general biology (bio107) chapter 4 – cells. cells are the smallest living units of life two types...

General Biology (Bio107)

Chapter 4 – Cells

Cells are the smallest living units of life

• Two types of cells exist on earth:1. Prokaryotic cells2. Eukaryotic cells

• All cells are surrounded by a phospholipid bilayer-made barrier called the plasma membrane.

• The semifluid substance within the membrane is the cytosol, containing the organelles.

• All cells contain chromosomes which have genes in the form of DNA.

• All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes.

• Besides showing a difference insize (prokaryotic cells are small), a major difference between prokaryotic and eukaryotic cellsis the location of chromosomes.

• In eukaryotic cells, chromosomes are contained in a membrane-enclosed organelle, the nucleus.

• In much smaller prokaryotic cells, the DNA is concentrated in thenucleoid without a membrane separating it from the rest of the cell.

The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the

eukaryotic cell.

• Eukaryotic cells are generally much bigger than prokaryotic cells.

• The logistics of carrying out metabolism set limits on cell size.– At the lower limit, the smallest bacteria,

mycoplasmas, are between 0.1 to 1.0 micron.– Most bacteria are 1-10 microns in diameter.– Eukaryotic cells are typically 10-100 microns in

diameter.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.5

• Metabolic requirements also set an upper limit to the size of a single cell.

• As a cell increases in size its volume increases faster than its surface area.– Smaller objects have a greater

ratio of surface area to volume.

• The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell.


Fig. 7.6

• The volume of cytoplasm determines the need for this exchange.

• Rates of chemical exchange may be inadequate to maintain a cell with a very large cytoplasm.

• The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells.

• Larger organisms do not generally have larger cells than smaller organisms - simply more cells.


• A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments and membraneous organelles.

• These membranes also participate in metabolism as many enzymes are built into membranes.

• The barriers created by membranes provide different local environments that facilitate specific metabolic functions.

Eukaryotic cell


• Different membranous organelles can be found in eukaryotic cells:1. Rough endoplasmic reticulum (rER)2. Smooth endoplasmic reticulum 3. Golgi apparatus4. Mitochondrion5. Lysosomes6. Peroxisomes

• Each type of membranous organelle has a unique combination of lipids, proteins and enzymes for its specific functions.– For example, those in the membranes of

mitochondria function in cellular respiration.



Fig. 7.7

• Cells of eukaryotic photosynthesizing life forms, e.g. algae and plants, contain unique sets of organelles, most namely:1. Tonoplast (central vacuoles)and 2. Chloroplasts

• Their plasma membrane is further surrounded by a thick, protective cell wall.


Fig. 7.8

Plasma membrane

• Phospholipid bilayer-made barrier between inside and outside of cell; also contains cholesterol

• Semi-permeable; only water, gases and lipophilic molecules can freely cross

• Controls entry of materials with the help of selective transport proteins, e.g. carrier, porins, channels

• Receives chemical and mechanical signals with the help of receptor proteins

• Transmits signals between intra- and extracellular spaces

Facilitated Diffusion

17

Active TransportSolutes are transported across plasma membranes with the use

of ATP-derived energy; capable to transport from an area of lower concentration to an area of higher concentration

Example: Sodium-potassium pump

1

3 Na+

K+

gradient

Cytosol

1

3 Na+ expelled

3 Na+

ADPP

P

2 K+

imported

K+

gradient

Na+

gradientNa+/K+ ATPase

Extracellular fluid

Cytosol

2K+

ATP2 3 4

• The nucleus contains chromosomal DNA and most of the genes in a eukaryotic cell.– The nucleus of each human cell contains 46

chromosomes.– Some genes are located in mitochondrial and

chloroplast DNA.• The nucleus averages about 5 microns in diameter.• The nucleus is separated from the cytoplasm by a

double membrane.– These are separated by 20-40 nm.

• Where the double membranes are fused, a nuclear pore allows gene-regulating proteins, large macromolecules and particles to pass through.

Nucleus


• The nuclear side of the envelope is lined by the nuclear lamina, a network of intermediate filaments that maintain the shape of the nucleus.


Fig. 7.9

• Within the nucleus, the DNA and associated proteins (histones) are organized into fibrous material, called chromatin.

• In a normal cell they appear as diffuse mass.• However when the cell prepares to divide, the

chromatin fibers coil up to be seen as separate structures, chromosomes.

• Each eukaryotic species has a characteristic number of chromosomes.– A typical human cell has 46 chromosomes, but

sex cells (eggs and sperm) have only 23 chromosomes.


• In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. – There, ribosomal RNA (rRNA) is synthesized

and assembled with proteins from the cytoplasm to form ribosomal subunits.

– The subunits pass from the nuclear pores to the cytoplasm where they combine to form ribosomes.

• The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA).– mRNA travels to the cytoplasm and combines

with ribosomes to translate its genetic message into the primary structure of a specific polypeptide.


• Ribosomes which contain rRNA and protein are responsible for synthesis of new proteins in cells.

• A ribosome is composed of two subunits that combine to carry out protein synthesis.

Ribosomes


• Cell types that synthesize large quantities of proteins (e.g., pancreas) have large numbers of ribosomes and prominent nuclei.

• Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.

• Other ribosomes, bound ribosomes, are attached to the outside of the rough endoplasmic reticulum.– These synthesize proteins that are either

included into membranes or for export from the cell.

• Ribosomes can shift between roles depending on the polypeptides they are synthesizing.


Endoplasmic Reticulum (ER)

• Structure: network of folded membranes

• Functions: synthesis, intracellular transport

• Types of E.R.– Rough E.R.: studded with ribosomes (sites of

protein synthesis & protein folding)– Smooth E.R. lacks ribosomes. Functions:

• lipid synthesis• release of glucose in liver cells into bloodstream • drug detoxification (especially in liver cells)• storage and release of Ca2+ in muscle cells (where

smooth E.R. is known as sarcoplasmic reticulum or SR)

Copyright 2010, John Wiley & Sons, Inc.

Endoplasmic Reticulum (E.R.)

• Structure: – Flattened membranes (cisterns) with bulging

edges (like stacks of pita bread)

• Functions: – Receive protein from rER, modify and sort

proteins glycoproteins and lipoproteins that:

• Become parts of plasma membranes• Are stored in lysosomes, or• Are exported by exocytosis

Golgi Complex


Golgi Complex

Small cell organelles• Lysosomes: contain digestive enzymes

– Help in final processes of digestion within cells– Carry out autophagy (destruction of worn out parts of

cell) and death of old cells (autolysis)– Important for phagocytotic cells (e.g. macrophages)– Tay-Sachs: hereditary disorder; one missing lysosomal

enzyme leads to nerve destruction

• Peroxisomes: special forms of metabolism, detoxify; abundant in liver; produce hydrogen peroxide

• Proteasomes: digest unneeded or faulty proteins– Faulty proteins accumulate in brain cells in persons with

Parkinson or Alzheimer disease.

Lysosomes

• Membrane-bounded sacs which contain many hydrolytic enzymes that recycle macromolecules, e.g. DNA, lipids, proteins, back into their monomers.

• Lysosomal enzymes work best at acidic pH of 5.– Proteins in the lysosomal membrane pump

hydrogen ions from the cytosol to the lumen of the lysosomes.

• While rupturing one or a few lysosomes has little impact on a cell, but massive leakage from lysosomes can destroy an cell by autodigestion.

• Lysosomes fuse with phagosomes which space allows the cell to digest macromolecules safely.

• Lysosomal enzymes and membranes are synthesized by rough ER and then transferred to the Golgi where they bud off.

• Lysosomes play crucialrole in following cellprocesses:1. Food digestion2. Autophagy (“organelle recycling”)3. Phagocytosis + bacterial kill

• Lysosomes also play a critical role in the programmed destruction of cells (“apoptosis”) in multicellular organisms.– This process allows reconstruction during the

developmental process.• Several inherited diseases affect lysosomal metabolism

(“lysosomal storage disorders”).– These individuals lack a functioning version of a normal

hydrolytic enzyme.– Lysosomes are engorged with indigestable substrates.– These diseases include Pompe’s disease in the liver

and Tay-Sachs disease in the brain.

Mitochondria

• Structure:– Sausage-shaped with many folded membranes

(cristae) and liquid matrix containing enzymes– Have some DNA, ribosomes (can make proteins)

• Function:– Nutrient energy is released and trapped in ATP;

so known as “power houses of cell”– Chemical reactions require oxygen

• Abundant in muscle, liver, and kidney cells– These cells require much ATP

Mitochondria

Chloroplast• Structure:

– Oval-shaped with many stacked phospholipid sacs (= thylacoids) and liquid stroma containing enzymes

– Have large ring-formed DNA, ribosomes (can make proteins)

• Function:– Convert solar energy into ATP and nutrient energy

(glucose) by a process called “photosynthesis”– Chemical reactions require carbon dioxide

• Abundant in cells of green algae and plants

Chloroplast

Thylacoid

Stroma

Inner ChloroplastMembrane

Centrosome• Structure:

– Two centrioles arranged perpendicular to each other

• Composed of microtubules: 9 clusters of 3 (triplets)

– Pericentriolar material• Composed of tubulin that grows the mitotic spindle

• Function: important for microtubule formation and assembly; important for movement of chromosomes to ends of cell during cell division (mitosis) and for vesicular transport, e.g. in neurons


Centrosome


Cytoskeleton

• Maintains shape of cell

• Positions organelles• Changes cell shape

• Includes: microfilments, intermediate filaments, microtubules

Microtubules• Build 9+2 protein core of cilia and flagella.• Associated with ATP-consuming motor proteins.• Important for movement of:

1. Cilia2. Flagella (sperm)3. Vesicles (axons)4. Chromosomes (mitosis & meiosis)

• Build from polymerizedtubulin monomers.

• Monomer: Tubulin

• Microtubules play a major role in cell motility.– This involves limited movements of parts of the

cell.• The microtubules interacts with ATP-consuming

motor proteins, e.g. dynein.– In cilia and flagella motor proteins pull

components of the cytoskeleton past each other. – This is also true

in muscle cells.

• A flagellum of a sperm has an undulatory movement.

• Cilia move more like oars with alternating power and recovery strokes.– They generate force perpendicular to the cilia’s

axis.

Microfilaments

• Build protein core of microvilli of epithelial cells.• Builds cortical network underneath cell membrane.• Important for pseudopodia

formation and movement of cells, e.g. white blood cells.

• Build from polymerized protein monomers.

• Monomer: Actin

Intermediate Filaments• Are intermediate in size with 8 - 12 nm diameter.• They are specialized for bearing tension.

– Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins.

• Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes.

• They reinforce cell shape and also fix organelle location.

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