Download Biology Cells Lecture B. Rife SOHI 2001

Biology Cells Lecture B. Rife SOHI 2001
Cell is the smallest unit of living matter. All organisms are made up of cells.
Cell Theory - Cells are capable of self-reproduction. Cells come only from preexisting cells.
Plasma membrane - is a thin layer of protoplasm, consisting mainly of phospholipids and proteins
(cholesterol), which is present on the surface of all cells.
The plasma membrane separates the cell interior from the outside environment and regulates the passage
of molecules into and out of the cell. (Fluid Mosaic Model)
Organelles - are small membranous bodies whose structure suits their function.
Cytoplasm - refers to the contents of a cell that lies between the nucleus (or nucleoid) and the plasma
membrane.
Microscopes Provide Windows To The World Of The Cell
Images formed by microscopes represent the object “under” the microscope.
Two aspects of microscope images are important.
1. Magnification: the number of times larger the image appears than the object actually is.
M total = M ocular x M objective
2. Resolution: clarity of the image
Light Microscope (LM) bend the light coming through an object. The bent light rays form larger
images in the viewer’s eyes. Well-resolved LM images are limited to 1000-2000 times larger
than life size.
Always start viewing with low power
The LM is particularly good for looking at living cells and tissues.
Scanning electron microscope (SEM) composes images on a TV screen, from electrons that bounce
off the surfaces of the object. SEM images are usually about 10,000-20,000 times larger
than life size. The SEM is particularly good for showing organismal and cellular surfaces
under high magnification.
Transmission electron microscopes (TEM) compose images on camera film, from electrons that
have traveled through very thin slices of the object and have been bent by magnetic lenses.
TEM images are usually about 100,000-200,000 times larger than life size. The TEM is
particularly useful for showing the internal structures of cells.
Chemical Basis of Life
Element - A substance that cannot be broken down into two or more different substances.
(C, H, O, N = 96% of the elements in the human body)
Compounds - A substance composed of two or more elements chemically combined.
(Lipids, Carbohydrates, Proteins, Nucleic Acids)
Cell Sizes Vary With Their Functions
Nerve cells are very long, to communicate between different parts of an animal’s body.
Bird eggs are very large, mostly composed of food reserves.
Blood cells are very small, to allow them to flow through blood vessels, and to provide a large surface
area (from many cells) for efficient gas exchange.
Natural Laws Limit Cell Size
Prokaryotic cells are between 1 µm and 10 µm
Eukaryotic cell are between 10 µm and 100 µm
1 µm = 1 x 10 -6 m (REVIEW ALL SI UNITS)
Nutrients enter a cell and wastes exit a cell at the plasma membrane.
A cell needs a surface area (plasma membrane) that can adequately exchange materials with the
environment. Surface area to volume relationships require that cells stay small.
1 x 1 x 1 mm cell
Surface Area
6 mm2
Volume
1 mm3
A:V Ratio
6:1
2 x 2 x 2 mm cell
24 mm2
8 mm3
3:1
4 x4 x 4 mm cell
96 mm2
64 mm3
1.5:1
Prokaryotic Cells Are Small And Structurally Simple
Prokaryotic cells - lack a true nucleus. Their DNA is in a single chromosome located within a region called
the nucleoid. There are few organelles.
Prokaryotic cells are bacteria and cyanobacteria (blue-green algae
Eukaryotic Cells Are Partitioned Into Functional Compartments
Eukaryotic cells - have a true nucleus, a membrane-bound compartment that houses DNA within
chromosomes.
All cells, aside from bacteria, are eukaryotic. Eukaryotic organisms include the algae, protozoa, and
fungi, in addition to plants and animals.
Eukaryotic organisms are internally complex, with organelles of two types: membranous and
nonmembranous.
Membranous organelles found in eukaryotic cells include the nucleus, endoplasmic reticulum, Golgi
apparatus, mitochondria, lysosomes, and microbodies.
Nonmembranous organelles found in eukaryotic cells include ribosomes, microtubules, centrioles, flagella,
and the cytoskeleton.
Animal cells are bounded by the plasma membrane alone, often have flagella, and lack a cell wall.
Plant cells are bounded by both a plasma membrane and a rigid cell wall. In addition, plant cells usually
have a central vacuole and chloroplasts, lack centrioles, and usually lack flagella.
The Nucleus Is The Cell’s Genetic Control Center
Nucleus - is a small mass of nucleoproteins surrounded by a double membrane. It contains
chromosomes (DNA), and functions in metabolism, growth, and reproduction.
DNA always remains within the nucleus, but messenger RNA (mRNA), carries the instructions from DNA
out into the cytoplasm where protein synthesis occurs (ribosomes)
Nucleoli - are small spherical bodies composed principally of ribosomal RNA (rRNA) located in the nucleus.
Nuclear membrane - The envelope surrounding the cell nucleus, separating the nucleoplasma from the
cytoplasm; composed of two membranes and contains numerous pores.
Many Cell Organelles Are Related Through The Endomembrane System
An extensive system of membranous organelles works together in the synthesis, storage, and export of
molecules.
A single membrane bound each of these organelles. Some are in the form of flattened sacs, some are
rounded sacs, and some are tube-shaped.
Rough Endoplasmic Reticulum Makes Membrane And Proteins
Ribosomes - are small organelles composed of proteins and RNA (ribonucleic acid) which synthesize
proteins from amino acids within the living cell.
Some ribosomes may lie free within the cytoplasm but most are attached to the endoplasmic reticulum
(rough ER)Rough ER specializes in protein synthesis.
The endoplasmic reticulum is a transport system. Protein molecules move from the lumen of rough ER to
that of smooth ER, which sends them enclosed within vesicles usually to the Golgi complex.
Vesicles move to the plasma membrane where they discharge their contents and add new membrane as
the vesicle fuses with the plasma membrane.
Smooth Endoplasmic Reticulum Has Many Functions
Smooth ER has a function suited to the particular type cell; sometimes it specializes in lipid synthesis.
In other forms of smooth ER, enzymes help process materials as they are transported from one place
to another. An example of this function is the detoxification of drugs (alcohol) by smooth ER
in liver cells.
The Golgi Apparatus Finishes, Stores, And Ships Cell Products
Golgi Complex - A cellular organelle that is part of the cytoplasmic membrane system; it is composed of
regions of stacked cisternae and it functions in manufacture, storage and secretory processes.
(via vesicles)
The hormone insulin is received by the Golgi in an inactive form, but after certain amino acids are removed,
the hormone is active and ready to be secreted.
Most common processing by the Golgi is the synthesis of glycoproteins
Golgi complexes produce lysosomes.
ILysosomes Digest The Cell’s Food And Wastes
Lysosomes - are membrane-bounded vesicles that contain hydrolytic digestive enzymes.
The lysosomal enzymes digest macromolecules into simpler subunits. They also digest bacteria and even
parts of the cell. (autodigestion - suicide packets)
Microbodies - are similar to lysosomes because they are vesicles bounded by a single membrane and
contain specific enzymes.
They are believed to help detoxify alcohol.
Vacuoles Function In General Cell Maintenance
Vacuole - A membrane-bound cavity within a cell; may function in digestion, storage, secretion, or
excretion.
Typically, plant cells have one or two large vacuoles so filled with a watery fluid that it gives added support
to the cell.
Plant vacuoles contain not only water, sugars, and salts, but also pigments and toxic substances.
Contractile vacuoles in cells of freshwater protists (both protozoans and algae) function in water balance.
Mitochondria Harvest Chemical Energy From Food
Cellular respiration, which occurs in mitochondria, is the process by which the chemical energy of
carbohydrates is converted to that of ATP (adenosine triphosphate). ATP energy is use for
synthetic reactions, active transport, and all energy-requiring processes in cells
C6H12O6 + 6O2 ---> 6CO2 + 6H2O + ATP ENERGY
Mitochondria - is a double membrane-bound organelle of the cell known as the powerhouse because it
transforms the energy of carbohydrates and fats to ATP energy.
The inner membrane is folded to form little shelves, called cristae, which project into the matrix, an inner
space filled with enzymes.
Like chloroplasts, mitochondria also contain their own DNA and ribosomes, and can reproduce themselves
by division.
Origin The endosymbiotic theory states that chloroplasts and mitochondria were originally prokaryotes that
came to reside inside a eukaryotic cell, establishing a symbiotic relationship known as mutualism.
Cells Have An Internal Skeleton
Eukaryotic cell shape and motility are dependent upon the cytoskeleton, an internal framework that
contains microtubules, intermediate filaments, and microfilaments that differ as to composition and
size.
Microfilaments - are long, solid and extremely thin fibers, composed of globular proteins, that usually occur
in bundles or networks.
Actin microfilaments are well known for their role in muscle contraction where they interact with other
protein microfilaments composed of myosin.
Intermediate Filaments - are intermediate in size between microtubules and microfilaments and are
composed of fibrous proteins.
In the skin, where the fibers are made of keratin, the the presence of intermediate filaments gives great
mechanical strength to cells.
Microtubules - are hollow tube-like filaments composed of globular proteins and are found free within the
cytoplasm or in definite patterns within cilia and flagella and are composed of repeating subunits
of the protein tubulin.
Cilia And Flagella Move When Microtubules Bend
Cilia - are relatively short, numerous and centriole-based, hairlike processes on certain motile organisms.
Flagella - are relatively long, whiplike, centriole-based locomotor organelles on some motile cells.
In both cases, these nonmembranous organelles are minute, tubular extensions of the plasma membrane
that surround a complex arrangement of microtubules.
The underlying structure consists of nine microtubule doublets arranged in a cylinder around a central pair
of microtubules. At the base within the cell body (basal body), the structure is slightly different.
Cilia and flagella function to move whole cells or to move materials across or into cells.
Eukaryotic Organelles Comprise Four Functional Categories
Manufacture: Nucleus, Ribosomes, Rough ER, Smooth ER, Golgi apparatus - synthesis of
macromolecules and transport within the cell
Breakdown: Lysosomes, Microbodies, Vacuoles
- elimination and recycling of cellular materials
Energy processing: Chloroplasts, Mitochondria
- conversion of energy from one form to another
Support, movement, and communication: Cytoskeleton, Cell walls, Cell surfaces, Cell junctions
- relationships with extracellular environments
Within each of the four categories there are structural similarities that underlie their functions.
All four categories work together as an integrated team, producing the emergent properties at the
cellular level.
Membranes Organize The Chemical Activities Of Cells (Function)
Membranes separate cells from their outside environments, including, in multicellular organisms, that
environment in other cells that performs different functions..
They control the passage of molecules form one side of the membrane to the other.
In eukaryotes, they partition function into organelles.
The provide reaction surfaces, and organize enzymes and their substrates.
The plasma membrane regulates the passage of molecules into and out of the cell.
The plasma membrane is selectively permeable - it has special mechanisms to regulate the
passage of most molecules into and out of the cell.
Membrane Phospholipids Form A Bilayer
Phospholipids are like fats, with two nonpolar (hydrophobic) fatty acid “tails” and one polar
(hydrophilic) phosphate “head” attached to the glycerol.
In water, thousands of individual molecules form a stable bilayer, aiming their heads (hydrophilic) out
and their tails (hydrophobic) in.
The hydrophobic interior of this bilayer offers an effective barrier (selectively permeable) to the flow
of most hydrophilic molecules.
The Membrane Is A Fluid Mosaic Of Phospholipids And Proteins
The fluid-mosaic model of membrane structure is widely accepted at this time.
It is fluid because the individual molecules are more-or-less free to move about laterally.
Cholesterol is a common constituent of animal cell membranes and helps stabilize the fluidity at
different temperatures.
Some proteins extend through both sides of the bilayer.
Proteins Make The Membrane A Mosaic Of Function
The movement of molecules through the plasma membrane is dependent upon its protein
component.
Functions for plasma membrane proteins:
1. cell to cell recognition (Identification tags - particularly glycoproteins)
2. receptors for chemical messengers (trigger cell activity when a messenger molecule attaches.
3. enzymes catalyzing intracellular and extracellular reactions
4. passage of (hydrophilic) molecules across the membrane
5. Cell junctions - either attachments to other cells or the internal cytoskeleton.
Permeable allows all molecules to pass through
Impermeable allows no molecules to pass through
Cell membranes are selectively permeable. They permit the free passage of some materials and
restrict the passage of others.
Passive Transport Is Diffusion Across A Membrane
Diffusion is the tendency for particles of any kind to spread out spontaneously from an area of high
concentration to an area of low concentration.
Only a few types of small molecules freely diffuse through the plasma membrane.
Passive transport across membranes occurs when a molecule diffuses down a concentration
gradient. At equilibrium, molecules continue to diffuse back and forth, but there is no net
change in concentration anywhere.
Different molecules diffuse independently of one another.
Passive transport is an extremely important way for small molecules to get into and out of cells. For
example, O2 moves into red blood cells and CO2 moves out of these cells by this process in
the lungs.
Osmosis Is The Passive Transport Of Water
Osmosis is the diffusion of water across a selectively permeable membrane. The presence of
osmotic pressure is evident when there is an increased amount of water on t he side of the
membrane that has the higher solute concentration.
When a cell is placed in an isotonic solution (same solute concentration), it neither gains nor loses
water.
When a cell is placed in a hypotonic solution (less solute conc. - more water conc.) the cell gains
water.
When a cell is placed in a hypertonic solution (greater solute conc. - less water conc.) the cells
loses water and the cytoplasm shrinks.
Water Balance Between Cells And Their Surroundings Is Crucial To Organisms
If a plant or animal cell is isotonic with its surroundings, no osmosis occurs, and the cells do not
change. however plant cells in such environments are flaccid or wilted, lacking the turgor that
helps support some plant tissues.
An animal cell in a hypotonic solution will gain water and lyse (burst)
A plant cell in a hypotonic solution, the cytoplasm and central vacuoles gain water, and plasma
membrane pushes against the rigid cell wall. The resulting pressure, called turgor pressure,
helps give internal support to the cell.
An animal cell in a hypertonic solution will lose water and shrivel. A plant cell in a hypertonic
solution will pull the plasma membrane away from the cell wall and lose turgor.
Specific Proteins Facilitate Diffusion Across Membranes
Facilitated diffusion occurs when a pored protein, spanning the membrane bilayer, allows a solute
to diffuse down a concentration gradient. No energy expenditure is required in this case.
The rate of facilitated diffusion depends on the number of such transport proteins, in addition
to the strength of the concentration gradient. Hormones often enter cells this way.
Filtration: movement of water and small solute particles, but not larger particles, through a filtration
membrane; movement occurs from area of high pressure to area of low pressure (Down a
pressure gradient). Materials are removed from a solution on the basis of size. Urine
formation involves filtration in the nephron.
Cells Expend Energy For Active Transport
Active transport involves the aid of a transport protein in moving a solute up a concentration
gradient.
Energy expenditure in the form of ATP-mediated phosphorylation is required to help the protein
change its structure and thus move the solute molecule.
Active transport proteins often couple the passage of two solutes in opposite directions across
membranes. A very important example of a coupled active transport system is the Na+ - K+
pump (ion pump), which functions in nerve impulse transmission