Explain how substances get across membranes and into the cell. Solution The plasma membrane surrounding animal cells is where the exchange of substances inside and outside of cells takes place. Some substances need to move from the extracellular fluid outside cells to the inside of the cell, and some substances need to move from the inside of the cell to the extracellular fluid. the plasma membrane is choosy about what substances can pass through it, it is said to be selectively permeable. Selectively permeable or semipermeable means that only certain substances are able to pass through the membrane. Transporting substances across the plasma membrane can require that the cell use some of its energy. If energy is used, the transport is called active. If molecules can pass through the plasma membrane without using energy, the molecules are using passive transport. ACTIVE TRANSPORT Sometimes, the molecules are just too big to easily flow across the plasma membranes or dissolve in the water so that they can be filtered through the membrane. In these cases, the cells must put out a little energy to help get molecules in or out of the cell. Osmosis osmosis, the concentration of substances in the water is taken into consideration. If a solution is isotonic, that means the concentrations of the substances (solutes) and water (solvent) on both sides of the membrane are equal. If one solution is hypotonic, there is a lower concentration of substances (and more water) in it when compared to another solution. If a solution is hypertonic, there is a higher concentration of substances in it (and less water) when compared to another solution. For example, the blood in your body contains a certain amount of salt. The normal concentration is isotonic. If suddenly there is too high a concentration of salt, the blood becomes hypertonic (too many salt molecules). This excess of salt forces water out of the blood cells in an attempt to even things out. But the effect this action has is actually that of shrinking the blood cells.

1. Explain how substances get across membranes and into the cell.
Solution
The plasma membrane surrounding animal cells is where the exchange of substances inside and
outside of cells takes place. Some substances need to move from the extracellular fluid outside
cells to the inside of the cell, and some substances need to move from the inside of the cell to the
extracellular fluid. the plasma membrane is choosy about what substances can pass through it, it
is said to be selectively permeable. Selectively permeable or semipermeable means that only
certain substances are able to pass through the membrane. Transporting substances across the
plasma membrane can require that the cell use some of its energy. If energy is used, the transport
is called active. If molecules can pass through the plasma membrane without using energy, the
molecules are using passive transport.
ACTIVE TRANSPORT
Sometimes, the molecules are just too big to easily flow across the plasma membranes or
dissolve in the water so that they can be filtered through the membrane. In these cases, the cells
must put out a little energy to help get molecules in or out of the cell.
Osmosis
osmosis, the concentration of substances in the water is taken into consideration. If a solution is
isotonic, that means the concentrations of the substances (solutes) and water (solvent) on both
sides of the membrane are equal. If one solution is hypotonic, there is a lower concentration of
substances (and more water) in it when compared to another solution. If a solution is hypertonic,
there is a higher concentration of substances in it (and less water) when compared to another
solution. For example, the blood in your body contains a certain amount of salt. The normal
concentration is isotonic. If suddenly there is too high a concentration of salt, the blood becomes
hypertonic (too many salt molecules). This excess of salt forces water out of the blood cells in an
attempt to even things out. But the effect this action has is actually that of shrinking the blood
cells