Exocytosis
Movement out of the cell
Endocytosis
Movement into the cell
Exocytosis is
Process of releasing large biomolecules through the plasma membrane
Define Ca2+-dependent Exocytosis (regulated)
Requires the presence of Ca2+ for the vesicle to be released
Important process in:
Neuronal communication
Converts electrical information into chemical information (at the synapse).
Hormone release
Insulin from β-cells (as shown before – secretory/zymogen granules)
Explain vesicle trafficking by using the terms tethering, docking, and fusion
Tethering: Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse
Tethering [by tethering proteins] involves links over distances of more than about half the diameter of a vesicle from a given membrane surface (>25 nm)
Docking: The holding of two membranes within a bilayer’s distance of one another (<5-10 nm).
Tethering and docking of a transport vesicle at the target membrane precedes the formation of a tight core SNARE complex
Fusion: Driven by SNARE proteins
Process of merging the vesicle membrane with the target one
Results in release of large biomolecules into the extracellular space (or in case of neurons in the synaptic cleft).
A process whereby cells absorb material (molecules such as proteins) from the outside by engulfing it with their cell membrane.
Endocytosis
Endocytosis is for cells that have an
Uptake of polar or large molecules
Cannot pass through the hydrophobic plasma membrane.
Phagocytosis is
The plasma membrane extends to surround the material to be engulfed.
Plasma membrane receptors may bind to antibodies on the surface of the foreign body.
An intracellular vacuole or heterophagosome is then formed.
The ingested material is on the inside of a plasma membrane-bound vesicle.
What is the vacuole in phagocytosis fused with?
The vacuole now moves into the cell and fuses with a primary lysosome.
Digestion proceeds in the newly formed secondary lysosome.
This process is most commonly seen in phagocytic cells like monocytes, neutrophils and macrophages.
Pinocytosis involves…
Involves materials of molecular or macromolecular dimensions in the surrounding medium.
Most cells exhibit pinocytotic activity though the extent varies.
Receptor mediated pinocytosis is when molecules bind to…
receptors on the plasma membrane and are selectively adsorbed.
Clathrins are…
Clathrin coated pits may be formed where receptors concentrate
Clathrin attaches to the intracellular portion of the receptor with the aid of adaptor proteins as endocytosis occurs
What does it mean when we say that exocytosis must balance endocytosis?
A large macrophage can engulf/endocytose 25% of its own volume of fluid each hour by the process of pinocytosis.
This removes 3% of its plasma membrane each minute (100% of its plasma membrane per hour).
The total surface area and volume of the cell remains unchanged.
This means that exocytosis must balance endocytosis with respect to the amount of membrane involved.
____________ is the transfer of small molecules and ions across cell membranes, both internal and external.
Microtransfer
True or False: Passive transport doesn’t require energy and examples include primary and secondary transports
False, although passive transport does not require energy, examples of passive transport are simple and facilitated diffusion.
What is active transport?
Requires energy
Primary and secondary active transport
Describe Brownian motion
First observed indirectly by the Scottish botanist Robert Brown.
Under the light microscope, pollen grains in water are seen to be moving here and there (random movement) in water. So called Brownian Motion applies to particles 2m and does not require energy consumption
It is due to collisions with the water molecules which we can’t see.
Describe diffusion
The tendency for all molecules in liquids and gases to move in all directions until they are evenly distributed in the available space.
- Dependent on movement of molecules, due to their kinetic energy, from regions of high concentration to low concentration
- Occurs whenever such a concentration gradient exists
- Movement ceases when the equilibrium constant reaches 0.
Simple diffusion is through the…
lipid bilayer
Facilitated diffusion is through…
a channel protein or carrier protein.
In simple diffusion, we can see…
This requires the membrane to be freely permeable to the molecules and ions at the time in question
Water and small non-polar molecules (O2, CO2) pass freely through the plasma membrane by diffusion.
Molecules diffuse across the membrane by passive transport
____________ enter some animal cells by a process of facilitated (or mediated) diffusion.
Glucose and other molecules (e.g. amino acids, nucleotides)
Two types of protein facilitate the diffusion, and each one uses a different mechanisms:
Channel proteins (e.g. K+ ion channels)
Carrier proteins (e.g. sugars, amino acids and nucleotides) are specific permeases which are highly selective often transporting only one type of molecule. The mechanism involves a conformational changes to transfer the bound solute
No energy is required for transport; essentially this is a passive transport mechanism
True or False: Macromolecules and charged ions (H+, Na+, K+, Cl-) DO NOT pass freely through cell membranes.
True, these molecules require the use of selective pores or channels (so called ‘membrane transport proteins’) to cross/transfer from one side of the membrane to the other.
