Biology - B2.3 Cell Specialization

Process of forming blastocyst

Firstly, there is an ovum which is fertilized by a sperm cell. After the fusion of a male and female gamete, a zygote is formed. After 72 hours of division through mitosis a mass of unspecialised cells is produced (the morula). At around 4-5 days after fertilisation, when cells start to differentiate, a blastocyst is formed.

Why do cells not use mitosis to produce specialised cells?

Mitosis produces genetically identical cells so to express certain genes and others not, cells must differentiate.

List examples of differentiated cells, cells that originate from a stem cell

Reproductive cells, muscle cells, fat cells, immune cells, bone cells, blood cells, nervous cells, epithelial cells

What is a gradient in an early-stage embryo?

There are signaling chemicals in the embryo, called morphogens, and the different concentrations of these form gradients. The gradients regulate gene expression

How do gradients in the early-stage embryo regulate gene expression?

In the different concentrations of these signaling chemicals (A, B, C, D) and they all regulate different genes expressing so different specialized cells are created

What are stem cells?

Stem cells are undifferentiated cells (or sometimes partially differentiated) and have the capacity to divide endlessly and differentiate along different pathways.

List the three different types of stem cells

Totipotent, pluripotent, and multipotent

Totipotent stem cells

Cells that can differentiate into any cell type.

Found in the early embryo stage up to the morula

Pluripotent stem cells

cells that can differentiate into a range of cell types, but NOT ALL

Found in the inner cell mass of a blastocyst, thus embryonic stem cells

Multipotent stem cells

Cells that can only differentiate into a limited type of cells

Found in adults(eg in hematopoietic stem cells in bone marrow)

Stem cell niches

Precise location within tissues where stem cells are found in adults

Stem cell niches role in cell specialisation

Stem cell niches have particular microenvironments that enable cells to continue dividing for self-renewal, as well as differentiation if necessary.

Hair follicle as example of stem cell niche

Hair follicles have a microenvironment where adult multipotent stem cells are maintained and their proliferation (rapid reproduction, self-renewal) and differentiation

Bone marrow as example of stem cell niche

Bone marrow has a microenvironment where adult multipotent stem cells are maintained and their proliferation (rapid reproduction, self-renewal) and differentiation

Compare and contrast a morula and a blastocyst

Morula is a mass of unspecialised cells (around 3 days after fertilisation whilst a blastocyst has some specialised cells (4-5 days after fertilisation)

Distinguish between a zygote and a morula

zygote is right after fusion, there is only one cell.

a morula is after around 3 days of mitosis and there are many more cells

what causes the cells in a morula to differentiate into different types of cell?

their position and thus the concentration of signaling chemicals they are experiencing - their gradient

Why is the surface area to volume ratio important?

The metabolic rate of a cell is proportional to its volume and the rate at which substances can enter or exit a cell depends on the surface area of it’s plasma membrane.

Therefore, there is a maximum size for cells otherwise they can’t appropriately absorb nutrients and water. The smaller the better

How is surface area to volume ratio calculated?

surface area (units²) / volume (units³)

Describe the relationship between size of a cube and surface area to volume ratio

As the volume increases, the surface increases but not in the same increments so the SA:V ratio decreases

Examples of implications of SA:V ratio

• Root hairs have high SA to absorb nutrients

• Lungs have alveoli with high SA to absorb oxygen

• Cells have microvilli for high SA

• On the other hand, high SA means it is easier to lose heat, e.g. elephants

Example cells that are adapted by cell size

Egg cells, sperm cells, red blood cells, white blood cells, motor neurons, striated muscle fibre

Red blood adapted in terms of size

Around 7 micrometres by 2 micrometres

Very small to fit through capillaries as oxygen has to be delivered everywhere

Very large SA:V ratio to diffuse oxygen

Biconcave shape to increase SA

White blood adapted by cell size

Have a diameter of around 10 to 20 micrometres

Are smaller because have to be transported around the body through blood vessels

A bit bigger than red blood cells to make antibodies

Motor neurons adapted by cell size

Up to 1m in length and the cell body has a diameter of around 50 micrometres

Very long to transmit from one place (spinal cord) to around the body

Adaptations of egg cells

Around 100 micrometres in diameter

• Large cell for the store of nutrients

• A glycoprotein layer around the outside called the zone pellucida prevents polyspermy and provides early-embryonic stage protection

Adaptations of sperm cell

Around 50 micrometres in length, head has a diameter of 5 micrometres

• Small size

• Tail/flagellum for movement/effective swimming

• Mid part of the sperm cell contains many mitochondria to power the movement of the flagella

• The head region of the sperm cell contains a very large vesicle (acrosome) that contains digestive enzymes to break down the zona pellucida during fertilization

Adaptations of striated (skeletal) muscle cells

Up to 1m in length and 50 micrometres in diameter

• Long length because have to stretch from one bone to the other

• Multinucleate (many nuclei) to produce proteins quickly when muscles are repairing or growing

• Contain contractile myofibrils for precise movement, as one myofibril can be stimulated at a time

Adaptations of cardiac muscle cells

• Contain contractile myofibrils that ensure coordinated contraction

• Cells are branched and join with adjacent cells by intercalated discs to ensure rapid spread of depolarisation (electrical impulse) across the whole muscle

Type 1 pneumocytes in alveoli adaptations

Found in lung/alveoli cells

Very flat and thin cells to reduce distance for diffusion of oxygen and CO2 as gas exchange

Type II pneumocytes in alveoli

• Many secretory vesicle (lamellar bodies) in cytoplasm to discharge surfactant (surfactant reduces surface tension in lungs so is released to prevent lungs from collapsing in on themselves)

The two types of cells in pneumocytes/alveoli and why

Type 1 (long and thin) and Type 2 (secretory vesicles that discharge surfactant).

There are two types because different adaptations are required for the overall function of the alveoli