2.5 Cell Membranes Flashcards
Roles of membranes
- Surface of cells (plasma membrane)
- within cells
Surface of cells (Plasma membrane)
- barrier - control of substances entering and leaving- partially permeable
(diffusion, osmosis, active transport) - Communication/ cell Signalling
- recognition (immune system)
Within the cell
- Compartmentalisation
- Surface area for reactions
- vesicles
Fluid Mosaic Model
- Model of cell membrane structure
- Lipid molecules give fluidity
- Proteins give a mosaic appearance
-2 layers of phospholipids and easily flow over each other
Cell Signalling
- Communication between cells using chemical messengers
- Drugs, hormones which travel to other cell and bind to receptor on cell membrane
Cell Membrane Receptors
- Complementary shape to a messenger molecule binds with Receptor
Components of Cell Membranes
- Phospholipids
- Cholesterol
- Proteins
- Glycolipids
- Glycoproteins
Phospholipid bilayer
- form a barrier to dissolved substances
- partially permeable
- Rows of phospholipids naturally arrange themselves into a bilayer
- head is hydrophilic- attracts water
- tail is hydrophobic- it repels water
- hydrophobic centre of bilayer- membrane
Cholesterol
- gives membrane stability
- between phospholipid bilayer
- made of lipid
- controls membrane fluidity
- bind to hydrophobic tails of phospholipids
- causing them to pack more closely together
- making membrance less fluid
- and more rigid
Proteins
- channel and carrier proteins in membrane
- receptors on cell membrane
Channel proteins
- ## form pores in the membrane for charged particles to diffuse through
Carrier proteins
- move large molecules out of Cell
- large molecule attaches to carrier protein in membrane
- protein changes shape
- releases molecule on opposite side of membrane
Receptors in proteins
- receptors cell signalling
- when molecule binds to protein, chemical reaction takes place in cell
Glycolipids
- carbohydrate on a lipid
- stabilise membrane by forming hydrogen bonds with surrounding water molecules
- sites where drugs, hormones and antibodies bind
- receptors for cell signalling
- antigens- involved in immune system
Glycoproteins
- carbohydrate on a protein
- stabilise membrane by forming hydrogen bonds with surrounding water molecules
- sites where drugs, hormones and antibodies bind
- receptors for cell signalling
- antigens- involved in immune system
Temperature
Increasing temperature increases membrane fluidity
- Temperatures below 0°C
- Phospholipids don’t have much energy so they can’t move much
- Packed closely together and the membrane is rigid
- Ice crystals form and pierce membrance making it highly permeable
- Carrier and channel proteins deform increasing permeability
- Temperatures between 0 and 45°C
- Phospholipids can move and aren’t packed as tightly together
- membrane is partially permeable
- as temperature increases, phospholipids move more as they have more energy
- increases permeability of membrane
- Temperatures above 45°C
- phospholipid bilayer starts to break down
- membrane becomes more permeable
- water inside the cell expands, putting pressure on the membrane
- Channel and carrier proteins denature
- increases permeability of membrane
-
Types of transport across cell membranes
- Diffusion
- Osmosis
- Active transport
- Bulk transport
Diffusion
1) the net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient
2) molecules diffuse both ways but net movement will be to area of lower concentration.
- This continues until particles are evenly distributed
- Concentration gradient
- passive
What factors affects the Rate of Diffusion?
-temperature
-
What factors affects the Rate of Diffusion?
- Temperature
- Concentration gradient
- Movement
- Surface area
- Distance
Temperature affecting rate of diffusion
- the warmer
- the faster rate of diffusion
- particles have more kinetic energy so they move faster
Temperature affecting rate of diffusion
- as temperature increases
- the faster rate of diffusion
- particles have more kinetic energy
- they move faster
Surface area affecting rate of diffusion
-
Surface area affecting rate of diffusion
- larger surface area of membrane
- faster rate of diffusuon
Distance affecting rate of diffusion
- The thinner the exchange surface
- The shorter the distance molecules have to travel
- The faster the rate of reaction
Facilitated diffusion
- Diffuse through channel proteins or carrier proteins in cell membrane
- Large molecules, ions and polar molecules don’t diffuse directly through phospholipid bilayer
- Doesn’t use energy- passive
Osmosis
Diffusion of water
-Movement of water from an area of high water potential to an area of low water potential down its water potential gradient
Water potential c (negative)
(Ψ)
kiloPascals (kPa)
Measure of how much water there is
- potential of water molecules to diffuse out of or into a solution
- measure of the tendency of water molecules to diffuse from one region to another
pure water
- highest water potential
- All solutions have a lower water potential than pure water
- water potential of zero kPa
Lower water potential
- Large amount of solute dissolved
-
Hypotonic
- Solution with a higher water potential than the cell
- Net movement of water molecules is into the cell
- in Animal cell, cell bursts (lysis)
- in Plant cell, vacuole swells
- vacuole+cytoplasm push against cell wall
- Cell becomes turgid (swollen)
Isotonic
- Solution with the same water potential as the cell
- water molecules pass into and out of the cell in equal amounts
- the cell stays the same
Hypertonic
- Solution with a lower water potential than the cell
- Net movement of water molecules is out of the cell
- in Animal cell, cell shrinks (shrivel)
- in Plant cell, Cell becomes flaccid (limp)
- cytoplasm and membrane move away from the cell wall
- this is called plasmolysis
Active transport
- Against concentration gradient
- Active - energy used from ATP
- Use carrier and channel proteins
Active transport process
- molecule attaches to Carrier protein
- Protein changes shape
- moves molecule across Membrane
- releasing it on the other side
-energy used to move Solute against Concentration gradient
Bulk transport
- Endocytosis- in
- Exocytosis- out
- Pinocytosis- liquid
- Phagocytosis- solid
Endocytosis
How large particles are brought into cell
Segment of plasma membrane surrounds and encloses particle
-brings it into the cell, enclosed in a vesicle
Exocytosis
- How large particles are exported out of cells
- Substance to be exported out of cell is moved towards and fuses with plasma membrane
- Fused site opens releasing contents of vesicle
