Bio 2.4 - Cell Processes

Active Transport

Passive Transport

SA:V Ratio

Links

• Active & Passive Transport Animations http://www.northland.cc.mn.us/biology/biology1111/animations/transport1.swf

• Transport Videos http://www.linkpublishing.com/video-transport.htm#Blood_-_Isotonic_Solution

•

Passive Transport

• Movement of a substance (eg oxygen) from an area of high concentration to an area of low concentration across a *selectively permeable membrane.

• Passive = no energy (ATP) used

*allow small polar molecules through including some water (water is polar) but not large molecules (eg protein, glucose). Mostly it is dissolved gases and lipid soluble molecules that pass through.

• Factors affecting rate– Conc gradient: diffusion rate higher when greater

difference in concentrations– Diffusion distance: faster over shorter distances– Surface area: larger the area for diffusion the faster

the diffusion rate– Temperature: molecules move faster at higher

temperatures (up to a point in living systems)– Size of molecule: larger molecules diffuse slower– State of matter: gases > liquids

• Types of passive transport– Simple (directly across membrane), – Osmosis (water only), – Facilitated diffusion:

• protein carriers / cannels assist the passage of certain molecules especially large molecules (glucose) and lipid insoluble molecules.

• this speeds up the rate of diffusion of these molecules. • water, oxygen and glucose enter cells through

facilitated diffusion

Net Diffusion

Equilibrium

Osmosis

• Diffusion of WATER – Across a selectively permeable membrane

• Water moves from [high water] [low water] – Or “water moves from lower to higher solute conc”– Think: “salt sucks”

Q What will happen?

Question

Which way will water travel across a semi-permeable membrane between 2 solutions of 5% and 20% dissolved starch?

AnswerWhich way will water travel across a semi-permeable membrane between 2 solutions of 5% and 20% dissolved starch?

98% water 95% water

Water (moves down gradient)

Water moves from an area of high concentration to an area of low concentration,

across the selectively permeable membrane

Net diffusion occurs until equilibrium is

reached (isotonicity), or water pressure pushes back or the

cell bursts…

Types of Solutions• Isotonic (iso – “same”)• Solute concentration equal both sides• No net movement of water (cell in equilibrium)• Plant cells flaccid (contain water but not swollen with it)

• Hypotonic (hypo = “less”)• Less solute outside of cell• Net movement of water into cell (high water to low water concs)• Plants: vacuole fills, cells turgid, cell wall prevents lysis (bursting)• Animals: cell swells, may burst

• Hypertonic (hyper = “more”)• More solutes outside of cell• Net movement of water out of cell(high water to low water concs)• Plants: vacuole empties, cells shrivel, plant wilts (cells

plasmolysed – cell membrane may detach from wall), may die• Animals: cell shrivels, may die

If in doubt remember: LOw SOlute = HypO

Cel

ls in

diff

eren

t sol

utio

ns

Cells in different solutions

Use your knowledge of osmosis to explain:1 Your mouth feels dry after eating chips

2 You can’t drink seawater

3 Retreating soldiers salt the fields of their enemies

4 Salt is able to preserve food

5 You gargle salt water for a sore throat

6 My dad used to put salt on thistles to kill them

7 Sea turtles have salty tears

8 You chop up tamarillos and sprinkle them with sugar, later there is juice all around the fruit

9 The bitter taste of eggplant (aubergine) can be removed by sprinkling the chopped up food with salt.

Active Transport• Movement of substances from an area of low concentration to

an area of high concentration, across a selectively permeable membrane.

Eg plant root cells absorb nitrate from soil (low conc) into cells (high conc).

• Requires energy (ATP)• Includes movement of molecules too large to move across the

membrane (eg glucose)• Types:

– Ion Pumps– Endocystosis

• Phagosytosis

• Pinocytosis

– Exocystosis

EndocytosisThe process by which large substances (or bulk amounts of smaller substances) enter the cell without travelling across the plasma membrane

An invagination of the membrane forms a flask-like depression which envelopes the material; the invagination is then sealed off forming a vesicle

There are two main types of endocytosis:

1.  PhagocytosisThe process by which solid substances (e.g. food particles, foreign pathogens) are ingested (usually to be transported to the lysosome for break down)

2.  PinocytosisThe process by which liquids / solutions (e.g. dissolved substances) are ingested by the cell (allows quick entry for large amounts of substance)

ExocytosisThe process by which large substances exit the cell without travelling across the plasma membraneVesicles (usually derived from the golgi) fuse with the plasma membrane expelling their contents into the extracellular environment 

Cytosis

Exocytosis

Compare Active and Passive Forms of Transport

Passive Transport

Active Transport• does not require energy

• across membrane or facilitated by proteins

• travels down concentration gradient

High concentration

Low concentration

Active transport

High concentration

Low concentration

Passive transport(diffusion)

• requires energy (ATP)

• only occurs through proteins (ion pumps), budding of membrane

• travels against concentration gradient

Compare Active and Passive Forms of Transport

Passive Transport

Active Transport

What types of molecules?

• Small ones Eg:

• Salts

• Dissolved gases (O2, CO2)

• Ions

• Water (osmosis)

• Glucose (fac. Diff)

•NOT starch, sucrose

What types of molecules?

• Large ones

•Where transport against the conc gradient is required:

•Excretion of Na+ ions from the gills of salt water fish

• Ones where the conc. is low in the environment but the cell needs it at a higher conc. Eg:

•Reabsorption of all glucose back into blood from kidney tubules

•Uptake of nitrates by root hairs

The Size of Cells

Cells tend to be small ie <100um (0.1mm)

Why?

By small I mean in volume (motor neurons in humans may be long - up to 1.3m long but are thin, so have a low volume)

Importance of Surface Area to Volume Ratio

Surface Area to Volume Ratio is a factor affecting cell size

•The rate of metabolism of a cell is a function of its mass / volume

•The rate of material diffused in and out of a cell is a function of its surface area

•As the cell grows, volume increases faster than surface area (leading to a decreased SA:Vol ratio)

•If the metabolic rate is greater than the rate of exchange of vital materials and wastes, the cell will eventually die 

•Hence the cell must consequently divide in order to restore a viable SA:Vol ratio and survive

•Cells and tissues specialised for gas or material exchange (e.g. alveoli, cells lining small intestine) will increase their surface area to optimise the transfer of materials by having structures such as a flattened shape, hairs, villi.

SA:V Ratio

As the cells below grow the volume increases at a faster rate than surface area so the SA:V ratio decreases and the rate of diffusion will be insufficient to keep the cell alive.

Still confused about cell size and scale?Go here: http://learn.genetics.utah.edu/content/cells/scale/

Something Interesting

• http://vce-biology.blogspot.co.nz/2012/08/why-are-cells-so-small-surface-area-to.html