Figure 8.1 Artificial membranes (cross sections)

Figure 8.4 The fluidity of membranes

Figure 8.5 Evidence for the drifting of membrane proteins

Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in cross section

Figure 8.7 The structure of a transmembrane protein

Figure 8.9 Some functions of membrane proteins

Movement of substances through the membrane May be passive – requires no energy expenditure by cell. Diffusion Osmosis Facilitated diffusion Active – does require energy expenditure

Passive transport Depends on concentration gradients. Materials move from area of high concentration to low concentration If occurs across a membrane, the membrane must be permeable to the substance. Size is a major factor Electrical charges Polar vs non-polar

Diffusion Movement of particles (solute) from high to low concentration will continue until there is an even distribution of particles. This is called equilibrium. Particles move across the membrane randomly at equilibrium.

Diffusion of two solutes If there are 2 or more kinds of solute, each moves independently of the other; 2 different concentration gradients exist. Membranes may be permeable to some substances and not others – selective permeability.

Figure 8.10 The diffusion of solutes across membranes

Osmosis Diffusion of water (solvent) is called osmosis. Water molecules will move from regions of lesser solute concentration to regions of greater solute concentration. Movement of water stops when equilibrium is reached.

Osmotic equilibrium In osmosis, water accumulating on one side of a membrane exerts pressure on the membrane. If this pressure is great enough, it may stop entry of water molecules from the other side, even though a concentration gradient still exists. This is important for plant cells!

Figure 8.11 Osmosis

Tonicity When comparing the concentrations of 2 solutions: Isotonic – same concentration of solute Hypertonic – higher concentration of solute Hypotonic – lower concentration of solute

Figure 8.12 The water balance of living cells

Movement through the membrane - passive In diffusion and osmosis, very small molecules may pass directly between the phospholipids as long as they are not charged. Ions can NOT pass through Some water can pass O2 and CO2 can pass Small lipids can pass

Facilitated Diffusion Charged particles and some molecules can move passively through protein channels. The channels facilitate passage through the membrane. The proteins may undergo a shape change as the particles pass, but no energy is expended.

Figure 8.14 Two models for facilitated diffusion

Figure 8.15 The sodium-potassium pump: a specific case of active transport

Figure 8.16 Review: passive and active transport compared

Figure 8.17 An electrogenic pump

Figure 8.18 Cotransport

Figure 8.19 The three types of endocytosis in animal cells