B5 - Homeostasis and response Flashcards

Question

Why is maintaining water levels important?

Answer 1

* Maintaining water levels in the body is vital to **prevent harmful changes occurring to cells of the body as a result of osmosis** * * **Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death** * **Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting**

Answer 2

* There are two sources of water in the body: water produced as a result of aerobic respiration and water in the diet * The cytoplasm of all cells is largely composed of water, as is the blood plasma * Water is lost from the body in the following ways: * Water l**eaves the body via the lungs during exhalation (breathing out)** * **Water**, **ions** **and urea are lost from the skin in sweat.** * **Urine - The kidneys are organs of the urinary system - which** r**emoves EXCESS water, salts and urea** * However, the lungs and skin have no control over how much water, ion or urea is lost via exhalation or sweating * **Controlled loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine**

Answer 3

In the liver these amino acids are deaminated to form **ammonia**. **Ammonia is toxic and so it is immediately converted to urea for safe excretion** as they cannot be stored by the body in the same way that excess glucose can . **Urea and water are released from the liver cells in to the bloodstream and transported to the kidneys where the blood is filtered and the urea is passed out of the body in the urine**. * **Deamination is the process of breaking down excess protein** and it predominantly occurs in the **liver** * **Enzymes** in the liver split up amino acid molecules, with the part containing carbon turned into glycogen and the other part containing nitrogen (the amino part) turned into ammonia (this is why we say the amino acid has been deaminated) * **Ammonia is toxic to cells and so it is immediately converted to urea which can be transported around the body via the blood safely for excretion by** the **kidneys**

Answer 4

The **kidneys** are organs of the urinary system - which **remove excess water, salts and urea.** Blood is transported to the kidney through the renal artery. The blood is f**iltered at a high pressure and the kidney selectively reabsorbs any useful materials such as glucose, salt ions and water**. After it has been purified, the blood **returns to the circulatory system through the renal vein.** ## Footnote The kidneys **produce urine and this helps maintain water balance.** The **urine is taken from the kidneys to the bladder by the ureters.** The **bladder stores the urine until it is convenient to expel it from the body.** Note that **'ureter'** differs from the word **'urethra'**. The ureters are tubes that carry urine from the kidneys to the bladder, whereas the urethra is the tube that carries urine out of the body.

Answer 5

The role of the kidney **Each kidney contains over one million microscopic filtering units called nephrons.** Each nephron is **made of a tubule and is responsible for 'cleaning' the blood by removing urea** and **excess water and mineral ions.** The kidney works in a number of different stages: **Stage 1 - Filtration** **Blood passes through the nephron inside the kidneys,** there are **many capillaries inside the kidney, and the blood is under high pressure at the start of the nephron**, which **aids the ultrafiltration of the blood. Small molecules are filtered out and pass into the nephron tubule**. These small molecules include **ureas, water, ions, and glucose**. However, **large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.** Stage 2 - Selective reabsorption Having **filtered out small essential molecules from the blood** - the **kidneys must reabsorb the molecules which are needed**, while **allowing those molecules which are not needed to pass out** in the **urine**. Therefore, **the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.** The reabsorbed molecules include: * a**ll of the glucose which was originally filtered out** * **as much water as the body needs to maintain a constant water level** in the blood plasma * **as many ions as the body needs to maintain a constant balance of mineral ions** in the blood plasma Stage 3 - The formation of urine The **molecules which are not selectively reabsorbed** (the **urea, excess water and ions**) continue along the **nephron tubule** as **urine**. This eventually passes **down to the bladder.**

Answer 6

* Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys * The kidneys contain structures called tubules which filtrate passes through on its way to the bladder * Water reabsorption occurs along these tubules; if the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed * The pituitary gland in the brain constantly releases a hormone called **ADH;** how much ADH is released depends on how much water the kidneys should reabsorb from the filtrate * ADH, therefore, affects the permeability of the tubules to water * If the **water** c**ontent of the blood is too high, the pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney** (**the tubules become less permeable to water**) * If the **water content of the blood is too low and the blood is too concentrated,** the pituitary gland releases **more** **ADH** which l**eads to more water being reabsorbed in the tubules of the kidney** (th**e tubules become more permeable** to water)

Answer 7

The kidney is **responsible for the removal of waste products from the blood.** Damage from **accidents or disease can lead to a build-up of poisonous wastes** in the body. Humans can **survive with one kidney**, but **for people who suffer from total kidney failure** this would be **fatal if not treated**. Treatment is available for kidney failure and can be by organ transplant or by using kidney **dialysis**. In this procedure, **patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and ion balance** of the **blood**. **Patients with kidney failure** can be **kept alive by using kidney dialysis** until a **transplant** **becomes available**, but they have several disadvantages: * they are **expensive** * the patient must have his or her blood **connected to the machine for several hours** every week * patients must **follow a very rigid diet to avoid complications** * they **only work for a limited time** for a patient

Answer 8

**Unfiltered blood that is high in urea is taken from a blood vessel in the arm, mixed with blood thinners or an anti-coagulant to prevent clotting,** and **pumped into the dialysis machin**e. Inside the machine the blood and dialysis fluid are separated by a **partially permeable membrane** the **blood flows in the opposite direction to dialysis fluid,** allowing **exchange to occur between the two where a concentration gradient exists.** Dialysis fluid contains: * a **glucose** **concentration similar to a normal level in the blood** * a **concentration of ions similar to that found in normal blood plasma** * no **urea** As the dialysis fluid **has no urea in it**, **there is a large concentration gradient** - meaning that urea moves across the **partially permeable membrane**, from the blood to the dialysis fluid, by **diffusion. This is very important as it is essential that urea is removed from the patients' blood.** As the dialysis fluid **contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient** exists. This is **very important as the patients' need to retain glucose** for **respiration**. And, as the dialysis fluid contains **an ion concentration** similar **to the ideal blood plasma** **concentration**, **movement of ions across the membrane** only occurs where there is an **imbalance**. * If the **patient's blood is too** **low in ions**, they will **diffuse from the dialysis fluid** into the blood, **restoring the ideal level** in the **blood**. * If the **patient's blood** is **too high in ions**, the **excess ions will diffuse from the blood** to **the dialysis fluid.**

Answer 9

Advantages of dialysis * **greatly reduced levels of urea** – **it is 'cleaned blood'** * **no overall change in blood glucose levels** * the **correct water and ion balance maintained or restored** (with only excess ions removed) Disadvantages of dialysis * Kidney d**ialysis requires highly specialised and expensive machiner**y. * The patient must be connected to this machinery **2-3 times a week for periods (on average) of between 4-6 hours at a time.** This is time consuming and restrictive, as this mainly happens in hospital. * **Diet** needs to be **controlled and monitored.** * Finally, d**ialysis will only work for a limited amount of tim**e before a **transplant is needed, and sadly many patients will die before a suitable one is found.**

Answer 10

Kidney transplants This is better than using a restrictive dialysis machine, as the recipient **can lead a normal life afterwards.** As **with all cells, the donor kidney cells will have protein** **antigens** on **their surface.** Antigens are **unique to each of us** (with the exception of identical twins), and **allow our body to identify our own cells from those of potential** **pathogens**. **Differences in the antigens of the donor kidney cells and those of the patient receiving the transplant would mean that the patient’s immune system** would **quickly form antibodies** **against the kidney cell antigens**, and **would ultimately destroy the kidney**. This is known as **organ** rejection. This is potentially very harmful for the patient. Precautions against rejection Two precautions can be taken to reduce organ rejection: 1. **Tissue typing** - a **kidney is given to patients who have antigens that are very similar to the antigens of the donor kidney.** This can lead to long waits for transplants while compatible donors become available - during which time patients must undergo dialysis, and in some cases they will die before a match is found. 2. **Immuno-suppressant drugs** – these **drugs must be taken by transplant patients for the rest of their lives.** They **suppress the immune system**, greatly reducing the immune response against the donor kidney. The negative effect of this is that it also suppresses the immune response against pathogens which enter the body, increasing the risk of infections. Even with these two precautions, most donor kidneys will only survive for an average period of **8-9 years** before the patient will **require a further transplant or a return to dialysis.** * **Kidney transplants** are a b**etter long term solution to kidney failure than dialysis**; however, there are several disadvantages to kidney transplants, including: * Donors **won’t have the same antigens on cell surfaces so there will be some** **immune response** to the **new kidney.** * This has to be **suppressed** by taking **immunosupressant drugs** for the **rest of their lives** – **these can have long term side effect**s and leave the patient **vulnerable to infections** * **There are not enough donors to cope with demand**

Answer 11

* However, if a healthy, close matched kidney is available, then the **benefits** of a transplant over dialysis include: * **much more freedom** **as they are not tied to having dialysis** several **times a week** in one place * Their **diets** **can be much less restrictive** * dialysis machines is **very expensive** **and so this cost is removed** * A kidney transplant is a **long term solution** whereas **dialysis will only work for a limited time**

Answer 12

In order to survive, plants require light and water for **photosynthesis**. They have d**eveloped responses called tropisms to help ensure they grow towards adequate sources of light and water.** **Phototropism** is a **response to the stimulus of light.** Responses to stimuli of different parts of the plant * In the **plant stem, responses to light** are **known as a** **positive phototropism**, which means the **stem grows towards the light** * In the **plant root, responses to light are known as a** **negative phototropism**, **which means the root grows away from the light**

Answer 13

Auxins **Auxins** are a **family of plant hormones**. **They are mostly made in the tips of the growing stems and roots**, which are known as apical meristems, and can **diffuse** to other parts of the stems or roots. Auxins **control the growth of plants by promoting cell division** and **causing elongation in plant cells (the cells get longer).** Stems and roots respond differently to high concentrations of auxins: * In the **shoots** **auxins** **promote** **cell elongation** (**growth**); more auxin = more cell elongation = more growth * In the **roots** **auxins** **inhibit** **cell elongation** (**growth**); less auxin = less cell elongation = less growth * The **distribution of auxin in the shoots is affected by light** and **gravity**, **whereas the distribution** in the **roots is primarily affected by gravity alone** * If **a shoot or root is placed on its side**, **auxins will accumulate** **along the lower side** **as a result of gravity**; **so the uppermost side has a lower** **auxin concentration** * In the **shoots, the lower side grows faster the upper side**, so the s**hoot grows upwards** * In the roots, the **lower side grows slower than the upper side** (as auxin inhibits cell elongation and growth in the roots), **so the root grows downwards** * **Unequal** **distributions** of auxin cause **unequal** **growth** rates in plant roots and shoots

Answer 14

* Plants need to be able to grow in response to light (**phototropism**) and gravity (**gravitropism** or geotropism) * The **shoots** must grow **upwards**, away from gravity and towards light, so that leaves are able to absorb sunlight – **shoots** show a **positive** **phototropic** response and a **negative** **gravitropic** response * **Roots** need to grow **downwards** into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles so **roots** show a **negative** **phototropic** response and a **positive** **gravitropic** response

Answer 15

* Auxin is m**ostly made in the tips of the growing shoots diffuses to the region behind the tip** * Auxin **stimulates the cells behind the tip to elongate (get larger);** the **more auxin** there is, **the faster they will elongate and grow** * This is an important point - only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation * If **light** **shines all around** the tip, auxin is **distributed evenly throughout and the cells in shoot grow at the same rate** - this is what normally happens with plants growing outside * When light **shines on the shoot predominantly from one side though**, **the auxin produced in the tip concentrates on the shaded sid**e, making the cells on that side **elongate and grow faster than the cells on the sunny side** * This **unequal** growth **on either side of the shoot** causes the **shoot** to **bend** and grow in the direction of the light

Answer 16

Geotropisms **Phototropism** is a response to the stimulus of light, whereas **geotropism** (also called gravitropism) **is a response to the stimulus of** **gravity**. Plants responses to gravity: * when the stem grows against the force of gravity, this is known as a negative geotropism * when a root grows in the direction of the force of gravity, this is known as a positive geotropism Just like phototropism, geotropism is also caused by an unequal distribution of auxin. In a **root placed horizontally**, the bottom side contains more auxin and **grows less** - causing the root to grow in the direction of the force of gravity. The opposite happens in a stem. When a **stem placed horizontally**, the bottom side contains more auxin and **grows more** - causing the stem to grow upwards against the force of gravity.

Answer 17

**Gibberellins are important in initiating seed germination** Ethene **controls cell division and ripening of fruits.**