NAME: ______H.R: ______DATE: ______

SCI & TECH 204

The Living World SECTION 3:Life Sustaining Processes


How Do Cells Work? (p. 280-283)

  • The three main needs essential to life are drinking, eating and breathing.
  • Thirst is a signal your cells send to your brain telling you they need water.
  • Cells use the air you breathe and the energyfrom the food you eat to fulfill their functions.

Inputs and Outputs

  • Inputs are substances that enter the cell and are indispensable to its activities.
  • The cell’s main inputs are water, nutrients and oxygen.
  • In the mitochondria of the cell, the nutrients release the energy they contain when they come into contact with oxygen.
  • Nutrients are also used for construction and repair.
  • The useless substances left after the cell has used nutrients are called wastes, which must leave the cell.
  • The main outputs of the cell are water, carbondioxide and other waste from cellular functions.

Exchanges Between the Cell and Its Environment

  • The cell (or plasma) membrane verifies materials entering or exiting the cell.
  • The membrane only allows certain substances to pass, so it has selectivepermeability.
  • This selective permeability of the cell membrane results from its structure.


  • To enter the cell, substances must move.
  • Diffusion is the movement of particles when they shift from a region where they are concentrated to a region where they are less concentrated.
  • Diffusion is how substances enter and exit the cell.


  • Water is the most abundant substance both inside and outside the cell.
  • If cells are deprived of water they will die.
  • The water in the cell contains dissolved particles (nutrients, carbon dioxide, wastes).
  • Because they are so small, water particles can enter and exit the cell easily.
  • Water moves from areas with lower amount of dissolved substances to areas with higher amounts.
  • In other words, water moves from one side of the cell membrane to the other to re-establish the equilibrium of (balance out) the concentration of dissolved particles. This process is called osmosis.

Draw an annotated diagram showing what happens to a stalk of celery when you leave it out on the counter and then when you put it in a glass of water.

In the space below, answer #4, 5a) & 5c), 6 & 7 from the Memory Check (p. 285)

4. a) diffusionb) osmosisc) osmosisd) diffusion

5. a) Initially, the concentration of particles is much higher on side A than on side B. The water moves through the selectively permeable membrane from side B (where the concentration of particles is higher) to side A (where the concentration of particles is lower) thus balancing out the concentration on either side of the membrane.

c) The water is lost through the leaves by osmosis (greater concentration of particles inside the leaves than in the air), causing the cells to eventually become dehydrated. Water will therefore move into the cells when the concentration of dissolved particles becomes high enough and this water must come through the roots (only source of water).