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Cell is the base of life

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How can one define life?

The simplest definition is that any living thing must have three general properties:

• metabolism

• growth

• reproduction

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Life's HierarchyLife on Earth is incredibly extensive and, to make it easier to study, biologists have broken living systems up into generalized hierarchical levels:

molecules; organells;

cells; tissues;

organs; organisms; populations

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Life's Hierarchy

The focus of this course is on the fundamentals of life;

that is, the properties that are held in common among all living things.

We will concentrate almost exclusively on the molecular through the cellular level.

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The cell theory

1. Cells are the fundamental units of life, because a cell is the simplest unit capable of independent existence.

2. All living things are made of cells.

This theory still holds true, with the minor caveat that viruses are only alive while infecting a cell.

Mattias Schleiden and Theodor Schwann (1839) called cells “units of life” – cell theory.

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Cell elemental compositionCells are 90% water.

The remaining molecules, the dry weight is approximately:

• 50% protein• • 15% carbohydrate

• 15% nucleic acid

• 10% lipid

• 10% miscellaneous

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Similarities between the pro- and eukaryotic types of cells

1. They both have DNA as their genetic material.

2. They are both membrane bound.

3. They both have ribosomes.

4. They have similar metabolism.

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Major differences 1. Eukaryotes have a nucleus and membrane-bound

organelles, while prokaryotes do not.

2. Prokaryotic DNA floats freely around the cell; eukaryotic is held within its nucleus.

3. Eukaryotic/prokaryotic size is 10/1.

4. The DNA of eukaryotes is much more complex.

5. Prokaryotes have a cell wall composed of peptidoglycan, a polymer of amino acids and sugar. Some eukaryotic cells also have cell walls, but none made of peptidoglycan.

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Eukaryote cells (protozoa, fungi, higher animals and plants)Each eukaryote has

•a surrounding membrane - a thin layer of protein and fat that restricts the flow of substances in and out of the cell and encloses the

•cytoplasm, a jellylike material containing the nucleus and other structures (organelles) such as mitochondria.

•the nuclei of some cells contain a dense spherical structure called the nucleolus, which contains ribonucleic acid (RNA) for the synthesis of ribosomes.

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Animal cell structure

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Liver cell

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Liver cell nucleus and nucleolus

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Plant cell structureThe plant cell is surrounded by:

a plasma membrane

and contains:

a nucleus, ribosomes, ER, Golgi apparatus, mitochondria, peroxisomes, microfilaments and microtubules.

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Plant cell structurePlant cell also contains a family of membrane-enclosed organelles called plastids.

The most important type of plastid is the chloroplast, which carries out photosynthesis, converting sunlight to chemical energy stored in sugar and other organic molecules.

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Plant cell structure

The vacuole stores chemicals, breaks down macromolecules, and, by enlarging, plays a major role in plant growth.

Another prominent organelle in many plant cells, especially older ones, is a large central vacuole.

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Plant cell structure

The vacuole membrane is called the tonoplast.

Outside a plant cell's plasma membrane (in fungi and some protists as well) is a thick cell wall.

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Plant cell structure

It helps to maintain the cell's shape and protects the cell from mechanical damage.

The cytosol of adjacent cells connects through trans-wall channels called plasmodesmata.

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Nucleus structure

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The nuclear envelope

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Nucleus with Nuclear Pores

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Ribosomes

Ribosomes are the sites of protein synthesis.

They are not membrane-bound and thus occur in both prokaryotes and eukaryotes.

Eukaryotic ribosomes are slightly larger than prokaryotic ones.

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Ribosomes

Structurally the ribosome consists of a small and larger subunit.

Biochemically the ribosome consists of ribosomal RNA (rRNA) and some 50 structural proteins.

Often ribosomes cluster on the endoplasmic reticulum, in which case they resemble a series of factories adjoining a railroad line.

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Structure of the ribosome

Small subunit

Large subunit

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RibosomesBoth free and bound ribosomes are abundant in this pancreas cell (TEM). The pancreas secretes hormones, including the protein insulin, into the bloodstream, and secretes digestive enzymes into the intestine.

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RibosomesBound ribosomes, those presently attached to the endoplasmic reticulum (ER), produce secretory proteins. Free ribosomes mainly make proteins that will remain dissolved in the cytosol. Bound and free ribosomes are identical and can alternate between these two roles.

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Ribosomes and Polyribosomes in liver cell

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The Endomembrane systemA membranous system of interconnected tubules and cisternae

Membranes of the endomembrane system vary in structure, composition, thickness and behavior

The endomembrane system includes:

Nuclear envelopeEndoplasmatic reticulumGolgi apparatusLysosomesVacuolesPlasma membrane (related to endomembrane)

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The Endomembrane systemA membranous system of interconnected tubules and flattened sacs called cisternae, the ER is also a continuous with the nuclear envelope.

The membrane of the ER encloses a compartment called the cisternal space.

Rough ER, which is studded on its outer surface with ribosomes, can be distinguished from smooth ER in the electron micrograph (TEM).

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The Endomembrane system

Membranes in the endomembrane system are related either through:

-direct physical continuity

or

-by the transfer of membrane segments as tiny vesicles.

Endoplasmic reticulum

is a mesh of interconnected membranes that serve a function involving protein synthesis and transport.

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Endoplasmatic reticulum (ER)

ribosomes

smooth ER

cisternae

cisternal space

rough ER

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Endoplasmatic reticulum

Rough endoplasmic reticulum (Rough ER) is so-named because of its rough appearance due to the numerous ribosomes that occur along the ER.

Rough ER

connects to the nuclear envelope through which the messenger RNA (mRNA) that is the blueprint for proteins travels to the ribosomes.

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Rough ER

Newly synthesized proteins fold into its native conformation

Most of the secretory proteins are glycoproteins (proteins covalently bound to carbohydrates

Secretory proteins are kept separate from the other proteins to be remained in cytosol

Secretory proteins leave the ER in transport vesicles

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Rough ER

Another very important function of rough ER is the “production of the membrane”

Proteins and phospholipids are added to the membrane

They are either transported to rough ER or synthesized in rough ER (phospholipids)

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Smooth ER

Smooth ER

lacks the ribosomes characteristic of Rough ER and

is thought to be involved in transport and metabolic processes including:

•synthesis of lipids (phospholipids and steroids),•metabolism of carbohydrates, and•detoxification of drugs and poisons

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Smooth ER

Among the steroids produced by SER are:

the sex hormones secreted by the adrenal glands;

testes and ovaries are rich in smooth ER

Role of smooth ER in carbohydrate metabolism is well presented by liver cells:

liver cells store carbohydrate in the form of glycogen;

phosphate molecule is removed from the glucose which can then live the cell

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Smooth ER

Detoxification of drugs in liver cells mainly occur by adding hydroxyl groups to drugs.

Hydroxylation makes drugs more soluble and more easier to flush from liver cells

Unfortunately the exposure to barbiturates or alcohol forces the growth of smooth ER and subsequent increase tolerance to one of these agents

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Summary

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Summary

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Reading

Campbell et al. Biology. Ch. 6 A tour of the cell, 110-107.