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 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)

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 bilayer

What 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 hypotonic

Beaker (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 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 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