Structure and Function of cell membrane 1.1 and 1.2 Flashcards
Why is chemical composition of ECF and ICF different?
Difference comes about due to the presence of the cell membrane (aka plasma membrane) which separates ECF and ICF.
What are the chemical differences between ECF and ICF?
ECF-
has higher composition of sodium and chloride ions.
More bicarbonate ions.
Protein only in plasma compartment.
ICF-
has higher concentration of potassium.
More phosphates and sulphates.
Protein more abundent in ICF.
What does cell membrane separate?
ECF and ICF
What is part of ECF ?
Plasma - interstitial
What is the distribution of fluids?
2 x ICF (28L) than ECF (14L)
What is distribution of fluid in ECF?
80% in interstitial (11L)
20% in plasma (3L)
What does capillary wall separate?
Separates plasma from interstitial
Where can water pass between compartments?
Water passes freely between all compartments; from plasma to ISF between the capillary cells of the blood vessel wall and from ISF to ICF via aquaporin channels.
How does cell membrane let water through?
The cell membrane is studded with aquaporin channels – channels that are permanently open to water. This allows water to move, unhindered, between the ECF and ICF.
There are no barriers to H2O movement
What about gases ability to move between compartments?
Gases also pass freely between all compartments
What about nutrients and waste’s ability to move between compartments?
Nutrients and waste also pass easily, but sometimes need help crossing the cell membrane (see later)
What about ion’s ability to move between compartments?
Ions pass freely across the capillary wall so exchange readily between plasma and ISF. They do not penetrate the cell membrane.
Ions distribution across capillary wall?
Composition of ions across plasma and interstitial fluid is the same
Difference in proteins between plasma and ISF?
Plasma has plasma proteins- different to ISF
How is ECF different to ICF in terms of ions?
Where sodium is high in ECF, low in ICF.
Where potassium low in ECF, high in ICF.
Why do we find lots of proteins in ICF?
Making up all the organelles
Why are these differences in the composition of fluid in the extracellular and intracellular compartments fundamental for life?
many physiological and biochemical processes rely on the concentration gradients between different compartments
Describe the way in which cell membrane is selective barrier?
It is freely permeable to some substances e.g. O2 and CO2, but the difference in composition between ECF and ICF shows that permeability is selective and not universal.
Can permeability of cell membrane vary?
Permeability can vary, may increase or decrease at different times, fundamentally important for various cell functions e.g. transmission of the nervous impulse.
Describe what a membrane being dynamic means?
Membranes are dynamic, continually being formed and maintained or dismantled and metabolized according to the needs of the cell.
Describe the physical characteristics of membranes?
Membranes are very thin double layers of sheets of lipids (the lipid bi-layer)
Describe physical characteristics of major membrane lipids?
Major membrane lipids are phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail
Major membrane lipid arrangement in aqueous environment?
In an aqueous environment (i.e. ECF ad ICF!) arrange themselves so that the polar (hydrophilic) part is on the outside of the membrane and the fatty acid chains are on the inside, away from H2O.
Hydrophilic = lipophobic (lipid repellent);
Hydrophobic = lipophilic (lipid loving)
Why are membranes very flexible?
because of fatty acids in vivo, behave like an oil, therefore can stretch, although may rupture if over-stretched.
Why are cell membranes good insulators?
Membranes are excellent insulators against movement of electrical charge. This prevents the passage of electrons – important in maintaining electrical stability of cell
What are the types of proteins involved in cell membrane?
Integral proteins - both sides of membrane
Peripheral proteins-one side of membrane
Where are carbohydrate groups in membrane?
Carbohydrate replacing phospholipid head, directly attached to lipid component (glycolipid)
Carbohydrate attaching to protein (glycoprotein)
Where are carbohydrate groups found?
always orientated on extracellular. Important in cell to cell communication. Flag to tell immune system the identity of the cell
What are the common functions of the membrane proteins?
Receptors
Transport (transporter and channel proteins)
Enzymes
Maintenance of cell structure (anchorage)
Communication
Characteristics of integral proteins?
integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal e.g. neurotransmitter or hormone, to the intracellular space to create a cellular response.
How is extracellular signal communicated to intracellular side of cell?
Ligand (extracellular chemical signal) binds to receptor protein.
Causes a conformational change in protein and this is transmitted to intracellular side.
Changes events in cell and cell behaviour.
Would all tissues respond to extracellular signal?
No. Only tissues that express receptors for a particular hormone or neurotransmitter will respond to chemical.
Characteristics of transport proteins?
Transporters – integral proteins which allow movement of ions or molecules across the membrane
What are the two forms of transport proteins?
Channels and Carrier mediated transport proteins (transporters)
What do channel proteins do?
– create a pore through the membrane through which molecules, usually water and ions, flow.
Types of channel proteins?
Can be open (water) or gated (ions).
Characteristics of open channels?
Create a water filled pore (Aquaporin)
Characteristics of gated channels?
Pores in membrane but most of time are closed and only open to respond to certain stimuli.
Characteristics of carrier proteins?
do not create a continuous pore from ECF to ICF. Open to ECF, then ICF, but never at the same time. Typically move larger molecules than channels e.g. glucose.
Where are glucose transporters present?
Glucose transporters permanently present in cell membranes are insulin independent e.g. nerve cells. Insulin only required for glucose uptake in muscle and adipose tissue.
What are important stores of glucose in body?
Muscle and adipose tissue are main storage sites.
Membrane proteins as enzymes, what is their role?
membrane enzymes catalyse chemical reactions on the cell membrane. Enzymatic part may be on external side of membrane
Give an example of membrane proteins found on external side in liver?
those found in small intestine which break down nutrients into smaller units, or internal side such as those associated with converting signals carried from receptors into an intracellular response e.g.:
Describe the process of membrane protein enzymes?
-when ligand binds to receptor, signal is transmitted to intracellular side. Change in activity of intracellular enzyme region
What do structural proteins of cell membranes do?
anchor the cell membrane to the intracellular skeleton, to the extracellular matrix (collagen) and/or to other cells
What happens if there is dysfunction or loss of structural proteins?
Dysfunction or loss can cause serious debility e.g. lack of dystrophin protein in Duchenne’s Muscular Dystrophy.
Membrane proteins role in communication?
this can be between cells or within cells.
What do receptor/enzyme complexes and G proteins do?
involved with communicating extracellular messages carried by hormones (or drugs) to the intracellular environment i.e. they convert extracellular chemical signals into intracellular responses
What do G proteins specifically do?
The latter are peripheral proteins that relay a signal along the membrane from an integral receptor protein to an integral enzyme protein.
What is the protein content of the myelin membrane?
a specialised type of membrane that serves as an insulator around myelinated nerve fibres has a low content of protein (18%), major component is lipid, very good insulator so ideal for function
Protein content of plasma membrane?
Plasma membranes of most other cells have much greater activity and protein content is typically 50%.
Energy transduction membrane protein content?
Membranes involved in energy transduction such as the inner membranes of mitochondria, have highest protein content, 75%.
