- Life Processes
- Plant Cells and Animal Cells
- Cells, Tissues, Organs and Organisms
- Single Cell Organism
- Human Cells
Life Processes
The seven life processes of living organisms are:
- Movement - moving parts of the body
- Reproduction - producing offspring
- Sensitivity - responding and reacting
- Nutrition - getting food, to stay alive
- Excretion - getting rid of waste
- Respiration - turning food into energy
- Growth - getting to adult size
Plant Cells and Animal Cells
All living things are composed of different kinds of cells. All cells have a nucleus, cytoplasm and cell membrane. In addition plant cells have chloroplast, vacuole and a cellulose cell wall:
- nucleus (核) - controls the working of the cell and contains the genes (DNA) that are passed on
- cytoplasm (細胞質) - jelly-like substance where the chemical reactions occur
- cell membrane (細胞膜) - thin skin holds the cell together and controls the food and oxygen passing in and waste products passing out
- chloroplast (葉綠體) - contains green chlorophyll that traps energy from the Sun and produce food by photosynthesis
- vacuole (液泡) - conatins cell sap, a weak solution of sugar and salts
- cellulose cell wall (纖維素的細胞壁) - gives the cell a rigid structure
The size of an animal cell is about 1/100th of a mm (~10μm). A plant size is 40 times bigger than an animal cell (0.4mm).
Cells, Tissues, Organs and Organism
Similar cells are usually grouped together to form a tissue (組織) where they work together. In a tissue all of the cells have a similar function. Examples of tissues are epidermis (表皮) (skin), blood, bone and nerves.
Different tissues are grouped together to form an organ (器官). An organ has a complex structure where different tissues have different functions. Examples of plant organs are stems, leaves, roots, flowers and seeds.
A group of organs working together form an organ sytem (器官系統), or even a whole organism (生物).
An example is: palisade cells (柵欄細胞) make up palisade tissue which, with other tissues make up a leaf (an organ), and leaves and other organs make up a full plant (an organism).
Single Cell Organism
Very simple organism can contain just a single cell. An example is the amoeba (阿米巴/變形蟲) which are found in ditches and stagnant water. It is an unicellular organism which does not have a definite shape. The 'cell' has to carry out all of the functions of an organism, including movement, nutrition and reproduction.
The amoeba has a nucleus, cytoplasm, cell membrane and pseudopodia (false feet). Movement is by 'stretching' the pseudopodia, and getting nutrition is by engulfing the food with its pseudopodia and pulling it into the cell - when the food enters its body the amoeba forms a food vacuole around it which contains certain enzymes (酶) to digest the food; when the food is digested the unwanted waste is released through its body surface. Reproduction of amoeba is asexual (binary fission) but recent research suggests that several other single-celled eukaryotes (真核生物) were in the past capable of sexual reproduction. In binary fission the nucleus is replicated, then the cell divides into two with the cells genetically identical, i.e. have the same genetic (DNA) material.
Human Cells
The human body is made up from over 100 billion tiny cells and different cells have different cell structures to facilitate their functions. All cells, apart from red blood cells, have a nucleus.
Intestininal absorptive cells (enterocytes (腸)) are simple columar epithelial (上皮) cells (cells that line the major cavities of the body) in the small intestines. A glycocalyx (糖萼) surface coat contains digestive enzymes. Microvilli on the apical (top) surface protude into the gut lumen increase the surface area for the digestion and transport of molecules from the intestinal lumen. The cells also have a secretory (分泌) role.
Adipocytes (脂肪細胞), also known as lipocytes and fat cells, are the cells that primarily compose adipose (脂肪) tissue, specialized in storing energy as fat. There are two types of adipose tissue, white adipose tissue (WAT) and brown adipose tissue (BAT), which are also known as white fat and brown fat, respectively, and comprise two types of fat cells. White fat cells or monovacuolar cells contain a large lipid (油脂) droplet surrounded by a layer of cytoplasm. The nucleus is flattened and located on the periphery. The fat stored is in a semi-liquid state, and is composed primarily of triglycerides (甘油三酯) and cholesteryl ester (膽固醇酯). Brown fat cells or plurivacuolar cells are polygonal in shape and have considerable cytoplasm, with lipid droplets scattered throughout. The nucleus is round, and, although eccentrically located, it is not in the periphery of the cell. Brown fat, also known as "baby fat", is used to generate heat and the brown colour comes from the large quantity of mitochondria (線粒體) (an organelle (細胞器) in animal cells that combines food with oxygen to supply energy to cells).
Neurons (神經元) are nerve cells that transport messages around the body by receiving and transmitting signals. A neuron contains a nerve cell body with a nucleus and cytoplasm. Branching off the nerve cell body are the dendrites (樹突), which act like tiny antennae picking up signals from other cells. At the opposite end of the nerve cell body is the axon (軸突), which is a long, thin fiber with branches at the end that sends signals. The axon is insulated by a myelin sheath made up of segments called Schwann cells. Nerve impulses are received by the dendrites, travel down the branches of the dendrites to the nerve cell body, and are carried along the axon. When the impulse reaches the branches at the end of the axon, it is transmitted to the next neuron. Impulses continue to be carried in this way until they reach their final destination.
Red blood cells (紅血細胞) transport oxygen around the body to all the tissues from the lungs and then transport the carbon dioxide from the cells back to the lungs. They have a unique shape for this function. It has a flattened, biconcave disc shape which is thinner in the centre and thicker around the edges, and is flexible with the ability to twist and bend through the blood vessels, as these narrow and widen. The biconcave shape increases the cell's surface area and facilitates diffusion of oxygen and carbon dioxide into or out of the cell. Red blood cell do not have a nuclei or mitochondria which helps contribute to increased haemoglobin (血紅蛋白) content and gas-carrying capacity. Red blood cells are produced in the bone marrow and the colour is due to haemoglobin. The thickness of a red blood cell is about 2μm and the diameter about 7.5μm.
Sperm cells are sex cells produced by the male. They are very small and about 10μm in length. The sperm cell is composed of three main parts: the head, midpiece and tail. The head contains the nucleus which in turn contains chromosomes (染色體) with all the genetic (遺傳) (DNA deoxyribo nucleic acid) 脫氧核酸) information from the father needed to successfully reproduce once the sperm cell has fertilized an egg. The head has a cap, or acrosome (enzyme cap), containing enzymes near the tip, is also used to penetrate the egg (break through the cell membrane of the egg). The midpiece contains various pieces of mitochondria to provide the sperm cell energy which are essential for it to successfully survive on its journey to the female egg. The flagellating tail propels the sperm cell, moving as quickly as possible to reach the egg.
Egg cells (ovum) are sex cells produced by the female. They are much larger than sperm (about 0.12mm or 120μm in diameter) and are unable to move on their own. In appearance and structure, the ovum differs little from an ordinary cell, but distinctive names have been applied to its several parts; thus, the cell substance is known as the yolk or oöplasm, the nucleus as the germinal vesicle, and the nucleolus as the germinal spot. The egg cell, or oocyte, has fewer mitochondria than the sperm, but has more energy reserves, in the form of ooplasm, which are utilized during the early development of the fertilized egg.