3.3 - Transport across the plasma membrane Flashcards
What is intracellular fluid?
- Fluid contained inside body cells
- 2/3 total fluid volume
- Cytosol
What is extracellular fluid? Give 4 examples
- Fluid outside body cells
- Interstitial fluid - between cells
- Plasma - in blood vessels
- lymph - in lymph vessels
- CSF - surrounds the brain and spinal cord
Describe the concentration gradient
- Difference in concentration between two areas.
- physiologically: difference in concentration between the ECF and ICF
- Solutes can either move along or against the concentration gradient
- Along: from high to low
- Against: from low to high
Describe the difference between a passive and active transport process
Passive: along the concentration gradient
Active: Against the concentration gradient. Requires ATP
Describe simple diffusion
- Diffusion of membrane permeable substances
- Directly across the lipid bilayer
Give examples of substances that move via simple diffusion
- Gases: O2, CO2, Nitrogen gases
- Lipid soluble compounds: Fatty acids, Steroids, Vitamins A D E K
- small polar molecules: Water, Urea
Describe facilitated diffusion
- Movement of substances that are membrane impermeable
- Requires channel or carrier proteins
What are the different types of transport proteins
- Ion channels: Movement of Ions, K+, Na+, Ca++, (mostly potassium)
- Carrier proteins: Larger polar molecules, sugars, polar vitamins
- Pump proteins: For active transport
Describe gated channel proteins
- Can be opened and closed by changing structure
- Signalling molecules bind to the channel protein to cause the change in shape
- Important in the generation in action potentials
Describe how carrier proteins work
- Substrate binds to a site on one side of the carrier protein
- Binding of the substrate causes the carrier protein to change shape
- Substrate moves across the membrane
- Substrate releases from the protein
- Protein reverts back to its original shape
Describe Osmosis
- Passive movement of water across a membrane
- Against the solute concentration gradient - low to high SOLUTE concentration
- Subject to osmotic pressure
Describe osmotic pressure
- Pressure required to stop fluid movement across a membrane
- Osmotic pressure of cytosol and interstitial fluid are the same
- Cell volume remains constant
Describe isotonicity, hypotonicity and hypertonicity
Isotonicity: external and internal osmotic pressure are the same. No net movement of water
Hypotonicity: lower extracellular solute concentration. Water enters the cell. Cell swells.
Hypertonicity: higher extracellular solute concentration. Water leaves the cell. Cell shrinks.
Describe active transport
- Transport of substances against the concentration gradient
- Requires ATP. ATP is cleaved into ADP + Pi
- ATP used to change the shape of a pump protein
- 40% of cellular ATP used for active transport
Give examples of ions transported using active transport
K+, Na+, H+, Ca2+, I-, Cl-
What is the sodium-potassium pump?
- Most important active transport pump
- Brings in potassium, expels sodium
- Acts as an enzyme to cleave ATP into ADP+Pi
- Maintains a low cytosol concentration of sodium and a high cytosol concentration of potassium
- Pump acts continuously: Na+ and K+ leak back across the membrane
- Vital in maintaining cell osmolality
- Describe the action of the Na/K pump
- 3 Na+ bind to the Na/K pump.
- ATP is cleaved and changes the shape of the pump. 3 Na+ expelled
- 2 K+ bind to the protein. Pi released
- Protein changes to it’s original shape. 2 K+ imported into the cell
Describe the types of vesicular transport
- Intracellular transport between organelles
- Endocytosis: movement of substances into a cell
- Exocytosis: movement of substance out of a cell
Describe endocytosis
- Substances are surrounded by a section of membrane
- Membrane section buds off inside the cell to form a vesicle
- Vesicle is inside the cell
Describe phagocytosis
- Important part of the immune response
- Phagocytes engulf large structures for digestion
1. Particle binds to receptors on the cell surface
2. Pseudopods extend from the membrane and surround the particle
3. Pseudopods fuse and release a phagosome into the cell
4. Phagosome fuses with a lysosome
5. Lysosomal enzymes digest the particle. Undigested material remains in a residual body
Describe bulk-phase endocytosis
- Cells take up small droplets of extracellular fluid
1. Plasma membrane folds inward and forms a vesicle
2. Vesicle enters the cell and fuses with a lysosome
3. Solutes are digested by the lysosome
4. Digested products exit the lysosome to be used by the cell
Describe exocytosis
- Vesicles of cellular products fuse to the plasma membrane
- Cellular products are expelled from the cell
- Secretion
Describe the two most important examples of exocytosis
- In secretory cells: liberate digestive enzymes, hormones, mucus etc.
- In nerve cells: Release of neurotransmitters at the synapse
