Cells

There’s no such thing as a typical cell. Cells are the building blocks of life, and all living things are made from them. Each cell has internal organelles they are all based in the cytoplasm of the cell which has the texture of sloppy jelly. It contains many dissolved substances such as proteins. This post explores the different cell types and the structures within cells.

Biology - Cells and organisms

An Organelle is a small structure within a cell, meaning that all cells are made of organelles. The nucleus is probably the most important organelle because it contains DNA coiled up as chromosomes. The DNA contains all of the information about which proteins to make inside the cell, which reactions should happen and (in a broader context) what the organism as a whole will look like. 

The cytoplasm is the structure in which most of the reactions take place - as a general rule, it is found around the outside of the vacuole in plant cells or everywhere within the cell membrane in animal cells. Most of the other structures can be found outside of the cytoplasm, for example the chloroplasts, which contain chlorophyll to allow them to generate energy from sunlight (photosynthesis, only found in plant cells), or the mitochondria which carry out the respiration reactions in the cell.

The vacuole is the structure that is usually found in the centre of a plant cell. They are filled with cell sap, a solution of sugars. Some specialised animal cells have temporary vacuoles which store food and water, but this is not common.


Around the outside of the cytoplasm will be a cell membrane. All cells have a cell membranes but it is very hard to see in plants because it is against the cell wall. A cell membrane is a very thin layer of protein and fat which controls what goes in and out of the cell. We describe it as a partially permeable; this means it lets some things through but prevents others from entry to the cell.

Cell walls can only be found in plants, and are made of a polymer called cellulose. A cell wall is a very strong covering for the cell. It protects and supports the cell. It can withstand the internal pressures of turgor (when the cell has become stiff because it has absorbed so much water through osmosis. Turgor keeps plants stiff so that they can grow tall and compete for light). Spaces between the cellulose fibres allow even large molecules to pass through; it is described as fully permeable.


There are a few interesting exceptions to the rules. Xylem vessels in plant stems do not have nuclei because they are technically dead cells. Red blood cells have no nuclei because they need to conserve room in the nucleus to carry as much haemoglobin as possible.


Remember - there are many exceptions. This is just an accurate generalisation that can be applied for most GCSE purposes.

Excretion

Biology - Homeostasis & Excretion - Excretion

‘Excretion is the process by which waste products of metabolism are removed from the body’

Excretion happens in both animals and plants. Plants also oxygen (a waste product of the photosynthesis reaction) and carbon dioxide (from the respiration reaction). However, most of the detail here is about excretion in animals.


Nitrogenous waste has to be excreted by all animals.This is because it cannot be stored in the body in the same way that carbohydrates and fats can. Excess proteins are the main source of nitrogenous waste, and are converted either into carbohydrates or into urine.


Humans have a special excretory system. Most of the body's waste products are removed in sweat and urine.

Urine contains:

• ·       Urea
• ·       Ammonia
• ·       Other nitrogenous waste
• ·       Potassium (not nitrogenous)
• ·       Phosphate (not nitrogenous)

Filtration happens in 3 stages:

1.     Blood from the aorta (the largest artery in the body) enters the renal artery (renal meaning to do with the kidneys). This blood is carried to the kidneys.

2.     The kidney filters out the waste products

3.     The ‘clean’ blood leaves via the renal vein and enters the vena cava (a larger vein).

The urine travels down the ureters (connecting tubes) into the bladder. At the top of the urethra, the two sets of sphincter muscles control whether the urine can flow out of the body.
The voluntary sphincter muscles can be relaxed consciously, but the upper sphincter muscles relax when the bladder is full.

The kidney has 3 important types of structure:

Ø  Cortex. The blood flows into the cortex from the renal artery. The cortex contains blood vessels that branch from there, as well as nephrons (also known as kidney tubules)

Ø  Nephrons/kidney tubules run from the cortex to the medulla. The urine is emptied from there into the pyramids.

Ø  Pyramids join to the medulla, and they take the urine from there that has been filtered by the nephrons. The urine collects in the pelvis, which leads to the ureter.

Focusing on near and far objects (accommodation)

Biology - Coordination - Eyes

The cornea does about 70% of the work involved in converging the light rays onto the retina. The lens will do the final adjusting. This is called Accommodation. Accommodation is a reflex action. The stimulus is the light, which is picked up by the rod and cone cells in the eye (the receptors). The signal as to whether the image is too far-focussed or short-focussed is sent via a sensory neurone to the CNS, which uses a relay neurone to send an impulse down a motor neurone to the effector, the muscles, which will relax or contract according to the original stimulus.

Accommodation is achieved through the relaxation and contraction of the antagonistic pair of muscles, the ciliary muscles and the suspensory ligaments.

Near Focusing: The ciliary muscles relax, and then the lens is pulled thin by the strain on suspensory ligaments exerted by the sclera under pressure from tissue fluid.

Far Focusing: The lens collapses more, due to its elasticity, when the strain on suspensory ligaments is reduced and ciliary muscles contract.

Long sighted people are unable to focus on near objects. Either the cornea or lens doesn’t bend the light enough, or the eyeball is too short. The image is brought into focus too far behind the retina.

Short sighted people cannot focus on distant objects. The cornea or lens bends the light too much or the eyeball is too long. The image is brought into focus in front of the retina.

With age, the lens loses flexibility and does not go back to a round shape as easily. This is why some older people have to wear reading glasses.