2.5 Biological membranes Flashcards
Simple diffusion
Movement of molecules from high to low concentration
Passive so doesn’t require ATP
may/may not be across a membrane
Facilitated diffusion
Diffusion across a membrane through protein channels from high to low concentration
Passive so doesn’t require ATP
Helps ions pass through
Osmosis
Movement of water over a partially permeable membrane/from a dilute solution to a more concentrated one
From high to low concentration
Passive so doesn’t require ATP
Active transport
Movement of molecules from low to high concentration
Active so requires ATP
E.g. movement of glucose up a xylem vessel
Uses carrier proteins
Bulk transport
Mass movement of molecules in/out of a cell from low to high concentration
Active so requires ATP
Examples of endocytosis/exocytosis
Roles of the membrane
Acts as a partially permeable barrier between the cell and its environment, between organelles and cytoplasm, within organelles
Site of chemical reactions
Site of cell communication (cell signalling)
What is compartmentalisation?
Formation of separate membrane-bound areas in a cell- containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions
What are the parts of a plasma membrane?
Phospholipids, glycoproteins, glycolipids, carrier/channel proteins, cholesterol
Phospholipids info
Comprised of glycerol, 2 hydrophobic fatty acid tails and a hydrophilic phosphate group, in bilayer heads point outwards and tails point toward themselves
Cholesterol
In between phospholipids, helps with fluidity/flexibility, maintains fluidity (from being too stiff/fluid), has a philic/phobic ends that interact with corresponding ends on phospholipids to pull membrane together
Glycoproteins and Glycolipids
Branching carb attached to outside of a protein/phosphate head, acts as recognition sites- membrane-bound receptor molecules to detect presence of hormones/drugs. Cell signalling occurs- when chemical binds to receptor it creates a response from the cell or sets off a cascade of events)
Channel proteins
Provide a hydrophilic channel that allows diffusion of polar molecules from high to low concentration through the membrane
Carrier proteins
Play a role in active and passive transport- shape of protein changes when a specific molecule binds to fit molecule
Intrinsic proteins
Transmembrane proteins (embedded through both layers)- have amino acids with hydrophobic R-groups on their surfaces that interact with the membrane’s hydrophobic core to keep them in place. Examples are channel and carrier proteins
Extrinsic proteins
Present in one side of the bilayer- normally have hydrophilic R-groups on their outer surfaces and interact with polar phospholipid heads or with intrinsic proteins. Examples are glycoproteins
How does temperature affect diffusion rate?
The higher the temperature, the higher the rate of diffusion as molecules have increased KE so move at higher speeds- increases permeability if molecules have increased KE
How does concentration affect diffusion rate?
The greater the difference in concentration, the steeper the gradient between 2 regions, the faster the rate of diffusion as there are more molecules able to diffuse
How does diffusion distance affect diffusion rate?
The thicker the membrane across which molecules have to diffuse, the slower rate of diffusions as molecules have to diffuse over a larger distance
How does surface area affect diffusion rate?
The larger the area of an exchange surface, the more space their is for molecules to be taken in, the higher the diffusion
How does concentration gradient affect diffusion rate?
The steeper the gradient, the faster the diffusion of molecules from the side with more molecules to the side with fewer, down the concentration gradient
How do molecules in simple diffusion move?
From their own KE, which is affected by the surface area of a cell, concentration, temperature and PH
Diffusion stops when equilibrium is reached (no longer high to low as the high concentration is no longer high)
Fick’s law with simple diffusion is the rate of movement of a substance across an exchange surface. It’s worked out by:
Surface area x concentration difference all ÷ by diffusion path
How do molecules in facilitated diffusion move?
Large, polar molecules like glucose sometimes unable to pass through membrane as phospholipid tails are hydrophobic, so they use channel/carrier proteins to transport them- carrier proteins change shape for the molecule
Rate of diffusion affected by temperature, concentration gradient, surface area, diffusion distance and amount of proteins present
How do molecules in Bulk transport move?
Membrane is fluid so movement of large quantities of materials move into cells (endo) and out of cells (exo)
Active process so requires ATP/energy to form vesicles and move them
How do molecules in active transport move?
Molecule binds to receptors in the channel of the carrier protein and ATP binds to the protein where its hydrolysed into ADP and a phosphate, phosphate molecule binds and causes shape to change opening a pathway to the cell, the molecule goes through into the cell and the phosphate is released to recombine with ADP forming ATP again, then the protein returns to its original shape
Endocytosis involves:
(Plus its process)
Phagocytosis- movement of solids
Pinocytosis- movement of liquids
Membrane bends when in contact with transported material and enfolds the material until eventually the membrane fuses forming a vesicle, vesicle separates and moves into the cytoplasm to transfer the materials for further processing like how bacteria are moved towards lysosomes to be digested
Phagocytosis
Bulk transport of solids, takes place in specialised cells called phagocytes
White blood cells ingest/engulf bacteria as part of our body’s defence system
Amoebas do this as a food source
Pinocytosis
Bulk transport of liquids, takes place in human egg cell to take up nutrients for possible zygote
Exocytosis involves:
(Plus its process)
Reversal of endocytosis where materials are removed from the cell
Vesicle carrying large molecules moves to and fuses with the membrane, the fused site opens and releases contents of the secretory vesicle
Happens in phagocytes and plant cells during cell wall binding
How does temperature affect membrane structure and permeability?
Increased temperature gives KE to phospholipids so they move more and membrane becomes more fluid (losing its structure), this loss increases permeability making it easier for molecules to diffuse across, channel/carrier proteins become denatured and if temp continues to increase then the cells break down completely
How do solvents affect membrane structure and permeability?
Water (a polar solvent) essential in bilayer formation as tails orientated away from water to from bilayer with phobic core ad heads interact with water keeping bilayer intact.
Organic solvents are less polar than water e.g. alcohols(benzene is a non-polar example)- organic solvents dissolve membranes, disrupting cells (like why alcohol used in antiseptic wipes as the alcohol dissolves the bacteria membranes in a wound, killing them and reducing infection risk)
Membrane becomes more fluid/permeable when disrupted
Actual correct terms used for osmosis
Water molecules move from a region of high water potential to a region of low water potential (down a water potential gradient), across a partially permeable membrane and equilibrium eventually reached
What is water potental?
Measure of tendency (how likely) water is to move from one place to another
The higher the KE, the higher the water potential
Always from higher to lower
Unit and symbol for water potential
Measured in kilopascals (kPa)
Pure water has a water potential of 0 kPa (the highest possible water potential)
Symbol looks like a trident (Ψ)
What does more of a solute do to water potential?
This lowers the water potential, it therefore becomes negative- the more concentrated a solution is, the more negative water potential is
Solute potential
The amount by which solute molecules lower the water potential
Isotonic
Solutions with the same solute potential so no net movement of water
Hypotonic
Solutions with low concentrations of solutes so high Ψ, water moves from hypotonic solutions to other solutions
Hypertonic
Solutions with high concentration of solutes so low Ψ, water molecules will move from other solutions to hypertonic solutions
The closer to 0 kPa, the…
Higher the water potential
Hydrostatic pressure
When water diffuses into a closed system like a cell it increases the hydrostatic pressure (diffusion of water into a solution increases the solution’s volume)
Water potential and animal cells with cytolysis and crenation
If an animal cell is placed in a solution with a higher Ψ than that of its cytoplasm, water will move into the cell by osmosis, increasing its hydrostatic pressure- the membrane cannot stretch and withstand the pressure so will burst- this is called cytolysis
If solution has lower Ψ than the cell’s cytoplasm then it will lose water by osmosis and “pucker”/shrivel and shrink, called crenation
Define turgid and flaccid in terms of plant cells
Turgid- plant cell that’s fully inflated with water
Flaccid- plant cell that’s limp through pressure reduction inside cell
Haemolysis
Rupturing of red blood cell- if placed in concentrated solution, cell absorbs water and possibly bursts/ruptures
Plasmolysis
When plant cells are put in solution with a lower Ψ than theirs, water is lost from the cell by osmosis and this leads to a reduction in the cytoplasm’s volume- this pulls the membrane away from the cell wall- plasmolysis occurs
