topic 4 Flashcards
Why is transport needed in cells?
- cells require supply of chemicals (glucose + oxygen for cellular respiration = must be transported from outside the organism into the cell.
- waste products such as CO2 must be transported out of the cell before it causes damage.
- cells need to transport substances made in one area to another or out of cell completely
The fluid mosaic model of the cell membrane
cell surface membrane controls transport of materials in/out of cells
- Fluid - phospholipid molecules can move around within each layer freely = membrane is flexible and can change shape.
- Mosaic - protein molecules are scattered around the membrane
Phospholipid bilayer with hydrophobic polar tails facing inwards on themselves and hydrophilic heads facing out.
Glycoproteins ( proteins with carbohydrate added)
- act as antigens on outer surface acting as receptors and important for cell recognition
Integral proteins
- can form pores / channels = allow specific molecules to move through (gated channels open + shut depending on conditions of cell) or simple gaps in lipid bilayer that allow ionic substances to move through in both direction
Peripheral proteins (can be enzymes)
- Form temporary bonds with the cell membrane, allowing them to detach + reattach at specific times - involved in regulating transport by eg cell signalling.
2 Main types of transport in cells?
Passive transport - involves no energy from cell, takes place due to concentration / pressure
Active transport - involves moving substances in or out cell up a conc gradient using ATP produced from cellular respiration.
3 Passive transport mechanisms?
-
Diffusion - movement of particles in liquid/gas down a concentration gradient from area of high conc to low conc as a result of random movements, until they reach a uniform distribution.
[after that equal numbers of particles move in all directions= doesn’t change conc]
- the phospholipids in bilayer just simply move apart to allow it pass through - Facilitated diffusion - diffusion that takes place through carrier proteins or protein channels
-
Osmosis - movement of water molecules down a conc gradient through a partially permeable membrane.
- water passes through channel protein to avoid the hydrophobic centre of phospholipid bilayer
3 Active transport mechanisms
-
Endocytosis - movement of large molecules into cells through vesicle formations.
[cell extensions (pili) engulf material to form a vesicle = which enters the cytoplasm] -
Exocytosis - movement of large molecule out of cells by the fusing of a vesicle containing the molecule with the surface cell membrane.
[Vesicles fuse with cell membrane to release the contents from cell] - Active transport - Movement of substances across membrane of cells directly using ATP (often up a conc gradient)
Facilitated diffusion through gated channels and carrier proteins
which type of substances use this?
- gated channels - open only when a specific molecule is present or there is an electrical change across membrane, and then close afterwards.
- Carrier proteins are specific for particular molecules, depending on the shape of the protein and the substance being carried.
- once the protein carrier of specific shape picks up substance from area of high conc, it changes shape
=allowing the molecules to get passed into the cell/area of low conc
- protein carrier then returns to its original shape - to allow more molecules to enter and then repeat.
it can only work when conc gradient is in the right direction as it does not use ATP (its passive)
Used by charged substances (eg glucose) because the cell membrane repels them.
Factors that affect rate of diffusion?
- surface area ( large SA = higher rate, as more particles can be exchanges at same time due to larger surface available)
- temperature (higher temp = move faster = higher rate)
- concentration (steeper conc gradient = the faster the particles move = higher rate)
- membrane thickness ( the distance they have to travel = shorter diffusion distance/thinner membrane = faster diffusion)
How might certain properties of a molecule affect how it’s transported?
- solubility = lipid-soluble molecules pass through membranes more easily.
- size = smaller molecules diffuse faster
- charge = charged molecules cannot diffuse by simple diffusion as membrane repels them so have to use facilitated
Osmotic concentration
- isotonic solution?
- hypotonic solution?
- hypertonic solution?
Isotonic solution - osmotic concentration of the solutes in the solution is the same as that in the cells
Hypotonic solution - Osmotic concentration of solutes in solution is LOWER than in cytoplasm of cells.
Hypertonic - Osmotic concentration of solutes in solution is HIGHER than in cytoplasm of cells.
Osmosis in :
-animal cells
-plant cells
animal cells:
- too much water moves out = cell shrivels as concentrated cytoplasm loses its internal structure and the chemical reactions stop working
- too much water moves in = cell burst
plant cells:
- too much water moves out = turgor is lost + cell membrane begins to pull away from cell wall as the protoplasm shrinks = called incipient plasmolysis + vacuole will be reduced
- too much water moves in = cytoplasm swells and presses on cell walls = becomes rigid = turgor = this state supports the stems + leaves of plant
[this is because the pressure of cytoplasm on cell wall is cancelled out by the inwards pressure of the cell wall on the cytoplasm (pressure potential) to stop water ]
in plants they do not swell/burst , only the contents change.
- Water potential?
Water potential - measure of the tendency of water to move by osmosis.
-pure water has highest water potential of 0.
-osmosis occurs from high water potential to low.
- Turgor pressure?
Measure of the inwards pressure exerted by the cell wall on the protoplasm of the cell as cell components expand + press outwards
-this force opposes/stops entry of water by osmosis
- turgor pressure has a positive value
- Osmotic potential?
Measure of the potential of a solution to cause water to move into the cell across a partially permeable membrane from high conc to low conc
-pure water has highest (least negative) osmotic potential
-solution with dissolves solute =lower (more negative)
-the greater solute cocn = the more negative
-osmotic potential has a negative charge
How can water potential be calculated?
Water potential = turgor pressure + osmotic potential
ψ = P + π
usually negative = usually positive + always negative
When turgor pressure is balanced with osmotic potential the cell is …
At turgor
How does active transport work
best example of active transport is sodium pump that actively moves potassium ions into cell and sodium ions out.
- Protein carrier of specific shape picks up glucose molecule of specifc shape
- Protein carrier changes shape to allow glucose molecules into the cell -this requires energy from the hydrolysis of ATP into ADP + P
- Glucose molecules are carried across membrane in protein carrier of matching shape from low conc to high conc
- Glucose molecules released and protein carrier returns passively to original shape to allow more glucose molecules to enter.
Explain the role of ATP in active transport
ATP binds to the carrier protein, providing enough energy for the protein to change shape, which carries the molecule in/out of cell.
Hydrolysis of ATP into ADP + P
How does ATP release energy?
When ADP is phosphorylated to form ATP, this requires energy which is then stored in the molecule.
Therefore when ATP is hydrolysed, the energy stored is released to be used were required.
Evidence for active transport using ATP?
- active transport takes place only in living , respiring cells
- The rate of active transport depends on temp + O2 conc. These also affect rate of respiration and so ATP production as well
- Many cells that are known to carry out active transport, have lots of mitochondria -site of aerobic respiration and ATP production
- Poisons that stop respiration or prevent ATPase from working, also stops active transport.
What is SA : V ratio and how does it affect gas exchange?
The relationship between the SA of an organism and its volume.
the larger the SA is compared to Volume =the more particles can be exchanged at same time = faster gas exchange
Gas exchange in small organisms?
why does it work like that?
For single-celled organisms (eg amoeba) nutrients/o2 can diffuse directly into cell from external environment + waste products directly out.
This works because:
- diffusion distance from outside to inside is very small
- SA:V ration is very large = there is big SA = more substances can diffuse in/out
- metabolic demands are low = don’t regulate own temp/ don’t use much o2 and produce much co2 etc = don’t need gas exchange to happen rapidly
= don’t need specialised gas exchange/ transport systems as diffusion is enough to supply their needs.
Gas exchange in larger organisms
Larger organisms made up of billions of cells = substances need to travel long distance from outside to reach cytoplasm of cells.
Metabolic rate is also higher as they control own body temp and are more active = demand for O2 + food and CO2 + waste produced is much higher than smaller organisms.
= have evolved specialised systems to exchange gases they need in and need to remove.
humans - in lungs
fish - in gills
insects - in tracheal system
plants - in leaves
What features make a gas exchange system effective?
- A large SA : V
- thin layers =minimise diffusion distance
- rich blood supply to maintain a steep concentration gradient
- Moist surfaces to allow gases to dissolve in it
- Permeable surfaces = allow free passage of respiratory gases
Nasal cavity in humans
main entrance for gases into the body
-the lining secretes mucus and is covered in hairs = external air is ‘filtered’ from dust /small particles and pathogens such as bacteria breathed in
-rich blood supply raises temp of air if needed
-moist surfaces increase level of water vapour in air
=all means that air entering lungs has little effect on internal environment.