Unit 3: Cellular Transport

Transport through cell membranes The phospholipid bilayer is a good barrier around cells, especially to water soluble molecules. However, for the cell to survive some materials need to be able to enter and leave the cell.

Moving stuff from one side of the membrane to the other There are two general ways that materials can be moved from one side of the membrane to the other. 1) Passive Transport (doesn’t require energy) 2) Active Transport (requires ENERGY) ** There are several specific types of both passive & active transport **

Passive Transport Definition: Movement of a substance across the cell membrane that does NOT require energy from the cell. Another way of saying this is movement of a substance across the cell membrane down its Concentration gradient. Concentration gradient- a difference in the concentrations of a substance across a space. What does ‘down’ the concentration gradient mean? Movement occurs from a region of high concentration to a region of low concentration

Types of Passive Transport 1) Diffusion 2) Osmosis 3) Ion Channels 4) Facilitated Diffusion

1. Diffusion Definition- movement of a substance from an area of high concentration to an area of lower concentration. In other words, the tendency of molecules of any substance to spread out in the available space Example: a drop of dye in a beaker of water

Example: A drop of dye in water If diffusion is allowed to continue, equilibrium results. 7

Equilibrium- a condition in which the concentration of a substance is equal throughout a space.

Diffusion in the cell Substances within a cell spread to all parts of the cytoplasm by diffusion. If there is a concentration gradient across the cell membrane for a particular substance, it may diffuse into or out of the cell until equilibrium is reached.

Molecules that diffuse through cell membranes a) Oxygen – Non-polar so diffuses very quickly. b) Carbon dioxide – Polar but very small so diffuses quickly. c) Water – Polar but also very small so diffuses quickly. AS Biology, Cell membranes and Transport 10

2. Osmosis Definition- The diffusion of water across a selectively permeable membrane. This involves the movement of water down its concentration gradient ( high to low). The amount of dissolved ions present on either side of the cell membrane determine in what direction free water will diffuse to.

What determines the direction water will diffuse? Free (unbound) H 2 O molecules will move from where they are abundant to the solution where they are rarer. Osmosis continues until the solutions are isotonic. (Latin breakdown: ‘same pressure’)

Osmosis AS Biology, Cell membranes and Transport 13 Cell membrane partially permeable. Inside cell Outside cell Water Molecule VERY Low conc. of water molecules. Sugar molecule DILUTE SOLUTION CONCENTRATED SOLUTION

Three possibilities for the direction of water movement. a) Hypertonic solution Description- A solution that causes water to move OUT of a cell, causing the cell to shrink. The solution outside of the cell has LESS free water than is inside the cell. This causes water to leave the cell and move into solution. Example ~ cell in salt water solution

Hypertonic Solution

b) Hypotonic Solution Description- A solution that causes water from the solution surrounding the cell to move INTO a cell, causing the cell to swell. Why does this happen? The solution inside of the cell has LESS free water than the solution outside the cell.

This can cause a cell to burst. Example ~ cell placed in a beaker of water.

c) Isotonic Solution Description- a solution that produces no net change in cell volume because of osmosis. The amount of free water is the same both inside and outside of the cell. (Amount of H 2 O moving out of the cell is the same as the amount of H 2 O moving into the cell) ** The cell stays in a state of equilibrium. Ex: Cell placed in its original bucket of vinegar

Isotonic Soultion

Tonicity

AS Biology, Cell membranes and Transport 21

Many polar molecules and ions are blocked from gaining entry to the cell by the nonpolar zone of the lipid bilayer (the fatty acid tails). Facilitated Diffusion - The passive movement of molecules down its concentration gradient via a carrier protein. Each carrier protein is specific as to the substances that it will move. 3. Facilitated Diffusion

Facilitated Diffusion Example This is how glucose is brought into our cells

4. Ion Channels (Channel Proteins) Description- a transport protein with a polar pore through which ions can pass. This allows ionic substances to pass through the non- polar region of the lipid bilayer Some ion channels are always open. Others can be closed by ion channel gates. They are opened and closed in response to several stimuli (ex: electrical charge)

Some Ion Channels can open and close

Take 2 minutes to summarize what you have learned regarding the various types of passive cellular transport. How are they similar? How are they different? Discuss with your shoulder partner- What do you understand well? What is a little confusing?

Active transport Definition- Cellular transport that requires the cell to use its own metabolic energy. Two types 1)Molecular Pumps 2)Vesicle Mediated Transport

1. Molecular Pumps Description- a carrier protein that pumps molecules into or out of a cell against their concentration gradient. (From a region of LOW concentration to a region of HIGH concentration) Example: Sodium-Potassium Pump This keeps the level of sodium nontoxic in our cells.

2. Vesicle Mediated Transport Also known as bulk transport; this method is used when substances are too big to fit through the pores found in membranes or through transport proteins) Two types: Endocytosis & Extocytosis

a) Endocytosis Definition- a form of bulk transport when a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane. The material is taken into the cell by means of infoldings (pockets) of the cell membrane, which pinches off to form a vesicle.

b) Exocytosis Definition- movement of a substance by a vesicle to the outside of the cell. Example This is how cells export proteins that are modified by the golgi apparatus.

Exocytosis The opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are released through the cell membrane. 32

Endocytosis & Exocytosis AS Biology, Cell membranes and Transport 33

Take 2 minutes to summarize what you have learned regarding the different types of Active transport. How are they similar? How are they different? Discuss with your shoulder partner- What do you understand well? What is a little confusing?

Diffusion Animation hill.com/sites/ /student_view0/chapter2/animat ion__how_diffusion_works.html

Osmosis Animation hill.com/sites/ /student_view0/chapter2/animat ion__how_osmosis_works.html

Facilitated Diffusion Animation hill.com/sites/ /student_view0/chapter2/animat ion__how_facilitated_diffusion_works.html

Sodium Potassium Pump Animation hill.com/sites/ /student_view0/chapter2/animat ion__how_the_sodium_potassium_pump_works.html

Phagocytosis Animation hill.com/sites/ /student_view0/chapter2/animat ion__phagocytosis.html

Aquaporin

Gated Channel Protein

Glucose enters the cell via facilitated diffusion

Carrier Protein (a) & Gated Carrier Protein (b)

Cholesterol helps to stabilize the membrane