2.5 biological membranes Flashcards
Why is the fluid mosiac model called ‘fluid’?
- phospholipids and proteins can move by diffusion
- phospholipids move side to side
Why is the fluid mosiac model called ‘mosiac’?
- scattered pattern produced by proteins within the phospholipid bilayer
Structure of a phospholipid
- hydrophillic head
- hydrophobic fatty acid tail (non-polar)
- fatty acid tails face inwards creating a barrier against water soluable substances (polar substances)
Role of cholesterol in the fluid mosiac model
regulate fluidity and stability of the membrane
* stops it being too rigid at low temperatures and too fluid at high temperatures
* stops phospholipids packing too close together
Role of glycoproteins and glyolipids in the fluid mosiac model
- forms hydrogen bonds with water to stabilise membrane
- cell adhesion in tissue formation
- receptors for cell signaling
- antigens for cell recognition
Role of extrinsic proteins in the fluid mosiac model
partly embedded in the bi-layer
contain mainly hydrophillic R-groups
What are the key internal membrane functions
- isolation
- comparmentalisation
- control what enters and leaves the cell
- site of chemical reactions
Definition of bulk transport
a form of active transport that requires ATP but doen’t require a concentration gradient
Definition of endocytosis
bulk transport into cells
2 types: phagocytosis (solid) and pinocytosis (liquid)
Function of the phospholipids in the bi-layer
to create a partially permeable membrane
Defintion for exocytosis
bulk transport out of the cell
Process of exocytosis
- transport out of the vesicles from the golgi appartus
- move towards and fuse with the cell surface membrane
- the contents are released outside of the cells
Process of endocytosis
- cell membrane invaginates (bend inwards) when in contact with the substance to be transported
- membrane enfolds until the membrane fuses to make a vesicle
- this pinches off and moves to the cytoplasm for further transfer ( moved by cytoskeleton )
Role of the cytoskeleton
- changes cell shape to engulf materials
- movement of secertory vesicles
- fusion with the cell membrane
Definition of cell signaling
communication between cells to trigger a reaction within the target cell
How do glycoproteins act as a receptor
specific shape which is complementary to the shape of the signalling molecule
Definition of diffusion
net movement of a substance from a region of high concentration to a region of low concentration
- caused by natural kinetic energy
What can pass through the PL bi-layer
- oxygen/carbon dioxide (small)
- lipid soluable molecules (alcohol, steriod hormones)
- water
What cannot pass through the PL bi-layer
- ions (charged - so they are repelled against non-polar fatty acid tails)
- polar/ large molecules (too large)
Definition of faciliated diffusion
net movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels
Functions of a channel protein
transport charged substances (ions)
acts as gates so part of the inside surface of the membrane can moves to close or open the pores
Function of carrier proteins
can change between two shapes
creates a binding site to be open
the direction of movement depends of the concentrations
Factors affecting diffusion
- concentration gradient
- temperature
- surface area
- distance
- membrane
- size of diffusing molecule
How does the concentration gradient affect diffusion
the steeper the concentration gradient the faster the rate of diffusion
How does temperature affect diffusion
the higher the temperature the more kinetic energy the particles will have to move around
it will occur faster
How does surface are affect diffusion
the larger surface area : volume ratio of the membrane the more particles can diffuse at once
it will occur faster
How does distance affect diffusion
the shorter the distance or the thinner the membrane the quicker the rate of diffusion
How does membranes affect diffusion
the greater the number of protein channels or carrier proteins present the higehr the rate of diffusion
How does the size of the diffusing molecule affect diffusion
the smaller the ions / molecule diffuses more rapidly than larger molecules
Definition of active transport
net movement of substances against the concentration gradient ( low to high ) across a cell membrane, using ATP and carrier proteins
energy is from hydrolosis of ATP
Defintion of osmosis
net movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane
Definition of water potential
the pressure exerted by water molecules as they collide with the membrane
What is the highest water potential
0
Definition of isotonic
no overall net movement
Definition of hypertonic
high solute concentration
low water concentration
Definition of hypotonic
low solute concentration
high water concentration
What happens in an animal cell if it is hypertonic
water leaves the cell so it crenates (shrivels)
What happens to a plant cell if its hypertonic
water leaves and the cell membrane becomes detached
they are plasmolysed
What happens to an animal cell if its isotonic
water goes in and out of the cell
becomes balanced (equilibrium)
What happens in a plant cell if its isotonic
flaccid
What happens in an animal cell if its hypotonic
it lyses (splits open) due to hydrostatic pressure (occurs in a closed system)
What happens in a plant cell if its hypotonic
water goes in and the cell becomes turgid
the pressure potential prevents any further entry of water
What is the water potential of a cell and why
negative water potential because it has dissolved solutes in their cytoplasm
How does temperture affect the permeability of the PL bi-layer
- PL are constantly moving
- the PL gain more kinetic energy meaning the membrane becomes more fluid and gaps begin to form between the PL
- if too fluid it will loose control of the entry and exit of substances and protein channels will denature
How does solvents affect membrane permeability of the PL bi-layer
organic solvents (ethanol or acetone) dissolve the membrane as they are lipid soluable
this disrupts membrane permeability
