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?

Sec C2.4Unit C

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

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

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…

Surface area of one side= s2

Volume of cube = s3

Total surface area? A = 6s2

Surface area to volume ratio A = 6s2 = 6V 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 cmb) 2.4 cmc) 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

Practice

Calculate the surface area to volume ratio for a rectangular prism with: Length l= 3.0 cm Width w= 2.5 cm Height h= 1.5 cm Formula= 2lw+2lh+2wh

lwh Answer: 2.8

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?

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

Is Bigger Better?

Read through the lab on page 290-291. You need to come up with an hypothesis as

well as make a data table (copy from textbook).