Cell Transport: moving things in and out of the cell

Material that surrounds all cells and certain organelles within the cell

- location of plasma membrane gives it more specialized name like cell membrane, nuclear membrane etc.

Plasma membrane:

Cell Transport

Phospholipid – primary molecule in the cell membrane consisting of two parts

Remember the Triglycerides?What’s the difference between them and a phospholipid?

Phosphate Group (POLAR)

Fatty Acid Tails (NON POLAR)

Phospholipids and Polarity• Fatty acid tails are

non-polar. – Hydrophobic

(water-haters)• Phosphate heads

are polar– Hydrophilic

(water-lovers)

Phospholipids Bi-layer

Plasma Membrane Structure• 1. Phospholipid Bi-layer• 2. Proteins

– Transmembrane– Integral– Peripheral

• 3. Carbohydrate Chains• 4. Cholesterol

Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in cross section

Membrane Model• Membrane as a whole is often described

as a fluid mosaic – two-dimensional fluid of freely diffusing lipids,

dotted or embedded with proteins

– Think of the lipid bi-layer as a body of water with various proteins and associated carbohydrate chains moving around.

- Proteins can change location by moving through the phospolipids.

Figure 8.5 Evidence for the drifting of membrane proteins

Figure 8.9 Some functions of membrane proteins

Movement of substances through the membrane

May be • PASSIVE– requires no energy

expenditure by cell.– Diffusion and Osmosis– Facilitated diffusion

• ACTIVE– does require energy expenditure– Why?– Ex. Transport Proteins, Bulk Transport

Diffusion- Molecules will move from areas of high

concentration to areas of lesser concentration (concentration gradient)

- Due to random collisions of molecules– If occurs across a membrane, the membrane

must be permeable to the substance.• Size is a major factor• Electrical charges• Polar vs non-polar

Factors that affect permeability:

Diffusion (cont.)• Movement of particles (solute) from

high to low concentration will continue until there is an even distribution of particles.

• This is called equilibrium.• Particles move across the membrane

randomly at equilibrium. (In other words, movement still happens, just no NET MOVEMENT!)

Special Type of Diffusion - Osmosis• Diffusion of water across a selectively permeable

membrane• Water moves from a high concentration of water

(less salt or sugar dissolved in it) to a low concentration of water (more salt or sugar dissolved in it)

Figure 8.11 Osmosis

Tonicity• When comparing the concentrations of

2 solutions:• Isotonic – same concentration of solute• Hypertonic – higher concentration of

solute• Hypotonic – lower concentration of

solute

Osmosis and Cells (cont.)

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Osmotic Pressure Effects on Cells• Inside of cells are usually hypertonic to fresh water

(hypotonic)• Animals cells tend to be surrounded by isotonic

solutions (blood, saliva, etc)• Plant cells have tough cellulose cell walls that

protect them from over-expanding

Facilitated Diffusion• Charged particles and some molecules

can move passively through protein channels. Glucose is a great example of a molecule that enters cells via facilitated diffusion

• Important to note that this is still happening through the process of diffusion using the concentration gradient to move molecules in/out of cell.– Energy Not Needed!

Facilitated Diffusion• Those molecules which cannot move freely

through the membrane have to diffuse through special integral membrane proteins

ACTIVE TRANSPORT Small molecules and ions can be “pumped” in/out of cell using energy (ATP). We will look at a couple really important examples of these when we look at cell respiration and photosynthesis. Also key in neuron function!

Active Transport• Two main categories

– Small Stuff – Transport Proteins• Molecules can move in/out of cell using transport

proteins. – Big Stuff – Vesicle Transport

• Large materials (ie. chunks or food, complex molecules, liquid and even cells) that cannot pass through a protein channel need to be brought in through complex movement of membrane vesicles

– Exocytosis

– Endocytosis

The three types of endocytosis in animal cells

SUMMARY OF WAYS TO MOVE STUFF IN AND OUT:

Passive Transport:

Requires no energy

- simple diffusion

- facilitated diffusion (via proteins)

Active Transport:

Requires energy in the form of ATP (energy molecule)

- transport proteins

- endo/exocytosis