Cell Structure and Functions

Question 1

Cell

Answer 1

The smallest unit of life which is capable of independent existence.

Question 2

Cell Components

Answer 2

1) Membrane organelles: nucleus (double membrane), mitochondria (double membrane), RER, SER, Golgi, lysosome, peroxisome.;

2) non-membrane organelles: ribosomes, microfilaments, microtubule, centriole; and

3) Inclusions: melanin, glycogen, lipid, secretory granules

Question 3

Cell Membrane

Answer 3

Semi-permeable, consists of three laminae, each lamina is 2-3 nm thick, made of lipids and proteins: 98% lipids (75% phospholipids, 20% cholesterol, 5% glycolipids) and 2% proteins. But make up 50% of the weight

Question 4

Phospholipid

Answer 4

Molecules are arranged in a bilayer, with hydrophilic heads facing outside and hydrophobic ends facing toward the center of the membrane. Cholesterol molecules are located amid the tails of phospholipid molecules. Proteins, although 2% of the membrane molecules, constitute 50% of the membrane weight. These include transmembrane and peripheral proteins.

Question 5

Transmembrane Proteins

Answer 5

Have hydrophilic regions facing the cytoplasm and extracellular fluid and hydrophobic regions passing back and forth through the lipid of the membrane; mostly, they are glycoproteins.

Question 6

Peripheral Proteins

Answer 6

Don’t protrude through the phospholipids but adhere to one face of the membrane.

Question 7

Functions of membrane proteins

Answer 7

1) receptors: usually specific for one ligand;
2) second-messenger system: e.g., cAMP that activates kinases which, in turn, activate physiological changes within a cell;
3) enzymes that carry out starch and protein digestion, e.g., in the intestine; 4) channel proteins that are transmembrane proteins with pores, which allow passage of water and hydrophilic solutes through the membrane; some channels are always open, while others are gated and open or close in response to three types of stimuli: ligand-regulated gates, voltage-regulated gates, and mechanically-regulated gates;
5) carrier proteins that are transmembrane proteins and help transferring glucose, electrolytes and other solutes across the membrane;
6) cell identity markers that are glycocalyx and help the body in distinguishing between its own cells and foreign invaders; and
7) cell adhesion molecules that help bind cells to one another and to extracellular material

Question 8

Mitochondria

Answer 8

Double membrane, cristae, matrix contains circular DNA (maternal origin), ribosomes, and calcium ions. Function: Powerhouse (ATP synthesis, Krebs Cycle), also contain enzymes associated with steroid synthesis; synthesize 13 mitochondrial proteins. Mitochondria are thought to be derived from bacteria because of similarities in DNA and ribosomes. Also, both can self-duplicate.

Question 9

Endoplasmic reticulum (ER)

Answer 9

Network of interconnected channels called cisternae enclosed by a membrane; Rough ER cisternae are covered with ribosomes; Smooth ER lacks ribosomes; both are continuous with each other and with the nuclear membrane. RER synthesizes proteins that are destined for other organelles such as lysosomes or secreted extracellularly such as enzymes, antibodies, and hormones. SER synthesizes steroids (testosterone, Leydig cells), detoxifies alcohol and drugs (liver), stores and releases calcium (skeletal and cardiac muscle).

Question 10

Ribosomes

Answer 10

Composed of ribosomal proteins and ribonucleic acids; three types of RNA: Transfer RNA (tRNA) carries amino acids to ribosomes; mRNA carries a code for amino acid sequence; and rRNA (ribosomal RNA) produces ribosomes with the ribosomal proteins that are organelles responsible for mRNA translation.

Question 11

Golgi Complex

Answer 11

Consists of a stack of flat cisternae with an inner face (cis, forming) and outer face (trans, maturing); cisternae of cis phase receive newly synthesized proteins from RER in the form of vesicles; after Golgi processing, these molecules are released from the trans face in larger vesicles to constitute secretory vesicles, lysosomes, or other cytoplasmic components. While lysosomes remain within the cell, secretory vesicles leave the cell by fusing with the cell membrane. The Golgi complex in most epithelial cells, especially simple epithelium, is supra-nuclear, and it can be stained in paraffin sections with silver stain.

Question 12

Lysosomes

Answer 12

re divided into three types: Primary lysosomes are homogenous (virgin, newly synthesized), secondary lysosomes are heterogeneous, and tertiary lysosomes contain undigested material and are also called lipofuscin pigment or residual body. The latter are more numerous in neurons and cardiac cells because these cells are not replaced. Lysosomal enzymes are acidic in pH and thus they can be stained in paraffin sections for acid phosphatase. All cells use lysosomes to digest old organelles, the process is called autophagy; neutrophils and macrophages use lysosomes to digest and dispose of foreign materials such as bacteria; liver cells use them to break down glycogen; the uterus after pregnancy uses them to shrink its size, the process is called autolysis. Lysosomal enzymatic failure leads to diseases because of accumulation of undigested material in different cell types.

Question 13

Peroxisomes

Answer 13

Spherical bodies, 0.5 to 1.2 μm in diameter; like mitochondria, they utilize oxygen, but this is used to oxidize organic molecules, which produces H2O2, which is broken to water and oxygen by catalase enzyme.

They are present in all cells but are abundant in liver and kidney cells, where they neutralize free radicals and detoxify alcohol, drugs, and blood-borne toxins.

Question 14

Cytoskeleton

Answer 14

includes: microfilaments (actin and intermediate filaments) and microtubules.

Question 15

Actin

Answer 15

6 nm thick, form a part of the cytoplasmic skeleton in all cells; however, in muscle cells, they form integration with myosin filaments. In other cells, they are associated with membrane activities such as pinocytosis, endocytosis, exocytosis, and cell’s migratory activities

Question 16

Intermediate filaments

Answer 16

10 nm thick, resist stress placed on a cell, form a part of the junctions between cells, especially developed in epidermal cells where they are also called tonofilaments. (Fig. 1-14). Types of intermediate filaments, depending upon protein composition, differ among tissue types: keratin (epithelium), vimentin (mesenchymal cells), desmin (muscle), glial fibrillary acidic protein (glial cells), and neurofilaments (neurons).

Question 17

Microtubules

Answer 17

Microtubules are cylindrical structures, 25nm in diameter, lumen is surrounded by 13 protofilaments of globular proteins called tubulin. Microtubules provide support to the cell and participate in moving organelles and other cytoplasmic contents within the cell. Microtubules are parts of the centriole, basal body, cilium, and flagellum. They can be easily assembled and disassembled, depending upon the cell’s requirements. The anti-mitotic alkaloids arrest microtubule formation and are used in preparing karyotypes (colchicine) and cancer chemotherapy (Taxol, vinblastine).

Question 18

Centrioles

Answer 18

Short cylindrical bodies consisting of nine groups of peripherally arranged three microtubules each; two centrioles lie at right angle to each other within the cytoplasm, usually above the nucleus, called centrosome. Centrioles play a role in cell division, are the source of basal bodies, which migrate to the plasma membrane and give rise to cilia or flagella (the process is called ciliogenesis).

Question 19

Cytoplasmic Inclusions

Answer 19

Are metabolic byproducts and are of two kinds: endogenous and exogenous. Endogenous inclusions include pigments (melanin, lipofuscin), glycogen, lipid, and secretory granules. Exogenous inclusions include dust, bacteria, and viruses. While secretory granules are surrounded by cell membrane; melanin granules, glycogen granules, and lipid droplets are not.

Question 20

Nucleus

Answer 20

Largest organelle and usually the only one visible with the light microscope, spherical to elliptical in shape and about 5 um in diameter, without nucleus in the case of mammalian RBC, multinucleate in the case of skeletal muscle, osteoclast (bone eating cell), giant cell; surrounded by two unit membranes, which are perforated by 30-100 nm nuclear pores, which are sites of transport between the nucleus and the cytoplasm (e.g., raw materials for RNA and DNA synthesis, certain enzymes and hormones that activate DNA enter the nucleus; RNA and ribosomes leave the nucleus); the nuclear material is called nucleoplasm, which includes chromatin (fine thread-like material composed of DNA and protein) and one or more dark-staining bodies called nucleoli where ribosomes are produced. Chromatin is classified into two types: heterochromatin appears as basophilic clumps at the LM or coarse granules in the EM; euchromatin is visible as lightly basophilic areas in the LM or finely dispersed particles in the EM. Sex chromatin is one of the two X chromosomes in the female nucleus. Nucleoli are composed of nucleolar organizer DNA, pars fibrosa, and pars granulosa. Note, the nucleolus synthesizes ribosomes, but not proteins.

Question 21

Necrosis ( Pathological) Nuclear degenerative changes are classified as:

Answer 21

pyknosis (clumping of chromatin), karyorrhexis (breaking down of chromatin), and karyolysis (dissolution of chromatin).

Question 22

Body cells, based upon the length of cycles can be divided into following types

Answer 22

1) Static Cells: never divide, stem cells not present (e.g. Neurons and Cardiac cells, come out of the cycle, G0 phase for life).
2) Stable Cells: normally don’t divide but can divide in response to disease and/or injury (e.g., liver cells, remain in G0 phase until injured).
3) Renewing Cells: are continuously replaced at a fixed interval. Examples: Blood cells are replaced after 120 days, leukocytes are replaced every 2-3 days, gastrointestinal epithelial cells are replaced every 2-3 days, skin cells are replaced every 2-3 weeks. Note: Only stem cells can divide. Differentiated cells do not divide.

Question 23

Apoptosis

Answer 23

Defined as the process of programmed cell death that occurs continuously in all organs throughout life.

Nuclear Fragmentation

Cytoplasmic blebbing

Question 24

Cell membrane transport

Answer 24

Membrane is selectively permeable, allowing some things to pass through (generally lipid soluble and small molecules), but prevents proteins and phosphates from entering or leaving the cell.

Two overlapping methods of moving substances across the membrane: passive, requires no energy (ATP), e.g., filtration and diffusion; active, requires ATP, e.g., carrier-mediated, and vesicular transports.

Question 25

Filtration:

Answer 25

Driven by hydrostatic pressure, e.g., transport of water, salts, nutrients, and other solutes from capillaries to the tissue fluid and vice versa, and filtration of wastes from the blood in the kidneys.

Question 26

Diffusion

Answer 26

Movement of particles from higher concentration to lower concentration, e.g., transport of oxygen into the bloodstream, diffusion of solutes across the artificial dialysis membrane (note, diffusion does not require cell membrane). The diffusion rate is directly affected by temperature, molecular weight, concentration gradient, and surface area, and inversely by molecular weight. Diffusion through a cell membrane depends on how permeable it is to the particles. For example, potassium ions diffuse more rapidly than sodium; nonpolar, hydrophobic, lipid soluble substances such as oxygen, nitric oxide, alcohol, and steroids diffuse easily through the phospholipid molecules of the membrane; conversely, water and small charged hydrophilic solutes such as electrolytes do not mix with lipids but diffuse through channel proteins

Question 27

Osmosis

Answer 27

Diffusion of water down its concentration gradient and it occurs through non-living membranes such as dialysis membranes, as well as through the plasma membrane. Significant amount of water even moves through hydrophobic regions of the membrane, but it occurs more rapidly through channel proteins called aquaporins

Question 28

Facilitated Diffusion

Answer 28

Is the carrier-mediated transport through a membrane down its concentration gradient, but it does not consume ATP, e.g., transport of glucose with a carrier protein.

Question 29

Active transport

Answer 29

Is a carrier-mediated transport of a solute through a membrane up its concentration gradient (from a lower to a higher concentration) and requires ATP;

Example: calcium pumps (transport calcium out of the cell in the extracellular fluid [ECF], which already has more calcium than within the cell), absorption of amino acids from the ECF to the cytoplasm; sodium-potassium pump, also called sodium-potassium ATPase, transfers sodium in the ECF and potassium in the cytoplasm (keeps potassium concentration higher and sodium concentration lower within the cell, this difference in sodium and potassium concentration maintains the resting membrane potential).

Question 30

Vesicular transport

Answer 30

Unlike carrier-mediated transport, moves large particles, droplets of fluid or numerous molecules at once, contained in vesicles or vacuoles.

Question 31

Endocytosis

Answer 31

Brings matter into a cell

Question 32

Exocytosis

Answer 32

Brings matter out of the cell

Question 33

Pinocytosis

Answer 33

“Cell drinking “is the process of taking in droplets of ECF containing molecules of some use to all body cells,

Example: pinocytic vesicles in the epithelial cells of the intestine, kidney, gall bladder.

Question 34

Phagocytosis

Answer 34

Absorption of large particles by means of pseudopods and occurs only in specialized cells,

Example: macrophages and neutrophils.

Question 35

Receptor-mediated endocytosis

Answer 35

Is phagocytosis or pinocytosis in which specific solutes bind to receptors on the plasma membrane and then are taken into the cell in clathrin-coated vesicles with a minimal amount of fluid, e.g., low-density lipoproteins (LDL, protein-coated droplets of cholesterol in the blood), insulin transport from the blood to the ECF (this process called transcytosis)