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 SURFACE AREA, VOLUME & CELL SIZE 

Surface Area to Volume Ratio

When Cells Get too large they have to divide

  • Cells do this because of the change in their surface area to volume ratio

  • A cell is a metabolic compartment where reactions occur

  • The larger the size (volume) of a cell the larger the number of reactions that can occur within it. 

  • Raw materials needed for metabolism enter through the cell membrane. Hence the more surface area the larger amount of raw materials that can enter (diffuse) at one time. 

  • However, each unit of volume requires a certain amount of surface area to supply enough raw materials for the reactions that are occurring. 

  • As a cell grows in size the surface area to volume ratio decreases and therefore, not enough raw materials can enter the cell and sustain the cells reactions and need to grow, this prevents the cell from getting bigger and causes it to divide.

increasing the Surface area to volume ratio

Cells diffuse nutrients (raw materials). However, diffusion is not rapid or very effective at distributing this nutrience and raw materials over a long cellular distance. As such when cells get too big they are further forced to divide or perish as they can no longer be supplied enough nutrients fast enough for the reactions that they need to sustain within them. 

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When this occurs cells can either divide, slow down metabolism and stop growing, fold, or change their shape into one that is longer and thinner to increase their surface area and allow more nutrients to enter the cell at a time. 

Movement of particles across the cell membrane

  • Plasma membrane is selectively permeable 

  • It controls the passage of particles in and out of the cell 

  • The size of the particle, its concentration gradient (low to high concentration in and out of the cell), its lipid solubility (polar or non-polar) affects how it moves through the membrane. 

    • Polar molecules and ions are lipid insoluble and CANNOT enter the membrane freely​ without help from a carrier protein

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METHODS OF TRANSPORT:

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1. PASSIVE TRANSPORT:

  • Way of transport across the plasma membrane that REQUIRES NO ENERGY (ATP)

  • Includes diffusion, osmosis, facilitated transport

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2. ACTIVE TRANSPORT: 

  • Way of transport that REQUIRES ENERGY (ATP) 

  • Includes carrier proteins "pumps", exocytosis, endocytosis (phagocytosis & pinocytosis) 

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