Cells structure and their functions

1. The Cell
Cell: is the structural and functional unit of life (and of disease processes). Cell also defines
as the smallest unit of protoplasm than can exist independently and reproduce its kind.
Human being possesses billions of cells, arranged in more than 200 types ,but on the
functional similarities, theses cells are classified into 4 basic groups: epithelial, connective,
muscular and nervous tissues.

2. The cell either:
• Eu-karoytic : contain nuclei.
• Pro-karotic : contain no nuclear envelop, no histones and no membranous
organells.

5. A- The cytoplasm
1. Cytoplasmic matrix ( cell sap): it is a colloidal gel-like solution of proteins, lipids,
carbohydrates, minerals, enzymes, small molecules, and ions.
2. Cytoplasmic organelles (organoids): these are specialized structures with specific
functions, and they can be enclosed in membranes (membranous organelles) or not (non-
membranes organelles).
3. Cytoplasmic Inclusions: these are
temporary, non-living components
which appear and disappear at
different periods in certain cells,
such as stored food(glycogen, fat),
Pigments(exogenous & endogenous)
and crystals.

6. Cell Organelles(Organoids)
Membranous organelles: these are permanent components, present in all nucleated
cells, and included:
1. Cell membrane(plasma membrane or plasmalemma).
2. Mitochondria.
3. Golgi Apparatus.
4. Lysosomes.
5. Rough Endoplasmic Reticulum (RER).
6. Smooth Endoplasmic Reticulum (SER).
7. Peroxisomes.

7. 1-Cell membrane(plasma membrane or plasmalemma):
Is the boundary that separates the living cell from it’s non-living surroundings.
Thickness: 7.5-10 nm(75-100 Angstrom) and consequently is visible only in electron
microscope, where it appears as if it’s formed of three layers, therefore it’s called
tri-lamellare membrane. Its outer and inner layers appear as dark lines, while its
middle layer appears as a light area.

8. Structure of the cell membrane

9. 1-Lipids
a. Phospholipids: It’s composed of:
i. Phosphate head:
• Has affinity for aqueous solution (hydrophilic )
• It's the charged polar head
• Directed outwards
ii. Fatty acid tails:
 Has no affinity for aqueous solution (hydrophobic )
 It's the non- polar (not charged )tail
 Directed inwards
Phosphate head
Fatty acid
Arranged as a bilayer

10. b. Cholesterol: fit into spaces between phospholipids and prevent water-soluble
molecules from diffusing across the membrane.

11. 2- Protein molecules
a. Extrinsic or Peripheral protein:
 Lie outside lipid bilayer
 forms a non-continuous layer loosely bound to both surface of membrane.
b. Intrinsic or Integral proteins:
-Small intrinsic proteins: small proteins embedded in the lipid bilayer.
-Large intrinsic proteins: called trans membrane proteins, they contain channels
through which ions can pass.
trans membrane proteins
Peripheral protein

12. Functions of membrane proteins
Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor
Enzyme
activity
Cell surface
identity marker
Attachment to the
cytoskeleton
Cell adhesion
“Antigen”
“Channel”

13. 3- Carbohydrates molecules
Membrane carbohydrates either linked to the protein molecules forming glycoproteins or to
lipid forming glycolipids.
Glycoproteins + Glycolipids = Glycocalyx (Cell Coat), it may be thick or thin according to
function of cell.It helps in adhesion adjacent cells and enables cells to recognize other cells
of their own special kind. ABO antigen. Form part of the basement membrane
Cell adhesion Cell surface identity marker
Dr.
Sabah
A.
AL-Qadasi

14. The functions of the cell membrane
1. protects the cell by acting as a barrier
2. regulates the transport of substances in and out of the cell(active transport,
selective transport, phagocytosis, pinocytosis and exocytosis)
3. receives chemical messages from other cell
4. acts as receptors
5. cell mobility, secretions, and absorptions of substances

16. 2-Endoplamic Reticulum(ER):
 They are membranous cell organelles formed of communicating wide and narrow
tubules (cisternae).
 They synthesize protein, carbohydrates, lipid and regulate mineral metabolism.
 They are 2 types of ER: Rough and Smooth
i. Rough Endoplasmic Reticulum(RER):
 is studded with ribosomes and is the site of protein synthesis and processing.
 They are basophilic substances (stains well with basic stains)
 Their number increases in protein
secreting cells such as fibroblast, plasma
cell,ameloblast, hepatic and pancreatic cells
 Protects the cytoplasm from the action
Of the hydrolytic enzyme.

17. ii. Smooth Endoplasmic Reticulum(SER):
 Can’t demonstrated by LM, while by EM
 With EM it appears as anastomosing tubules with no ribosomes.
 It’s the site of phospholipids and carbohydrates synthesis
 Fund in great amount in endocrine cells which synthesize steroid hormones.
 Regulates muscles contraction (calcium ions).
 Play role in platelets formation
 Play role in HCL formation
 Detoxification of excess
drugs or hormones

18. 3- Golgi Apparatus:
 The Golgi structure is a smooth, curvy structure. It is a flattened stack of membranes.
 It has a front end and a back end.
 The front end is called the cis face and the back end is called the trans face.
 Golgi apparatus has cisternae which are the flattened membrane folds and secretory
vesicles which are what the cell discharges.
 Golgi apparatus is responsible for collecting, concentrating, packaging, sorting and
adding specific products to the secretion.
By TEM

19. • In nerve cells it surrounds the nucleus, while in secretory epithelial cells it’s found
between the nucleus and cells free border.
• With silver stain: it appears as brown
fibrillar or granular network called positive image.
• With H&E: it appears as an unstained area called
negative image in a highly basophilic cells
(e.g. plasma cells).
• With EM: it has 3 forms:
i. Saccules: small sacks arranged one above the other to form stacks.
Each stack has concave mature face and a convex immature face.
ii. Transfer vesicles: small round membranous vesicles containing protein and originate
from RER.
Transfer vesicles fuse with saccules of immature convex face.

20. iii. Secretor vesicles: when transfer vesicles are concentrated and enveloped by
membrane they known as secretory vesicles.
They arise from the periphery of saccules. Secretory vesicles are discharged outside the
cell by exocytosis, others remain intracellular as lysosomes.
 Functions of Golgi apparatus:
1. Accumulate, concentrate and package secretory products of cells.
2. Rich in sulfotransferase enzyme which adds sulfates to certain secretory products.
3. Rich in sugar transferase enzyme which adds carbohydrates to certain secretory
products.
4. Keep cell membranes and cell coat in good condition.

21. 4- Lysosomes:
• Lysosomes are enzyme-filled sacs, generally spherical, surrounded by a single membrane,
present in all kind of the cells.
• Their main function is an intracellular digestion.
• They are very common in white blood cells, where disease and sickness are fought so a
lot of bacteria needs to be digested.
• Their shape and size vary depending on what material is digested.
• They contain about 40 different enzymes (ex. nucleases, proteases, lipases, and
carbohydrases, phosphatase).

22.  With EM they appear in 2 forms:
i. Primary lysosomes: homogeneous rounded vesicle.
ii. Secondary lysosomes: heterogeneous rounded
bodies because they contain ingested and digested elements.
 Fate of the primary lysosomes:
These are newly formed lysosomes which have budded off from the Golgi apparatus.
They may circulate in cytoplasm and remain such as or fuse with some foreign particles or
cytoplasmic bodies like old organelles to form secondary lysosomes.

23.  Secondary lysosomes formed through the following processes:
1. Phagocytosis: (cell eating) the phagocytosed foreign bodies (phagosomes) fused with
primary lysosomes to form digestive vacuole.
2.Pinocytosis: (cell drinking) the pinocytosed foreign bodies (pinocytic vesicles) fused with
primary lysosomes to form multivesicular body.
3. Autophagocytosis: old membrane bound mitochondria fused with primary lysosomes to
form autophagic vacuole. They may be expelled by exocytosis or remain in cytoplasm.
4. Residual bodies: after digestion of the contents of secondary lysosomes diffuse into the
cytoplasm, the remaining vacuole is called
residual body. They may be expelled by
exocytosis or remain in cytoplasm.
Long lifespan cells (nerve& cardiac cells)
contain many residual bodies (lipofuscin) .
They may be expelled by exocytosis or
remain in cytoplasm.

24.  Functions of the lysosomes:
1. Intracellular digestion
2. Defense the body against invading organisms
3. Digest old mitochondria
4. Facilitate penetration of sperm into the oocyte
5. Concerned with post-mortem changes (suicide bags) such as hypoxia, ischemia and
bacterial infection.

25. 5- Mitochondria(The Powerhouse) :
• Mitos= thread + chondrons = granules, so with LM they appear as rods, granules or
filaments.
• With EM they appear as vesicles surrounded by 2 membranes; an outer smooth, and an
inner rough because it projects into the cavity of the mitochondrion forming shelves called
cristae which increase the surface area.
• Mitochondrial matrix contains respiratory apparatus, DNA &RNA , ribosomes, calcium and
magnesium granules.
• Mitochondrion has its own
strand of DNA, so they can
divide to increase their number.
• They are concerned with
oxidative phosphorylation and ATP
production and also play role in lipid synthesis (except in RBCs)

26. • Each cell contains a different number of mitochondria. The number present is
dependent upon how much energy the cell requires. The more energy a cell needs the
more mitochondria that will be present. Cells have the ability to produce more
mitochondria as needed.
• Mitochondria are in constant movement and they are able to expand, contract, divide
and fuse to make larger ones (giant mitochondrion) .
• They are sensitive to temperature, PH, osmotic pressure.
• Their life span is about 10 days.
 Functions of the mitochondria:
1. Supply energy (ATP) to all cellular activities.
2. Storage of calcium and magnesium ions as dark granules.
3. Catalyze the enzymatic reactions of mitochondria.
4. Play role in lipid synthesis.

27. 6- Peroxisomes or Microbodies:
• Peroxidase are self-replicating organelles that contain oxidative enzymes.
• They are somewhat larger than primary lysosomes.
• In rats, they are distinguishable from lysosomes by an electron-dense granule nucleoid of
urate oxidase
• They contain more than 40 oxidative enzymes, especially urate oxidase, D-amino acid
oxidase, hydroxyacid oxidase, and catalase.
 Functions of Peroxisomes:
Hydrogen peroxidase (H2O2) degradation: beta oxidation of long chained fatty acids
produces H2O2 a potentially toxic substance to cells that must be eliminated.
Peroxisomes contain catalase enzyme which destroys H2O2. Excess H2O2 that accumulates in
cells from other sources can also be eliminated by peroxisomes.