1. Chapter 5: Membrane Structure and Function (Outline)
Plasma Membrane Structure and Function
Phospholipids and proteins
Fluid-Mosaic Model
Carbohydrate Chains
Functions of the Proteins
Plasma Membrane Permeability
Diffusion
Osmosis
Active Transport Across a Membrane
Bulk Transport

2. Structure and Function: The Phospholipid Bilayer
The plasma membrane is a phospholipid bilayer with partially or wholly embedded proteins
Phospholipids are amphipathic – molecules that have both hydrophilic and hydrophobic regions
Nonpolar tails (hydrophobic) are directed inward
Polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid
Cholesterol – a lipid found in animal plasma membranes that helps modify the fluidity of the membrane
The proteins are scattered throughout the membrane forming a mosaic pattern

3. Plasma Membrane of an Animal Cell

4. Plasma Membrane Structure
Membrane proteins may be integral (embedded) or peripheral
Integral proteins are found in the membrane and are held in place by the cytoskeleton and the extracellular matrix
Peripheral proteins are found on the inner membrane surface
The plasma membrane is asymmetrical; the distribution of integral proteins can be analyzed by freeze-fracture technique 4

6. Fluid-Mosaic Model
The fluid-mosaic model describes the plasma membrane
The fluid component refers to the phospholipids bilayer of the plasma membrane (PM)
The mosaic component refers to the protein content in the PM
Fluidity of the plasma membrane allows cells to be pliable (flexible)
Protein movements are limited by interactions with the cytoskeleton and other proteins

7. Membrane Fluidity Four main factors contribute to membrane fluidity
Temperature – at body temperature, the phospholipid bilayer has the consistency of olive oil
Membrane phospholipid tail length – shorter hydrocarbon tails can move sideways (lateral) more easily; rarely flip-flop, why?
The degree of unsaturation of membrane phospholipid tails
Amount of cholesterol - keeps the hydrocarbon tails fluid at cold temperatures, and stabilizing them at high temperatures

8. Membrane Fluidity With Cholesterol Phospholipid Movement
Unsaturated/Saturated

9. Carbohydrate Chains
Membrane contain carbohydrate chains linked to phospholipds “Glycolipids” & proteins “Glycoprotein” on the extracellular surface
Glycocalyx – a ‘sugar coat’ that facilitates cellular adhesion, protection, signal reception and cell-cell recognition
Carbohydrate chains vary by number (from 15 to 100’s), sequence of sugars and whether the chain is branched (a “fingerprint”)
The cell surface of blood cells is covered with glycoproteins – A, B, and O blood groups

10. Functions of Membrane Proteins
The manner in which a protein associates with a membrane depends on its structure and can be categorized as follows
Channel proteins
Carrier proteins
Cell Recognition proteins
Receptor proteins
Enzymatic proteins
Junction proteins

11. Channel Proteins
Allows passage of molecules or ions freely through membrane
They facilitate diffusion by forming hydrophilic transmembrane channels
H+ ions across mitochondrial inner membrane during ATP production
Faulty Cl- channel causing cystic fibrosis
Channel proteins are only responsible for passive transport

12. Carrier Proteins
Selectively interact with a specific molecule so that it can cross the plasma membrane to enter or exit the cell
This process often requires energy (ATP)
When ATP is involved with actively moving molecules through the membrane the process is called active transport
Example: Sodium and potassium ions across the plasma membrane of nerve cells

13. Cell Recognition Proteins
Glycoproteins and some glycolipids serve as surface receptors for cell recognition and identification (cellular fingerprint)
Important in that the immune system cells can distinguish between one’s own cells and foreign cells
The major histocompatibility complex (MHC) glycoprotiens are different in each individual
MHC determines organ transplant acceptance or rejection

14. Receptor Proteins
Receptor proteins serve as binding or attachment sites
Protein has a specific shape so that specific molecules can bind to them
Binding of a molecule (e.g. insulin hormone) can influence the liver to store glucose
Pygmies are short due to their faulty plasma membrane hormone receptors that cannot interact with growth hormone

15. Enzymatic Proteins
Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface
Catalyzes a specific reaction
Adenylate cyclase is a membrane bound enzyme that is involved in ATP metabolism
Cholera toxin activates the adenylate cyclase enzyme in the intestinal cells
Results in the loss of H2O, Na+ and K from the intestinal cells (dehydration)

16. Junction Proteins Form various types of junctions between animal cells
Signaling molecules that pass through gap junctions allow the cilia of cells lining the respiratory tract to beat at the same time
Tight junctions joining cells in order to form a specific function
Example – nervous system in animal embryos 16

17. Types of Transport: Active vs. Passive
Plasma membrane is differentially (selectively) permeable
Allows some material to pass freely
Inhibits passage of other materials
Some materials enter or leave the cell only by the using cell energy
Passive Transport:
No ATP requirement
Molecules follow concentration gradient
Concentration - the number of molecules of a substance in a given volume
Gradient - a physical difference between two regions so that molecules will tend to move from one of the regions toward the other (i.e. concentration, pressure & electrical charge)

18. Active vs. Passive
When the distribution of molecules is not equal, and we have a gradient, there is a net movement of molecules along the gradient
Example: Cellular respiration
Concentration of O2 is lower inside a cell than outside
Concentration of CO2 is higher inside the cell than outside
Active Transport
Requires carrier protein
Molecules move through the membrane against the concentration gradient
Requires energy in form of ATP
Movement out of the cell involving changes of the membranes & formation of vesicles is exocytosis
Movement of materials into the cell is endocytosis

19. Types of Passive Transport: Diffusion
A solution consists of:
A solvent (liquid) , and
A solute (dissolved solid)
Diffusion – the net movement of solute molecules from where there is more of it along a concentration gradient to where there is less of it, until molecules are equally distributed
In terms of cellular activity, diffusion:
Requires no energy
However, the cell has no control over diffusion, and the rate of diffusion is quite slow

20. Diffusion The rate of diffusion can be affected by:
Temperature (higher temperature, faster molecule movement)
Molecule size (smaller molecules often move more easily)
Concentration (Initial rate faster with higher concentration)
Electrical & pressure gradients of the two regions (greater the gradient differential, the more rapid the diffusion)

21. Membrane Transport
Materials that may move through membranes freely by simple diffusion include:
CO2 (carbon dioxide)
O2 (oxygen)
Small lipid-soluble molecules
Passive transport (carrier proteins):
H2O (aquaporin)
Glucose
Many small ions
Some amino acids

22. Types of Passive Transport: Osmosis
Focuses on solvent (water) movement rather than solute
Diffusion of water across a differentially (selectively) permeable membrane
Solute concentration on one side high, but water concentration is low
Solute concentration on other side low, but water concentration is high
Water diffuses both ways across membrane but solute can’t
Net movement of water is toward low water (high solute) concentration

23. Osmosis Demonstration
Osmotic pressure is the pressure that develops due to osmosis
The more solute particles present, the higher the osmotic pressure

24. Significance of Osmosis
Absorption of water from the soil by plant roots
Turgidity is developed by the process of osmosis which mechanical strength in plants
Re-absorption of water by the kidneys
Absorption of water by the digestive tract - stomach, small intestine and the colon

25. Types of solutions: Isotonic
Isotonic Solution
Solute and water concentrations are equal on both sides of membrane
This results in no net movement of water into or out of cells
Isotonic solutions are osmotically balanced
Physiological or normal saline consists of 0.9% NaCl in water, which is isotonic to red blood cells

26. Types of solutions: Hypotonic
Hypotonic Solution
The solution has a lower solute concentration (more water) than the cell
This results in a net movement of water into the cells
Cells placed in a hypotonic solution will swell
May cause cells (animal) to burst – lysis

27. Hypotonic Environments
Cells which typically exist in hypotonic solutions (fresh water), use various mechanisms to oppose this inevitable influx of water
The contractile vacuoles found in protists (e.g. paramecium) are used to expel excess water
Well-developed kidneys in freshwater fish to excrete large volume of diluted urine
Plant cells use osmotic pressure to their advantage
When plant cells immersed in water, the vacuole (containing the stored molecules) gain water which increases the turgor pressure
This pressure forces the cytoplasm against the plasma membrane and cell wall, helping to keep the cell rigid

28. Types of solutions: Hypertonic
Hypertonic Solution
The solution has a higher solute concentration (less water) than the cell
Cells placed in a hypertonic solution will shrink – Plasmolysis
Antibiological activities used in food preservation (i.e., meats, fruits, & vegetables are pickled, salted, or mixed with concentrated sugar solutions to prevent bacterial & fungal growth)

29. Hypertonic Environments
Salt water, for example, is hypertonic to the cells of freshwater organisms
Central vacuole in plants lose water and the plasma membrane pulls away from the cell wall
Marine animals cope in various ways
Sharks increase/decrease urea in blood
Fishes excrete salts across their gills

30. Facilitated Transport: Carrier Proteins
Small molecules (i.e., glucose, amino acids)
Can’t get through membrane lipids
Combine with carrier proteins
Follow concentration gradient (i.e., no ATP)

31. Types of Transport: Carrier Proteins
Active Transport
Small molecules (i.e., glucose, amino acids)
Move against concentration gradient
Requires energy
Requires two carrier protein active sites:
one to recognize the substance to be carried
one to release ATP to provide the energy for the protein carriers or "pumps“
The sodium-potassium pump

32. The Na+-K+ Pump

33. Bulk Transport: Exocytosis
Macromolecules are transported into or out of the cell inside vesicles
Vesicle formation requires ATP
Exocytosis – vesicles fuse with plasma membrane and secrete contents
Hormones, neurotransmitters and digestive enzymes are secreted by exocytosis
Example: insulin, made in pancreatic cells, are secreted by exocytosis
Regulated secretion occurs when plasma membrane receives a signal (i.e. rise in blood sugar)

34. Exocytosis

35. Bulk Transport: Endocytosis
Endocytosis - substances that enter the cell by vesicle formation
There are a variety of endocytosis processes:
Phagocytosis – Large, solid material into vesicle, such as a bacterium
Pinocytosis – Liquid or very small particles, such as macromolecules, go into the vesicle
Receptor-Mediated Endocytosis – Specific form of pinocytosis using a receptor protein
Endocytosis takes up large amounts of the plasma membrane and is balanced by the return of membrane components to the plasma membrane by exocytosis

36. Methods of Endocytosis: Phagocytosis
Membrane surrounds and engulfs object, used for larger objects & pinches off placing the particle in a phagocytic vacuole
The phagocytic vacuole then fuses with lysosomes and the material is degraded
Examples:
Unicellular organisms (amoebas)
White blood cells (macrophage)

37. Methods of Endocytosis: Pinocytosis
The process by which a cell takes in extracellular fluid or very small particles by the invagination of the cell membrane
A pocket then forms and pinches off to form a vesicle
The liquid contents of the vesicle is then slowly transferred to the cytosol
Examples:
Blood cells
Plant root cells

38. Methods of Endocytosis: Receptor-Mediated Endocytosis
A form of pinocytosis, occurs when specific macromolecules (e.g. vitamin, lipoprotein, etc.) bind to plasma membrane receptors
The macromolecules are taken into the cell via coated vesicles that pinch from the plasma membrane
Molecules first bind to specific receptor proteins, which are found at one location in the plasma membrane
This location is a coated pit with a layer of fibrous protein on the cytoplasmic side; when the vesicle is uncoated, it may fuse with a lysosome

39. Receptor-Mediated Endocytosis
Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood)
This system is selective and more efficient than pinocytosis, why?
Defects in receptor-mediated endocytosis are responsible for certain diseases such as hypercholesterolemia
LDL receptors cannot bind to the coated pit, thus the cells are unable to take up cholesterol
Access cholesterol accumulates in the circulatory system
Will cause heart attacks & atherosclerosis 39

40. Receptor-Mediated Endocytosis