Homeostasis and Transport

Movement of cellular materials in and out of the cell.

Cell membranes help cells maintain homeostasis by controlling what substances may enter or leave cells.

Cell membranes are selectively-permeable in that they allow only certain substances to pass.

I - Passive Transport

• No energy input from the cell..

No ATP energy molecules used.

Simplest type of Passive Transport: (simple) Diffusion

It is the (random) movement of molecules from an area of higher concentration to an area of lower concentration.

In other words…

• Materials move from where there’s more to where there’s less.

This difference in concentration of molecules (over a given area)is called the concentration gradient

Diffusion movement occurs with, (or down) the concentration gradient.

Diffusion is driven by the kinetic energy of the molecules.

Kinetic Energy – Energy of Movement

from an area oflow tohigh

concentration

• Eventually the process of diffusion reaches equilibrium.

• At equilibrium the concentration of molecules are the same throughout the space. (The concentration gradient disappears)

Are the molecules still moving at this point (equilibrium)?

• Yes. Molecules of a substance are always in motion to some degree except at absolute zero.

The net movement is zero.

Osmosis The process by which H2O molecules

diffuse across a cell membrane (selectively permeable membrane) from an area of HIGH concentration to LOW concentration.

The net direction of water movement depends on the relative amount of solutes (dissolved substances) on the two sides of the cell membrane.

THE THREE BASIC TYPES OF SOLUTIONS:

• Hypotonic

• Hypertonic

• Isotonic

Hypotonic

• solute concentration outside the cell is lower than inside the cell cytosol.

• water diffuses into the cell

Hypertonic

• solute concentration outside the cell is higher than inside the cell cytosol.

• water diffuses out of the cell

Isotonic

• solute concentration outside and inside the cell are equal thus, NO net movement of water into or out of the cell

How do cells deal with osmosis?

• Isotonic – usually no difficulties

interesting chromosomepattern makes thiscell have a happy

face

Hypotonic environment• Unicellular freshwater organisms rid themselves

of excess water by an organelle called a contractile vacuole. (Other cells cannot get rid of the water and they may burst)

What happens to ananimal cell under

different conditions?

What happens to aplant cell under

different conditions?

Facilitated Diffusion It is a type of passive transport in

which molecules pass through pores in the cell membrane. Carrier proteins help move molecules across the membrane.

II - ACTIVE TRANSPORT

Cell expend energy to move materials.

Active transport always needs ATP energy molecules

Active Transport is usually against the concentration gradient.

Materials move from low to high concentration.

EXAMPLES OF ACTIVE TRANSPORT

• Cell Membrane Pumps (such as the Sodium-Potassium pump) It involves Carrier proteins

Sodium-Potassium Pump is important for nerve cells.

• Endocytosis - cells take in fluids, large molecules, large particles and other cells.

Vesicles are formed to ingest materials.

Two major types of endocytosis: • pinocytosis – movement of solutes or fluids

(Cell-drinking)

• phagocytosis – movement of large particles or whole cells. (cell-eating)

Amoeba uses pseudopodia to engulf food.

Exocytosis (the reverse of endocytosis)

• Vesicles fuse with the cell membrane, releasing their contents to the outside environment.

• Unicellular organisms may get rid of wastes through exocytosis.