From The Student Room

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Core Principles

---

Organisms are organised on a cellular basis.

Life depends on precise compartmentalisation and organisation of certain molecules. The plasma membrane defines the external boundary of cells and makes the cell the basic unit of life.

Look at the diagram of an epithelial cell from mammal small intestine using a light microscope.

This is an example of an animal cell.

Look at the diagram of a palisade mesophyll cell in a leaf using a light microscope. (compare the plant cell with the animal cell).

Life depends on cell chemistry so we need to study the molecules inside cells.

Contents 1 Inside a cell 1.1 Biological molecules 1.2 Inorganic constituents of cells 2 The Electron Microscope 3 The Role of Membranes within a cell 4 What is cytoplasm? 5 The Organelles which you need to know about 5.1 There is a phospholipid bi-layer 5.2 Cholesterol is present 5.3 Proteins are present 5.4 Glycoproteins (protein complexed with carbohydrate) 5.5 Glycolipids 6 UNITS OF MEASUREMENT FOR CELLS AND ORGANELLES 7 Comments

Inside a cell

Biological molecules

(To a chemist - organic molecules) These are complex molecules of carbon - e.g.

• Carbohydrates
• Proteins
• Lipids
• Nucleic acids (DNA)

Inorganic constituents of cells

• Water - most cells are approximately 80% water
• Bulk ions - Na⁺, Cl^-, K_­⁺, Ca²⁺, acids, bases and salts dissociate into ions when dissolved in water.
• Trace elements

The Electron Microscope

Since the 1950's, biologists have used this to study cell structure.

The Function of the Cell Membrane (plasma membrane) (plasmalemma)

1. Separates contents of cells from their external environment, i.e. the collection of organic molecules must be surrounded by a barrier and kept together for life activities to be possible. (The cell can be chemically different from its surroundings).
2. Regulates the entry and exit of molecules, (it is differentially permeable).
3. Acts as receptor sites for recognising hormones and other chemicals.

The Role of Membranes within a cell

2 main functions: -

1. establish a number of compartments inside the cell. These are called organelles
2. To provide a neat spacial organisation of enzymes and pigments

What is cytoplasm?

It is a collection of organelles surrounded by cytosol. Cytosol is a complex solution containing some molecules which are too large to form a true solution.

The Organelles which you need to know about

Structure and function of organelles.'

• Nucleus
• Mitochondrion
• Rough and smooth Endoplasmic
• Reticulum (rough and smooth ER)
• Golgi body
• Ribosome
• Chloroplast (not in animal cells)
• Cell wall (not surrounding animal cells)

The structure of the Plasma Membrane (remember to relate structure to function)

The Fluid Mosaic model (proposed by Singer and Nicholson 1972).

So called because it is thought that there are nuggets of protein freely moving in a phospholipid sea.

There is a phospholipid bi-layer

Glycerol end - water soluble - can form H bonds with water molecules (hydrophilic) (polar)

2 long hydrocarbon chains (fatty acid chains) - insoluble in water (hydrophobic) (non-polar)

If a thin layer of phospholipid is spread over the surface of water, the molecules orientate themselves into a single monolayer.

AIR

• non-polar tails are hydrophobic
• polar heads are hydrophilic (water loving)

WATER

• If phospholipid is present in a large enough amount to more than cover the surface of the water it spontaneously forms a bi-layer owing to the molecules having polar heads and non-polar tails.

The double layer of phospholipid is the means of keeping 2 watery phases apart, i.e. the solution inside the cell is kept separate from the surrounding solution by the lipid barrier.

Cholesterol is present

Cholesterol is overwhelmingly hydrophobic. In a plasma membrane there is almost 1 cholesterol molecule/phospholipid, (10 times less cholesterol in the internal membranes).

It is essential for the function of plasma membranes and has 2 important roles:

• To reduce the passive permeability of cells, i.e. uncontrolled leakage of small molecules (ions and water). A lot of energy would be wasted if these materials leaked in the wrong direction.
• To modify the physical properties of the membrane phospholipids. Cholesterol molecules pull the lipid fatty acid chains in phospholipids together, restricting their movement but not making them solid. This enhances mechanical stability and flexibility of the membrane and strengthens it. The usual consistency of lipids is similar to olive oil but differing proportions of cholesterol, unsaturated fatty acid and protein alter fluidity and this affects membrane activity.

Proteins are present

NB. Proteins make the membrane selectively permeable.

• Some are fixed and some are moving about in a 'phospholipid sea'.
• Some penetrate only part way through the membrane.
• Some completely span the membrane.
• Various sizes and shapes - thousands of different proteins.
• Some purely structural, i.e. giving strength.
• Some act as carrier molecules, i.e. transporting specific substances through the membrane. ('Pumps' and/or 'gates')
• Some are enzymes.
• Some are specific receptor molecules.
• Some form pores (hydrophilic channels through the lipid bi-layer)

Glycoproteins (protein complexed with carbohydrate)

(glyco carbohydrate)

Carbohydrates are exposed at the cell surface. Branching carbohydrate produces many different but precisely defined patterns.

These are important recognition sites.

1. Sites of 2 neighbouring cells may bind together to produce cell to cell adhesion. Cells can orientate themselves and form tissues.
2. Basis of immune response; glycoproteins act as antigens.
3. Certain molecules in solution can be taken in by cells with a specific recognition site, e.g. the hormone insulin.

Typically 2-10% of a plasma membrane is carbohydrate.

Glycolipids

These also act as recognition sites.

UNITS OF MEASUREMENT FOR CELLS AND ORGANELLES

The micrometre - lm - 1000lm = 1mm

i.e. 1lm = 10^-3mm = 10^-6m

The nanometre - nm - 1000nm = 1lm

i.e. 1nm = 10^-3lm = 10^-9m

Comments