NOTES: Cross section of leaf

Leaf Structure:
A leaf is made of many layers that are sandwiched between two layers of tough skin cells (called the epidermis). The epidermis also secretes a waxy substance called the cuticle. These layers protect the leaf from insects, bacteria, and other pests. Among the epidermal cells are pairs of sausage-shaped guard cells. Each pair of guard cells forms a pore (called stoma; the plural is stomata). Gases enter and exit the leaf through the stomata.

Most food production takes place in elongated cells called palisade mesophyll. Gas exchange occurs in the air spaces between the oddly-shaped cells of the spongy mesophyll.

Veins support the leaf and are filled with vessels that transport food, water, and minerals to the plant.

Cross section of skin: organ that forms the outer covering of a human body.
Pores: minute holes from which sweat and sebum are secreted.
Hair shaft: a filament that grows from the skin.
Epidermis: outer layer of the skin.
Dermins or true skin: middle layer of the skin.
Sebaceous gland: gland that secretes sebum, which lubricates the skin and hair.
Subcutaneous tissue: deep subcutaneous layer.
Connective tissue: tissue that consists of cells and fibres and which connects and supports.
Matrix: cells that allow hair growth copyright bernard dery infovisual.
Nerve ending: part of the skin that senses stimuli.
Adipose tissue (fat): fat-producing cells.
Arteriole: network of blood vessels that carries blood from the heart to the organs.
Venule: network of blood vessels that carries blood from the organs to the heart.
Sweat gland: gland that produces and secretes sweat.
Pilo erectile muscle: muscle that elevates the hair.
Capillaries: blood vessels that allow the exchange of various nutriments and wastes among cells.
Sweat pore: minute hole that secretes perspiration.
Skin surface: top of the epidermis.

Structure and function

The structure and function can be described both as gross anatomy and at a microscopic level. The intestinal tract can be broadly divided into two different parts, the small and large intestine.^[2] People will have different sized intestines according to their size and age.

The lumen is the cavity where digested food passes through and from where nutrients are absorbed. Both intestines share a general structure with the whole gut, and are composed of several layers. Going from inside the lumen radially outwards, one passes the mucosa (glandular epithelium and muscularis mucosa), submucosa, muscularis externa (made up of inner circular and outer longitudinal), and lastly serosa.

§  Along the whole length of the gut in the glandular epithelium are goblet cells. These secretemucus which lubricates the passage of food along and protects it from digestive enzymes. In the small intestine, villi are vaginations (folds) of the mucosa and increase the overall surface area of the intestine while also containing a lacteal, which is connected to the lymph system and aids in the removal of lipids and tissue fluid from the blood supply. Microvilli are present on the epithelium of a villus and further increase the surface area over which absorption can take place. Pocket-like invaginations into the underlying tissue are termed Crypts of Lieberkühn. In the large intestines, villi are absent and a flat surface with thousands of crypts is observed.

§  Underlying the epithelium is the lamina propria, which contains myofibroblasts, blood vessels, nerves, and several different immune cells.

§  The next layer is the muscularis mucosa which is a layer of smooth muscle that aids in the action of continued peristalsis and catastalsisalong the gut. The submucosa contains nerves (e.g. Meissner's plexus), blood vessels and elastic fibre with collagen that stretches with increased capacity but maintains the shape of the intestine.

§  Surrounding this is the muscularis externa which comprises longitudinal and circular smooth muscle that again helps with continued peristalsis and the movement of digested material out of and along the gut. In between the two layers of muscle lies Auerbach's plexus.

§  Lastly there is the serosa which is made up of loose connective tissue and coated in mucus so as to prevent friction damage from the intestine rubbing against other tissue. Holding all this in place are the mesenteries which suspend the intestine in the abdominal cavity and stop it being disturbed when a person is physically active.

The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to break down itself.^{[citation needed]} This is an example of symbiosis. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence when eliminated through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus) and the reabsorption of sodium, as well as any nutrients that may have escaped primary digestion in the ileum.