Cell theory test #1 Revision notes

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Biology test #1 Cell theory

Revision notes:

1.1 Introduction to cells

Unicellular organisms can carry out all the functions of life.

Surface area to volume ratio is an important factor in limiting cell size.

Cells cannot grow to be very large. As the cell grows, it has proportionately less surface area to obtain the materials it needs and dispose of waste. Beyond the limit of the cell size, it must divide and an organism must become multicellular.

Interactions between their cellular components lead to new emergent properties in multicellular organisms.

Their cellular components interact so that the organism can carry out a range of more complicated functions. (Violin vs. Orchestra)

Multicellular organisms have specialised tissues, which develop as a result of cell differentiation.

The cells differentiate, meaning they take on different functions so that the organism can grow in complexity as well as size. Differentiation allows for new properties to emerge as different cell types interact with each other to allow more complex functions to take place.

Cell differentiation results from the expression of some genes but not others.

Differentiation involves the expression of some genes from the organism's genome in the cell, but not others.

Stem cells are able to divide and differentiate along different pathways and are essential for embryonic development. This ability makes the suitable for therapeutic uses.

1.2 Ultrastructure of cells

Prokaryotes have a simple cell structure with no compartmentalisation

Prokaryotic cells are usually much smaller in volume than more complex cells because they have no nucleus. Their means of division is also simple, as they grow, their DNA replicates and separates into two different areas of the cytoplasm which then divides in two (binary fission).

Eukaryotes have a compartmentalised cell structure with membrane-bound organelles in the cytoplasm

Animals, plants, fungi and protoctists all have eukaryotic cells. They contain organelles, each of which forms a compartment in which specific functions take place.

The resolution of the electron microscopes is much higher than that of light microscopes, which allows identification of cell structures and organelles.

1.3 Membrane structure

Phospholipids form bilayers in water due to the amphipathic properties of the molecules.

Amphipathic compounds: posses both hydrophilic (water-loving, polar) and lipophilic (fat loving) properties. The amphipathic properties of phospholipids in a membrane explain the way a membrane structure forms.

Phospholipids arrange themselves into bilayers, with their polar groups facing the surrounding aqueous medium, and the hydrophobic chains face towards the inside of the bilayer. Therefore, a non-polar region is formed between the two polar ones.

Membranes contain a range of proteins, which differ in their structure, function and position.

Integral proteins are embedded in the bilayer, and the peripheral proteins are attached to the surface.

Many proteins on the outside are glycoproteins-they have carbohydrate groups attached to them. Some serve as hormone binding sites and have special shapes to recognise the specific hormones to which the cell will respond.

Others are important to cell-to-cell communication + adhesion.

Some integral proteins are enzymes immobilised within the membrane structure and perfectly placed to carry out sequences of metabolic reactions.

Channel proteins- have an interior which is hydrophilic, so water soluble materials can pass through them. They are specific, which means that they only allow a particular substance to move through. Some channels are permanently open, whereas others are gated and only open when stimulated to do so.

Some proteins act as channels for ions and molecules to pass by passive transport, or form pumps that use active transport to move molecules into or out of the cell.

Cholesterol is an important component of the membranes of animal cells.

Cholesterol is often present in mammal cells, usually found in the plasma membrane. One end of the cholesterol molecule associates with the polar heads of phospholipid molecules, while other parts of it are embedded in the membrane. This makes the membrane more rigid and less permeable to water-soluble molecules.

1.4 Membrane transport

Particles move across membranes by osmosis, active transport, simple diffusion and facilitated diffusion.

Materials can be taken into cells by endocytosis and leave cells by exocytosis due to the fluid nature of the membrane.

Diffusion and active transport do not work when the cell has to transport large chemical molecules or material in bulk across the membrane. During endocytosis (uptake) the plasma membrane surrounds the material, and it becomes closed in a vesicle, which is thereafter drawn into the cell.

Within a cell, vesicles move materials around

1.1 + 1.2

1.3 + 1.4