Cells Information Booklet Cell Structure

Animal cells usually have an irregular shape, and plant cells usually have a regular shape

Cells are made up of different parts. It is easier to explain what these parts are by using diagrams like the ones below.

Animal cells and plant cells both contain: cell membrane, cytoplasm, nucleus

Plant cells also contain these parts, not found in animal cells: chloroplasts, vacuole, cell wall

The table summarises the functions of these parts.

Part Function Found in

Cell membrane Controls what substances can get into and out of the cell. Plant and animal cells

Cytoplasm Jelly-like substance, where chemical reactions happen. Plant and animal In plant cells there's a thin lining, whereas in animal cells cells most of the cell is cytoplasm.

Nucleus Controls what happens inside the cell. Carries genetic Plant and animal information. cells

Chloroplast Where photosynthesis happens – chloroplasts contain a Plant cells only green substance called chlorophyll.

Vacuole Contains a liquid called cell sap, which keeps the cell Plant cells only firm.

Cell wall Made of a tough substance called cellulose, which Plant cells only Part Function Found in

supports the cell. Immune System

Defending against infection Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens. They can produce antibodies to destroy pathogens, and antitoxins to neutralise toxins.

In vaccination pathogens are introduced into the body in a weakened form. The process causes the body to produce enough white blood cells to protect itself against the pathogens, while not getting diseased.

Antibiotics are effective against bacteria, but not against viruses. Some strains of bacteria are resistant to antibiotics.

Bacteria

Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - 10 millionths of a metre. Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include:

 food poisoning cholera  typhoid whooping cough  & gonorrhoea - a sexually transmitted disease

A salmonella bacterium cell

Viruses

Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes. Hepatitis C virus. DNA is Diseases caused by viruses include: enclosed in a protein coat

 influenza - flu colds measles mumps rubella  chicken pox  AIDS Defending the Body

The body has different ways of protecting itself against pathogens. The first defence aims to stop the pathogen entering the body in the first place. The body has structures including the skin, mucus: [Slimy white protein, which lines the respiratory tract and alimentary canal. ] and cilia: [Tiny hairs, which line the respiratory tract. They beat continuously to move mucus and dirt

Cross-section of skin up the bronchi and trachea. ] in the respiratory system, acid in the stomach, and enzymes: [proteins which catalyse or speed up chemical reactions inside our bodies ] in tears which prevent pathogens entering the body.

If a pathogen still manages to get into the body, the second defence takes over. This is the immune system, and the white blood cells have key functions in this.

More about white blood cells

White blood cells can:  ingest pathogens and destroy them  produce antibodies to destroy pathogens  produce antitoxins that neutralise the toxins released by pathogens  antibodies and antitoxins are not living things – they are specialised proteins produced by the white blood cells of the immune system.

There are several different types of white blood cells, each with different functions, but they can be put into two main groups:

 phagocytes or macrophages  lymphocytes

Phagocytes

Phagocytes can easily pass through blood vessel walls into the surrounding tissue and move towards pathogens or toxins. They then either:

 ingest and absorb the pathogens or toxins  release an enzyme to destroy them Having absorbed a pathogen, the phagocytes may also send out chemical messages that help nearby lymphocytes to identify the type of antibody needed to neutralise them. Lymphocytes

Pathogens contain certain chemicals that are foreign to the body and are called antigens. Each lymphocyte carries a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly, and makes many copies of the antibody that neutralises the pathogen.

Antibodies neutralise pathogens in a number of ways:

 they bind to pathogens and damage or destroy them  they coat pathogens, clumping them together so that they are easily ingested by phagocytes  they bind to the pathogens and release chemical signals to attract more phagocytes Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging the body.

Vaccination

People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens.

Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain:

 live pathogens treated to make them harmless  harmless fragments of the pathogen  toxins produced by pathogens  dead pathogens These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen.

Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated person is not in danger of developing disease - although some people may suffer a mild reaction. If the person does get infected by the pathogen later, the required lymphocytes are able to reproduce rapidly and destroy it.

Vaccines & Boosters

Vaccines in early childhood can give protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.

Sometimes vaccine boosters are needed, because the immune response 'memory' weakens over time. Anti-tetanus injections may need to be repeated every ten years. Antibiotics

Antibiotics are substances that kill bacteria or stop their growth. They do not work: it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

How some common antibiotics work antibiotic how it works

penicillin stops cell wall* synthesis in the bacteria

erythromycin stops protein synthesis in the bacteria

neomycin stops protein synthesis in the bacteria

vancomycin stops protein synthesis in the bacteria

ciprofloxacin stops DNA replication in the bacteria

* Bacterial cell walls are not made of cellulose, but are formed from a substance called peptidoglycan.

Penicillin

The first antibiotic - penicillin - was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould. Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most antibiotics used in medicine have been altered chemically to make them more effective and safer for humans.

A bacterium damaged and distorted by penicillin

Antibiotic Resistance

Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.

MRSA is methicillin-resistant staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. It is important to avoid over-use of antibiotics, so we can slow down, or stop, the development of other strains of resistant bacteria.

Cleanliness

One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed. . How did cells first start?

Living things (even ancient organisms like bacteria) are enormously complex. However, all this complexity did not leap fully-formed from the primordial soup. Instead life almost certainly originated in a series of small steps, each building upon the complexity that evolved previously:

1. Simple organic molecules were formed.

Simple organic molecules, similar to the nucleotide shown below, are the building blocks of life and must have been involved in its origin. Experiments suggest that organic molecules could have been synthesized in the atmosphere of early Earth and rained down into the oceans. RNA and DNA molecules — the genetic material for all life — are just long chains of simple nucleotides.

2. Replicating molecules evolved and began to undergo natural selection.

All living things reproduce, copying their genetic material and passing it on to their offspring. Thus, the ability to copy the molecules that encode genetic information is a key step in the origin of life — without it, life could not exist. This ability probably first evolved in the form of an RNA self-replicator — an RNA molecule that could copy itself.

Many biologists hypothesize that this step led to an "RNA world" in which RNA did many jobs, storing genetic information, copying itself, and performing basic metabolic functions. Today, these jobs are performed by many different sorts of molecules (DNA, RNA, and proteins, mostly), but in the RNA world, RNA did it all.

Self-replication opened the door for natural selection. Once a self-replicating molecule formed, some variants of these early replicators would have done a better job of copying themselves than others, producing more "offspring." These super-replicators would have become more common — that is, until one of them was accidentally built in a way that allowed it to be a super-super-replicator — and then, that variant would take over. Through this process of continuous natural selection, small changes in replicating molecules eventually accumulated until a stable, efficient replicating system evolved.

3. Replicating molecules became enclosed within a cell membrane.

The evolution of a membrane surrounding the genetic material provided two huge advantages: the products of the genetic material could be kept close by and the internal environment of this proto- cell could be different than the external environment. Cell membranes must have been so advantageous that these encased replicators quickly out-competed "naked" replicators. This breakthrough would have given rise to an organism much like a modern bacterium.

4. Some cells began to evolve modern metabolic processes and out-competed those with older forms of metabolism. Up until this point, life had probably relied on RNA for most jobs (as described in Step 2 above). But everything changed when some cell or group of cells evolved to use different types of molecules for different functions: DNA (which is more stable than RNA) became the genetic material, proteins (which are often more efficient promoters of chemical reactions than RNA) became responsible for basic metabolic reactions in the cell, and RNA was demoted to the role of messenger, carrying information from the DNA to protein-building centers in the cell. Cells incorporating these innovations would have easily out-competed "old- fashioned" cells with RNA-based metabolisms, hailing the end of the RNA world.

5. Multicellularity evolved.

As early as two billion years ago, some cells stopped going their separate ways after replicating and evolved specialized functions. They gave rise to Earth's first lineage of multicellular organisms, such as the 1.2 billion year old fossilized red algae in the photo below.

These fossils of Bangiomorpha pubescens are 1.2 billion years old. Toward the lower end of the fossil on the left there are cells differentiated for attaching to a substrate. If you look closely at the upper part of the

fossil on the right, you can see longitudinal division that has divided disc-shaped cells into a number of radially arranged wedge-shaped cells, as we would see in a modern bangiophyte red alga.

Nerve Cells & Nerves

Function: To transmit messages from one part of your body to another

Neurons: Messenger cells in your nervous system

Nerve impulses: Electrical signals carrying messages

Neurotransmitters: Chemicals released by one neuron to excite a neighbouring one

Millions of messengers

Your nervous system contains millions of nerve cells, called neurons. Neurons are highly specialised to transmit messages from one part of your body to another.

All neurons have a cell body and one or more fibres. These fibres vary in length from microscopic to over 1 metre. There are two different kinds of nerve fibres: fibres that carry information towards the cell body, called dendrites, and fibres that carry information away from it, called axons. Nerves are tight bundles of nerve fibres.

Teamwork

Your neurons can be divided into three types:

 Sensory neurons, which pass information about stimuli such as light, heat or chemicals from both inside and outside your body to your central nervous system  Motor neurons, which pass instructions from your central nervous system to other parts of your body, such as muscles or glands  Association neurons, which connect your sensory and motor neurons

Electrical and chemical signals

Your neurons carry messages in the form of electrical signals called nerve impulses. To create a nerve impulse, your neurons have to be excited. Stimuli such as light, sound or pressure all excite your neurons, but in most cases, chemicals released by other neurons will trigger a nerve impulse.

Although you have millions of neurons that are densely packed within your nervous system, they never actually touch. So when a nerve impulse reaches the end of one neuron, a neurotransmitter chemical is released. It diffuses from this neuron across a junction and excites the next neuron.

Protecting cells Over half of all the nerve cells in your nervous system do not transmit any impulses. These supporting nerve cells are located between and around your neurons to insulate, protect and nourish them Seahorse Reproduction

Seahorses reproduce in an unusual way. The male seahorse becomes pregnant instead of the female. Most seahorse pregnancies lasts approximately 2 to 3 weeks. The male seahorse has a brood pouch where he carries eggs deposited by the female.

Seahorses are renowned for their mating rituals in which they dance together before mating. View the short video on the laptop to see these graceful creatures perform their mating dance.

The mating pair entwine their tails and the female aligns a long tube called an ovipositor with the males pouch. The eggs move through the tube into the male’s pouch where he then fertilizes them. The embryos develop in ten days to six weeks, depending on species and water conditions. When the male gives birth he pumps his tail until the baby seahorses emerge.

The males pouch regulates salinity (dissolved salt content of a body of water) for the eggs, slowly increasing in the pouch to match the water outside as the eggs mature. Once the offspring hatch, the male releases the fry (baby seahorses) and does not provide parental care for them. Most will not consume their own offspring, however, it has been known for them to do so.

Once released, the offspring are independent of their parents. Some spend time among the ocean plankton developing before settling down.

At times, the male seahorse may try to consume some of the previously released offspring. Other species such as the Dwarf Seahorse (Hippocampus zosterae) hatch immediately and begin life in the benthos (organisms and habitats of the sea floor) .

Seahorses are generally monogamous, though several species such as the Big-Bellied Seahorse (Hippocampus abdominalis) are highly gregarious. In monogamous pairs, the male and female will greet one another with courtship displays in the morning and sometimes in the evening to reinforce their pair bond. Seahorses spend the rest of the day separate from each other hunting for food.

The seahorses means of propulsion (process of propelling) are its pectoral fin, located just behind the gill opening and its dorsal fin, which joins the trunk at the tail.

On male seahorses, a brood pouch is found beneath the anal fin and when empty, tapers gradually to the tail.

During courting or when pregnant, the pouch is very pronounced and protruding, it features a vertical opening into which the female deposits her eggs and from which fry (baby seahorses) emerge after gestation.

Some species of seahorses have spindly appendages, called cirri, in the area of the facial spines and trunk ridges.

Unusual Animal Reproduction

Argonaut: Detachable Penis

Argonaut or paper nautilus is a weird species of octopus. First, they have a highly divergent sexual dimorphism. That’s science-speak for the difference in body sizes between males and females. A female argonaut grows up to 10 cm (~ 4 in.) with shells as large as 45 cm (~ 18 in.) The male, however, is only 2 cm (3/4 in) long!

But that’s not why argonaut is on this list. The male argonaut produces a ball of spermatozoa in a special tentacle called a hectocotylus [wiki]. When meeting a female it fancies, the male then detaches its penis to swim by itself to the female!

Hectocotylus (Image Credit: Julian Finn, Macalogist)

This detachable swimming penis was actually first noted by an Italian naturalist back in the 1800s, who mistook it for a parasitic worm!

Whiptail Lizard: Sex? No Thanks! We’ll Clone Ourselves Instead.

Whiptail Lizard in pseudocopulation (Image Credit: Tino Mauricio, Daily Texan)

How does a whiptail lizard have sex? Trick question! There are no males – all whiptail lizards are females, so they can’t have sex at all. Wait a minute – so how do they reproduce? By cloning themselves:

In the bizarre life of a whiptail lizard, reproduction is preceeded by pseudocopulation, where two females act out the roles of a male mounting a female (they switch roles later on).

Apparently, this is required to stimulate egg production in both lizards. When the eggs hatch, they will be all-female clones of the mother lizard.

Banana Slug: Penis Stuck? Chew It Off!

Banana slugs checking each other out for size (Image Credit: Husond, Wikipedia)

Banana slug, the beloved mascot of UC Santa Cruz, has a weird mating habit. First of all, they have an enormous penis. (In fact, their latin name dolichyphallus translates to "giant penis.") The average size of a banana slug penis is 6 to 8 inches. This is incredibly impressive, considering their entire body length is 6 to 8 inches as well!

Banana slugs are hermaphrodites, so two slugs will try to fertilize each other. To mate properly, a slug must choose a mate roughly its own size – if it miscalculates, its penis will get stuck during copulation.

This isn’t just an embarrassing faux pas, the other slug will actually bite off the stuck penis, a term scientists euphemistically called "apophallation."

Barnacle: Inflatable Penis

Yes, that long thing is a barnacle penis mating with its neighbor (Image Credit: Sue Scott, MarLIN)

Barnacles, those crustaceans that stick themselves to the bottom of boats (much to the consternation of sailors everywhere), are stuck in one position all their lives.

So, how do they mate? The solution, turns out, is brilliantly simple: the barnacle has an inflatable penis that is up to 50 times as long as its body. In fact, it has the longest penis in the animal kingdom, relative to body length! Fire Ant: Queen and Workers "Negotiate" the Colony’s Sex Ratio

Ants have a complex social structure. Case in point: some scientists used to think that worker ants are all females who control the queen (a simple egg-laying machine) and kill their brothers while still larvae.

It turns out the queen has more say than this: she controls the number of females and male eggs she lays.

But why does a colony’s sex ratio matter? A queen wants to propagate her line by producing another queen, which needs male drones to mate and produce a colony. Worker ants, on the other hand, have no use for males (which die after mating).

So, the queen and her daughters negotiate a rather violent solution: when she needs male drones, the queen will "overwhelm" the colony with male eggs. The female workers will kill many of their brothers, but they can’t kill them all!

Bowerbird: Obsessive Decorator of Bachelor Pad

Satin Bowerbird in front of his bower (Image Credit: bdonald [Flickr])

To attract a mate, the male bowerbird [wiki] builds an amazingly complex structure called a bower. It is made of twigs and often shaped like a small hut.

The male bird then decorates his "bachelor pad" bower with a variety of objects as gifts: flowers, feathers, stones, and even bits of discarded plastics and glass. Hundreds of pieces are carefully arranged in monochromatic themes (i.e. all blue items). The bird is so anal that it will get really angry if you mess up its pile (say, by putting one differently colored pebble in its pile).

The male bowerbird spends hours sorting and arranging things. In fact, it will break its focus only to go to a different males’ bowers to steal stuff and mess the place up! Red Velvet Mite: The Love Gardener

Red Velvet Mite (Image Credit: erica_naturegirl [Flickr])

Red velvet mite, which is as big as one of the letters in this sentence, has a peculiar mating habit.

The male releases its sperms on small twigs or stalks in what scientists call the "love garden", then lays down an intricate silken trail to the spot. When a female stumbles upon this trail, she will follow it to seek out the "artist". If she likes his work, then she will sit on the sperm.

However, if another male spots the garden, he will trash it and lay his own instead!

Porcupine: Wee Marks the Spot.

Quick: how do porcupines mate? If you answer: "carefully," you’d only be half right – it’s also "bizarrely." Indeed, porcupines have a very bizarre mating habit:

First of all, female porcupines are interested in sex only about 8 to 12 hours in a year! Second, to court a female during the short mating season, a male porcupine stands up on his hind legs, waddles up to her, and then sprays her with a huge stream of urine from as far as 6 feet away, and drench his would-be paramour from head to foot!

If the female wasn’t impressed, she’ll scream and shake off the urine. But, if she is ready, then she’ll rear up to expose her quill-less underbelly and let the male mount her from the behind (that’s the only safe position for porcupines!). Once mating begins, the female is insatiable: she forces the male to mate many times until he is thoroughly exhausted. If he gets tired too quickly, she will leave him for another male!

Bedbug: Traumatic Insemination

Here’s chivalry for you: the male bedbugs don’t even bother with the female’s sex organs. Instead, a male bedbug uses its scimitar-like sexual organ to impale the female bedbug’s body and deposit his sperm!

Scientists even have a cute name for this sort of thing: "traumatic insemination." Ouch Garden Snail: Love Darts

Roman snails mating: the gallery (Image Credit: Robert Nordsieck)

Snails’ genitals are on their necks, right behind their eye-stalks. Not weird enough? Read on.

Snails are hermaphrodites, meaning they have both male and female sexual organs, but they do not self-fertilize.

Before two snails mate, they shoot "love darts" made of calcium at each other. People used to think that these sharp darts are nutritional gifts, like you give someone you love a box of chocolate.

Snail love dart (Image Credit: Prof. Ronald Chase)

Scientists now think, however, that these darts serve a more sinister purpose. The mucus on the darts allow more sperms to be stored in the snail’s uterus (and thus helped it gain an edge in reproduction).

There’s no advantage to the target snail (getting hit may even be dangerous as snails are really, really bad shots). Indeed, snails jostle each other not only to get into a better position to fire their darts, but also to avoid getting hit themselves!