B2.3 Cell Specialisation SL

Size of red blood cells

Red blood cells need to squeeze through narrow capillaries and so have a diameter of only 7 - 8 µm

Size of Neurons

Neurons need to transmit signals throughout the body and can be over 1 m in length

Size of striated muscle fibres

Striated muscle fibres consist of fused muscle cells, resulting in widths of 100 µm and lengths of >12 cm

Size of Adipocytes (fat cells)

Adipocytes (fat cells) may vary greatly in size (<20 - 300 µm) due to their role in the storage of lipids

Size of sex cells

A human ovum is one of the largest cells (diameter = 120 µm), while a sperm cell is very small (~ 5 µm)

How do cells survive

Cells need to produce chemical energy to survive (via metabolism) and this requires the exchange of materials with the environment

What is metabolism a function of in cells

The rate of metabolism of a cell is a func-on of its mass / volume (larger cells will need more energy)

What is material exchange rate a function of in cells

The rate of material exchange is a func-on of a cell's surface area (more membrane = more exchange)

Why do cells require a high SA:Vol ratio

If the metabolic rate (volume) exceeds the rate of material exchange (surface area), the cell will be unable to maintain homeostasis and will eventually die. Hence, a cell will require a high SA : Vol ratio in order for it to survive

What happens to the ratio when cells grow

As a cell grows the volume (cubic units) increases more rapidly compared to the surface area (squared units), leading to a decreased ratio

How can cells combat this decreased ratio

Growing cells will tend to divide and remain small in order to maintain a viable SA : Vol ratio

Certain cells and tissues that are specialised for material exchange (e.g. lungs) may modify their structure to increase their surface area and optimise material transfer (e.g. cell membranes may be ruffled to form microvilli).

Are cells in multicellular organisms identical

Yes

All cells in a multicellular organism are clones but groups of cells may differentiate in order to specialise and perform specific functions

Tissue

A group of cells that perform a specific function.

Organs

Groups of tissues that work together to perform a specific function or related functions

Organism

A living thing

What are stem cells

Stem cells are unspecialised cells from which all other cell types may be derived in a mul-cellular organism.

What two traits to stem cells have allowing them to form the myriad of different tissues and organs in a body

Self-Renewal

Potency

Self-Renewal

They have an unlimited capacity for growth (they can con-nuously divide and replicate)

Potency

They retain the capacity to differentiate into specialised cell types (they are progenitor cells)

Four types of stem cells

Totipotent

Pluripotent

Multipotent

Unipotent

Totipotent

Can form any cell type, as well as extra-embryonic -ssues like the placenta (e.g. zygote)

Pluripotent

Can form any cell type, but cannot form autonomous life (e.g. embryonic stem cells)

Multipotent

Can differentiate into a number of closely related cell types (e.g. adult stem cells)

Unipotent

Cannot differentiate but are capable of self-renewal (e.g. precursor cells / tissues)

What happens to the unspecialised zygote after fertilisation

Following fertilisation an unspecialised zygote will divide and develop into a mass of specialised cells (early embryo) via differentiation

Morphogens

Gene regulating chemicals

What determines the impact of morphogen

The impact of the morphogen will be determined by its relative concentration

Cells closer the the morphogenic source vs further away

Cells closer to the morphogen source will receive higher concentrations of morphogen resulting in the activation of more genes

Cells further away from the morphogen source receive lower concentrations of morphogen resulting in the expression of fewer genes

What controls the expression of genes within an early stage embryo

morphogen gradients

What do multicellular organisms share

Identical genome - each cell contains the entire set of genes for that organism

What is differentiation

Differentiation is the process whereby new cells become more specialised and distinct as they mature

What causes differentiations

Caused by the activation of different genes within a given cell (triggered by chemical signals)

inactive vs active package

Active genes are packaged in an expanded form

Inactive genes are packaged in a condensed form

Within the nucleus of eukaryotes the DNA is packaged to form

chromatin

How are specialised cells packaged

Specialised cells have their identical genomes packaged differently according to their function

Gastrulation

Gastrulation is an early phase of embryogenesis whereby embryonic stem cells differentiate into three germ layers the ectoderm (outer layer) the mesoderm (middle layer) and the endoderm (inner layer)

Ectoderm

Forms the nervous system and outer surfaces (such as skin, pigment cells and hair cells)

Mesoderm

Forms the majority of body organs, including muscle, vessels, kidney, heart and skeleton

Endoderm

Forms the respiratory and digestive tracts, along with associated organs (liver, pancreas)

What are adult stem cells

Adult stem cells are found in differentiated tissue, such as bone or skin - they divide to replace damaged cells

What stem cells are used to treat diseases

Embryonic and adult stem cells by replacing damaged cells with healthy ones

Embryonic stem cell for treating diseases

Embryonic stem cells have a greater potency (can treat more conditions) but there are ethical issues associated with their use (involves the genera-on and destruction on an embryo).

Adult stem cell for treating diseases

Adult stem cells have less ethical issues and a lower chance of graft rejection (involves use of patient's own cells) but have lower potency and are therefore limited in their potential use (can only treat very specific conditions)