Environments of cells• All cells exist in a watery environment –

extracellular fluid

• Mullticellular organisms have an outer layer that acts as a barrier and creates an internal environment

Aspects of the internal environment that need to be regulated:

•Salt concentrations

•Temperature

•Levels of nutrients

•pH

•Removal of wastes

•Ion concentrations

CELL MEMBRANE (look pg 58)

The role of the membrane is very important in maintaining cell stability and enabling the cell to operate at optimum levels.

The cell membrane (plasma membrane),

- selectively controls the molecules entering & leaving the cell.

- allows the cytoplasm to have a different composition from the the surrounding environment

Membrane Structure.

• The membrane is partially permeable. Some substances pass freely across the membrane while others are excluded.

Phosopholipid bi-layer

• The cell membrane is formed from a double layer of lipids with proteins embedded in it forming ion channels.

• The lipid layer is capable of much movement, i.e. it is fluid.

• Cholesterol molecules reduce the membrane fluidity

• It is impermeable to water soluble substances.

• Lipid soluble substances e.g. alcohol pass through.

Hydrophobic,

‘water hating’ end

Hydrophillic,

‘water loving’ endWhen in contact with an aqueous solution, the phospholipid molecules line up with tails pointing in (away from sol)

EXAMPLE! Detergents cause fats to form micelles.

Protein Channels• These are large proteins with

openings on both sides of the membrane.

• Allow flow of

ions

•Carbohydrates on the outer surface are important in cell adhesion and cell recognition. These are attached to recognition proteins.

Phospholipid Bilayer

Embedded Cholesterol Protein channel

Embedded protein

Carbohydrate

(1) Fluid (2) Mosaic Model (Draw)

Surface area to volume ratio

Movement in and out of Cell

• Diffusion (simple)

• Osmosis

• Facilitated diffusion

• Active Transport

• Endocytosis and Exocytosis

Diffusion (simple)

Comments: Moves down Con~ gradient (high to low) The diffusion rate increases with a high concentration gradient, heat, small particles and with gaseous substances. Movement is from a strong solution to a weak solution. (think sugar cube /water) (lungs, CO2)

Where? What? Energy?

Through the phospholipid bilayer

small uncharged particles (water, urea, Oxygen, carbon dioxide) & lipid soluble substances such as alcohol, ether, chloroform.

NO

Facilitated DiffusionWhere? What? Energy?

Through the protein molecules spanning the membrane. Either by channel proteins or by carrier proteins.

Charged ions &larger molecules (glucose).

- the lipid insoluble molecules.

NO

Comments: The process is the same as for simple diffusion.

Osmosis

Comments: Osmosis is a special type of diffusion. The process is the same but only water moves to even up concentrations. Movement of water is from a weak solution into a strong solution.Example: water from the gut (diarrhoea) Cell wall in plants

Where? What? Energy?

Through the phospholipid bilayer

Water only NO

Diffusion across the (partially permeable) membrane.

12

Active Transport.

Comments: Active transport is movement against the concentration gradient to ‘build up’ rather than to‘even up’.

Where? What? Energy?

Through the protein molecules spanning the membrane.

Nutirent molecules, ions such as chloride ions (Cl-), sodium ions (Na+), potasssium ions (K+) and water

Yes

Bulk Transport (endocyctosis and exocytosis).

Comments: Phagocytosis (solids - cell eating) and pinocytosis (cell drinking – entry of liquid) are forms of endocytosis.

Where? What? Energy?

Small sections of the plasma membrane.

Large molecules such as enzymes, hormones and antibodies and foreign material.

YES