Cells, Tissues & Organisms Key Notes

Additional Science – Biology

A cell is the basic building block for both animals and plants

Cells are extremely small (we need a microscope to see them)

Many organisms are multi-cellular - they are made up of lots of cells, not just one!

Many of these cells are specialised, sharing out the life processes (they work together as a team, supporting the organism)

Specialist cells occur in both animals and plants…

Bacteria are single celled organisms – they do not have DNA in a membrane bound nucleus and are termed prokaryotes (from the Greek meaning before nucleus)

Their genetic material is in a long strand and is sometimes found in circles called plasmids

Yeast are a single-celled organism in the fungi kingdom, containing a nucleus, cytoplasm, cell membrane and a cell wall…

Diffusion is the net movement of particles from an area of high concentration, to an area of low concentration

So the particles gradually move from where there are lots of them, to areas where there are less (net just means the overall)

Dissolved substances have to pass through the cell membrane to get into or out of a cell – diffusion is one of the processes that allows this to happen

Diffusion occurs when gas particles or particles dissolved in a solution spread

They move from a region where they are in high concentration to a region where they are in low concentration (this is how the smell of cooking travels around the house from the kitchen)

Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration – the greater the concentration difference, the quicker this occurs

Water can move across cell membranes because of osmosis – osmosis is the net movement of water molecules across a partially permeable membrane from a high concentration to a low concentration

For osmosis to happen you need: -

Two solutions with different concentrations
A partially permeable membrane to separate them (a partially permeable membrane lets some substances pass through them, but not others

Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls

When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions – at this point, the net exchange of water is zero and there is no further change in the liquid levels

In the gut digested food goes from an area of high concentration from the gut cavity to low concentration in the capillaries of the villi. In the lungs, the blood will continue to take in oxygen from the alveolar air spaces provided the concentration of oxygen there is greater than in the blood – oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away

Osmosis is important to plants – they gain water by osmosis through their roots as well as controlling the water content of some specialised cells

Stomata are tiny holes which allow CO2 into the leaf (they also allow water to escape) – they are pores which open and close automatically controlled by guard cells

When there is lots of water they become turgid so the pore opens

When there is not much water they become flacid which closes the pore preventing water loss but it also stops CO2 getting in so photosynthesis stops too!

A group of similar cells is called a tissue

A group of different tissues is called an organ

An organ is made from a group of different tissues, which all work together to do a particular job

Both animals and plants have organs, e.g.

Heart
Lung
Stomach
Brain
Kidney
Liver

Organs can work together to perform specific jobs – these are called organ systems

Circulatory system (heart; blood vessels; blood)
Respiratory system (lungs; trachea)
Digestive system (mouth; stomach; intestines; liver; anus)
Nervous system (brain; spinal cord; nerves)
Excretory system (kidneys; urinary bladder; urethra)
Reproductive system (ovaries; testes)
Muscular system (skeletal muscles)

Leaves are designed for one thing – making food via photosynthesis

Leaves are broad and flat to capture lots of light

Veins carry water to the leaf, and food away to the rest of the plant (veins also support the leaf)

Small holes called stomata in the underside of the leaf allow gases in and out

Cells in the epidermis make wax which covers the leaf surface (especially at the top) which limits water loss and acts as a protective barrier

Palisade cells are full of chloroplasts (containing chlorophyll) which it where photosynthesis takes place

Xylem transports water
Phloem transports food

Green plants absorb light energy using chlorophyll in their leaves – they use it to react carbon dioxide with water to make glucose (used for respiration, or converted into starch and stored)

Plants also need mineral ions, including nitrate and magnesium, for healthy growth – they suffer from poor growth in conditions where mineral ions are deficient

Photosynthesis is the chemical change which happens in the leaves of green plants

It is the first step towards making food - not just for plants but ultimately every animal on the planet

Photosynthesis takes place in leaf cells – these contain chloroplasts, which are tiny organelles containing chlorophyll which absorbs light energy for the reaction to take place

Plants make their own food via photosynthesis (chlorophyll in the leaves) with the energy coming from the Sun

Photosynthesis can be reduced by a variety of factors, including

Temperature
Carbon dioxide concentration
Light intensity

Light intensity – without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and CO2

Increasing the light intensity will boost the speed of photosynthesis

CO2 – sometimes photosynthesis is limited by the concentration of CO2 in the air

Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient CO2

Temperature – if it gets too cold, the rate of photosynthesis will decrease

Plants cannot photosynthesise if it gets too hot and enzymes within the chlorophyll begin to denature above 45oC

During photosynthesis glucose is produced (a simple sugar)

However, plants cannot store glucose, so they convert it into starch (many small glucose molecules are joined together)

Starch is insoluble, meaning it can be stored easily (when you wash some rice you can see the starch wash off)

Roots are adapted to absorb the minerals needed to remain healthy (although some plants have adapted some special ways to get more minerals, e.g. a Venus fly trap)

There are 3 essential minerals: -

Nitrates – needed for protein manufacture
Phosphates – needed for respiration and photosynthesis
Potassium – needed for enzymes involved in respiration and photosynthesis

Iron and magnesium are also needed in trace amounts for chlorophyll production

Organisms in a community compete for the resources there – they are in competition with each other

Animals compete for: -

Food
Water
Space / Shelter
Mates

Plants compete for: -

Light
Water
Space
Minerals (within the soil)
The number of predators or prey affect the number of organisms

Disease also affects the number of organisms in a habitat

Organism distribution can be identified by measuring how common an organism is in sample areas (using quadrats) or by studying how the distribution changes across an area using quadrats along a transect…

Population size is the mean number of organisms per m2 multiplied by the total area of the habitat

E.g. 800m2 habitat has a mean of 50 tulips per m2 then the population is 800 x 50 = 40’000

A quadrat is a square frame enclosing a known area (e.g. 1m2). It is placed on the ground at a random point and sampled. This is repeated, then a mean taken which allows for the population size to be worked out as before…

Transects are used to study the distribution of organisms along a line (ideally using the quadrats)

Enzymes are biological catalysts - catalysts are substances that increase the rate of chemical reactions without being used up

Enzymes are specifically proteins that are folded into complex shapes that allow smaller molecules to fit into them – the place where these substrate molecules fit is called the active site

If the shape of the enzyme changes, its active site may no longer work – the enzyme has been denatured

They can be denatured by high temperatures or extremes of pH

*It is wrong to say the enzyme has been killed – although enzymes are made by living things, they are proteins, and not alive

Enzymes are important catalysts in a variety of reactions within the body, including respiration and digestion, protein synthesis and photosynthesis

They are also important within industry, such as developing biological detergents and sports drinks…

As the temperature increases, so does the rate of reaction

Very high temperatures denature enzymes

The graph shows the typical change in an enzyme's activity with increasing temperature

The enzyme activity gradually increases with temperature until around 37°C

As the temperature continues to rise, the rate of reaction falls rapidly, as heat energy denatures the enzyme

The enzymes involved in respiration, photosynthesis and protein synthesis work inside cells

Other enzymes are produced by specialised cells and released from them (the digestive enzymes are like this)

They pass out into the gut, where they catalyse the breakdown of food molecules

Enzymes are not living! They are special proteins which can break large molecules down.

There are specific enzymes, which break down specific nutrients: -

Amylase (carbohydrase) enzymes break down starch into simple sugars
Protease enzymes break down proteins into amino acids
Lipase enzymes break down fats into fatty acids and glycerol

Digested food molecules are absorbed in the small intestine, passing through the wall of the small intestine into the bloodstream (where they are carried around the body to where they are needed)

Only small, soluble substances can pass across the wall of the small intestine

Large insoluble substances cannot pass through - this is why we need enzymes!

The stomach produces hydrochloric acid, helping to begin digestion (and killing many harmful microorganisms that might have been swallowed along with the food)

The enzymes in the stomach work best in acidic conditions (low pH)

After the stomach, food travels to the small intestine – the enzymes in the small intestine work best in alkaline conditions, but the food is acidic after being in the stomach

A substance called bile (produced in the stomach and stored in the gall bladder) neutralises the acid to provide the alkaline conditions needed in the small intestine (as well as breaking down fat)

Changes in pH alter an enzyme’s shape

Different enzymes work best at different pH values – the optimum pH for an enzyme depends on where it normally works

E.g. intestinal enzymes have an optimum pH of about 7.5 and enzymes in the stomach have an optimum pH of about 2

Enzymes are used in washing powders to help digest fats and proteins in food stains – biological washing powders will only work at >400C

Enzymes are used in baby foods to “pre-digest” the proteins

Enzymes are used to convert starch into sugar which can then be used in food

Conversion of glucose into fructose – glucose and fructose are “isomers” (they have the same chemical formula), but fructose is sweeter

Respiration is not the same thing as breathing (ventilation)

Respiration is a chemical process in which energy is released from food substances, such as glucose (sugar)

Aerobic respiration needs oxygen to work – most of the chemical reactions involved in the process happen in specialised cell organelles – the mitochondria

Respiration takes place in the cells (it is not “breathing in and out”)

Respiration is the release of energy (from glucose) in our cells

Animals and plants respire

Aerobic Respiration: -

glucose + oxygen  carbon dioxide + water + (energy)

C6H12O6 + 6O2 6CO2 + 6H2O + (energy)

Anaerobic respiration is used if there is not enough oxygen available – it is the incomplete breakdown of glucose which produces lactic acid…

glucose  lactic acid + (energy)

C6H12O6 2C3H6O3 + (energy)

Anaerobic respiration causes lactic acid to build up which results in cramp (muscle fatigue which causes the muscle to stop contracting efficiently)

It also does not produce nearly as much energy as aerobic respiration – though is useful to allow the muscles to work a little while longer…

Fitness comparisons – recovery time is a very good indication of fitness…

After digestion the pancreas produces insulin to store glucose as glycogen in the muscles

During exercise the glycogen is broken back down into glucose by glucagon (produced in the pancreas)

Glycogen is mainly stored in the liver but each muscle also has its own store