4.2 Cell transport mechanisms Flashcards
What is the purpose of a cell surface membrane?
- Maintains integrity of cell.
- Barrier for substances entering + leaving cell.
What is the cell surface membrane made almost entirely of?
- Protein + lipid.
- Small + variable amount of carbohydrate.
What is a model to show a model of the molecular structure of the cell surface membrane called?
- Fluid mosaic model.
Why is it called a fluid mosaic model?
- Mosaic - proteins are scattered.
- Fluid - components are able to move past each other in a linear plane.
What are the components of a cell surface membrane (6)?
- Lipid bilayer.
- Integral proteins - embedded in lipid bilayer (carrier + channel proteins).
- Cholesterol.
- Glycoproteins.
- Glycolipids.
- Peripheral proteins ( attached to surface of lipid bilayer).
What is important to remember about energy transfer in cells?
- We cannot create or destroy energy.
What does ATP stand for?
- Adenosine triphosphate.
What type of molecule is ATP?
- A nucleotide.
What are the components of ATP?
- Adenine.
- Ribose.
- 3 Phophates.
What is ATP formed from?
- ADP and phosphate ion (Pi).
Why is ATP referred to as an ‘energy currency’?
- Can be used for many purposes + is constantly recycled.
Why is ATP special?
- Reservoir of stored chemical energy.
- Common intermediate between energy-yielding + energy-requiring reactions.
What are some examples of energy-requiring reactions?
- Synthesis of cellulose from glucose.
- Synthesis of proteins from amino acids.
- Contraction of muscle fibres.
How is ATP converted to ADP?
- Hydrolysis of ATP leads to ADP + phosphate.
What does ATP mostly react with?
- Other metabolites + forms of phosphorylated intermediates, making them more reactive.
What are 3 important features of ATP?
- Move easily within cells by facilitated diffusion.
- Involved in cellular respiration + many reactions of metabolism.
- Transfer energy in relatively small amounts, sufficient for individual reactions.
What substances get moved across cell surface membranes (5)?
- Water, respiratory gases, nutrients, ions + excretory products.
What other processes happen across cell surface membranes?
- Receptors - recognition of hormones, antigens + cells.
- Secretion - neurotransmitter substance + hormones.
- Enzymes secreted.
What is diffusion?
- Free passage of molecules (+ atoms + ions) from a region of their high concentration to a region of low concentration until they are evenly distributed.
What is kinetic energy?
- Energy possessed by a particle because it is in continuous motion.
When does diffusion occur across cell surface membranes?
- Fully permeable to solute - lipid bilayer permeable to non-polar substances.
- Pores in membrane - channel proteins + tiny spaces between phospholipid molecules.
Why is facilitated diffusion used?
- For substances otherwise unable to move across cell surface membrane.
How does facilitated diffusion work?
- Molecules of globular proteins that form pores of channels.
Where does the energy for facilitated diffusion come from?
- Kinetic energy from all molecules involved.
- Energy from metabolism is not required.
What is an important example of facilitated diffusion?
- Movement of ADP into + out of mitochondria.
What is the meaning of osmosis?
- Net movement of water molecules from a region of high conc. of water molecules to an area of lower conc. of water molecules, across a partially permeable membrane.
When is the movement of water molecules resticted?
- Water molecules held together by hydrogen bonds.
- Organic substances (sugars, AA, polypeptides, proteins + charged inorganic ions) have this effect on water, restricting random movement.
When are water molecules the most stationary in solution?
- When the solution is stronger - More solute dissolved per volume of water.
Do both animal + plant cells experience lysis?
- Animal cell - burst when too much water by osmosis.
- Plant cell - strong cell wall, only expands a little.
What is turgor pressure (P)?
- Pressure inside plant cells caused by water entering cell.
What does it mean when a plant cell is fully turgid?
- When turgor pressure inside is so high that it prevents further entry of water by osmosis.
What is a plasmalemma?
- Cell surface membrane surrounding outside of cytoplasm in plant + animal cells.
What is plasmolysis?
- This occurs as water leaves plant cell by osmosis, causing cytoplasm to shrink away from contact w cell wall.
What does is mean when a plant cell is flaccid?
- When water is withdrawn by osmosis + plant cells loose their firmness.
What are the symbol and units for water potential?
- Ψ.
- Kilopascals (kPa).
What is water potential?
- Measure of tendency for water to pass from one place to another.
- Tendency of water to move in or out of a cell.
How do we know that the water potential of pure water is zero?
- Water moves from dilute solution to conc. solution.
- Making solution more conc. means water molecules less free to move.
- No dissolved solutes, water most free to move.
- Pure water has highest water potential.
- Any other solution must have lower Ψ than water ∴ must be -ve.
- ∴ means that Ψ of water must be 0.
What is the Ψ of more concentrated solutions?
- More conc. solutions = more -ve Ψ.
What is osmotic potential (π)?
- Increased Ψ of a solution caused by solutes dissolved in it.
- Sometimes referred to as solute potential, it will always be -ve.
What is the word equation for water potential?
- Water potential = turgor pressure + osmotic potential.
What is the symbol equation for water potential?
- Ψ = P + π.
What is active transport?
- Movement of substances across a cell surface membrane against a conc. grad. using energy in form of ATP.
What direction of a conc. gradient does active transport?
- Can go against conc. gradient.
- Conc outside cell is less than inside cell.
Is active uptake selective or not?
- Highly selective.
- K+ and Cl- ions available to animal cell, K+ ions more likely to be absorbed.
- Na+ and NO3- ions available to plant cell, NO3- ions absorbed more rapidly.
- Important in ensuring needs of cell are met.
What special molecules in membranes are involved in active transport?
- Carrier proteins.
- Movements by carrier proteins require reaction w/ ATP.
What is an example of two way active transport?
- Na+ and K+ molecules.
What are gated ion channels?
- Ion channels in carrier proteins have controlled closing + opening.
What are voltage-gated ion channels?
- Controlled by small potential differences.
What are ligand-gated ion channels?
- Sensitive to chemical signals.
What are the steps of active transport of a single substance?
- Molecule enters carrier protein in lipid bilayer.
- Carrier protein activated by reaction w/ ATP.
- Molecule released along w/ ADP + Pi.
- Change in shape + position of carrier protein back to receptive shape.
How does the sodium-potassium ion pump work?
- Carrier protein activated by reaction w/ ATP.
- Na+ ions enter carrier protein.
- Change in shape + position of carrier protein.
- ADP released + Na+ ions released + K+ ions loaded.
- K+ ions + Pi released.
What is another mechanism of transport across the cell surface membrane?
- Bulk transport.
What is bulk transport?
- Movement of vesicles of matter across the membrane by process known generally as cytosis.
What are vesicles?
- Membrane-bound cell organelles containing liquid or solid particles.
What is cytosis?
- Bulk transport of materials across membranes contained in vesicles.
What is bulk transport uptake?
- Endocytosis.
What is bulk transport export?
- Exocytosis.
Does bulk transport require ATP?
- yes.
What is phagocytosis?
- Cells (phagocytes) use their membranes to surround external particles to form vesicles within their own cytoplasm.
What are macrophages?
- Large WBCs that engulf cell debris + foreign particles by process of phagocytosis.
What is the difference between phagocytosis + pinocytosis?
- Phagocytosis - uptake of solid particles.
- Pinocytosis - uptake of liquid.
What types of substances are transported in vesicles + why?
- Highly active substances such as enzymes + hormones.
- Releasing these into cytoplasm would cause major disruption.
What are the important feature of particles which determine the way that they are transported?
- Size of particle.
- Solubility of particle.
- The charge present.
How are small non-polar molecules (O2, CO2, N2) transported?
- Direct diffusion.
How are small polar molecules (H2O) transported?
- Facilitated diffusion through channel proteins.
- Sometimes through phospholipid bilayer.
How are large polar molecules (Glucose) transported?
- Facilitated diffusion using specialised carrier proteins.
How are lipid-soluble molecules (Glycerol, fatty acids) transported)?
- Direct diffusion.
How are small charged atoms (Ions) transported?
- Active transport using carrier proteins.
How are large polar molecules (ADP + ATP) transported?
- Move in + out of mitochondria by facilitated diffusion.
