Chapter 2 - Cells and their composition

2.1 Cells

Functional units of life, cells are the basic building blocks that make up all living organisms. They vary in size, shape, and function, but all share common features such as a cell membrane, cytoplasm, and genetic material.


Cell Theory —

  • all organisms are composed of cells

  • all cells come from pre-existing cells

  • the cell is the basic organisational unit of living things


All types of cells perform similar basic processes and many also carry out highly specialised functions. The activities of cells require considerable energy and produce a variety of biological molecules. These biological molecules are called biomolecules, are used to build new organelles, used for repair or exported from the cell.

  • All processes are by enzymes and are precisely regulated. These enzymes can be affected by factors such as temperature, pH, and substrate concentration, which can influence the rate of cellular reactions.


Types of cells -

  • Prokaryotes - unicellular organisms that lack a nucleus and membrane-bound organelles, such as bacteria and archaea.

    • Include: bacteria and archaea.

    • Less complex than eukaryotes and comprise mainly of prokaryotic cells.

  • Eukaryotes - organisms that have a defined nucleus and membrane-bound organelles, including plants, animals, fungi, and protists. They are generally more complex than prokaryotes and can be unicellular or multicellular.


Common features of cells -

  • Plasma membrane: semipermeable structure that separates the interior of the cell from the exterior environment.

  • Cytoplasm: Comprises the cytosol (gel-like substance). In Eukaryotes, the cytoplasm includes the cytosol and the organelles present in the cell except for the nucleus.

  • Deoxyribonucleic acid (DNA): carries hereditary information, directs the cell’s activities and is passed on accurately from generation to generation.

  • Ribosomes: structures that assist in the synthesis of proteins. Not membrane bound, however are often grouped with organelles.


Prokaryotic cells

  • Unicellular and have simple cell structure.

  • Can be found everywhere, even in extreme environments.

  • Small and lack membrane-bound organelles, including a distinct nucleus.

  • Cytoplasm contains scattered ribosomes that are involved in the synthesis of proteins.

  • Genetic material is usually: single, circular DNA chromosome called geneophore, irregularly shaped region called the nucleoid.

    • does not have nuclear membrane.

    • may contain rings of double-stranded DNA called plasmids.

  • Plasma membrane of most prokaryotic cells is surrounded by an outer cell wall of protein and complex carbohydrate (peptidoglycan).

  • Many bacteria also have capsule outside the cell wall, which protects cell from damage and dehydration.

  • Most prokaryotes also have flagella (singular flagellum) that enable them to move freely and small hair-like projections called pili (singular pilus), which are involved in the transfer of DNA between organisms and which van also help generate movement.

  • Specialised pili that can attach to surfaces are called fimbriae.


Eukaryotic cells -

  • Have plasma membrane surrounding cytoplasm

  • have internal (non-plasma) membranes that form specialise membrane-bound compartments within the cell. = Cell compartmentalisation.

  • Membrane-bound structures are called organelles

Organelles of eukaryotic cells

  • Sub-cellular structures that have a specific function

  • Their presence depends on the needs of the cell

  • Some are membrane-bound and some are not

Ribosomes - rough and smooth endoplasmic reticulum.

Golgi apparatus - vesicles are all important organelles involved in protein manufacture and movement within and cell.


Nucleus

Structure:

  • membrane-bound: double membrane

  • contains DNA

Fuction:

  • contains genetic information (used for synthesis of proteins)

  • directs the activities of the cell

Present in plants: Yes

Present in animals: Yes


Mitochondria

Structure:

  • membrane-bound: is double membrane; the inner membrane is highly folded

  • contains DNA

Function:

  • release energy from organic compounds (cellular respiration)

Present in plants: Yes

Present in animals: Yes


Rough endoplasmic reticulum (RER)

Structure:

  • membrane-bound

  • composed of a network of membranous tubules and sacs (called cisternae)

  • ribosomes bind to the membrane

Function:

  • synthesis and processes proteins (often by adding carbohydrates to proteins produced by the ribosomes to form glycoproteins)

Present in plants: Yes

Present in animals: Yes


Ribosome

Structure:

  • non-membrane-bound

  • composed of proteins and ribosomal RNA

  • found free in the cytosol or attached to endoplasmic reticulum (ER)

Function:

  • synthesises proteins (translate messenger RNA into proteins)

  • RER-bound ribosomes synthesises proteins for export from the cell

Present in plants: Yes

Present in animals: Yes


Golgi Apparatus

Structure:

  • membrane-bound

  • stack of cisternae that are not connected to each other

Function:

  • further processes and packages proteins into vesicles for export from the cell (except lysosomes, which remain in the cell)

Present in plants: Yes

Present in animals: Yes


Chloroplast

Structure:

  • membrane-bound: double membrane

  • contains thylakoids (disc-shaped membranous sacs)

  • contains DNA

Function:

  • uses light energy, carbon dioxide, and water to produce glucose (photosynthesis)

Present in plants: Yes

Present in animals: No


Smooth endoplasmic reticulum

Structure:

  • membrane-bound

  • network of cisternae

Function:

  • synthesises lipids

Present in plants: Yes

Present in animals: Yes


2.2 Molecular composition of organisms

Organic molecules

  • Organisms produce characteristics complex compounds that contain carbon and hydrogen.

  • Most large organic molecules are composed of many smaller organic molecules linked together.


4 Main types of organic molecules:

  • carbohydrates

  • proteins

  • nucleic acids (huge and known as biomacromolecules)

    • biomacromolecules - chain-like molecules called polymers.

      • polymers are formed by joining together many smaller units to form a chain.

In organisms, organic molecules can be converted from one form into another.

For example, glucose units

may be linked together to form larger carbohydrates such as starch, glycogen or cellulose.


When food is plentiful, carbohydrates are converted into fats for storage, when it is scarce, the reverse will occur. Even proteins can be converted into small molecules to use for energy


Carbohydrates

  • Most abundant organic molecules in nature.