Theme B: B2 Cells - B2.3 Cell Specialisation Flashcards
the development progression for humans
gametes –> zygote –> embryo –> foetus –> infant
gametes
the 2 cells that fuse together in sexual reproduction. Gametes are haploid sex cells (sperm in males and eggs in females) that carry half the genetic material of an organism.
zygote
A zygote is the diploid cell formed when two haploid gametes (sperm and egg) fuse during fertilization. It is the first stage of development in a multicellular organism.
cell differentiation
Cell differentiation is the process by which unspecialized cells, such as stem cells, develop into specialized cells with distinct structures and functions. This occurs through the selective expression of certain genes while others are suppressed.
after fertilization
how do cells intitially develop?
(in humans)
the zygote can divide very rapidly, and while the cells initially produced are unspecialised, the cells of the zygote also rapidly differentiate. this process results in the formation of specialised cells driven by the selective expression of certain genes while others remain inactive.
each cell will develop in a very specific manner depending on which genes become active and which signals the cell recieve.
cell signalling
the process by which infomrtaion is transferred from the cell surface to the nucleus. this is essential in controlling gene expression and thus differentiation.
morphogens
what are they? how does it effect the development of cells?
signal molecules that control cell differntiation. they occur in gradients (areas of concentration differences) in different regions of the early embryo.
the concentration of the signal molecules controls the regional development of teh first cells into head and tail structures.
the gradient of the signal molecules results in diffferent genes being expressed in different parts of the embryo. this leads different parts of the embryo to develop different structures.
as embryo develops, other signalling molecules become factors in differentitaion.
stem cells
population of cells within organisms that retain the ability to divide indefinetely and can differentiate along different pathways, resulting in all the cell types an organism possesses.
meristematic tissue
Where stem cells are contained in plants. it’s composed of rapidly reproducing cells that can form various types of tissue within the root or stem (a.k.a stem cells).
gardeners take advantage of these cells when they take cuttings from stems or roots and use them to grow new plants.
2 unique properties of stem cells
1) self renewal
2) recreate functional tissues
self renewal
when a stem cell divides, there are several possible outcomes:
1) both daughter cells remain stem cells
2) a stem cell and a differentiated cell may be formed
3) both cells differentiate
whatever the outcome is, the stem cells are maintained.
this process allows the continual production of a particular tissue whilse also providing for the continuation of stem cells.
stem cell niches
in a stem cell niche, the stem cells are present in high numbers due to regular proliferation.
e.g. bone marrow and hair follicles (in humans)
stem cell niches in humans are also studied in the contral nervous system, intestinal system, and in the muscle fibre bundles.
bone marrow
(as a stem cell niche)
stem cells that produce blood cells are found in the bone marrow alongside self renewing stem cells.
as blood cells are produced, differentiated cells are transported away via a large array of supporting blood vessels.
bone marrow tissue is multipotent.
the soft, spongy tissue that is in the medullary cavities (centers) of bones
hair follicles
as stem cell niches
they exist in the skin, and large numbers of epithelial stem cells are found in the bottom rounded area of a hair follicle. these stemc ells are multipotent.
these stem cells are also involved with hair growth, skin and hair follicle regeneration, and the production of sebaceous (oil-producing) glands associated with hair follicles.
a tube-like structure (pore) that surrounds the root and strand of a hair
what feature do all stem cell niches all have in common?
the presence of signalling factors that bring about both self renewal and differentiation.
types of stem cells
(in order of decreasing potency)
1) totipotent (total - most ‘potent’)
2) pluripotent (plural)
3) multipotent (multiple)
4) unipotent (one - least potent)
totipotent
- capable of continued dividiion and possesses the ability to produce any tissue in the organism.
- very few cells are totiptent.
- only exist in very early stages of embryo development.
- they may form a complete organism.
pluripotent
- arise form totipotent cells and only exist in early embryonic stage
- can mature into almost all the differnet cell types that exist in an organism
- unlike totipotent, they cannot produce a complete organism.
multipotent
- only forms limited number of cell types.
- bone marrow tissue is multipotent
- they occur later in the development of the embryo and present during the remainder of an organism’s life
unipotent
- only forms a single cell type, e.g. sperm cells in mammals
- usually form late in the embryonic stage and exist in the funcioning organism
one problem with using stem cells to treat disease
stem cells cannot be distinguished by appearance. they can only be isolated from other cells based on their behaviour.
therapuetic cloning
recent research directed towards growing large numbers of embryonic stem cells in culture so that they can be used to replace differentiated cells lost to injury or disease. this involves therapuetic cloning.
e.g.
* parkinsons and alzheimer diseases are caused by loss of proper functioning brain cells, where this technique could be used to replace these lost or defective brain cells, thus relieving symptoms.
* forms of diabetes
cell size according to various common cell types
red blood cell: 7.5
white blood cell: 12-15
sperm cell: 3 diameter, 50 length
skeletal muscle fibre: 10-50 width, 40 length
fat cell: 50-150
egg cell: 120
neuron (nerve cell): 350 length
all in micrometers
what is cell size determined by?
FUNCTION of a cell determines its size
sperm cell size importance
sperm cells are relatively small because they only carry out the function of transporting genetic material so VIABLE ZYGOTE can be formed.
red blood cell adaptations to carry oxygen through the organism
1) contain haemoglobin that combine with and release oxygen
2) biconcave disk shape allowing more surface area for oxygen absorption
3) lack mitcohondria and nucleus
4) flexible and limited size because they need to move through narrow blood capillaries
cell size of white blood cells
- bigger than red blood cell
- main function is defense against infections
- retain nucleus throughout lifetime
- several distinct types and each have a specific function
- many possess vesciles with enzymes that can kill microorganisms. the enzymes are also used to break down harmful cellular debris brought in by phagocytosis
- their size is increased due to necessary presence of nucleus, granules, and organelles like mitochondria
specifically discusses motor neurons
adaptations of neurons for specialised roles
there are several types of neurons, the distinct types exhibit adaptations for specialised roles
e.g. motor neruons carry impulses from brain or spinal cord that allow muscles to appropriately respond to stimuli:
* this neuron type has long fires called axons that can carry impulses up and down body over long distances
* the axon of motot neurons can extent up to 1 meter in human body
* this allows efficient and rapid transmission of nerve impulses from brain and spinal cord to muscles in limbs to produce movement
adaptations of skeletal muslce fibres for specialised roles
skeletal muslce fibres are specialised cells found in skeletal muscle:
* the fibres are cylindrical and surrounded by membranes capable of impulse propagation
* skeletal muslce fibres can be up to 12 cm long and longer than muscle fibres in smooth muscle or cardiac muscle
* this muscle type can only produce movement by contraction. becaus ethe fibre is relatively long, all the units contracting together produce a significant movement
in short:
* a long, cylindrical shape
* a membrane capable of impulse propagation
* multiple nuclei (multinucleated)
* visible bands capable of shortening to produce voluntary movement.
cell size is largely dictated by 2 factors:
1) Basic processes of cell physiology, such as the need for materials to move in and out of cell. this usually involvves the surface-area-to-volume ratio
2) cell division apparatus. If cells are too large/toosmall the mitotic spindle will not function properfly.
why is cell size important?
cell size is set as it goes through its differentiation process to become a particular cell type. All the adaptations come together to produce the most efficient cell possible for the specific function it has
how does the surface-area-to-voume ratio of a cell effect its size?
the surface-area-to-voume ratio limits the size a cell can reach. Most of the chemical reactions of life occur inside a cell, and the size of the cell affects the rate of those reactions.
Volume:
* In a cell, the rate of heat and waste production, and rate of resource consumption, are dependent on the volume.
Surface area:
* The surface of the cell, the membrane, controls what materials move in and out of the cell.
Hence, A cell with more surface area per unit volume can move more materials in and out of the cell, for each unit volume of the cell. This means that a large cell, compared to a small cell, has less surface area to bring in materials that are needed and to get rid of waste. Because of this, cells are limited in the size they can reach and still be able to carry out the functions of life. This means that large animals do not have larger cells; instead, they have more cells.
how does the surface-area-to-voume ratio work?
As the width of an object such as a cell increases, the surface area also increases, but at a much slower rate than the volume:
* The volume increases by a factor calculated by cubing the radius
* At the same time, the surface area increases by a factor calculated by squaring the radius.
Adaptations that give a more favourable surface-area-to-vlume ratios in cells
1) changes in shpae
2) cellular projections, both inward and outward
3) Location relative to sources of nutrients and means of transporting away wastes
4) How the cells fit together at a specific location
chatgpt also includes:
* Being small to maximize diffusion efficiency.
* Developing thin, elongated, or flattened shapes (e.g., red blood cells, nerve cells).
* Having microvilli to increase surface area (e.g., intestinal epithelial cells).
* Possessing internal compartmentalization (e.g., organelles in eukaryotic cells) to optimize metabolic processes.
erythrocytes
A type of blood cell that is made in the bone marrow and found in the blood. Erythrocytes contain a protein called hemoglobin, which carries oxygen from the lungs to all parts of the body. (Red blood cell)
surface-area-to-volume ratio in red blood cells
the biconcave disc shape of red blood cells allow a greater surface-area-to-voume ratio. Their size coupled with their flexibility als allows them to squeeze through small capillaries to deliver oxygen to all cells of the body.
& the adaptations of the cells lining the tubule
proximal convoluted tubule
a part of the human kidney. the cells lining this tubule have several unique adaptations that increase their ability to reasorb fluids and secrete ion:
1) cube shaped cells closely packed together to use space efficiently
2) cells have tiny projections called microvilli pointing outwards into the lumen of the tubule in whcih fluid flows. this brush border increases the surface area
3) large numbers of mitochondria, allowing active transport of ions and other substances
4) invaginations (channels) are on the opposite side of the cell (thus on the opposite side from of the lumen as well) increases the surface area of the cell to help transport
Alveolus (alveoli)
the functional unit of the lungs.
* the alveoli increase surface area of the lung to maximise gas exchange
* they must also be able to function as the lungs and expand and contract during breathing process
* they’re at the very end of the respiratory tract and can be thought of as empyt sacs lined by a wall that’s one cell thick
process of gas exchange in alveoli
1) Deoxygenated blood arrives: Blood with low oxygen (O₂) concentration and high carbon dioxide (CO₂) concentration flows into the lungs via the pulmonary artery.
2) Oxygen diffusion: The alveoli have a high concentration of O₂ from inhaled air, while the blood in the capillaries has a low O₂ concentration. Oxygen diffuses across the thin alveolar membrane into the blood, binding to hemoglobin in red blood cells.
3) Carbon dioxide removal: The blood has a high CO₂ concentration, while the alveolar air has a low CO₂ concentration. CO₂ diffuses from the blood plasma into the alveolus to be exhaled.
4) Oxygenated blood transport: The now oxygen-rich blood leaves the lungs through the pulmonary vein, delivering oxygen to body tissues.
5) Maintaining diffusion gradients: Continuous blood flow and ventilation sustain the oxygen and carbon dioxide concentration gradients, ensuring efficient gas exchange.
active cell types on the surfeace of each alveolus
1) Type I pneumocytes
2) Type II pneumocytes
3) Alveolar macrophages
Type I pneumocytes
Type I pneumocytes cover 95% of the alveolar surface. Their major function is to allow gas exchange between the alveoli and the capillaries. They have several interesting adaptations:
* they are thin and flat in shape to increase the surface area and minimize diffusion distance
* a shared basement membrane with the endothelium (lining) of lung capillaries, which minimizes diffusion distances for respiratory gases
* they are tightly joined to each other so that fluids cannot enter the alveoli from the capillaries.
Type II pneumocytes
Type Il pneumocytes make up less than 5% of the surface, found between the type I pneumocytes. Their role is to produce pulmonary surfactant, which reduces surface tension and prevents the alveoli from collapsing and sticking to each other during the breathing process. Adaptations include:
* a cube shape, providing a larger cytoplasmic area for the organelles producing the surfactant
* microvilli oriented towards the alveolar sac, increasing the surface area and allowing more surfactant secretion
* a cytoplasm that contains many organelles involved with surfactant production and its secretion, including secretory vesicles (called lamellar bodies)
* the ability to transform into type I pneumocytes when needed.
alveolar macrophages
The alveoli contain macrophages, which are white blood cells.
You do not need to know about these for the IB, but they perform endocytosis to remove harmful substances and organisms from the inhaled air.
adaptations of cardiac muscle fibres
Cardiac muscle fibres occur in the heart, and they have banded cells like the striated skeletal muscle cell. However, they also have further adaptations to perform their unique functions. They:
* are composed of branching, striated cells
* have a single nucleus per fibre/cell
* are connected at the ends by intercalated discs.
Cardiac muscle fibres are shorter in length than striated skeletal muscle fibres. Because of their branching cells, and the connections between them, cardiac muscle fibres coordinate a contractile process involving the whole heart in order to pump blood successfully throughout the body.
why is there a debate around whether striated skeletal muscle fibres are cells?
It has been debated whether striated skeletal muscle fibres should be considered cells:
* These cells are larger than most animal cells.
* Once they are formed, skeletal muscle fibres do not follow the usual pattern of cell division. They do not expand by producing more cells in adult organisms, but grow when their supporting satellite cells fuse with the existing fibres.
* Also, when muscle fibre cells are severely damaged, they do not go through the usual process of cell death (apoptosis) followed by cell replacement.
However, even with all these unique characteristics, striated muscle fibres still have many of the organelles and features discussed when describing cells.
sperm cell adaptations
- One of the smallest human cells 3 um in width, 50 um in length
- Flagellum (mid-piece) present that allows motility. Mitochondria located near the flagellum to supply energy for movement
- Shape includes a head and a tail region and is streamlined for speed and efficiency
- Very few cytoplasmic organelles such as endoplasmic reticulum, Golgi apparatus or ribosomes
- Continually produced in vast numbers throughout the life of a male
- Head has a specialized secretory vesicle called the acrosomal vesicle that helps the sperm penetrate the egg’s outer coat
- Contains a haploid nucleus
egg cell adaptations
- One of the largest human cells
120 um - No flagellum present, the cell is non-motile
- Shape is non-streamlined. It is spherical
- Most cytoplasmic organelles are present plus specialized storage structures for initial embryo development
- All the early gamete-forming cells are present before birth. No new egg-forming cells are produced after birth
- Has special secretory vesicles just under the plasma membrane that release their contents after one sperm penetrates the egg to prevent other sperm from entering
- Contains a haploid nucleus
