Y7 SCIENCE

Cells and Movement

Home Learning Booklet

What are cells?

• All living things are made up of cells.

• Most cells are so small that you can only see them with a microscope.

• Cells have different components and each performs its own function within the cell.

What’s in a cell?

Cells are the basic building blocks of all animals and plants.

Inside cells are various structures that are specialised to carry out a particular function. Both animal and plant cells have these components:

• Cell membrane – this surrounds the cell and allows nutrients to enter and waste to leave it.

• Nucleus – this controls what happens in the cell. It contains DNA, the genetic information that cells need to grow and reproduce.

• Cytoplasm – this is a jelly-like substance in which chemical reactions happen.

• Mitochondria – these are the powerhouse of the cell. They are structures where respiration takes place.

How are plant and animal cells different?

Plant cells have all the parts in the list above, plus a few extra structures:

• Cell wall - this is an outer structure that surrounds the cell and gives it support.

• Vacuole - this is a space within the cytoplasm of plant cells that contains sap.

• Chloroplasts - these contain chlorophyll and are the site of photosynthesis.

How big are cells?

Cells can vary greatly in size. You need a microscope to see most human cells.

Red blood cells are some of the smallest cells in the human body. These have a diameter of 0.008 mm, meaning a line of 125 red blood cells is only 1 mm long.

The ovum (or egg cell) is one of the largest cells in the human body. It has a diameter of roughly 0.1 mm, so you can see them without a microscope. A line of 10 egg cells is 1 mm long.

What are specialised cells?

• Specialised cells have a specific role to perform.

• Each specialised cell has a different job to do. They have special features that allow them to do these jobs.

• Muscle cells, for example, are held together in bundles, which pull together to make muscles contract.

Most cells share features such as having a nucleus, a cell membrane, cytoplasm and mitochondria.

There are differences between cells, too. Each type of cell, has its own job to do. These cells have special features that allow them to perform their functions effectively.

Here are some examples of specialised cells and the features they have to help them with their role:

Red blood cells

Red blood cells carry oxygen around the body. They are well suited to this function because:

• They contain haemoglobin, which carries oxygen molecules.

• They don't have a nucleus, allowing more space to carry oxygen.

• They are a flat disc shape (bi-concave) which gives them a large surface area, and the best chance of absorbing as much oxygen as they can.

Nerve cells

Nerve cells transmit electrical signals. They are well suited to their function because:

• They are thin, and can be more than 1 metre long. This means they can carry messages up and down the body over large distances.

• Nerve cells have branched connections at each end. These join to other nerve cells, allowing them to pass messages around the body.

• They have a fatty (myelin) sheath that surrounds them. The fatty sheath increases the speed at which the message can travel.

Muscle cells

Muscle cells bring parts of the body closer together. They are well suited to this function because:

• Muscle cells are held together in bundles, which pull together to make muscles contract (get shorter and fatter).

There are different types of muscle cell, each perfectly adapted to its function:

• Cardiac (heart) muscle cells are branched, and they join together to make a net. Cardiac muscle cells contract rhythmically, even outside the body. They never get tired.

• Skeletal muscle is joined to bones. Its cells contract to make bones move and joints bend.

• Smooth muscle cells make up thin sheets of muscle, such as the stomach lining. They can also be arranged in bundles, or rings, like that in the anus.

How to observe cells under a microscope

• All living organisms are made up of cells.

• Cells are the smallest part of a living organism and are around 0.01 mm - 0.03 mm long.

• To look at a cell close up we need a microscope.

How to use a microscope

1. Move the stage (the flat ledge the slide sits on) down to its lowest position.

2. Place the glass slide onto the stage. Be careful pushing it under the clips that the cover slide doesn't move or crack.

3. Select the lowest power objective lens.

4. Turn the coarse focus knob slowly until you are able to see the cells.

5. Turn the fine focus knob slowly until the cells are in focus and you can see them clearly.

6. Repeat steps 1-5 using the higher power magnification to see the cells in more detail.

Calculating magnification

To see an object, the eye piece lens and the objective lens magnification are multiplied together to give the total magnification.

Total magnification = eye piece lens magnification × objective lens magnification

For example: 10 × 20 = 200

Magnification = 200x

Layers of organisation

The human body is made up of many cells, so it is an example of a multicellular organism. A multicellular organism has five layers of organisation, called a hierarchy. Here they are in order of complexity:

• Organelles - tiny structures which carry out various functions within a cell.

• Cells - the basic unit of all living organisms.

• Tissues - groups of similar cells that work together to perform a specific function. For example, brain tissue, muscle tissue and heart tissue.

• Organs - different tissues working together to carry out a particular function.

• Organ system - a group of organs that work together to do a job.

Organs

In an organ, different tissues work together to carry out a particular function. These are the main organs, as well as their primary function:

• The brain controls thoughts, memory and other organs.

• The heart pumps blood around the body.

• The lungs separate oxygen from the air and remove carbon dioxide from the blood.

• The stomach helps to digest food.

• The intestines absorb nutrients from food.

• The liver removes poisons from the blood.

• The kidneys filter blood and produce urine.

• The bladder stores urine.

• The skin protects and contains the other organs.

Organ systems

Organs are organised into organ systems. An organ system is a group of organs that work together to do a job.

Key human organ systems:

• The circulatory system includes the heart, veins and arteries. Its function is to transport substances in the blood, around the body.

• The respiratory system includes the nose and the lungs. It takes in oxygen and removes carbon dioxide.

• The digestive system includes the stomach and intestines. It breaks down food and absorbs nutrients.

• The reproductive system includes the uterus and vagina in women, and the penis and testes in men. Its function is to create offspring.

• The musculoskeletal system includes bones and muscles. It supports the body and allows movement.

• The immune system includes bone marrow. It protects the body from infection.

Life processes

Living organisms have certain life processes in common. There are seven things that they need to do to count as being alive. The phrase MRS GREN is one way to remember them:

• Movement - all living things move, even plants

• Respiration - getting energy from food

• Sensitivity - detecting changes in the surroundings

• Growth - all living things grow

• Reproduction - making more living things of the same type

• Excretion - getting rid of waste

• Nutrition - taking in and using food

It can be easy to tell if something is living or not. A teddy bear might look like a bear, but it cannot do any of the seven things it needs to be able to do to count as being alive.

A car can move, it gets energy from petrol (like nutrition and respiration), it might have a car alarm (sensitivity), and it gets rid of waste gases through its exhaust pipe (excretion). But it cannot grow or make baby cars. So a car is not alive.

Uni-cellular organisms

A unicellular organism is a living thing that is just one cell. There are different types of unicellular organism, including:

• bacteria

• protozoa

• unicellular fungi

You might be tempted to think that these organisms are very simple, but in fact they can be very complex. They have adaptations that make them very well suited for life in their environment.

What are bacteria?

Bacteria are among the smallest living things. A single bacterium consists of just one cell, and is called a single-celled, or unicellular organism.

Even though it is just a single cell, it can carry out all seven life processes (movement, respiration, sensitivity, growth, respiration, excretion and nutrition).

What’s in a bacterium?

There are similarities between plant and animal cells and a bacterium. They all share a cell membrane and cytoplasm. Bacterium also have a cell wall around them, like plant cells. However unlike most plant and animal cells, it has no nucleus. Instead, it has a circular strand of DNA, called a plasmid, that floats in the cytoplasm.

There are millions of different types of bacteria. Different types of bacteria have different shapes.

Many types of bacteria have extra cell structures to help them to survive. For example, they may have:

• Flagella, which are tail-like structures that allow bacteria to move through liquids.

• A slime capsule, outside the cell wall, to protect them and stop the bacterium drying out.

Why is it useful to know about bacteria?

Some bacteria cause diseases, including food poisoning, meningitis and scarlet fever. Inside the body, these bacteria reproduce quickly. They produce poisons that make you feel ill.

Among the people that need to know about bacteria are chefs. Anyone who cooks needs to know how to prevent bacteria spreading in their kitchens, to ensure all the food they produce is safe to eat.

Not all bacteria are harmful. For example, yoghurt and cheese makers use bacteria to make their products. Bacteria like intestinal flora can be helpful to our bodies whereas E.coli can be harmful and cause disease.

More importantly, there are millions of bacteria in your digestive system. Having the right mix of bacteria here is vital. Gut bacteria help to digest food. They also help to prevent diabetes, obesity and some types of cancer.

Protozoa

Protozoa are unicellular organisms that live in water or in damp places. The amoeba is an example of one. Although it is just one cell, it has adaptations that let it behave a bit like an animal:

• it produces pseudopodia (“false feet”) that let it move about

• its pseudopodia can surround food and take it inside the cell

• contractile vacuoles appear inside the cell, then merge with the surface to remove waste

Yeast

You may be familiar with fungi from seeing mushrooms and toadstools. Yeast are unicellular fungi. They are used by brewers and wine-makers because they convert sugar into alcohol, and by bakers because they can produce carbon dioxide to make bread to rise.

Yeast have a cell wall, like plant cells, but no chloroplasts. This means they have to absorb sugars for their nutrition, rather than being able to make their own food by photosynthesis

Yeast can reproduce by producing a bud. The bud grows until it is large enough to split from the parent cell as a new yeast cell.

The skeleton

Our skeleton is made of more than 200 bones. Calcium and other minerals make the bone strong but slightly flexible. Bone is a living tissue with a blood supply. It is constantly being dissolved and formed, and it can repair itself if a bone is broken.

Function of the skeleton

The skeleton has four main functions:

• to support the body

• to protect some of the vital organs of the body

• to help the body move

• to make blood cells

Support

The skeleton supports the body, e.g. without a backbone we wouldn’t be able to stay upright.

Protection

Here are some examples of what the skeleton protects:

• the skull protects the brain

• the ribcage protects the heart and lungs

• the backbone protects the spinal cord

Movement

Some bones in the skeleton are joined rigidly together and cannot move against each other. Bones in the skull are joined like this. Other bones are joined to each other by flexible joints. Muscles are needed to move bones attached by joints.

Making blood cells

There are different kinds of blood cells, including:

• red blood cells, which carry oxygen around the body in the blood

• white blood cells, which are involved in destroying harmful microorganisms in your body

These cells are made in the bone marrow. This is soft tissue inside our larger bones which is protected by the hard part of the bone which surrounds it.

Joints

Bones are linked together by joints. Most joints allow different parts of the skeleton to move. The human skeleton has joints called synovial joints.

The synovial joint

If two bones just moved against each other, they would eventually wear away. This can happen in people who have a condition called arthritis. To stop this happening, the ends of the bones in a joint are covered with a tough, smooth substance called cartilage. This is kept slippery by a liquid called synovial fluid. Tough ligaments join the two bones in the joint and stop the joint falling apart.

Movement

Different types of synovial joint allow different types of movement. The table describes two types of joint:

Type of joint Examples Movement allowed

Hinge joint Knee, elbow The same as opening and closing a door, with no rotation (turning)

Ball and socket

Hip, shoulder

Back and forth in all directions, and rotation

The bones cannot move on their own - they need muscles for this to happen.

Muscles

Muscles work by getting shorter. We say that they contract, and the process is called contraction. Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.

Antagonistic muscles

Muscles can only pull and cannot push. This would be a problem if a joint were controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. This problem is solved by having muscles in pairs, called antagonistic muscles.

For example, your elbow joint has two muscles that move your forearm up or down. These are the biceps on the front of the upper arm and the triceps on the back of the upper arm:

• to raise the forearm, the biceps contracts and the triceps relaxes

• to lower the forearm again, the triceps contracts and the biceps relaxes

Muscles exert a force on bones when they contract. This happens for example when you lift or hold an object, or when you move a part of your body.

If you hold an object weighing 10 N, keeping it still and with your forearm horizontal and your upper arm vertical, you would be exerting an upwards force of 10 N on the object. You could work out the force exerted by the biceps muscle to do this using the idea of moments. The way in which muscles and bones work together to exert forces is called biomechanics.