4.2 - Cell Size

when the cell size is doubled the surface area...

increases by a factor of 4

when the cell size is doubled the volume...

increases by a factor of 8

what does the cell do about this?

1. stay small single cells BUT some cells, however, must be large, or at least, very long so... 2. add extensions ex: microvilli in intestines, long thin neurons [A neuron running down a giraffe's neck may be several meters long, but it's not shaped like a cube - instead, it's very long and thin. This gives the neuron a large surface area with little volume] 3. divide ex: form tissues, specialization notes: Pseudopodia - extension of plasma with plasma membrane surrounding it (fake feet) Compartmentalization = divides up cell I think

if cell is too large...

1. there won't be enough proteins to carry out all of the cell's functions because there's only a limited amount of DNA so it can only make so many proteins at a time. 2. it won't be able to transport nutrients, waste products, oxygen & other gases in and out of the cell, and if it cannot do this, it could starve or poison itself (a.k.a. rate of diffusion)

if cell is too small...

1. they can't hold all the organelles and molecules they need to maintain homeostasis (if a cell does not have enough mitochondria, it cannot make enough energy to live) ( 1.5 ) there won't be enough space to house organelles, store nutrients & wastes 2. DNA won't be able to make enough proteins to direct the cell's activities (DNA limits size because it must be able to make enough proteins to direct cell activities)

surface area

= (Length) X (Width) X (Number of Sides) [or 6 X S2 ] cm (squared-2)

*read

A further increase in the size of a cell could result in a surface area too small for adequate exchange of materials. Google: A large surface to volume ratio means that a small amount of living matter has a large surface through which nutrients, oxygen and wastes can diffuse. so surface area matters because there is so much space to get through (volume) but also there is not that much space that it can pass through on the surface? PPT: Large cells have less surface area per unit volume and are less capable of transporting materials across plasma membrane in a sufficient amount of time.

do you think molecules will more efficiently pass in and out of a cell with a large surface to volume ratio or a small surface area to volume ratio?

a large surface area to volume ratio. the larger ratio is more efficient b/c there's more surface area than volume (distance in amount of time it needs it to take)

rate of diffusion

is always the same rate, the only thing that changes is the distance it has to diffuse through, so it is never faster or slower, just inefficient over larger distances (the diffusion rate for large cells is the same as the rate for small cells, larger cells just need a longer time to receive nutrients/export waste products) (As a cell increases its size, its volume increases faster than its surface area)

what happens to the surface area to volume ratio as a cell increases in size?

it decreases [larger cells=smaller ratio]

what happened to the cubes as you poured the NaOH into the beaker? why did this change take place?

it turned pink phenolphthalein (was in the augur) is an indicator that when mixed with NaOH turns pink (the part that the NaOh went through turned pink) sodium hydroxide reacts with phenolphthalein that turns the augur pink

*read (from book)

needs to have a surface area large enough to service the volume of a cell (active cells have a large amount traffic across their outer surface) very thin, elongated shapes provide a large surface area relative to a cell's volume

do surface area and volume of a cell increase at the same rate?

no, the surface area increases by a factor of 4 when the cell size is doubled, and the volume increases by a factor of 8 when the cell size is doubled

what did you notice when you measured the width of the pink area of each cube? what do you think this is so?

the pink area was 0.5cm (width) in all the agars/cells shows that 100% of the small cell was reached which showed that it was most like to survive and it was the most accurately representation of a real cell shows that diffusion occurs at the same rate regardless of cell size

as a cell increases in size...

the volume increases faster than the surface area the ratio of SA:V decreases

why is cell size limited?

to maintain homeostasis

can you speed it up? (rate of diffusion)

yes. 1. heat it up 2. use a transport protein

volume

= (Length) X (Width) X (Height) [or S3 (cubed)] cm (cubed-3)

for cellular respiration to occur, oxygen must diffuse into a cell and carbon dioxide must diffuse out. which cube accurately represents an efficiently respiring cell? use the data that you have calculated in this activity to support your choice.

the 1cm cube (a.k.a. the smallest cube) a large cell with a small surface area to volume ratio will require more oxygen and nutrients than the cell membrane can take in. the large cell will also produce more O2 and other wastes than the cell membrane can excrete. the models in this activity showed that 100% of the small cell was reached by diffusion. small cells with large surface area to volume ratios can take in the required amounts of oxygen and still release wastes, as required for survival.

what is indicated by the width of the pink area of each cube?

the DISTANCE that the NaOH diffused into agar/cell