Transport Across Membranes Flashcards
The Cell-Surface Membrane
The entry and exit of molecules in cells is controlled by the cell-surface membrane, which surrounds the cytoplasm of a cell.
The membrane is described as semi-permeable.
How is the cell surface membrane described?
Semi-permeable.
Function of the cell surface membrane
Controls what enters or leaves the cell
The fluid-mosaic model
Phospholipid molecules form a BILAYER that are constantly moving around, giving the membrane a fluid structure.
The different protein molecules are unevenly distributed throughout the membrane, forming a mosaic.
The semi-permeability of the cell-surface membrane is related to the type and distribution of specific proteins and phospholipid molecules present in the membrane.
Why is the cell surface membrane fluid?
Phospholipid molecules form a BILAYER that are constantly moving around
Why is the cell surface membrane a mosaic
The different protein molecules are unevenly distributed throughout the membrane, forming a mosaic.
What is the semi-permeability of the cell-surface membrane is related to?
The type and distribution of specific proteins and phospholipid molecules present in the membrane.
Phospholipids in the fluid mosaic model
The hydrophobic fatty acid tails of phospholipid molecules are attracted towards each other
The hydrophilic polar heads are facing towards water, either in the cytoplasm, or outside of the cell, forming the phospholipid bilayer.
Most abundant molecule found in ALL membranes.
The phospholipid bilayer allows small lipid-soluble (non-polar) molecules to pass through by simple diffusion but prevents the passage of small polar/charged molecules (like ions) and larger molecule (e.g, Glucose)
What does the phospholipid bilayer allow to pass through?
Lipid soluble molecules
Non polar small molecules
What does the phospholipid bilayer prevent from passing through?
Large molecules
Charged molecules like ions
What is the most abundant molecule found in ALL membranes?
Phospholipids
What type of diffusion happens at the phospholipid bilayer?
Simple diffusion
Cholesterol in the fluid mosaic model
Decreases permeability of the membrane
Increases the stability of the membrane.
MORE cholesterol = LESS fluidity of the membrane
What does more cholesterol do to the membrane?
Makes it less fluid and more stable
Channel proteins structure
Proteins have specific tertiary structures, so they are specific and can only transport molecules that are complementary to the shape of the channel protein
Channel proteins allow which type of molecules to pass through?
Specific charged ions
Small molecules to move across the membrane by facilitated diffusion
What type of diffusion happens with channel proteins?
Facilitated diffusion
Example of channel protein
Na+ ions can only pass through Sodium ion channel proteins embedded within and through the membrane.
Carrier proteins allow which type of molecules to pass through?
Ions
Large molecules
Receptor proteins
Other protein molecules act as specific receptors for complementary molecules.
Receptor protein example
Hormones, such as insulin, may bind to the insulin receptor protein, which allows a cell to respond by increasing the cells permeability to glucose.
Receptor protein structure
Have specific tertiary structures.
Only specific molecules can bind to specific receptor proteins.
It’s worth noting that specific cells have specific receptors.
Enzymes
Enzymes are also found embedded in the cell membrane
The shape of the enzymes active site is specific and complementary to its substrate, allowing them to bind and form enzyme-substrate complexes.
Why are glycoproteins important?
Glycoproteins are important in cell recognition, often acting as antigens.
The immune cells detect the specific shapes of glycoproteins to identify the cells as SELF or if they are NON-SELF Glycoproteins are produced in the golgi apparatus within the cell that displays them.
All cells have glycoproteins on their cell-surface membrane.
