7.3 Cell Boundaries

Fluid compartments in our bodies are separated by membranes

Composition of body fluids Greater number of osmotically active particles

Some organisms have a CELL WALL Plants (cellulose) Algae (polysaccharide) Fungi (-chitin) Prokaryotes(peptidoglycan)

Why have a cell wall? 1.Mechanical support 2.Protection 3.Cell-cell communication 4.Maintenance of structure (turgor) 5.Prevent water loss

Membranes

Where are membranes located? Plasma membrane (double membrane) Membrane-bound organelles (single or double) Secret Universe Introduction to cell membrane 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 barrier 2.Cell-Cell signalling 3.Transport of nutrients, products and waste products 4.Localisation of function within organelles 5.Semi-permeable: controls entry and exit of substances 6.Self-sealing! 7.Flexible, mobile fluid mosaic Cell membrane function

Let’s meet the components of the cell membrane Phospholipid bilayer Cholesterol Carbohydrates (glycoproteins) Proteins

Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

Membrane Proteins Proteins determine a membrane ’ s specific functions Cell membrane & organelle membranes each have unique collections of proteins Channels, pumps, receptors

NH 2 H+H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic)  -helices in the cell membrane H+H+ Porin monomer  -pleated sheets Bacterial outer membrane 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 MECHANISMS These don’t require energy Simple diffusion Facilitated diffusion Osmosis ACTIVE MECHANISMS These require energy (usually ATP) to transport substances (often against their concentration gradient) Protein pumps Endocytosis (cotransport)

Simple Diffusion 2nd Law of Thermodynamics governs biological systems: the universe tends towards disorder (entropy) Diffusion : movement of small, soluble particles from high  low concentration

Diffusion Movement is from HIGH to LOW concentration “ passive transport ” no energy needed diffusionosmosis movement of water

Let’s watch some an animation of diffusion Diffusion animation 2

Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids inside cell outside cell lipid salt aa H2OH2O sugar NH 3 What molecules can NOT get through directly?  polar molecules  H 2 O  ions  salts, ammonia  large molecules  starches, proteins

Facilitated Diffusion Diffusion 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 diffusion Membrane becomes semi-permeable with protein channels : specific channels allow specific material across cell membrane inside cell outside cell sugaraa H2OH2O salt NH 3

Which substances are transported by facilitated diffusion? Glucose Urea Amino acids Animation

Osmosis Movement of water across the cell 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

Concentration of water 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 hypotonichypertonic water net movement of water

freshwaterbalancedsaltwater

Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which 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 Structure, function and dynamics of aquaporins Peter Agre John Hopkins Roderick MacKinnon Rockefeller

Active Transport

Cells 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 summary simple diffusion facilitated diffusion active transport 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