OLT 1

Overview

• Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.

• The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.

• The function of an organ system depends on the integrated activity of its organs. For instance, the organs of the digestive system cooperate to process food.

• The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.

Multicellular organisms need specialised systems

If you were a single-celled organism and you lived in a nutrient-rich place, staying alive would be pretty straightforward: if you were an amoeba living in a pond, you could absorb nutrients straight from your environment. The oxygen you would need for metabolism could diffuse in across your cell membrane, and carbon dioxide and other wastes could diffuse out. When the time came to reproduce, you could just divide yourself in two.

However,  things aren’t quite so simple for large, many-celled organisms like human beings. Your complex body has over 30 trillion cells, and most of those cells aren’t in direct contact with the external environment.  A cell deep inside your body—in one of your bones, say, or in your liver—can’t get the nutrients or oxygen it needs directly from the environment.

How, then, does the body nourish its cells and keep itself running?

Most cells in large multicellular organisms don't directly exchange substances like nutrients and wastes with the external environment, instead, they are surrounded by an internal environment of extracellular fluid—literally, fluid outside the cells. The cells get oxygen and nutrients from this extracellular fluid and release waste products into it. Humans and other complex organisms have specialized systems that maintain the internal environment, keeping it steady and able to provide for the needs of the cells.

Different systems of the body carry out different functions. For example, the digestive system is responsible for taking in and processing food, while the respiratory system—working with the circulatory system—is responsible for taking up oxygen and getting rid of carbon dioxide. The muscular and skeletal systems are crucial for movement; the reproductive system handles reproduction; and the excretory system gets rid of metabolic waste.

Because of their specialisation, these different systems are dependent on each other. The cells that make up the digestive, muscular, skeletal, reproductive, and excretory systems all need oxygen from the respiratory system to function, and the cells of the respiratory system—as well as all the other systems—need nutrients and must get rid of metabolic wastes. All the systems of the body work together to keep an organism up and running.

Overview of body organisation

The cells in complex multicellular organisms like people are organized into tissues, groups of similar cells that work together on a specific task. Organs are structures made up of two or more tissues organized to carry out a particular function, and groups of organs with related functions make up the different organ systems. This is called a hierarchy (a system of organisation based on importance), or a hierarchical structure.

ANIMALS

At each level of organisation—cells, tissues, organs, and organ systems—structure is closely related to function. For instance, the cells in the small intestine that absorb nutrients look very different from the muscle cells needed for body movement. The structure of the heart reflects its job of pumping blood throughout the body, while the structure of the lungs maximises the efficiency with which they can take up oxygen and release carbon dioxide.

Tissues

Every organ is made up of two or more tissues, groups of similar cells that work together to perform a specific task. Humans—and other large multicellular animals—are made up of four basic tissue types: epithelial tissue (the thin tissue forming the outer layer of a body's surface and lining the alimentary canal and other hollow structures ), connective tissue (connects, supports, binds, or separates other tissues or organs ), muscle tissue, and nervous tissue.

Epithelial tissue

Epithelial tissue consists of tightly packed sheets of cells that cover surfaces—including the outside of the body—and line body cavities. For instance, the outer layer of your skin is an epithelial tissue, and so is the lining of your small intestine.

Epithelial cells are polarised, meaning that they have a top and a bottom side. The apical, top, side of an epithelial cell faces the inside of a cavity or the outside of a structure and is usually exposed to fluid or air. The basal, bottom, side faces the underlying cells. For instance, the apical sides of intestinal cells have finger-like structures that increase surface area for absorbing nutrients.

Epithelial cells are tightly packed, and this lets them act as barriers to the movement of fluids and potentially harmful microbes. Often, the cells are joined by specialised junctions that hold them tightly together to reduce leaks. 

Connective tissue

Connective tissue consists of cells suspended in an extracellular matrix. In most cases, the matrix is made up of protein fibers like collagen and fibrin in a solid, liquid, or jellylike ground substance. Connective tissue supports and, as the name suggests, connects other tissues.

Loose connective tissue, show below, is the most common type of connective tissue. It's found throughout your body, and it supports organs and blood vessels and links epithelial tissues to the muscles underneath. Dense, or fibrous, connective tissue is found in tendons, which connect muscles to bones, and ligaments, which connect bones to each other.

Muscle Tissue

Muscle tissue is essential for keeping the body upright, allowing it to move, and even pumping blood and pushing food through the digestive tract.

Muscle cells, often called muscle fibers, contain the proteins actin and myosin, which allow them to contract. There are three main types of muscle: skeletal muscle, cardiac muscle, and smooth muscle.

Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialised regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell.

Skeletal muscle, which is also called striated—striped—muscle, is what we refer to as muscle in everyday life. Skeletal muscle is attached to bones by tendons, and it allows you to consciously control your movements. For instance, the quadruceps in your legs or biceps in your arms are skeletal muscle.

Cardiac muscle is found only in the walls of the heart. Like skeletal muscle, cardiac muscle is striated, or striped. But it's not under voluntary control, so—thankfully!—you don’t need to think about making your heart beat. The individual fibres are connected by structures called intercalated disks, which allow them to contract in sync.

Smooth muscle is found in the walls of blood vessels, as well as in the walls of the digestive tract, the uterus, the urinary bladder, and various other internal structures. Smooth muscle is not striped, striated, and it's involuntary, not under conscious control. That means you don't have to think about moving food through your digestive tract.

Nervous tissue

Nervous tissue is involved in sensing stimuli—external or internal cues—and processing and transmitting information. It consists of two main types of cells: neurons, or nerve cells, and glia.

The neurons are the basic functional unit of the nervous system. They generate electrical signals called conducted nerve impulses or action potentials that allow the neurons to convey information very rapidly across long distances. The glia mainly act to support neuronal function.

Organs

Organs, such as the heart, the lungs, the stomach, the kidneys, the skin, and the liver, are made up of two or more types of tissue organised to serve a specific function. For example, the heart pumps blood, the lungs bring in oxygen and eliminate carbon dioxide, and the skin provides a barrier to protect internal structures from the external environment.

Most organs contain all four tissue types. The layered walls of the small intestine provide a good example of how tissues form an organ. The inside of the intestine is lined by epithelial cells, some of which secrete hormones or digestive enzymes and others of which absorb nutrients. Around the epithelial layer are layers of connective tissue and smooth muscle, interspersed with glands, blood vessels, and neurons. The smooth muscle contracts to move food through the gut, under control of its associated networks of neurons.

Organ systems

Organs are grouped into organ systems, in which they work together to carry out a particular function for the organism.

For example, the heart and the blood vessels make up the cardiovascular system. They work together to circulate the blood, bringing oxygen and nutrients to cells throughout the body and carrying away carbon dioxide and metabolic wastes. Another example is the respiratory system, which brings oxygen into the body and gets rid of carbon dioxide. It includes the nose, mouth, pharynx, larynx, trachea, and lungs.

We will look at these in more detail later in this module.

PLANTS

Plants are also organised along the lines of cell, tissue, organ, organ system.

Plant systems

A plant has two organ systems: 

1) the shoot system - generally grows above ground where it absorbs the light needed for photosynthesis;  consists of two portions: 

• the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems

• the reproductive parts of the plant, which include flowers and fruits (if the plant has any). 

2) the root system: includes those parts of the plant below ground, such as the roots, tubers and rhizomes; supports the plants and absorbs water and minerals.

Plant organs

• Roots: The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis (eg carrots, turnips, and beets ). Some roots are modified to absorb moisture and exchange gases. Most roots are underground. 

• Stems: Stems are a part of the shoot system of a plant. Their main function is to provide support to the plant, holding leaves, flowers and buds. Of course they also connect the roots to the leaves, transporting absorbed water and minerals from the roots to the rest of the plant, and transporting sugars from the leaves (the site of photosynthesis) to desired locations throughout the plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. 

• Leaves: Leaves are the main sites for photosynthesis: the process by which plants synthesise food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.  Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Typical leaves are attached to the plant stem by a petiole, though there are also leaves that attach directly to the plant stem. The edge of the leaf is called the margin. The vascular tissue (xylem and phloem) run through veins in the leaf, which also provide structural support. The center vein is called the midrib. 

Plant tissues

Plants have 3 tissue types: 

1) Dermal Dermal tissue covers the outer surface of herbaceous plants. Dermal tissue is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss.

2) Ground The ground tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma, and sclerenchyma cells are common in the ground tissue. 

 3) Vascular  Vascular tissue transports food, water, hormones and minerals within the plant. Vascular tissue includes xylem, phloem, parenchyma, and cambium cells.

Plant cells:

Plant cell types are formed by mitosis (cell division) from a meristem (a region of localised mitosis.) Meristems may be at the tip of the shoot or root (a type known as the apical meristem) or in cylinders extending nearly the length of the plant. 

We will look at ground tissue as an example:

There are three types of plant cells in ground tissue:

1. parenchyma: a general plant cell type, function in storage, photosynthesis, and make up the bulk of ground and vascular tissues, including spongy mesophyll (really spongy parnechyma found in the mesophyll tissue) and palisade mesophyll (palisade parenchyma, also found in the leaf mesophyll tissue.) Parenchyma cells also occur within the xylem and phloem of vascular bundles.

2. collenchyma: support tissue of living long cells with thick deposits of cellulose in their cell walls. They are polygon shaped when cut across (cross-section.)

3. sclerenchyma: mature sclerenchyma cells are usually dead cells that have heavily thickened secondary walls containing lignin. The cells are rigid and nonstretchable and are usually found in nongrowing regions of plant bodies, such as the bark or mature stems.  

A hierarchical structural organisation of multicellular organisms enables organisms to function, and to function efficiently. Individual cells are limited in size by the surface area/volume ratio, so for multicellular organisms they form smaller collections of the same cell = tissue. Different tissues can cooperate with each other to perform the same function = organ, and similarly different organs cooperate to achieve the same outcome = systems. Systems working together allow the organism to carry out processes necessary for it to remain alive.