Cell Structure & Function
Cell membranes | Cells, cytoplasm, & organelles | DNA & protein synthesis | Cell environment | Movement across membranes | Cellular metabolism
Physiology - science that describes how organisms FUNCTION and
survive in continually changing environments
Levels of Organization:
CHEMICAL LEVEL - includes all chemical substances necessary for life (see, for example, a small portion - a heme group - of a hemoglobin molecule); together form the next higher level
CELLULAR LEVEL - cells are the basic structural and functional units of the human body & there are many different types of cells (e.g., muscle, nerve, blood, and so on)
TISSUE LEVEL - a tissue is a group of cells that perform a specific function and the basic types of tissues in the human body include epithelial, muscle, nervous, and connective tissues
ORGAN LEVEL - an organ consists of 2 or more tissues that perform a particular function (e.g., heart, liver, stomach, and so on)
SYSTEM LEVEL - an association of organs that have a common function; the major systems in the human body include digestive, nervous, endocrine, circulatory, respiratory, urinary, and reproductive.
There are two types of cells that make up all living things on earth: prokaryotic and eukaryotic. Prokaryotic cells, like bacteria, have no 'nucleus', while eukaryotic cells, like those of the human body, do. So, a human cell is enclosed by a cell, or plasma, membrane. Enclosed by that membrane is the cytoplasm (with associated organelles) plus a nucleus.
Cell, or Plasma, membrane - encloses every human cell
Cells, cytoplasm, and organelles:
DNA (Deoxyribonucleic acid) - controls cell function via transcription and translation (in other words, by controlling protein synthesis in a cell)
Transcription - DNA is used to produce mRNA
Translation - mRNA then moves from the nucleus into the cytoplasm & is used to produce a protein
Used with permission of John Kimball
Used with permission of John Kimball
COMPONENTS OF THE CELLULAR ENVIRONMENT
Movement Across Membranes
1 - Passive processes - require no expenditure of energy by a cell:
Shown here is one way that active transport can occur. Initially, the membrane transport protein (also called a carrier) is in its closed configuration which does not allow substrates or other molecules to enter or leave the cell. Next, the substance being transported (small red spots) binds to the carrier at the active site (or binding site). Then, on the inside of the cell, ATP (Adenosine TriPhosphate) binds to another site on the carrier and phosphorylates (adds one of its phospate groups, or -PO4, to) one of the amino acids that is part of the carrier molecule. This attachment of a phosphate group to the carrier molecule causes a conformational change in (or a change in the shape of ) the protein so that a channel opens between the inside and outside of the cell membrane. Then, the substrate can enter the cell. As one molecule of substrate enters, the phosphate group comes off the carrier and the carrier again 'closes' so that no other molecules can pass through the channel. Now the transport protein, or carrier, is ready to start the cycle again. Note that as materials are transported into the cell, ATP is used up and ADP and -PO4 accumulate. More ATP must be made by glycolysis and the Kreb's cycle.
Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials; see graph below).
Cells require energy for active transport, synthesis, impulse conduction (nerve cells), contraction (muscle cells), and so on. Cells must be able to 'capture' and store energy & release that energy in appropriate amounts when needed. An important source of energy for cells is glucose (C6 H12O6):
C6H12O6 + O2 ----------> CO2 + H2O + ENERGY
However, this reaction releases huge amounts of energy (for a cell). So, cells gradually break down glucose in a whole series of reactions & use the smaller amounts of energy released in these reactions to produce ATP (Adenosine Triphosphate) from ADP (Adenosine Diphosphate). Then, cells can break down ATP (as in this reaction):
A----P++P++P <-----> A----P+++P + P + 7700 calories*
(*Those of you who know about food Calories may be surprised by this number. After all, an entire candy bar may contain only 200 food Calories. The explanation lies in the capital C. One food Calorie, spelled with a capital C, is 1000 times larger than one physiologist's calorie, spelled with a small c.)
The energy released in this reaction is used by cells for active transport, synthesis, contraction, and so on. Cells need large amounts of ATP &, of course, must constantly make more. But, making ATP requires energy. The breakdown of glucose does release energy. But, how, specifically, is the energy released in the breakdown of glucose used to make ATP.
A primary source of ENERGY is OXIDATION. Specifically, cells use a type of oxidation called HYDROGEN TRANSFER to generate energy:
XH2 + Y ------> X + YH2 + ENERGY
These hydrogen transfer reactions are so-named because pairs of hydrogens are 'transferred' from one substance (XH2 in the above reaction) to another (YH2 in the above reaction). Because the reactants (XH2 + Y) represent more energy than the products (X + YH2), this reaction releases energy.
In a cell, hydrogen transfer reactions occur in MITOCHONDRIA. Pairs of hydrogens are successively passed from one substance to another, and these substances are called HYDROGEN CARRIERS.
XH2 + NAD ----> NADH2 + FAD ----> FADH2 + Q ----> QH2 + C-1 ----> C-2 ---->
C-3 ----> C-4 ----> H2O + X
These hydrogen transfer reactions release energy that is used to make ATP from ADP (in other words, to add a third phosphate to adenosine diphosphate in a reaction called phosphorylation). So, what occurs in mitochondria involves hydrogen transfer (a type of oxidation) + phosphorylation, or, in other words, OXIDATIVE PHOSPHORYLATION. Oxidative phosphorylation produces lots of energy but requires hydrogen. Where do the hydrogens come from?
Sources of hydrogen include GLYCOLYSIS and the KREB'S CYCLE.
Glycolysis involves the breakdown of glucose. Cells obtain glucose from the blood. Blood glucose levels are maintained by the interaction of two processes: glycogenesis and glycogenolysis. Glycogenesis is the production of glycogen from glucose and occurs (primarily in the liver and skeletal muscles) when blood glucose levels are too high (for example, after a meal).
Glycogenolysis is the reverse process - the breakdown of glycogen to release individual molecules of glucose. This occurs when blood glucose levels begin to decline (for example, several hours after a meal). The interaction of these two processes tends to keep blood glucose levels relatively constant.
Glucose taken up by cells from the blood is used to generate energy in a process called glycolysis.
In the first few steps of glycolysis, glucose is converted into fructose-1,6-diphosphate. These reactions, like all chemical reactions, involve making and breaking bonds between atoms, and this sometimes requires energy. Even though glycolysis, overall, releases energy, some energy must be added initially to break the necessary bonds and get the energy-producing reactions started. This energy is called activation energy. In the above diagram, energy (i.e., a molecule of ATP) is needed at steps 1 & 3. So, before the energy-producing reactions of glycolysis begin, a cell must actually use two molecules of ATP.
Overall, glycolysis can be summarized as:
Glucose ----> 2 Pyruvic Acid (or pyruvate) + 2 net ATP + 4 hydrogens (2 NADH2)
So, glycolysis produces 2 direct ATP (ATP produced directly from the reactions that occur during glycolysis) and 6 indirect ATP (the 4 hydrogens produced in glycolysis will subsequently go through oxidative phosphorylation and produce 3 ATP per pair, i.e., 4 hydrogens equals 2 pair and 2 pair times 3 ATP equals 6 ATP). Thus, glycolysis produces a total of 8 ATP.
Next comes an intermediate step (called oxidative decarboxylation):
Used with permission of Gary Kaiser
the 2 Pyruvic Acid are converted into 2 Acetyl CoA & this reaction produces 4 hydrogens (2 NADH2). Those hydrogens (i.e., 2 pair of hydrogens) go through oxidative phosphorylation and produce 6 more ATP (2 pair @ 3 ATP per pair).
Finally, comes the Kreb's Cycle:
2 Acetyl CoA go through this cycle of reactions and produce 2 ATP (= GTP in the above diagram) + 16 hydrogens (6 NADH2 + 2 FADH2) plus the waste products carbon dioxide + water. The 16 hydrogens go through oxidative phosphorylation and produce 22 ATP [22 because 12 of these hydrogens (6 NADH2) go completely through the reactions of oxidative phosphorylation and produce 18 ATP (6 pair @ 3 ATP per pair), while 4 of these hydrogens (2 FADH2) go through only some of the reactions and produce 4 ATP (2 pair @ 2 ATP per pair).
Overall, therefore, the Kreb's cycle produces 24 ATP (2 direct & 22 indirect).
OVERALL ATP PRODUCTION from glucose = 8 (from glycolysis) + 6
(from the hydrogens produced when the 2 pyruvic acid are converted into
2 acetyl CoA) + 24 (from the Kreb's cycle) for a GRAND TOTAL OF 38:
Overall Total = 38 ATP
Glucose (carbohydrates) are not the only source of energy for cells. Fats (or lipids), like triglycerides, are also metabolized to produce energy.
Triglycerides ----> Glycerol + Fatty Acids:
This reaction not only produces lots of Acetyl CoA (or acetate) but
lots of hydrogens. The Acetyl CoA goes through the Kreb's Cycle, while
the hydrogens go through Oxidative Phosphorylation.
Proteins are also used as a source of energy.
Proteins are first broken down into amino acids. The nitrogen component of amino acids is then removed (in a reaction called DEAMINATION), and these deaminated amino acids are then converted into Acetyl CoA which passes through the Kreb's Cycle to make more ATP.
Used with permission of Gary Kaiser
Related (and Useful) Links:
Cell Biology Topics
DNA Workshop: You Try It
What are cell organelles?
Membrane Transport Mechanisms
Tutorial: Cellular Respiration
to the BIO 301 syllabus
Lecture Notes 2 - Neurons & the Nervous System I
Lecture Notes 2b - Neurons & the Nervous System II
Lecture Notes 3 - Muscle
Lecture Notes 4 - Blood & Body Defenses I
Lecture Notes 4b - Blood & Body Defenses II
Lecture Notes 5 - Cardiovascular System
Lecture Notes 6 - Respiratory System