2.3-Transport across membranes

what are the three main functions of plasma membranes?

1) control transport of substances

2) act as a receptor site

3) separate off the cell from the environment

why is the cell wall of plants and prokaryotes thicker than the plasma membrane?

freely permeable- all molecules can move thru it as it contains large pores

what is the membrane like, why does it act as a barrier and what does that suggest?

partially permeable; doesn’t let water-soluble, charged or polar substances pass thru easily

suggests that it contains lipids, as these don’t mix with water

what do phospholipids form in water and what is the structure?

bilayer; hydrophobic tails point towards each other + hydrophillic tails point out into the water

what is the PLB a barrier to, and what does it allow to pass thru?

barrier to polar/charged/water-soluble

allows non-polar/lipid-soluble thru

what do the membrane proteins form?

2 types?

where is it found?

why may the proteins that span the membrane have hydrophillic channels?

fluid mosaic structure

intrinsic (whole membrane) or extrinsic (one layer)

fixed in one place or floating in PLB

allow transport of some substances more easily

what are glycoproteins and what is their function?

short, branching carbohydrate chains are attached to proteins in the membrane

act as receptors for hormones and neurotransmitters or antigen

what are glycolipids and what is their function?

made of carb covalently bonded to lipid

act as recognition sites, help to maintain stability of the membrane and help cells attach to one another, so forming tissues

what is cholesterol and what is its function?

restricts movement of other molecules making up the membrane and increases strength and stability of membranes by making them less flexible

helps prevent loss of water and dissolved ions from the cell

(more cholesterol-less fluid-less permeable the membrane)

why is it fluid?

why is it mosaic?

what can the molecules in the membrane do?

what does this fluidity mean for cells?

all different molecules can MOVE around

different TYPES of molecule- proteins and carbs ‘float around’ within the PLB

move sideways about the membrane, leave the membrane to form vesicles in the cell’s cytoplasm, join the membrane from vesicles in the cytoplasm

cells able to change their shape e.g. phagocytes

what are the two types of membranes?

example of single PLB and double membrane?

within the cell, and surrounding it

single e.g. the cell membrane and those around vesicles, vacuoles and some organelles e.g. ER

double e.g. nucleus

what is simple diffusion?

what is the difference in conc between the two areas called?

is it passive or active?

what happens as a result of diffusion?

movement of substances from a region of higher conc. to a region of their lower conc.

conc. gradient

diffusion- movement down conc. gradient- therefore passive

substances tend to reach a dynamic equilibrium when they are evenly spread, altho still moving

how does the size of diffusing molecule affect diffusion?

temperature?

water solubility/polarity?

smaller the diffusing molecule, the faster the rate of diffusion

higher the temp, the faster the rate of diffusion

non polar molecules diffuse faster than polar molecule

what is fick’s law?

what should be done to increase diffusion rate in relation to these three factors?

rate of diffusion = sa x diff in conc / thickness of exchange surface (or length of diffusion pathway)

maximise sa, maximise conc difference, minimise thickness of exchange surface

which molecules can rapidly diffuse across the PLB?

which molecules is the PLB a barrier to?

non polar molecules as they are lipid soluble

smaller molecules can diffuse thru more easily as they can fit in between PL molecules

oxygen molecules are non-polar and small + diffuse rapidly thru bilayer

barrier to polar molecules- these can pass using intrinsic proteins using faciliated diffusion

some molecules are polar, but are so small they can diffuse rapidly thru the bilayer e.g. water

what is facilitated diffusion?

movement of large polar molecules and ions down their conc gradient (passive) using carrier/channel proteins that are specific to certain molecules

what is osmosis?

movement of water molecules across a partially permeable membrane down a water potential gradient i.e. from higher (less negative) to lower (more negative) water potential

what does WP mean and what is it measured in?

when high water potential mean?

low water potential?

tendency of water to move from one place to another + measured in kPa

high WP- high conc of water + low conc of solute

very dilute solution e.g glucose or pure water

low WP- low conc of water + high conc of solute

animals cells have no cell wall + change in volume and conc when placed in diff solutions

what is a hypertonic solution?

what is a isotonic solution?

why is a hypotonic solution?

conc solution/low WP- conc salt soltuon lower WP than cell, so water leaves cell by osmosis- causes cell to shrink or crenate

same conc and WP as the cell- no net osmotic movement of water- shape + size of cell is the same

dilute solution/high WP- dilute salt solution higher WP than cell, water enters cell by osmosis- causes cell to burst (osmotic lysis)

plant cell walls are prevented from large changes in volume and the cell wall will prevent the cell from taking into too much water + bursting by osmotic lysis. plant cell walls are freely permeable

what is a hypertonic solution?

what is an isotonic solution?

what is a hypotonic solution?

conc solution/low WP- conc. salt solution will have a lower WP than the cell, so water leaves the cell by osmosis- cytoplasm shrinks away from cell wall leaving gaps between cell membrane and cell wall (plasmolysis)

same conc + WP- no net osmotic movement of water

dilute solution/high WP- dilute salt solution will have a higher WP than the cell, so water enters cell by osmosis- cytoplasm gains some water, due to tensile strength of cellulose cell wall, cell won’t burst (turgid)

what is active transport?

the movement of molecules against their conc gradient using energy in the form of ATP (active) using specific carrier proteins

what is endocytosis?

what is exocytosis?

cells use vesicles to transport large amount of materials across their plasma membranes either into or out of the cell

import = endocytosis

export = exocytosis

both processes require ATP

what happens in the co-transport of glucose and amino acids?

1) Na+ pumped out of the cell into the blood by active transport

2) lowers Na+ conc in the cell

3) Na+ can then diffuse from lumen down their conc gradient into cell by co transporter protein, bringing either glucose or amino acids along with it, and are transported into the cell against their conc gradient

4) glucose moves into the blood from the cell by facilitated diffusion