And movement through membranes

PhospholipidsPhospholipids are made up of a

Phosphate head andTwo fatty acid tails

The head portion “loves water”:It is HYDROPHILIC

The tails “hate water”:They are HYDROPHOBIC

Phospholipid Bilayer

Cytoplasm of cells is water based and most cells live in a watery environment.

The phosphate heads are attracted to this water.

Watery environment

Watery cytoplasm

Phospholipid BilayerThe fatty acid tails form a

lipid layer in the middle of the membrane.

Water and water soluble substances do not move easily through this lipid layer.

The membrane can therefore be a barrier or partition between two parts of a cell.

Lipid Layer

Proteins in MembranesGlobular proteins can be a

variety of shapes.These can be found attached

to (peripheral), or within membranes(integral).

Proteins can also have hydrophobic and hydrophilic surfaces, and this affects how they will sit in the phospholipid bilayer.

Proteins in MembranesHydrophobic portions of

the proteins are repelled by water.

Proteins will lie in the membrane so that the hydrophobic areas are surrounded by fatty acid tails.

Hydrophilic areas will be exposed to water .

Cell Membrane Structure

Glycocalyx

Channel protein

Intrinsic protein

Exterior of Cell

Cytoskeleton fibrils

Other non-globular proteins and cholesterol also in membrane

Transmembrane Protein

Interior of Cell

Fluid Mosaic Model

The membrane is fluid in the sense that the proteins can move within the membrane.

It is a mosaic because the distribution of the molecules is not regular.

Diffusion through MembranesMovement of dissolved

substances between the phospholipid molecules is very limited. (So, it happens, but not much!)

The phospholipids are always moving past each other, and this leaves occasional gaps.

Channel ProteinsSome proteins are shaped

like channels.The inner surfaces of the

channel are hydrophilic.This allows water and

dissolved substances to move through the membrane.

The channels are like pores in the membrane.

Facilitated DiffusionSome channels are

specifically adapted to transport certain molecules across the membrane.

Facilitated diffusion is a passive process and uses no energy.

membrane

molecule

Channel protein

Facilitated DiffusionThe molecule

interacts with the channel protein and causes it to change shape.

Only certain molecules can cause this to happen

Facilitated DiffusionThe molecule is released

by the channel protein

It has been moved across the membrane, down its concentration gradient

The channel protein will return to its original shape.

Active TransportActive transport occurs in

a similar manner to facilitated diffusion EXCEPT that energy is required to make it happen.

Like active transport there will be an interaction between the molecule and the channel protein.

Active TransportEnergy is

supplied by ATP

The molecule is moved across the membrane AGAINST

its concentration gradient.

Active TransportActive Transport is used when

the cells need to accumulate certain substances at a higher concentration than they are found in the environment.

ATP = Adenosine Tri PhosphateADP = Adenosine Di Phosphate

ATP is regenerated in the mitochondria

Membrane movements:EXOCTYOSIS: The process of exporting

material from the cell. Vesicles are released from the Golgi body and fuse with the cell membrane during this process.

ENDOCYTOSIS: The process of taking material into the cell. (Think “entrance”). There are two types:PHAGOCYTOSIS – like “cell eating”PINOCYTOSIS – like “cell drinking”

ExocytosisExocytosis is a

process that allows substances to be exported from a cell.

The membrane of the secretory vesicle moves towards the cell membrane.

Digestive enzymes are exported this way.

Exocytosis: A vesicle moves

towards the cell membrane.

When the vesicle touches the cell membrane the process of exocytosis can start.

vesicle

Cell membrane

Substance for export

ExocytosisThe cell membrane

and the membrane of the vesicle fuse.

The contents of the vesicle are expelled from the cell.

ExocytosisThe membrane that

surrounded the vesicle becomes part of the cell membrane.

The vesicle membrane is shown here in orange before and after exocytosis.

Vesicle membrane

Endocytosis: PhagocytosisLarge particles that are

outside the cell may be taken in by a process called PHAGOCYTOSIS (a type of ENDOCYTOSIS)

The cell membrane moves around the particles to form a food cup.Food cup

The out-foldings of the cell membrane are called pseudopodia (pronounced sudo-pode-ee-aa).

Pseudo = false. Pod = foot.

A food vacuole is formed.

Endocytosis: PhagocytosisPseudopodia

Food vacuole

Endocytosis: PhagocytosisThe contents of the food

cup will usually be digested by enzymes from a lysosome.

The membrane of the lysosome fuses with the membrane of the food vacuole.

lysosome

enzymes

Active and Passive ProcessesProcesses that are PASSIVE require no input

of energy. They include: Diffusion Osmosis Facilitated Diffusion

Processes that are ACTIVE require energy in the form of ATP to happen. They include:

Active Transport Exocytosis Endocytosis