Homeostasis and Cell Transport

Chapter 5 Section 1

Passive Transport

Movement of substances across the cell membrane without any input of energy by

the cell.

Remember: The cell membrane’s function is to help organisms maintain homeostasis by controlling what substances enter or

leave the cell.

Diffusion

Simplest type of passive transport.Movement of molecules from an area of

higher concentration to an area of lower concentration.

Concentration Gradient: the difference in the concentration of molecules across a distance.

Equilibrium: the same throughout

Important!!!

Keep in mind that molecules are constantly moving even if they appear to be still.

The many movements of all molecules in all directions balances each other out so

equilibrium is maintained.

Why does sugar dissolve faster in hot tea than in cold tea?

Simple Diffusion

Diffusion of molecules across a membrane.

Depends on size, type, and chemical charge. of molecule◦Small molecules, molecules that can dissolve in

lipids, and non-polar molecules may pass through the membrane.

◦Non soluble lipids may diffuse across the membrane through pores in the membrane.

Solution, solute, & solvent

Solution: composed of a solute dissolved in a solvent.

Solvent: Part of the solution that most abundant: Water

Solute: the “stuff” dissolved in the solvent. Ex. Salt

Osmosis

The process by which water molecules diffuse across an area of higher concentration to an are of lower concentration.

No energy is used for this process therefore it is still passive transport.

Direction of Osmosis

HIGH CONCENTRATION OF WATER TO LOW CONCENTRATION OF WATER

Hypotonic: external concentration has lower concentration of solute (higher water) than then internal concentration

Direction of Osmosis

Hypertonic: concentration of solute molecules outside the cell is higher (water lower) than inside the cell.

Direction of Osmosis

Isotonic: concentration of solutes on inside and outside of the cell are equal.

NO NET MOVEMENT!

Cells and Osmosis

Most cells are typically exposed to an isotonic external environment.

Freshwater fish are examples of an organism in a hypotonic solution. Why?

Contractile vacuoles: a structure in some organisms that remove excess water from the organism. NOT PASSIVE TRANSPORT!!

Plant Cells

Plant roots are typically always in a hypotonic environment. This is important to the survival of the organism.

Water moves into the roots by osmosis and the cells swell (where is the water stored?)

When it fills with water, the cell membrane swells and pushes up against the cell wall; this pressure is called turgor pressure. The condition when pressure is lost is called Plasmolysis

Facilitated Diffussion

Another type of passive transportUsed for the molecules that cannot readily

diffuse through cell membranes even if there is a concentration gradient.

Travel through proteins called carrier proteins◦These molecules may not be soluble in lipids or

too large to pass through pores.

Carrier Proteins

Facilitate in movement down the concentration across the membrane.

Does not require energy!!!Molecules bind to carrier protein and once

bound, the protein changes shape to allow molecule to enter the cell.

Ex. Glucose transfer.