transport in cells

PHOSPHOLIPIDS

PHOSPHOLIPIDS

• hydrocarbon is non-polar so is hydrophobic- repels water

• phosphate is polar so is hydrophobic- negative charge means that it forms bonds with water

PHOSPHOLIPIDS IN WATER

• form spheres called micelles

• tails face each other and heads face water

CELL MEMBRANE STRUCTURE

• form phospholipid bilayer

• head point outwards towards aqueous environment

• tails point inwards towards centre of membrane

FATTY ACIDS- SATURATED AND UNSATURATED

• saturated fatty acids have single bonds between carbon atoms- chain is straight and easy to pack tightly

• unsaturated fatty acids have double bonds between carbon atoms- create kinks in chain so is harder to pack tightly

• kinks increase space between phospholipids, making the molecules harder to compact at lower temperatures

• increased space allowed certain small molecules (e.g. CO2 and O2) to cross membrane quickly and easily

INTRINSIC PROTEINS

• proteins randomly arranged amongst phospholipids

• span/extend through the membrane from one side to the other

• some act as carriers to transport water soluble material across the membrane while others are enzymes

• GATED CHANNELS- some act as channels through the membrane and can be open or shut depending on conditions

EXTRINSIC PROTEINS

• occur either on surface of bilayer or only partially embedded

• either give support to the membrane or in conjunction with glycolipids act as cell receptors for molecules (e.g. hormones)

PROTEINS- FUNCTIONS

• provide structural support

• act as carriers transporting water-soluble substances across the membrane

• allow active transport across membrane by forming ion channels for sodium, potassium etc.

• form recognition sites by identifying cells

• help cells adhere together

• act as receptors

CHOLESTEROL

• they are randomly distributed across the phospholipid bilayer, helping it stay fluid in different environmental conditions

• they hold the phospholipids together so that they don’t separate too far, letting unwanted substances in, or compact toot tightly, restricting movement across the membrane

FLUID MOSAIC

• FLUID- bc phospholipid molecules can move relative to one another- gives the membrane a flexible structure that is constantly changing its shape

• MOSAIC- bc the proteins that are embedded in the membrane vary in shape, size and pattern

PASSIVE TRANSPORT MECHANISMS

• passive transport takes place bc of concentration, pressure or electrochemical gradients and involves no energy from the cell

• diffusion, facilitated diffusion and osmosis

DIFFUSION

• particles in gas or liquid are in constant random motion due to kinetic energy

• they’re constantly colliding

• particles that are concentrated together will distribute themselves evenly

• diffusion is the net movement of molecules or ions from a region where they’re highly concentrated together one where the conc is lower

FACILITATED DIFFUSION

• movement of particles from high to low concs using proteins (assist/help/facilitate movement)

• particles move down a conc gradient so process is passive-no energy required

• involves 2 intrinsic proteins:

  • channel proteins

  • carrier proteins

OSMOSIS

• net movement of solvent molecules from a region of high water potential (dilute) to lower water potential (concentrated) through a partially permeable membrane

ACTIVE TRANSPORT MECHANISMS

• involves moving substance in or out of the cell using adenosine triphosphate (ATP) produced during cellular respiration

• e.g. endocytosis, exocytosis and active transport

ENDOCYTOSIS

• movement of large molecules into cells through vesicle formation

• surface membrane encases molecule in a bilayer

• possible bc membrane fluid

EXOCYTOSIS

• movement of large molecules out of cells through vesicle formation

• vesicle fuses with CSM and molecule released from cell

• possible bc membrane is fluid

ACTIVE TRANSPORT

• movement of particles/molecules across the membrane of cells directly using ATP

• proteins in membrane act as carriers a or enzymes making ATP available to move ions or molecules through membrane

• happens against concentration gradient