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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?

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!

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 = 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

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

Homework

- P. 288
- questions: p 288 #2-6

- Read Sec C2.4 (p. 289-293)
- Questions: p. 289 #1-2
- p. 293 #1-5

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