2. CELL FUNCTIONAL MORPHOLOGY

By Musenge E.(07/08/2015)

I. Background

• The word cell comes from the Latin cella, meaning "small room“ and was coined by Robert Hooke (Lodish, 2007)

• Entire human body contains about 100 trillion cells, of which about 25 trillion are the RBCs (Guyton and Hall, 2010)

• Each of the 100 trillion cells is a living structure that can survive for months or many years, provided its nourished appropriately

• Cell is the basic structural, functional and biological unit of all known living organisms (Lodish, 2007).

18/04/2023 MME 2

Background cont…• Specific types of cells are adapted to perform

particular function• Body cells often differ markedly from one

another, but all have certain basic characteristics that are alike

• To understand the function of organs and other structures of the body, it is essential to first understand the basic organisation of the cell, its structures and their functions

18/04/2023 MME 3

II. Cell organisation• A typical cell, under light microscope has two major

parts; the nucleus (enclosed by nuclear membrane) and the cytoplasm (enclosed by cell membrane)

• Protoplasm is a colourless material comprising the living part of cell, including the cytoplasm, nucleus, and other organelles

• Protoplasm is composed mainly of five basic substances:

• Water (70-85%), electrolytes, proteins (10-20%), lipids (2%), and carbohydrates (1-6%) (Guyton and Hall, 2010)

18/04/2023 MME 4

III. Functions of cell organelles and other structures

18/04/2023 MME 5

Structure of Animal Cell

i. Cell membrane• Cell membrane is a thin, lipid bilayer, pliable, elastic

structure of only 7.5 to 10 nm thick enclosing the cell• It comprises almost entirely proteins and lipids

(proteins-55%, phospholipids-25%, cholesterol-13%, other lipids-4%, and carbohydrates-3%) (Guyton and Hall, 2010)

• Lipid bilayer comprises:Hydrophilic head portions constituting the two

surfaces of the complete cell membraneHydrophobic tail of the membrane in the middle is

impermeable to the usual water-soluble substances, such as ions, glucose, and urea

18/04/2023 MME 6

Cell membrane cont…• Equally, fat-soluble substances, such as O2, CO2, and

alcohol, can penetrate the hydrophobic tails with ease

• Cholesterol molecules; are dissolved in membrane mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids

Cholesterol controls much of the fluidity of the membrane as well

• Cell membrane proteinsAre globular masses floating in the lipid bilayer, most

of which are glycoproteins18/04/2023 MME 7

Cell cembrane cont…• Two types of proteins: Integral, and peripheral

proteins. • Functions of integral proteins Provide structural channels Act as carrier proteins Act as enzymes Can also serve as receptors e.g. for peptide hormones• Functions of peripheral protein Molecules are often attached to the integral proteins Function almost entirely as enzymes or as controllers

of transport of substances through the cell membrane “pores.”

18/04/2023 MME 8

Cell membrane cont…• Membrane Carbohydrates• Occur almost invariably in combination with proteins or

lipids in the form of glycoproteins (most integral proteins) or glycolipids (one tenth of lipid molecules), many other carbohydrate are proteoglycans

• Important functions of carbohydrates Gives most cells an overall negative surface charge Provide for cell to cell attachment Receptors for binding hormones e.g. insulin Some carbohydrate moieties enter into immune reactions

18/04/2023 MME 9

Structure of phospholipid

18/04/2023 MME 10

Structure of cell membrane

18/04/2023 MME 11

ii. Endoplasmic reticulum (ER)• ER is a network of lipid bilayer membrane walled

tubular and flat vesicular structures in the cytoplasm filled with endoplasmic matrix

• Granular ER and Ribosomes• Ribosomes comprises a mixture of RNA and proteins,

and function to synthesize new protein molecules in the cell and the initial folding of polypeptide chains with the formation of disulfide bonds (Barrett et al., 2010).

• Agranular (smooth) ER• Steroid synthesis in steroid-secreting cells and the

site of detoxification processes in other cells (Ibid).18/04/2023 MME 12

Granular ER

18/04/2023 MME 13

iii. Golgi apparatus (GA)• GA is closely related to the ER and has membranes similar

to those of the agranular ER• GA functions in association with the ER, small “transport

vesicles” (ER vesicles) continually pinch off from the ER• The transported substances are then processed in the GA

to form lysosomes, secretory vesicles, and other cytoplasmic components

• Synthesize certain carbohydrates that cannot be formed in the ER e.g. hyaluronic acid and chondroitin sulfate

• Mainly provide additional processing of substances (proteins) already formed in the ER

• Transport vesicles instantly fuse with the GA and empty their content into the vesicular spaces of the GA where, additional carbohydrate moieties are added to the secretions

18/04/2023 MME 14

GA cont…• GA compact and processing of ER secretions into

highly concentrated packets• Both small and large vesicles continually break

away from the GA, carrying with them the compacted secretory substances, and in turn, the vesicles diffuse throughout the cell

• Two types of vesicles formed by GA are; secretory and lysosomes

• Mainly the secretory vesicles containing proteins for extrusion or intracellular use

• Replenishes the membranes as they are used up18/04/2023 MME 15

Golgi apparatus functions

18/04/2023 MME 16

iv. Lysosomes• Are lipid bilayer membranous vesicular

organelles that form by breaking off from the GA and then dispersing throughout the cytoplasm

• They are quite different in different types of cells, but it is usually 250-750 nm in diameter.

• Are filled with large numbers of small granules 5 to 8 nm in diameter, which are protein aggregates of as many as 40 different hydrolase enzymes.

18/04/2023 MME 17

Lysosomes cont…• Functions of lysosomes• Provide an intracellular digestive system that allows the cell to

digest:-damaged cellular structures, food particles, unwanted matter such as bacteria

Pinocytotic or phagocytic vesicle inside a cell, attach one or more lysosomes and empty their acid hydrolases to the inside of the vesicle

Body tissue regression to a smaller size e.g. uterus, muscles, mammary glands etc and autolysis.

Contain bactericidal agents e.g. Lysozyme-dissolves bacterial cell membrane; Lysoferrin-binds iron and other substances before promoting bacterial growth; acid at a pH of about 5.0, activates hydrolases and inactivates bacterial metabolic systems

18/04/2023 MME 18

v. Peroxisomes• Are similar physically to lysosomes, but they are

different in two important ways:-Believed to be formed by self-replication (or

perhaps by budding off from the smooth ER) rather than from the GA

Contain oxidases rather than hydrolases• Form H2O2 a highly oxidizing substance and is

used in association with catalase e.g. about half the alcohol a person drinks is detoxified by the peroxisomes of the liver cells in this manner

18/04/2023 MME 19

vi. Secretory granules (Secretory vesicles)• Nearly all of the secretory substances are formed by

the ER–GA system and are then released from the GA into the cytoplasm in the form of storage vesicles

• One example of vesicles is the secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes

• The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and thence into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract

18/04/2023 MME 20

vii. Mitochondria• Mitochondria are called the “powerhouses” of the cell as

without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease

• Total number of mitochondria per cell varies from less than a 100 up to several thousand, depending on the amount of energy required by the cell

• Additionally, the mitochondria are concentrated in those portions of the cell that are responsible for the major share of its energy metabolism

• Some are only a few 100 nm in diameter and globular in shape, others are elongated as large as 1 µm in diameter and 7 µm long; others are branching and filamentous

18/04/2023 MME 21

Mitochondria cont…• Basic structure of the mitochondrion is composed mainly

of two lipid bilayer protein membranes: an outer membrane and an inner membrane

• Many infoldings of the inner membrane form shelves onto which oxidative enzymes are attached

• The inner cavity of the mitochondrion is filled with a matrix that contains large quantities of dissolved enzymes that are necessary for extracting energy from nutrients

• The enzymes operate in association with the oxidative enzymes on the shelves to oxidize nutrients, thereby forming CO2 and H2O and at the same time releasing energy

18/04/2023 MME 22

Mitochondria cont…• The liberated energy is used to synthesize a “high-

energy” substance, ATP• ATP is then transported out of the mitochondrion,

and it diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions

• Mitochondria are self-replicative, as such one mitochondrion can form a second one, a third one, and so on, whenever there is a need in the cell for increased amounts of ATP (Barret et al., 2010).

18/04/2023 MME 23

Mitochondria structure

18/04/2023 MME 24

Mitochondria cont…• Function of the Mitochondria• Cells extract energy from carbohydrates, fats, and

proteins that react chemically with O2, to glucose, amino acids and fatty acids

• Inside the cell, nutrients react chemically with O2, under the influence of enzymes that control the reactions and channel the energy released in the proper direction

• Almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP

18/04/2023 MME .25

Mitochondria cont…• Then, ATP, not the original foodstuffs, is used

throughout the cell to energize almost all the subsequent intracellular metabolic reactions

• High-energy bonds contains about 12,000 calories of energy per mole of ATP

• The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where its energy is used to energize multiple cell functions

• This overall process for formation of ATP is called the chemiosmotic mechanism of ATP formation

18/04/2023 MME 26

ATP synthesis

18/04/2023 MME 27

viii. Cytoskeletons• These are the fibrillar proteins of the cell usually

organized into filaments and tubules structures• These originate as precursor protein molecules

synthesized by ribosomes in the cytoplasm• The precursor molecules then polymerize to form

filaments e.g. large numbers of actin filaments frequently occur in the outer zone of the cytoplasm, called the ectoplasm, to form an elastic support for the cell membrane

• Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction

18/04/2023 MME 28

Cytoskeleton cont…• A special type of stiff filament composed of polymerized

tubulin molecules is used in all cells to construct very strong tubular structures (microtubules) and include;

Tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium

Both the centrioles and the mitotic spindle of the mitosing cell are composed of stiff microtubules

• The primary function of microtubules is to act as a cytoskeleton (three types; microfilament, microtubules and intermediate filaments), providing rigid physical structures for certain parts of cells, anchor organelles and help substances move

18/04/2023 MME 29

Types of cytoskeleton

18/04/2023 MME 30

Cytoskeletons in the Cell

18/04/2023 MME 31

ix. Nucleus• The nucleus is the control center of the cell• Nucleus contains large quantities of DNA, which are the

genes• The genes determine the characteristics of the cell’s

proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities

• Genes also control and promote reproduction of the cell itself

• The genes first reproduce to give two identical sets of genes; then the cell splits by a special process called mitosis to form two daughter cells, each of which receives one of the two sets of DNA genes.

18/04/2023 MME 32

Nucleus cont…• Unfortunately, the appearance of the nucleus under the

microscope does not provide many clues to the mechanisms by which the nucleus performs its control activities

• The light microscopic appearance of the interphase nucleus (during the period between mitoses), reveal darkly staining chromatin material throughout the nucleoplasm

• During mitosis, the chromatin material organizes in the form of highly structured chromosomes, which can then be easily identified using the light microscope

18/04/2023 MME 33

Nucleus cont…• Nucleus is encased in the nuclear membrane• Nuclear membrane is actually two separate bilayer

membranes, one inside the other• The outer membrane is continuous with the ER of the

cell cytoplasm• The space between the two nuclear membranes is also

continuous with the space inside the ER• Nucleolus is non-membrane bound structure

comprising proteins and nucleic acids found within the nucleus of eukaryotic cells

• Its function is to transcribe rRNA and combine it with proteins to form almost-complete ribosome

18/04/2023 MME 34

Nucleus structure

18/04/2023 MME 35

x. References• Barrett E. Kim, Barman M. Susan, Boitano Scott and

Brooks L. Heddwen,(2010). Ganong’s Review of Medical Physiology, 23rd Ed., San Francisco: McGraw-Hill Companies Inc.

• Guyton, A. C. and Hall, J. E., (2010). Text Book of Medical Physiology, Philadelphia: Elsevier Inc.

• Kerfeld, C. A.; Sawaya, M. R; Tanaka, S; Nguyen, C. V.; Phillips, M; Beeby, M; Yeates, T. O. (5 August 2005). "Protein structures forming the shell of primitive bacterial organelles.". Science 309 (5736): 936–8.

• Lodish (2007). Molecular Cell Biology,6e. W.H.Freeman and Company. ISBN 0-7167-7601-4.

18/04/2023 MME 36