Session 1: Cell structure

TEM (Transmission Electron microscope )

• Beam of electrons transmitted through specimen to form an image

• Specimen is very thin, 100nm

• Electrons interact with specimen and image captured

• High resolution

Interpreting E.M.

• Light areas: attract fewer electrons

• Dark areas: Electron dense

Phospholipid Bilayer:

• Amphipathic: A molecule with both hydrophobic and hydrophillic parts

• To form a bilayer

Proteins on membranes:

All membranes contain proteins that can move in the bilayer => Fluid Mosaic Model

• Some proteins are attached to cytoskeleton

• Others are glycosylated (have short polysaccharide chains)

KEY: Eukaryotic cells contain many types of membranes (RER is single convoluted layer, plasma membrane contains triple layer)

About the cell membrane:

Glycocalyx:

• Sugar coat formed by glycoproteins and glycolipids.

• Involved in protection, cell recognition, adhesion, and immune responses (the glycoproteins and glycolipids are recognised by the bodys immune system => immunogenicity)

Functions of the plasma membrane:

Exocytosis => Exit so the vesicle forms a membrane with the cell and leaves

Endocytosis +> Entering so the vesicle fuses its membrane with the cell and enters

Nucleus

• Contains DNA, RNA and nucleoproteins

• TEM reveals electron dense heterochromatin (DNA and associated nucleoproteins not active in RNA synthesis) => coiled structure // Electron-lucent (light) euchromatin (active in RNA synthesis) => uncoiled

• Inactive cells = small nuclei containing heterochromatin

• Active cells = large nuclei containing euchromatin

• Nuclei not present in differentiated cells such as erythrocytes (most common RBCs), lens fibre cells

Nuclear envelope:

• Has inner and outer membrane

• The membrane of nucleus connects directly to the membrane of the ER

• Nuclear pores that allow transport of molecules

RER (Rough endoplasmic reticulum)

Contain interconnecting set of membranes, vesicles and cisternae (flattened sacs)

Ribosomes on surface, undergoing protein synthesis

RER = Flattened + Ribosomes + protein production

SER (Smooth endoplasmic reticulum)

SER = Tubular + no Ribosomes + lipid production

Site of steroid production (Cortisol, Oestrogen, progesterone), cells that make a lot of steroid hormones = lots of SER

Golgi apparatus:

Proteins move from nucleus to the glogi at the cis face (smaller face) and travel to the trans face (larger face), the proteins then transport out to the lysosomes.

CIS (COMES IN) TRANS (TRANSPORT OUT)

Mitrocondria:

Large numbers in liver and skeletal muscles

Mitrocondria can divide => have their own genetic information

Lysosomes:

• Produced by the Golgi apparatus

• They digest and re-use carbohydrates

• Contain acid hydrolases at pH 5 (nucleases, proteases…)

• Lysosomes membranes are highly glycosylated (lots of carbohydrate chains)

• Wide variety of shapes => spherical or oval

How do Lysosomes digest ?

Where does intracellular digestion occur?

• Secondary lysosomes => Where the lysosomal enzymes meet the material that needs to be digested

Peroxisomes:

• Small membrane-bound organelles found free in the cytoplasm

• They react with oxygen, and form Hydrogen peroxide

• H2O2 is a reactive and harmful chemical

  • However, catalase uses H2O2 to detoxify other substances

Abundant in liver and kidney, to remove toxins from body.

Neutrophils (white blood cell) + H2O2 = to kill bacteria

Cytoskeleton:

• The skeleton of the cell

  • allows to maintain, and allow flexibility of cell

Microfilaments:

Microfilaments: Thin and flexible fibres involved in shape and movement, made from double stranded helix of 2 actin proteins joined together.

• They maintain the cells’ shape

• Allow movement (assemble = actin molecules join the filament) // (disassemble = actin molecules leave the filament)

Microtubules:

A microtubule is a hollow protein tube inside the cell, part of the cytoskeleton, so helps organise cell.

Main function: To provide tracks along which things can be transported through the cell. Motor proteins (Kinesin and Dynein), which have attached cargo in a vesicle and they walk along the microtubule.

Microtubules are made from alpha-tubulin and beta-tubulin.

Microtubules originate from centrosomes.