TRANSPORT AND THE PLASMA MEMBRANE

NELSON 11

CHAPTER 2

CELL TRANSPORT

All cells must exchange materials with their environment to maintain homeostasis. The cell can use a variety of methods to move things from the internal environment to the external environment and vice versa.

There are two main types of cell transport; passive transport and active transport.

CELL TRANSPORT

PASSIVE TRANSPORT

Passive transport is the movement of materials across the plasma membrane without the use of cellular energy and it includes diffusion, osmosis, and facilitated diffusion.

DIFFUSION The random

movement of molecules from an area of high concentration to an area of low concentration.

BROWNIAN MOTION

This can be defined as the random movement of molecules.

Molecules will move about randomly in a solution or gas and collide with one another with equal frequency.

This random motion is the reason why molecules move from an area of high concentration to one of low concentration.

CONCENTRATION GRADIENT

The difference in concentration between a region of high concentration and a region of low concentration.

EQUILIBRIUM

A condition in which all acting influences are balanced resulting in a stable environment.

Once equilibrium has been reached there are now as many molecules moving into an area as there are moving out.

DIFFUSION AND THE PLASMA MEMBRANE

Particles that are soluble in lipids or small enough to fit through the spaces between the polar heads will diffuse across a plasma membrane IF a concentration gradient exists.

Diffusion will continue until equilibrium is reached.

OSMOSIS

The diffusion of water across a semi-permeable membrane from an area of high concentration to one of low.

Water will also move as a result of osmotic pressure from high to low.

In the image on the right water is represented by the small red dots.

OSMOTIC BALANCE

Water molecules are small and can diffuse easily across the plasma membrane. The direction of osmosis will depend on the concentration gradient.

When equilibrium is reached in osmosis it is known as osmotic balance.

EFFECTS OF OSMOSIS

Cells will react differently to different solutions.

Plant cells and animal cells will also react differently to different solutions.

There are three types of solutions that have an effect on cells; isotonic, hypotonic and hypertonic solutions.

ISOTONIC SOLUTION

A solution where the concentration of water and solute molecules outside a cell is equal to the concentration of water and solute molecules inside the cell.

Isotonic solutions are the ideal solution for cells to be contained in. A cell will be at equilibrium in this solution.

HYPOTONIC SOLUTION

A solution where the concentration of water outside a cell is greater than that found inside the cell. (so less solutes)

Cells placed in this solution will experience a net movement of water into the cell by osmosis. This will result in increased pressure inside the cells.

Effects of a hypotonic solution

Animal cells cannot withstand a high amount of osmotic pressure and the plasma membrane will burst.

Plant cells build up turgor pressure. Plant cells can stand up to the pressure due to the presence of the cell wall. Turgor pressure helps keep a plant cell rigid.

HYPERTONIC SOLUTION

A solution where the concentration of water molecules outside a cell is lower than that found inside the cell. (more solutes)

Cells placed in this solution will experience a net movement of water out of the cell by osmosis.

The cytoplasm of the cell will shrink due to the loss of water. The shrinking of the cytoplasm in cells is known as plasmolysis.

Effect of Solutions on Plant and Animal Cells

FACILITATED DIFFUSION

Many particles needed by the cell are too large to diffuse across the membrane on their own. Proteins located in the plasma membrane can act as carriers to aid or facilitate passive transport.

FACILITATED DIFFUSION

ACTIVE TRANSPORT

Active transport is the movement of particles across the cell membrane against the concentration gradient with the use of cellular energy and it includes endocytosis and exocytosis.

ACTIVE TRANSPORT

Active transport plays a very important role in homeostasis. Many molecules needed by the cell cannot enter the cell through passive transport because they are too large.

The cell must use energy to move these molecules across the plasma membrane.

There are two types of active transport; endocytosis and exocytosis.

ENDOCYTOSIS

Endocytosis is the process used to ingest materials and bring them inside the cell.

Due to the fluidity of the plasma membrane it is able to fold around materials in the external environment and bring them inside within a small pouch called a vesicle.

Once inside the cell these vesicles often fuse with a lysosome that contains the digestive enzymes needed to break down the ingested molecules.

There are two types of endocytosis; pinocytosis and phagocytosis.

PINOCYTOSIS

Pinocytosis involves liquid droplets being engulfed by the cell.

For example, the cells in your small intestine engulf fat droplets by pinocytosis.

PHAGOCYTOSIS

Phagocytosis involves solid particles being engulfed by the cell.

For example, your white blood cells engulf foreign microbes that invade your blood stream by phagocytosis.

EXOCYTOSIS

Exocytosis is the process used by cells to move large molecules to the external environment.

These molecules are often waste products. Some molecules released by exocytosis are

products that have been made by that cell and they are needed elsewhere in the organism. For example, our nerve cells produce transmitter chemicals that leave the cells by exocytosis.

EXOCYTOSIS