7.3 Cell Boundaries
Fluid compartments in our bodies are separated by membranes
Composition of body fluids
Greater number of osmotically active particles
Distribution of Total body fluids
Some organisms have a CELL WALL
• Plants (cellulose)• Algae (polysaccharide)• Fungi (-chitin)• Prokaryotes(peptidoglycan)
Why have a cell wall?1. Mechanical support2. Protection3. Cell-cell communication4. Maintenance of structure (turgor)5. Prevent water loss
Membranes
2007-2008
Where are membranes located?
• Plasma membrane (double membrane)
• Membrane-bound organelles (single or double)
• Secret Universe• Introduction to cell
membrane
Why do we need plasma membranes made
Our cells inhabit an aqueous environment• …but they must let ‘stuff’ (nutrients, ions,
molecules,waste products) in and out• …..and they need to communicate with each
other…How is this achieved?....
Functions of the Cell membrane
1. Protective barrier2. Cell-Cell signalling3. Transport of nutrients, products and waste products4. Localisation of function within organelles5. Semi-permeable: controls entry and exit of
substances6. Self-sealing!7. Flexible, mobile fluid mosaicCell membrane function
Let’s meet the components of the cell membrane
• Phospholipid bilayer• Cholesterol• Carbohydrates
(glycoproteins)• Proteins
Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
Membrane Proteins
• Proteins determine a membrane’s specific functions• Cell membrane &
organelle membranes each have unique collections of proteins
• Channels, pumps, receptors
NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)a-helices in thecell membrane H+
Porin monomerb-pleated sheets
Bacterialoutermembrane
proton pump channel in photosynthetic bacteria
water channel in bacteria
function through conformational change (shape change)
Examples
Membrane glycoproteins (carbohydrates)
‘Chemical identification cards’Play a key role in cell-cell recognition• ability of a cell to distinguish
one cell from another– Antigens
• basis for rejection of foreign cells by immune system
Let’s Review…
Let's build a membrane from scratch…And now…let’s make a membrane!
Movement across the Cell Membrane
How do things get into and out of our cells?
PASSIVE MECHANISMSThese don’t require energy
Simple diffusionFacilitated diffusion
Osmosis
ACTIVE MECHANISMSThese require energy (usually ATP) to transport substances (often against their concentration gradient)
Protein pumpsEndocytosis
(cotransport)
Simple Diffusion2nd Law of Thermodynamics governs biological systems: the universe tends towards disorder (entropy)
Diffusion: movement of small, soluble particles from high low concentration
DiffusionMovement is from HIGH to LOW concentration• “passive transport”• no energy needed
diffusion osmosis
movement of water
Let’s watch some an animation of diffusion
Diffusion animation 2
Diffusion through phospholipid bilayerWhat molecules can get through directly?• fats & other lipids
inside cell
outside cell
lipid
salt
aa H2Osugar
NH3
What molecules can NOT get through directly? polar molecules
H2O ions
salts, ammonia large molecules
starches, proteins
Facilitated DiffusionDiffusion through protein channels– channels move specific molecules across
cell membrane– no energy needed
open channel = fast transport
facilitated = with help
high
low
Channels for facilitated diffusionMembrane becomes semi-permeable with protein channels : specific channels allow specific material across cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
Which substances are transported by facilitated diffusion?
• Glucose• Urea• Amino acids• Animation
OsmosisMovement of water across
the cell membrane
2007-2008
Osmosis is facilitated diffusion of water
Facilitated diffusion of water from
high concentration of water to low
concentration of water across a semi-permeable
membrane
Concentration of waterDirection of osmosis is determined by comparing total solute concentrations on either side of the membrane:• Hypertonic - more solute, less water• Hypotonic - less solute, more water• Isotonic - equal solute, equal water
hypotonic hypertonic
water
net movement of water
freshwater balanced saltwater
Cell (compared to beaker) hypertonic or hypotonicBeaker (compared to cell) hypertonic or hypotonicWhich way does the water flow? in or out of cell
.05 M .03 M
Osmosis…
Water passes through special water pores - Aquaporins
Aquaporins
• Structure, function and dynamics of aquaporins
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
Active Transport
Active TransportCells may need to move molecules against concentration gradient• shape change transports solute from
one side of membrane to other • protein “pump”• “costs” energy = ATP
“The Doorman”
conformational change
ATP
low
high
Transport summarysimplediffusion
facilitateddiffusion
activetransport
ATP
Endocytosis
• A simple one...• McGRaw Hill phagocytosis
Exocytosis• Protein produced by the ribosomes enter the lumen of the ER• Protein exits the ER and enters the cis or trans side of the Golgi
aparatus• Protein is modified through Golgi, and vesiculated• Vesicle moves to and fuses with the plasma membrane• Exocytosis