cell membranes

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  • Created by: Kate
  • Created on: 24-11-12 14:10

Roles of cell membranes

Function-Seperates the cells contents from the external environment and controls passage of materials between the cell and its external environment 

Membranes are partially permeable-Allow small uncharges particles through . Channel and transport proteins let large and charged in .

Compartmentalisation-Membranes isolate reactions eg mitochondria.

Cell signaling-Substances dock into receptor on membrane and make sommat happen.              

Lets electrical signals through-Membranes of nerve cells can transmit action potentials within nervous system.   

Attachment of enzymes-EG mitochondria have enzymes on membranes for ATP.              

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Plasma membrane structure fluid mosaic model

A plasma membrane is partially permeable

 A normal membrane is 7nm

A membrane is a fluid mosaic model because 

Fluid because the phospholiped vibrate .

Mosaic because when viewed from above the proteins embedded in the phosopholipid bi layer look like a mosaic.

Model because its a representation of the real structure in 3D

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The roles of components of the cell membrane

Phospholipid bi-layer-Outside is hydrophillic . Inside is hydrophobic.

Phospholipid attached to to carbodydrate chain -glycolipid. Protein attched to carb chain=glycoprotein

cholesterol-gives mechanical stability.Makes barrier more completestops water and ions getting though membrane.

Channel proteins-lets substances through that are too big and too hydrophillic.

Carrier proteins - Use ATp to pump ions across the membrane

Glyco proteins and lipids-act as receptors . Glyco ps can join cells into tissues

Enzymes-Take part in reactions.

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Cell signalling

Responding to a stimulus.

The glycoprotein recieves the signal and acts as a receptor . The glycoprotein needs t be specific and complementary to signal molecule.

Each signal molecule has a target cell and they must have complementary receptors.

Hormone molecules bind to receptors on the target cells surface membrane because of them being complementary making the cell respond.

Eg insulin receptor . Insulin released from islets of langerhans in response to increased blood sugar insulin attaches to insulin receptors on plasma membrane make the cell make more glucose channels so it can take up more glucose.

Some drugs block receptors.

Some molecules act as signals during processes in the cell and out the cell.

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Diffusion and active transport

Diffusion-random net movement of molecules from an are of high conc to an area of low conc.

Small uncharged particles eg oxygen and carbon dioxide . NO ATP needed.

Facilitated diffusion-As diffusion yet with channel or carrier proteins.

Channel proteins-charged molecules and ions eg hydrogen and sodium

Carrier proteins-Large molecules eg glucose and proteins  carrier protein specific to molecule.

Active transport- USES ATP

against conc gradient , a pump ,eg hydrogen ions in the phloem glucose into cells

Molecules moving from an area of low conc to an area of high conc. Uses transport /carrier proteins.

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Exocytosis and endocytosis

Exocytosis-Movement of substances out of the cell

1) Vesicle moves towards plams membrane

2)Vesicle fuses with membrane

3)Vesicle contents released(secreted or excreted) vesicle membrane is now part of the plasma membrane

Endocytosis-Movement of substances into the cell

1)Foreign material recognised by cell and cell spreads around it

2)Encolsed area becomes a vesicle

3)Contents is absorbed and vesicles come from membrane- recycled

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Osmosis and water potential

Osmosis-the movement of water from an area of high water potential to an area of low watre potential through a partially permeable membrane.

Waterpotential- Conc of water (Psi or KPa) High water potential =0

Distilled water=0

Every solution= -x water potential

Water always moves to most negative water potential.

When water moves out of cell animal becomes crenated an star shaped plant victim of plasmolysis + cytoplasm shrinks . Incipient plasmolysis when a cell has lost 50% of its cytoplasm.

When water moves into a cell animal bursts and has lyses if a red cell haemolysis.Plant cell becomes turgid .

When normal equal balance theres an isotonic solution outside the cell.

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Change of temp affect on membrane permeability

The hotter the membrane gets the more the phospholipids vibrate creating temporary gaps in the membrane.

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