Download The Cellular Level of Organization

The Cellular Level of Organization
Objectives
List the structure and functions of the cell membrane.
Specify the routes by which different ions and molecules can enter or leave a cell
Explain transmembrane potential
Describe the structure and function of each organelle
Explain the structure and function of the nucleus
Explain the genetic code
Summarize the process of protein synthesis
Describe the stages of the cell cycle
Describe the process of mitosis
An Introduction to Cells
• Cell theory (Robert Hooke, 1665)
- Cells are the building blocks of all plants and animals.
- All cells come from division of preexisting cells.
- Cells are the smallest units that perform all vital physiological functions.
- Each cell maintains homeostasis at the cellular level.
• Cytology: the study of cell structure and function, a division of cell biology.
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Two classes of cells in the human body:
1. sex cells (germ cells): reproductive cells [male sperm, female oocytes
(eggs)]
2. somatic cells (soma = body): all body cells except sex cells.
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Human body cells are surrounded by a cell membrane which separates the inside
(cytoplasm) from the outside (extracellular) fluid. Fluid between cells is called
interstitial fluid. (interstitium = something standing between).
•
Cell membrane is more than just a barrier; it selectively transports nutrients into,
and cellular products and wastes out of, the cell.
The Cell Membrane or Plasma Membrane
 The cell membrane is made up of a double layer of phospholipid molecules
(phospholipid bilayer) with their hydrophilic heads toward the watery
environment on both sides.
Membrane Proteins
 There are 2 structural classes of membrane proteins:
1. integral proteins: embedded within the membrane.
2. peripheral proteins: stuck to the inner or outer surface of the membrane.

These large protein molecules are divided into 6 specialized functions:
1. anchoring proteins: attach cell membrane to inside or outside structures.
2. recognition proteins: label cells as normal or abnormal to the immune
system.
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3.
4.
5.
6.
enzymes: catalyze reactions inside or outside the membrane.
receptor proteins: bind to extracellular molecules (ions, hormones).
carrier proteins: transport solutes through the cell membrane using energy.
channels: pores that regulate water and specific solute movement
Membrane Carbohydrates
 Carbohydrates in the cell membrane extend outside the cell membrane
1. Lubrication and protection
2. Anchoring and locomotion
3. Specificity in binding (receptors)
4. Recognition (immune response)
The Cytoplasm
 Cytoplasm includes all materials inside the cell membrane but outside the
nucleus. The 2 components of cytoplasm are:
1. cytosol (intracellular fluid): thick liquid with dissolved nutrients, ions,
soluble and insoluble proteins, waste products.
2. organelles: structures with specific functions inside the cell..
The Organelles
 The 6 types of nonmembranous organelles are: cytoskeleton, microvilli, centrioles,
cilia, ribosomes and proteasomes.
1. cytoskeleton: structural proteins for shape, strength,
2. microvilli: finger-shaped projections on the surfaces of some cells, increase
surface area for absorption, attached to cytoskeleton.
3. centrioles form spindle apparatus during cell division,
4. cilia: long extensions of cell membrane move fluids across the cell surface
5. ribosomes: carry out orders from the nucleus for protein synthesis.
i. -Protein synthesis begins when 2 subunits join a strand of
messenger RNA (mRNA).
ii. -2 types of ribosomes in cells are:
free ribosomes: scattered through cytoplasm
fixed ribosomes: attached to endoplasmic reticulum
•
The 5 types of membranous organelles are: endoplasmic reticulum, Golgi apparatus,
lysosomes, peroxisomes and mitochondria.
1. endoplasmic reticulum a network of membranes forming storage chambers
Smooth endoplasmic reticulum (SER) has no ribosomes. Its main
function is to synthesize lipids
Rough endoplasmic reticulum (RER) is rough because of
ribosomes attached to its surface. Functions in protein and
glycoprotein synthesis.
2. Golgi apparatus: a stack of 5 or 6 flattened membrane containers (cisternae)
modifies and packages proteins for exocytosis
3. Cells clean up their internal environment with powerful enzyme-containing
vesicles called lysosomes (lyso = dissolve, soma = body).
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•
4. Peroxisomes are enzyme-containing vesicles that break down fatty acids and
other organic compounds, producing the dangerous free radical hydrogen
peroxide (H2O2).
5. Cells require energy to function. Mitochondria (singular: mitochondrion)
produce energy in the form of ATP. Produce energy by breaking down foods,
such as the sugar glucose. This process is called aerobic metabolism or cellular
respiration.
(*) Mitochondria provide most of the energy needed to keep your cells alive.
The Nucleus
 The cell’s control center, the nucleus, is the largest organelle in the cell. It’s
surrounded by a double membrane (the nuclear envelope) with communication
passages called nuclear pores. The nucleus contains all of our DNA
- chromatin: loosely coiled DNA (cells not dividing)
- chromosomes: tightly coiled DNA (cells dividing)
Information Storage in the Nucleus
 How is genetic information stored?
- DNA carries instructions to produce every protein in the body. Each
protein is a unique sequence of amino acids.
- The DNA instructions for one protein make up a gene. The chemical
language of DNA instructions is the genetic code.
- Genetic code is the order (sequence) of the nitrogenous bases (A, T, C
and G) in the DNA molecule. Every group of three bases (the triplet code)
represents one amino acid.
- There is more than 1 code for each amino acid, e.g. TGT and TGC are
both genetic code for the amino acid cysteine.
- Not every gene codes for a protein, some code for RNA, or have
unknown functions.
Gene Activation and Protein Synthesis
The first step in protein production is gene activation (uncoils DNA).
Complementary strands of DNA uncoil in the specific area of one gene
(marked by start or promoter codes and stop codes).
The gene is read by an enzyme called RNA polymerase.
- One half of the DNA (the coding strand) codes for the protein.
RNA polymerase copies the complete code for the protein (gene) onto a strand
of messenger RNA (mRNA) in the process of transcription.
- Starting with the promoter or start sequence, RNA polymerase
binds free nucleotides together to form a strand of mRNA, duplicating the
DNA coding strand, but substituting uracil nucleotides for thymine
nucleotides.
- In the resulting mRNA strand, each group of 3 nucleotides
(triplet) is a codon representing one amino acid.
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At the stop signal, the mRNA strand detaches from the DNA molecule and its
code is edited (RNA processing).
The mRNA leaves the nucleus through a nuclear pore and carries the
instructions to a ribosome in the cytoplasm, where protein synthesis occurs.
- Translation begins when an mRNA strand is bound between a
small and a large ribosomal unit.
- Each mRNA codon (sequence of 3 nitrogenous bases) translates
to one amino acid in a polypeptide chain.
- In the cytoplasm, transfer RNA (tRNA) delivers free amino acids
to the mRNA. Each t-RNA is identified by a specific anticodon, which
identifies the type of amino acid it carries. The anticodon binds to a
specific mRNA codon, creating the correct sequence of amino acids for a
specific protein.
- Enzymes join the amino acids together in a polypeptide chain.
- One mRNA can be read by many ribsomes (polyribosome) to
synthesize multiple copies of the same polypeptide.
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(*) Genes are functional units of DNA containing instructions for one or more
proteins. Protein synthesis requires several enzymes, ribosomes, and 3 types of
RNA.
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(*) A mutation is a change in the nucleotide sequence of a gene, caused by
chemical or radiation exposure or by mistakes during DNA replication, and can
change gene function.
How Substances Get Into and Out of Cells
• The cell membrane is a barrier -- but nutrients must get in, and products and
wastes must get out.
• Permeability determines which materials move in and out of a cell.
- Selectively permeable membranes allow some materials to move freely and
restrict others
• Transport through a cell membrane can be active (requiring energy/ATP) or
passive (no energy required).
-The 3 major categories of transport are diffusion (passive), carriermediated transport (passive or active), and vesicular transport (active).
The Transmembrane Potential
• The electrical charge inside the cell membrane is slightly negative, and the
outside slightly positive (more positive ions outside, more negative ions inside).
• Transmembrane potential provides electrical energy to muscles, the nervous
system and some glands.
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The Cell Life Cycle
• A human being grows from 1 cell to 75 trillion cells by cell division.
- Cells grow, age and die at a genetically determined rate (apoptosis)
- Most of a cell’s life is spent in a non-dividing state (interphase)
- For body (somatic) cells to divide, their genetic material must be
duplicated exactly (DNA replication) and divided equally (mitosis)
- Cytokinesis divides the cytoplasm and organelles into 2 daughter cells
- The life cycle of a cell from one division to another is the cell cycle.
-Sex cell activity is different (meiosis) discussed later in reproductive unit.
Interphase
• Body cells spend more time working than dividing. The working period is called
interphase.
• In DNA replication, DNA strands unwind and DNA polymerase begins attaching
complementary nucleotides along each strand.
Mitosis
• Mitosis occurs in 4 stages: prophase, metaphase, anaphase, telophase.
Please make another two
Stage 1. Prophase:
The 2 DNA copies coil tightly into chromatids, connected at a centromere.
This visible structure is a chromosome.
Stage 2. Metaphase:
Chromosomes align in a plane between the centrioles (metaphase plate)
Stage 3. Anaphase:
Chromosomas pull apart into 2 daughter chromosomes
Stage 4. Telophase:
- nuclear membranes reform
- chromosomes uncoil
- nucleoli reappear
- cell has 2 complete nuclei
• (*) Mitosis is the duplication of chromosomes in the nucleus in preparation for
cell division.
Cytokinesis
• Cytokinesis is the division of the cytoplasm of the cell into 2 daughter cells.
Cell Division and Cance
- A tumor or neoplasm is an enlarged mass of cells produced by abnormal
cell growth and division.
- A benign tumor is contained and not life threatening.
- Cells in a malignant tumor spread into surrounding tissues and start new
tumors (metastasis).
- An illness that disrupts normal cellular controls and produces malignant
cells is cancer.
- Mutated genes that disrupt normal cellular controls are oncogenes.
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Cancer develops in series of steps:
- abnormal cell
- primary tumor
- metastasis
- secondary tumor
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(*) Cancer results from mutations that disrupt normal controls over cell growth
and division. Cancers often begin where stem cells are dividing rapidly. More
chromosome copies mean greater chance of error.
Cell Diversity and Differentiation
• All cells carry a complete set of DNA, with instructions for all bodily functions.
- In order for tissues to form, cells must specialize or differentiate
(e.g. liver cells, fat cells. neurons).
- This requires turning off all the genes not needed by that cell.
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(*) All your body cells (except sex cells) contain the same 46 chromosomes. What
makes cells different from one another is which genes are active, and which genes
are inactive.
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