Week 3

• If volume is too large, surface area surrounding the cell REDUCES

  • Low surface area = things can't move things efficiently (move in or out)/cannot survive

  • MUST have a high surface area

• Volume increasing faster than surface area can cause problems in exchanging materials with the environment

• e.g. small sponge = absorbs quickly and large sponge = slower absorption --> like cells: small = fast exchange and large = slow exchange

• When cells are too big:

Disadvantages (Bigger cells/smaller SA:V ratio):

• diffusion becomes too SLOW to supply the center of the cell

• Waste build up

• Cells may overheat (low surface area to volume makes heat removal harder)

• Aim is that cells either stat small or divide when they grow too large

Advantages (Smaller cells/bigger SA:V ratio):

• Diffusion pathways are shorter --> MORE EFFICIENT!! (molecules do not have to travel so far to get in/out of the cell, so it takes less time and (if it is active transport) energy

• Concentration gradients are easier to generate --> diffusion becomes more efficient!!

Disadvantages (Smaller cells/bigger SA:V ratio):

• Small, warm-blooded mammals lose heat very quickly due to their large SA: Vol --> Need to eat constantly (similarly to how humans get hungry on a cold day)

• Desert plants lost water quickly with flat leaves (so they minimise their SA:Vol ratio in order to conserve water) --> some plants change their metabolism to save water

• How SA: V can be maximised

  • Increase folds --> efficient gas exchange/loads of oxygen going in + CO2 out of cell into blood

  • Examples:

    • Alveoli in lungs --> thing membranes that maximise the surface for gas exchange

    • Intestines fold to maximise the ratio, making absorption of food molecules more efficient

    • Roots are long and branched, with root hairs on the cells to maximise the surface area for water uptake

Practice questions:

1. Define surface area to volume ratio

Ratio between the surface area of a single cell and its volume.

1. Why is a high SA: V ratio important for cells

It makes diffusion more efficient since the diffusion pathways would be shorter, and concentration gradients would also be easier to generate

1. Explain why cells divide instead of growing very large

Because if it gets too large, it can decrease the efficiency of cell transport (SLOW DOWN the transport process), cause waste build up, and may cause the cell to overheat

1. Describe one way cells or organisms can increase their SA:V ratio without getting smaller

Developing protrusions or folds to increase the surface area, and thus allow more efficient diffusion/cell transport.