Eukaryotic cells

Larger and more complex than prokaryotes

Can be unicellular or part of multicellular organisms

Have linear DNA molecules packaged as chromosomes enclosed in nucleus

Chromosomes number, ploidy (number of copies of the genome), and C-value (amount of DNA in cell) widely variable.

Have membrane bound organelles

Unicellular eukaryotes

• Some of the most complex eukaryotic cells

• Need to be able to perform all functions in harsh environments

• Some exist as unicellular organisms when food supplies available, but aggregate and specialise to form primitive multicellular organisms when food is scarce

Structure of the cell membrane

Phospholipids are a key component of cell membranes

• They are amphipathic in nature, polar head, hydrophobic tail

• They form closed bilayers headgroups facing out, hydrophobic tails in the middle.

They are complex with different components embedded into the plasma membrane ie proteins.

The cell membrane is asymmetric as some are only displayed on one face of the membrane. The lipids and proteins can be shown in one face or both.

Function of plasma membrane

• Regulation of transport - nutrients into cell, waste out of cell

• Maintains balance of chemical conditions in the cell such as pH- homeostasis

• Provides a site for chemical reactions not likely to occur in an aqueous environment

• Detects signals in the extracellular environment

• Interacts with other cells or the extracellular matrix - in multicellular organisms

Organelles

Nucleus

A membrane-bound structure usually visible by light microscopy, contains the cell’s genetic material

• Electron microscopy reveals two membranes each about 5-7 nm thick spaced by 11-40nm

• Inner membrane is in contact with the nuclear contents

• Outer membrane appear to be continuous with endoplasmic reticulum

• Contents are in contact with cytoplasm via nuclear pores which pass theough both membranes

• Large dense region- Nucleolus - visible within nucleus by electron microscopy

• Rich in protein and RNA- involved in synthesis of ribosomal RNA (rRNA) and ribosomes

The nucleus separates DNA from cytosol

Transcription from translation separated in space.

Key features- outer membrane, inner membrane, nuclear pores, nucleolus

Endoplasmic reticulum

Extensive membrane structure forming interconnected sacs and tubules

Responsible for most lipid synthesis

Most membrane protein synthesis

Ca2+ ion storage

Detoxification

Rough ER

• Ribosomes attached to surface

• Plays a role in synthesising membrane-bound and secreted proteins

• Extensive in cells synthesising these proteins

Smooth ER

• Carries no ribosomes

• Plays a role in producing lipids eg membrane lipids and steroid hormones

Ribosomes

Complex multi-subunit structures comprised of toughly 50% proteins and 50% ribosomal RNA (rRNA)

rRNA are key to the structure and function of ribosomes

Involved in the synthesis of proteins

Eukaryotic ribosomes consist of 40S and 60S subunits- assembled give 80S

Often associated with endoplasmic reticulum or as cytoplasmic polyribosomes

Golgi complex/ apparatus

Stack of flattened membranous sacs which vary in number

Sacs form from parts of rough ER which breaks off and fuse

Inner cis face is close to the nucleus

membrane at nuclear face 5-7nm thick like nuclear membrane

At outer trans face it is 7-9nm thick like plasma membrane ensures that vesicles budding off outer face can fuse with plasma membrane

packages lysosomal proteins and proteins to be secreted from the cell

• composed of 3 regions cis (entry), medial, trans (exit)

• each region contains different sets of modifying enzymes

Mitochondria

Typically 0.5-10 µm (size of typical bacterium), but can vary in shape from cylindrical to almost spherical

Contains DNA and ribosomes which are smaller than normal eukaryotic type

Can direct production of some of own proteins

Self-replicating; reproduces via binary fission

Multiple mitochondria per cell 1 to1000

Aqueous environment

Site of ATP production via aerobic metabolism

Plays important role in apoptosis- programmed cell death

structures

• Outer membrane

• Intermembrane space

• Inner membrane

• Matrix

Mitochondrial DNA

Contains multiple mtDNA molecules

Genes in mtDNA exhibit cytoplasmic inheritance and encode rRNA and tRNA and some mitochondrial proteins

The size and coding capacity of mtDNA varies considerably in different organisms

The products of mitochondrial genes are not exported

Mutations in mtDNA cause several genetic diseases in humans ie Leigh syndrome, Leber Hereditary Optic Neuropathy

Lysosomes

Appear as electron-dense bodies up to 400nm by EM

Single membrane bound organelles containing hydrolytic enzymes

Degrade materials taken up by endocytosis and cell debris and organelles

Degrade damaged newly synthesised proteins

Interior pH is low about 3.5-5.0

Enzymes eg acid hydrolases, they contain could cause serious cell damage but cannot function at neutral cytoplasmic pH

If a lysosomal enzyme is missing due to genetic defect, results in accumulation of material: lysosomal storage disease- usual fatal in adolescence

Not present in plant cells- vacuole plays some roles of lysosomes in plants

Peroxisomes

Single membrane-bound organelles

Can be diverse in size and enzyme composition

Contains catalase and urate oxidase

Breaks down very long chains of fatty acids via beta oxidation spiral

Oxidation of toxins (alcohol)

Cytosol

The portion of the cell enclosed by the plasma membrane but not part of any organelle.

Not static, the contents of the cell are continuously moving

Key features include;

• Cytoskeleton

• Polyribosomes

• Metabolic enzymes

Cytoskeleton

• Lattice like array of filaments and fine tubules

• Involved in cell movement, cell division, maintenance of cell shape, Intracellular trafficking or organelles, coordinated movement of tissues

• 3 major components: microfilaments, microtubules and intermediate filaments

Microfilaments

• F-actin filaments are double helices of polymerised G-actin subunits

• Fibers expand and contract by further polymerisation and depolymerisation

• ATP dependent

• Interact with other filaments and motors to create movement: contraction can cause shape change

Actin + myosin

• Actin microfilaments work with myosin in muscle fibres

• Myosin filaments walk along the tethered actin hence pulling the filaments towards the centre to cause muscle contraction

Microtubules

• Tubes of tubulin: grow by polymerisation from specific organising centres

• Can form trackways in cells along which motor proteins (kinesins) drag vesicles, organelles etc

• Fundamental role in portioning the chromatids in cell division

• Motor protein complexes carry cellular components along the microtubule tracks

• Different motors for different directions of movement

Intermediate filaments

• Structures about 10nm in diameter

• Many different types that differ in composition and function

• May have role in maintaining cell shape, tissue integrity

Chloroplasts- plant cells only

Double membrane structure found in photosynthetic plant cells

Up to 10µm by 2µm- larger than mitochondria

Contain their own DNA molecules

Stacked inner thylakoid membranes contain green pigment chlorophyll and other pigments that absorb light and generate NADPH and ATP during photosynthesis- used to produce organic molecules.

Vacuoles- plant cell

Occupy up to 80% of the plant cell

Stores water, ions and nutrients and degrade macromolecules

Inflow of water by osmosis causes vacuole expansion and maintenance of turgor pressure

Expansion of vacuoles involved in cell elongation

Plasmodesmata- plant cells

Plant cells are surrounded by a rigid cell walls designed to withstand turgor pressure

Comprised mainly of cellulose cross-linked by hemicellulose, pectin and lignin-insoluble

Plasmodesmata directly connect the cytosol of adjacent cells in higher plants

Specialised peroxisomes

One type is found in the leaves where it involved in photorespiration- utilises oxygen and generated carbon dioxide.

Glyoxysomes are found in germinating seeds- carry out glyoxylate cycle to convert fatty acids into sugars