Topic 3: Cell structure and function
Definitions can be found at
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Objectives
·
Describe
and explain the cell theory of life.
·
What is
the plasma membrane and what does it have to do with cell life?
·
Describe
the fluid mosaic model
of the plasma membrane
·
Describe
the physical processes of osmosis and diffusion.
·
What is
the difference between Active and passive transport.
·
Differentiate
between
Exocytosis and endocytosis.
·
What
happens when a person is given an IV with the wrong "tonicity"
·
Describe
the cells
organelles that carry out specific functions
·
Describe
the cells organelles that are used for support and
movement.
·
What is
metabolism?
·
How
does a cell get energy from sugar and oxygen?
1. A
cell is the smallest part of the body that can still believe
A. (History) Early Microscopists who first observed cells and their contents
1) Robert Hook use a simple lens to observe cork. He observed tiny compartments he called cellulae and from then on we have been calling them cells.
2) Van Leeuwenhoek was able to see human sperm and even bacteria, he made his own microscopes.
B. Once biologists started looking at cells they developed the cell theory: remember it’s three generalizations:
1) All organisms are composed of one or more cells.
2) The
cell is the smallest unit having the properties of life
3) The continuity of life arises directly from the growth and division of single cells ( cells come only from other cells)
2.
Basic Aspects of Cell
structure and function (two types of cells exist in nature)
A. Prokaryotic cells (bacteria) do not have a separation of DNA from the remainder of the cell parts; Eukaryotic cells have a definite nucleus.
B. Eukaryotic cells ( Humans and all other visible animals) have three basic parts:
1) A plasma membrane separates each cell form the environment, permits the flow of molecule across the membrane, and contains receptors that can affect the cell’s activities. Is like the rubber part of a balloon.
2) The nucleus contained within the plasma membrane is a second membrane that holds the DNA of a cell
3) The cytoplasm is a fluid that fills and gives the cell bulk. It is where many cellular activities occur.
C. Cell size and shape
1) Because of their small size, most cells can only be seen by using microscopes
2) Human cells come in many shapes including long, thin and frilly
3) Why aren't humans made of one massive cell?
a) This is a pretty old question and part of this question is answered in figure 3.3 of your text. The larger the cell the smaller the surface to volume ratio. This means that large cells cannot easily move materials across the plasma membrane into the cell. The cell surface to volume ratio is a physical limit on the size of the cell. Now certain cells can increase their surface area by having structures called microvilli, while this will increase the surface area to volume ratio, it still does not help in the movement, by diffusion of materials across the large cell.
3. Membrane Structure and Function (Fluid Mosaic model)
1) A double layer of molecules called Phospholipids makes up the membrane of a cell
a) A phospholipid molecules is composed of hydrophilic head and two hydrophobic tails
b) If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results: hydrophilic heads are at the outer faces of a two layer sheet
c) Bilayers of Phospholipids are the structural foundation of cell membranes.
2) A variety of different proteins are embedded in the bilayer or positioned at its two surfaces
a) The scattered islands of protein in the sea of lipid creates a “mosaic” effect.
b) Membrane proteins (most are glycoproteins) serve as transport proteins, receptor proteins, recognition proteins and adhesion proteins
3) The beauty of the Fluid Mosaic model is that it explains the properties on not only the plasma membranes of human cells but of basically all membranes. If Phospholipids are added to a test-tube and shaken rudimentary membranes will form. This self-assembly ensures that basically all membranes are similar in structure.
A. In order to live a cell must be able to move molecules from one side of a membrane to another
1) Normally molecules in water and air move from areas of high concentration to low concentration
a) When you pass gas in a car the person in the back seat will still smell it, why?
2) The
net movement of molecules down a concentration gradient (from high near you to
low near your victim) is called simple diffusion
3) Gradients in pressure, temperature and electric charge can also influence movements of molecules into and out of each of the cells in your body.
1) Only
certain small molecules will move across a cell membrane. This property is called selectively
permeable. A chain link fence will
not allow a child to pass but mice will run freely across it.
2) Osmosis is the passive movement of water across a selectively permeable membrane in response to water concentration gradients
3) Osmotic movements are affected by the relative concentrations of solutes in the fluids inside and outside the cell (tonicity)
a) In isotonic fluid has the same concentration of solutes as the fluid in the cell: immersion in it causes no net movement of water
b) in hypotonic fluid has a lower concentration of solutes than does the fluid in the cell: cells immersed in it may swell
c) in hypertonic: fluid has a greater concentration of solutes than does the fluid in the cell: cells in it may shrivel.
4) Osmotic water movements across a membrane produce osmotic pressure (the tendency of water to move form a hypotonic solution to a hypertonic one) may be counterbalanced by hydrostatic pressure. We will look at this later when we study blood pressure.
C. How molecules cross Membranes
1) Lipid soluble molecules and small, electrically neutral molecules (for example, oxygen, carbon dioxide, and ethanol) cross the lipid bilayer by simple diffusion.
2) Larger molecules (such as glucose) and charged ions (such as Na+, Ca+, HCO3-) must be moved by membrane transport proteins.
5.
Transport Mechanisms of
the Cell membrane (Which defines the living cell)
A. Passive and Active Transport
1) In passive transport, molecules pass through channel proteins in accordance with the concentration gradient: also called “facilitated diffusion”, because no energy is required
2) In active transport, solutes can move against concentration gradients with assistance form transport proteins that can change their shape with energy supplied by ATP; examples: Na+ - K+ pump.
1) Vesicles, small sacs made of membranes, can transport and store substances within the cytoplasm
2) Exocytosis moves substances form cytoplasm to plasma membrane during secretion
3) Endocytosis (also known as phagocytosis) encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm
C. Cell to cell recognition.
1) The plasma membrane of the all cells contain specific molecules that determine the origin and purpose of that cell. So if you have a lung cell growing in your hand, your immune system has a way of determining this .
6.
Cells and other Organelles
A. All Eukaryotic cells contain organelles that perform a function in the cell
1) Organelles form compartmentalized portions of the cytoplasm
2) Organelles separate metabolic reactions with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity).
a) Like an assembly line where the product is not moved on until something is attached or removed.
B. A brief summary of the major cellular structures (you need to memorize these)
1) The nucleus is the location of the cell’s DNA
2) The Ribosome’s, can be free or attached to membranes and is where protein synthesis occurs
3) The Endoplasmic reticulum (ER) modifies proteins and is also involved with lipid synthesis
4) Golgi bodies modify, sort, and ship proteins: they also play a role in the synthesis of lipids for secretion or internal use
5) Vesicles transport or store substances
6) Mitochondria produce ATP (A molecule who’s energy is used to perform work in a cell)
7) The cytoskeleton determines cell shape and provides for motility. Go to
A. The nucleus encloses DNA, its membrane isolates DNA, which contains the code for protein assembly, from the sites (ribosome’s in cytoplasm) where proteins will be assembled
1) the nuclear membrane helps regulate the exchange of signals between the nucleus and the cytoplasm
B. The nucleus has the following components:
1) The nucleolus is a region where subunits of ribose’s are prefabricated before shipment out of the nucleus
2) The nuclear envelope consists of two lipid bilayers with pores
3) Chromosomes are composed of DNA and associated proteins (some serve as enzymes, others for support); DNA is duplicated and condensed before cell division occurs
A. Within the cytoplasm, newly formed polypeptide chains assembled on the ribosome’s may enter the cytomembrane system:
1) The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs, continuous with the nuclear membrane
a) Rough ER consists of stacked, flattened sacs with many ribosome’s attached: oligosaccaride groups (carbohydrates) are attached to polypeptides as they pass through the ER membrane on their way to other organelles or to secretary vesicles
b) Smooth ER has no ribosome’s: it is the area from which vesicles carrying proteins and lipids are budded: it also inactivates harmful chemicals.
2) In the Golgi bodies, proteins and lipids undergo final processing, sorting, and packaging in stacks of flattened sacs whose edges break away as vesicles.
B. A variety of Vesicles
1) Lysosomes are vesicles that bud from Golgi bodies: they hold powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign amino acids.
2) Peroxisomes are membrane bound sacs of enzymes that break down fatty acids and amino acids.
3) Caveolae organelles that apparently take in proteins from the outside of the cell and slowly release them in the cytoplasm.
A. Mitochondria are the primary organelles for transferring the energy in carbohydrates to the form used by the cell’s metabolic enzymes. ATP.
1) Oxygen is required for the release of this energy.
2) Hundreds of thousands of mitochondria may be present in cells, depending on the energy requirements.
B. Each Mitochondrion has compartments formed by an inner folded membranes surrounded by a smooth outer membrane.
A. The cytoskeleton is an interconnected system of bundled fibers, slender threads, and lattices extends for the nucleus to the plasma membrane
1) The main components are microtubules, microfilaments, and intermediate filaments all are assembled from protein subunits
2) Some portions are transient, such as the “spindle” microtubules used in chromosome movement during cell division: others are permanent, such a filaments operational in muscle contraction
B. Flagella
and cilia
1) Microtubular extension f of the plasma membrane has a 9+2 cross sectional array and are useful in propulsion.
2) Flagella are quite long, are usually not numerous, and are found on one celled Protozoans and animal sperm cells
3) Cilia are shorter and more numerous and can provide locomotion for free living cells or may move surrounding water and particles if the ciliated cell is anchored
C. Centrioles and MTOCs
1) The microtubules of flagella and cilia arise form centrioles which are associated with basil bodies
2) In many cells, centrioles are located in MTOCs (microtubule organizing centers).
11.
Life and Energy: Metabolism
A. Metabolic Pathways
1) Metabolism is the controlled capacity to acquire energy and use it to build, break apart and eliminate substances.
2) Metabolic pathways form series of reactions that regulate the concentration of substances within cells by enzyme-mediated linear and circular sequences
a) In biosynthetic pathways also called anabolism small molecules are assembled into large molecules for example, simple sugars are assembled into complex carbohydrates
b) In degradative pathways (called catabolism), large molecules such as carbohydrates, lipids, and proteins are broken down to form products of lower energy. Released energy can be used for cellular work.
3) Some terms used in describing metabolism:
a) Substrates (reactants) are substances that enter a reaction
b) Intermediates are substances that form between the start and conclusion of a metabolic pathway
c) End products are the substances present at the conclusion of the pathway
d) Enzymes are proteins that speed up a reaction
e) Cofactors are organic molecules that assist enzymes in their action
f) Energy carriers donate energy to substances by transferring functional groups to them
g) Transport proteins adjust concentration gradients at cell membranes in ways that influence the direction of metabolic reactions
4) “redox” reactions
a) Many metabolic pathways involve oxidation-reduction reactions (=redox)
b) The donor molecule loses an electron and is oxidized
c) The receptor molecule gains an electron and is reduced
B. Enzymes
1) enzymes are protein that serve a catalysts: they speed up reactions
2) Enzymes have four features in common
a) Enzymes do not make anything happen that could not happen on its own. they just make it happen faster
b) Enzymes can be reused
c) Enzyme actions are reversible
d) Enzymes are selective and act upon specific substrates, which are the molecules that enzymes (at their active sites) recognize and bind to.
3) Because enzymes operate best within defined temperature ranges, high temperatures decrease reaction rate by disrupting the bonds that maintain three-dimensional shape (denaturation occurs).
4) Most enzymes function best at pH near 7: higher or lower values disrupt enzyme shape and halt function
C. Enzyme cofactors
1) These are nonprotein groups that bind to many enzymes and make them more reactive.
2) Coenzymes are large organic molecules such as NAD+, FAD, and HADP+ (partly derived form vitamins), which transfer protons and electrons form one substrate to another.
3) Inorganic metal ions such as Fe++ also serve as cofactors when assisting membrane cytochyrome proteins in their electron transfers in mitochondria
D. Control of Enzyme Function
1) Enzyme activity can be controlled by cellular regulation of enzyme synthesis, stimulation, and inhibition.
2) Hormones also have significant enzyme regulating effects
12. Overview of ATP Production
A. Before cells can use the energy of sunlight or that stored in carbohydrates, they must transfer the energy to molecules of ATP.
1) ATP is composed of adenine, ribose, and three phosphate groups.
2) ATP transfers energy to many different chemical reactions: almost all-metabolic pathways directly or indirectly run on energy supplied by ATP.
3) ATP can donate a phosphate group (phosphorolation) to another molecule, which then becomes primed and energized for specific reactions
B. In human cells, cellular respiration releases energy form energy right organic molecules and changes ADP into ATP
1) Aerobic respiration is the main ATP producing pathway
2) Anaerobic respiration produces much less ATP and can only be used for short periods of time, such as in vigorous muscle exercise
13. Summary of Energy
Harvests from Glucose Breakdown.
Most of the energy used by our bodies is in the form of sugar or
glucose. This sugar energy must be
converted into another form in order for your cells to use it. An analogy would be gasoline and
diesel. While diesel has plenty of
energy it cannot be used by gas engines, and if we were able to perform the
transition we could use diesel in our cars.
Our bodies convert glucose energy to ATP which you will learn later are
used to perform work in all our cells.
A. Net ATP yield of aerobic respiration
1) The aerobic metabolic energy producing rout
is summarized C6H12O6 + 6 O2 -->
6CO2 + 6 H2O
2) Electron transport yields thirty two ATP; glycolysis yields two ATP; Krebs yields two ATP for a grand total of thirty six ATP per glucose molecule
3) Actual yield can vary with cell type, but is somewhere near 34 ATP per molecule of glucose.
B. ATP from Anaerobic Pathways
1) Anaerobic pathways operate when oxygen is absent (or limited): pyruvate from Glycolysis is metabolized to produce molecules other than Acetyl-CoA.
2) In humans during lactate fermentation, glycolysis produces two pyruvate, two NADH molecules, two ATP molecules, and two lactate, which tend to build up and cause temporary muscle cramps. The net energy gained is 2 ATP molecules, which is significantly lesser than the aerobic pathway.
C. The ADP/ATP cycle
1) The ADP{/ATP cycle is a method for renewing the supply of ATP that is constantly being used up in the cell.
2) Energy imput couples inorganic phosphate to ADP to form energized ATP
3) In your muscle cells, ADP is continuously converted to ATP by metabolic pathways. ATP is converted to ADP by the working muscles.
D. http://www.almaz.com/nobel/medicine/1953a.html Well all of these are kind of correct, but I will try and put it a slightly different way. All of the cells in our body get energy in a process called Catabolism. In this process larger molecules are broken down into smaller molecules and energy is released. Typically your body breaks down glucose (6 carbon ring)into 6 carbon dioxide molecules. This provides ATP. This process is divided into 3 processes called Glycolysis, the Krebs cycle and the electron transport system. During glycolysis 2 ATP are generated. If no oxygen is present then Lactic acid is produced. If oxygen is present the Krebs cycle occurs and 2 more ATP are made, however the main function of the Krebs cycle is to produce High energy intermediates that are used by the Electrontransport system in the mitochondria of cells to produce 32ATP for a total of 36 ATP. Yes the Krebs cycle is something that we can understand and there was even a Nobel Prize awarded for its discovery.
14. Alternative Energy Sources in the Human Body
1) The 4 types of organic molecules discussed in chapter 1 can not only be used to construct a body, but these molecules contain energy that can be used to energize the human body.
B. Carbohydrates are the body’s first source of energy
1) Excess carbohydrate intake is stored as glycogen in liver and muscle for future use
2) Free glucose is used until it runs low: then glycogen reserves are tapped.
C. Energy from Fats
1) Lipids are used when carbohydrate supplies run low.
a) Excess fats are stored away in cells of adipose tissue
b) Fats are digested into glycerol (which enters glycolysis) and fatty acids, which enter the Krebs cycle
2) Because fatty acids have many more carbon and hydrogen atoms, the are degraded more slowly and yield greater amounts of ATP
D. Energy form Proteins
1) Proteins are used as the last resort for supplying energy for the body
2) Amino acids are released by digestion and travel in the blood
3) After the amino group is removed, that amino acid remnant if fed into the Krebs cycle
E. Nucleic Acids can be used in little amounts by the body. Toxic buildups can occur if they are too much in the diet.
Possible assignments: things to think about
1. Why is the structure of the plasma membrane basically the same among all organisms?
2. What would happen to a blood cell if suddenly placed in saltwater?
3. What is the difference between diffusion and osmosis
4. Based on your knowledge of membrane and solubility, which poison you expect to kill people faster: one that is water-soluble or lipid soluble?
5. What would the result be on blood cells of a substitution of pure water for physiological saline in an IV bottle.
6. Does the cytoplasm have any functions of its own, or is it just a filler matrix in which other organelles float.
7. What would happen if the body temperature of humans changes to 105’F or the body pH rose to 9.5.
8. Can a person gain energy from just eating fats or proteins and no carbohydrates?
.
To view another instructors description of organelles see http://cellbio.utmb.edu/cellbio/ [SFD1]
Some lecture notes on homeostasis
http://bioserve.latrobe.edu.au/vcebiol/cat1/aos2/u3aos21.html
Review
Quiz: Multiple-choice examples. top
Click on the linked words to see if you are correct. You can get back to your question by clicking its number.
1.
Two types of
cells exist in nature. Which of the
following statements are totally correct?
a) Prokaryotic cells have a nuclear membrane and many chromosomes
b) Eukaryotic cells have no internal membrane bound organelles
c) Prokaryotic cells have no internal membrane bound organelles
d) Eukaryotic cells are the types of cells humans have
e) Eukaryotic cells are much more complex than prokaryotic cells
2. How we view the structure of a membrane is called the
fluid mosaic model. This is because
a)
it is a really cool name
b) a membrane looks like tiles on a wall
c) a membrane is made up of many different subunits imbedded in the phospholipid. It looks like a mosaic but these units may move around within the structure.
d) A membrane is like a glass of water
e) The colder the temperature the more solid the membrane
3.
Which of the
following are examples of simple diffusion
a) You have a sundae with separate layers of ice cream and fudge that do not mix.
b) You place some food coloring in water and you come back a half hour later and the water is all red
c) You walk by a doorway in SanFrancisco and you can smell the stench of urine.
d) When you stay in the water too long you become wrinkled.
e) Why blood tends to stay in one spot on the carpeting
4. Which of the following statements are true concerning isotonic
a) Iso literally means the same
b) Solutions that are isotonic have the same amount of materials dissolved in them.
c) The IV solution that you receive in the hospital is isotonic
d) Blood serum is isotonic to the solution within a blood cell
e) All IV solutions only contain water
5. Which of the following are correctly matched
a) the nucleus is the site of protein production in a cell
b) the cytoskeleton is a semiliquid substance in which all the cell organelles float
c) the Golgi bodies are used to modify and store nucleotides produced by the mitochondria
d) vesicles produce ATP (they convert the energy within glucose into ATP molecules)
e) The endoplasmic reticulum contains a cells DNA and is responsible for transferring genes form one generation to the next.
Answers 1) c,d,e 2) c 3) b,c 4) a,b,c,d 5) trick question none are correct
Return to the quiz by clicking on the question number.
This is a trick question. They are all wrong. Go back
Return to the quiz by clicking on the question number.
Web pages that you can use.
Cells and cell biology are discussed in a nice way at http://mindquest.net/biology/cell-biology/cell-biology.html
A nice set of pages describing the cell organelles is at http://www.brigadoon.com/~schaffer/biology/plasmame.html