Last lecture was spent talking about basic chemistry and some of the kinds of molecules found in living organisms. Today, we are going to consider the cell.
Cells are the lowest level of the biological hierarchy and are the smallest units that possess the characteristics of life: organization, metabolism, ability to reproduce.
Cells have three major components:
1. an outer plasma membrane.
2. a region containing DNA.
3. cytoplasm -- everything else except the DNA.
A cell is separated from the external environment (or from other cells) by a membrane called the plasma membrane.
Plasma membrane -- membrane composed of 1) a double layer of phospholipids and 2) proteins that are embedded in the phospholipid bilayer.
Putting phospholipids into water results in the formation of a lipid bilayer -- phospholipids have a polar "head" that is attracted to water and a nonpolar, fatty acid "tail" that is repelled by water -- bilayer is fluid-like and displays quite a bit of movement.
Figure 4.3.
Imbedded into the fluid-like bilayer of lipids are proteins -- now we have what is called the "fluid-mosaic model."
Figure 4.4.
Proteins in plasma membrane have at least two important functions:
1. transport of materials in and out of cell -- more in a minute.
2. receptors of chemical messages coming to cell from somewhere else.
Besides holding the cell together, plasma membrane regulates what moves between cell and its environment -- only certain molecules can move through the membrane -- membrane is selectively permeable -- small molecules pass through more easily than large ones.
There are several processes by which materials move in and out of cells through the plasma membrane:
1. diffusion -- movement of molecules from region of high to region of low concentration-- a concentration gradient must exist between inside of cell and environment -- some small molecules are able to pass through lipid bilayer by diffusion -- e.g. CO2, O2 enter/leave cells by diffusion.
2. osmosis -- special case of diffusion = diffusion of water across a cell membrane -- concentration of water (solvent) varies with amount of dissolved material (solute) -- 100% pure H2O is most concentrated -- difference in conc. of water on both sides of a membrane referred to as osmotic pressure (greater the difference, the greater the osmo. pressure).
Note: Diffusion and osmosis take place without the cell expending any energy.
3. Active transport -- it is sometimes necessary for the cell to move molecules against the concentration gradient -- cell must expend energy (ATP) to do this -- proteins imbedded in cell membrane act as carriers for specific ions or molecules.
4. Endocytosis and exocytosis -- large items can be brought into cell by engulfing the object and storing them in a membrane-line compartment called a vesicle (endocytosis) -- likewise, material stored internally in a vesicle can be released into the environment by the reverse process (exocytosis).
Let’s now talk about what cells have inside of them.
Cells can be classified into two general types:
1. prokaryotes -- examples are the cells known as bacteria -- prokaryotes are the most primitive cells -- have the following characteristics:
a. DNA not enclosed in a nucleus -- "prokaryote" means "before nucleus."
b. plasma membrane surrounded by a semi-rigid cell wall -- composed of a polysaccharide.
c. cytoplasm contains only one type of organelle: ribosomes -- ribosomes are the organelle in which protein synthesis occurs.
2. eukaryotes -- cells having a membrane-bound nucleus.
a. DNA enclosed by a membrane called the nuclear envelope
b. cytoplasm contains ribosomes and other organelles -- organelles are membrane-bound structures that have a specific metabolic function within the cell.
Eukaryotic cells can further be classified as either plant or animal -- both have some common characteristics -- plant cells have several things animal cells do not have.
Let’s look at characteristics common to both plants and animals.
Nucleus = membrane-bound control center that contains the cell’s genetic material -- components of nucleus:
1. nuclear envelope -- membrane surrounding nucleus -- contains pores to allow chemical messages to move in and out of nucleus.
2. chromosomes -- DNA and protein -- the hereditary material that is passed on when the cell divides -- carry instructions as to how cell functions, reproduces, etc.
3. nucleolus -- 1 or more sphere-like structures composed primarily of RNA -- function is the production of ribosomes.
Other cellular organelles --
1. ribosomes -- small, spherical structures found throughout cell -- composed of ribosomal RNA and some protein -- ribosomes are the site of protein synthesis -- may bound linked together in groups called polysomes.
2. endoplasmic reticulum (ER) -- complex of membranous passageways found throughout cell --
a. rough ER -- studded with ribosomes -- produces proteins to be used outside the cell.
b. smooth ER -- no ribosomes -- some hormones and fats are synthesized by the smooth ER -- also used for internal transport of materials from one part of cell to another.
3. Golgi bodies -- stack of flattened, interconnected sacs -- involved in the modification of lipids and proteins -- packaged into vesicles for transport to specific locations in the cell.
4. lysosomes -- vesicles produced by Golgi bodies that contain powerful digestive enzymes -- fuse with other vesicles containing materials brought in by endocytosis and then digest the material.
5. vacuoles -- membrane-bound sacs (large vesicles) -- more often found in plants than in animals -- storage of such things as sugars, starch.
6. mitochondrion -- powerhouse of the cell -- site of process called cellular respiration = conversion of the chemical energy in glucose into ATP, which is the form of energy that the cell can use -- consists of a double membrane --scattered throughout the cell.
7. cytoskeleton -- fibers and threads that give cells their shape and internal organization -- basic unit is the microtubule -- filament made up of protein called tubulin -- filaments anchor cell organelles -- two other important structures are made up from bundles of microtubules:
a. flagella -- long, whip-like structures that give cells motility (e.g. sperm) -- usually just one or several per cell.
b. cilia -- shorter structures -- more abundnant on cell surface -- used for motility or for "stirring" surroundings (e.g. ciliated cells of respiratory tract).
Here are figures from the book showing a typical animal cell.
Figure 4.7
Now, let’s look at some cellular organelles and structures that are unique to plants:
1. plastids -- membranous sacs containing various pigments -- give plants the green color that we are familiar with -- yellows, reds, and oranges also -- special type called chloroplasts contain chlorophyll pigments and are the site of photosynthesis in green plants -- photosynthesis is the process by which plants use energy from sunlight to remove CO2 from the air and chemically convert it to glucose -- much more on this later.
2. central vacuole -- large, fluid-filled vesicle -- may occupy 50%-90% of cell’s interior -- stores amino acids, sugars, and other materials.
3. cell wall -- plants cells encased in a layer of cellulose (polysaccharide) -- gives plant cells rigidity and helps them keep their shape.
Here's a typical plant cell.
Figure 4.7
NEXT TIME: Energy Pathways in Cells.