Transport across membranes

what is the cell surface membrane

the plasma membrane that surrounds the cell and forms a boundary between the cell cytoplasm and the external environment

what does the cell membrane allow

different environment inside the cell to outside the cell

a phospholipid contains

hydrophilic phosphate head and hydrophobic fatty acid tails x2

phospholipids form a

bilayer

what type of molecule can move through the bilayer

lipid soluble

functions of phospholipid bilayer

allow lipid-soluble molecules to enter and exit

prevent water soluble entering and exiting

make the membrane flexible and self sealing

extrinsic proteins are found

on the surface of the membrane and never span across it

what do extrinsic proteins do?

give mechanical support, and act as cell receptors

what are intrinsic proteins

channel or carrier proteins

channel proteins transport

water soluble ions

carrier proteins transport

large molecules e.g. glucose or amino acids (change shape)

functions of intrinsic proteins

structural support, transport large and charged ions, act as receptors

what does cholesterol do

add strength by wedging gaps between phospholipids. prevent leakage of water and dissolved ions. restricts movement of other molecules

what are glycolipids made of

carbohydrate and lipid

what do glycolipids do?

act as recognition sites

help cells to attach to each other

what are glycoproteins made of

carbohydrates and proteins

what do glycoproteins do

act as cell surface receptors for hormones and neurotransmitters - lymphocytes recognise its own cells

why do most molecules not freely diffuse across the membrane

polar (difficult to pass through non-polar phospholipids)

large (can’t fit through channels)

why is it called the fluid mosaic model

fluid - individual phospholipids are flexible, constantly changing shape

mosaic- proteins are embedded in membrane vary in size and shape

simple diffusion is an example of

passive transport (through phospholipid bilayer)

definition of diffusion

the net movement of molecules or ions from a region of high concentration to an area of low concentration until evenly distributed

what molecules can simply diffuse through the membrane

small, non-polar molecules e.g. oxygen or carbon dioxide

facilitated diffusion is a

passive process

what molecules use facilitated diffusion

large and charged (phosphate head is positively charged)

channel proteins transport

ions

carrier proteins transport

too large molecules (glucose, amino acids etc)

what is an aquaporin

water can flow more rapidly in and out of a cell.

what are the 5 factors that influene diffusion

temperature

concentration gradient

surface area

layer of cells

number of channel/carrier proteins

as temperature increase diffusion

increasing, molecules have more kinetic energy so they move faster

as the concentration increase diffusion

increases, an increased concentration gradient increasing diffusion

as the surface area increases

diffusion increases as there is more space for molecules to diffuse

as the number of channel/carrier proteins increase

diffusion increases as there are more proteins so more things can diffuse across quicker

as the layer of cells (distance) decrease diffusion

increases as the distance decreases

definition of osmosis

the movement of water from an area of high water potential to an area of low water potential through a partially permeable memebrane

what does partially permeable

allow some molecules e.g. water to pass through but not other larger molecules

what is water pressure measured in

kPa

under standard conditions and pressure pure water has a water potential of

0kPa

if you add solute to water

its water potential decrease (becomes more negative)

water potential is never

greater than 0 (0 or negative)

water will move

from a region of higher water potential (less negative) to an area of lower water potential (more negative)

animal cells in higher water potential

have thin cell surface membranes so will burst if placed in a higher water potential(less negative) than itself (swell and burst)

animal cell in lower water potential

water leaves cell via osmosis and cell shrinks

plant cell in higher water potential (pure water)

water enters cell and cell becomes turgid( vacuole fills up). doesn’t burst as there is a cell wall

plant cell in lower water potential

vacuole shrinks and the cell membrane pulls away from the cell wall

what is plasmolysis

when the cell membrane pulls away from all the water leaving the cell

Hypertonic means

low water potential

isotonic means

same water potential

hypotonic means

high water potential

definition of direct active transport

carrier proteins in the membrane move molecules or ions from a region of low concentration to an area of higher concentration using ATP (against concentration gradient)

active transport

is not a passive process- using energy from hydrolysed ATP (metabolic process)

how does direct active transport work?

1) molecule/ions attaches to receptor site of the carrier protein (side of membrane of lower concentration)

2) molecule of ATP binds to carrier protein

3) ATP molecule undergoes hydrolysis producing phosphate and ADP

4) Phosphate attaches to carrier protein and causes it to change shape ( Carrier protein transports molecule/ion)

5) Phosphate leaves - original shape

6) Phosphate recombines with ADP during respiration to form ATP

where is the ileum

small intestine ( absorbs molecules produced by digestion) → lumen → epithethial cells → bloodstream

after digestion there is

a high concentration of glucose in the lumen of the ileum which facilitated diffuses using carrier proteins into the epithelial cells and again into the bloodstream and is carried away

as glucose is diffused into the epithelial cells

the glucose concentration decrease so facilitated diffusion cant absorb all glucose

how does cotransport work?

1)sodium ions are actively transported out of epithethial cells by the sodium potassium pump using a carrier protein of the cell surface membrane

2) this creates a high concentration gradient of higher sodium ions in the lumen than the EC

3) Sodium ions diffuse through proteins (co-transport protein) using the concentration gradient. As sodium ions diffuse across, they carry a glucose molecule with them against the concentration gradient (sodium glucose cotransporter)

4) The glucose transports into the blood stream using facilitated diffusion and is quickly taken away

why do the epithelial cells have lots of mitochondria

uses ATP in the sodium potassium pump

the membrane of the epithelial cells are

folded into microvilli which increases surface area so more space for membrane proteins

why are glucose molecules absorbed rapidly into the blood

maintain a steep concentration gradient