Plasma Membrane: Composition & Functions Flashcards
What are the main functions of the cell membrane?
- boundary that separates the living cell from its surroundings
-Essential for all cells
-A barrier to prevent the contents of the cell from escaping and mixing with molecules in the surrounding environment (e.g. nutrients must pass inward, waste products must move out)
-exhibits selective permeability, allowing some substances to cross it more easily that others
- protection/support to the cell, regulating its shape
In Eukaryotic cells, inner membranes allow what?
-the compartmentalisation of individual organelles (specific composition for specific reactions)
Diagram of Bacterial and Eukaryotic cell membrane?
Cell membranes are composed of what?
different kinds of lipids and proteins (different functions) and share a common general structure
What is a lipid bilayer?
-lipids are arranged in two closely apposed sheets, forming a lipid bilayer, in which proteins are embedded and some carbohydrates
What are Phospholipids?
-(phosphoglycerides) are the most abundant lipids in the plasma membrane arranged in two-layer sheets (phosphatidycholine is the most abundant)
-Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions
What does amphipathic mean?
What does hydrophobic mean?
-Their hydrophobic (’water repellent’) tails are arranged inwards, to shield water and form a sealed comportment
What does hydrophilic mean?
-The Phosphate group or other hydrophilic (’waterloving’) heads are arranged to face outwards, exposed to the aqueous solutions
Hydrophillic and Hydrophobic diagram?
What is the Fluid Mosaic?
- dynamic and complex structure of the plasma membrane
What are the components of the mosaic ?
-phospholipids, Glycolipids, sterols (cholesterol in mammalian cells) and various proteins and glycoproteins are contained.
Fluidity of a lipid bilayer depends on what?
-its composition
What can a lipid molecule do?
-2-dimensionally move along the plane of the bilayer
What is a Glycoprotein?
-A protein with a carbohydrate attached
What is a Glycolipid?
-A lipid with a carbohydrate attached
Phospholipids movements can lead to what?
-Rapid lateral diffusion of lipid molecules within the plane of each monolayer
- Spin in place (rotation -up to 500 revolutions per second)
- Flexion: contraction movement
- Flip-flop from a side to the other (very rarely)
-Proteins (usually do not move and they are anchored to the cytoskeleton)
- Only some proteins can slightly and slowly move driven by the motor proteins
Diagram of Phospholipids movements?
Other lipids of the plasma membrane ?
-Cholesterol (only in animal cell membranes), and glycolipids are also amphipathic
- Cholesterol (20% of the membrane lipids) modulates the membrane fluidity and improving its stability
- Stiffen the membrane (less flexible and less permeable). Reducing molecules passage
Other lipids of the plasma membrane diagram?
What are Glycolipids?
-lipids containing sugars representing their hydrophilic head. Sugar groups always facing the cell exterior (The bilayer is asymmetrical)
What are membrane Proteins?
–synthesised in the rough ER, (secretory pathway)
- Most are trans-membrane / integral membrane proteins, crossing the membrane
-They are amphipathic
-Hydrophobic regions (non-polar amino acids), often coiled into alpha helices, lie in the interior of the bilayer
-Hydrophilic regions of amino acids are exposed to the external aqueous environment.
- Other proteins are embedded in the membrane, exclusively on the cytosolic
- Or peripheral membrane proteins, on either side of the membrane and covalently attached to membrane lipids or other integral membrane proteins.
Extracellular Space diagram?
What are the functions of membrane proteins?
- Transport of molecules
-Protect the cell surface from mechanical damage and involved in cell-to-cell communication
- Enzymatic activity
- Signal transduction/cell communication
- Structural support/Attachment to the extracellular matrix (ECM)
Most membrane proteins are what?
-Glycoproteins
Glycoproteins/ membrane proteins are what?
-short chains of sugars linked to amino acids (facing extracellular space)
What is Protein glycosylation?
-post-translational modification in Rough ER and Golgi apparatus
Cell/plasma membranes are what?
-selectively permeable, regulating the traffic of molecules and ions in/out of cells. Also valid for the membrane of organelles
What are the two main types of cell/plasma membranes?
-Passive transport
- Active transport
What is PASSIVE transport?
-Movement of molecules across membranes without the expenditure of energy
- Movement DOWN the concentration gradient
- Movement from areas with high concentrations to a low concentration of the molecule, with the aim to reach an equilibrium
- Works in both directions
What are the 3 main types of PASSIVE transport?
- Diffusion
- Osmosis
- Facilitated diffusion
Passive Transport - What is diffusion?
-FREE movement of molecules across the lipid bilayer such as O2 and CO2 and small uncharged polar molecules, such as urea and ethanol
- No energy ATP cost
- Down their gradient concentrations until an equilibrium is reached
-Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.
-Many of the organic molecules that a cell uses as nutrients are too large and polar to pass efficiently through diffusion
Diagram of diffusion and what can pass through the lipid bilayer ?
Passive transport- What is Osmosis?
-Movement of of water/solvents across a semipermeable membrane from a solution with higher water potential (lower solute concentration) to a solution with lower water potential (higher solute concentration)
- Water potential of a solution is inversely proportional to the solutes concentration
- When the solutes cannot go across the membrane, osmosis occurs
Diagram of Osmosis?
What is Osmotic pressure?
-Hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different water potentials
-The rate of osmotic movement of water across the plasma membrane of cells is limited. Water can flow into/out of cells also by aquaporins (facilitated diffusion)
What are the 3 Osmotic solutions?
-Isotonic Solution
-Hypertonic Solution
-Hypotonic Solution
What is an Isotonic Solution ?
-a solution with the same concentration of solutes both inside and outside the cell
What is a Hypertonic Solution?
-a solution that has a higher solute concentration outside than inside the cell
What is a Hypotonic Solution?
- a solution that has a higher solute concentration inside than outside the cell
What does Tonicity: mean?
-The ability of a surrounding solution to cause a cell to gain or lose water
Changes in osmotic conditions cause cells to ..?
-swell or shrink rapidly
-The movement of water across the plasma membrane determines the volume of an individual cell, which must be regulated to avoid damage to the cell.
Hypertonic Solution causes red blood cells to do what?
-Shrivel, cell loses water
Hypotonic Solution causes red blood cells to…?
-Swell/ lysed, gain water
Isotonic solution causes red blood cells to..?
-Act as normal due to no net water movement
Passive Transport- What is Facilitated Diffusion?
- The passive movement of molecules down a concentration gradient (or electrical potential) via a transport protein (transmembrane proteins)
-Protein channel (ions or water)
-Protein carrier/transporters (of specific small molecules or ions)
- Highly specific (glucose vs. fructose)
- Reversible
- No energy ATP cost
- Specific ions (e.g Cl-)
- Hydrophilic small molecules (e.g glucose)
- For facilitated diffusion of water (aquaporins)
Facilitated diffusion- What are protein channels?
–Channels (pore membrane forming) form hydrophilic ‘tube’ across the membrane through which specific molecules/ions diffuse (down a concentration gradient)
- Do not interact with the molecules, although specific
- Move at a very rapid rate
- Passive transport (no energy cost)
Facilitated Diffusion- What are Aquaporins?
-for facilitated diffusion of water to specifically increase the permeability of cellular membranes to water
Facilitated diffusion- What are protein carriers/ transporters?
-Three transporter types ( uniporters, symporters and anti-porters)
- Uniporter is the only transporter based in passive transport
- Shift/transport a single type of molecule (ions or small molecules) across the membrane (not as a channel)
- It involves the interaction with the specific molecule (shape complementary) that induces a conformational change of the protein to mediate the transport of the molecule
- Work at much slower rate than channels
What are the 3 protein transporter types?
-uniporters
- symporters
-anti-porters)
What are Symporters and anti-porters?
-Symporters and anti-porters (based on co-transport of more than 1 molecule) are active transport (against the concentration/electron gradient)
What is a Uniporter?
-is the only transporter based in passive transport
What is Active Transport ?
-Moves substances against their concentration or electrochemical gradients
- Requires energy, usually in the form of ATP
-Performed by specific proteins embedded in the membranes
- Allows for stockpiling
What are the Common features of transport proteins and enzymes ? -Active Transport
-Specific binding sites for solute
- Can be saturated
- Can be inhibited by molecules that resemble the normal ‘substrate’
- Catalyse a physical process
Transporters that shift more than 1 molecule type at the same time are…?
-are active transport based on binding with the molecule (shape complementarity)
How many molecules do Symporters transport?
-Transport 2 different molecules across the cell membrane (same direction)
How many molecules do Anti-porters transport?
-moving 2 distinct molecules across the membrane (opposite directions) E.g Na+ - K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in.
What is Vesicle Transport?
-Transport of large macromolecules
-Bulk transport requires vesicle transport (endomembranous system).
What are the two main types of Vesicle Transport?
according to the direction:
1) Endocytosis
2) Exocytosis
What is Endocytosis?
- Uptake (from outside to inside) of macromolecules and large particles. Invagination of the cell surface to form an intracellular membrane-bounded vesicle containing extracellular-fluid.
What is Exocytosis?
- Secretion or excretion (from inside to outside) e.g. insulin (protein hormone) by pancreatic cells. Transport vesicles (from Golgi) migrate to the plasma membrane, fuse with it, and release their contents,
Endocytosis and Exocytosis Diagram?
What are the 3 main types of endocytosis ?
1) Phagocytosis (’cellular eating’)
2) Pinocytosis (’cellular drinking’)
3) Receptor-mediated endocytosis
the cell takes macromolecules from outside by forming vesicles, what is this process called?
-Endocytosis
What does the The lipid bilayer provide?
-The basic structure and barrier function of all cell membranes
Membrane lipid molecules are what?
-amphipathic, having both hydrophobic and hydrophilic regions. This property promotes their spontaneous assembly into bilayers when placed in water, forming closed compartments.
The lipid bilayer is a ?
-fluid mosaic, composed of many different lipids, embedded proteins, glycolipids and glycoproteins. Lipid molecules are able to move along the bilayer
-Membrane proteins are responsible for what?
-most of the functions of cell membranes and they are in general trans-membrane proteins or associated to other proteins or lipids.
