Cell Biology

Cell Differentiation

Specialized tissues can develop by cell differentiation in multicellular organisms

Differentiation involves the expression of some genes and not others in a cell's genome

Stem Cells

Stem cells are unspecialised cells that can give rise to a wide range of body cells by differentiating along different pathways. They retain the capacity to divide indefinitely and have the potential to differentiate into specialised cell types when given the right stimulus.

Types of stem cells: 

Use of stem cells to treat diseases

Treating Stargardt's disease with stem cells

Stargardt's disease is a disease of the eye. It generally appears in late childhood to early adulthood. It is caused by a recessive genetic mutation in gene ABCA4, which causes an active transport protein on photoreceptor cells to malfunction. This ultimately causes the photoreceptor cells to degenerate.

To treat it, patients are given retinal cells derived from human embryonic stem cells, which are injected into the retina. The results obtained have been quite positive as the inserted cells attach to the retina and become functional, suggesting that it may be possible to restore sight to affected individuals using stem cells.

Treating Leukaemia with stem cells

Leukemia, a type of cancer of the blood or bone marrow, is caused by high levels of abnormal white blood cells. People with leukaemia have a higher risk of developing infections, anaemia and bleeding.

Treatment, in this case, involves harvesting hematopoietic stem cells (HSCs), which are multipotent stem cells. HSCs can be taken from bone marrow, peripheral blood or umbilical cord blood. The HSCs may come from either the patient or from a suitable donor. The patient then undergoes chemotherapy and radiotherapy to get rid of the diseased white blood cells. The next step involves transplanting HSCs back into the bone marrow, where they differentiate to form new healthy white blood cells.

Ethics of Stem Cell Research

Eukaryotic cell Ultrastructure

Animal Cell.

A Plant Cell (palisade mesophyll cell of a leaf)

Electron microscopy

Electron microscopes have a much higher resolution than light microscopes. The resolution of a light microscope is 200 nm compared to 0.1 nm for an electron microscope.

Microscope resolution is the shortest distance between two separate points in a microscope’s field of view that can still be distinguished as distinct objects. 

It can differentiate objects as small as 0.001 µm. It played a part in understanding the structure of organelles, such as the thylakoid membranes of chloroplasts, and the actin and myosin filaments of muscle.

Eukaryotic cells can be observed using a light microscope. At this magnification, only larger structures such as cell membrane, cell wall, nucleus, central vacuole and chloroplasts can be seen.

For a detailed view of cells, an electron microscope is usually used.

1.3 Membrane structure

Phospholipid bilayers 

The Fluid Mosaic Model

Typical exam drawing question: 

Cholesterol in membranes

Cholesterol is a type of liquid, but is not a fat or oil. It belongs to the group called steroids. 

Its role in the phospholipid bilayer: 

• Cholesterol forms an integral part of an animal membrane and plays an important role in controlling membrane fluidity and permeability to some solutes.
• The cholesterol in the membrane restricts the movement of phospholipids and other molecules, thus reducing membrane fluidity.
• At low temperatures, it disrupts the regular packing of the hydrocarbon tails of phospholipid molecules, which prevents the solidification of the membrane. This enables the membrane to stay more fluid at lower temperatures, allowing the membrane to function properly.
• It reduces membrane permeability to hydrophilic molecules and ions such as sodium and hydrogen.

1.4 Membrane transport

Endocytosis: 

The fluidity of membranes allows materials to be taken into cells by endocytosis

Passive methods of transport across membranes

Simple diffusion 

Simple diffusion occurs in a gas or liquid medium and only requires a concentration gradient. Simple diffusion across membranes involves particles passing between the phospholipids in the membrane. It can only happen if the phospholipid bilayer is permeable to the particles. Non-polar particles such as oxygen can diffuse through easily. 

Facilitated diffusion 

Ions and particles that cannot diffuse between phospholipids can pass into or out of cells if there are channels for them through the plasma membrane. Because these channels help particles to pass through the membrane, from a higher concentration to a lower concentration, the process is called facilitated diffusion. 

Osmosis

Water is able to move in and out of most cells freely. This net movement is osmosis.

Osmolarity refers to the concentration of a solution in terms of moles of solutes per litre of solution.

The Endosymbiotic Theory

The theory of endosymbiosis helps to explain the evolution of eukaryotic cells. It states that mitochondria were once free-living prokaryotic organisms that had developed the process of aerobic cellular respiration. Larger prokaryotes that could only respire anaerobically took them in by endocytosis. They allowed them to live in their cytoplasm. As long as the smaller prokaryotes grew and divided as fast as the larger ones, they could persist indefinitely inside the larger cells. The cells evolved to develop a mutualistic relationship turning it into a eukaryote with membrane-bound organelles. In plant cells, the same thing happened with chloroplasts. If a prokaryote that had developed photosynthesis was taken in by a larger cell and was allowed to survive, grow and divide, it could have developed into the chloroplasts of photosynthetic eukaryotes. 

1.6 Cell division

Cell cycle

Cyclins

Mitosis

Cytokinesis

It is the division of the parental cytoplasm between the two daughter cells after mitosis (though it often starts in telophase).

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