2-1-5-eukaryotic-cells

Eukaryotic Cell Structure

Cell surface membrane

The structure of the cell surface membrane – although the structure looks static the phospholipids and proteins forming the bilayer are constantly in motion

• 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’

• The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10 nm

Cell wall

The cell wall is freely permeable to most substances (unlike the plasma membrane)

• Found in plant cells but not in animal cells

• Cell walls are formed outside of the cell membrane and offer structural support to cell

• Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells

• Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

Nucleus

The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is the genetic material of the cell

• Present in all eukaryotic cells (except red blood cells), the nucleus is relatively large and separated from the cytoplasm by a double membrane (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 (eg. 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 can be observed – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production

Mitochondria

A single mitochondrion is shown – the inner membrane has protein complexes vital for the later stages of aerobic respiration embedded within it

• The site of aerobic respiration within all eukaryotic cells, mitochondria are just visible with a light microscope

• Surrounded by double-membrane with the inner membrane folded to form cristae

• The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP

• Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

Chloroplasts

Chloroplasts are found in the green parts of a plant – the green colour a result of the photosynthetic pigment chlorophyll

• Found in plant cells

• Larger than mitochondria, also surrounded by a double-membrane

• Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana

• Grana are joined together by lamellae (thin and flat thylakoid membranes)

• Chloroplasts are the site of photosynthesis:

  • The light-dependent stage takes place in the thylakoids

  • The light-independent stage (Calvin Cycle) takes place in the stroma

• Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Ribosomes

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

• Found in all cells

• Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells

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

• 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells

• 70S ribosomes (composed of 50S and 30S subunits) in prokaryotes, mitochondria and chloroplasts

• Site of translation (protein synthesis)

Endoplasmic reticulum

The RER and ER are visible under the electron microscope – the presence or absence of ribosomes helps to distinguish between them

Rough Endoplasmic Reticulum (RER)

• Found in plant and animal cells

• Surface covered in ribosomes

• Formed from continuous folds of membrane continuous with the nuclear envelope

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