Physiology of Cells

Movement of Substances Through Cell Membranes

If a cell is to survive, it must be able to move substances to places where it is needed. Membrane transport processes can be classified as either passive, not requiring any energy expenditure, or active, requiring energy to pull substances across the membrane.

Passive Transport Processes

Simple Diffusionmolecules or ions spread spontaneously from regions of higher concentration to regions of lower concentration until equilibrium is reached

Dialysis – form of diffusion in which the selectively permeable membrane causes the separation of smaller solute particles from larger ones.

Passive Transport Processes

Osmosis – the diffusion of a solvent through a selectively permeable membrane. Occurs when there is a difference in solute concentration on the two sides of the membrane. Osmotic water flows are extremely important in determining the ditribution of water in the various fluid-containing compartments of the body (cells, interstitial fluid, blood, etc.)

Osmosis

Hypertonic – movement of water out of an area

Isotonic - equal movement of water into and out of

Hypotonic – movement of water into an area

Passive Transport Processes

Facilitated Diffusion – molecules (ex. Sugar) combine with a carrier molecule at the surface of the membranes. This union changes the shape of the carrier, which moves glucose to the other side of the membrane. Only moves molecules from regions of higher to lower concentrations.

Active Transport Processes

Active Transport – movement of molecules from an area of lower concentration to an area of high concentration; requires energy to pump molecules against the concentration gradient

Active Transport ProcessesEndocytosis – bringing in bulk

molecules or substances through the plasma membrane by forming vacuole around it – Phagocytosis – solid

particles; cellular eating– Pinocytosis – water with

dissovled substances; celular drinking

– Receptor-mediated – bringing in molecules attached to surface markers

Exocytosis – sending out bulk packages; usually things packages by the Golgi Apparatus and waste removal

Cell Metabolism

• Metabolic pathways can either be catabolic or anabolic.

• Catabolism – cellular respiration is an example

• Anabolism – DNA replication and protein synthesis are examples

The Role of Enzymes

• The activation energy needed to start a chemical reaction is too great for a number of chemicals to react by themselves. A catalyst is needed to reduce the amount of activation energy needed to start a chemical reaction.

• Enzymes act as chemical catalyst in the cell. Enzymes are functional proteins.

Classification and Naming of Enzymes

Two systems used for naming enzymes:1. The suffix –ase is used with the root name of

the substance whose chemical reaction is catalyzed, or

2. with the word that describes the kind of chemical reaction catalyzed.

Enzymes investigated before these methods of nomenclature were adopted are still called by older names, such as pepsin and trypsin.

Enzymes are classified into several groups according the chemical reactions catalyzed:

• Oxidation-reduction enzymes – (oxidases, hydrogenases, dehydrogenases) Energy release from muscular contraction and all physiological work depends on these enzymes.

• Hydrolyzing enzymes – (hydrolases) Digestive enzymes belong to this group. Named after the substrate they act on (lipase, sucrase, maltase).

• Phosphorylating enzymes – add or remove phosphate groups and are known as phosphorylases or phosphatases

chemical reactions catalyzed

• Enzymes that add or remove carbon dioxide – carboxylases or decarboxylases

• Enzymes that rearrange atoms in a molecule – mutases or isomerases

• Hydrases – these add water to a molecule without splitting it, as do hydrolases

General Functions of Enzymes

Most enzymes are specific in their action (key-in-a-lock action).

Enzymes

Various physical and chemical agents activate or inhibit enzyme reaction by changing the shape of the enzyme. An allosteric effector is any molecule or agent that alters the function of the enzyme by changing it’s shape.

Some allosteric effectors are:

1. antibiotic drugs

2. changes in pH

3. changes in temperature

4. inhibition or activation molecules

Most enzymes catalyze a chemical reaction in both directions.

Growth and Reproduction of Cells

The Cell Life CycleG1 Phase – first growth phase; protein synthesis occurs; synthesis of large organelles and plasma membraneS Phase – synthesis phase; DNA replication occursG2 Phase – second growth phase; continued growth and synthesis of other molecules and organelles

MitosisMitosis means nuclear

division.

Prophase – chromosomes coil; centrioles move to opposite poles, spindle fibers begin to form; nuclear membrane and nucleolus diappear

Metaphase – chromosomes align across the equator of the spindle fibers and attach to them

MitosisAnaphase – centromeres split

detaching two chromatids which are then pulled to opposite poles of the cell

Telophase/Cytokinesis – Nuclear envelope and nucleolus reappear around each set of chromosomes; spindle fibers disappear; chromosomes uncoil; plasma membrane divides into two new identical cells

Meiosis

Meiosis is the process of reducing the chromosome number in half to form gametes (egg or sperm). 46 chromosome (23 pair) to 23 chromosomes

• Meiosis I • Meiosis II

Regulating the Cell Cycle

The cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs). The number of CDK enzyme molecules stays about the same throughout the cell’s life cycle, but the number of cyclin molecules varies widely. Cyclins act as mechanisms that triggers the CDK and moves the cell from one phase to the next.

Cellular Disease

Disorders Involving Cell TransportCystic fibrosis

Duchenne muscular dystrophy

Disorders Involving Cell Membrane Receptors

Diabetes mellitus (type 2 diabetes)

Cellular Disease

Disorders Involving Cell ReproductionCancers

Disorders Involving DNA and Protein Synthesis

Sickle-cell anemia

Hemophelia

Infections Bacteria and viruses