A-level Biology/Biology Foundation/cell structure

Cell Structure

Microscopy

Different types of microscope

There are two types of microscope - light & electron. Light uses light waves as it's source of radiation and electron microscopes use electrons. This has to do with the wavelength of the radiation - visible light ranges from 400-700 nm, and for the light microscope to see a structure, that structure has to be greater than half the wavelength used, so as to interfere with the light waves and produce an image. This means the maximum resolution of a light microscope is around 200 nm.

Electrons have a much smaller wavelength (at least as small as x-rays) and because they are negatively charged, they can be focused using electromagnets, whereas x-rays cannot). This allows them to see much smaller structures than light microscopes - up to and including 0.5 nm, and a single DNA molecule is 2 nm. Electron microscopes however do have a drawback - the specimen must be scanned in a vacuum, and since water boils at room temperature in a vacuum, all specimens must be dehydrated before being examined, and thus only dead material can be seen.

Magnification and Resolution

Optical magnification is defined as the ratio between the apparent size of an object (or its size in an image) and its actual size. It can be calculated as thus;

$magnification={\frac {size-of-image}{actual-size-of-specimen}}$

Resolution on the other hand, is defined as the ability to distinguish between two separate points. If the light microscope cannot distinguish between the two separate points, those two points become one in the resulting image. For example, ribosomes are approximately 25 nm in diameter and when viewed with a light microscope cannot be discerned as it does not interfere with the light waves, whereas a 1000 nm mitochondrion does.

Cell Organelles

An organelle is defined as both a functionally and structurally separate part of the cell and are often surrounded by membranes of their own. This is known as compartmentalisation.

Nucleus

The parts of a cell nucleus

Structure of nucleus

• Largest organelle
• Dual-membrane (nuclear envelope) - porous, allowing exchange between nucleus and cell.
• Sub-structure - nucleolus

Function

• Controls cells activities
• mRNA leaves the nucleus to perform protein synthesis
• Contains chromosomes

Mitochondria

The diagram shows a section of a eukaryotic cell's mitochondrion.

Structure is scientifically very important

• dual membrane forming an envelope
• Inner membrane folded to form cristae, projecting to the inside of the mitochondria, known as the matrix

Function

• Perform later stages of aerobic respiration, a metabolic process that creates ATP
• Also involved in lipid synthesis

Endoplasmic Reticulum

1. Nuclear membrane 2. Nuclear pore 3. Rough endoplasmic reticulum (REM) 4. Smooth endoplasmic reticulum 5. Ribosome attached to REM 6. Macromolecules 7. Transport vesicles 8. Golgi apparatus 9. Cis face of Golgi apparatus 10. Trans face of Golgi apparatus 11. Cisternae of Golgi apparatus 12. Secretory vesicle 13. Cell membrane 14. Fused secretory vesicle releasing contents 15. Cell cytoplasm 16. Extracellular environment

Structure-.

• Rough endoplasmic reticulum has its membrane surface lined with ribosomes.
• Smooth endoplasmic reticulum do not have ribosomes
• Both form a series of sheets which enclose flattened sacs called a cisternae.

Function:

• Protein synthesis takes place in ribosomes on the rough endoplasmic reticulum
• Provide a large surface area for chemical reactions and a pathway for transport of materials through the cell
• Smooth endoplasmic reticulum is involved in lipid and carbohydrate synthesis and the detoxication of drugs

Golgi Apparatus

Structure

• Similar to smooth ER, more compact

Function

• Collect, process and sort molecules.

The stack of flattened sacs (cisternae) are constantly being formed by vesicles budding off at the end of smooth ER and being broken down at the other end to form Golgi vesicles.

Ribosome

Ribosome structure

Structure

• One large and one smaller subunit
• Comprise of RNA (ribosomal) and protein
• 20 nm size

Function

• Protein synthesis

Lysosome

Structure

• Around 0.1-1.0 µm in diameter
• Membrane surrounding digestive enzymes known as hydrolases

Functions

• Digesting worn out organelles, or bacterium taken in during phagocytosis
• Bind to the cell membrane and release their enzymes outside of it in a process known as exocytosis like exocism

Cilia and flagella

Numerous bunched cilia extending from a common pore. Also evident are pores lacking obvious bunched cilia.

These two organelles are almost identical except that cilia are shorter and more numerous. Their structure is a two central micro tubules, surrounded by nine pairs of micro tubules on the outside, wrapped in a plasma membrane, in a long elongated shape similar to a hair. Their function can either be to move an entire organism or to move material within an organism. An example of the latter is the cilia in the trachea moving mucus along the throat.

Centriole

A centriole is a hollow cylinder formed from a ring of microtubules and used to grow the spindle fibres used in nuclear division.

Plasma Membrane

This is a very thin phospholipid bi-layer. It controls the movement of substances in and out of the cell; further explanation can be found later in this book.

Plants Only

Chloroplast

Chloroplast

These are relatively large organelles which are only in photosynthesising cells, are green in colour due to the presence of the pigment chlorophyll, and at high magnifications grana can be seen in them. Grana are used in photosynthesis which will be discussed more later.

Vacuole

The vacuole is a fluid filled sac bound by a single membrane - it contains a solution of sugars, amino acids, waste products and mineral salts. It can serve as a temporary store, for waste or food, and can also contain hydrolytic enzymes. They also support some plants by providing an osmotic system which creates a pressure potential.

Cell Wall

Functions of the cell wall in plant cells include preventing the cell bursting when osmosis allows water to enter and provides the cell with structure and a definite shape.

Plasmodesmata

These fine strands of cytoplasm are linked through the cell wall, connecting neighbouring cells.

Animal cell diagram

Plant cell diagram

Refer to the below table for the differences between plant and animal cells.

Table 1: Comparison of structures between animal and plant cells

	Typical animal cell	Typical plant cell
Organelles	Nucleus Nucleolus (within nucleus) Rough endoplasmic reticulum (ER) Smooth ER Ribosomes Cytoskeleton Golgi apparatus Cytoplasm Mitochondria Vesicles Lysosome Centrosome Centrioles Vacuoles	Nucleus Nucleolus (within nucleus) Rough ER Smooth ER Ribosomes Golgi apparatus Cytoplasm Mitochondria Vesicles Chloroplast Vacuole(large)
Additional structures	Plasma membrane Flagellum Cilium	Plasma membrane Flagellum (only in gametes) Cell wall Plasmodesmata

Prokaryotes and Eukaryotes

Karyote means 'nucleus' and thus prokaryote means 'before nucleus' which is an easy way to remember that these are the less complex cell type. They are also 1,000 to 10,000 times smaller than eukaryotes.

The table below is a comparison of prokaryotic and eukaryotic cells:

Table 1: Comparison of features of prokaryotic and eukaryotic cells

	Prokaryotes	Eukaryotes
Typical organisms	bacteria, archaea	fungi, plants, animals
Typical size	~ 1-10 µm	~ 10-100 µm (sperm cells, apart from the tail, are smaller)
Type of nucleus	no membrane bound nucleus or no nucleus at all	real nucleus with double membrane
DNA	circular (usually)	linear molecules (chromosomes)
RNA-/protein-synthesis	coupled in cytoplasm	RNA-synthesis inside the nucleus protein synthesis in cytoplasm
Ribosomes	Smaller (18 nm)	Larger (22 nm)
Cytoplasmatic structure	very few structures	highly structured by endomembranes and a cytoskeleton
Mitochondria	none	one to several thousand (though some lack mitochondria)
Chloroplasts	none	in algae and plants
Organization	usually single cells	single cells, colonies, higher multicellular organisms with specialized cells
Cell division	Binary fission (simple division)	Mitosis (fission or budding) Meiosis

Tissues and Organs

• Tissue: A collection of cells specialised to perform one or more functions, and the cells do not have to be of the same type - merely collectively perform the same functions.
• Organ: Composed of more than one type of tissue which forms a structural and functional unit. Leaves, the brain, the heart are all examples of organs.
• System: Collection of organs with the same function, such as the digestive system.

Epithelium

Epithelial tissue is tissue that forms sheets covering surfaces and both the types you are required to learn about are said to be simple epithelia as they are one cell thick. They rest on a basement membrane, which is NOT a cell membrane, but rather a network of collagen and glycoproteins and holds the epithelium cells in place.

Squamous Epithelium

Squamous Epithelium is very smooth, flat and thin - they fit together like floor tiles providing a smooth almost frictionless surface over which fluids can move easily. The thickness (or lack thereof) in epithelium aids in processes like diffusion. This type of epithelium cell covers many surfaces in the body, for example the inner surfaces of atria and blood vessels. It also forms alveoli walls.

Ciliated Epithelium

This is epithelium with cilia and is found for example in the trachea, where it is tall and narrow and the cilia is used to waft mucus along the trachea.

Glandular Epithelium

This is epithelium with cells that excrete some substance in the body. Some substances it secretes include hormones, mucous, and lubricating fluid.