Communication & Homeostasis (OCR A Level Biology): Flashcards

• Glucose is an essential respiratory substrate for cellular respiration (e.g. brain cells are rapidly damaged without a sufficient supply)

• The amount of glucose affects the water potential of the blood, so too high a concentration has a dramatic effect on it

• Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves

• This controls the entry of carbon dioxide needed for photosynthesis

• Regulating stomatal aperture balances carbon dioxide uptake against minimising water loss by transpiration

Physiological control systems maintain the internal environment within restricted limits through a process known as homeostasis.

Hormones are chemical substances produced by endocrine glands and carried by the blood to alter the activity of target organs.

True

False — it keeps the internal environment relatively constant, fluctuating within restricted limits.

The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

A control mechanism in which a change in a physiological factor triggers a response that reduces the initial effect of the stimulus, returning the factor towards its normal range.

A control mechanism in which the original stimulus produces a response that causes the factor to deviate even more from the normal range, enhancing the effect of the original stimulus.

Negative feedback, which keeps physiological factors (such as internal temperature or blood glucose concentration) within certain limits.

• Receptor (sensor) – detects a stimulus/change in a physiological factor\n\n- Coordination system (nervous and endocrine systems) – transfers information between different parts of the body\n\n- Effector (muscles and glands) – carries out a response

The factor is continuously monitored.\n\nIf there is an increase in the factor, the body responds to make the factor decrease.\n\nIf there is a decrease in the factor, the body responds to make the factor increase.

So that the factor can be returned to its normal range whether it deviates too high or too low:\n\n- One corrective mechanism operates when the factor becomes too low\n\n- One corrective mechanism operates when the factor becomes too high

The magnitude of correction is monitored and regulated by negative feedback. As the factor gets closer to its normal value, the level of correction reduces.

In a negative feedback loop, receptors detect deviations from the normal range, which results in a corrective mechanism to return the factor back to its normal range.

A positive feedback loop enhances the effect of the original stimulus, causing the factor to deviate even more from the normal range, whereas a negative feedback loop reduces the initial effect of the stimulus.

• Osteoblasts – secrete the hormone osteocalcin (a protein) in an inactive form\n\n- Osteoclasts – secrete acid, lowering the pH

The low pH alters the hydrogen and ionic bonds in the protein, changing its tertiary structure. This converts the inactive form of osteocalcin into the active form.

Active osteocalcin binds to receptors on beta (β) cells in the pancreas, stimulating them to release insulin.\n\nOsteoblasts possess insulin receptors; when stimulated by insulin they release more inactive osteocalcin.\n\nThis enhances the effect of the original stimulus, making it a positive feedback loop.

False — most rely on negative feedback to keep physiological factors within certain limits.

True

The process by which cells communicate with each other.

It allows multicellular organisms to control and coordinate their bodies and to respond to their environments.

• A stimulus is received by a receptor cell

• The stimulus is converted to a signal (nearly always chemical) that can be passed on — this is transduction

• The signal is transmitted to a target cell (effector) that can detect it via receptors in its cell membrane

• An appropriate response is made

The process by which a stimulus is converted into a signal (nearly always a chemical) that can be passed on.

The cell that detects the signal and produces an appropriate response is known as the target cell, or effector.

Paracrine and endocrine signalling, categorised by how far the signal must travel.

Cell signalling between cells that are close together.

Cell signalling between cells that are far apart, in which the signalling molecule (always a hormone) is transported in the circulatory system.

Proteins, glycoproteins, amino acids, lipids and phospholipids.

In endocrine signalling, the signalling molecules are always hormones.

They control which molecules (including signalling molecules) can move between the internal and external environments of the cell, which is essential for transmitting messages across barriers.

So that they can be transported easily across cell membranes.

They are proteins or glycoproteins.

It causes specific changes in the receiving (target) cell, bringing about the appropriate response.

• Paracrine signalling occurs between cells that are close together

• Endocrine signalling occurs between cells that are far apart, with the signalling molecule transported in the circulatory system

Usually on or in the cell surface membrane.

Some receptors, such as oestrogen receptors, are found in the cytoplasm, because steroid hormones can diffuse through the cell membrane.

True

False — some receptors, such as oestrogen receptors, are found in the cytoplasm.

The control of internal body temperature.

An animal that possesses physiological mechanisms for the maintenance of internal body temperature (e.g. mammals and birds).

An animal that relies on behavioural mechanisms to maintain internal body temperature, such as moving into or out of the sun (e.g. reptiles and amphibians).

A stable core temperature is essential for enzyme activity:

• Lower temperatures reduce the kinetic energy of molecules and slow down chemical reactions

• Higher temperatures speed up reactions up to a point, above which the rate drops sharply as enzymes begin to denature

• Peripheral receptors (thermoreceptors) in the skin and mucous membranes detect external heat and cold, sending impulses to the hypothalamus

• The hypothalamus also contains receptors that monitor the temperature of the blood flowing through it

• The hypothalamus processes this information and initiates homeostatic responses when temperature gets too high or too low

• The muscles in the walls of arterioles relax, causing them to dilate

• This allows more blood to flow into the skin capillaries

• Heat is then lost to the environment by radiation

• The muscles in arteriole walls contract, causing arterioles near the skin to constrict so that less blood flows through the skin capillaries

• Blood is diverted through deeper shunt vessels, which do not lose heat to the environment

• This reduces heat loss by radiation at the skin surface

Sweat is secreted onto the skin by sweat glands and cools the skin by evaporation, as heat energy from the body converts liquid water into water vapour.

Muscles contract and relax repeatedly in quick succession; the metabolic reactions powering this release heat energy that warms the blood and raises core body temperature.

Hair erector muscles contract, making hairs stand on end and trap an insulating layer of air, which reduces heat loss by radiation.

• Seek out the sun or warmer surfaces and 'bask'

• Huddle together to retain heat gained from the sun earlier in the day

They save a lot of energy, so their nutritional requirements are lower, allowing them to survive where food is limited.

Water temperatures are significantly less variable than those on land, owing to the high specific heat capacity of water.

Temperature control in endotherms is an example of negative feedback, in which a change is detected and responses act to reverse it.

In humid environments there is a reduced water vapour concentration gradient between the skin and the air, so evaporation (and therefore cooling) is less effective.

The hormone thyroxine (released by the thyroid gland) raises the basal metabolic rate, and the exothermic metabolic reactions increase heat production in the body.

Hair erector muscles relax so hairs lie flat, preventing an insulating layer and allowing air to circulate over the skin to remove heat.

• Seek shade

• Move their bodies into water

False — they are caused by muscles in the walls of the arterioles; capillaries have no muscle in their walls.

True