Cell membranes

Roles of membranes

• Membranes seperate organelles contents from the cytoplasm to provide attachment and other molecules involved in metabolic reactions

• controls what goes in and out

• cell recognition and communication

• Every living cell is surrounded by a membrane PLASMA MEMBRANE

• separtes the cells contents from its external enviroment

• controls what goes in and out PARTIALLY PERMEABLE

Glycoproteins and gylcolipids- allow recognition by the immune system, glycoproteins can bind cells together in tissues

Cholestrol- gives the cell mechanical stability and fluidity

Channel proteins - allow some substances across the membrane which are too large or have a charge to pass thrugh the channel protein

Carrier proteins - actively move substances across the membrane

Phospholipid - head (polar) is hydrophillic tail is hydrophobic (non polar)

Also called a fluid mosaic model as all of it is free to move around and called a mosaic as its like a make up of components in a mosaic.

Recognition and communication

• Receptor sites- some allow hormones to bind with the cell so that the cell response can be carried out.
• Cells can respond only to a hormone if it has a receptor for that hormone on its cell surface.

Metabolic processes

• Enzymes and coenzymes.
• Some reactions in photosythesis take place in membranes inside the chloroplast.
• Respiration takes place in membranes of mitocondria
• Enzymes and coenzymes may br bound to these membranes
• More membranes - more enzymes and co enzymes can be held

Membranes and temperature

• More temperature means increased kinetic energy so they move faster and increased movement

• Leaky so substances that couldnt pass before now are able to

In multicellular organisms, communication between cells is often mediated by hormones. Hormones are CHEMICAL MESSENGERS. They are produced in a specific tissue and then released into the organism. Any cell with a RECEPTOR for the hormone molecule is called a TARGET CELL.

Insulin receptors

• INCREASED blood glucose levels
• INSULIN RELEASED from BETA CELLS in the pancreas
• TRIGGERS a RESPONSE in cells that lead to more glucose channels being present in plasma membrane
• Allows cell to take up glucose from the blood
• REDUCED blood glucose levels

Beta blockers - prevent heart muscle increasing the heart rate in people who's increased heart rate can be dangerous. i.e if you have high blood pressure, angina (restricted oxygen supply to the heart).

Block the action of natural substances (noradrenaline) at receptors in arteries and on heart muscle

Noradrenaline causes arteries to narrow and the heart beat faster

BETA BLOCKERS - BLOCK NORADRENALINE so stop the narrowing of vessels

results in a drop of blood pressure and less work for the heart to do

some drugs mimic neeurotransmitters which some people cant produce

• some viruses can bind on to the receptors which orginally bind to the host signalling molecules.

• viruses such as hiv is caused by the virus having a simillar shape to the receptor and binds onto the the cell surface of immune cells such as helper T-lymphocytes. After a period of inactivity of the t cell the hiv cell can reproduce in the cell and destroy it.
• Some poisons also bind with receptors such as a bacterium called clostridium botulinum. which bind on the the muscle cells disabling them from working properly causes PARALYSIS. The toxin is lethal but it is used in small quanitities in the cosmetic surgey under BOTOX.

• Botox signals from nerves to the muscles
• Injected into muscles so they are no longer able to contract which causes wrinkles to relax and soften
• Small quantities of clostridium botulinum causes paralysis
• but used in botox as it paralyses small musclles on the skin.

Membranes is the net movement of molecules (or ion) from a region of their HIGH CONCENTRATION to a region of their LOW CONCENTRATION.

The molecules move down a concentration gradient.

What determines the rate of diffusion;

• The steepness of concentration gradient
• Temperature
• Surface area
• Type of molecules
• Thickness of membrane

Bigger Molecules or molecules with a charge will go through a channel protein.

• some particles such as ions diffuse through a channel protein embedded within the membrane
• only one type of molecules goes in
• Specific molecules fits into the carrier protein in either direction depending on the diffusion gradient when the specific molecules fits the protein changes shape to allow the molecules to the other side of the membrane
• Channel protein is also gated which means they can be open or closed
• This is a type of PASSIVE TRANSPORT

Active Transport

The energy consuming transport of molecules or ions against a concentration gradient made possible by transferring energy by respiration.

• A phosphate group from the molecule ATP attaches to the carrier protein causing it to change shape
• As the protein changes shape it pumps the molecule or ion into the cell
• goes against the concentration as it goes from a low concentration to a high concetration

• Bulk transport is large quantities into or out of a cell

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Active transport Diffusion Facilitated diffusion Osmosis Uses ATP Can go against concentration gradient Requires protein carrier molecules tick the box for yes n cross for no

Water potential - is the measure of water molecules to diffuse from one place to the other

• Water always diffuses from a high water potential to a lower water potential
• Water potential of water is always 0
• Plants contain cytoplasm in which solutes have been dissolved in the water potential is therefore lower.
• Water potential in plant cells are always negative

Osmosis is the net movement of water from a high water potential to a lower water potential through a partially permeable membrane.

Exdocytosis

• The vesicle containing a hormone or enzyme with fuse with the cell membrane
• Then the contents will be secreted out of the cell

Endocytosis

• Foreign cell or food particle will try to enter the membrane.
• So membrane invaginates around the particle.
• Vesicle is created. which fuses with lysosomes and contents will be broken down.