Grade 8 - Cells and Systems

Section 1: Living Things

All living things require oxygen, water, food and a place to live. Oxygen is a gas found in the air and in water and it is very important for living things.

Living things can be differentiated from non-living things because living things carry out five life functions as follows:

Living things reproduce: this means they can make more of their own kind. Birds produce chicks and people produce babies. The chicks (and babies) can be called offspring.
Living things grow and develop. Living things get bigger with time and undergo developmental changes.
Living things consume food as a source of energy and other nutrients. Plants make their own food using energy from the sun.
Living things have the ability to get rid of waste from the food they eat, the air they breath and the liquids they drink. This waste is released from the body as urine, feces or carbon dioxide (the air we breath out). Not all living things produce the same waste material.
Living things respond/react to their environment. For example, plants grow toward the light, animals cover themselves from cold weather and others.
Living things respond/react to their environment. For example, plants grow toward the light, animals cover themselves from cold weather and others.

Organisms have systems which perform the functions that keep them alive. Systems are made up of organs, and organs are made from tissue. Tissues are composed of cells.

Section 2: Cells and The Microscope

All living things are made up of one or more cells. A cell is the basic unit of life and the smallest part of a living thing that is capable of life. Most cells are too small to be seen with the unaided eye. We need to use an instrument called a microscope to be able to visualize cells.

The term 'cell' was coined by a scientist called Robert Hooke, who was the first to develop a microscope and used it to study thin slices of cork.

Anton van Leeuwenhoek was a Dutch scientist who developed an improved version of the microscope that was almost ten times more powerful than the one developed by Hooke.

In 1831, another scientist named Robert Brown discovered the nucleus of a plant cell. After several sudies observing multiple tissues from plants, the German scientist Matthias Schleiden concluded that plants were made up of cells. Theodor Schwann conducted similar studies in animals and concluded that animal were also made up of cells. It was the work of these many scientists that resulted in the cell theory.

Technology improvements have lead to the development of compound light microscopes (2000X magnification) and electron microscopes (2,000,000X magnification). There are two types of electron microscopes:
TEM (transmission electron microscope) and
SEM (scanning electron microscope)
The microscope is a valuable tool for the investigation of the microorganisms.

The cell theory is a scientific theory first formulated in the mid-nineteenth century, that organisms are made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from pre-existing cells. Cells are the basic unit of structure in all organisms and also the basic unit of reproduction. The three tenets of the cell theory are:
All organisms are composed of one or more cells.
The cell is the basic unit of structure and organization in organisms.
Cells arise from pre-existing cells.

Organisms can either be unicellular (made up of only one cell) or Multicellular (made up of more than one cell, and may be made up of trillions of cells.) Cells can specialize to perform specific functions. For example, blood cells, muscles, nerves are all specialized to perform their function.

Section 3: The Cell and Its Structures

Parts of a Cell: The human body has more than 200 different kinds of cells. Plant and animal cells have several basic structures in the cell, called organelles.

Plant and animal cells differ in:

Plant cells contain chloroplasts, which are necessary for energy metabolism.
Plant cells contain a large central vacuole.
Plant cells contain a cell wall, which gives the cells a regular shape.
Animal cells contain lysosomes, plant cells do not.
Animal cells have centrioles, which are necessary for cell division.

Cell Organelles

The Nucleus

The nucleus is the brain of the cell. It is a large dark round spot inside Plant & Animal cells. It houses the nuclear DNA and controls the daily activities of the cell. Because it holds the genetic material in the DNA, it can be termed as the library of the cell. Inside the nucleus is a jelly-like fluid medium called the nucleoplasm, that provides support to the contents in the nucleus.

Cytoplasm

Inside the cell, but outside the nucleus is the cytoplasm, which is a gel-like substance that dissolves nutrients to be used for various physiological functions. The cytoplasm also suspends the organelles preventing them from crushing into each other.

Mitochondria

The mitochondria is the powerhouse of the cell. It produces ATP (Adenosine Triphosphate); which is the most common form of energy in the cell. The process involved in energy metabolism is referred to as Cellular Respiration. It is shaped like a sausage on the outside, but contains a double membrane on the inside (outer and inner membrane).

The structure of a mitochondria. (Source: genome.gov)

Vacuole

The vacuole appears hollow when viewed under a microscope. The organelle is more prominent in plant cells and is located somewhat at the center of the cell. Vacuoles function as storage organelles storing water, nutrients and waste.

Chloroplasts

Chloroplasts are only found in plant cells. They contain chlorophyll which s responsible for capturing light energy for photosynthesis.

Cell Wall

A cell wall is found in plant cells only. It is the rigid thick outer wall surrounding the cell. It gives the cell its regular shape and provides support to the plant. It's made up of cellulose which is also a structural molecule.

Cell Membrane

Also called Plasa membrane. It's present in both plants and animal cells. In plants, the cell wall is located on the inside surface of the cell wall. The cell membrane is made up of a phospholipid bilayer with other large molecules scattered. It retains cell contents inside the cell and allows movement of substances inside and outside the cell.

Structure of the Plasma Membrane

The plasma membrane is composed of phospholipids and proteins. The phospholipids are organized in two layers thus referred to as a phospholipid bilayer. As the name suggests, a phospholipid has a phosphate head that is hydrophilic (water loving - can interact with water) and a lipid tail that is hydrophobic (water hating - does not interact with water). Due to this property, the phosphate heads are oriented towards the side of the plasma membrane where water molecules are located.

Chemical structure of a phospholipid and the phospholipid bilayer. (Source: Wikipedia-CC BY-SA 3.0)

Section 4: Fluid Movement in Cells

Transport across the cell membrane is either Active (requires energy) or Passive (does not require energy to occur).

Passive transport include Diffusion and Osmosis.

Diffusion

Simple Diffusion

The random movement of molecules from an area of higher concentration to an area of lower concentration until the concentration is uniform throughout the space, that's why when you add a drop of red dye into a clear glass of water it will spread throughout the water until it establishes an equilibrium.

Diffusion across a membrane

Membranes can be classified as:

Impermeable: does not let anything pass through the membrane

Permeable: allows all materials to pass through the membrane

Semi-permeable: allows some particles to pass through the membrane while excluding other particles

Diffusion can occur across a semi-permeable membrane, from the side with higher solute concentration (lower water concentration) to the side with lower solute concentration (higher water concentration), until the concentration is equal.

Dynamic Equilibrium

Dynamic Equilibrium describes a scenario where the diffusing particles are still moving, (diffusing) but the movement no longer results in a net change in concentration at one location, more than another location. When there is a membrane, dynamic equilibrium is a state where the diffusing molecules move across the membrane at almost the same rate in either direction.

Factors that influence rate of diffusion

Concentration Gradient: The difference in concentration between two locations. Molecules of substances move from high concentration to low concentration, so the larger the concentration gradient the faster the diffusion rate.

Temperature: Higher temperature results in faster diffusion rate.

Particle size: The larger the particle the slower the movement.

Facilitated Diffusion:

Facilitated diffusion uses transport proteins to facilitate the diffusion of particles across the plasma membrane. There are 2 types of transport proteins and they are recognized based on their shape, size, and electrical charge: Carrier Proteins are those protein that change shape to allow certain molecules to cross the membrane. Channel Proteins are proteins that form tunnel-like pores in the cell membrane, allowing electrically charged ions in and out of the cell.

Osmosis

Osmosis is the movement of water through a selectively permeable (semi-permeable) membrane from an area of higher water concentration to an area of lesser water concentration.

A solute are molecules that are dissolved in a solvent to form a solution. A solvent is substance that dissolves the solute. In most biological reactions, the solvent will be predominantly water. A solution is the result of dissolving a solute in a solvent.

The environment inside a cell can be described as Intracellular while the environment outside the cell is termed as Extracellular. Usually these two environments differ in their chemical composition.

In plant cells, when cells lose water, they shrink and the plant appears wilted. However, in cases where water is entering the cells, the cell walls in plant cells allow them to resist the pressure so they do not burst. This pressure created by water moving into the cells is called Turgor pressure.

Plants rely on a process called photosynthesis to obtain their energy. They utilize energy from the sun to produce food in form of glucose. The main ingredients (reactants) needed for photosynthesis to occur are carbon dioxide and water. The process produces glucose and oxygen.

Photosynthesis takes place inside chloroplasts. These organelles in plant cells contain the green pigment called chlorophyll. Chlorophyll captures energy from the Sun and the energy powers photosynthesis. The glucose produced in the process is stored within the plant. Oxygen, a waste product of photosynthesis, is released into the atmosphere.

Plants and animals use glucose as an energy source to perform various biological processes through a process called cellular respiration. Cellular respiration is like burning fuel to produce energy. It takes place in the mitochondria, an organelle found in the cell.

Cellular respiration can either be aerobic (requiring oxygen) or anaerobic (not requiring oxygen). Aeorbic respiration is more common when the oxygen supply to the cells meets the demand. Anaerobic respoiration (also called fermentation) occurs when the cells are not receiving sufficient oxygen for the needs. For example, during strenous exercise, the cells are utilizing energy much faster than can be produced by aerobic respiration.

Section 5: Cell Specialization & Organization

Cell Type	Special Structures	Functions
Muscle	Elongated and tapered on either end	Contracts and relaxes, moving parts of the body
Skin	Flat and thin, brick-shaped or honeycomb	Fit closely together to form a continuous protective layer
Nerve	Long branched fibres running from the main part of the cell	To carry nerve signals from one part of the body to another
Blood	Red Blood Cells are thin, disc-like	Increases the surface area making it more efficient to transport oxygen in the bloodstream
Bone	Thick, mineral matrix	To provide support

Advantages of Multi-cellular organisms

They can live in diverse environments
They grow very large
They have multiple sources of food
They have complex bodies with specialized cells that work in harmony with other cells

Biological Organization

The single cell of a unicellular organism carries out all the functions necessary to keep the organism alive. A group of similar cells that perform the same function make up a Tissue. Animals are mostly composed of 4 types of tissue:

Muscle tissue: Cells and fibers that move bones, pump blood and push substances through the digestive system.
Connective tissue: The tissue present in bone, cartilage, tendons, fat and blood.
Nerve tissue: Responsible for carrying messages throughout the body.
Epithelial tissue: Include the outer layer of the skin and the cells lining the digetive system.

An Organ is a group of two or more types of tissue that work together to carry out a specific function. The skin is the largest organ with several layers made up of different tissues. The heart is also an organ that is made up of muscle tissue, connective tissue and nerve tissue. The brain, lungs and eyes are more examples of organs.

Plants also have organs namely the roots, stem and leaves. These support photosynthesis, absorption of water absorption and transport.

A group of organs working together is called an organ system. The circulatory system in humans combines the heart, blood vessels and blood to deliver oxygen and nutrients to various parts of the body and eliminate waste material. The repiratory system obtains oxygen from the environment and carries it to the blood where it enters the circultory system. Carbon dioxide from the blood enters the respiratory system and is released as a waste product.

Section 6: Body Systems in Humans

The human body has ten major systems which include the skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and the reproductive system. These systems will be discussed in more detial in subsequent years.

In this section, you will learn the basics of some of the animal systems.

Living things obtain energy from food. Photosynthetic organisms can make their own food using the energy from the sun. Most other organisms obtain their food by consuming it in a process called ingestion. After the organism obtains (ingests) food, the food goes through a process where it is gradually broken down into smaller and simpler structures that the body can utilize for energy. Digestion is the process where ingested food is broken down into molecules that are usable by the body. Excretion is the removal ot waste material from the body. This waste material usually has little value to the body and in some cases, it can be toxic.

Digestive System

Invertebrates have several ways to digest food and release wastes. Sponges are filter feeders, the pores filter food from the water. In other invertebrates such as cnidarians and flatworms, food enters the body and leaves from the same opening. Invertebrates with more advanced digestive systems such as earthworms use a tube-within-a-tube system. These have separate openings for ingested food and for excreted wastes.

Vertebrates are more complex and so is their digestive system. They have many structures in order to handle different diets from teeth specialised to chew the type of food/feed, to digestive systems that have bacteria to help digest plant material.

HUman digestion starts in the mouth. Nutrients are absorbed in the small intestines and then move into the blood. Solid wastes are processed and eliminated from the body. The kidneys, liver, lungs and skin also help eliminate other types of waste material.

The product of digestion of carbohydrates is glucose, which is a simple sugar. Glucose is used by most of the body cells to make energy through a process called cellular respiration.

Respiratory System

Alhtough there is a connection between cellular respiration and the respiratory system, students should not be confused. The respiratory system relates to how to body obtains oxygen from the environment and releases carbon dioxide and moisture, which are waste products. The oxygen is needed for cellular respiration, and the carbon dioxide and moisture are the waste products from cellular respiration.

Some soft bodied invertebrates such as flatworms rely on simple diffusion for their exchange of gases. Diffusion is the movement of molecules from an area of higher concentration to areas of lower concentration. For oxygen to diffuse effectively, the surface must be moist, which is why most worms and snails stay in moist places.

Other invertebrates such as crustaceans have gills specialised for the gas exchange. Gills are feathery structures with a rich supply of blood vessels and the gas exchange occurs in these blood vessels.

As indicated previously, vertebrates are more complex in structure, which also means they have more complex systems. Amphibians live in water when they are young and on land when they are adults. Young amphibians have gills where gas exchange occurs. Adults have lungs for gas excahnge. Gas exchange can also occur through their skin in both young and adults.

Birds, mammals and reptiles use lungs exclusively for respiration.

In humans, air enters through the nose and mouth and passes through the pharynx then into the larynx and then trachea. The trachea divides into bronchi, then bronchioles and finally ends with sac-like structures called alveoli. Alveoli are very thin-walled and have high blood supply to allow gas exchange.

Circulatory System

The circulatory system is the body's transport system, it moves important materials such as oxygen, glucose and waste materials throughout the body to areas where the materials can be utilized, or eliminated.

Circulatory sytem can either be open or closed. In open circulatory system, the blood is not fully enclosed within blood vessels. Instead of moving into smaller blood vessels, the blood is released directly into the tissues. A closed circulatory system is where the blood is contained inside blood vessels. Materials diffuse in and out of the blood through the walls of the vessels.

Thermoregulation

Many processes in the body occur at a certain temperature. Therefore there is need for animals to maintain an internal temperature that will allow them to function properly. Some animals do not have to ability to regulate their body temperature and they rely on the environmental conditions. For example, when its hot, reptiles will burrow under rocks to stay cool, when its cold, they will bask in sunlight to stay warm. These animals that cannot regulate their internal tempeature and rely on the environment are named cold-blooded animals. Amphibians, reptiles and most fish are cold-blooded.

Section 7: Body Systems and Human Health

Blood

The blood vessels of the circulatory system form a complex network linking the outside environment with the internal environment of the body. The blood supplies all the living cells in the body with the nutrients they need to carry out their functions. About 8% of an adult's body weight is blood.

Blood Components

Plasma: Makes up about 55% of blood by volume. Its the clear liquid part of the blood. It carries nutrients, waste products, hormones, and acts as a medium for blood cells.
Red Blood cells: Make up 44% of blood. They have hemoglobin, which makes them able to carry oxygen.
White blood cells make up less than 1% of the blood by volume and are important in immune responses against pathogens.
Platelets also make up less than 1% of the volume of blood and are important in blood clotting process.

The circulatory system must work closely with the respiratory system (which supplies the oxygen) and the digestive system (which supplies the nutrients)

High blood pressure may be associated with several factors including high salt content, high epinephrine levels as in stress/freight, cancer, obesity etc. High blood pressure can cause heart attack, stroke, kidney damage etc. Low blood pressure is usually caused by low blood volume resulting from either bleeding, dehydration or severe anemia.

High fibre diet is important because fibre is used by the colon to process waste materials (low-fibre can irritate the colon wall and lead to colon cancer). Long-term stress, smoking, excessive use of alcohol or aspirin can lead to a peptic ulcer.

Poisons in cigarette smoke and pollutants irritate the lining of the lungs, causing certain cells to produce more mucus. If this lining becomes inflamed, it can lead to bronchitis, which can lead to emphysema. Lung cancer is caused by the tar and smoke in cigarettes, which cause the lung cells to grow out of control and overcome healthy cells.