Unit 3A

Human Form & Function

Cells, metabolism & regulation

Cellular Activity

Some useful websites for Cell Biology

• http://www.bscb.org/?url=softcell/centrioles

• http://biology.tutorvista.com/animal-and-plant-cells/centrioles.html

The cell membrane

Drawing by Mariana Ruiz Villarreal

The Fluid Mosaic Model

What is the fluid mosaic model??

• The cell membrane is said to be fluid because the molecules it is made of are constantly changing position.

• It is said to be mosaic because it is composed of many different kinds of molecules.

http://www.youtube.com/watch?v=Rl5EmUQdkuI

The cell membrane

• The cell membrane (or plasma membrane) is the outer boundary of a cell.

• It separates the intercellular components of the cell from the extracellular components.

• The cell membrane is selectively permeable (differentially)– i.e. it allows some substances to pass through more easily than others.

Structure of the cell membrane

• The membrane is a phospholipid bi-layer (i.e. two layers of phospholipid molecules.

• The phosphate ‘head’ of the phospholipid molecules are hydrophilic (attracted to water), and form the outer and inner boundaries of the membrane.

• The lipid ‘tail’ of the phospholipid molecules are hydrophobic (repelled by water), and form the inner layer of the membrane.

• The phospholipid bi-layer also contains the steroid compound cholesterol

• It helps to maintain the stability of the membrane by being wedged between the phospholipids.

• A variety of protein molecules are embedded in the membrane to allow the movement of substances into and out of the cell either passively(no energy required) or actively(using energy in the form of ATP).

Functions of the cell membrane

The cell membrane:

• Isolates the cell from the surrounding tissue fluid – physical barrier

• Determines what substances enter and leave the cell – regulation of the passage of materials (the entry of ions and nutrients and, the removal of wastes and the release of secretions).

Functions of the cell membrane

• Sensitivity – contains a variety of receptors that respond to substances in the cell’s environment e.g. hormones, antigens

• Support – the internal part of the cell membrane is attached to the microfilaments of the cells cytoskeleton.

Membrane Proteins

• Are very large molecules and make up 55% of the membrane weight.

• They include:- Channel Proteins

Carrier Proteins

Receptor Proteins

Cell Identity Markers

Membrane transport proteins

• Membrane proteins that aid the movement of substances in and out of the cell include:

• Channel proteins (ion channels) – open channels that allow simple diffusion.

• Carrier proteins that allow facilitated diffusion (e.g. glucose) and active transport (specific membrane pumps).

Membrane transport proteins

• Receptor Proteins – sensitive to molecules outside the cell.

• When present they bind with the receptor protein to trigger changes within the cell.

• Some hormones work in this way e.g Insulin• The hormone Insulin binds to a specific

receptor protein which leads to an increase in glucose absorption by the cells

Membrane transport proteins

• Receptor proteins are specific e.g. only bind with specific molecules.

• There are a limited number of receptor proteins in the membrane of each cell.

• Once each receptor is bound to a molecule there can be no further increase in the rate of the cell’s activity.

Membrane transport proteins

• E.g When each insulin receptor in the membrane is bound to insulin, the rate of glucose uptake cannot increase further even if the amount of insulin increases.

• Different cells have different types and numbers of receptor proteins which is why there is a variation in the sensitivity of cells to hormones and to other substances.

Membrane transport proteins

• Receptor proteins are important in communication between cells.

• Substances produced in one cell can stimulate and initiate changes in adjacent cells e.g. nerve cells ( one nerve cell releases a neurotransmitter that is detected by receptor proteins of an adjacent cell which responds by producing a nerve impulse)

Cell Identity markers

• These are various protein and carbohydrate molecules exposed on the surface of the cell membrane.

• They act as markers and indicate to cells of the immune system whether a cell is normal or abnormal.

• Major histocompatibility complex (MHC) is a group of genes that are unique in every individual they code for small protein molecules that act as ‘self’ markers on all body cells.

• MHC molecules initiate the immune response.

• http://www.youtube.com/watch?v=OlHez8gwMgw

• http://www.youtube.com/watch?v=o4Srx4mUmaI

Membrane transport• Transport processes are either

passive or active.

• Passive processes require no cellular energy and include diffusion, osmosis & facilitated diffusion.

• Active processes require ATP and include specific membrane pumps and vesicular transport.

• http://www.youtube.com/watch?v=2UPqLm-uDnI

• http://www.youtube.com/watch?v=RPAZvs4hvGA

Membrane Transport

• Diffusion -2 ways in cell membranes

1.Substances soluble in lipids i.e. alcohol, steroids and fatty acids can diffuse through the lipid part of the membrane.

So can carbon dioxide and oxygen

molecules

Membrane Transport

Diffusion 2. Substances can pass through membrane channels formed by channel proteins.

Water soluble molecules can enter or leave through the membrane channels but only if they are small enough.

If the channels are really small only water and sodium, calcium and chloride ions will be able to pass through. Glucose would be too BIG even though they are relatively small.

Membrane Transport

• Osmosis (diffusion of water)

• Passive process

• Water can travel across the membrane through carrier proteins when there is a concentration gradient.

• Movement from high concentration to low concentration

Membrane Transport

• Carrier Mediated – proteins in the cell bind to molecules to be transported and help their passage across the membrane.

• Carrier proteins are specific; they will only bind to a particular molecule.

• They become saturated; once all the carriers are occupied, any increase in concentration is ineffectual. The rate of transportation does not increase.

• Carrier activity is regulated by substances like hormones.

Membrane Transport

• Facilitated Diffusion - passive• Molecules such as glucose attaches to a binding

site on the carrier.• The carrier changes shape and the molecule is

released on the other side of the membrane.• Happens across a concentration gradient!

http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter3/animation__how_facilitated_diffusion_works.html

Simple diffusion (left) & facilitated diffusion

Diagram created by LadyofHats

Membrane Transport

• Active transport – requires energy from ATP.

• Substances are transported across the membrane against the concentration gradient from a region of low concentration to a region of high concentration.

Active transport (sodium pump)

Diagram created by LadyofHats

Membrane Transport• Vesicular transport – Movement across the cell

membrane in structures called vesicles.

• Active process – energy is required to form the vesicle.

• Endocytosis – taking liquids and solids into the cell.

• Pinocytosis – taking liquids into the cell.

• Phagocytosis – taking solids into the cell

• Exocytosis – taking solids and liquids from the cell. The vesicle fuses with the cell membrane then pushes the contents out into the extracellular fluid.

Endocytosis

Diagram created by LadyofHats

Exocytosis

Diagram created by LadyofHats

Other Web Links

• http://prezi.com/fztruy_c7jfa/biology-mind-mapcell-membrane/