• To identify the factors that effect diffusion rates

• To state Fick’s Law

•That facilitated diffusion is a process where channel or carrier proteins transport ions or polar molecules across the membrane

• To understand that the tertiary structure of each protein means that it recognises, binds with and transports a specific substance

•To be able to answer questions on diffusion and Fick’s Law

I fink that cos the stuff in chamber A is weaker, not like my Peter, but it means that cos its just a big wus it can’t move and this water

potato stuff means that you can have chips like

dunnit?

Jordan discusses osmosisJordan discusses osmosis

Glucose Glucose solution 0.2 solution 0.2

mol dm mol dm -3-3

Glucose Glucose solution 0.5 solution 0.5

mol dm mol dm -3-3

AA BB

Explain in which of the water Explain in which of the water chambers the water potential chambers the water potential is higheris higher

The water potential is less The water potential is less negative (higher) in chamber negative (higher) in chamber

‘A’ because there is less ‘A’ because there is less solute (or a lower solute (or a lower

concentration of solute) in concentration of solute) in chamber A, and therefore chamber A, and therefore

more free water molecules. more free water molecules. Duh, its easy innit??Duh, its easy innit??

Jordan discusses osmosisJordan discusses osmosis

Glucose Glucose solution 0.2 solution 0.2

mol dm mol dm -3-3

Glucose Glucose solution 0.5 solution 0.5

mol dm mol dm -3-3

AA BB

Explain in which of the water Explain in which of the water chambers the water potential chambers the water potential is higheris higher

1.Diffusion

• A few substances can diffuse directly through the lipid bilayer part of the membrane.

• The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2.

• For these molecules the membrane is no barrier at all. Since diffusion is (obviously) a passive process, no energy is involved and substances can only move down their concentration gradient.

• Diffusion cannot be controlled by the cell, in the sense of being switched on or off.

Factors Effect on the rate of diffusion

The concentration gradient

The distance

Size of molecules

Temperature

Surface Area

particles diffuse faster when there is a high concentration gradient

the shorter the distance the faster diffusion

small molecules diffuse faster than large ones

high temperatures mean that particles have more kinetic energy and so diffuse faster

the larger the surface area the faster the rate of diffusion

The rate at which a substance diffuses can be worked out using Fick’s Law;

Rate of Diffusion

α Surface area × difference in concentration

Thickness of membrane

Diffusion rate is increased by a large surface area, a large difference in concentration and a small diffusion distance

2. Facilitated Diffusion.

channel protein carrier protein

or

Facilitated diffusion (passive transport) is the transport of substances across a membrane by a transmembrane protein molecule.

•The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it has the appropriate shape.

•Facilitated diffusion is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient.

There are two kinds of transport protein:

•Channel Proteins form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes.

•Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.

•Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane.

•The substance will bind on the side where it at a high concentration and be released where it is at a low concentration

Questions

1)Which molecules can pass through the cell membrane by diffusion?

Lipid molecules, Oxygen, carbon dioxide and water

2) Define diffusion

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

3) Why can lipid soluble molecules such as steroids pass easily through the membrane?

The cell membrane is composed of a lipid bilayer which hydrophobic lipids can pass through

4. Active Transport (or Pumping).

protein pump

ATP ADP + Pi

active site

Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. It requires: a transmembrane protein (usually a complex of them) called a transporter and energy. The source of this energy is ATP.

•Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule.

•The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side.

•The proteins are highly specific, so there is a different protein pump for each molecule to be transported.

•The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule.

•Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.

The Na+K+ Pump.

• This transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps.

The cytosol of animal cells contains a concentration of potassium ions (K+) as much as 20 times higher than that in the extracellular fluid.

Conversely, the extracellular fluid contains a concentration of sodium ions (Na+) as much as 10 times greater than that within the cell.

These concentration gradients are established by the active transport of both ions.

The same transporter, called the Na+/K+ ATPase, does both jobs. It uses the energy from the hydrolysis of ATP to actively transport 3 Na+ ions out of the cell for each 2 K+ ions pumped into the cell.

outside cell

3 Na+

2 K+

inside cell

-

+

ATP ADP + Pi

Na+K+

pump

•The Na+K+ pump is a complex pump, simultaneously pumping three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP split.

•This means that, apart from moving ions around, it also generates a potential difference across the cell membrane.

•This is called the membrane potential, and all animal cells have it. It varies from 20 to 200 mV, but and is always negative inside the cell.

•In most cells the Na+K+ pump runs continuously and uses 30% of all the cell's energy (70% in nerve cells).

This accomplishes several vital functions:

•It helps establish a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior.

•This resting potential prepares nerve and muscle cells for the propagation of action potentials leading to nerve impulses and muscle contraction.

•The accumulation of sodium ions outside of the cell draws water out of the cell and thus enables it to maintain osmotic balance (otherwise it would swell and burst from the inward diffusion of water).

Active transport is used to: 

1. Generate charge gradients. For example in the mitochondrion, hydrogen ion pumps pump hydrogen ions into the intermembrane space of the organelle as part of making ATP. 

2. Concentrate ions, minerals and nutrients inside the cell that are in low concentration outside. 

3. Keep unwanted ions or other molecules out of the cell that are able to diffuse through the cell membrane. 

In all these cases the key is that active transport uses energy to send substances against the direction they would travel by simple diffusion: that is from a region of low concentration to a region of high concentration. 

5.Vesicles

The processes described so far only apply to small molecules.

Large molecules (such as proteins, polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.

•Endocytosis is the transport of materials into a cell.

•Materials are enclosed by a fold of the cell membrane, which then pinches shut to form a closed vesicle.

•Strictly speaking the material has not yet crossed the membrane, so it is usually digested and the small product molecules are absorbed by the methods above.

• When the materials and the vesicles are When the materials and the vesicles are small (such as a protein molecule) the small (such as a protein molecule) the process is known as process is known as pinocytosispinocytosis (cell (cell drinking).drinking).

• If the materials are large (such as a white If the materials are large (such as a white blood cell ingesting a bacterial cell) the blood cell ingesting a bacterial cell) the process is known as process is known as phagocytosisphagocytosis (cell (cell eating).eating).

Exocytosis•Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. •Materials to be exported must first be enclosed in a membrane vesicle, usually from the RER and Golgi Body. •Hormones and digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands

Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in by endocytosis at one end of a cell and passing out by exocytosis at the other end.

•The rate of diffusion of a substance across a membrane increases as its concentration gradient increases; lipid diffusion shows a linear relationship,

• Facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate being limited by the number of transport proteins.

• The rate of active transport also increases with The rate of active transport also increases with concentration gradient, but most importantly it has a concentration gradient, but most importantly it has a high rate even when there is no concentration high rate even when there is no concentration difference across the membrane. difference across the membrane.

• Active transport stops if cellular respiration stops, Active transport stops if cellular respiration stops, since there is no energysince there is no energy

concentration difference

lipiddiffusion

facilitateddiffusionra

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f tra

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Biopress fact sheetBiopress fact sheet

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