Authors Yamni Nigam is professor in biomedical science; John Knght s associate With the exception of inges tion, the small and large intestines carry out all the major functions of the digestive system. This is where the ‘real business’ of digestion takes place. The intestines take up most of the space in the abdominal cavity and constitute the greatest portion of the gastrointestinal (GI) tract in terms of mass and length. Part 4 in this sixpart series on the GI tract described the anatomy and function of the small intestine (Bit.ly/NTGITract4). Part 5 describes the anatomy and functions of the large intestine, as well as common pathologies that affect both the small and large intestine. Anatomy of the large intestine The large intestine is approximately 1.5m long and comprises the caecum, colon, rectum, anal canal and anus (Fig 1). The structure of the large intestine is very sim ilar to that of the small intestine (see part4), except that its mucosa is com Caecum and appendix Chyme that has not been absorbed by the time it leaves the small intestine passes through the ileocaecal valve and enters the large intestine at the caecum. On receipt of the contents of the ileum, the caecum con tinues the absorption of water and salts. The caecum is about 6cm long and extends downwards into the appendix, a winding tubular sac containing lymphoid tissue. The appendix is thought to be the vestige of a redundant organ; its narrow and twisted shape makes it an attractive site for the accumulation and multiplica tion of intestinal bacteria. Colon At its other end, the caecum seamlessly joins up with the colon, this is the longest portion of the large intestine (Fig 1). Food residue starts by travelling upwards through the ascending colon, located on the right side of the abdomen. The ascending colon bends near the liver at the right colic flexure (or hepatic flexure) and becomes the transverse colon, passing professor in biomedical science; Nkki Wiiams is associate professor in respratory physiology; all at the Colege of Human Heath and Sciences, Swansea Unversity. Abstract In the large intestine the fina section of the gastrointestna tract absorption of water and eectrolytes takes pace and coonic bacteria complete the process of chemica digestion. The large intestine is aso where faeces are formed from the remains of food and fluid combined with byproducts of the body. Intestinal content s pushed back and forth by haustra contractions and antiperistatic contractions, unti faeces are finally pushed towards the anal cana by mass movements. Ths article, the fifth n a sixpart series exporing the gastrointestinal tract, describes the anatomy and functions of the large intestine. Citation Nigam Y et a (2019) Gastrontestina tract 5: the anatomy and functons of the arge intestne Nursing Times [onine]; 115: 10, 5053 pletely devoid of villi.

1. LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
The Structure and Function of Proteins
Chapter 5 (continued)
Lectures modified by Garrett Dancik

2. Concept 5.4: Proteins include a diversity of
structures, resulting in a wide range of
functions
• Proteins account for more than 50% of the dry
mass of most cells
• Protein functions include structural support,
storage, transport, cellular communications,
movement, and defense against foreign
substances
© 2011 Pearson Education, Inc.
2

3. Figure 5.15a
Enzymatic proteins
Enzyme
Example: Digestive enzymes catalyze the hydrolysis
of bonds in food molecules.
Function: Selective acceleration of chemical reactions
3
• Enzymes are a type of protein that acts as a
catalyst to speed up chemical reactions
• Enzymes can perform their functions
repeatedly, functioning as workhorses that
carry out the processes of life
• http://www.biotopics.co.uk/other/morinf.html

4. Figure 5.15b
Storage proteins
Ovalbumin Amino acids
for embryo
Function: Storage of amino acids
Examples: Casein, the protein of milk, is the major
source of amino acids for baby mammals. Plants have
storage proteins in their seeds. Ovalbumin is the
protein of egg white, used as an amino acid source
for the developing embryo.
4

5. Figure 5.15c
Hormonal proteins
Function: Coordination of an organismʼs activities
Example: Insulin, a hormone secreted by the
pancreas, causes other tissues to take up glucose,
thus regulating blood sugar concentration
High
blood sugar
Normal
blood sugar
Insulin
secreted
5
Insulin protein entry:
https://www.ncbi.nlm.nih.gov/protein/AAA59172.1

6. 6
Insulin and Glucose transport

7. Figure 5.15d
Muscle tissue
Actin Myosin
100 µm
Contractile and motor proteins
Function: Movement
Examples: Motor proteins are responsible for the
undulations of cilia and flagella. Actin and myosin
proteins are responsible for the contraction of
muscles.
7

8. Figure 5.15e
Defensive proteins
Virus
Antibodies
Bacterium
Function: Protection against disease
Example: Antibodies inactivate and help destroy
viruses and bacteria.
8
COVID example: https://www.ncbi.nlm.nih.gov/Structure/pdb/7R7N

9. Figure 5.15f
Transport proteins
Transport
protein
Cell membrane
Function: Transport of substances
Examples: Hemoglobin, the iron-containing protein of
vertebrate blood, transports oxygen from the lungs to
other parts of the body. Other proteins transport
molecules across cell membranes.
9

10. Figure 5.15g
Signaling
molecules
Receptor
protein
Receptor proteins
Function: Response of cell to chemical stimuli
Example: Receptors built into the membrane of a
nerve cell detect signaling molecules released by
other nerve cells.
10

11. Figure 5.15h
60 µm
Collagen
Connective
tissue
Structural proteins
Function: Support
Examples: Keratin is the protein of hair, horns,
feathers, and other skin appendages. Insects and
spiders use silk fibers to make their cocoons and webs,
respectively. Collagen and elastin proteins provide a
fibrous framework in animal connective tissues.
11

12. p53 stress response pathway
Harris, S. L., & Levine, A. J. (2005). The p53 pathway: positive and negative feedback loops. Oncogene, 24(17), 2899–2908.
Stress signals such as
DNA damage
12
activation of p53
The activated p53 protein binds
to a specific DNA sequence…
composed of RRRCWWGYYY
(spacer of 0–21 nucleotides)
RRRCWWGYYY, where R is a
purine, W is A or T, and Y is a
pyrimidine.
https://www.nature.com/articles/1208615

13. • Amino acids are the building blocks
(monomers) of proteins
– Amino acids are organic molecules with carboxyl and
amino groups
– Amino acids differ in their properties due to differing
side chains, called R groups (see next slide)
• Polypeptides are unbranched polymers built
from the same set of 20 amino acids
• A protein is a biologically functional molecule
that consists of one or more polypeptides
© 2011 Pearson Education, Inc.
13
Proteins

14. Figure 5.UN01
Side chain (R group)
Amino
group
Carboxyl
group
a carbon
14

15. Figure 5.16
Nonpolar side chains; hydrophobic
Side chain
(R group)
Glycine
(Gly or G)
Alanine
(Ala or A)
Valine
(Val or V)
Leucine
(Leu or L)
Isoleucine
(Ile or I)
Methionine
(Met or M)
Phenylalanine
(Phe or F)
Tryptophan
(Trp or W)
Proline
(Pro or P)
Polar side chains; hydrophilic
Serine
(Ser or S)
Threonine
(Thr or T)
Cysteine
(Cys or C)
Tyrosine
(Tyr or Y)
Asparagine
(Asn or N)
Glutamine
(Gln or Q)
Electrically charged side chains; hydrophilic
Acidic (negatively charged)
Basic (positively charged)
Aspartic acid
(Asp or D)
Glutamic acid
(Glu or E)
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H) 15

16. Amino Acid Polymers
• Amino acids are linked by peptide bonds
• A polypeptide is a polymer of amino acids
• Polypeptides range in length from a few to
more than a thousand monomers (amino
acids)
• Each polypeptide has a unique linear
sequence of amino acids, with a carboxyl end
(C-terminus) and an amino end (N-terminus)
© 2011 Pearson Education, Inc.
16

17. Figure 5.17
Peptide bond
New peptide
bond forming
Side
chains
Back-
bone
Amino end
(N-terminus)
Peptide
bond
Carboxyl end
(C-terminus)
17
How are
peptide bonds
formed?
Methionine (M) Tyrosine (Y)
Cysteine (C)
Methionine (M) Tyrosine (Y)
Cysteine (C)

18. (a) A ribbon model (b) A space-filling model
Groove
Groove
18
• A functional protein consists of one or more polypeptides
precisely twisted, folded, and coiled into a unique shape
• The sequence of amino acids determines a proteinʼs
three-dimensional structure
• A proteinʼs structure determines its function
• Bioinformatics uses computer programs to predict protein
sequence, structure and function from amino acid
sequences

19. Figure 5.19
Antibody protein Protein from flu virus
19

20. Four Levels of Protein Structure
• The primary structure of a protein is its unique
sequence of amino acids
• Secondary structure, found in most proteins,
consists of coils and folds in the polypeptide
chain
• Tertiary structure is determined by interactions
among various side chains (R groups)
• Quaternary structure results when a protein
consists of multiple polypeptide chains
© 2011 Pearson Education, Inc.
20

21. Figure 5.20a
Primary structure
Amino
acids
Amino end
Carboxyl end
Primary structure of transthyretin
21
• Transthyretin
polypeptide
• Transports thyroid
hormones
• Transports retinol
(Vitamin A)
• Primary structure
– The sequence of
amino acids in a
protein
– Determined by
inherited genetic
information

22. Secondary structure
Hydrogen bond
a helix
b pleated sheet
b strand, shown as a flat
arrow pointing toward
the carboxyl end
Hydrogen bond
Figure 5.20c
22
• Secondary
structure results
from hydrogen bonds
between parts of the
polypeptide
backbone
• Includes the a helix
and b pleated sheet

23. 23
• Tertiary structure is the shape
of a polypeptide in three
dimensions
• Quaternary structure results
when two or more polypeptide
chains form one macromolecule
(not all proteins have a quaternary structure)
– Transthyretin is composed of 4
identical polypeptides

24. Figure 5.20b
Secondary
structure
Tertiary
structure
Quaternary
structure
Hydrogen bond
a helix
b pleated sheet
b strand
Hydrogen
bond
Transthyretin
polypeptide
Transthyretin
protein
(composed of 4
identical
polypeptides)
24
Putting it all together…
(but primary structure is not shown)

25. Sickle-Cell Disease: A Change in Primary
Structure
• A slight change in primary structure can affect
a proteinʼs structure and ability to function
– How does the primary structure change?
• Sickle-cell disease, an inherited blood
disorder, results from a single amino acid
substitution in the protein hemoglobin
• Genpept:
– https://www.ncbi.nlm.nih.gov/protein/4504349
© 2011 Pearson Education, Inc.
25

26. Hemoglobin is a protein made up of 4 polypeptide chains:
two a and two b subunits
Heme
Iron
a subunit
a subunit
b subunit
b subunit
26

27. Figure 5.21
Primary
Structure
Secondary
and Tertiary
Structures
Quaternary
Structure
Function
Red Blood
Cell Shape
b subunit
b subunit
b
b
a
a
Exposed
hydrophobic
region
Molecules do not
associate with one
another; each carries
oxygen.
Molecules crystallize
into a fiber; capacity
to carry oxygen is
reduced.
Sickle-cell
hemoglobin
Normal
hemoglobin
10 µm
10 µm
Sickle-cell
hemoglobin
Normal
hemoglobin
1
2
3
4
5
6
7
1
2
3
4
5
6
7
b
b
a
a
27
V
H
L
T
P
E
E
V
H
L
T
P
V
E