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minimum requirements for a cell
plasma membrane
maintains a selective barrier between inside and outside of the cell
controls what can come into the cell and what can leave
cytoplasm
water soluble layer where al the chemical reactions that make up the cell happen
DNA
hereditary material
encodes the instructions for how the cell how to make itself
ribosomes
translate the instructions from the DNA to the proteins
limits/constraints on cells
Lower limit: the cell needs enough volume to carry the DNA that encodes its components, the basic machinery to express those genes
Upper limit: enough surface area : volume ration to satisfy diffusive entry of O2, nutrients and removal of waste
surface area and volume
plasma membrane: the only access point for nutrients to get into the cell
the total area constraining the cell controls how much nutrients can get into the cell
what can pass through a plasma membrane?
difficult for many particles to get through the cell membrane
molecules that are very small and uncharged can pass through easily
e.g. O2 and CO2
water is small, but because it is highly polar it moves slower through the plasma membrane
plasma membrane
cell membranes are a mosaic of lipids and proteins
come in all shapes and sizes
two main types of proteins
periphery proteins
integral membrane proteins
phospholipids present (the fluid part)
fluid phospholipid bilayer
diverse phospholipids comprise the bilayer
types of phospholipids
glycolipids assists with signal transduction within the plasma membrane
sterols assist in regulating the fluid (membrane) e.g. cholesterol
high temps: sterols reduce the fluidity of the membrane
low temps: sterols reduce the rigidity of the membrane
plasma membrane; periphery proteins
not permanently embedded into the membrane
they don’t have a hydrophobic region
tend to be bound or associated with an integral membrane protein or some other feature of the plasma membrane (such as the glycolipid)
plasma membrane; integral membrane proteins
permanently bound into the membrane
a region which is hydrophobic (fully embedded into the membrane)
some are fixed and will stay in the same position (if they are attached to a part of the cytoskeleton or some other part outside the cell)
some can move freely throughout the membrane
diffusion
movement of solutes from regions with higher concentration into regions with lower concentration
net diffusion ceases when an equilibrium concentration is reached throughout
diffusion example
Sugar on both sides of the tube
MEMBRANE IS PERMEABLE TO WATER, BUT IMPERMEABLE TO SUGAR (selectively permeable membrane)
The water will move to the area where the water is at a lower concentration (side with more sugars)
Water will move to the right in order to reach equilibrium
This will happen until the concentration of the solute is the same on both sides of the membrane (sugar)
transport proteins
allow large or charged molecules to be transport across membranes
solute will move down an electrochemical gradient
the transport of the solute into membrane will be assisted by a transport protein
PASSIVE TRANSPORT
uniports (one direction of movement of molecules)
transport proteins; channel-mediated transport
open hydrophilic space where the molecule can be transported through the membrane
transport proteins; carrier-mediated transport
a specific part of the protein that recognises a specific solute
then binds it
and then facilitates its transport across the membrane
simple diffusion through the plasma membrane
can occur depending on the size and charge of the molecule
slower, rarer and the cell has less control over this process compared to the use of a channel or carrier protein
active transport through the plasma membrane
when the molecule goes up the electrochemical gradient
energy needs to be used to get the molecule across the membrane
often ATP
occurs with the use of electronic pumps
generate concentration and electrochemical gradient across membranes
e.g. Na+ or K+ ATPase protein pump
small, positively charged ions can be moved through this way
ACTIVE TRANPORT PUMPS
ATP synthase operates like an ion channel in reverse
ATP synthase is the smallest molecular rotary known in nature
membrane proteins
symports: two molecules transported in the same direction
antiports: two molecules transported in opposite directions
COTRANSPORT
able to recognise two different molecules
anti-port example
Sodium is toxic for the cell in high concentrations
Potassium is the major cellular cation for eukaryotes and prokaryotes
ATP powers the sodium transport out of the cell and changes the shape of the protein, then it is ready for potassium to enter the cell though binding (constant shape change)
eukaryote vs prokaryote
Eukaryote cells are much bigger than prokaryote cells (10-100x bigger)
Genome: all the genes of an organism
Prokaryote genomes: come in one copy (haploid) , and are generally circular plasmids inside the cell; do not have a special compartment for a nucleus
Eukaryote genomes: tend to be linear chromosomes; have two copies of the genome (diploid); compartmentalisation of the genome in the nucleus (as seen in other parts of the cell)
Eukaryotes tend to have compartments for the different roles
Prokaryote cells tend to be carried out all within the cytoplasm