1. the cell 2 © zanichelli editore 2015 cells are complex microsystems 3 © zanichelli editore 2015

The cell

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© Zanichelli editore 2015

Cells are complex microsystems

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From biomolecules to cells

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There is a hierarchical organization in cells: atoms constitute biomolecules; biomolecules and water are used to build up structures that work together so that the cell can grow, develop and reproduce.


Why are cells small? /1

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Cells are small in order to have a large surface compared to their volume. This ratio allows for efficient exchanges with the external environment.

Same volume but different surface


Why are cells small? /2

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Volume 64 cm3 64 cm3 64 cm3

Surface 96 cm2 192 cm2 384 cm2

Surface/Volume 96/64 = 1.5 192/64 = 3 384/ 64 = 6


Observing cells

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Cells can be observed with the light microscope or the electron microscope which has a higher resolving power.


Prokaryotic cells /1

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Prokaryotes are unicellular organisms that belong to Archaea or Bacteria domains.

Prokaryotic cells always present four structures:• plasma membrane;• cytoplasm;• nucleoid with a circular DNA molecule;• ribosomes.


Prokaryotic cells /2

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Many prokaryotic cells present additional structures:• cell wall;• capsule;• flagella;• fimbriae or pili.


Eukaryotic cells

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Eukaryotes are organisms that can be unicellular or multicellular.

Eukaryotic cells, besides the base structures of prokaryotic cells, have membrane-bound organelles and a nucleus.


Animal and plant cells /1

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Both animal and plant cells have common structures:•mitochondria;•Golgi apparatus;•nucleus;•endosplamic reticulum;•cytoskeleton.

Animal cells also have lysosomes and centrioles.


Animal and plant cells /2

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Plant cells also present a cell wall, vacuolesand chloroplasts.


Organelles in eukaryotic cells

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Nucleus and nucleolus

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The nucleus contains the DNA with the genetic information of the organism. The DNA controls the synthesis of proteins and it replicates during cell division.

The subunits of ribosomes are assembled in the nucleolus.

The synthesis of proteins takes place in the cytoplasm, on ribosomes.


nucleus

nucleolus

Mitochondria and cell energy

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The energy necessary for the cell is produced by cell respiration in the mitochondria.These organelles contain their own DNA and ribosomes.


They are divided into compartments:• the outer membrane;• the intermembrane space;• the inner membrane; • the matrix.

outer membrane

inner membrane

matrix

Intermembrane space

The endoplasmic reticulum:rough and smooth

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The main function of the rough endoplasmic reticulum is to store and modify proteins.

The smooth endoplasmic reticulum is important for the synthesis of many lipids and for the degradation of toxic substances and metabolic waste.


The Golgi apparatus

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The Golgi apparatus is a membrane system. It receives the molecules synthesized in the endoplasmic reticulum, modifies them and stores them in vesicles.

The vesicles transport the molecules to different partsof the cell.


Lysosomes

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nucleus(green colored)

Lysosome (red colored)

Lysosomes are vesicles formed in the Golgi apparatus.

They contain enzymes which degrade macromolecules and metabolic waste or destroy bacteria.

Cytoskeleton, cilia and flagella

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The cytoskeleton consists of a dense network of filaments and microtubules.

It provides shape and support to the cell and an anchoring site for organelles. It is involved in the transport of organelles and molecules inside the cell.


Many cells can present external, mobile structures, called cilia and flagella.

They are involved in cell movement and they can be used to move the external fluids.

Plant cells: the cell wall

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The cell wall in plant cells provides support and protection and reduces water loss.

It is mainly made up of cellulose and surrounds the cell membrane.


Plant cells: vacuoles

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Up to 90% of a plant cell’s volume may be occupied by the vacuole.


This organelle stores nutrients, degrades waste, accumulates toxic substances that repel herbivores and insects, contains pigments that attract pollinators and provides mechanical support for the plant.

Plant cells: chloroplast

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The chloroplast, which is present in plants and algae, is the organelle responsible for photosynthesis - the transformation of light energy into chemical energy.

It contains chlorophyll and possesses its own DNA.


outer membrane

thylakoids

stroma

inner membranegranum

Cell adhesion and communication

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Cells communicate each other /1

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Nearly all the cells of multicellular organisms are connected to, and communicate with, other cells.

In animal cells, the main types of cell adhesion are:• tight junctions – they form a thin watertight layer to keep the cells locked together;• desmosomes – they connect cells in robust layers and bundles;• gap junctions – they form open channels between two cells and are important for intercellular communication.


Cells communicate to each other /1

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In plants, cells can exchange substances through plasmodesmata: they are microscopic channels that enable communication and transportation between cells.


1. the cell 2 © zanichelli editore 2015 cells are complex microsystems 3 © zanichelli editore 2015

Documents

plant cells

animal cells

eukaryotic cells

observing cells

cell energy

cell division

cell respiration

rough endoplasmic reticulum