Differentiation- The development and changes seen in cells as they mature to form specialised cells. This is achieved due to the switching on and off of relevant genes.
Stem cells can be classified as totipotent , pluripotent or multipotent.
- Totipotent - Embryonic stem cells from an 8 cell stage embryo (morula). These are able to differentiate into any type of cell, including embryonic cells. Totipotent stem cells have a wide range of clinical applications
- Pluriponent - From the inner cell mass of an embryo (blastocyst). These are able to differentiate into most types of cells but not embryonic cells
- Multipotent cells - Adult stem cells found in the bone marrow and cells from umbilical cord blood. These are able to differentiate into only one cell type – its own type. Pluripotent and multipotent stem cells have a narrow range of clinical applications.
Plants also have ‘stem cell like’ cells.- These are found in growing regions (meristem tissue) – e.g. tips of shoots and tips of roots. Stem cells in the cambium differentiate into vascular (transport) tissue - xylem and phloem. You will learn more about these tissues in ‘Transport in Plants’. Stem cells are unspecialised cells that have two key qualities:
- 1 Self renewal – they can continuously divide and replicate by mitosis.
- 2 Potency – they have the capacity to differentiate into specialised cell types.
Diseases that could be treated using stem cells
- Ectoderm- Brain, Skin
- Mesoderm- Muscle, blood, bone, cartilage
- Endoderm- Lung, Gut, Liver
- Germline- Sperm, Egg
- Heart disease – to replace cardiac muscle tissue damaged by a heart attack
- Type 1 diabetes – to replace damaged insulin producing cells in the pancreas
- Parkinson’s disease – to replace damaged dopamine producing cells in the brain
- Alzheimer’s disease – to replace brain cells damaged by a build up of abnormal proteins
- Macular degeneration – to replace damaged retinal cells in the eyes
- Spinal injuries – to replace damaged nerve cells in the spinal cord
Developmental biology – stem cells can be used to study changes which occur as multicellular organisms grow and develop and why things go wrong and how to prevent this.
Ethnics of stem therapy
Many people object on moral and religious grounds to embryos being created for research.
Researchers in the U.K. are allowed to create embryos for research purposes but their creation is strictly regulated by the HFEA (Human Fertility and Embryology Authority).
A possibility for future stem cell therapy may be a technique which takes mature differentiated cells and reverses the process of differentiation called induced pluripotency.
Erythrocytes (red blood cells) - carry oxygen in the blood
Contain haemoglobin (an iron containing respiratory pigment) to transport O2 to tissues (cells) from the lungs
Haemoglobin requires iron for its structure as a respiratory pigment – O2 is carried, from the lungs to the tissues bound to the iron in haemoglobin
Thin outer membrane - creates a short diffusion distance – allows rapid diffusion of oxygen into erythrocytes in the lungs (alveoli) and out of erythrocytes in tissues
Bi-concave disc shape - increases the surface area for diffusion of gases allows more haemoglobin to be packed around the edge (in contact with capillary wall) – reduces the diffusion distance
Flexible membrane framework – allows red blood cells to squeeze through the narrow capillaries; ensures maximum contact between red cell membrane and capillary wall
No nucleus - there is more room for haemoglobin, with the whole cell full of haemoglobin
Disadvantage – limits life span to ~ 120 days
Transports some CO2 – from tissues to lungs
Multicellular organisms & Stem cells
Multicellular organisms function as a result of many different cell types that are specialised for their function. For example:
- Neurones (nerve cells) – specialised for the transmission of electrical nerve impulses
- Erythrocytes – specialised for the carriage of oxygen around the body
- Xylem and phloem – cells in the cambium tissue of plants differentiate to form vascular tissue specialised for transport
All cells in multicellular organisms originate from stem cells. Stem cells are produced after two gametes fuse during fertilisation. The fertilised egg divides by mitosis to produce a morula, blastocyst and eventually an embryo. As all of the cells are produced by mitosis they are genetically identical. Therefore each and every cell nucleus contains ALL of the genetic information and genes to become any type of cell.
Stem Cell- A cell that is unspecialised / not differentiated and capable of division by mitosis. A stem cell is able to specialize / differentiate to become other cell types.
Stem cells have a unique ability to renew themselves and give rise to the more specialised cell types that do the work of the body. Stem cells remain unspecialised until a signal from the body tells them to develop into specific cells of the body like a heart, nerve, or skin cell.
Neutrophils (white blood cells; phagocytes) - specialised for defence against disease
Flexible shape – allows engulfing of foreign particles or pathogens by the process of phagocytosis (“cell eating”)
Migrates to and from the tissues, through pores in the capillary endothelium
Multi-lobed nucleus – allows flexibility
Contains many lysosomes – contain hydrolytic/digestive enzymes to break down engulfed particles.
Contains large amounts of rough endoplasmic reticulum – for protein (enzyme) synthesis
Granular appearance – contains granules - granules contain hydrolytic enzymes
Many receptor sites on the cell surface membrane for attachment to cells and pathogens (antigens)
Sperm cell (male sex cell; male gamete) - specialised to fertilise the ovum (female gamete)
Fertilisation – fusion of male and female gamete to form a zygote – fusion of sperm and egg nuclei
Nucleus contains half the number of chromosomes of an adult somatic (body)cell in order to fulfil its role as a gamete
Streamlined head and body – to reduce resistance during movement through fluid
Head contains genetic information (DNA) in the haploid nucleus, and an acrosome (lysosome)
Acrosome contains hydrolytic (digestive) enzymes – digests the egg cell membrane for the penetration of the sperm head (nucleus)
Cell surface membrane in head region has receptors for binding to egg cell surface membrane
A flagellum (tail) to propel the sperm to the egg
Mid-piece is packed with mitochondria – to release energy (ATP) for movement of flagellum
Leaf Palisade mesophyll cell
Leaf palisade mesophyll cell - specialised to carry out photosynthesis
Packed with chloroplasts containing the light absorbing pigment chlorophyll
Converts light (solar) energy to chemical energy – i.e. synthesis of glucose (organic) from inorganic materials (CO2 and H2O) using light energy of the sun
Regular shaped, closely packed columnar palisade cells forming a continuous layer for maximum absorption of sunlight
Thin walled cells – for rapid diffusion of gases (carbon dioxide and oxygen)
Chloroplasts can move within the cell, aided by the cytoskeleton– for maximum light absorption
Vacuole – membrane bound organelle containing cell sap; helps to maintain turgidity to support plant
Cellulose cell wall – protects the cell, confers strength, and prevents the cell from bursting
- Epithelial – forms layers and linings
- Connective tissue – holds structures together and provides support
- Muscle tissue – consists of cells specialised to contract and move certain parts of the body
- Nervous tissue – coverts stimuli into electrical impulses and conducts these electrical impulses
- Squamous epithelial tissue: consists of flattened thin cells. Together the cells form a thin, smooth flat surface, making them ideal for linings tubes such as blood vessels, as they do not impose the path of fluids, and liquids can easily pass over them. They also are good for forming walls of exchange surfaces, such as the alveoli. They are thin and so create a short diffusion pathway. They are held together by a basement membrane which is made from glycoproteins and collagen and is secreted from the cells themselves. It attaches them to connective tissue.
- Ciliated epithelial tissue: consists of long column shaped cells. It forms the inner surface of some tubes, such as the trachea and oviducts. The surface exposed to the lumen is covered is ciliated. The cilia move in a synchronized rhythm and waft mucus over the tissue surface. The mucus is produced and secreted by goblet cells.
Root hair cell
Root hair cell - specialised for absorbing water by osmosis and dissolved minerals by active transport and facilitated diffusion from the soil into the root
Numerous, long hair-like extensions of the cell wall and cell membrane of root epidermal cells in young plants, extending into the soil
Provide a large surface area for efficient absorption of water by osmosis down a water potential gradient
Minerals are absorbed by active transport against a concentration gradient
Some minerals are transported down a concentration gradient from the soil by facilitated diffusion through channel proteins
Cell wall of root hair cell is thin and permeable – reduces diffusion distance
Large number of mitochondria in root hair cells provide energy (ATP) for the active transport proteins located in the cell surface membrane
Cartilage and Muscle
Cartilage- Cartilage is connective tissue which may be structural (e.g. outer ear, nose) or used as a shock absorber and to reduce friction between or at the end of bones. Cartilage is firm and flexible and has a smooth surface.
Cartilage is composed of chondrocyte cells embedded in an extracellular matrix which contains the fibrous proteins elastin and collagen..
Muscle tissue is able to shorten or contract. There are 3 types of muscle –
Skeletal - attached to bones, contraction brings about movement
Cardiac –heart muscle cells, contraction forces blood through heart and blood vessels
Smooth – found in walls of alimentary canal, contraction causes peristalsis and moves food through the gut.
Skeletal muscle is comprised of muscle fibres and connective tissue and has a rich blood supply. Muscle fibres contain myofibrils made of the contractile proteins actin and myosin.
Tissues & Organs
Smallest to Largest
- Organelle-cell-tissue-organ-organ system-organism
- Nucleus-muscle cell-muscle-heart-circulatory system- human
Tissue- a group of specialised cells of one or more types that work together with a common function
Organ- Collection of a group of specialised tissues of one or more types that work together with a common function, e.g- Stomach.
How are cells organised in multicellular organisms
- Cells differentiate and become specialised
- group of cells form tissues
- group of tissues form organs
- group of organs form organ systems
- group of cells/tissues/organ/organ systems work together
Transport tissues in plants
Xylem and phloem come from dividing meristem cells located in the vascular cambium. Meristem cells undergo differentiation to form the different kinds of cells in the transport tissues.
Xylem tissue consists of xylem vessels with parenchyma cells and cell fibers.
Meristem cells divide and differentiate to produce long cells that line up end to end. Their walls become waterproofed and reinforced with lignin. This also kills cell contents. Their end walls break down, forming a long continuous column of cells – a tube with a large lumen.
Phloem tissue consists of sieve tube elements and companion cells. When meristem cells divide they form long cells and line up end to end. The end walls do not break down totally, but form perforated end walls, called sieve plates. They allow movement of material up and down the phloem vessel. The companion cells are very metabolically active and are used to transport the products of photosynthesis up and down the vessel.
Guard cells - specialised to open and close leaf pores (stomata) – used for gas exchange and transpiration (loss of water vapour from leaves by evaporation)
- Regulate the size of leaf pore – allow entry and exit of gases and water vapour
- Change shape easily
- Swell up when the vacuole is filled with water and become turgid (firm)
- In light (photosynthesis) - K ions are moved into the guard cell actively – water follows down a water potential gradient and – makes the guard cells turgid.
- ATP for active transport
- Contain mitochondria to generate ATP for active transport
- This causes the thin outer walls to stretch and the thickened inner walls to bend outwards – opening the stomata – allowing gas exchange for photosynthesis.
- When flaccid (e.g. in the dark – when there Is no photosynthesis), the thickened inner walls move inwards to close the stoma