cell-structure-and-organisation eukaryotic-cells

Eukaryotic Cells: Structure

• Cells can be divided into two broad types; eukaryotic and prokaryotic cells

• Eukaryotic cells have a more complex ultrastructure than prokaryotic cells

  • The term ultrastructure refers to the internal structure of cells

• The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles

• Animal and plant cells are both types of eukaryotic cells that share key structures such as

  • Membrane-bound organelles, including a nucleus

  • Larger ribosomes known as 80S ribosomes

• Key differences between animal and plant cells include

  • Animal cells contain centrioles and some have microvilli while plant cells do not

    • Microvilli are folded regions of the cell surface membrane that increase cell surface area for absorption, e.g. in the small intestine

  • Plant cells have a cellulose cell wall, large permanent vacuoles, and chloroplasts while animal cells do not

Animal cells are eukaryotic cells

Plant cells are eukaryotic cells with a cellulose cell wall, permanent vacuole, and chloroplasts

Cell surface membrane

• All cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment

  • The membrane is described as being partially permeable, meaning that some substances can pass through the membrane while others cannot

• Cell membrane is formed from a phospholipid bilayer spanning a diameter of around 10 nm

• Many organelles inside cells are surrounded by cell membrane, so when referring to the outer membrane of a cell it is always a good idea to refer to it as the cell surface membrane

  • The cell surface membrane can also be referred to as the plasma membrane

The cell surface membrane surrounds the cell, separating it from its external environment

The nucleus

• Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane called the nuclear envelope, which has many pores

  • Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes, e.g. DNA polymerases, and signalling molecules to travel in

• The nucleus contains chromatin, the material from which chromosomes are made

  • Chromosomes are made of sections of linear DNA tightly wound around proteins called histones

• Usually, at least one or more darkly stained regions of the nucleus can be observed under a microscope; these regions are individually termed nucleolus (plural nucleoli) and are the sites of ribosome production

The nucleus of a eukaryotic cell is surrounded by the nuclear envelope and contains chromatin as well as a region called the nucleolus. Note that the nucleus is shown here surrounded by another organelle; the endoplasmic reticulum

Mitochondria

• The site of aerobic respiration within eukaryotic cells, mitochondria (singular mitochondrion) are just visible with a light microscope

• Mitochondria are surrounded by a double-membrane with the inner membrane folded to form structures called cristae

• The matrix of mitochondria contains enzymes needed for aerobic respiration, producing ATP

• Small circular pieces of DNA, known as mitochondrial DNA, and ribosomes are also found in the matrix

  • These are needed for replication of mitochondria before cell division

Mitochondria are the site of aerobic respiration in eukaryotic cells

Ribosomes

• Ribosomes can be found as free organelles in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells

• They are not surrounded by a membrane

• Each ribosome is a complex of ribosomal RNA (rRNA) and proteins

• 80s ribosomes are found in eukaryotic cells

• 70s ribosomes are found in prokaryotes, mitochondria, and chloroplasts

• Ribosomes are the site of translation

Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein

Endoplasmic reticulum

• There are two types of endoplasmic reticulum; rough and smooth

• Rough Endoplasmic Reticulum (RER)

  • RER is formed from folds of membrane continuous with the nuclear envelope

  • The surface of RER is covered in ribosomes

  • The role of the RER is to process proteins made on the ribosomes

• Smooth Endoplasmic Reticulum (SER)

  • SER is also formed from folds of membrane but its function is distinct from the RER, being involved in the production, processing and storage of lipids, carbohydrates and steroids

  • SER does not have ribosomes on its surface