Movement of cells across cell surface membrane
- Created by: cathymm
- Created on: 29-10-18 13:40
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- Movement of cells across cell surface membrane
- Simple diffusion
- Water soluble substances unable to pass by simple diffusion because of hydrophobic centre of phospholipid bilayer
- Diffusion is net movement of water from a region of high concentration to a region of low concentration- against the concentration gradient
- Diffusion across membrane in biological systems
- Factors affecting diffusion across membrane:
- concentration gradient- greater concentration gradient, faster rate of diffusion
- size of molecule- smaller molecules diffuse faster than large molecules
- Temperature- diffusion takes place faster at higher temperatures as molecules are given more kinetic energy
- Thickness of exchange surface- thinner surfaces give more rapid diffusion
- Surface area of membrane- greater the surface area- faster the diffusion. Microvilli can increase surface area that diffusion takes place
- Factors affecting diffusion across membrane:
- passive process- doesn't require use of metabolic energy
- Facilitated diffusion
- Supported by proteins
- carrier proteins-
- Binding sites for specific molecules
- take in diffusing molecule, change shape and release molecule on other side of membrane- move molecules across membrane.
- Ion (channel) proteins
- channels with a central pore formed by proteins for ions to pass through
- channels permanently open or gated. Gated ions can open or close controlling ion movement
- channels with a central pore formed by proteins for ions to pass through
- carrier proteins-
- affected by number of carrier or channel proteins in the membrane
- transport molecules against the concentration gradient- from high to low concentration
- passive process- doesn't use metabolic energy
- Supported by proteins
- Active Transport
- Molecules move from low to high concentration- against concentration gradient
- metabolic energy required in form of ATP= non passive process
- cells carrying out a lot of active transport have lots of mitochondria which supply ATP energy required
- metabolic energy required in form of ATP= non passive process
- uses protein carriers (pumps)
- substance to be transported binds against protein carrier. Carrier changes shape releasing substance on other side of membrane
- Carrier specific to a type of molecule or ion
- Molecules move from low to high concentration- against concentration gradient
- Cytosis
- Substances transported into and out of cell without passing through membrane
- Transports large molecules too big for carriers
- Transports small molecules e.g. water
- Endocytosis movement of substances into the cell
- cell surface membrane invaginates around substance to be digested forming a vesicle or membrane bound sac. When vesicles taken into cell, fluid nature of cell surface membrane allows it to reform and fill the gap from cytosis
- Phagocytosis= transport of solid material into the cell e,g engulfing bacteria by phagocytes
- Pinocytosis= transport of fluid into the cell
- Exocytosis- movement of substances out of the cell
- Secretory vesicles (e.g. from Golgi body) fuse with cell surface membrane, releasing contents outside the cell. Creation of gap reformed in cell surface membrane after exocytosis
- secretes proteins from cells e.g. digestive enzymes and hormones
- Osmosis
- net movement of water from a high water potential to a low water potential across a selectively permeable membrane (high to low water concentration- dilute to more concentrated solution).
- called net movement a water molecules can move in both directions
- Water moves through selectively permeable phospholipid membrane by osmosis
- Water moves through channel proteins called aquaporins
- Stronger solution (less water) is hypertonic solution to the weaker solution (more water)= hypotonic solution
- If two solutions are of equal concentration called isotonic solutions
- Osmosis is movement of water molecules from a less negative water potential to a more negative water potential across a selectively permeable membrane
- Osmosis in plant cells
- As plant takes in water by osmosis, its contents become less concentrated so solute potential rises. No pressure potential to restrict intake of water so, solute potential= water potential of cell (membrane and wall not in contact)
- As pressure potential becomes positive, hinders water entering cellso water potential of cell and solute potential diverges
- Solute potential still negative at full turgor as there are solutes present in cell.
- Water potential of water is zero as no further water can enter.
- At full turgor, maximum water potential between cell membrane and cell wall
- As cell begins to gain water by osmosis, becoming turgid, the membrane exerts a fore on the cell wall- incipient plasmolysis.
- At point where cell membrane begins to lose contact with cell wall =incipient plsmolysis
- Plants e.g. herbaceous (non-woody)need turgor for support
- Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
- Opposing forces of cell wall and cell membrane create the turgor support
- Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
- Plant tissues with a shortage of water are no longer turgid and are flaccid
- If a large number of cells are flaccid, wilting occurs.
- If a plant cell loses too much water by osmosis, the cell membrane (protoplast) can pull way from the cell wall (except at points where adjacent points are joined by plasmodesmata)- called plasmolysis and cell is plasmolysed
- Protoplast= cytoplasm (and vacuole and surrounding cell membrane
- A plasmolysed cell is unlikely to survive.
- When plasmolysis occurs
- Plants growing in a field with too muhch fertiliser
- A seed from a woodland tree carried to a salt marsh and starting to germinate in its environment
- Plants growing in a field with too muhch fertiliser
- When plasmolysis occurs
- Osmosis in animal cells
- Animal cells don't have a cell wall , so nothing to stop cell membrane until it bursts (lysis)
- Lysis occurs when animal cell is placed in hypotonic solution
- if animal cells placed in hypertonic solution, lose water, shrink and shrivel up (crenation)
- In healthy animals, blood and tissue fluid kept at correct water potential so lysis and crenation don't occur
- Animal cells don't have a cell wall , so nothing to stop cell membrane until it bursts (lysis)
- net movement of water from a high water potential to a low water potential across a selectively permeable membrane (high to low water concentration- dilute to more concentrated solution).
- Water potential
- Tendency to take in pure water by osmosis from pure water across a selectively permeable membrane
- Affected by space available within cell e.g. turgid cell has potential to take in water but no space
- Measured in kilopascals (kPa)
- Pure water has water potential of O kPa- unable to take in any more water by osmosis
- Pure water has water potential of 0 kPa as all water molecules are free- not forming associations with other molecules
- Pure water has water potential of O kPa- unable to take in any more water by osmosis
- indication of free energy of water molecules
- In solutions, not all water molecules are free as they are forming hydration shells around solutes.
- Solutes reduce ability of water molecules to diffuse throughout the solution
- Always have a negative water potential- always have some water in hydration shells.
- More concentrated a solution is, more negative the water potential as less water molecules are free- in hydration shells.
- More likely to take in water by osmosis
- In solutions, not all water molecules are free as they are forming hydration shells around solutes.
- Solute potential
- Potential of a solution to take in water
- potential= solute concentration only
- Potential of a solution to take in water
- Pressure potential
- effect of pressure on a solution
- pressure influences cells ability to take in or lose water by osmosis
- positive greater than or equal to 0kPa
- effect of pressure on a solution
- written using Greek letter psi
- Water potential of cell= solute potential + pressure potenial
- Tendency to take in pure water by osmosis from pure water across a selectively permeable membrane
- Simple diffusion
- Osmosis in plant cells
- As plant takes in water by osmosis, its contents become less concentrated so solute potential rises. No pressure potential to restrict intake of water so, solute potential= water potential of cell (membrane and wall not in contact)
- As pressure potential becomes positive, hinders water entering cellso water potential of cell and solute potential diverges
- Solute potential still negative at full turgor as there are solutes present in cell.
- Water potential of water is zero as no further water can enter.
- At full turgor, maximum water potential between cell membrane and cell wall
- As cell begins to gain water by osmosis, becoming turgid, the membrane exerts a fore on the cell wall- incipient plasmolysis.
- At point where cell membrane begins to lose contact with cell wall =incipient plsmolysis
- Plants e.g. herbaceous (non-woody)need turgor for support
- Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
- Opposing forces of cell wall and cell membrane create the turgor support
- Plant cell wall's strength limits expansion of cell membrane as water enters plant cell by osmosis.
- Plant tissues with a shortage of water are no longer turgid and are flaccid
- If a large number of cells are flaccid, wilting occurs.
- If a plant cell loses too much water by osmosis, the cell membrane (protoplast) can pull way from the cell wall (except at points where adjacent points are joined by plasmodesmata)- called plasmolysis and cell is plasmolysed
- Protoplast= cytoplasm (and vacuole and surrounding cell membrane
- A plasmolysed cell is unlikely to survive.
- When plasmolysis occurs
- Plants growing in a field with too muhch fertiliser
- A seed from a woodland tree carried to a salt marsh and starting to germinate in its environment
- Plants growing in a field with too muhch fertiliser
- When plasmolysis occurs
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