1. Cell membranes and transport
Chapter 4:
Cell membranes and transport

2. Phospholipids Hydrophilic polar heads Hydrophobic non polar tails
Spherical micelles form in water if the tails all face each other
Two layered structures are called bilayers and will form sheets

4. Structure of membranes
Phospholipid bilayer is only visible using an electron microscope with a magnification of at least 100,000x
Shows as a double black line with a width of 7nm
Proteins are spread throughout the membrane and serve various functions
Individual phospholipids and proteins can move about: fluid mosaic model

5. Features of the fluid mosaic model
Bilayer of phospholipids, move about in their own monolayer
Non polar, hydrophobic tails point inward toward each other (inner membrane space)
Polar, hydrophilic heads face the aqueous environment inside the cell and outside
Tails are saturated and some are unsaturated, the more unsaturated, the more fluid the membrane

7. Features of the fluid mosaic model (cont.)
Most proteins float like icebergs, some are fixed like islands to internal structures
Some proteins are embedded in the inner layer, some the outer, some span the entire membrane.
Proteins stay in place due to hydrophilic places and hydrophobic places on their membranes
Glycoproteins and glycolipids are short, branching carbohydrate chains attached to the external surface of the membrane
Cholesterol molecules are also found distributed throughout (adds stability)

8. Roles of components of the cell membranes
Phospholipids: basic structure, barrier to most water soluble substances since it is difficult for ions to pass through them
Cholesterol: philic heads and phobic tails, help regulate fluidity, stability, helps to prevent ions or polar molecules from passing through

9. Roles cont.,
Proteins:
Transport proteins: provide hydrophilic channels for ions and polar molecules to pass through. Each protein channel is specific for a different ion or mc
Enzymes: found on cell surfaces
Involved in processes such as photosynthesis and cell respiration
Glycolipids and glycoproteins: stabilizers, receptor molecules for hormones or neurotransmitters. When initiated, starts a chain of reactions.
Antigens: cell recognition of foreign substances

11. Transport across the plasma membrane
Exchange with the surrounding environment is essential
Diffusion and facilitated diffusion
Osmosis
Active transport
Bulk transport

13. Diffusion
Diffusion: net movement of mc’s from a region of higher concentration to a regions of lower concentration until they are spread out evenly. Called moving down a concentration gradient, natural kinetic energy.
Rate depends on
Steepness of gradient: higher the concentration, faster they spread out
Temperature: increase in temp increases kinetic energy
Surface area: the larger their surface area, the more mc’s or ions can cross it
Nature of mc’s or ions: small mc’s like water (polar), oxygen and carbon dioxide (non polar) slip through unhindered

16. Facilitated diffusion
Large molecules such as glucose, amino acids, Na+ or Cl-.
Must pass through hydrophilic channels created by protein molecules
Protein channels are specific for a substance
Still passive, down a concentration gradient
Rate depends on how many channels there are and if they are open
Cystic fibrosis is caused by a defect in a protein which should be present in the plasma membrane of cells lining the lungs, since it is not there, the channels are not open for Cl- to get out.

18. Osmosis Diffusion involving water molecules only
Solute + solvent = solution. Whereas the solvent is always water
The solvent (water) will cross a semi permeable membrane if the solute (sugars) are more concentrated on the other side of that membrane. Solutes cannot pass through that membrane
Looking for an equilibrium
Water and solute potential = next slide

20. Water potential
What is the likelihood or tendency for water to cross that membrane? Called water potential and it written as the Greek letter psi ψ
Water always moves down a concentration gradient, therefore osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Think of water held back by a dam, but do not confuse with gravity
If A has more solutes than B, then solution A has lower water potential than solution B
Pure water has the highest water potential, it is set at zero – 0., therefore by adding solutes, it will lower the potential and make the potential a negative number.

21. Solute potential
If pure water is set at 0, and if you add solutes, that makes the water potential <0, or a negative number
The more solute, the more negative the number becomes (farther from 0)
Solute potential is the amount that the solute will lower the water potential of a solution
Solute potential is always negative, the symbol is Greek letter ψs

22. Osmosis in animal cells
Important to maintain a constant water potential inside the bodies of animals
Water potential should equal and balance to solute potential (=no net movement = homeostasis)
If the water potential of the solution surrounding the cell is too high, the cell will swell and burst (salt water fish in fresh water) or (red blood cell in pure water) (=turgid)
If it is too low, it will shrivel and shrink (fresh water fish in salt water) or (rbc in a solution with a high concentration of glucose) (=plasmolysis)

23. Pressure potential
The application of a physical force will decrease the amount of osmosis
The greater the pressure applied, the greater the tendency for water to be forced back from solution B to solution A.
Solution B now has greater water potential
Pressure potential makes the water potential less negative and therefore positive

24. Osmosis in plant cells
Turgidity is when a plant cell is fully inflated with water and presses against an inflexible cell wall.
Water potential = solute potential + pressure potential
Plasmolysis: when a plant cell is placed in a sucrose solution (lower water potential) water leaves the plant cell and the protoplast shrinks

26. Active transport
How to accumulate ions against a concentration gradient
Achieved by special transport proteins, each specific
Requires energy, supplied by ATP
ATP allows the carrier protein to change its shape and transfer mc’s across the membrane
The energy consuming transport of mc’s or ions across a membrane against a concentration gradient made possible by transferring energy from respiration
Reabsorption in the kidneys where certain useful molecules and ions have to be reabsorbed into the blood after filtration
Digestion of some products from the gut
Used to load sugar from the photosynthesizing cells of leave into the phloem tissue for transport around the plant
Load inorganic ions from the soil into the root hairs

28. Bulk transport
Endocytosis: engulfing larger material by the plasma membrane to form a small sac that will pinch off and enter the cell
Phagocytosis: cell eating, wbc’s eating infectious bacteria, brought to the lysosome for total destruction.
Pinocytosis: cell drinking, the follicle of the human egg
Exocytosis: removal, secretion of digestive enzymes from cells in the pancreas. Plants getting their cell wall building materials to the outside of the plasma membrane

31. Gaseous exchange in mammalian lungs
Most cells of multi cellular organisms are a great distant from the external environment from which oxygen is obtained
They have a specialized gaseous exchange surface where oxygen from the external environment can diffuse into the body and the carbon dioxide out
Human: surface is alveoli, tiny individually, collectively it is a huge area, about 70m2. greater surface area = greater gas exchange

32. Alveoli cont.,
Alveoli have extremely thin walls, consisting of a single layer of squamous (flattened or squashed) epithelial (lines the organs, cavities and external surfaces) cells (endothelial lines the lumen of blood vessels)
Pressed close to them are capillaries, also thin, one celled walls. Thinness of this barrier ensures diffusion across all the membranes
For proper diffusion down across a concentration gradient, a steep concentration gradient must be maintained. Done by inhalation and exhalation

33. Uptake of mineral ions in a plant root
Root hairs of an angiosperm (flowering plant) are a specialize exchange surface
Very thin extensions of the cells of the epidermis
Thousands on a tiny branch root for an enormous surface area
Root hairs make contact with water in the soil, absorb it by osmosis because there is a higher concentration of solutes in the root hair. Water moves from higher potential to a lower potential down a concentration gradient.
Mineral ions: taken passively if there is a higher concentration gradient outside the hair, active if it is against a concentration gradient.
Cell wall does not provide a barrier to water nor mineral ions