Cell Transports

Passive and Active Transport

Transportation and the Plasma Membrane

• Just as the world depends on transportation to get goods and people to different locations the survival of the cell depends on the transportation of macromolecules into and out of the cell.

• The plasma membrane however does not just let any molecules come in and out. It regulates the transportation.

Structure of the Plasma Membrane

• There are 4 main parts to the plasma membrane– Phospholipid bilayer– Integral Proteins– Peripheral Proteins (attached to the outside of the

membrane and do not affect the movement of molecules.)

– Cholesterol molecules (these do not affect the movement of molecules into and out of the molecule and are just there to stabilize the membrane.)

Phospholipid Bilayer

• A phospholid is a molecule that has a polar phosphate molecule on one side and a non-polar fatty acid tail on the other.

• The phospholipid bilayer is constructed so that the fatty acid tails are pointing towards each other creating a hydrophobic inside and a hydrophilic outside.

Phospholipid Bilayer• Most cells reside in a watery environment

which requires the outside of the cell to be hydrophilic. Additionally, the inside of the cell is mostly water and also must be hydrophilic.

• This is why the phosphates are on the outside.• However, because the center is hydrophobic,

only non-polar molecules are allowed to pass through the membrane.

• Polar molecules require movement through the proteins.

Integral Proteins

• Proteins that are embedded into the plasma membrane.

• They have a non-polar area on the outside that interacts with the fatty acid section of the phospholipid bilayer.

• The inside of the protein is hydrophilic and allows water polar molecules to pass through.

Transportation

• There are two types of transport– Passive– Active

Passive Transport

• Passive transport occurs when the cell does not need to use any energy for the movement of molecules to occur.

• There are two forms of passive transport– Diffusion– Osmosis

Diffusion

• Diffusion is the movement of molecules across a membrane without using energy.

• There are two types of diffusion– Simple– Facilitated

Simple Diffusion

• Simple diffusion occurs without having to use any of the proteins that are embedded into the membrane.– This occurs with molecules that are small enough

and have the right polarity to just go directly through the cell.• Example:

– Oxygen and carbon dioxide exchange

Concentration Gradient

• Diffusion is based on the concentration.• When there is more of substance A in a

smaller area than there is of substance B, we say that there is a higher concentration.

• When molecules diffuse, they move from an area of higher concentration to lower concentration. – They move DOWN the concentration gradient.

Oxygen and Carbon Dioxide Diffusion

• Oxygen and Carbon Dioxide are both non-polar molecules and move easily through the plasma membrane.

• Since the concentration of oxygen is higher on the outside of the cell than it is on the inside, it can move into the cell by diffusion.

• The opposite is true for carbon dioxide.

Facilitated Diffusion

• In facilitated diffusion, polar molecules must diffuse into the cell through the use of the pores created by the integral proteins in the plasma membrane.

• The movement of sodium, potassium, calcium, or chloride ions (all of which are polar) can be done through the use of these protein pores. – Diffusion of these molecules is only completed if

no energy is required.

Osmosis

• Osmosis is specific form of diffusion and only involves water.

• Since water is a polar molecule is requires the use of the integral proteins.

Concentration Gradient and Water

• The osmosis of water depends on what type of solution the cell is placed in. Movement will occur so that the water concentration is equal inside and outside the cell.

• Hypotonic – more water and less dissolved substances

• Hypertonic – less water and more dissolved substances

• Isotonic – equal concentrations

A Cell in a Hypotonic Solution

• When a cell is placed in a hypotonic solution, that means that the solution has less dissolved substances than the cell.

• This means that the concentration of water is higher in the solution than it is in the cell.

• Therefore, water will move from the solution into the cell.

Affects on the Cell

• When a cell is placed in a hypotonic solution too much water can enter the cell, cause the cell to swell and burst.

A Cell in a Hypertonic Solution

• When a cell is placed in a hypertonic solution, that means that the solution has more dissolved substances than the cell.

• This means that the concentration of water is lower in the solution than it is in the cell.

• Therefore, water will move from the cell into the water.

Affects on the Cell

• When a cell is placed in a hypertonic solution, too much water can leave the cell, causing the swell to shrivel.

A Cell in an Isotonic Solution

• When a cell is placed in an isotonic solution, that means that the solution and the cell has the same amount of dissolved substances as the solution.

• This means that the concentration of water is equal inside and outside of the cell.

• Because the concentrations are equal, water will enter and leave the cell at the same rate.

Affects on the Cell

• When a cell is placed in an isotonic solution, the same amount of water will both enter and leave the cell. Therefore the cell will remain the same size.

• This is important when getting a saline drip in the hospital. The concentration of the saline IV, must be the same concentration as the cells within the human body.

Regulation

• Passive transport is regulated by the concentrations of substances inside and outside of the cell.

Active Transport

• Active Transport is the movement of substances into and out of the cell while using energy.

• Three types of active transport– Pumps – Contractile vacuoles– Cytosis

Pumps

• Moves substances against the concentration gradient.– Example: Sodium Potassium Pump• Moves sodium and potassium ions against the

concentration gradient • Enables a nerve cell to return to its resting state.

Contractile Vesicles

• Pumps water out of a cell– Example: Freshwater Protista• Since freshwater protista live in a hypotonic

environment, an excess amount of water diffuses into the cell. To get rid of the water the vesicle pumps it out back into the environment.

Cytosis

• Exocytosis and Endocytosis are the processes of removing or engulfing larger particles that cannot fit through membrane pores.

• Involves attaching to the membrane

Exocytosis

• In exocytosis, materials packaged in vesicles fuse to the plasma membrane, split the membrane, release the packaged material and become part of the membrane.

Endocytosis

• The cell membrane alters its shape to surround the incoming material.

• After the material is surrounded, it fuses together, and a small section splits off surrounding the new material.

Regulation

• Regulation of active transport is done through cell to cell recognition.

• A certain form of endocytosis is called Phagocytosis and occurs when white blood cells engulf bacteria cells to break them down.– This only occurs if white blood cell receptor’s

detect the peripheral proteins on the outside of the bacteria cells.