2.1.6 Flashcards
What is the cell cycle?
Highly ordered sequence of events that takes place in every cell - resulting in cytokinesis (division of the cell into two genetically identical daughter cells)
Phases of the cell cycle
Interphase - G1, S, G2
Mitotic phase
Interphase
Long periods (90%) of growth and normal functioning separates division phases - it is not JUST resting phase but instead very active with the cell carrying out all of its major functions, including producing enzymes or hormones while also preparing for cell division
What occurs during interphase?
G1 - proteins from which organelles are synthesised are produced and organelles replicate with the cell increasing in size
S - DNA is replicated in the nucleus
G2 - Duplicated DNA checked for errors, cell continues to grow in size and energy stores are increased
NORMAL METABOLIC PROCESSES OF CELLS OCCUR (respiration continues throughout division too)
Mitotic phase involves…
Cell division
Mitosis - nucleus divides
Cytokinesis - cytoplasm divides and two cells are produced
What is G0?
Name given to the phase when the cell leaves the cycle - either temporarily or permanently
Reasons for G0?
Differentiation - cell becomes specialised to carry out a particular function and therefore cannot divide ; it carries out this function indefinitely and does not enter the cell cycle again
DNA gets damaged - in this case if is not longer viable and a damaged cell can no longer divide (now in permanent cell arrest) ; normal cells only divide a set number of times and become SENESCENT
As you age - number of senescent cells increases… has been linked to cancer and arthritis
G0 and lymphocytes?
Can be stimulated to go back into the cell cycle and start dividing again - WBCs in an immune response for example
How is the cell cycle controlled?
Vital to ensure a cell only divides when at the right size and with the right nurtures and with correct DNA etc (fidelity of cell division - 2 identical daughter cells)
THIS IS MONITORED BY CHECKPOINTS - they ,omit or whether processes at each stage have been accurately completed before moving on to the en t phase
3 checkpoints
G1 - At the end of G1 (into S) ; checks for cell size, nutrients, growth factors and DNA damage ; on,y then can it proceed to S phase IF NOT THEN IN G0
G2 - At the end of G2 - going into mitotic ; check for cell size, DNA damage and if DNA has been replicated correctly (without error) - if this checkpoint is passed, the cell intimidates the molecular processes that signal the beginning of mitosis.
Spindle assembly checkpoint - checks if all the chromosomes are attached to spindles correctly and have aligned (mitosis cannot process unless this is passed)
How are cells difference from each other?
They all have common features like membranes and nuclei but they are specialised to perform different roles and organised into efficient biological structures - each with a particular funcyion
Organisation of a multicellular organism
Specialised cells - tissues - organs - organ systems - whole lrgansim
Specialised cells
Cells in an organism are differentiated, meaning they are specialised to carry out very specific functions
Erythrocytes differentiation
Red blood cells with a flattened bacon cave shape - increases their SA:V ration ; essential to their role of transporting O2 around the body. They do not have nuclei in mammals which increases the space for haemoglobin. FLEXIBLE TO SQUEEZE THROUGH CAPILLARIES
Neutrophils
Type of WBC that plays an essential role in the immune system - they have multi-lobed nuclei which makes it easier to squeeze through small gaps to get to the site of infections. GRANULAR cytoplasm contains many lysosomes that contain enzymes used to attack pathogens
Sperm Cells
Function is to deliver genetic information to ovum - flagellum to propel through liquids and a lot of mitochondria to supply the energy needed. A do some contains digestive enzymes that break down the protective layers of ovum, allowing the sperm to penetrate and fertilise (microtubules in 9+2 arrangement)
Palisade Cells
Present in mesophyll - chloroplasts to absorb large amounts of light for photosynthesis ; they are rectangular which allows them to be closely packed to form a continuous layer ; thin cell walls which increases rate of diffusion of CO2. A large vacuole to maintain turbos pressure and chloroplasts can move within the cytoplasm to absorb more light
Root Hair Cells
Surfaces of roots near growing tips - long extensions called root hairs which increases SA which maximises uptake of water (osmosis) and minerals (active transport) from the soil
Guard cells
Form openings called stomata ; necessary for CO2 to enter for photosynthesis. When guard cells lose water and become less swollen they change shape and the stoma close to prevent further water loss. The inner wall is THICK so cell does not change shape symmetrically
What is a tissue?
A collection of differentiated cells that have a specialised function ; each tissue is adapted for a particular function within the organism
4 main categories of tissues in animals
Nervous tissue - transmission of electrical impulses
Epithelial tissue - covers body surfaces (internal and external)
Muscle tissue - adapted to contract
Connective tissue - hold other tissues together or as a transport medium
Squamous epithelium tissue
Made up of specialised squamous cells - they are very flat cells and very thin (only one cell thick) ; allows for rapid diffusion across a surface and forms the lining of the lungs and allows rapid diffusion of oxygen into the blood
Ciliated epithelium tissue
Made up of ciliated epithelial cells that beat in a synchronised pattern to move mucus produced by goblet cells (tiny projections) ; line the trachea causing mucus to be swept away from the lungs. Goblet cells releas mucus to trap the unwanted particles from reaching the alveoli.
Cartilage
Connective tissue - outer ear/nose etc and contains fibres elastin and collagen… for, and flexible composed of Chondrocyte cells embedded in an extracellular matrix. THIS PREVENTS ENDS OF BONES FROM RUBBING TOGETHER AND CAUSING DAMAGE
Muscle
Tissue that needs to be able to contract to move the bones (which moves different parts of the body) - skeletal muscle fibres contain myofibrils which contain contractile proteins
The skeletal micrograph has several bands of muscle fibres separated by connective tissue
Tissues in plants
Epidermis tissue - covers plant surfaces
Vascular tissue - adapted for transport of water and nutrients (xylem + phloem = vascular bundle)
Epidermis tissue (plants)
Single layer of closely packed cells covering the surfaces of plants - covered by a waxy cuticle (prevents evaporation fo water) to reduce loss of water. Stomata that can open and close are present in the epidermis ; allowing CO2 in and out and water vapour and oxygen in and out
Xylem tissue
Transport of water and minerals - composed of vessel elements which are elongated dead cells… strengthened with a waterproof LIGNIN (pink rings on micrograph) which provides structural support
Phloem Tissue
Another type of vascular tissue in plants - transports organic nutrients from leaves and stems to everywhere it is needed. COMPOSED OF SIEVE TUBE CELLS SEPARATED BY SIEVE PLATES (allow food to pass through) - has parenchyma cells
What is an organ?
Collection of tissues adapted to perform a specific function in an organism - the heart is adapted to pump blood around the body and is made up of muscle and connective tissue
Leaf contains vascular and epidermis tissue adapted for photosynthesis
What are organ systems?
Composed of a number of organs working together to carry out a major function in the body
Examples of organ systems
Digestive system - breaks down food into small soluble bits and absorbs the nutrients while retaining water and removing any undigested material from the body
Cardiovascular - moves blood around the body to provide an effective transport system for the substances it carries
Gaseous exchange system - brings air into the body so oxygen can be extracted for respiration and CO2 can be expelled
What is the process of a cell becoming specialised?
Differentiation
Properties of all cells
Despite being differentiated in structure and function - all body cells have the same DNA (except RBCs that don’t have a nucleus) - differentiation just involves the expression of some genes but not others in the cell’s genome
What are stem cells?
They are undifferentiated cells that have not adapted to any particular function - have the potential to differentiate to become any one of the range of specialised cell types
Purpose of stem cells
They can undergo cell division several times and are the source of new cells necessary for growth and development and tissue repair - once they become specialised, they are unable to divide and enter the G0 phase of the cell cycle
Why do stem cells have to be controlled?
If they do not divide fast enough - then tissues are not efficiently replaced, leading to ageing. BUT if there is uncontrolled division, then they form masses of cells called tumours (which can lead to cancer)
What is potency?
A stem cells ability to differentiate into different cell types - greater the number of cell types = greater potency
Totipotent
Can differentiate into any type of cell - a fertilised egg and the first 16 cells from its first few mitotic divisions rare totipotent ; they can also differentiate into extra-embryonic tissues like umbilicus
Pluripotent
Can form all tissue types but not whole organisms - present in early embryos and are the origin of different types of tissue in an organism
Multipotent
Can only form a range of cells within a certain type of tissue (bone marrow stem cells are multipotent - gives rise to different blood cells)
Evolution of stem cells?
Multicellular organisms like animals and plants have evolved from unicellular organisms because groups of cells with different functions work together as one unit can make use of resources more efficiently than single cells
Link between adaptation and function
When cells differentiate, they become adapted to their specific role - wheat form this adaptation takes is dependent on the function of the tissue/organ/organ system to which the cell belongs. ALL BLOOD CELLS FROM BONE MARROW
Replacement of RBCs
Erythrocytes essential for the transport of oxygen - adapted to maximise oxygen carrying capacity by having only a few organelles so there is more room for haemoglobin. Due to lack of nuclei and organelles they have very short lifespan (120 days) - therefore need to be replaced Constantly with stem cell colonies producing 3 billion erythrocytes per kg of body mass each day
Replacement of Neutrophils
They live for only 6 hours - need to rpdpice 1.6 billion per kg per hour ; have essential role in immune system and FIGURE RISES DURING INFECTION
Embryonic stem cells
Totipotent - after about 7 days of embryo development, a blastocyst forms and the cells are now in a pluripotent state ; they remain like this till birth
Tissue (adult) stem cells
Found in specific areas like bone marrow and multipotent but can be triggered to become pluripotent - stem cells can also be harvested from umbilical cords ; NOT INVASIVE AND NO SURGERY REQUIRED WITH PLENTIFUL CORDS - CAN BE STORED IF NEEDED BY THE INDIVIDUAL IN THE FUTURE - TISSUE CULTURED FROM SUCH STEM CELLS WOULD NOT BE REJECTED AS IT IS OWNER
Sources of plant stem cells
Meristematic tissue (meristems) - tissue found at the tips of roots and shoots. Also found between phloem and xylem (vascular cambium) - cells differentiate into different cells in xylem and phloem ; now vascular tissue grows as plant grows. THEY ARE PLURIPOTENT THROUGHOUT LIFE OF THE PLANT
Diseases stem cells can treat
Heart disease - muscle tissue can be replaced as a result of a heart attack (some success)
Type 1 diabetes - body’s own immune system destroys insulin producing cells in pancreas (must inject insulin) ; has some success
Parkinson’s - symptoms caused by death of dopamine-producing cells in the brain (drugs only delay the deisease)
Alzheimer’s - brain cells are destroyed as a result of abnormal protein build up (drugs only alleviate symptoms)
Macular degeneration - causes blindness in the elderly and diabetics (research)
Birth defects - have successfully reversed them in mice
Spinal injuries - have restored movement to rats using stem cell implants
Where are stem cells used?
Treatment of burns - stem cells grown on biodegradable meshes produces new skin (quicker than taking a graft)
Drug trials - potential new drugs can be tested on cultures of stem cells before being tested on animals
Developmental biology - ability to divide indefinitely, important area of study of the changes that occur as multicellular organisms grow and develop from a single cell (such as a fertilised egg)
Ethical dilemma?
Removal of stem cells from embryos normally kills the embryo - religious objections but moral ones too as many believe that life begins at conception and therefore the destruction of an embryo is murder ; who owns the genetic material being used for research?
HOLDS BACK progress that could lead to the successful treatment of many incurable diseases - use of umbilical cord overcomes these issues but these are merely multipotent but not pluripotent, restricting their usefulness. Adult tissue stem cells are more likely to have acquired mutations. Induced pluripotent stem cells are adult stem cells that have been genetically modified to act like embryonic stem cells so are pluripotent
USE OF PLANT STEM CELLS DOES NOT RAISE THE SAME ISSUES AS ANIMAL CELLS
