lecture 3; overview of cell structure

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