Movement in and out of the cell membrane Fluid compartments in our bodies are separated by...
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Transcript of Movement in and out of the cell membrane Fluid compartments in our bodies are separated by...
Movement in and out of the cell membrane
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 monomer
b-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)
Endocytosis/ exocytosisProtein pumps(cotransport)
Active Mechanisms: Endocytosis and Exocytosis
• A simple one...• Animation• Animation 2
Endocytosis/ exocytosis are
import/export of materials by
infolding/outfolding of 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)
Endocytosis/ exocytosisProtein pumps(cotransport)
Simple diffusion…in the real world
• Gas exchange in the lungs• Gas exchange in plants
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
Factors affecting diffusion
1. Temperature2. Surface area for diffusion
3. Distance for diffusion4. (size of particle)
5. (charged or uncharged)
Diffusion in the real world
Diffusion animation 2• Gas exchange at the alveoli — oxygen from air to blood,
carbon dioxide from blood to air. • Gas exchange for photosynthesis — carbon dioxide from air
to leaf, oxygen from leaf to air. • Gas exchange for respiration — oxygen from blood to tissue
cells, carbon dioxide in opposite direction. • Transfer of neurotransmitter substance at a nerve synapse. • Osmosis — diffusion of water through a semipermeable
membrane.
Facilitated DiffusionFacilitated diffusion is diffusion of specific molecules through protein channels in the cell membrane– no energy is required
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
Factors affecting the rate of facilitated diffusion are the same as for simple diffusion
1. Temperature2. Surface area for diffusion
3. Distance for diffusion
Osmosis
A special case: facilitated diffusion of water molecules
across a semi-permeable membrane
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
Examples of Osmosis in Biology
• Absorption of water by plant roots. • Re-absorption of water in the kidney. • Re-absorption of tissue fluid into blood
capillaries. • Absorption of water in the GI tract
Water passes through special water pores - Aquaporins
Aquaporins
• Structure, function and dynamics of aquaporins
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
Water moves across a membrane from
the hypotonic solution to the
hypertonic solution
• Animation of osmosis 1• Animation of osmosis 2:
why water balance matters
Comparing ‘water concentration’ of different solutions
Direction 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
Water potential?• It can be confusing to talk about the concentration of
water molecules, since ‘concentration’ is usually reserved for a solute (e.g. glucose,
• Instead we use the term water potential• High water potential = lots of water (high water
concentration/ low solute concentration• LOW water potential = little water (low water
concentration/high solute concentration)
Water moves from a region of high water potential to a region of low water potential
Low water potential (relatively few water molecules compared to solute molecules)
High water potential (relatively many water molecules compared to solute molecules)
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)
Active transport 1: Protein pumps
• Active transport uses energy (ATP) to transport substances AGAINST a concentration gradient into/out of the cell
• The energy is used to change the shape of the ‘protein pump’ and thus import/export specific molecule
• Animation• protein pumps in plants
ATP
low
high
Active transport in the real world
• Re-absorption of glucose, amino acids and salts in the kidney.
• Sodium/potassium pump in cell membranes (especially nerve cells)
• Uptake of nutrients (e.g. nitrate ions)in plant root hair cells
Transport summarysimplediffusion
facilitateddiffusion
activetransport
ATP