EXCRETION AND HOMEOSTASIS (PLANT AND ANIMAL)

Introduction

What is Excretion? Excretion is the process by which living organisms separate and eliminate waste products of metabolism from body cells. If these substances were left to accumulate, they would be toxic to the cells.

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Egestion is the removal of undigested materials from the alimentary canals of animals. Secretion is the production and release of certain useful substances such as hormones, sebum and mucus produced by glandular cells. Homeostasis is a self-adjusting mechanism to maintain a steady state in the internal environment

Excretion in Plants

Plants have little accumulation of toxic waste especially nitrogenous wastes. This is because they synthesise proteins according to their requirements. In carbohydrate metabolism plants use carbon (IV) oxide released from respiration in photosynthesis while oxygen released from photosynthesis is used in respiration.

Gases are removed from the plant by diffusion through stomata and lenticels. Certain organic products are stored in plant organs such as leaves, flowers, fruits and bark and are removed when these organs are shed. The products include tannins, resins, latex and oxalic acid crystals.

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Some of these substances are used illegally. Khat, cocaine and cannabis are used without a doctor’s prescription and can be addictive. Use of these substances should be avoided.

Plant Excretory Products, their source and uses

Excretion and Homeostasis in Unicellular Organisms

Protozoa such as amoeba depend on diffusion as a means of excretion. They have a large surface area to volume ratio for efficient diffusion. Nitrogenous waste and carbon (IV) oxide are highly concentrated in the organism hence they diffuse out.

In amoeba excess water and chemicals accumulation in the contractile vacuole. When it reaches maximum size the contractile vacuole moves to the cell membrane, bursts open releasing its contents to the surroundings.

Excretion in Human Beings

Excretion in humans is carried out by an elaborate system of specialised organs. Their bodies are complex, so simple diffusion cannot suffice. Excretory products include nitrogenous wastes which originate from deamination of excess amino acids. The main excretory organs in mammals such as human beings include lungs, kidneys, skin and liver.

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Structure and function of the human skin

Nerve Endings:

These are nerve cells which detect changes from the external environment thus making the body to be sensitive to touch, cold, heat and pressure.

Subcutaneous Fat:

Is a layer beneath the dermis. It stores fat and acts as an insulator against heat loss. The skin helps in elimination of urea, lactic acid and sodium chloride which are released in sweat.

The Lungs

Carbon (IV) oxide formed during tissue respiration is removed from the body by the lungs. Mammalian lungs have many alveoli which are the sites of gaseous exchange. Alveoli are richly supplied with blood and have a thin epithelium.

Blood capillaries around the alveoli have a high concentration of carbon (Iv) oxide than the alveoli lumen. The concentration gradient created causes carbon (IV) oxide to diffuse into the alveoli lumen. The carbon (IV) oxide is eliminated through expiration.