cell membranes Flashcards
role of membranes at cell surface?
-controls which substances enter or leave cell
-cell communication
-cell recognition
role of membranes within cells?
-form vesicles to transport substances
-compartmentalise cell
-control what enters or leaves organelle
-act as barrier between membrane contents and rest of organelle
-site of chemical reactions
model name to describe arrangement of molecules in membrane?
fluid mosaic model
5 main components of membrane?
-phospholipids
-cholesterol
-proteins
-glycolipids
-glycoproteins
how is the bilayer formed?
hydrophobic/ polar heads face outwards, towards water
hydrophilic/ non-polar tails face inwards, away from water
what substances can’t diffuse through membrane and why?
water soluble substances (e.g. ions or polar molecules)
as the centre of the bilayer is hydrophobic
what substances can enter membrane and why?
fat soluble substances (e.g. fat soluble vitamins) dissolve in the bilayer and pass through membrane
what does cholesterol do?
provides stability to membrane
-at high temperatures, binds to tails, causing them to pack together, making membrane less fluid
-at low temperatures, prevents phospholipids from packing together, making membrane more fluid
two types of co-transporter proteins, and what they transport?
carrier proteins- larger/charged particles
channel proteins-
smaller/ charged particles
what stabilises the membrane and how?
glycoproteins and glycolipids
form hydrogen bonds with surrounding water molecules
what roles do glycoproteins and glycolipids have?
-receptors for messenger molecules in cell signalling
-site for drugs, antibodies and hormones to bind to
-antigens involved in cell recognition
how do solvents affect membrane permeability?
some solvents (e.g. ethanol) increase permeability by dissolving lipids in membrane
beetroot experiment variables?
I.V.- temperature or solvent concentration
D.V.- absorbance from colorimeter
C.V.- beetroot used, size of beetroot pieces, volume of water in test tube
how do high temperatures (above 45.C) affect membrane permeability?
phospholipid bilayer melts, increasing permeability
co-transporter proteins denature, no control over what enters or leaves
how does low temperature (below 0.C) affect membrane permeability?
little energy, pack closely together, but proteins denature, increasing permeability. Ice crystals increase permeability by piercing membrane
what state is the membrane at, between 0-45.C?
partially permeable
what process does cell communication use?
cell signalling-
cell released a messenger molecule
the messenger molecule binds to receptor
message is detected by new cell, change triggered
how do the messages reach correct cells?
messenger molecules bind to target cells due to complementary shape
how do drugs work in relation to cell membranes?
drugs bind to receptors in cell membrane
to trigger a response or block receptor and stop it from working
diffusion definition?
net movement of particles from an area of high to low concentration
is diffusion active or passive?
passive
4 factors affecting diffusion rate?
-concentration gradient
-thickness of exchange surface
-surface area (larger= faster rate)
-temperature
osmosis definition?
diffusion of water molecules across a partially permeable membrane, down a water potential gradient
(higher to lower water potential)
does adding solutes to pure water increase or decrease the water potential?
decrease (pure water has a water potential of 0)
what is an isotonic solution?
what happens to cells placed in one?
if two solutions have the same water potential, they are isotonic
no net movement of water, so no change
what is a hypotonic solution?
solutions with higher water potential than the inside of the cell
what happens when an animal/ plant cell is placed into a hypotonic solution?
plant- vacuole swells, contents of vacuole and cytoplasm push against cell wall = cell becomes turgid
animal- swell and burst = cytolysis
what is a hypertonic solution?
solutions with lower water potential than the inside of cell
What is facilitated diffusion?
The diffusion of large/ charged particles from an area of high to low concentration, using co transporter proteins
How do carrier proteins work?
A large molecule attaches to the carrier protein
The protein changes shape
The molecule is released on other side of membrane
How do channel proteins work?
Form pores in the membrane for smaller ions/ polar molecules to diffuse through
What is active transport?
The movement of particles against their concentration gradients, active process, uses ATP
How are active transport and facilitated diffusion different/ the same?
Both involve carrier protein transporting molecule
Active transport is against concentration gradient, so requires energy
What is endocytosis?
A cell surrounds substance with a section of plasma membrane
Which pinches off into a vesicle inside the cell, containing ingested substance
Is endocytosis active or passive?
Active
What is exocytosis?
Vesicle containing substance (e.g. hormones) is pinched off from Golgi
Vesicle fuses with plasma membrane and releases contents
Is exocytosis active or passive?
Active
