Cell Biology - chapter 3

Lecture 6:  September 21, 2000 – Lipids and cell membranes

Cholesterol:

Last lecture, we talked about the amphipathic structure of phospholipids, which make up cell membranes.  Another component of cell membranes is cholesterol:  see fig. 2.19  a steroid.  Steroids are types of lipids with a ring structure.  The rings are extremely hydrophobic (non-polar), and cholesterol has a small hydroxyl group (OH) that is hydrophilic – the only polar part of the molecule.  Cholesterol is therefore amphipathic.

Several important hormones (estradiol, testosterone, cortisol) are derivatives of cholesterol. 

Cell membrane (also known as the plasma membrane)

The cell membrane is a flexible yet sturdy barrier that surrounds and contains the cell cytoplasm.  The fluid mosaic model (fig. 3.2) describes its structure.  The membrane consists of proteins afloat in a sea of lipids.

Lipid bilayer: 

The lipid bilayer forms the basic framework of the cell membrane.  It forms because of the amphipathic nature of lipid molecules.  When it comes to polar and non-polar molecules,  “like seeks like” – polar molecules are attracted to one another, due to their charges, and non-polar molecules stay together, to get away from charges.  If you float lipids on top of a layer of water, they will orient themselves with their polar head groups associating with the water, and the hydrophobic tails as far away from the water as they can get (i.e. pointing upwards).  Small groups of lipid molecules form micelles, which are little droplets of lipid, with the polar head groups on the outside and the non-polar tails inside associating with one another.  More lipid in an aqueous environment will tend to form lipid bilayers, with the polar head groups oriented toward water.  In the cell membrane, the hydrophilic head groups are oriented toward the outside (extracellular fluid = ECF) and inside (intracellular fluid = ICF = cytosol). 

Cell membranes contain about 75% phospholipids, 20% cholesterol, and 5% glycolipids. 

Lipids are free to move within the membrane in all directions. Cholesterol in the membrane acts to stabilize the phospholipids, providing some rigidity and making it less fluid. 

Membrane lipids can be glycolipids (=sugar –lipid), i.e they can have attached chains of sugar molecules.
 Floating in the “sea of lipid” are proteins, some anchored, some able to move freely.  These can be two types:

1.      integral proteins; embedded in the plasma membrane, either exposed on both ECF and ECF sides, or only one side – the protein molecules have to be amphipathic to have parts able to interact with both the hydrophilic ECF and ICF and the hydrophobic interior of the cell membrane

2.      peripheral proteinsloosely attached to either the inside or outside cell surface

Glycoproteins: 

Found only on the outside surface of the cell membrane – this gives the cell membrane a specific orientation, making it quite different on each side – are glycoproteins.  These have chains of sugar molecules, in varied sequences, attached to integral or peripheral proteins.  This results in the cell being sugar coated – the sugar coat is called the glycocalyx.  This has several functions:

1.      signature sequence (some common to many individuals, like the A-B-O blood group sequences, some specific to a single individual) – the glycocalyx allows cells to recognize one another and is important in immune function

2.      adherence – sugars allow cells to adhere together

3.      protection from digestion – keeps enzymes in the ECF from being able to digest cells

4.      attracts water molecules – this is important in making RBC’s slippery and able to travel through the bloodstream 

Functions of membrane proteins – fig 3.3

Proteins in membranes are very important – proteins are the molecules in a cell that do all the work, the work that they do being dependent on their 3-D shape (tertiary structure).

Membrane proteins can be:

1.      channels - form pores in the membrane that ions which are charged and polar and can’t move through the lipid bilayer by themselves can pass through

2.      transporters – moves things across the cell membrane by changing shape – involved in facilitated diffusion – eg. glucose transporter (we’ll talk about it in detail next lecture) – glucose binds on outside of cell, protein changes shape and moves it to the inside

3.      receptors – recognize a specific ligand and change the cell function in some way when bound – the ligand (eg. hormone like insulin) can deliver a message into the cell without being transported into the cell

4.      enzymes – catalyzing a reaction inside or outside of cell – eg. lactase on surface of intestinal epithelial cells breaks down lactose (milk sugar), which is a disaccharide of galactose & glucose

a.      only lactase can break down lactose (enzyme specificity)

b.      if it’s not broken down, bacteria in intestine act on it, producing gas as by-product

c.      genetic composition can lead to an individual lacking lactase, so lactose-intolerant – gas produced by bacteria causes discomfort

d.      Lactaid tablets are lactase, or person can drink milk which has the lactose pre-digested

5.      Cell-identity markers – distinguish your cells from anyone else’s (except identical twins) – often glycoproteins, cause problems with organ transplant because your immune system knows which cells are your own (self) & which are not (non-self) and works to destroy non-self

6.      Linkers – proteins can link one cell to another, or link proteins outside the cell to proteins inside – important in cell movement and cell signaling

Membrane permeability

Cell or plasma membrane is selectively permeable – it allows some things to move across easily and other things not.

        Permeable to O2, CO2, steroids – generally things soluble in lipid

        Impermeable to ions, glucose – generally things soluble in H2O – these things can only cross membrane through channels

Exception is H2O itself, which is very permeable to cell membranes – moves through cells in two ways: 

1.      through special membrane protein channels called aquaporins

2.      by slipping through temporary spaces between membrane lipids caused by their movement

Concentration gradients – figure 3.4:

Because the membrane is differentially or selectively permeable, the concentration of some substances is higher inside, others are higher outside

            Eg.  [Na+] is higher outside, [K+] is higher inside

This results in a concentration gradient across the cell membrane (this is a chemical gradient).  Overall, more anions end up inside the cell, and more cations out – resulting in a charge difference on either side – a charge gradient.  Together these two gradients from the electrochemical gradient.   This is very important to a cell’s function, especially an excitable cell like a neuron or muscle cell.

Diffusion:

-          movement of molecules by random thermal motion from areas of high concentration to areas of low concentration (eg. sugar cube in cup of coffee – sugar molecules move away from cube – eventually equilibrium is reached) 

How do things move across membranes?

4 ways:

1.      diffusion directly through the lipid bilayer – must be non-polar and relatively small to do this easily – large non-polar molecules can get stuck in membrane

2.      diffuse through a channel – some are open all the time (leakage channels) others are gated and can by opened or closed depending on what signals the cell is receiving from its environment

3.      diffusion through a transporter – facilitated diffusion – change in protein shape moves something across

4.      active transport – transport against the concentration gradient – involves a membrane protein is the only way to move something from where it’s present in low concentration to high

1, 2, & 3; are passive processes, free to the cell, don’t require the input of energy – move things with the gradient

4 is an active process, costs the cell ATP to move things against the gradient


Lecture 7 – September 26, 2000

Active transporters can be of three types: 

1.    Uniporters move only a single substance across the membrane.

2.    Coupled transporters move two substances across the membrane and are either symporters or antiporters.  Symporters move the two substances in the same direction

3.     while antiporters move them in opposite directions.

 

Principles of Diffusion

Diffusion: movement of molecules by random thermal motion from areas of high concentration to areas of low concentration

Diffusion rate across plasma membranes is influenced by several factors:

1.    steepness of the concentration gradient –steeper is faster

2.    temperature – faster with higher temp  (how does having a fever affect diffusion?)

3.    size or mass of the diffusing substance –smaller is faster

4.    surface area – greater area, more diffusion

5.    diffusion distance – shorter is obviously faster

Osmosis and tonicity:

Water molecules penetrate the membrane by diffusion through the lipid  bilayer or through aquaporins, transmembrane proteins that function as water channels.  Water moves from an area of lower solute concentration to an area of higher solute concentration.  Osmosis is the movement of water from an area of higher concentration (of water) to an area of lower concentration across the membrane (Fig. 3.7).  Please read pages 67-68 and be sure you understand osmosis.

Tonicity of a solution relates to how the solution influences the shape of body cells.

a.      In an isotonic solution, red blood cells maintain their normal shape (Fig. 3.8a) – water molecules enter and exit at the same rate.

b.      In a hypotonic solution, red blood cells undergo hemolysis (Fig. 3.8b) – the concentration of solutes outside of the cell is less than inside – water moves through the cell membrane to where it is present in low concentration, causing the cell to burst. 

c.      In a hypertonic solution, red blood cells undergo crenation (Fig. 3.8c)- the concentration of solutes inside the cell is less than outside, so water moves out of the cell, causing the cell to shrink, or crenate. 

More about transport mechanisms:

Diffusion through membrane channels:

Most membrane channels are ion channels.  Each is specific for a particular type of ion – channels that allow K+ to pass through don’t allow Na+ to pass through, for instance.  The rate of transport depends on how many channels are in the membrane – but it’s still very fast – a million K+ ions per second can pass through a K+ channel.  Channels may be open all of the time or they may be gated - we'll talk about them more when we do the nervous system.

The final way to move things across the cell membrane doesn’t involve actually crossing the cell membrane.   Instead, vesicular transport involves the formation of membrane-surrounded vesicles to move materials into by endocytosis or out of the cell by exocytosis.   A bit of the membrane surrounds the substance to be imported, and it is pinched off and taken in to the cell as a membrane bound vesicle. 

Facilitated diffusion

Facilitated diffusion isn’t really diffusion – it does involve movement from high to low concentration but it isn’t due to random thermal motion.  Facilitated diffusion involves a transport protein that binds to a specific substance on the outside of the cell.  The most famous example is the glucose transporter (fig. 3.10).  Molecules of glucose bind to the transporter on the extracellular side of the membrane, and this binding causes the transporter protein to change it’s shape, opening up a channel to the other side.  When the other side opens, the glucose molecule is released to the inside of the cell.  If the cell is constantly moving glucose in, how does it keep up a concentration gradient?  As soon as glucose enters the cell, it’s acted upon by an enzyme, a kinase, that adds a phosphate group to it to form glucose-6-phosphate, which then takes part in Kreb’s cycle.  The concentration of glucose itself is thus always low inside of the cell, and glucose will keep moving in. 

Active transporters

Active transport is movement against a concentration gradient, from low concentration to high.

The Na+/K+ ATPase, or pump (fig 3.11) 

This is the most common primary active transporter – in fact, a large portion of your body’s energy (~40%) is spent just making this pump run.  Every cell of your body has thousands of these pumps in their membranes, going all the time.  They maintain the concentration gradients of Na+ and K+ across the cell membrane – remember, Na+ is always high outside, K+ inside.

The Na+/K+ ATPase works like this:

1.    three sodium ions in the cytosol bind to the pump

2.    when Na+ binds, it causes ATP to be hydrolyzed to ADP, and the phosphate group that is removed from ATP is attached to the pump protein.  Adding this phosphate changes the shape of the pump protein, causing it to release the Na+ molecules into the cytoplasm

3.    Now, in it’s new shape, two spots are available for K+ ions to bind to the pump.  When they bind, they make the pump release the phosphate that’s bound to it – without the phosphate, it springs back to its original shape. 

4.    The two K+ are released into the cytoplasm.  The pump is now back to its original state, ready to bind more Na+ and go again.  Each cycle uses one ATP molecule. 

 

The final type of transport into or out of cells is vesicular transport.  A vesicle is a small membrane bound sac, formed when a bit of cell membrane pinches off.  Vesicles can move in two directions:  endocytosis is movement of materials into a cell in a vesicle formed from the plasma membrane, and exocytosis is movement out. 

Exocytosis is used by cells that produce hormones, like the beta cells of the pancreas, which produce insulin.  Insulin is a protein hormone, that’s made inside of the beta cells, on the ribosomes in the rough E.R.  Vesicles bud off from the RER and move to the Golgi where they fuse with the Golgi membrane.  The Golgi serves as a sorting station, and packages up the proteins it receives from the RER for delivery to different parts of the cell.  Things that are meant to go outside of the cell are packaged in carrier vesicles and brought to the plasma membrane – the membrane of the vesicle fuses with the plasma membrane and the is delivered to the extracellular fluid.

There are three types of endocytosis:

1.    phagocytosis  

2.    pinocytosis

3.    receptor mediated endocytosis

Phagocytosis (fig 3.14) = “ cell eating “ = uptake of particulate material (eg. Bacteria).  When taken into the cell, the particle is enclosed in phagosome, then the phagosome fuses with a lysosome.  While almost every cell type can do phagocytosis, macrophages and neutrophils are cells of the immune system that do this for a living. They have lysosomes that are perfect for killing ingested microorganisms:

·        they contain lysozyme, an enzyme that digests the cell walls of bacteria and some other specific enzymes that are deadly to bacteria

·        also contain acids, that can kill bacteria by their pH

Pinocytosis (fig 3.15) = “ cell drinking” – the non-selective uptake of fluid surrounding the cell – allows the cell to sample its surroundings.  From this figure, you could get the impression that it’s simply a tour through the cell, but this isn’t so – whatever is taken up in pinocytic vesicles is digested by lysosomes and delivered across the lysosome membrane into the cell – only what isn’t useful to the cell is collected in a residual body and dumped outside the cell. 

Receptor-mediated endocytosis (Fig 3.13) brings in specific molecules that are found outside of the cell.  The surface of the cell membrane has areas that are specialized for receptor-mediated endocytosis.  In these areas, there are receptors (proteins that bind specific ligands and mediate some effect in the cell) that are specific for particular ligands – found in pits or depressions in the membrane.  In the area surrounding these receptors, but on the inside of the cell membrane, are many molecules of a special kind of protein called clathrin.  When a ligand binds to a receptor, these clathrin molecules join together and surround an inward projection of the cell membrane – each clathrin molecule is a triskelion (if you watched the original Star Trek series, there were aliens from a planet called Triskelion who had emblems on their uniforms with the same shape as these proteins.  The scientists who discovered the protein were fans and named the proteins for the planet).  The triskelions fit together to form a ball-shaped structure containing the receptor and its ligand, and the neck gets pinched off by another type of protein, bringing the vesicle, now coated with clathrin molecules, into the cell.  Once inside the cell, the clathrin coat drops off, and those clathrin molecules go back to the cell membrane.  The contents of the vesicle can have several fates – they can be recycled – this often happens with the receptors, so the cell doesn’t have to make more, it just sends them back to the plasma membrane.  Some things are digested by lysosomes, and some things may just make the trip across the cell and be delivered to the other side – this is transcytosis – this happens with the delivery of maternal antibodies across an infants intestinal epithelial cells – the antibody molecules are found in mother’s milk and are delivered right through the babies intestinal epithelium into the baby’s bloodstream, which gives nursing babies some immunity. 

Lecture 8 – September 28, 2000

Table 3.1, page76, summarizes the topics covered in the last two lectures:  the processes by which materials are transported into and out of cells.

CYTOPLASM

The cytoplasm consists of the cytosol and the cell organelles, other than the nucleus.  Cytosol, the intracellular fluid, is the semifluid portion of cytoplasm contains inclusions and dissolved solutes (Fig 3.1).  The cytosol is composed mostly of water, but is jelly-like due to dissolved proteins, carbohydrates, lipids, and inorganic substances.The chemicals in cytosol are either in solution or in a colloidal (suspended) form.  Functionally, cytosol is the medium in which many metabolic reactions occur.

NUCLEUS

The nucleus is usually the most prominent feature of a cell (Fig. 3.25).  Most body cells have a single nucleus; some (red blood cells) have none, whereas others (skeletal muscle fibers) have several.

The parts of the nucleus include the nuclear envelope which is perforated by channels called nuclear pores, nucleoli, and genetic material (DNA),Within the nucleus are the cell’s hereditary units, called genes, which are arranged in single file along chromosomes.  Human somatic cells (body cells, not reproductive cells) have 46 chromosomes arranged in 23 pairs.  Each chromosome is a long molecule of DNA that is coiled together with several proteins (Fig. 3.26).   DNA is a long, double stranded molecule.  In S phase of mitosis all of the DNA in a cell is duplicated.  DNA is a polymer made up of monomers of nucleotides - the four nucleotides are adenine (A), guanine (G), thymine (T) and cytosine (C).  Each nucleotide on one strand of DNA is paired with a complementary nucleotide on the second strand.  Due to size restraints, A always pairs with T, C always pairs with G.  During replication, the two strands separate, and new nucleotides  (these are found in the cell nucleus) match up with their complementary nucleotides in each strand of DNA, producing two identical copies.  If an error is made in matching, a mutation has occurred.  When the next round of cell division occurs, the mutation gets passed on to the new DNA strands formed.  DNA sequences are genes that code for the production of proteins, so a mutation in a gene sometimes results in the production of an abnormally shaped protein, that is unable to do it's normal job as effectively. 

NORMAL CELL DIVISION

Cell division is the process by which cells reproduce themselves. It consists of nuclear division (mitosis or meiosis) and cytoplasmic division (cytokinesis). Cell division that results in an increase in body cells is called somatic cell division and involves a nuclear division called mitosis, plus cytokinesis.

Cell division that results in the production of sperm and eggs is called reproductive cell division and consists of a nuclear division called meiosis plus cytokinesis.  We'll talk about meiosis when we talk about reproduction - the following discussion is about mitosis.

The Cell Cycle in Somatic Cells

The cell cycle is an orderly sequence of events by which a cell duplicates its contents and divides in two. It consists of interphase and the mitotic phase (Fig. 3.31).

Interphase During interphase the cell carries on every life process except division. Interphase consists of three phases: G1, S and G2 (Fig. 3.31).  In the G1 phase, the cell is metabolically active, duplicating its organelles and cytosolic components except for DNA.  In the S phase, chromosomes are replicated (Fig. 3.32).  In the G2 phase, cell growth continues and the cell completes its preparation for cell division.  A cell in interphase shows a distinct nucleus and the absence of chromosomes (Fig. 3.33a).

Mitotic Phase : The mitotic phase consists of mitosis (or nuclear division) and cytokinesis (or cytoplasmic division).

Nuclear division: Mitosis is the distribution of two sets of chromosomes, one set into each of two separate nuclei.  Stages of mitosis are prophase, metaphase, anaphase, and telophase.  Don’t worry about the details of these stages for this course.

Cytoplasmic Division: Cytokinesis is the division of a parent cell’s cytoplasm and organelles.  When cytokinesis is complete, interphase begins (Fig. 3.33f).

Control of Cell Destiny

A cell can do one of three things:  it can maintain itself without dividing, it can grow and divide, or it can die.  Cell division is necessary to replace worn out cells, or to grow.  Cell death is also necessary – during embryonic development, and to eliminate body cells, such as cancer cells or virally infected cells.  Cells can die in two ways; via necrosis – which is death due to trauma or injury and is rather messy – cells swell, burst and their contents are spilled out – this causes an immune response because our immune systems don’t recognize intracellular contents as “self”, only cell-surface components.  This results in inflammation.  Cells that have to be eliminated during development or due to cancer or viruses are dealt with more tidily – they die by a process called apoptosis.  In apoptosis, the cell doesn’t burst, but it is digested from the inside, and forms small membrane bound vesicles that are cleaned up by neighbouring phagocytic cells, without causing an immune response. 

There are many proteins inside of cells that control life and death – determine whether a cell divides or undergoes apoptosis.  For example, p53 is a protein that normally inhibits cell division – if a defect develops in the gene that codes for the p53 protein, the protein that is made is not the right shape to do its job, and cell division that is normally inhibited is allowed to carry on.  This can result in the formation of a tumour.  There are many other genes that code for proteins involved in either suppressing cell division when it shouldn’t occur, or making apoptosis happen when it should, and if these genes are mutated, a tumour may grow. 

We are constantly subject to mutations in genes, sometimes due to exposure to chemical substances called carcinogens – a chemical that has been shown to cause cancer - (abundant in cigarettes, for instance).  As we age, DNA replication becomes less error-proof, and we gradually accumulate mutations in our cells.  Cancer doesn’t start from a single mutation in a single gene, however;  a cell must accumulate as many as 10 distinct mutations to lose control of its growth and be on its way to growing into a tumour.

Tumours are masses of cells that are dividing uncontrollably.  A tumour can be benign, or it can be malignant.  A benign tumour is relatively harmless, except that its size may interfere with normal body function – eg. A tumour in your brain - or it may be disfiguring, and then they will have to be removed.   A benign tumour isn’t cancerous, however. 
Malignant tumours are cancerous.  They are made up of cells that have not only lost track of their growth, but have also lost track of their place in the body, and the normal controls that keep cells in the organs where they belong.  The movement of cancerous cells throughout the body is called metastasis.

  Cancers are named for the tissue that they develop in. 

1.    Carcinomas (the most common cancer type) arise from epithelial cells, eg. melanomas are skin epithelial cells that produce the pigment melanin.

2.    Sarcomas arise from bone or muscle, eg. Osteogenic sarcoma destroys blood tissue.

3.    Leukemias arise from hemopoetic (blood forming) organs

4.    Lymphomas are cancers of the lymph nodes. 

Growth and Spread of Cancer

Cancer cells divide rapidly and continuously, and trigger angiogenesis, the growths of new networks of blood vessels, to provide nourishment for themselves. 

Treatment of Cancer

Treating cancer is a subject of a lot of medical research, and we lack really effective ways of specifically killing only cancer cells.  Cancer cells differ from normal body cells only in that they aren’t controlling their growth and their location – and this makes it difficult to kill them, as there is nothing dramatically different about them that makes them easy to target with drugs and other treatments.  Various treatments include surgery, chemotherapy, and radiation therapy.  Surgery removes a tumour, but doesn’t always remove metasitized cells, so follow-up therapy with chemicals or radiation is necessary to kill those.  The chemicals used target dividing cells, and radiation has the same effect – it kills cells that are in the process of dividing.  Unfortunately, these methods also kill other dividing cells in the body, particularly developing blood cells, and cause illness and discomfort in the patients. 

Lecture 8 also includes an intro to the integumentary system - look under that chapter heading for notes.