HL Bio - Cellular Bio (II): B2.2 Cell Specialisation

Define fertilisation

The fusion of a male sperm and a female egg to form a zygote

In an early stage embryo, embryonic cells are_________

unspecialised

To form the embryo, why do zygote cells divide in mitosis?

To ensure that the cells are genetically identical

Define differentiation

Differentiation is the proesss by which an unspecialised cell develops into a specialised cell with a specific form and function

Describe the proces of differentiation in the embryo

Concentrations of morphogens indicate to a cell its position in the embryo - indicates the pathway of differentiation the cell should follow

What are stem cells?

Unspecialised cells that are able to produce specialised cells

Why are stem cells able to undergo extensive prolieferation?

For long term self-renewal via mitosis

Describe the two types of divisions that stem cells may undergo

1. Symmetrical - produces two daughter cells to maintain a constant pool of stem cells

2. Asymmetrical - produces one daughter cell and one progenitor cell to replace a population of cells in a specific tissue that died

Define stem cell niche

The precise location of stem cells within a tissue

What is the function of the microenvironment in the stem cell niche?

Provides a microenvironment for stem cells to stay inactive and undifferentiated, and for them to polieferate rapidly and differentiate when chemical signals are present for differentiation

Describe two stem cell niches

1. Bone marrow - has haemotopoietic stem cells that produces large volumes fo RBC, WBC and platelets

2. Hair follicles - stem cells at the base of hair follicles divide repeatedly to generate many cells needed for hair growth

Explain the different types of stem cells

Totipotent - can differentiate into all cell types that make up the entire organism

Pluripotent - can differentiate into all cell types except extraembryonic tissue e.g. placenta

Multiptent - has limited range of cell types that can divide into e.g. haemotopoietic

The metabolic rate of a cell is proportional to the:

Volume of the cell

Why does a large surface area to volume ratio limit the growth of the cell?

1. Substances required by the cell may not enter the cell as quickly as they are required

2. Waste products are not removed fast enough - accumulation of waste products

3. Overheating

Describe adaptations by cells to increase SA:V

1. Flattening - make cells very wide and thin e.g. RBC, type I pneumocytes

2. Microvilli - finger like projections e.g. epithelial cells in small intestine, PCT cells

3. Invagination - inner membrane foldings e.g. PCT, cell membrane

Named example: proximal convoluted tubule cells

One cell thick

Apical membrane has microvilli to increase SA:V

Basal membrane has invaginations to incrase SA:V

Membrane provides a large SA for channel proteins and pump proteins to ensure cells only reabsorb useful substances back into the blood stream

Explain the adaptation of Type I and Type II pneumocytes

Type I: flattened and very little cytoplasm; cells are extremely thin and permeable; shorter distance for diffusion of oxygen and carbon dioxide

Type II: dense cytoplasma and many vesicles to allow for higher rate of secretion of surfacant to prevent sides of alveoli from sticking together

Describe the adaptations of cardiac muscle cells

Striated - arrangement of actin and myosin for contraction

Branched - connects cells into a network

Intercalated discs - junctions that help cells work as a unit

Gap junctions - allows ions to pass quickly between cells, rapid spread of electrical impulses

Desmosomes - junctions that prevent cells from seperating during contraction

Numerous mitochondira - synthesis of ATP - contraction

Myogenic activity - can produce its own electrical sygnals, does not have to rely on nerves

Rich supply of blood - provision of oxygen and nutrients for ATP synthesis

Slow and sustained contractions - prevents tetanus

Describe the adaptations of striated muscle cells

Many mitochondria - ATP for muscle contraction

Multinucleated - manage large cytoplasmic volume

Striations - arrangement of actin and myosin for contraction

Sarcoplasmic reticulum - stores and releases calcium ions to initiate contraction

Transverse tubules - spreads action potentials deep into fibre for efficient contraction

Mycrofibrils with repeating sacromeres - densely packed contractile units that maximse force production during contraction

Describe the adaptations of the human egg

Large cytooplasmic volume - to store nutrients, organelles and mRNA required for the early development of the zygote

Haploid nucleus - n=23, ensures correct chromosome number after fusion with sperm

Zona pellucida - protects egg, prevents multiple sperms fusing with egg

Cortical granules - releases enzymes after fertilisation to harden zona pellucida and to prevent polyspermy

Corona radiata - provides nutrients

Sperm receptors on membrane - ensures sperm binding and intitiates fertilisation

Describe the adaptations of the human sperm

Small and streamlined shape - reduced resistance for swimming

Long flagellum - enables motility

Many mitochondria - ATP synthesis to power flagellum for swiming

Haploid nucleus - n = 23, ensure correct number of chromosomes after fertlisation

Acrosome - contains digestive enzymes to break down zona pellucida for fertlisation

Minimal cytoplasm - reduced mass for faster swiming

Compact head - for efficient penetration the egg

Surface proteins on head - allows the sperm to recognise and bind to receptors on the egg