Review What are two main types of transport? What is the difference between simple diffusion and facilitated diffusion? What is a hypertonic, hypotonic and isotonic solution? Why do we use endocytosis/exocytosis? What special structures do we use in endo/exocytosis?

Is a Bigger Cell Better?

Objectives calculate surface to volume ratios relate these ratios to size, efficiency of diffusion, and structures in humans and plants

Introduction Cells are microscopic and carry out all life processes What structure in the cell is important for transport of materials? Cell membrane Transport of materials must be kept at a maximum

Cell Size

The Biggest Cells Most cells are in the order of a few micrometers in diameter, and are visible only under the microscope What are the largest cells in the human body? Oocyte (egg cell) – is 1000 micrometers (1 mm) in diameter and is visible with the naked eye Neural Cells – although only a few micrometers across, can be 1 metre long!! The pseudounipolar cell (in the spine), is only 135 micrometers across, but can be the height of a person in length!

Why don’t huge cells exist?

If cells were larger … What happens to transport if the cell were larger and its volume increases? More molecules needed to be transported Distance to travel to the cell’s surface also increases Must have a greater surface area to match need to transport Need to look at surface area to volume ratio

Cells Have Large Surface Area-to-Volume Ratio

Calculating Surface Area to Volume Ratio Need to find both total surface area and volume Ex. Determine the surface area to volume ratio for cubes with following side lengths: a) 1.0 cm b) 2.5 cm c) 4.0 cm How do we find total surface area of a cube? Volume?

Example cont… A = 6s2 = 6 V s3 s Surface area of one side= s2 Volume of cube = s3 Total surface area? A = 6s2 Surface area to volume ratio A = 6s2 = 6 V s3 s Note: We can only use this expression for a cube where 6 sides are equal

Example Using the expression we derived for a cube, find the surface area to volume ratio for a cube with sides: a) 1.0 cm b) 2.5 cm c) 4.0 cm Answers: a) 6.0 cm b) 2.4 cm c) 1.5 cm

What does this mean? Larger surface area to volume ratio means more efficient cell transport Ie. higher surface area and smaller volume

The Size and Shape of Organisms Surface area determines opportunity for transport i.e. little surface area, transport very limited In cells, bigger is not necessarily better Cells are specialized though in terms of function This determines their size and shape If a cell needed to transport a lot of material (ex. A liver cell), what might their size be in relation to a cell which doesn’t need to transport very much?

Cell Shape Cells’ surface areas (SA) are increased in ingenious ways so there is lots of cell membrane surface to increase sites for diffusion and activity.

Maximizing Potential Need to maximize surface area to volume ratio Look at the two pictures; which plant has an easier time of transporting materials? Why? What might the larger plant do to increase its surface area?

Internal Transport Systems Systems developed to reduce dependence on diffusion and surface area Animals: circulatory, digestive and respiratory systems Plants: xylem and phloem

Specialized Structures These structures increase the overall surface area to volume ratio Ex. Alveoli in lungs  small sacs to increase surface area for gas exchange Ex. Small intestine  villi and microvilli (projections) for absorption of nutrients