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This material has been developed independently by the publisher and the content is in no way connected with nor endorsed by the International Baccalaureate Organization. Chapter 2 — Molecular biology 2. Chapter 3 — Genetics 3. Chapter 4 — Ecology 4. Chapter 5 — Evolution and biodiversity 5. Chapter 6 — Human physiology 6. Chapter 7 — Nucleic acids 7. Chapter 8 — Metabolism, cell respiration and photosynthesis 8. Chapter 9 — Plant biology 9. Chapter 11 — Animal physiology Core topics A. Core topics D.
For a very long time, people did not know cells existed. The discovery of cells was linked to the developments in technology, in particular the ability to produce high quality lenses for microscopes. A series of steps led to the discovery of cells, most of them being related to the advances in technology. It involves a sequence of discoveries, often made by scientists in different countries.
The cell theory is widely accepted and advances in technology have allowed us to greatly increase our knowledge of cells. In particular the development of the electron microscope EM has allowed us to study the ultrastructure of cells in more detail than Robert Hooke could ever have imagined.
They have also led to the discovery of some cells which differ from most of our generalised models of plant or animal cells. Although different, these cells still support the cell theory. A compound microscope has 2 lenses which provides greater magnification. Intuition usually tells us if something is living or nonliving, but the Natural Sciences have used reason to develop lists of characteristics of organisms that distinguish them from their non-living environment.
This reductionist approach reducing a complex concept to a series of parts or a list is powerful in assisting Biologists in determining what is and is not alive. However, both ways of understanding knowledge have their flaws. Intuition is dependent upon and therefore limited by our sense perception, memory and preconceptions. Viruses and prions are a challenge as they have some characteristics of living organisms as well as those of the non-living environment. A combination of muscles and bones allows us to move: the bones provide the structure and the muscles can become shorter which moves the bones relative to each other.
Generally, two main types of muscle are distinguished: smooth muscle and striated muscle. Striated muscles are usually involved in voluntary movement. Acetabularia transports mRNA in inactive form from one side of the cell to the other to be activated and translated there. Refer to Figures and The cells of striated muscle are called myocytes and they originate from the fusion of several myoblasts.
Each myoblast has a nucleus so myocytes have several nuclei. Refer to Figures , Hyphae are the threads that make up some fungi. In aseptate or coenocytic hyphae, the usual cross walls, separating the cells, in their hyphae are lacking. Instead there are a number of nuclei in a continuous cytoplasm. The advantage to being aseptate is the greater ease and speed of transport no cross walls but when damage occurs to a part of the hypha, a coenocyte will suffer more extensive damage.
Pore Septum. Giant algae Several species of giant algae exist which have their own adaptations to their size. Valonia ventricosa bubble algae is a large cell with several nuclei. Refer to Figure All unicellular organisms need to carry out all functions of life inside their one cell. Two examples will be considered to illustrate this point. Scendesmus Scenedesmus is a unicellular but sometimes colonial photosynthetic organism.
This green algae is found in fresh water in most parts of the world. See Figure Unicellular organisms are fully functioning individual organisms so they need to be able to carry out all functions of life.
The functions of life include:. Paramecium is a unicellular organism in the kingdom of the Protista. These single celled eukaryotic organisms live in water and feeds on other organisms such as bacteria. Take up dissolved oxygen from the surrounding water via diffusion through its cell membrane.
Carbon dioxide will leave the cell in the same way. Take up dissolved carbon dioxide from the surrounding water via diffusion through the cell membrane. Oxygen will leave in the same way. Ammonia nitrogenous waste is excreted via the contractile vacuole.
At night no light, no photosynthesis carbon dioxide will be excreted via the cell membrane Carbon dioxide diffuses out via the cell membrane. When it Scenedesmus will respond to a light stimulus by hits an obstacle, it will reverse the beat of its cilia and swim growing towards the light.
If necessary, the procedure will be repeated until it has avoided the obstacle. In general, Paramecium will go towards light and away from heat. A Paramecium derived from binary fission will simply regrow to its original size. These cells will experience a loss of vitality clonal aging but can be rejuvenated when it undergoes sexual reproduction. Scenedesmus from asexual reproduction is created as a daughter colony of small cells. The individual cells will grow to adult size.
Paramecium has a large nucleus which controls day to day function. The smaller nucleus is involved in sexual reproduction. Paramecium can reproduce asexually via binary fission or it can exchange genetic material of the micronucleus with another Paramecium.
Scenedesmus reproduces asexually by repeated divisions within one cell, forming a daughter colony which is released. Living in a fresh water environment, the concentration of dissolved particles in the cytoplasm of Paramecium will constantly have water coming in through osmosis.
The canals arranged around the contractile vacuoles will collect water, pass it to the vacuole which will contract to expell the surplus water. Paramecium is a heterotroph so it does not make its own food.
It uses its cilia to create a water current which sweeps its food mainly bacteria into its gullet where they are taken up via endocytosis. Enzymes from the cell will enter the formed food vacuole and digest the food.
The digested food will diffuse from the vacuole into the cytoplasm. It rarely reproduces sexually, usually this process would be stress induced. Depending on circumstances, Scenedesmus can form colonies.
In water with low phosphorus and salt concentration, Scedemesmus is unicellular. Scenedesmus is an autotroph which uses photosynthesis to produce its own food. Each cell has one chloroplast. Scenedesmus produced starch inside the chloroplast and stores it.
The size of a cell is limited by its need to exchange materials with its environment. If a cell becomes too large, its diffusion distance becomes too long to be efficient diffusion through a liquid such as cytoplasm is a slow process and its surface to volume ratio becomes too small to allow the necessary exchange.
Chapter 1 As a structure such as a cell increases in size, the volume increases faster than the surface area as the following calculation will show. This means that every 1 cm3 of volume has 6 cm2 of surface area. This means that every 1cm3 of volume has 0. This is one tenth of the surface area per cm3 volume if you compare it with the smaller cube. It is even possible that not enough heat can be given off.
Evolutionary solutions to this are to increase the surface area by protruding extensions or by flattening the cell. Multicellular organisms face the same problem. This is why, for example, we have lungs structures in lungs increase the surface area available for gaseous exchange and a circulatory system blood carries materials round the body, reducing the diffusion distance.
The contraction of one muscle cell has very little effect. The contraction of all the cells in a muscle causes e. This is not an unexpected property but rather an expansion of the property of one cell. In populations or other biological systems, the system can have properties that may not have been expected by looking at the individual. Individuals vary, both within and between populations and the result their interaction is not always predictable: the whole is greater than the sum of its parts.
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Biology: International Baccalaureate