Cell Membranes and the Movement of Substances Flashcards
Phospholipids
THREE components:
- Two fatty acids
- Glycerol
- Phosphate group
Contain both a hydrophobic and hydrophilic region:
- Fatty acids ~ HYDROPHOBIC (non-polar)
- Phosphate group ~ HYDROPHILIC (polar)
Phospholipid bilayer
When phospholipids are placed in water:
- Hydrophilic part positions itself so it can interact with water
- Hydrophobic parts are buried in the centre, away from any water.
This is known as the phospholipid bilayer and is the BASIS of all cell membranes.
Includes:
- Cell-surface membranes
- Membranes that surround ORGANELLES e.g lysosomes & mitochondria.
Function of phospholipids
Act as a BARRIER to most substances, helping control what enters and exits the cell.
- What substances can pass through depends on several factors which determine their ability to diffuse across.
SMALL & NON-POLAR:
- Oxygen and carbon dioxide
- Rapidly diffuse across a membrane
SMALL & POLAR:
- Water and urea
- Diffuse across , but much more slowly , usually transported by channel proteins in facilitated diffusion.
CHARGE PARTICLES:
- Ions
- Cannot pass through the phospholipid
- Need to pass through a channel or carrier protein
LIPID SOLUBLE MOLECULES:
- Some vitamins
- Dissolve in the bilayer and pass through.
FUNCTIONS of the cell surface membrane
COMPARTMENTALISATION ~ Separates the internal contents of the cell from its external environment.
Also SEPARATES:
- the contents of an organelle from the cytoplasm.
- one part of an organelle from another part.
- Site of CHEMICAL REACTIONS
CELL SIGNALLING ~ communication between cells.
Cholesterol
STRUCTURE:
- Polar hydrophilic group at one end
- The rest of the molecule is non-polar and
hydrophobic.
INCREASES STRENGTH & STABILITY:
- The hydrophilic group attracts the polar head groups on the phospholipid molecules.
- The hydrophobic part attracts the non-polar fatty acids in the phospholipids.
REDUCES SIDEWAYS MOVEMENT OF PHOSPHOLIPIDS:
- Helps to control the fluidity of the membrane.
- Prevents the membrane from becoming:
- Too FLUID. under warm conditions
- Too RIGID under cool conditions
Why is the structure of the cell surface membrane called the :
FLUID MOSAIC MODEL
FLUID:
- The phospholipid molecules can move around within each layer.
- This means the membrane is flexible and can change shape.
MOSAIC:
- The membrane is studded with protein molecules.
- The arrangement of these proteins varies just like the tiles in a mosaic.
Integral / intrinsic membrane proteins
Fully EMBEDDED in the membrane from one side to the other.
Because they pass right through the lipid bilayer:
-they have hydrophobic amino acids on the outside surface of the protein.
- These can INTERACT with the hydrophobic fatty acid tails in the phospholipid bilayer
Examples:
- protein channels
- carrier proteins
Protein channels
- Has a CENTRAL PORE lined with HYDROPHILIC amino acids and contains water.
- Hydrophilic substances can pass through the channel, from one side of the membrane to the other.
- SELECTIVE for the chemical that can pass through.
- Some are ALWAYS OPEN
- Others open in response to a certain TRIGGER:
For Example:
- A chemical binding to the protein channel e.g neurotransmitter
- A change n the VOLTAGE across the membrane.
- INTRINSIC PROTEIN involved in FACILITATED DIFFUSION.
Carrier proteins
- Have a BINDING SITE for a SPECIFIC CHEMICAL.
- When that chemical binds, the TERTIARY STRUCTURE of the carrier protein CHANGES
- This brings the chemical ACROSS the membrane, where the chemical is now RELEASED
- INTRINSIC PROTEIN involved in FACILITATED DIFFUSION
Extrinsic proteins
- DO NOT span the membrane.
- Found on one side of the membrane OR the other.
- Can be attached to INTRINSIC proteins.
ROLES:
- Structural role
- Enzymes
- Receptors for other molecules such as
hormones
Glycoproteins
A PROTEIN attached to a CARBOHYDRATE molecule.
- Allow cells to ATTACH to each other to form TISSUES e.g nervous tissue
- Play a role in the IMMUNE SYSTEM ~ presenting antigens to T cells.
- Receptors for hormones
Glycolipids
CARBOHYDRATE attached to a PHOSPHOLIPID molecule.
- The glycolipids on the surface of one cell can be RECOGNISED by another cell.
- This can determine where cells come into CONTACT.
- Act as ANTIGENS e.g determining blood group.
Describe the FUNCTIONS of GLYCOPROTEINS & GLYCOLIPIDS in the cell surface membrane (5)
- Help stabilise the membrane by forming HYDROGEN BONDS with the surrounding water molecules.
- Act as RECEPTORS for messenger molecules in CELL SIGNALLING.
- Act as a site for drugs, hormones and antibodies to BIND.
- Act as ANTIGENS for CELL RECOGNITION and IMMUNE response.
Diffusion
The NET (or overall) movement of particles from a region of HIGHER concentration to a region of LOWER concentration ( down the concentration gradient)
PASSIVE PROCESS ~ does not require metabolic energy ( energy released by respiration).
The ACTION of diffusion in and out of a cell using CARBON DIOXIDE as an example.
- HIGH amount of CO2 particles inside the cell and LOW amounts outside the cell.
- This means we have a CONCENTRATION GRADIENT for CO2.
- The CO2 molecules will diffuse from the region with a higher concentration ( inside) to the region with a lower concentration (outside).
- Eventually , the concentration of CO2 will be the SAME both inside and outside.
- This means EQUILBRIUM is reached and there is now now NO NET movement of particles so diffusion has STOPPED.
Factors that influence the RATE of diffusion in and out of cells
CONCENTRATION GRADIENT:
- The greater the concentration gradient ,
the greater the rate of diffusion.
PARTICLE CHARGE:
- the cell membrane contains a hydrophobic core.
- therefore, charge particles such as ions WILL NOT be able to diffuse through the membrane
- WATER as an exception although polar as it is very small.
- uncharged molecules such as oxygen CAN diffuse rapidly across the membrane
PARTICLE SIZE:
- Generally, smaller particles will diffuse faster than larger ones.
TEMPERATURE:
- In warmer conditions , particles have more KINETIC ENERGY and diffuse faster than in colder conditions.
- not an issue in MAMMALS & BIRDS as they maintain a constant body temperature.
SURFACE AREA:
- Diffusion takes place more rapidly of the membrane has a LARGER SA compared to a smaller SA.
DISTANCE:
- The GREATER the diffusion distance, the SLOWER the rate of diffusion.
- Why membranes are extremely THIN.
CHANNELS / CARRIER PROTEINS:
- Facilitated diffusion.
Why do different substances diffuse across the cell membrane at different RATES?
What does this mean for hydrophobic & hydrophilic substances?
The centre of the cell membrane is HYDROPHOBIC due to the fatty acid tails of the phospholipid bilayer.
HYDROPHOBIC:
- e.g steroid hormones
- diffuse rapidly across the membrane
HYDROPHILIC:
- e.g ions & polar molecules
- cannot diffuse across the membrane
- Exception ~ WATER ( very small)
FACILITATED diffusion
- Hydrophilic substances can diffuse across the cell membrane via PROTEIN molecules.
- Without interacting with the HYDROPHOBIC CENTRE of the phospholipid bilayer.
Two types of molecules involved :
- Carrier proteins
- Protein channels
( both are INTRINSIC proteins)
Active transport
CARRIER PROTEINS in the membrane transport a chemical from a region of LOWER concentration to a region of HIGHER concentration.
- substances are moved AGAINST the concentration gradient
- Requires METABLIC ENERGY ~ provided by
ATP.
How does active transport work?
- The molecule or ion attaches to a RECEPTOR SITE on the carrier protein
- Takes place on the side of the cell membrane where the chemical is at a LOWER concentration.
- A molecule of ATP binds to carrier protein, which undergoes HYDROLYSIS , producing PHOSPHATE + ADP.
- The PHOSPHATE attaches to the carrier protein causing it to CHANGE SHAPE.
- This cause the carrier protein to TRANSPORT the molecule or ion from one side of the membrane to another.
KEY POINTS on active transport
- Uses a lot of ATP ~ often find a lot of MITOCHONDRIA in cells which carry out a lot of active transport.
- The carrier proteins used in active transport are SPECIFIC ~ only carry one type of molecule or ion.
EXAMPLES of active transport
- uptake of GLUCOSE & AMINO ACIDS in the small intestine.
- Absorption of MINERAL IONS by PLANT ROOTS.
- Excretion of H+ ions & UREA by kidneys
- Exchange of Na+ and K+ ions in neurones and muscle cells
Osmosis
- The MOVEMENT of WATER molecules
- From a region of HIGHER WATER POTENTIAL to a region of LOWER WATER POTENTIAL
- Through a PARTIALLY PERMEABLE MEMBRANE
- PASSIVE process ~ does not require any metabolic energy
Water potential
- The PRESSURE applied to the MEMEBRANE caused by WATER molecules COLLIDING with the membrane
- Unit ~ KILOPASCALS
Higher concentration of water molecules:
- Greater pressure due to water molecules
- Greater water potential
Equilibrium ~ osmosis
- BOTH sides of the membrane have the SAME water potential.
- Osmosis STOPS
- Water molecules will still be moving across the membrane , but the same number of water molecules will be moving in each direction.
- The NET or OVERALL MOVEMENT of water molecules will be ZERO
Pure water Vs concentrated solution
PURE WATER:
- Highest water potential ~ 0KPa
- no solute dissolved
DILUTE SOLUTION:
- Lower water potential ~ -10KPa
- small amount of solute dissolved
CONCENTRATED SOLUTION:
- Very low water potential ~ -500KPa
- Large amount of solute dissolved
Effects of osmosis on animal cells
- CYTOPLASM contains a number of solutes dissolved in water meaning it has a LOW water potential
Example ~ RED BLOOD CELLS
- Are surrounded by blood plasma that has the SAME water potential as the cytoplasm
- Osmosis is not taking place ~ no overall movement of water molecules
What happens when a red blood cell is placed in PURE WATER with a water potential of 0KPa?
Water potential outside > Water potential inside
- Water moves IN to the cell by OSMOSIS
- The HYDROSTATIC PRESSURE inside the cell increases
- The cell membrane is not strong enough to withstand a SIGNIFICANT INCREASE in hydrostatic pressure
Enough water molecules move inside :
- Cell membrane ~ RUPTURES
- Cell ~ BURSTS —— CYTOLYSIS
What happens when a red blood cell is placed in GLUCOSE SOLUTION with a lower water potential?
Water Potential outside < Water potential inside
- Water moves OUT of the cell by osmosis
- The cell will SHRINK ~ CRENATED
Plant cell components & control of environment
- Cellulose cell wall ~ strong
Permanent vacuole :
- filled with as solution of sugars and mineral ions
- give the interior a relatively LOW water potential
- Protoplast ~ between cell wall and vacuole which consists of the cytoplasm and the organelles
- Unlike animals, plants CANNOT control the environment around their cells
What happens if a plant cell is placed in PURE WATER with a water potential of 0KPa?
Water potential outside > Water potential inside
- Water ENTERS by OSMOSIS
- The HYDROSTATIC PRESSURE inside INCREASES
- The PROTOPLAST pushes OUTWARDS against the cell wall
- Cell becomes TURGID
- Internal pressure is known as TURGOR PRESSURE ~ at a certain point prevents any more water entering by osmosis
What happens if a plane cell is placed in a solution with a LOWER water potential?
- Water moves OUT of the cell by OSMOSIS
- The PROTOPLAST PULLS AWAY from the cell wall
- PLASMOLYSIS ~ the plant cell is PLASMOLYSED ~ FLACCID
- The space between the cell membrane and the cell wall is now filled with the solution which surrounds the plant cell.
Bulk transport
- The transfer of LARGE SUBSTANCES across a membrane
- The particles are too large to cross the cell membrane by facilitated diffusion or active transport.
TWO TYPES:
- Endocytosis
- Exocytosis
- Both required energy provided by ATP
Endocytosis
- The BULK TRANSPORT of material INTO the cell. e.g bacteria or antigens
PROCESS:
- The cell membrane FOLDS INWARDS to form a CAVITY around the particles ~ INVAGINATION
- The membrane completely ENCIRCLES the particles to form a VESICLE.
- Vesicle moves INTO the cell.
What happens next, depends on material inside vesicle:
- BACTERIA ~ digested in lysosomes
- OTHER MATERIAL ~ delivered to different parts of the cell
TWO types of endocytosis
PHAGOCYTOSIS :
- When SOLID materials are taken into cells
- Example ~ bacteria
PINOCYTOSIS:
- When FLUID is transferred into cells
- Example ~ liquid surrounding the cell
Exocytosis
- The BULK TRANSPORT of material OUT of the cell.
- Example ~ SECRETED proteins such as hormones and certain enzymes
PROCESS:
- Proteins are found in the GOLGI APPARATUS where they are modified
- Vesicles containing the protein PINCH OFF the golgi apparatus
- Make their way to the cell membrane
- The vesicles FUSES with the cell membrane and the protein is SECRETED.
