Chapter 3

Cells

You are responsible for the following figures and tables:

Fig. 3.1 - The cells of the human body. Fig. 3.2 - Specialized cells. Fig. 3.42.Fig. 3.3 - Generalized diagram of a universal animal cell. Fig. 3.6 - Phospholipid bilayer Fig. 3.7 - Embedded in this cell membrane are transmembrane proteins.Fig. 3.8 - cells communicate with each other Fig. 3.12 - representation of a cell. Fig. 3.12, 3.29, 3.30, 3.31 – endocytosis and exocytosis.Fig. 3.21 - Passive transport does not require energy. Tab. 3.4 - summary of transport mechanisms in and out of the cell.Fig. 3.36 - cell cycle - Name the phases.Fig. 3.37 - Name the phases of cell division, describe them.Define ' cancer'. Tab. 3.6.

Cells

• The adult human body consists of about 75 million cells, the basic units of the human body. • All precursor cells, also called stem cells, are alike and omnipotent. They only differentiate into specialized cells as they mature (Fig. 3.42).• Whether precursor or specialized, all body or somatic cells have 46 chromosomes. • Mature cells vary in size, shape, and function (Figs. 3.1 and 3.2).

Cells – They consist of:

Cell membrane

Cytoplasm

Cytoplasmic organelles

Nucleus

Cell Membrane

• Outermost limit of the cell consisting of a phospholipid bilayer (Fig. 3.6).

• Thin, flexible, elastic

• Maintains integrity of the cell

• Controls entrance and exit of substances, selectively semipermeable

• Receives and responds to messages, signal transduction

Cell Membrane Structure

• It is a phospholipid bilayer.

• It contains lipids, proteins, and glycolipids or glycoproteins with some carbohydrates.

• The surface of the membrane is formed by water-soluble heads made up of phosphate groups (hydrophilic or polar)

• The interior of the membrane is formed by water-insoluble tails composed of fatty acids (hydrophobic or nonpolar)

Membrane Transport

• Molecules that are soluble in lipids can easily pass through the membrane.

• Molecules that are water-soluble do not move through the membrane.

• Cholesterol molecules embedded inside of the lipid bilayer make the cell membrane even more impermeable and make it inflexible.

Membrane Proteins• Fibrous proteins that span the membrane

function as receptors.• Globular, integral proteins spanning the

membrane allow passage of certain molecules or ions such as the cystic fibrosis transporter protein (CFTR) that transports chlorine across the cell membrane.

• Globular, peripheral proteins that do not span the membrane function as enzymes or signal transducers. These often aid in cell recognition and cell binding of ‘growth factors’.

Figure 3.7

Cytoplasm

• The cytoplasm is a clear gel called cytosol.

• It contains a network of membranes around organelles that are suspended in the cytosol.

• Protein rods and microtubules form the cytoskeleton, a supportive framework.

• The many different organelles perform specific cellular functions that aid in the growth of cells and the body.

CELL COMPONENT/ORGANELLES

DESCRIPTION/STRUCTURE FUNCTION(S)

CELL or PLASMA MEMBRANE a phospholipid bilayer with transmembranous proteins / protein channels dispersed throughout it

semipermeable cell boundary (controls active/passive Transport)

CYTOPLASM jelly-like plasma / fluid (70% H2O) suspends organelles in cell; translation of m-RNA into protein using t-RNA

NUCLEUS control center of the cell; bound by phospholipid bilayer= nuclear membrane; contains DNA

DNA replication = synthesis of DNA double strand; DNA transcription into mRNAm-RNA synthesis r-RNA synthesis

NUCLEOLUS dense spherical body within nucleus; area of r-RNA synthesis / protein

synthesis of ribosomes

RIBOSOMES consist of r-RNAs / proteins; dispersed throughout cytoplasm and on RER

protein synthesis using m-RNA as template and t-RNA adding on amnio acids to the growing protein = translation

ROUGH ER( RER )

Membranous network studded with ribosomes protein synthesis = translation

SMOOTH ER ( SER )

Membranous network lacking ribosomes lipid / cholesterol synthesis

GOLGI Membrane stacks looking like “Stack of Pancakes”; cisternae posttranslational modification transport and packaging of proteins in vesicles =endocytosis / exocytosis

LYSOSOMES Membranous sac of digestive enzymes degradation of worn cell parts (“autolysis) and foreign particles

PEROXISOMES Membranous sacs filled with catalase enzymes (catalase) detoxification of harmful substances(i.e.ethanol,drugs)

MITOCHONDRIA“Powerhouse”

kidney shaped; inner membrane is folded into “cristae”; cellular respiration; uses O2,

releases CO2; makes 38 ATP =

energy

CYTOSKELETON protein filaments=microtubules intracellular transport along the protein filaments

Cell Components and Organelles

Nucleus• Large, spherical structure

• Enclosed in a phospholipid bilayer called nuclear envelope, composed of an inner and outer lipid bilayer

• Layers are joined at openings, the nuclear pores

• Nuclear pores are channels made up of more than 100 different proteins that allow movement in and out of the nucleus

Nucleolus and Chromatin

• The nucleolus is a small, dense body composed of RNA and protein.

• It is the site of ribosome production, that is, ribosomal RNA or r-RNA is synthesized.

• Chromatin fibers are composed of continuous DNA strands.

• Around these DNA molecules are wrapped eight proteins called histones.

Figure 3.12

Fig. 3.12 shows some specific functions of organelles:

Nucleus - transcription of DNA into mRNA

Ribosomes on Rough ER – translation of mRNA into protein

Rough ER – modification of proteins

Golgi Apparatus – modification of proteins and lipids by

addition of carbohydrates to the structure

Vesicles – transport of products outside the cell by exocytosis

• Exocytosis is an active energy requiring process

• Smooth endoplasmic reticulum (Smooth ER) lacks ribosomes (Fig. 3.10)

Mitochondria

• Fluid-filled sacs that contain their own DNA and can divide on their own.

• There are two layers, an outer membrane and an inner membrane.

• The inner membrane is folded into cristae.

• Enzymes of the mitochondrion control reactions of energy release from nutrients.

Figure 3.13b

Lysosomes

• Tiny, membranous sacs containing enzymes that break down proteins, carbohydrates, and nucleic acids

• White blood cells contain lysosomes which aid in the digestion of bacteria and degradation of bacterial components with lysosomal enzymes

• Lysosomes dismantle worn cells and cell parts

Peroxisomes

• Membranous sacs found in all cells, but abundant in liver and kidneys that break down lipidic/fatty toxins, drugs and alcohol

• Enzymes, peroxidases, catalyze these metabolic degradative reactions that release hydrogen peroxide as a byproduct

• Catalase decomposes hydrogen peroxide (H2O2)

Figure 3.3 – Hypothetical Composite Cell

Other Cell Components

Figure 3.3Figure 3.3 – Hypothetical Composite Cell

Other Cell Components

CELL COMPONENTS that modify cell surfaceOr are specialized during mitosis

DESCRIPTION/STRUCTURE FUNCTION(S)

FLAGELLA long, tail-like extension; sperm locomotion

CILIA short, eyelash-like extensions; respiratory tract & fallopian tube to push substances through passageways

MICROVILLI microscopic folds of cell membrane increase surface area

CENTRIOLES paired cylinders of microtubules at right angles near nuclear poles; they function during cell division;synthesis and resorption of protein filaments called microtubules

aid in chromosome movement during mitosis

Figure 3.16 – cilia move mucus or eggs in tubes

Microfilaments and Microtubules

• Threadlike structures in the cytoplasm

• Microfilaments are tiny rods of the protein actin. They function in cellular movements

• Microtubules are long, slender tubes composed of the protein tubulin. They form the cytoskeleton and help move organelles within the cells

Cells Also Contain Inclusions

• Chemicals stored temporarily in the cell.

• Nutrients such as glycogen and lipids can be stored.

• Pigments such as melanin can be stored.

Movements Into and Out of the Cell

• The cell membrane controls the movement of substances into and out of the cell.

• Movements involve physical or passive processes such as diffusion, facilitated diffusion, osmosis, and filtration.

• Movements can be physiological or active processes such as active transport, endocytosis, and exocytosis.

Diffusion• Diffusion is the tendency of molecules, and

ions in solution to move from areas of higher concentration to areas of lower concentration.

• The difference in concentration is the concentration gradient.

• Diffusion occurs because particles are in constant motion attempting to equilibrate across a membrane or inside an organelle.

Diffusion• Diffusion progresses until there is no net movement any

longer called diffusional equilibrium.

Diffusion in Living Systems

• Diffusional equilibrium is more correctly referred to as seeking of a physiological steady state.

• Diffusion of substances occurs if the membrane is permeable to that substance and if a concentration gradient exists

• Examples: oxygen, carbon dioxide, cholesterol

Facilitated Diffusion• Substances that are insoluble in lipids and

too large to move through pores move by facilitated diffusion.

• Facilitated diffusion includes protein channels and protein carriers.

• Molecules fit into the carrier and are transported across the membrane.

• The number of carriers limits the rate of movement.

Figure 3.24

Osmosis• Osmosis is the diffusion of water molecules from a

region of higher water concentration to a region of lower water concentration across a selectively permeable membrane.

Osmosis• One might also say: Osmosis is the diffusion of water

molecules from a region of low solute concentration to a region of high solute concentration across a selectively permeable membrane.

• Red Blood Cells (RBC) exhibit how osmosis works nicely: when in isotonic or plasma solution, they appear donut shaped.

• When they are placed into a hypotonic solution, they take up water and swell and might even burst.

• When they are placed into a hypertonic solution, they loose water and shrink in size.

Figure 3.26a

Figure 3.26b

Figure 3.26c

Filtration• Molecules can be filtered by force (such as pressure

differences) through membranes as is done in the kidney nephrons.

Active Transport• Active transport

occurs when the net movement of particles passing through membranes is in the opposite direction, from a region of lower concentration to one of higher concentration.

Active Transport

• Active transport utilizes protein carriers.

• This process requires energy from cell metabolism.

Endocytosis• Endocytosis

moves particles too large to move by diffusion or active transport.

• Pinocytosis is the intake of liquid droplets.

Endocytosis• Phagocytosis is

the intake of solids, often with the fusion of lysosomes which digest the material.

• Pinocytosis and phagocytosis bring material indiscriminately into the cell.

Exocytosis

• Exocytosis often expels the residue after lysosomal enzymes have digested solids brought in through phagocytosis.

• Exocytosis allows cells to secrete material produced by the cell, for example, neurotransmitters.

Other Functions of Cells: Growth.The Cell Cycle

• The series of changes that a cell undergoes from the time it forms until it divides is called the cell cycle.

The Cell Cycle: Interphase

• Interphase is the period of cell growth and function.

• Interphase is composed of G1, S Phase, and G2.

Interphase• G 1 is the first

Gap or Growth phase. During this period, cell growth occurs.

• S phase is the period of DNA synthesis as the cell prepares for cell division.

• G 2 is the second Gap or Growth phase. During this period, the cell replicates organelles in preparation for cell division.

Interphase

Mitosis• Mitosis occurs in somatic cells, all body cells with

the exception of sex cells.• Cell division results in two genetically identical

daughter cells with 46 chromosomes each.• Karyokinesis is nuclear division.• Cytokinesis or the movement of the daughter cells

apart from each other is the division of the cytoplasm.

Figure 3.37

Mitosis: Prophase• Prophase:

chromosomes condense from chromatin and centrioles move to the poles. The nuclear envelope disintegrates. The spindle apparatus forms.

Mitosis: Metaphase• Metaphase:

spindle fibers attach to centromeres and chromosomes align in the center or equatorial plate of the cell.

Mitosis: Anaphase• Anaphase:

Spindle fibers contract and the chromosomes move to the opposite poles of the future daughter cells.

Mitosis: Telophase• Telophase:

Nuclear envelope reforms and chromosomes unwind into chromatin.

Last Stage Of The Cell Cycle: Cytokinesis

• Cytokinesis or cytoplasmic division begins in anaphase when the cell membrane starts to constrict in the middle.

• Cytoplasmic inclusions and organelles are divided between the two new cells.

Control of the Cell Cycle

• How often cells divide is strictly controlled.

• Cells appear to keep track of their divisions, the “mitotic clock”.

• DNA at the tips of chromosomes, telomeres, wear down as the cell divides and may be a signal for the cell to stop dividing.

• Size, space, hormones, and growth factors influence cell division.

Abnormal Cell Growth• Abnormal growth or a neoplasm can produce a

disorganized mass called a tumor. Tumors often are initially benign becoming malignant only later by additional signals

• Benign tumors remain in place and grow, interfering with function of healthy tissue.

• Malignant (cancerous) tumors are invasive or metastatic and extend into surrounding tissue.

• Cancer or uncontrolled cell cycling, can affect different tissues.

Genes and Cancer

• Environmental factors may induce cancer by altering oncogenes and tumor suppressor genes in cells.

Figure 3.40

Cellular Differentiation

• The process by which stem cells develop different structures and specialized functions is called cellular differentiation.

• Differentiation begins in the first weeks after fertilization in the embryo.

• It reflects genetic control of development. Special proteins (hormones, growth factors) activate some genes and repress others.

Figure 3.42

Clinical Applications 3.1 – Cloning to produce

therapeutic stem cells

• In this as yet hypothetical scenario, embryonic cells can be altered to contain the normal unmutated genes.

• When these cells mature and start to specialize, they can then synthesize normal proteins.

Cloning